Plastic Deformation of B19' Martensite: Where it Matters in NiTi Technology

Abstract

Nitinol technology, besides utilizing the functional thermomechanical properties derived from the B2 cubic to B19' monoclinic martensitic transformation, also exploits the excellent plastic deformability of NiTi in the martensite state. It originates from the unique mechanism of plastic deformation of the B19' martensite by kwinking involving dislocation slip based kinking assisted by deformation twinning. Although the mechanism of plastic deformation of martensite by kwinking was revealed only very recently, various unusual phenomena that can only be rationalized by kwinking, have been reported in literature in the last 50 years. These phenomena include: 1) cold working with a high degree of reduction without introducing cracks, 2) excellent plastic deformability in the martensite state (plastic deformation up to~80% strain at stresses >1GPa), 3) refinement of austenitic microstructure to a quasi-amorphous state by tensile deformation, 4) observation of high density of {114} deformation bands in austenitic microstructures, 5) systematic ruptures of strengthened NiTi wires in tensile tests via necking at the onset of plastic yielding, 6) localized plastic deformation in tensile tests via propagation of Lüders band fronts with very large localized strain (~40%), 7) unusually long upper stress plateaus in superelastic tensile tests (>8% strain), 8) large plastic strains (> 20 %) generated in a single closed-loop cooling/heating cycle under constant stress, 9) shape setting of already annealed NiTi by heating under external constraint. Finally, we discuss how kwinking deformation was considered in constitutive modelling of thermomechanical behaviors of NiTi and, particularly, what is the role of the kwinking deformation in NiTi technology.