abstract

Unlike most of the shape memory alloys, NiTi is plastically deformable in its low-temperature phase, that is, in monoclinic B19′ martensite. Large plastic strains (over 30% for textured polycrystals under tension) can be achieved even at room temperature, without significant hardening and without any significant impact on the functional behavior. At the same time, B19′ is strongly anisotropic in terms of dislocations slip, having only one easy slip system, which should preclude NiTi polycrystals from experiencing homogeneous plastic flow. Thus, a question arises – what is the mechanism of the forming? The talk will answer this question based on the following thoughts, that comprehend the experimental and theoretical research on NiTi plastic forming over past two decades:

A closer look at the atomistic length-scale reveals that B19′ martensite can be understood as consisting of rigid layers that are easy to glide along each other. This suggests that the plastic deformation may proceed through formation of so-called kink bands, typical for layered media. Simultaneously, martensite has an additional degree of freedom because of reversible twinning. We can therefore propose a coupled mechanism, in which the twins merge with plastic kinks, giving rise to new type of deformation bands – twinning-assisted kinks or kinking-assisted twins, that are suggested to be called ‘kwinks’. Using the non-linear elasticity theory of martensite, one can predict the energy-minimizing crystallographic orientations of such kwink bands, and they turn out to be very close to (20-1) planes, which are, indeed, the planes on which ‘plastic twins’ are known to appear in irreversibly strained NiTi. The kwinking concept further elucidates formation of numerous experimentally observed patterns, such as the crossing-twins or V-shaped microstructures, and brings new intriguing insights into the behavior NiTi martensites, ranging from theoretical (understanding dislocations slip as a translation between different Eriksen-Pitteri neighborhoods) to technological (opening a pathway to superplastic forming of NiTi).

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