If I have a deformed specimen and I used wire EDM cutting to cut the material into samples of different thicknesses. Assuming the HAZ from the EDM wire cutting is removed, if I were to heat treat all the samples to same conditions of temperature (above Trx) and time, would the grain sizes be the same for all the samples? Is there any relationship between sample size and recrystallisation and grain growth?
Refer this link:www.fracturae.com/index.php/MST/article/download/994/949
Refer this link:https://en.wikipedia.org/wiki/Recrystallization_(metallurgy)
Refer this link:link.springer.com/content/pdf/10.1007%2FBF01699497.pdf
Refer this link:https://www.erowid.org/archive/rhodium/.../recrystallization.html
Refer this link:www.scielo.br/scielo.php?pid=S1516-14392005000300002&script...
Babu Bhaskaran, the links are not working properly. They seem to be incomplete.
In this topic:
Please note that the tests usually carried out on polycrystalline materials. For that to happen, the thickness of the sample must be at least 10 full grains. If it is less, it is going in the direction of a single crystal.
II hope to have understood the question correctly.
Whether and the HAZ is how thick after wire cutting, depends on the process parameters. The oft-quoted today static pulse generators with pulse durations greater than 2 microseconds and pulse currents of 20 A automatically lead to very thick HAZ. This HAZ can hardly be reduced by subsequent thermal treatment. Likewise, a problem is the many micro-cracks in the HAZ.
The HAZ can be avoided or neglected if the pulse durations of less than 500 ns can be selected. In these pulse durations even the current amplitude of up to 100 A and more can be increased.
If you want to disconnect from a sample many narrow sectional areas, then the problem is that hardly any energy can be stored or dissipated in the work-piece. They can then only with needle pulses of less than 400 ns work. Then they have some assurance that the sections do not have to be reworked.
Dear Dr. Schulze,
Thank you for the great answer. What I needed to know is about the recrystallisation of deformed sheets. The sheets were cut from a cold rolled bigger bulk block. But even after rolling the initial block has a very large grain size (2000 microns) whereas the sheet used for heat treatment is of 100 microns only. If this thin sheet were to be heat treated, would it show the same recrystallisation phenomena as the bulk?
With such large grain sizes, you can assume that certainly has the eroded surface a much smaller grain size. When "normal" WEDM they have plasma-foot points, which are only a fraction of their grain size. By how much the grain size is reduced depends on the pulse duration and pulse energy - must be tried. It would be interesting to know yet how deep is this grain change - I have not yet examined.
Dear Rohit,
The extend of grain growth in a bulk sample can be dependent on the thickness. The reason is the grain boundary pinning on the surface which can inhibit the growth of the grains. Except that you perform your heat treatment on perfectly clean surface and in a very clean atmosphere where no GB pining due to adsorption of impurities at the GB happens.
The rate of recrystallization will vary with your percentage deformation, Since in a thick specimen there will a higher deformation on the surface ( due to friction between tool and work piece) compared to core, recrystallization will be more on surface , some times growth too . If the difference in thickness are small, they can be negligible. Again recrystallization temperature plays a role too
Thank you Afrooz and Barnoush. Yes at the surface the deformation is usually the highest and is mostly neglected for very thin sheets. I think the grain boundary pinning is most likely the case that might happen if the sample is too thin. Also if that is the case then wouldn't it grown in the other 2 dimensions where its not thin?
Yes the growth will be typically on internal GBs which are free to move. Therefore, it is very likely that in the case of thin samples you end up with the so called oligocrystalline microstructure
BTW I'm not sure if I understood you correctly. I assumed you have a bulk cold worked sample then you will cut specimens with different thickness out of it and heat treat them. In this case you can expect to have a uniform strain and comparable strain in all your samples. (Take a look at the attached paper)
Though, if you cut samples with similar thickness's and hard work them to different thickness's then it will be a completely different situation.
Article An insight into the role of the grain boundary in plastic de...
It dependents on the relative length scales. If the grain size and sample thickness length scales are comparable, then yes thickness matters due to interfacial energy contributions. In thin films, recrystallization behavior to a very high extent depends on the film thickness due to internal stress. The Oligocrystalline case is addressed by Barnosh which is expected to happen in some cases.
If I understand correctly you will have samples of varying thickness but all (or most?) significantly less than the original grain size, so you will have, as noted above, an oligocrystalline structure. Different slices will have different grain orientations. In most metals different grain orientations recrystallize at different rates so with this oligocrystalline strructure you will probably get very different recrystallization rates and therefore final grain sizes according to the grain orientation, i.e; in your case the slice. This would lead to very unpredictable and unreproducible results.
The grain boundary forms a groove on the surface, and the shape of it is determined by the grain boundary energy and the surface energy, due to the mechanical equilibrium. I totally agree with Afrooz Barnoush that the grooves at the surface are the pins preventing the grain boundary from migrating. If the grain size is comparable with the thickness, surely the recrystallization and the subsequent grain growth would depend on the sample thickness. I would suggest you take a look at the book by Gottstein and Shvindleman.
You may find interesting this work
Article: TEM in situ annealing of severely deformed Ni 3Al intermetallic compound
W. Zieliński · Z. Pakieła · K. J. Kurzydłowski ·
[Show abstract]
Materials Chemistry and Physics 08/2003; 81(2):452-456. DOI:10.1016/S0254-0584(03)00045-2 · 2.26 Impact Factor
Prof. Julian Driver, I would like to ask if the samples are infact oligocrystalline, wouldn't the pre deformation result in a deformed orientation/texture from which the recrystallised new orientations should evolve or are those phenomena observed only in bulk polycrystalline materials?
The predeformation would create a limited number of grain orientations but they would certainly not be all the same. Hence the problem of orientation dependent recristallization in your oligocrystalline structure. In contrast, bulk polycrystals with fine grains possess a large number of grains, so that this effect of particular orientations is reduced, and you get a more reproducible recrystallization behaviour.
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