Re: Microsegregation question

G'day Stu,
How has your analysis been done? And how have the samples been taken?

I'm not up to date with SEM or similar techniques, the type of instruments
I'm used to are OES/spark spectrometers which will analyse an area of a
sample some 10mm in diameter and up to about 0.1mm deep, so testing a
particular grain boundary is out of the question.

If you are testing just a particular grain boundary, then a number of test
sites across the sample and averaging the results would seem to be in order.

Pete

<Stugrad98@xxxxxxx> wrote in message
news:1157293210.955799.115360@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Thank you for your response. This is more of an after-the-fact
assessment. What I'm trying to get at is this: in trying to assess
whether or not the "damage has been done", would it be fair to look
for correlations between the elements added (in this specific case,
antimony, copper) in specific grain boundaries? Or am I making an
unwarranted assumption: could antimony accumulate in the grain
boundaries in levels completely unrelated to the accumulation of
copper? If someone came to me and said-- "since there is no
correlation between the alleged spike in antimony and the alleged spike
in copper in each of the given samples, we can place faith in our
analysis"--- would that person be making a fatal assumption?

Thanks,
Stu


Bushy Pete wrote:
Some of this will depend on the elements added, but sampling is and has
been
a problem for many years.

There are a number of approaches to gaining a representative sample, some
of
which are more effective than others. There are methods that allow for
cleaning the surface of the furnace prior to dipping a (maybe coated)
sampling ladle or a vacuum operated sampler, different approaches to
moulding, and different ways to cut or prepare a face to analyse. These
change depending on the type of metal you are trying to test. Hard metals
like iron and steel are normally prepared with a grinding operation,
while
softer metals are often cut on a lathe or milling machine to prevent
smearing of "stuff" (*technical term) from one part of the sample to
another.

There are also a range of matrix effects that occur simply because of the
way the sample has been taken or prepared. For example, MnS inclusions
will
occur at varying depths from the face of a sample simply due to the way
the
sample has cooled. Reading this sample with say an optical emission
spectrometer (spark machine or spectrograph depending on how old you
are!)
will give different readings if varying amounts of metal are removed from
the face of the sample.

For spectrometers, which are the principal machine used in typical
foundries
for metal analysis, the samples are normally taken such that a thin
sample
that cools rapidly on a copper chill block or water cooled mould allows
for
very little crystal or plate or grain or whatever growth, and the sample
is
as homogonous as practically possible.

Some additions are particularly problematic, such as lead in free cutting
steel. The Pb form into little balls that provide a chip breaker function
for machining, like on a lathe. Of course, this makes it difficult to
obtain
stable results.

Hope this helps,
Peter

<Stugrad98@xxxxxxx> wrote in message
news:1157228562.946980.198010@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi,

I have a question about the chemical analysis of an alloyed metal, one
that had such elements as copper, antimony and arsenic added as
hardening agents. My understanding is that if one examines very small
samples, the grain boundaries that accumulate those elements (Cu, Sb,
etc.) could skew the results: in other words, using a small sample
would amplify your chances of mischaracterizing the chemical content of
the overall piece of metal. This is a result of microsegregation.
My question is this: within each sample, would the "spike" in antimony
or copper correlate with each other? If there was a jump in copper
in one sample, should there be a close/promixate jump in antimony in
the same sample, or would their relative accumulation in the grain
boundaries have nothing to do with each other?

Thanks,
Stu




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