van Humbeeck, J. "Shape Memory Alloys: A Material and a Technology." Advanced Engineering Materials 3 (2001): 837-850.
Taillard, K., et al. "Phase Transformation Yield Surface of Anisotropic Shape Memory Alloys." Materials Science and Engineering A 438-440 (2006): 436-440.
Calloch, S., et al. "Relation Between the Martensite Volume Fraction and the Equivalent Transformation Strain in Shape Memory Alloys." Materials Science and Engineering A 438-440 (2006): 441-444.
(a) Explain the physical basis of superelasticity.
(b) You are asked to quantify the Young's elastic modulus of a shape memory alloy such as Ni-Ti. Based on your understanding from the van Humbeeck review, how would you respond?
(c) How, then, do you explain the values of E and G in his Table 1?
(d) What in terms of material structure and continuum level mechanical properties determines the maximum mechanical energy possible to obtain from a superelastic alloy in an actuator application?
(b) Taillard, et al. summarize Bouvet, et al.'s attempt at a yield surface for superelastic alloys; this is a modification of the von Mises equivalent stress and criterion that we have discussed. Restate this criterion and its components in terms of the symbols used in class.
(c) Taillard, et al.'s Fig. 1 and 2 demonstrate the yield surface in biaxial tension/compression and torsion for xCu-yAl-cBe, respectively. They claim Fig. 3 shows a difference between this response and that of NiTi. Graph the biaxially stressed yield surface for NiTi.
(d) Contrast the Bouvet yield surface with the other non-asymmetric yield surfaces considered in PS3. Is this equivalent to one of them or different, and how? What is the atomistic origin of the asymmetry justifying the Bouvet yield criteria?
(b) Are the superelastic alloys you've read about here subject to creep failure in tension? Think especially about paper 2, and graph a rough (but justified) deformation mechanism map for one particular superelastic alloy of your choice.
(c) Compare the fracture strengths of superelastic alloys of your choice to those of the alloy components. For example, compare the fracture strength (in terms of fracture stress and plane strain fracture toughness) of Ni and Ti for the NiTi alloy. Discuss implications.
"Superelastic Materials: Shape Memory Alloys." (PDF)
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