Investigations of Deformation Behavior and Microstructure of Al Tailored Ti–Mo High Temperature Shape Memory Alloys during Isothermal Holding at 393 K

Abstract

To utilize β-Ti based high temperature (HT) shape memory alloys (SMAs), a high Al concentration of 14 mol% was designed for sufficient suppressing the undesired ω-phase. HTSMA exhibits shape memory effect (SME) above 373 K, and thus the operating temperature is over 373 K. However, the SME and the mechanical properties of most β-Ti SMAs deteriorate after holding at elevated temperatures due to the ω-embrittlement. The Ti-4.5Mo-14Al alloy (mol%) and the Ti-6Mo-7Al alloy as a comparison, both of which possess the identical reverse martensitic transformation start temperature of 407 K, were isothermally held at 393 K for up to 360 ks, and deformation behaviors and microstructures were investigated. It was found that after the isothermal holding, the deformation behavior of the Ti-6Mo-7Al alloy altered significantly; on the other hand, that of the Ti-4.5Mo-14Al alloy remained almost intact. Transmission electron microscopy observations revealed that the isothermal ω-phase (ωiso) was successfully suppressed in the Ti-4.5Mo-14Al alloy, while the ωiso phase grew in Ti-6Mo-7Al alloy. Moreover, the isothermal α″-phase coexisted in the Ti-4.5Mo-14Al alloy. It is concluded that a high Al concentration is a crucial prerequisite in the practical β-Ti HTSMAs. The presented design could be a useful guideline for developing Ti SMAs with comparable Mo- and Al-equivalents.