Optimum microalloying to tune the stress-induced martensitic transformation of Cu50Zr50 alloy

Optimum microalloying to tune the stress-induced martensitic transformation of Cu50Zr50 alloy

National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, UK
Faculty of Engineering and Environment, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK

Introduction

The effect of microalloying, with Co or Ni individually or combined, on the stress-induced martensitic transformation of Cu50Zr50 at. % shape memory alloy was assessed at nano- and macroscale.

Keywords: Shape memory alloy; Microalloying; Co-microalloying; Martensitic transformation

Methods
Samples were fabricated by suction casting. The structure of the as-cast and precompressed samples (at a force of 19.2 KN, held for 5 min) was studied by X-ray diffraction (XRD), thermal behaviour of the samples were studied by Differential Scanning Calorimetry (DSC) and the microstructure was investigated by Scanning Electron Microscopy (SEM) equipped with energy dispersive X-ray (EDX) analysis.
Nanoindentation were performed with a high temperature vacuum system.

Results
The stress-induced martensitic transformation of Cu50Zr50 at. % shape memory alloy was tuned through microalloying. The effect of microalloying elements Co or Ni individually or combined (i.e., co-microalloying) was investigated and compared at the macro- and nanoscale. From nanoindentation experiments, change in the slopes of (P/h)-h curves, recovery ratio after annealing and plastic index were studied. Partial replacement of Cu by 1 at. % Ni promotes twinning while for 1 at. % Co the twinning propensity decreased and co-microalloying using 0.5 at. % Co and Ni had an intermediate effect. The recovery ratio of the Cu50Zr50 alloy after annealing at 400C for 5 min increased from 15.6 % to 19.5 % when substituting Cu by 1 at. % Ni. Microalloying therefore opens up possibilities for the development of cost-effective CuZr alloys for commercial actuators.


Conclusions
Partial replacement of Cu by 1 at. % Ni from the parent Cu50Zr50 alloy, enabled to improve the recovery ratio after annealing at 400°C for 5 min, from 15.6 % to 19.5 %. Better twinning propensity of B2-austenite was observed, favouring the formation of martensite. Co-microalloying using equal concentrations of Co and Ni did not have a significant effect on the mechanical and thermal behaviour of Cu50Zr50 alloy due to the effect of Co in suppressing the formation of martensite.

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