NiTi and NiTi-TiC composites: Part IV. Neutron diffraction study of twinning and shape-memory recovery

Abstract

Neutron diffraction measurements of internal elastic strains and crystallographic orientation were performed during compressive deformation of martensitic NiTi containing 0 vol pct and 20 vol pct TiC particles. For bulk NiTi, some twinning takes place upon initial loading below the apparent yield stress, resulting in a low apparent Young's modulus; for reinforced NiTi, the elastic mismatch from the stiff particles enhances this effect. However, elastic load transfer between matrix and reinforcement takes place above and below the composite apparent yield stress, in good agreement with continuum mechanics predictions. Macroscopic plastic deformation occurs by matrix twinning, whereby (1 0 0) planes tend to align perpendicular to the stress axis. The elastic TiC particles do not alter the overall twinning behavior, indicating that the mismatch stresses associated with NiTi plastic deformation are fully relaxed by localized twinning at the interface between the matrix and the reinforcement. For both bulk and reinforced NiTi, partial reverse twinning takes place upon unloading, as indicated by a Bauschinger effect followed by rubberlike behavior, resulting in very low residual stresses in the unloaded condition. Shape-memory heat treatment leads to further recovery of the preferred orientation and very low residual stresses, as a result of self-accommodation during the phase transformations. It is concluded that, except for elastic load transfer, the thermal, transformation, and plastic mismatches resulting from the TiC particles are efficiently canceled by matrix twinning, in contrast to metal matrix composites deforming by slip.