Probe Software Users Forum

General EPMA => Discussion of General EPMA Issues => Topic started by: Probeman on October 29, 2014, 03:48:31 pm

Title: Dealing With Magnetic Specimens
Post by: Probeman on October 29, 2014, 03:48:31 pm
Hi all,
We are running some Fe-Ni-Cr alloys and some of the samples can cause the beam to deflect from the intended position by 50 um or more due to the magnetic field of the sample. Besides not analyzing where we want to, the Bragg defocus effects are killing our quant.

I tried the hysteresis button but that didn't help, so we are looking for a demagnetizer of some type, but wonder if anyone has a good solution for demagnetizing magnetic samples prior to loading them... or will the specimen magnetic field reappear once inside the instrument or the beam is on the sample?
Title: Re: Dealing With Magnetic Specimens
Post by: Karsten Goemann on October 29, 2014, 04:00:48 pm
How thick are the samples? We used to encounter problems with massive magnetite, but when we prepare them as petrological polished thin sections (30 microns thick) on glass slides, the problem usually goes away.
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on October 29, 2014, 04:04:24 pm
How thick are the samples? We used to encounter problems with massive magnetite, but when we prepare them as petrological polished thin sections (30 microns thick) on glass slides, the problem usually goes away.

Good question: unfortunately these are massive samples (1.25" to 1.5" diameter and at least a cm thick), and they aren't ours so they cannot be modified.
Title: Re: Dealing With Magnetic Specimens
Post by: Gseward on November 05, 2014, 10:51:52 pm
I have similar issues with Fe and Co alloys. It took me along time to convince the researchers that this was the issue. Thankfully I have a BIG demag unit that is very effective at randomizing the magnetic domains within these samples. I don't remember the numbers exactly, but as I recall a particularly 'bad' sample would have a field of 100's of gauss in some directions; after 3 cycles of demag in orthogonal directions, I might get it down to 0.1 . Obviously the direction and magnitude of deflection can be very anisotropic, both macroscopically and, I suspect in my samples, on a phase by phase basis.

The equipment is a relict of our old geomag lab and was built by Bob Dunn, a former geomag wizard in our department. So, I can demag them for you, but you'll have to send them. I think I could do 1.5" but I'll check tomorrow.

Perhaps an nth order effect, but out of interest, I wonder if the angle of deflection could be severe enough to also affect the depth of e- interaction and the absorption path length?
Title: Re: Dealing With Magnetic Specimens
Post by: Gseward on November 17, 2014, 07:47:45 pm
John,

Did you have any success? Where you able to measure the field strength before/after demag?

Gareth
Title: Re: Dealing With Magnetic Specimens
Post by: John Donovan on November 17, 2014, 11:51:27 pm
Did you have any success? Where you able to measure the field strength before/after demag?
Did not find a way to measure the field, but I think we're seeing a magnetic sample beam deflection (in scan mode and in spot mode) which, after degaussing and inserting in the instrument, starts at zero deflection (relative to the optical image which does not drift at all) and increases to about 50 um over 10 minutes. It is definitely not charging. Seems to settle eventually. See images below.

Running the sample using *only* EDS elements now... to avoid Bragg defocusing.

Is it possible a magnetically susceptible specimen can be re-magnetized by the electron beam? As evidence I offer the observation that the beam deflection only drifts when the faraday cup is out. If we blank the beam and wait a few minutes and then unblank, the beam deflection as observed on the electron image, has not shifted relative to how it last looked, but then immediately and slowly starts shifting again, as long as we leave the beam on the sample.
john
Title: Re: Dealing With Magnetic Specimens
Post by: Les Moore on November 19, 2014, 03:11:06 pm
A few thoughts...
1. Could it be a heating effect?
     Is the effect the same on all corners of the sample?
2. Could it be a charging effect?
     I recall that someone saying 40% of the 2nd'y electron yield comes from the polepiece via BSE. 
     So, these BSE could be BSE'd back onto the sample somewhere else to possibly charge.
     I would imagine that this would be cylic though.
3. Lastly, the sample should not be magnetised by the beam unless the beam creates a magnetic field.
    M=BILsin(theta).  'I' you have, the rest is anyones guess.
4. if the beam is magnetising the sample, it should be 'I' dependant. 
    But then again, so should heating and charging.

Now really pressing hard...
5. Local heating may allow the magnetic domain boundaries to migrate.  This may cause localised magnetic variations but this is just a guess. 

There's a lot of work being done on domain boundaries in TEM using low loss EELS and other techniques that are beyond my ken.

 
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on November 19, 2014, 03:23:59 pm
A few thoughts...
1. Could it be a heating effect?
     Is the effect the same on all corners of the sample?

Hi Les,
Thanks for chiming in.

I don't see how it could be a heating effect... that would *reduce* the magnetism (if we could get beyond the Curie temperature, which we can't in a thermally conductive material).

The effect (degree of beam deflection) is different in every part of the sample, but it is a complex sample of cm sized blocks of various pure metals including Ni, Fe and Cr...), so that would be expected?

2. Could it be a charging effect?
     I recall that someone saying 40% of the 2nd'y electron yield comes from the polepiece via BSE. 
     So, these BSE could be BSE'd back onto the sample somewhere else to possibly charge.
     I would imagine that this would be cylic though.

No, it is not. The degree and direction of beam deflection goes with the sample rotation- yes, we tested that on Monday!

Also the SE image shows zero charging.

3. Lastly, the sample should not be magnetised by the beam unless the beam creates a magnetic field.
    M=BILsin(theta).  'I' you have, the rest is anyones guess.

