Instrumented micro-indentation of NiTi shape-memory alloys

We study the instrumented Vickers micro-indentation of single-crystal Ti-50.9 at% Ni shape-memory alloys with systematically varied surface normal orientations ([100], [210], [111] and [221]) and Ti₃Ni₄ precipitate sizes (0 nm, 10 nm, 50 nm, 100 nm, 300 nm and 500 nm). Based on transmission electron microscopy observations, indentation of solutionized NiTi induces inelastic deformation via dislocation activity and a stress-induced martensitic transformation. The room-temperature hardness, Hv, and recoverable energy, E_r, of NiTi are shown to be a maximum for very small precipitate sizes, decrease for intermediate precipitate sizes, and increase for large precipitate sizes. The maximization of Hv and E_r at small precipitate sizes (10 nm) is attributed to the relatively high resistance to both dislocation motion and a recoverable stress-induced martensitic transformation. The decreases in Hv and E_r at intermediate precipitate sizes (50-300 nm) are attri buted to a decrease in the resistance to dislocation motion and a measured increase in the transformation temperatures with respect to the indentation temperature. The increases in Hv and E_r at large precipitate sizes (500 nm) are attributed solely to measured decreases in the transformation temperatures with respect to the indentation temperature, since the resistance to dislocation motion remains constant as the precipitates grow from 300 nm to 500 nm. For nearly all heat treatments, the [100] and [221] surfaces demonstrate the highest and lowest values of Hv and E_r, respectively, an effect attributed primarily to orientation of favorable slip systems.