Effect of Al Addition on Martensitic Transformation Stability and Microstructural and Mechanical Properties of CuZr Based Shape Memory Alloys

Carlo Alberto Biffi,Ausonio Tuissi,Jacopo Fiocchi,Mauro Coduri

doi:10.3390/met11071141

Abstract

In this work, the effect of the Al content (x = 5, 10, and 15 at. %) on the martensitic transformation (MT) and microstructure and mechanical properties of Cu(50−x)Zr50Alx alloys was studied. The microstructure of the alloys was characterized at room temperature by means of scanning electron microscopy and X-ray diffraction. An increase in Al content reduces the amount of transforming CuZr phase, and consequently the secondary phase formation is favored. The evolution of the MT upon thermal cycling was investigated as a function of the Al content by differential scanning calorimetry. MT temperatures and enthalpies were found to be decreased when increasing the Al content. Al addition can induce a sudden, stable MT below 0 °C, while the binary alloy requires ten complete thermal cycles to stabilize. Finally, the mechanical properties were investigated through microhardness and compression testing. No linear dependence was found with composition. Hardness lowering effect was observed for 5–10 at. % of Al content, while the hardness was increased only for 15 at. % Al addition with respect to the binary alloy. Similarly, compressive response of the alloys showed behavior dependent on the Al content. Up to 10 at. % Al addition, the alloys indicate a superelastic response at room temperature, while higher Al content induced untimely failure.

Highlights

IntroductionPublisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations
Scanning electron microscopy (SEM) images of the samples and related energy dispersion spectroscopy (EDS) analyses are shown in Figure 1 and
These microstructural changes strongly influenced the characteristics of martensitic transformation (MT): while in the binary alloy the transformation is stabilized only after eight thermal cycles, samples containing aluminum exhibit stable MT ab initio

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. An innovative way based on a prediction strategy for the formation of different sized CuZr-based shape memory bulk metallic glass composites (BMGC) was proposed for evaluating the competition among the vitrification, the precipitation of metastable B2 CuZr, and room-temperature equilibrium phases. Due to this lack of stability during phase transformations, as well as to improve the alloys’ workability, efforts have been made to study the addition of several alloying elements, like Co, Cr, Ni, Al, and Ti [10,11,12,13,14,15,16]. Mechanical properties were evaluated using microhardness measurements and compression testing, which were done at room temperature

Experimental

Results and Discussion

BSE-SEM images

Microhardness as aoffunction of the

Conclusions