Multiscale structure and properties of cast and deformation processed polycrystalline NiTi shape-memory alloys

Abstract

The objective of this study is to examine fundamental processing-structure-property relationships in polycrystalline NiTi bars. Three different polycrystalline Ti-50.9 at. pct Ni (Ti-55.7 wt pct Ni) materials were examined: (1) cast, (2) cast then hot rolled, and (3) cast, hot rolled, then cold drawn. The structure of the materials was investigated at various scales ranging from nanometers to micrometers. The cast materials contained random crystallographic textures along the loading axis of the extracted samples. The hot-rolled and cold-drawn materials contained a strong Ͻ111Ͼ texture parallel to the deformationprocessing direction. The high-temperature hot-rolling process facilitated recrystallization and recovery, and curtailed precipitate formation, leaving the hot-rolled and cold-drawn materials in near solutionized states. The cold-drawn material contained a high density of dislocations and martensite. After a mild aging treatment, all three materials contained distributed coherent Ti 3 Ni 4 precipitates on the order of 10 nm in size. The cast material was capable of full shape-memory transformation strain recovery up to approximately 5 pct strain at room temperature under both tension and compression. The hot-rolled and cold-drawn materials demonstrated significant tension-compression stress-strain asymmetry owing to their strong crystallographic texture. Under compression, the deformation-processed materials were only capable of 3 pct transformation strain recovery while under tension they were capable of nearly 7 pct transformation strain recovery. Based on the present results, the presence of small coherent Ti 3 Ni 4 precipitates is determined to be the driving force for the favorable strain transformation strain recovery properties in all three materials, despite drastically different grain sizes and crystallographic textures. The unique dependence of elastic modulus on stress-state, temperature, and structure is also presented and discussed for the deformation-processed materials. In addition, we demonstrate that the appearance of a Lüders band transformation under tensile loading can be controlled by material structure. Specifically, the presence of significant martensite and dislocations in the cold-drawn materials was shown to mitigate the Lüders band propagation and result in a more gradual transformation.

Phase volume fractions and strain measurements in an ultrafine-grained NiTi shape-memory alloy during tensile loading

“…3, the macroscopic stress-strain curve initially exhibits linear-elastic behavior with an apparent austenitic elastic modulus of 56 GPa (calculated from the stress-strain data ranging from 0 to 150 MPa), which is within the range of values reported in the literature (E austenite = 40-90 GPa [62][63][64]). At a load above 150 MPa, a bend in the stress-strain curve is observed.…”

Challenges During Microstructural Analysis and Mechanical Testing of Small-Scale Pseudoelastic NiTi Structures

“…Another important aspect is the effect of novel processing techniques on the resulting microstructures in small samples: conventional bulk NiTi is typically produced by melting, casting, followed by rolling, swaging, or wire drawing [3]. The resulting microstructures are typically characterized by considerably deformed grains, high dislocation densities (after cold work), and pronounced textures [4]. Physical vapor deposition (PVD) methods have emerged as an interesting alternative to these conventional processing routes; these PVD methods offer certain advantages, such as high purity (because samples are deposited in high vacuum), and because (pre-alloyed) targets can be carefully fine-tuned to the desired alloy composition [5][6][7].…”

Effect of Cu Content on Atomic Positions of Ti50Ni50−xCux Shape Memory Alloys Based on Density Functional Theory Calculations

“…TiNi-based shape memory alloys (SMAs) have been widely used in the aerospace and biomedical fields because of their superior shape memory effect (SME), high superelasticity, and favorable biocompatibility [1][2][3][4][5]. In order to fulfill the requirement of high response frequency in the automotive and aerospace industries, TiNiCu SMAs in which Cu substitutes for Ni have been widely studied due to their narrow martensitic transformation (MT) temperature hysteresis in comparison with TiNi binary alloys [6][7][8][9].…”