Formation of Nanoscale Aging Products and Their Effects on the Mechanical Properties of a Metastable β-Ti Based Shape Memory Alloy

Metastable β-titanium alloys exhibit excellent mechanical properties and shape memory behavior, making them promising candidates for high-temperature shape memory alloys. Among the nanoscale precipitates that form during aging, the ω phase is well-known for inducing embrittlement. Recently, other nanoscale phases, such as the isothermal α″ phase (αiso″), have been discovered. However, the formation mechanisms and mechanical impacts of these precipitates remain unclear. This study reviews the current understanding of nanoscale precipitates, including ω phase, O′ phase, and αiso″, with a focus on their formation mechanisms and microstructural evolution in β-titanium alloys. Based on this review, we conducted experimental investigations on Ti–(3.5–5.0)Mo–(11,14)Al alloys. The alloys were solution-treated and aged at 573 K for 3.6 ks to study the formation behavior of αiso″ and its influence on mechanical properties. X-ray diffraction (XRD) analysis confirmed the formation of αiso″ in alloys with high Mo and Al concentrations, characterized by peak broadening. Scanning electron microscopy (SEM) revealed fine needle-like precipitates and surface undulations associated with αiso″ formation. Alloys without αiso″ exhibited no significant microstructural changes. Tensile testing revealed that the alloys containing αiso″ suffered significant ductility loss, confirming, for the first time, that αiso″ plays a critical role in embrittlement. This study provides further insights into the role of nanoscale precipitates in β-titanium alloys, emphasizing the importance of controlling αiso″ formation. These findings highlight the need for composition optimization to suppress detrimental precipitates, ensuring the structural integrity of β-titanium alloys for high-temperature applications.