Improved functional stability of a coarse-grained Ti-50.8 at.% Ni shape memory alloy achieved by precipitation on dislocation networks

Abstract

In this work, a new process is developed to improve the functional stability of Ni-rich NiTi alloys. Repetitive temperature-and stress-induced phase transformation is first conducted to generate dislocation networks in the grain interior. Dislocations serve as nucleation sites for Ni4Ti3 nanoprecipitates, which are formed after subsequent low-temperature (523 K) aging. With the presence of dislocations, a homogeneous distribution of nanoprecipitates in the grains is expected, enhancing the strength of the NiTi matrix and resisting plastic deformation during the martensitic transformation. As a result, an improved functional stability of NiTi alloys is achieved.

Effect of operating parameters on functional fatigue characteristics of an Ni-Ti shape memory alloy on partial thermomechanical cycling

“…The rapid variation in permanent deformation during the initial stages can be attributed to the generation of dislocations. Unlike conventional materials, shape memory alloys generate dislocations by two mechanisms: permanent deformation by slip and incompatibility between the transforming phases, also known as transformation-induced plasticity or TRIP (Bo and Lagoudas, 1999; Gao et al, 2017; Lim and McDowell, 1994; Wang et al, 2019). The generation of dislocations is rapid in the initial cycles because the pinning action by the already generated dislocations is less effective.…”

Recent Development of TiNi‐Based Shape Memory Alloys with High Cycle Stability and High Transformation Temperature

“…The irrecoverable strain of Ti 49.2 Ni 50.8 alloy processed by such a process was reduced to 0.5% after 20 cycles. [25] Another modified aging process was reported by Chen et al, in which precipitation and grain refinement strengthening were combined. [26] The solution-treated Ti 49.2 Ni 50.8 alloy was first aged at 500 C to obtain the fine and coherent Ti 3 Ni 4 phase, and then the aged alloy was cold rolled, followed by annealing at 400 C, to obtain a nanocrystalline microstructure.…”

“…For coarse‐grained alloys, the distribution of the Ti 3 Ni 4 phase is not homogeneous between the grain interior and grain boundary. Wang et al proposed a new process primarily consisting of two steps: 1) introduction of a dislocation network by transformation cycle, serving as nucleation sites of the Ti 3 Ni 4 phase and 2) homogeneous precipitation of the Ti 3 Ni 4 phase by low‐temperature aging. The irrecoverable strain of Ti 49.2 Ni 50.8 alloy processed by such a process was reduced to 0.5% after 20 cycles .…”

“…Wang et al proposed a new process primarily consisting of two steps: 1) introduction of a dislocation network by transformation cycle, serving as nucleation sites of the Ti 3 Ni 4 phase and 2) homogeneous precipitation of the Ti 3 Ni 4 phase by low‐temperature aging. The irrecoverable strain of Ti 49.2 Ni 50.8 alloy processed by such a process was reduced to 0.5% after 20 cycles . Another modified aging process was reported by Chen et al, in which precipitation and grain refinement strengthening were combined .…”

“…It has been suggested that the HPT method and postdeformation annealing may be a possible way to achieve this. Another way is that developed by Wang et al for TiNi binary alloys (Section ). Both these methods are expected to be effective for tailoring the distribution, morphology, and size of the H‐phase.…”