Properties of rare earth added Cu–12wt%Al–3wt%Ni–0.6wt%Ti high temperature shape memory alloy

Abstract

In this study a Cu–12wt%Al–3wt%Ni–0.6wt%Ti + RE high temperature shape memory alloy, where RE denotes the rare earth doping elements, was prepared by vacuum arc melting and characterized by means of scanning electron microscopy, field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction at room and elevated temperatures, differential scanning calorimetry, tensile test and dynamic mechanical analysis. The results demonstrated that the addition of a 0.04 wt% RE to the initial alloy caused a significant grain size reduction, improved the ductility and enhanced the thermo-mechanical properties. As a consequence of the addition of RE, both the tensile fracture stress and tensile fracture strain of the alloy increased from 550 MPa and 4.50% to 660 MPa and 5.0%, respectively. The maximum recoverable strain was raised from 1.2% to 1.8%. A significant improvement of the mechanical properties and shape memory response of this alloy was attributed to the extra grain refinement mainly due to the formation of the RE-rich precipitates.

Introduction

Cu–based high temperature shape memory alloys (HTSMAs) are very attractive from the applied point of view, especially, due to relatively low cost of production in comparison with the other types of shape memory alloys [[1], [2], [3]]. Among the various compositions of Cu–based shape memory alloys, Cu–Al–Ni alloys with the transformation temperature as high as 200 °C, are the most promising candidates for HTSMAs [4]. However, their practical utilization in a polycrystalline state is limited due to the brittleness and poor mechanical properties resulted from large elastic anisotropy of the crystal lattice and usually large grain size [[5], [6], [7], [8], [9]]. Therefore, apart from the powder metallurgical approach [10,11], an extensive work has been done in order to refine the grains by adding fourth and fifth alloying elements to the alloy content [[12], [13], [14]]. Previous studies demonstrated that the addition of a small amount of Ti has a significant effect in decreasing grain size in Cu–Al–Ni ternary alloy [[15], [16], [17]]. The grain refinement due to the Ti addition is attributed to the development of finely dispersed Ti-rich precipitates of X-phase, which hinder the grain growth. The X-phase is formed owing to the low solid solubility of Ti in the β-phase (less than 0.05 wt%) [18]. According to the previous investigation of Cu–Al–Ni–Ti alloy, X-phase has a remarkable contribution to the reduction of as-cast grain size and also suppression of grain growth during hot working and heat treatment. It has been proved that the grain refinement enhances the mechanical properties and alters the fracture mode through increasing ductility [[18], [19], [20], [21], [22]]. However, in order to improve the properties, only a small amount of Ti should be added to the composition, beyond that limit the degree of the grain refinement becomes independent of the Ti addition [18].

An efficient influence of doping by the Rare Earth elements (RE) on a grain refinement is well known for different SMAs [[23], [24], [25], [26]]. However, few works studied the effect of RE doping on Cu–based SMAs. Lu et al. [27] have found that adding the appropriate amount of Ce to Cu–Al–Mn can reduce the grain size and improve the shape memory properties significantly. Yang et al. [28] studied the role of addition of the misch metal (alloy of rare earth elements) in Cu–Zn–Al alloys. According to their findings, an addition of the misch metal causes a grain size reduction and an increment of the fracture strength. The examination of Gd addition in Cu–Zn–Al done by Xu [29] demonstrated the improvement of shape recovery ratio. Bhattacharya et al. [30] stated that, besides decreasing the tendency for grain coarsening without notable affecting the shape recovery behavior of the alloy, the addition of grain refining elements, such as Ti and RE, promotes the phase transformation through retarding the precipitation of γ₂ phase in Cu–Al–Ni alloys.

Considering the aforementioned positive effects of adding Ti or RE separately on the performance of the Cu–based SMAs, it was reasonable to design a new alloy by a combined adding of these elements to the Cu–Al–Ni base system and to investigate its structural, mechanical and shape memory properties. The adding of a second grain refinement element, not RE, to Cu–Al–Ni–Ti alloy was probed in ref. [31], but the approach to use namely RE as second grain refining agent is used for the first time in the present work. The aim of this work was to prepare new Cu–Al–Ni–Ti–RE HTSMA and investigate its properties compared with the same initial alloy but without doping by RE. For this purpose 0.04 wt% misch metal (alloy of Ce and La) was added to the initial Cu–12wt%Al–3wt%Ni–0.6wt%Ti quaternary alloy and then its structure, transformation behavior and shape memory properties were studied systematically by a variety of characterization methods. Cyclic stability study revealed that new alloy is promising for high temperature shape memory applications.