Yes, that is the question.

4. If the beam is magnetising the sample, it should be 'I' dependant. 
    But then again, so should heating and charging.

We have not tried different beam currents, but it is a good suggestion... we'd have to demag the sample each test and carefully time the degree of beam deflection.  It is very strange...

Now really pressing hard...
5. Local heating may allow the magnetic domain boundaries to migrate.  This may cause localised magnetic variations but this is just a guess. 

Again I don't think so. See above, #1
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on December 03, 2014, 03:12:07 pm
I'm posting this response (with his permission) from one of our physics faculty who had some thoughts on this issue:

Quote
At 11:15 PM 11/20/2014, Benjamin McMorran wrote:
Hi John,

Interesting!. Yes, it sounds like Lorentz deflection of the beam to me. The question is why is there is a dose dependence? If it's only Fe-Ni-Cr in the sample, then I'd be surprised if it's heating up and switching domains significantly enough to deflect the beam by that amount. The azimuthal magnetic field from the beam itself is pretty small and drops off linearly with distance from the beam, so I don't think this is changing the magnetization (though that would be really cool!).

Maybe it's the fringing fields from the objective lens? A lot of SEMs can have a pretty high magnetic field from the OL, even if it's not purposefully using an immersion lens design. This might appear to depend on the scan range just because of the domain structure - different initial trajectories of the beam would get deflected different amounts.

For the demag, are you using a tabletop one, or are you talking about the instrument's demag? A tabletop one can be made with just a coil (I have one in the lab) and a variac.

Again, interesting problem!
Ben

Dr. Benjamin McMorran
Assistant Professor of Physics
Materials Science Institute and Oregon Center for Optics
Department of Physics
1274 University of Oregon
Eugene, OR 97403-1274
office phone:‚ 541-346-8624

My response was:

Quote
Hi Ben,
Thanks for the interesting and informative response.

The dose dependence could also be described as a time dependence as well, as it appears to increase the beam deflection with a constant beam current over time, and not while the beam is blanked.

We tried a hysteresis cycle several times on the objective lens, and to a first order it made no difference to the beam deflection degree or rate of change.

The demag is a table top device that Julie bought recently.

If you'd like to discuss this further just drop by the lab Mon through Wed when I'm here.  It's causing us a lot of problems- basically we had to give up on WDS for these samples!
john
Title: Re: Dealing With Magnetic Specimens
Post by: Les Moore on December 04, 2014, 02:57:53 am
Clutching at straws...

Could you cover all but a small window on the sample with Al tape.
It might shield the interaction volume from bulk sample effects.

Les
Title: Re: Dealing With Magnetic Specimens
Post by: John Donovan on December 04, 2014, 05:21:10 pm
Could you cover all but a small window on the sample with Al tape.
It might shield the interaction volume from bulk sample effects.

Hmmm... an interesting idea I must say.

Although using PFE for automated EDS acquisition works fine because there is no Bragg defocusing and and long as we plan the traverese coordinates using the BSE image (which already contains the beam deflection effects) it all seems to work pretty well.

So I know that Al shielding can be used for EMI because that is what we did for our power panels in the equipment gallery behind our instruments. We put 1/4" Al between the wall and the back of the power panel and it produces at least a 20 db drop in EMI into the instrument room as seen here:

(http://probesoftware.com/smf/oldpics/i57.tinypic.com/24g6ogz.jpg)

However, I wonder if Al would work for magnetic fields?  I know people usually employ "mu metal" which is a soft iron for magnetic shielding when I worked at LBL.

But I guess it's worth a try and heck, at the very least it sounds a like a great idea for a student project! Just get some tiny magnets and mount them one each in epoxy pucks and try different orientations and so forth.
Title: Re: Dealing With Magnetic Specimens
Post by: Les Moore on December 05, 2014, 04:15:13 pm
As I said, clutching at straws.

Could you cover all but a small window on the sample with Al tape.
It might shield the interaction volume from bulk sample effects.

But I guess it's worth a try and heck, at the very least it sounds a like a great idea for a student project! Just get some tiny magnets and mount them one each in epoxy pucks and try different orientations and so forth.

How cruel.  ;)

Especially if you ask for high mag pictures.  I once tried to get a SEM picture of the fracture surface on a hard magnet; not possible.

Les
Title: Re: Dealing With Magnetic Specimens
Post by: Gian Colombo on September 16, 2016, 07:12:04 am
I recently encountered this issue as well, also on metallic specimens primarily containing iron-chromium-nickel.  The job request was to perform multiple point analyses in a square grid pattern that effectively covered the entire specimen face.  The analytical totals shifted wildly (~99wt% to ~55wt%) and displayed a repeating periodic pattern as the grid passed through regions of magnetic domains with different orientations that would shift the beam different amounts and in different directions which took different spectrometers in and out of focus depending on sample location.

Here's a summary of the few things that we tried to address the problem:

-De-magnetizing helped reduce the overall magnitude of the beam displacement, but prior regions that showed severe deflection still pushed the beam far enough off axis that analytical totals were not recovered.

-Working at a higher accelerating voltage (25-30kV) had a centering effect on the beam.  It was still not quite enough to eliminate the deflection in all areas, but analytical totals were recovered over a significant area of the specimen.

-The best results we achieved were accomplished by "re-aligning" the beam with a user-defined beam offset (much like one would use to analyze different features in a SEM field of view without moving the stage) to counteract the deflection induced by the specimen.  The optical image was centered on an easily recognizable feature in the SE image which was used as a target to define the beam offset such that the beam would now be hitting the correct spot on the specimen.  This worked beautifully over small areas where the magnetic effects were relatively consistent, but it became very tedious as I worked across the specimen because I had to change the offset multiple times to account for the changes in the magnetic beam deflection.
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on September 16, 2016, 08:26:26 am
Hi Gian,
That is very interesting (and tedious sounding!).

