Neutron Diffraction Studies of the Stress Effect on Texture Transformations in NiTi Shape Memory Alloys

Abstract

Texture transformations accompanying B2 cubic to B19′ monoclinic martensitic transformation in NiTi shape memory alloy occurring upon stress-free cooling and cooling under applied compressive stress of 250 MPa were investigated by neutron diffraction. In the case of stress-free cooling, the martensite texture was found to be basically inherited from that of the parent austenite phase through the lattice correspondence between the austenite and martensite phases. In the case of cooling under stress, however, the martensite texture is different since it is strongly modified by the preferential martensite variant selection due to the compressive stress applied. It is found that the stress effect on the texture transformation can be to a first approximation rationalized by assuming that mainly the lattice correspondent variants of the martensite phase providing maximum strain into the direction and sense of load in individual crystallites form upon cooling under applied stress.