Effect of Severe Plastic Deformation and Post-Deformation Heat Treatment on the Microstructure and Superelastic Properties of Ti-50.8 at.% Ni Alloy

Abstract

Severe plastic deformation via high-ratio differential speed rolling (HRDSR) was applied to the Ni-rich Ti-50.8Ni alloy. Application of HRDSR and a short annealing time of 5 min at 873 K leads to the production of a partially recrystallized microstructure with a small grain size of 5.1 μm. During the aging process for the annealed HRDSR sample at 523 K for 16 h, a high density of Ni3Ti4 particles was uniformly precipitated over the matrix, resulting in the formation of an R phase as the major phase at room temperature. The aged HRDSR sample exhibits excellent superelasticity and superelastic cyclability. This achievement can be attributed to an increase in strength through effective grain refinement and particle strengthening by Ni3Ti4 and a decrease in the critical stress for stress-induced martensite (B19′) due to the presence of the R-phase instead of B2 as a major phase at room temperature. The currently proposed method for using HRDSR and post-deformation heat treatment allows for the production of Ni-rich NiTi alloys with excellent superelasticity in sheet form.

On Structural Sensitivity of Young’s Modulus of Ni-Rich Ti-Ni Alloy

“…Following the theory of internal friction, which describes the interaction of dislocations with point defects, a decrease in Young's modulus after plastic deformation is a natural consequence of an increase in the dislocation density [6,27,34,35]. It is also known that any discontinuities, such as nanopores that appear in the process of significant plastic deformation, lead to a decrease in the E magnitudes [36][37][38].…”

“…The scope of their clinical use in surgery is extremely wide, including various manipulators, cava filters, septal occlusion devices, vascular stents and stents for tubular structures, prostheses (osseous, ligamental, dental), compressive devices, and implants [1][2][3]. The traditional processing of NiTi alloys includes a thermomechanical treatment as well as subsequent post-deformation annealing (PDA) [3][4][5]; varying the aging modes makes it possible to precisely control aging-induced microstructure, transformation, and functional response over a wide temperature range [6][7][8][9].…”

“…The cited publications, however, do not provide an exhaustive knowledge of the structural responses of Young's modulus, since the comparative tests in the above cited studies are restricted by one or two kinds of structures (mainly recrystallized [15,17] or as-deformed [16,18]). Meanwhile, post-deformation annealing (PDA) is an obligatory procedure to provide a necessary temperature range for the shape recovery as well as other functional characteristics [3][4][5][6][7][8][9]19].…”