Hello everyone,
Can anybody please explain what are the influential microstructural/processing features that control the shape of a superelastic NiTi alloy? I am looking for some quantitative relationship between some variables (e.g., grain size or texture) and features of the stress-strain response of this alloy, such as start and finish strains/stresses during loading and unloading, as well as the size of the loop formed during one cycle of loading. In other words, is it possible to make an acceptable estimation of those values before mechanically testing a real NiTi specimen?
Thanks,
Mohammad
Thank you Piyas and João.
Let me be more specific. I have a few samples (D=10 mm rods of 100 mm length), taken from the same batch. I have heat treated them in an exact same way, but the stress-strain response is significantly different: Some of them are superelastic at room temperature some are not. Even between the superelastic ones there are some with high stress plateau (like 650 MPa) and some with low stress plateau (~350 MPa). Now the questions are:
1. What are the main causes for this different behavior?
2. Is there anyway to control the output to be very consistent (I should say that the heat treatment process is the same for all the specimens)?
3. Is there any way to estimate the plateau stress (preferably in a non-destructive way)?
I should also add that, since I am doing fatigue study, once the specimen reaches a large strain (~1%) in tension, which determines the plateau stress, if it is not consistent with the previous tests, the specimen is not usable anymore.
Thanks again,
João,
We have not manufactured the rods ourseleves. We have purchased them. But I am sure that they have been through the same thermomechanical history, before and after receiving them. Sometimes, even when I have a bar cut into two pieces and I heat treat the pieces together (i.e. exact same procedure), they exhibit two different stress-strain response!
Thanks,