Strengthening Mechanisms In Solution-Annealed Ni-Rich NiTi Alloys

Two most closely associated properties of NiTi alloys are shape memory and superelasticity.  As such, most NiTi research has concentrated on near equiatomic compositions.  In the present study, we have investigated the Ni-rich 60NiTi (Ni₅₅Ti₄₅ at.%) composition.  This composition still possesses superelasticity, but also exhibits elevated Vickers micro-hardness values.  In the  hot-rolled condition, the alloy contained the NiTi (B2), Ni₄Ti₃, and Ni₃Ti phases with a Vickers micro-hardness value of 375 Hv.  This alloy was then solution annealed at 1050 deg. C for 10 hours and water quenched. This  resulted in a significant increase  in Vicker’s microhardness to 645 Hv. Similar solution ageing for a 57NiTi (Ni₅₂Ti₄₈ at.%) alloy only  resulted in a Vicker’s microhardness of 450 Hv. The retained phases, post-solution annealing, for both compositions was B2 and Ni₄Ti₃. Through TEM and Atom Probe analysis, the increase in hardness is discussed in terms of nanoscale precipitates and local chemical modulations.