Isothermal ω and α″ Phase Evolution and Mechanical Behavior of Aged Ti–Al–Mo Shape Memory Alloys

To utilize β-Ti based high-temperature shape memory alloys (HTSMA), a high Al concentration of 14 mol% was designed to suppress the ω phase. Conventional NiTi alloys are unsuitable for high-temperature aerospace applications due to their limit of 373 K. β-Ti HTSMA are considered for these applications because of their lightweight and corrosion resistance but suffer from embrittlement from ω phase formation during high-temperature holding. This study aims to enhance the shape memory effect (SME) and mechanical properties of β-Ti HTSMA by exploring high Al concentrations. The alloys used were Ti–14Al–4.5Mo and Ti–7Al–6Mo, both with an identical reverse martensitic transformation start temperature (A_s) of approximately 407 K. These alloys were isothermally held at 393 K for up to 360 ks to investigate their deformation behaviors and microstructures. High-purity titanium, aluminum, and molybdenum were used as raw materials. Ingots were prepared by arc melting in argon, homogenized at 1273 K, and quenched in water. The ingots were hot-rolled and cold-rolled into sheets, followed by solution treatment at 1273 K and quenching in iced water. Differential scanning calorimetry (DSC) evaluated transformation temperatures. X-ray diffraction (XRD) identified phase constituents at room temperature (RT). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) provided microstructural observations. Tensile tests assess mechanical properties and shape memory behavior.
DSC curves indicated both alloys had an A_s of approximately 407 K. XRD profiles revealed both alloys consisted of β phase and α″ phase at RT, suggesting M_s is above RT and M_f below RT. SEM images showed equiaxed β phase grains and acicular martensitic α″ phase near grain boundaries. The Ti–14Al–4.5Mo alloy had a lower β phase and higher α″ phase fraction compared to the Ti–7Al–6Mo alloy. Tensile tests showed both alloys exhibited shape memory properties with residual strains of about 2.5% entirely recovered upon heating. Stress–strain curves showed distinct two-stage yielding behavior in the Ti–14Al–4.5Mo alloy, remaining almost unchanged after isothermal holding. In contrast, the Ti–7Al–6Mo alloy displayed significant changes in deformation behavior and increased yield stress after isothermal holding, with two-stage yielding behavior disappearing in the 36 ks and 360 ks specimens. TEM observations revealed the isothermal ω phase (ω_iso) was suppressed in the Ti–14Al–4.5Mo alloy but grew significantly in the Ti–7Al–6Mo alloy. The α″_iso phase presence in the Ti–14Al–4.5Mo alloy indicated high Al concentration suppressed ω_iso phase formation and growth. The yield stress of the Ti–14Al–4.5Mo alloy increased slightly with isothermal holding time, while the Ti–7Al–6Mo alloy showed a significant increase in yield stress early in isothermal holding. Microstructural analysis using high-resolution TEM indicated in the Ti–14Al–4.5Mo alloy, both ωiso and α″iso phases were complementarily dispersed at a nanometric scale, leading to minor changes in mechanical properties. Conversely, in the Ti–7Al–6Mo alloy, ω_iso phase growth significantly increased yield stress and altered deformation behavior. High Al content in Ti–14Al–4.5Mo alloy not only suppresses ω_iso phase but also ensures α″_iso phase stability, maintaining favorable mechanical properties. High Al concentration is crucial for suppressing ω phase and maintaining mechanical properties in β-Ti HTSMA. The Ti–14Al–4.5Mo alloy, with its stable microstructure and consistent deformation behavior even after prolonged high-temperature exposure, is particularly promising for HTSMA applications. Further research on β-Ti alloys with varying Mo and Al contents is recommended to optimize their performance for high-temperature shape memory and superelastic applications.