Staged induction of uniform subgrain nucleation and growth in 60NiTi intermetallic through multi-pass deformation

Abstract

60NiTi intermetallic is an attractive bearing material for centrifugal pumps in space life-support systems due to its low density, high hardness and non-magnetic. Hot-deformation process and recrystallization mechanism have been clarified for single-pass deformation. However, single-pass processing tends to produce heterogeneous necklace-like structure. In this work, three multi-pass deformation paths were designed to regulate deformation microstructure of 60NiTi intermetallic: Isothermal, Continuously decreasing and Decreasing-increasing. The decreasing-increasing path produced uniform subgrains and Ni₄Ti₃ phase through stored-energy accumulation at low temperature and meta-dynamic recrystallization during reheating. Stage I at 950 °C fragmented the long-lath Ni₃Ti phase into short rods and introduced dislocations. High Schmid factors facilitated dislocation glide on the {110}〈111〉 and {123}〈111〉 slip systems. Stage II at 850 °C promoted dislocation accumulation within grains and around Ni₄Ti₃ phase, creating high stored energy for substructure nucleation. However, lower Schmid factors hindered dislocation glide and weakened intergranular deformation, thereby increasing the flow stress. During Stage III at 900 °C, the inter-pass interval promoted dislocation rearrangement and meta-dynamic recrystallisation. Subgrains grew from grain boundaries towards grain interiors, and their misorientation increased from 0.77° to 4.48°. The equiaxed subgrains were refined to 4.55 μm without strong crystallographic orientation and necklace-like structure. Meanwhile, the concurrent evolution of subgrains and Ni₄Ti₃ phase consumed dislocations as strain increased, reducing flow stress. These findings provide theoretical and processing guidance for multi-step forging of 60NiTi intermetallic.