On our magnetic samples we tried higher keV which also didn't help enough, but because the customer only wanted specific traverses in specific locations on the sample, we ended up using PFE in EDS acquisition mode (without any WDS elements). (We have a Thermo NSS system on our SX100 interfaced to PFE for full spectra acquisition and quantification).

So, we would get to the correct sample locations using BSE electron imaging (which would always be off by some amount from the optical image), digitize the stage positions from the BSE image using the Digitize Image feature, and then just run them with the "Acquire EDS spectra for each point option. That worked pretty well as the EDS system doesn't have Bragg defocus issues of course.
john
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on December 21, 2017, 04:22:13 pm
We had a client in our lab recently wanting to look at very large (4.5 cm diameter) mild steel samples. In fact we had to make a special sample holder for our Sx100 to accommodate them.

Unfortunately the sample was highly magnetized, so much that when the optics and electron image were the same FOV, the electron image was so deflected that the central feature (as viewed in the light optics), was barely visible on the edge of the electron image.

We tried a "de-gausser" device (Kandrick & Davis, model 19-217, see attached image below) which we had purchased last time for some Ohio State Fe samples, but it only slightly decreased the magnetic effect.

We wanted to run WDS on these samples for the oxygen and carbon contents, but had to utilize EDS instead because of the magnetic deflection from the sample.  Unfortunately the EDS sensitivity for oxygen and carbon in steel is pretty awful due to the large Fe L emission peaks, though I'm interested in discussing that issue in a separate topic.

In the meantime does anyone who works with steel samples in EPMA have a suggestion for a more effective de-magnetizer that we can buy/build for such large samples?
john
Title: Re: Dealing With Magnetic Specimens
Post by: Les Moore on December 25, 2017, 05:06:51 am
Hi guys,

The interesting issue is why the sample needed to be so big.
If the whole sample is needed to be examined by mapping, then the pixel size will probably need to 50um or so and the deflection will possibly be within the pixel. Also, with this level of deflection you probably wont defocus the oxygen or carbon even if the beam moves within the pixel. Mn, Cr, Co, Ni on a LIF and Si on a TAP would be a different issue.
 
I would imagine the defection will be a function of the centre of mass/magnetic field.
"Mild steel" should not be a hard magnet and should demagnetise easily.  Ask some more questions.

If they want true microanalyses, can you cut the sample up?

Another issue wrt mild steel is the request for carbon and oxygen.  Are you talking to Metallurgists?
In Mild steel, Carbon will be present (barely) in the ferrite, tied up in Fe3C (6.7 wt%) which is present as a grain boundary carbide in low carbon steels ~0.05 wt% C.  This turns into an eutectoid mixture (pearlite) with a local concentration of 0.8wt%. So, the carbon content will be 0.008 in ferrite, 6.7% in Fe3C and a 'bulk' localised analysis of 0.8wt% in pearlite.

Higher cooling rates just make this finer with different transformation mechanisms such as the formation by bainite or martensite; this latter one freezes in the carbon into a metastable phase but you certainly aren't talking about mild steel any more.

The request for oxygen is equally puzzling.  The oxygen, (down to below 20ppm) should all be tied up in oxides.  Oxygen will react with carbon during solidification to form bubbles so it is tied up in strong oxides such as Al2O3 or manganese silicates depending on the steelmaking route.  Again, the oxygen should be 20ppm or 30-40wt% in the oxides which are a few um in size.

Happy to contribute more once more detail available.

Les
PS C maps of large areas tend to just be critiques of the sample preparation process - fingerprints, remnant polishing lubricant, polishing medium, mounting medium etc. 

Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on December 25, 2017, 10:22:31 am
Hi Les,
Good questions.

The large sample is from an engineering company and is a "forensic" sample from a vehicle accident litigation involving multiple fatalities where a modified axle assembly failed.  So the sample cannot be modified as it came from the NTSB, according to the metallurgist I am working with.

They don't care about oxygen, but care very much about carbon in relation to a SS part that was welded on.  I just added oxygen in because I had one spectrometer doing carbon and threw in oxygen since we had doubled up the other spectrometers for other elements. But since the sample was so magnetized, we couldn't use WDS anyway, so all moot.   They don't want maps, they just want point analyses from several regions on the sample near where they performed hardness testing.

We tried using the de-magnetizer (pictured above), several times and could not demagnetize the sample significantly.  I'm hoping someone has a more robust de-magnetizer suggestion for us...

We had good totals using EDS with standards, but oxygen and carbon were either zero or extremely large values due to the large Fe L emission lines they are sitting on.

I agree trace carbon analysis is challenging even with WDS EPMA.
john
Title: Re: Dealing With Magnetic Specimens
Post by: Les Moore on December 29, 2017, 07:12:53 am
Ooo errr, serious stuff.

The large sample is from an engineering company and is a "forensic" sample from a vehicle accident litigation involving multiple fatalities where a modified axle assembly failed.  So the sample cannot be modified as it came from the NTSB, according to the metallurgist I am working with.

So let me get this right, have they have welded a stainless steel component to a steel axle?

They don't care about oxygen, but care very much about carbon in relation to a SS part that was welded on. 

If so, why?
Steelmakers try very hard to get the C content in SS as low as possible (unless they are making a martensitic stainless for knife/blade usage and you certainly wouldn't weld that).  They even add strong carbide forming elements such as Ti, V and Nb. This is to avoid CrxCy forming in the heat affected zones or in the weld metal.

