The Investigation of Microstructure and Mechanical Behavior and the Fractographic Analysis of the Ti49.1Ni50.9 Alloy in States with Different Activation Deformation Volumes

Anna Churakova,Dmitry Gunderov,Elina Kayumova

doi:10.3390/app11073052

Abstract

In this article, the microstructure and mechanical behavior of the Ti49.1Ni50.9 alloy with a high content of nickel in a coarse-grained state, obtained by quenching, ultrafine-grained (obtained through the equal-channel angular pressing (ECAP) method) and nanocrystalline (high pressure torsion (HPT) + annealing), were investigated using mechanical tensile tests at different temperatures. Mechanical tests at different strain rates for determining the parameter of strain rate sensitivity m were carried out. Analysis of m showed that with an increase in the test temperature, an increase in this parameter was observed for all studied states. In addition, this parameter was higher in the ultrafine-grained state than in the coarse-grained state. The activation deformation volume in the ultrafine-grained state was 2–3 times greater than in the coarse-grained state at similar tensile temperatures. Fractographic analysis of samples after mechanical tests was carried out. An increase in the test temperature led to a change in the nature of fracture from quasi-brittle–brittle (with small pits) at room temperature to ductile (with clear dimples) at elevated temperatures. Microstructural studies were carried out after the tensile tests at different temperatures, showing that at elevated test temperatures, the matrix was depleted in nickel with the formation of martensite twins.

Highlights

TiNi (Nitinol) alloys are an important class of shape memory alloys (SMAs)
As a result of equal-channel angular pressing (ECAP), an ultrafine-grained (UFG) structure with a grain size of about 500 nm is formed in TiNi alloys, which leads to a significant increase in the mechanical properties and functional characteristics of shape memory effects [9,10]
The main goal of this paper is to elucidate the correlation between the microstructure, fracture, mechanical behavior, and the strain rate/temperature sensitivity of the TiNi alloy processed by ECAP and high pressure torsion (HPT), which have not been considered comprehensively

Summary

Introduction

TiNi (Nitinol) alloys are an important class of shape memory alloys (SMAs). They are well known for their shape memory effect and superelasticity, and have found several important applications. These alloys are used as functional materials in mechanical actuation systems, couplings [1], actuators, biomedical equipment and biomedical implants [1,2]. As a result of equal-channel angular pressing (ECAP), an ultrafine-grained (UFG) structure with a grain size of about 500 nm is formed in TiNi alloys, which leads to a significant increase in the mechanical properties and functional characteristics of shape memory effects [9,10]. In UFG materials, an increase in the rate of diffusion creep and an increase in the contribution of grain-boundary slip to deformation have been observed [25,26,27,28,29,30,31,32]

Objectives

Methods

Results

Conclusion