The Mechanism of Inhomogeneous Grain Refinement in a NiTiFe Shape Memory Alloy Subjected to Single-Pass Equal-Channel Angular Extrusion

Shuyong Jiang,Yanqiu Zhang

doi:10.3390/met7100400

Abstract

Based on electron backscattered diffraction analysis and transmission electron microscopy observation, the mechanism of inhomogeneous grain refinement in a NiTiFe shape memory alloy (SMA) subjected to single-pass equal-channel angular extrusion (ECAE) was investigated. The results show that refined grains are mainly nucleated near grain boundaries and a small fraction of them emerges in the grain interior. The size of refined grains increases as deformation temperature increases, which indicates that a higher deformation temperature is adverse to grain refinement in the ECAE of NiTiFe SMAs. It is the accumulation and rearrangement of geometrically necessary dislocations as plastic strain increases that leads to the transition of lower angle subgrain boundaries, and finally higher angle subgrain boundaries are induced and finer grains are formed. Due to the limitation of slip systems, the mechanism of grain refinement in a NiTiFe SMA subjected to ECAE is different from that in face-centered cubic and body-centered cubic crystals. Dislocation cells and shear bands are two transition microstructures of grain refinement in the ECAE of NiTiFe SMAs. The nucleation of fine grains mainly occurs along shear bands or grain boundaries, which leads to the inhomogeneity of grain refinement.

Highlights

As a functional material, NiTi-based shape memory alloys (SMAs) have been extensively applied in medical and engineering fields because they possess excellent superelasticity and perfect shape memory
Based on the aforementioned results, it can be concluded that single-pass equal-channel angular extrusion (ECAE) performed at
400–500 ◦ C is able to refine the grains of NiTiFe SMA

Summary

Introduction

NiTi-based shape memory alloys (SMAs) have been extensively applied in medical and engineering fields because they possess excellent superelasticity and perfect shape memory. Researchers have investigated the grain refinement of NiTi-based SMAs via various severe plastic deformation (SPD). Researchers have investigated the deformation behavior of ECAE in many conventional metals, such as magnesium alloys [12], steel [13], aluminum alloys [14], pure titanium [15], and pure copper [16]. These investigations mostly focused on the microstructures or mechanical properties of materials that have experienced multi-pass ECAE; the mechanism for grain refinement has not been thoroughly revealed

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Results

Conclusion