In corrosion scenarios, the CrxCy (X & Y depend on the alloy level) forms and locally depletes the areas around the carbides of Cr.  This is called sensitization.  This is quite significant and creates a galvanic cell between the Cr carbides and the Cr depleted regions and corrosion channels in - often called knife edge corrosion.

Background ref:
https://www.corrosionpedia.com/definition/1334/sensitization-stainless-steel
http://www.ssina.com/corrosion/igc.html

A good micrograph of HAZ sensitization:
https://www.hindawi.com/journals/ijc/2011/305793/fig3/
in article:
https://www.hindawi.com/journals/ijc/2011/305793/

Going back to what is going on, if they have welded SS to a high carbon steel axle then you need to map at various scales especially in the heat affected zone of the SS.

I would also be worried about solidification cracking within the weld as the C will significantly change the solidification path and possibly locally form low melting point eutectics that could rip apart.

Note the Vertical axis in the following diagram....
https://www.researchgate.net/profile/Johan_Pilhagen/publication/237660542/figure/fig3/AS:298763013902338@1448242061226/Figure-323-Schaeffler-constitution-diagram-for-stainless-steel-weld-metal-modified-for.png

The C has a scale factor of 30, much higher than any other deliberate alloy.  This can locally change the structure:

eg:
http://products.asminternational.org/fach/content/fach004/graphics/inline/i0017635.jpg

In this case, the situation was similar, a stainless weld on a high carbon steel:
http://products.asminternational.org/fach/data/fullDisplay.do?database=faco&record=642&search=
The problem here was dilution resulting in a drop down the Schaeffler diagram rather than C pickup but you get the idea.

Cheers

Les

My axiom would be to map first and ask questions later.
   
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on December 29, 2017, 10:24:37 am
So let me get this right, have they have welded a stainless steel component to a steel axle?

Yes. Axle housing I think...

They don't care about oxygen, but care very much about carbon in relation to a SS part that was welded on. 

If so, why?

Steelmakers try very hard to get the C content in SS as low as possible (unless they are making a martensitic stainless for knife/blade usage and you certainly wouldn't weld that).  They even add strong carbide forming elements such as Ti, V and Nb. This is to avoid CrxCy forming in the heat affected zones or in the weld metal.

They are only interested in the carbon in the steel part, not the SS.   Not sure why.

In corrosion scenarios, the CrxCy (X & Y depend on the alloy level) forms and locally depletes the areas around the carbides of Cr.  This is called sensitization.  This is quite significant and creates a galvanic cell between the Cr carbides and the Cr depleted regions and corrosion channels in - often called knife edge corrosion.

Background ref:
https://www.corrosionpedia.com/definition/1334/sensitization-stainless-steel
http://www.ssina.com/corrosion/igc.html

A good micrograph of HAZ sensitization:
https://www.hindawi.com/journals/ijc/2011/305793/fig3/
in article:
https://www.hindawi.com/journals/ijc/2011/305793/

Going back to what is going on, if they have welded SS to a high carbon steel axle then you need to map at various scales especially in the heat affected zone of the SS.

I would also be worried about solidification cracking within the weld as the C will significantly change the solidification path and possibly locally form low melting point eutectics that could rip apart.

This is really interesting stuff. Thanks for posting.  Yes, there is cracking at the weld, which is the concern I think.

My axiom would be to map first and ask questions later.

That would be my preference as well, but they insist they only want a few data points, apparently to keep the costs down. I would have started with WDS mapping of the traces, but the strong magnetism of the sample killed that idea.

Do you have any suggestions for a "professional" grade de-magnetizer?   One of the professors here suggested hooking up a Variac to the demagnetizer and turning it on at zero AC volts and then increasing the AC volts to max and then decreasing it to zero without moving the sample within the demagnetizer.  That's the next thing I'm going to try unless someone has a better idea.
john
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on January 20, 2018, 09:06:46 am
Do you have any suggestions for a "professional" grade de-magnetizer?   One of the professors here suggested hooking up a Variac to the demagnetizer and turning it on at zero AC volts and then increasing the AC volts to max and then decreasing it to zero without moving the sample within the demagnetizer.  That's the next thing I'm going to try unless someone has a better idea.

OK, I managed to find the old Variac that I knew I used to have somewhere in my garage...  found this pic on the web which is the exact same model as mine (and just about in the same condition!):

(http://probesoftware.com/smf/gallery/395_20_01_18_9_01_05.png)

brought it to the lab, hooked it up to our de-magnetizer unit, and as my colleague suggested, we set the Variac to zero, placed a strongly magnetized sample inside the de-magnetizer, shown here:

(http://probesoftware.com/smf/gallery/395_20_01_18_9_08_46.jpeg)

We slowly increased the AC to 120v or so, then slowly decreased the AC and then tested with with a magnetometer.  Result?  Completely demagnetized!

So this is how I will be demagnetizing steel samples from now on.
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on October 10, 2018, 01:53:18 pm
Warning, this post might be painful to read.   :D

I recently got some large (1.5") metallurgical Fe diffusion samples from a colleague to characterize the diffusion profiles. Unfortunately the samples are quite magnetic and even using the above mentioned demagnetizer did not help. So depending exactly where on the sample I am, the electron beam is significantly deflected away from the WDS x-ray focus location.

