Transcending the rule-of-mixtures via coefficient of thermal expansion mismatch-driven interfacial strengthening in additively manufactured Ti alloy heterostructures

Heterostructured alloys have recently gained prominence due to their exceptional capability to simultaneously enhance strength and ductility. However, despite these mechanical improvements, the properties of such alloys typically conform to the rule-of-mixtures, whereby overall performance falls between that of the hard and soft zones. This work explores how the inherent thermomechanical mismatch induced during additive manufacturing can be leveraged to surpass this limitation. In a layered Ti-3.5Al/Ti-6Al-4V heterostructure, the substantial thermomechanical mismatch between the constituent layers led to significant thermal expansion differences during rapid solidification. This process generated pronounced thermal mismatch stresses and promoted dislocation accumulation at the interfaces. These interfaces facilitated hetero-deformation-induced strengthening, governed by substantial back-stress effects and effective strain accommodation within the soft Ti-3.5Al layers. Furthermore, fractographic analyses confirmed that the soft Ti-3.5Al layers effectively arrested and deflected cracks, thereby mitigating premature fracture. These combined mechanisms resulted in strength for the Ti-3.5Al/Ti-6Al-4V heterostructure that exceeds that of both homogeneous Ti-3.5Al and Ti-6Al-4V, while maintaining sufficient ductility. This outcome breaks the rule-of-mixtures observed in typical heterostructured alloys. Consequently, this research demonstrates an innovative approach to utilize additive manufacturing-induced thermomechanical mismatch for designing high-performance titanium alloys, offering a pathway for property enhancement beyond conventional optimization strategies.