TEM observation of twins in surface grains of superelastic NiTi wire after cyclic loading

Abstract

Since the surface dominates fatigue of medical grade NiTi wire by yet unexplained causes, for comparison, we carried out a TEM observation of surface grains, both after one thousand superelastic cycles and on the unloaded wire. Crystallographic defects consisting of aggregates of austenite twins stacked in the bands were observed after cycling deformation. The nuclei of aggregates are already present in an unloaded wire that originates from cold drawing or heat treatment. Severe plastic deformation induced by a stress increasing in the surface during a cyclic phase transformation enables an accumulation of these aggregates into bands at inclined angle of 55° to the wire axis corresponding to the spreading of diffused martensite band front paths during their propagation. An unrecovered strain, suppression of transformation and acceleration of crack propagation can be expected while the aggregates grow. Based on investigation of the grain orientation and boundaries in the twin lamellae using nano beam diffraction mapping, it can be proposed that, with exceeding martensite twin variant stress, twinning plane {1 1 4}B2 is a result of non-transforming pathway.