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 ori entations ([100], [210], [111] and [221]) and Ti3Ni4 precipitate sizes (0 n m, 10 nm, 50 nm, 100 nm, 300 nm and 500 nm). Based on transmission electron microscopy observations, indentation of solutionized NiTi induces inelasti c deformation via dislocation activity and a stress-induced martensitic tra nsformation. The room-temperature hardness, Hv, and recoverable energy, E-r of NiTi are shown to be a maximum for very small precipitate sizes, decrea se for intermediate precipitate sizes, and increase for large precipitate s izes. The maximization of Hv and Er at small precipitate sizes (10 nm) is a ttributed to the relatively high resistance to both dislocation motion and a recoverable stress-induced martensitic transformation. The decreases in H v and E-r at intermediate precipitate sizes (50-300 nm) are attributed to a decrease in the resistance to dislocation motion and a measured increase i n the transformation temperatures with respect to the indentation temperatu re. The increases in Hv and E-r at large precipitate sizes (500 nm) are att ributed solely to measured decreases in the transformation temperatures wit h respect to the indentation temperature, since the resistance to dislocati on motion remains constant as the precipitates grow from 300 nm to 500 nm. For nearly all heat treatments, the [100] and [221] surfaces demonstrate th e highest and lowest values of Hv and E-r respectively, an effect attribute d primarily to orientation of favorable slip systems. (C) 2001 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.