Now normally when we have run into samples that cannot be demagnetized, we simply run the analyses using EDS with full standards in Probe for EPMA as this allows us to perform spectral interference corrections, but unfortunately these analyses require characterization of carbon.  And trace carbon by EDS in an Fe, V, Nb alloy is just not going to cut it in my opinion (though I'd be interested to learn otherwise).

Now carbon characterization in a microprobe is hard enough (need to have an uncoated sample and turn off the coating correction the unknowns but leave it on for the standards, use the TDI correction, deal with spectral interferences, etc., etc.), but when one adds severe beam deflection from magnetic samples into the mix, it really is a pain.

For example, when comparing the optical image with the electron image, in one analysis location I'm seeing 49 um of deflection in X and 12 um in Y. In other areas the deflection is just as large but different!  If I simply go ahead and run normal WDS analyses I get ~90% totals because of Bragg defocussing.

So I'm sitting there thinking how the heck am I going to do this?  I could have used the digitized image feature that allows one to acquire a BSE image and then digitize the points right on the image. And since the image would be deflected by the magnetic field just like the spot beam, the beam deflection should all null out.  But for analysis areas larger then the 20 um or so, one is still going to have Bragg defocus issues, and I can't use EDS because they want trace carbon.

So then I wondered about image shift, because the deflection from the magnetic sample is roughly constant over a mm or so. But image shift doesn't really work in spot mode- or does it?

So I switched to scan mode, and looking at the optical image, I adjusted the image shift until the center of the electron image coincided with the optical image. I then manually switched the beam mode to spot, and low and behold, the beam spot showed up and was deflected, even though the image shift controls are now non responsive (as one would expect)!  At least this is the case on my Cameca SX100.

So, then what I did was to utilize a checkbox in PFE that I don't think I've ever used before, but in this situation it is essential, and that is this one here that is normally checked, but I simply unchecked it:

(https://probesoftware.com/smf/gallery/395_10_10_18_1_43_51.png)

Now the deal is to be sure that the beam and column conditions are exactly as you want them otherwise, and then the software acquires the data with the deflected spot mode beam that hopefully is compensating for the beam deflection caused by the magnetic sample.

And after this, I calculated the results and instead of 90% totals we're doing much better:

n    7 1AV, 3

Un    7 1AV, 3
TakeOff = 40.0  KiloVolt = 15.0  Beam Current = 30.0  Beam Size =    0
(Magnification (analytical) =  40000),        Beam Mode = Analog  Spot
(Magnification (default) =     2245, Magnification (imaging) =    800)
Image Shift (X,Y):                                         .00,    .00
Number of Data Lines:  48             Number of 'Good' Data Lines:  48
First/Last Date-Time: 10/08/2018 05:27:47 PM to 10/08/2018 07:22:20 PM
WARNING- Using Exponential Off-Peak correction for c ka
WARNING- Using Exponential Off-Peak correction for v ka
WARNING- Using Exponential Off-Peak correction for o ka
WARNING- Using Time Dependent Intensity (TDI) Element Correction
WARNING- Using Blank Trace Correction
WARNING- Quantitation is Disabled For o ka, Spectro 4

Average Total Oxygen:         .000     Average Total Weight%:   98.565
Average Calculated Oxygen:    .000     Average Atomic Number:   24.215
Average Excess Oxygen:        .000     Average Atomic Weight:   51.558
Average ZAF Iteration:        2.96     Average Quant Iterate:     3.98

No Sample Coating and/or No Sample Coating Correction

Un    7 1AV, 3, Results in Elemental Weight Percents
 
ELEM:       Fe       C      Nb       V       O
BGDS:      LIN     EXP     LIN     EXP     EXP
TIME:    40.00   40.00   40.00   40.00     ---
BEAM:    30.10   30.10   30.10   30.10     ---

ELEM:       Fe       C      Nb       V     O-D   SUM 
   327    .009   -.019    .000 100.017     --- 100.006
   328    .020   1.103    .000  98.865     ---  99.989
   329    .033    .053   -.002  99.280     ---  99.364
   330    .030    .141   -.006  98.915     ---  99.081
   331    .017    .113    .008  98.143     ---  98.281
   332    .042   -.065   -.011  98.559     ---  98.525
   333    .034   -.045    .016  98.617     ---  98.622
   334    .035    .042    .002  98.318     ---  98.398
   335    .467   4.042   -.011  95.773     --- 100.271
   336  16.404    .070   -.021  82.303     ---  98.756
   337  36.791    .044    .002  62.380     ---  99.217
   338  42.883    .098    .004  55.452     ---  98.437
   339  46.199    .123   -.013  51.821     ---  98.130
   340  48.929    .131   -.004  49.326     ---  98.382
   341  38.853  18.902   -.013  39.861     ---  97.603
   342  51.870    .570    .001  45.361     ---  97.803
   343  54.801    .174   -.006  44.297     ---  99.265
   344  55.998    .147    .007  43.141     ---  99.293
   345  57.067    .168   -.017  41.637     ---  98.855
   346  57.834    .162   -.003  40.631     ---  98.624
   347  59.383    .220    .007  39.498     ---  99.107
   348  59.763    .174    .004  38.741     ---  98.681
   349  61.193    .263   -.012  37.850     ---  99.294
   350  60.820    .499   -.010  36.774     ---  98.083
   351  62.245    .269   -.009  36.380     ---  98.885
   352  62.695    .059    .002  35.104     ---  97.860
   353  61.348   3.872    .012  33.015     ---  98.248
   354  64.923    .270   -.008  34.661     ---  99.846
   355  65.745    .363   -.029  34.140     --- 100.219
   356  65.329    .336    .016  33.287     ---  98.968
   357  66.446    .480    .000  32.807     ---  99.732
   358  66.232    .437   -.009  32.246     ---  98.906
   359  68.073    .282    .000  31.814     --- 100.169
   360  67.658    .306    .013  31.365     ---  99.342
   361  68.051    .358    .004  31.018     ---  99.431
   362  68.374    .390    .013  30.217     ---  98.994
   363  29.768  28.703    .018  14.383     ---  72.872
   364  68.313    .380   -.003  30.694     ---  99.384
   365  71.621    .462    .010  27.391     ---  99.484
   366  71.649    .362   -.025  27.720     ---  99.706
   367  71.290    .327   -.010  27.765     ---  99.372
   368  71.315    .398   -.003  27.864     ---  99.575
   369  71.726    .282   -.007  27.620     ---  99.621
   370  72.036    .304   -.006  27.436     ---  99.770
   371  72.546    .254    .008  26.966     ---  99.774
   372  72.605    .206    .004  26.705     ---  99.519
   373  73.202    .244    .014  26.185     ---  99.644
   374  73.589    .216   -.010  25.956     ---  99.751

