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.
Highlights
To date, the conventional NiTi (Nitinol) shape memory alloys (SMAs) have been widely put into practice in various industrial communities [1]
The endothermic peaks were observed clearly in both alloys, which correspond to the reverse martensitic transformation start temperature (As ) from α” to β phase
No apparent exothermic peaks corresponding to the forward m tensitic transformation temperatures could be observed in the cooling process in both martensitic transformation temperatures could be observed in the cooling process in both loys
Summary
The conventional NiTi (Nitinol) shape memory alloys (SMAs) have been widely put into practice in various industrial communities [1]. Much effort has been put into studying new hightemperature shape memory alloys (HTSMAs) towards the high-temperature applications above 373 K [4,5]. Among the HTSMAs, the metastable β-Ti based SMAs (called β-Ti. SMAs), which possess the advantage of being relatively lightweight, meet the fundamental weight-reduction requirements for the applications of the aircraft materials in the aerospace industry [6]. To utilize the β-Ti alloys as HTSMAs, essential requirements, such as possessing shape recovery behavior at relatively high operating temperature and being stable at the relatively high operation temperature, are demanded. Ω-embrittlement could be attributed to the formation and growth of the Ti-rich isothermal ω (ωiso ) phase in the matrix
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