NITI AND NITI-TIC COMPOSITES .1. TRANSFORMATION AND THERMAL CYCLING BEHAVIOR

The transformation behavior of titanium-rich NiTi containing 0 vol pct , 10 vol pct, and 20 vol pct equiaxed TiC particles was studied by dif ferential scanning calorimetry. The thermoelastic phase transformation of the unreinforced matrix exhibits multiple steps. Upon multiple tra nsformation cycles, the rhombohedral phase (R phase) appears and all t ransformation temperatures decrease. The TiC particles inhibit the R p hase and also lower some of the transformation temperatures. These eff ects can be explained by the internal misfit stresses resulting from b oth thermal expansion and transformation mismatch between matrix and r einforcement. The measured transformation enthalpy of bulk and reinfor ced NiTi is discussed in light of a thermodynamical model, taking into account the elastic energy stored upon cycling. The model indicates t hat a significant fraction of the matrix is stabilized and thus does n ot contribute to the transformation enthalpy.