AVER:   49.089   1.389   -.002  48.090     ---  98.565
SDEV:   26.548   4.891    .011  26.706     ---   3.846
SERR:    3.832    .706    .002   3.855     ---
%RSD:    54.08  352.14 -597.27   55.53     ---
STDS:      526     506     541     523     ---

STKF:   1.0000   .9635   .9933  1.0000     ---
STCT:  11795.9 74090.3 15247.8 26182.7     ---

UNKF:    .4792   .0050   .0000   .4958     ---
UNCT:   5768.6   398.1     -.2 13218.3     ---
UNBG:     32.2    41.4    18.5    55.1     ---

ZCOR:   1.0343  3.2420  1.1506   .9586     ---
KRAW:    .4890   .0054   .0000   .5048     ---
PKBG:   169.93    5.72     .99  228.95     ---
INT%:     ----    -.02    ----    ----     ---

TDI%:     ----   9.396    ----    ----     ---
DEV%:     ----      .7    ----    ----     ---
TDIF:     ---- LOG-LIN    ----    ----     ---
TDIT:     ----   66.77    ----    ----     ---
TDII:     ----    384.    ----    ----     ---
TDIL:     ----    5.95    ----    ----     ---
BLNK#:    ----      12    ----    ----     ---
BLNKL:    ---- .000000    ----    ----     ---
BLNKV:    ---- .933238    ----    ----     ---

Note also that I utilized the blank correction for carbon by analyzing some points on a portion of the sample that was uncoated pure iron. The analysis above shows a carbon blank of 0.933 wt%, which is then subtracted out during the matrix iteration in Probe for EPMA.  Does this 0.9% represent the native hydrocarbon contamination layer on the sample?  I don't know, but I also noticed that the carbon TDI plots are unusual but I'll post those in another post.
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on October 17, 2018, 12:47:17 pm
These magnetic samples are a real pain since the demagnetizer doesn't seem to do much. To give you a better idea of what I'm dealing with here is an image of the sample. And you can see first of all, the damn thing is a horseshoe magnet!  Actually two horseshoe magnets!

(https://probesoftware.com/smf/gallery/395_17_10_18_12_32_38.png)

Second, I've marked the magnitude of the magnetic beam deflection, in microns, from the sample (compared to the optical image) in three places on the sample.

By the way, I tried cleaning the surface with ethanol and scrubbing with a Kimwipe, then drying it in a warming oven, but the TDI measurement on an new area on the Fe sample looks pretty much the same as before, and again yields a ~0.9% weight percent carbon blank.

(https://probesoftware.com/smf/gallery/395_17_10_18_12_44_36.png)

The average TDI correction for the first carbon blank was a 16.9% correction, on the second blank (in this post) it's a 15% TDI correction, probably due to the one point that has a flat slope!

But here's my question: does the direction and/or magnitude of the three beam deflection measurements I made seem to correlate with what one might expect for a horseshoe magnet?

(https://probesoftware.com/smf/gallery/395_17_10_18_12_49_46.png)
Title: Re: Dealing With Magnetic Specimens
Post by: Gian Colombo on October 19, 2018, 02:21:40 pm
I've used the image shift/static beam deflection strategy before on my magnetic specimens with pretty good results, at least over the limited distances where the magnetic deflection is uniform.

Recently, I've also had a modicum of success by doing a peak search for each measured element on the unknown magnetic specimen before each quant point and using the shifted peak locations as the reference points for measurement.  I'm assuming this is working because the magnetic beam deflection is causing the spectrometers to be in focus at some new sine theta value without affecting intensity.  Then again, maybe I've just been lucky enough on these specimens that the beam is not far enough off axis to render enough intensity loss that the peak shift is not able to recover.  Sadly, I currently have no way of including an automated peak search before each acquisition point during my quant routines, so I'm doing it manually as needed....ugh.

Even if you could automate it, having to wait for a peak search routine to complete could negatively impact the carbon quant since you'd have to account for contamination buildup as the beam dwells during the peak search.  C'est la vie.
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on October 20, 2018, 09:19:37 am
Even if you could automate it, having to wait for a peak search routine to complete could negatively impact the carbon quant since you'd have to account for contamination buildup as the beam dwells during the peak search.  C'est la vie.

Indeed!   Or as I sometimes say: "there are many more ways to be wrong than right!"    :)

The problem for me is that the user wants these analyses placed extremely accurately on specific sample locations, but since the magnetic deflection varies slightly over distances of even a few hundred microns, getting the beam exactly where it should be is painful.

