In Alloy 617, in its as-received condition the twins are more prominent, but as aging proceeds twins disappear, Why? Can one measure stacking fault energy (SFE) with TEM?
dear Aditya Narayan Singh,
The two primary methods of deformation in metals are slip and twinning. Slip occurs by dislocation glide of either screw or edge dislocations within a slip plane. Slip is by far the most common mechanism. Twinning is less common but readily occurs under some circumstances.
Twinning occurs when there are not enough slip systems to accommodate deformation and/or when the material has a very low SFE. Twins are abundant in many low SFE metals like copper alloys, but are rarely seen in high SFE metals like aluminum. This is because the SFE for twinning is lower than for dislocation glide Formation. However by the presence of multiple twin dislocations the material is also at a higher stress state.
Are you talking about the Nickel-based alloy A617? what are the aging conditions?
Depending on the aging conditions stacking fault cross slip or gliding through the Formation of partial dislocations can occur.
An earlier researchgate question posed by Michael William Spindler on "What effects does stacking fault energy have on plasticity and creep of FCC metals?" might also be of interest to you.
There is also a paper on Damage and Fracture Behaviours in Aged Austenitic Materials entitled "During High-Temperature Slow Strain Rate Testing" by Mattias Calmunger (et al.) it inculdes alloy 617.
Regarding your question of determination of the SFE I would like to refer to "Further Critical assessment 19: stacking fault energies of austenitic Steels" by Y.-K. Lee, S.-J. Lee & J. Han, as well as others. It Shows how to calculate the SFE. It is not measured directly, but by micro structure measurements using the TEM you can calculate as shown in this paper.
Best regards, Ruediger
If the answers helped vote them up.
Thank You Prof. Rudiger..
My aging conditions vary from 1000 to 20000 h, at a fixed temperature. The problem I find is my twins disappear after aging? Why do they disappear or what is reason for so?
Is it because the grain growth occurs in it at that temperature? Kindly elaborate your answer if your please enough to spend few more time on it.
Many thanks for your patience.Your's
Aditya
dear Aditya,
what are the specific temperatures ? do you apply any stresses? do the twin dislocations disappear at all conditions?
If you observe a significant grain growth it could be the cause for the twin reduction, since also the growth means a total energy reduction. As stale dislocations require a certain volume of material (max. dislocation density) a shrinking grain with a dislocation in it will contribute to further grain growth, respectively cause dislocation silp at some Point when the local energy reaches a certain Level.
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