Phase transformation in shape memory alloys

[Fabian_m9]

Dear all,

As part of my MSc thesis I am using molecular dynamics simulation to study the pseudoelastic effect of Cu-Zr shape memory alloy during a tensile and shear tests. My question is related to the stress induced phase transformations during both tests.

During the tensile test I can clearly see a stress-induced phase transformation (B2 to B19') which agrees well with the literature but for some reason, I can't see any phase transformations during the shear test. In few words, during the shear test I can see an elastic region and a plastic deformation region with no phase transformations. Unfortunately there is no information on phase transformations in Cu-Zr during shear tests in the literature. My question is, why can we see a phase transformation during a tensile test but not during a shear test?

My interpretation is the following but I do wish anyone can help in case I am wrong. I am assuming that in the shear test the energy required for dislocation nucleation is lower than the energy required for phase transformation hence the alloy deforms plastically with no prior phase transition. The opposite happens during a tensile test, where the energy required for dislocation nucleation is higher than the energy required for phase transformation, hence we can see the pseudoelastic effect before the alloy deforms plastically.
If my interpretation was the correct one it would mean that the energy required for dislocation nucleation changes depending on the crystal orientation where the load is being applied but I am not fully sure if this is the case.

If anyone can help explain why there is no phase transformation during a shear test but there is during a tensile test I would greatly appreciate it!

Many thanks,
Fabian

 

[Achy47]

Dear Fabian,

Thank you for sharing your research topic with us. Your question is an interesting one and it is not uncommon to see different results in different types of tests. In the case of your research, it is possible that the energy required for dislocation nucleation is indeed lower during a shear test compared to a tensile test. This could be due to the crystal orientation and the direction of the applied load.

Another factor to consider is the stress distribution within the material during the two different tests. In a tensile test, the stress is primarily applied in one direction, whereas in a shear test, the stress is applied in multiple directions. This could also affect the energy required for phase transformation.

It is also important to note that the pseudoelastic effect in shape memory alloys is highly dependent on temperature and loading rate. It is possible that the conditions in your simulation are not conducive for a phase transformation during a shear test.

I would suggest looking into the stress distribution and loading conditions in your simulation to better understand why there is no phase transformation during a shear test. It would also be helpful to compare your results with experimental data, if available, to validate your findings.

I hope this helps in your research. Best of luck with your thesis!