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Elastic anisotropy and twin boundary stresses in NiTi shape memory alloys

Date: 25.05.2009
Place: Institut für Werkstoffe, Fakultät für Maschinenbau, Ruhr-Universität Bochum, Germany

Martin Wagner, Department of Werkstoffwissenschaft (WW), TU Chemnitz, Chemnitz, Germany

NiTi shape memory alloys exhibit exceptional functional properties because of a fully reversible martensitic phase transformation from B2 type austenite to monoclinic B19 type martensite. The resulting martensitic microstructures are nely twinned in order to accommodate transformation strains. When mechanically loaded, twinned martensites may deform by multiple deformation processes, including but not limited to de-twinning (growth of one variant at the cost of another variant), reorientation (formation and growth of new variants), or dislocation-mediated plasticity. Because of the occurrence of various modes of deformation at the same time, the experimental methods available today do not allow for a straight-forward determination of the elastic constants of NiTi martensites. First-principles calculations in the framework of density functional theory, on the other hand, can be utilized to determine free energies of arbitrarily strained crystal structures (and hence all elastic constants) at zero K; this provides a good approximation of room temperature elastic properties and allows for the rst time to fully appreciate elastic anisotropy of B19. This presentation focuses on the determination of the elastic constants of B19 from rst principles. The relative mechanical stability of several martensite polymorphs is discussed. Moreover, several investigations based on the novel information on elastic anisotropy of B19 are presented: Compatibility stresses due to external loading at (100)-type compound twin boundaries and at <110>-type II twin boundaries are calculated with the Finite Element Method in an elastically anisotropic framework. The results are compared to simple analytical solutions (elastic half-spaces) originally derived for grain boundary analysis. It is highlighted that the B19 structure exhibits auxeticity (i.e., it expands in some crystallographic directions when subjected to tensile loading). Finally, microstructural evolution and martensite variant selection under multi-axial loading conditions are brie y considered via micromechanics modelling. These examples demonstrate that anisotropy strongly a ects the mechanical behaviour of B19 martensite, and they provide a rich picture of elastic and inelastic deformation processes in NiTi.

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