I can often get good totals in many places but in some places the beam deflects to outside the Bragg focus and then I know I'm not analyzing the spot I hoped I would be analyzing.  So it's re-align the image shift and try again, and then of course the filament blows and then the P-10 gas low tank warning alarm alerts me.

Perhaps I should say: "There are many more ways to be dead than alive!" 

I'm leaving it until Monday!   >:(
Title: Re: Dealing With Magnetic Specimens
Post by: Probe321 on October 29, 2018, 07:33:22 am
I encounter magnetism all the time especially with ferrous based alloys and some nickel alloys.  I stress to users to degauss  their sample before examination in the SEM or microprobe.  Been called in to assist users with imaging issues and once I learn what is being examined is an alloy, I have them degauss the sample and usually problems go away.  If not look at the ground path the sample mounting media may have pulled away from the sample giving a marginal ground path.

IT learned the Vertiy degausser did not erase hard drives, but it works for large magnetic samples that don't fit into the small degausser.  Metallographic sample prep can trigger magnetism in a sample,  prime examples of this is cold worked (polished) 304 stainless steel and high chromium white cast iron. Once in awhile the sample holders may be slightly magnetic, which also effects imaging/analysis, but it does not happen very often.

And finally it may not be your sample, it may be some previous sample(s) left some debris in the chamber.  After looking at numerous (a few years worth) of corrosion coupons the service engineer discovered rust particles on the pole piece and after cleaning the imaging issue was resolved.
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on October 29, 2018, 01:41:29 pm
Hi Keith,
The imaging isn't affected when the sample is not loaded, so we're pretty sure there's no magnetic particles on the objective lens. But that's a good thing to keep in mind.

The demagnetizer we're using can handle the complete shuttle for the Cameca EPMA which we utilize with a Variac transformer to be most effective:

https://probesoftware.com/smf/index.php?topic=354.msg6656#msg6656

Interestingly, some samples just don't seem to demagnetize very well.  Even worse is that some samples demagnetize fine, but as soon as the electron beam is applied in the instrument, they start visibly drifting (based on the electron image). Furthermore this image drift is perfectly correlated with the time the electron beam is actually unblanked and impacting the sample.

So it appears that the sample is not being affected merely by the magnetic field created by the objective lens. The sample is completely conductive and grounded to the holder using two large pieces of copper tape.  No image drift effect is seen when the electron beam is on the sample holder, but rather only for certain (very rare) materials.  In other words, as soon as we unblank the beam, the image starts drifting, and if we blank the beam and wait a minute of two, and then unblank the beam again, we observe the image drift starting again exactly where we last blanked the beam. The absorbed current is stable and does not change from the time the beam is unblanked.

It is very hard to understand how an electron beam at nA levels of current could induce a magnetic field in a material.  We ran this by one of electron physicists here at UofO and they cannot come up with any explanation for this, so maybe there's a Nobel Prize in it for someone who figures it out!   :D
Title: Re: Dealing With Magnetic Specimens
Post by: Probe321 on October 31, 2018, 09:54:13 am
We get drifting samples once an awhile also.  Usually it is a ground path issue of some type which can be a challenge to find. 
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on October 31, 2018, 10:03:23 am
We get drifting samples once an awhile also.  Usually it is a ground path issue of some type which can be a challenge to find.

I know, right.  That's  usually what is going on when one sees this, but it doesn't seem to be the case here. But we'll try again!
Title: Re: Dealing With Magnetic Specimens
Post by: DavidAdams on November 01, 2018, 05:19:24 am
Have you tried cooling the samples down to LN2 temperatures and trying the demagnetizing when the samples are cooled?
Title: Re: Dealing With Magnetic Specimens
Post by: Probe321 on November 01, 2018, 10:46:48 am
If this is the failure "modified axle assemble" I would suggest not cooling to LN2 temperatures.  It is possible you could change the microstructure of the sample, which maybe crucial to case. The LN2 temperature could force uncompleted metallurgical transformations to completion.  Why back in the good old days, when I did tool steels for a living an LN2 quench forced all retained austenite to convert to martensite the desired microstructure.  Retained Austenite is unstable and when it converts to martensite there is a volume change which can result in cracks forming in the part.

I would suggest checking with the university machine shop (or a local shop) and see if they have a large degausser.

If this is not the "modified axle assemble" it is worth a try though according to the Adams Magnetic Products when you return to room temperature the magnetic field returns that is for magnets anyway, nothing about ferrous magnets.

One question I have is how did the part get magnetized?
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on November 01, 2018, 12:51:05 pm
Hi Keith,
The latest "magnetic nightmare" sample in question is the diffusion study sample. It's the one that has a piece of pure Fe in a horseshoe shape(!), wrapped around a piece of V in the first case and another horseshoe shaped piece of Fe wrapped around a piece of Nb in the second case.  I posted a picture of it above.  I have no idea how they get magnetized. I know they run these diffusion experiments in a furnace for hundreds of hours at very high temperatures.

Both samples show highly variable beam deflections depending on the exact sample area on the order of 5 to 50 microns or so.  I worry that dipping the mount in LN2 could crack the epoxy. In any case I'd have to ask the researcher first.

I'll look around for a stronger demagnetizer when I attempt these samples again.  Thanks for all of your and Dave's suggestions.
john
Title: Re: Dealing With Magnetic Specimens
Post by: JonF on November 01, 2018, 02:38:57 pm
At risk of sounding a bit daft, but have you tried just dropping the sample on to the desk a few times?
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on November 01, 2018, 02:49:55 pm
At risk of sounding a bit daft, but have you tried just dropping the sample on to the desk a few times?

