Rapid Optimization of Microstructure-Hardness Relationship in Ti-Al-V-Zr-Nb-Mo Alloy via Gradient Heat Treatment

This study employed a gradient heat treatment strategy to efficiently acquire microstructure parameters and establish the microstructure-hardness relationship in Ti-6Al-4V-1.5Zr-1.0Nb-0.5Mo alloy, addressing the knowledge gap in rapid optimization of heat treatment windows. Gradient solution treatment in the α + β region (859-928 °C) revealed that hardness reaches a minimum at a Vα_p/Vβ_t ratio of approximately 0.5, a condition to be avoided if aging is not applied. Subsequent aging at 500 °C, a common temperature for such alloys, highlighted the solution-treated sample at 908 °C as possessing high hardening potential, attributed to its high β_t fraction (Vβ_t = 70%) and sufficient retained β phase that promoted fine α_s precipitation. Gradient aging (502-590 °C) of this optimized microstructure further showed that peak hardness (>350 HV1, measured under a 1 kg load) was achieved at 502 °C and 551 °C, where the Vα_p/Vβ_t ratio remained near the optimal 3:7, and the precipitated refined α_s exhibited minimal width. The hardness of the bimodal microstructure is governed by two principal factors: the Vα_p/Vβ_t ratio (optimum near 3:7) and the precipitation efficiency of refined α_s from retained β phase. The gradient approach proves to be an effective high-throughput method for rapidly correlating heat treatment parameters with microstructure and properties, accelerating the design of heat treatments for titanium alloys.