The role of microalloying on tuning the mechanical performance of Cu-Zr based shape memory alloys

Younes, Abdurauf K. M. (2021) The role of microalloying on tuning the mechanical performance of Cu-Zr based shape memory alloys. Doctoral thesis, Northumbria University.

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Abstract

Shape Memory Alloys (SMAs) are an interesting class of materials that possess sensing and actuation functions due to Shape Memory Effect (SME). Although SME is observed in NiTi where high recoverable stress-strain is observed, there are limitations associated with implementation of NiTi such as high cost compared to cost-effective elements (i.e., Cu) and the inability to observe SME at temperatures above ~100oC. Therefore, a cost-effective and temperature-adaptive replacement (CuZr-based SMAs) may replace NiTi in sensing and actuation applications.

The tribological performance of CuZr-based SMAs is investigated by wear and hardness tests at Room Temperature (RT) and high temperature. CuZr-based SMAs are obtained by rapid solidification and the microstructure is examined using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Due to poor SME of Cu50Zr50 at.% SMA compared to NiTi, a microalloying element (i.e., Co) is introduced in place of Cu at different at.% in order to promote martensitic transformation, leading to enhanced mechanical performance. This research found that microalloying with Co0.5 at.% at RT decreased mass loss by 46% compared to Cu50Zr50. However, when operating at 100oC, mass loss of the same alloy only decreased by 10%.

Additionally, other microalloying elements such as Fe and Mn were investigated to improve the wear performance of CuZr-based SMAs, it was found that partial replacement of Cu by 0.5 at.% Fe, results in a lifetime enhancement of about 30.5% while for 0.5 at.% Mn the lifetime enhancement is about 21%. Microalloying is therefore an efficient strategy to develop CuZr-based SMAs as temperature adaptive shaft seals for engines.

The synergistic effect of cooling rate control and concentration of microalloying element on mechanical performance of CuZr-based SMAs is also novel. In the case of fast cooling rate, 0.5 at.% Fe addition promotes formation of B19’ martensite upon wear testing, hence improving wear resistance. However, for slower cooling rate of the same alloy, relatively large volume fraction of B33 martensite is formed resulting in reduction of wear resistance.

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