Hi Jon,
You know, that's actually not a bad idea at all.  Certainly worth trying!    8)

I'll let you know if it works as soon as I get a chance to try it.  Hopefully next week or the week after.
john
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on August 14, 2019, 01:28:54 pm
I apologize for resurrecting this topic but I am really at my wits end and about to give up trying to quantify WDS elements on these highly magnetic samples.

Here's what I've learned over the last year:

1. One can completely demagnetize these magnetic samples and they will re-magnetize in the instrument.

I checked this by demagnetizing the samples in a de-magnetizer setup as shown here:

https://probesoftware.com/smf/index.php?topic=354.msg6656#msg6656

Then once the newly demagnetized sample is in the instrument, I first navigate over to the steel sample (H13 and 4140 in the most recent effort), and place the beam over epoxy but just adjacent to the steel sample and when I unblank the beam it burns a spot right where it is supposed to.

2. The sample remagnetizes over time. But here's the interesting part: the re-magnetization appears to occur *not* from the sample sitting in the instrument (though there may be some effect from this), but mostly from the time when the beam is impinging on the steel sample. 

My febrile hypothesis at the moment is that at these currents (~50nA for getting good quant for light and trace elements), it is the current *flow* that is that is re-magnetizing the sample.

I have two line of evidence for this.

First when I am imaging the sample, the image is slowly drifting as soon as I am imaging the sample. The longer the beam is imaging the sample, the greater the drift. But if I blank the beam and wait a minute, then unblank the beam, the image drift starts again as before, but it starts *exactly* where it previously left off when the beam was blanked!

Second, I noticed that on these samples I had applied conductive tape on the top of the sample, on the edge away from the edge I am analyzing.  And the beam is being deflected only in the Y direction and towards the copper tape. My completely crazy idea that the current flow of electrons towards the grounded side of the sample is inducing a magnetic field that deflects the beam in the direction of the current flow.

So now the even more crazy idea is that instead of grounding the sample on the opposite side, perhaps I could ground *both* adjacent sides of the sample and the current flow might be balanced enough that they might cancel each other out? 

Well after an hour of acquisition I checked the beam, again by unblanking it on some nearby epoxy, and indeed the deflection was now much smaller and only in the direction of one of the adjacent sides.

Crazy I know.

But here's my question: can anything else be done? I mean besides running the sample above it's Curie temperature!

The beam deflection we are seeing in these 5 mm sized samples is indeed smaller than the 5 centimeter sized sample we were previously struggling with, so would cutting the sample down even further help?  What about plasma fibbing them out?  We only need a 600 um traverse distance from the edge.  Has anyone tried plasma fibbing out small samples to reduce induced magnetism?

What do people in the steel industry do?  Where is Les Moore when you need him?
Title: Re: Dealing With Magnetic Specimens
Post by: Les Moore on August 14, 2019, 05:52:28 pm
I've never looked at magnets. Low core loss and magnetisable steels yes but not something  magnetised already.

How about a string of microhardness indents across the sample.  This will give you a static datum. you can then park the beam beside them.   
If the flow is towards the conductor, could you out the conductor on the surface - a thin film of some highly conductive metal (Al perhaps)
The latter will absorb your light trace elements a bit but it might be modellable in PENEPMA.

Perhaps you could use the new fandangled sputter coaters that deposit only a couple of nm of tantalum (is that the element).
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on August 14, 2019, 06:03:29 pm
Hi Les,
These are *not* magnets. But I do often de-magnetize them if I see big (~20 um or more) beam deflection effects when they are loaded the first time.

The particular samples we're running now are H13 and 4140 steels (we've seen the same thing on many other compositions that contain Fe). The thing is, they seem to *become* magnetic in the instrument (after de-magnetization!), but apparently only when exposed to a relatively high beam current (~30 nA or more) over time.

Haven't you (or anyone else!) seen this effect?  My lab manager Julie confirms that she sees this on many cm sized Fe alloy samples.  On these particular samples the customer wants several hundred 1 um points analyzed with C, N and O, so using reference points is OK to locate the beam position, but then the Bragg defocus brings the totals down to 90% or less. And to make things even worse the beam deflection is quite variable and depends on the actual stage position!  Not to mention the change in magnetism as the sample continues to magnetize.

It's an analytical nightmare!

We have a Cameca SX100, but I find it hard to believe it's only our instrument that has this issue!
john
Title: Re: Dealing With Magnetic Specimens
Post by: Probeman on September 10, 2019, 12:58:51 pm
To continue Probeman's sad (magnetic) sample saga, we had Julie Chouinard cut down the 4140 and H13 tool steel samples in an attempt to reduce the amount of magnetic material. I noted that when mounting the samples, they were strongly magnetic and so ran them through our demagnetizer unit as shown here:

https://probesoftware.com/smf/index.php?topic=354.msg6656#msg6656

Once the samples were in the instrument again, I aligned the light optics to the beam, then I went near the edge of the steel samples and unblanked the beam over some nearby epoxy, and the beam burn showed the beam to be aligned perfectly on the beam centered position.

I then re-peaked all the spectrometers and acquired the standards. Then (~2 hours later) I went back to the steel samples, and alas, the beam was now deflected anywhere from 10 to 30 um depending on the distance from the edge of the samples.  In both cases the deflection was towards the sample edges and varied with the distance to the edges (beam was pulled more towards the sample, as I approached the edge).

It really does appear that just sitting in the instrument for a few hours re-magnetizes the samples and causes significant beam deflection.  This is a Cameca Sx100, but has anyone seen this effect in their instruments?