Composition and Microstructure Design for an Ultra-High Strength and High-Temperature Dual-Phase Ti Alloy Using Cluster Formula Approach

Abstract

Dual-phase Ti alloys are known for good processabilities but their mechanical strengths are not sufficient both at room and at high temperatures. We here report a dual-phase Ti alloy Ti-6Al-14Zr-2V-2Mo-1Nb (wt. %), designed on the basis of the cluster formula {Al2Ti14}12{Al1Ti15V2}5 of popular Ti-6Al-4V alloy, after additional (Zr, Mo, Nb) co-alloying. This alloy is solutioned at different temperatures, and finally aged at 550 °C for 4 h. The 840 °C /1 h solutioning-plus-aging specimen exhibits a nearly ultra-high strength level at room temperature, with ultimate tensile strength Rm = 1366 MPa, yield strength Rp0.2 = 1330 MPa, and elongation to failure A = 7.0 %. This specimen also shows a superior 500°C performance, Rm = 1058 MPa, Rp0.2 = 991 MPa, and A = 16.0 %. Such a mechanical performance is attributed to a bimodal morphology consisting of about 30 vol.% of equiaxed αp grains imbedded in a fine β-transformed lamellar matrix. A microstructure model is proposed, mimicking the cluster-plus-glue-atom model, to explain this αp proportion as the maximum volume fraction before the αp grain becomes agglomerated. It is also confirmed that, at the aging state, the α and β phases reach the designed chemical composition, indicating that the 840 °C / 1 h solution plus 550 °C /4 h aging is the ideal heat treatment scheme that fully realizes the potential of the cluster formula design.

Keywords: dual-phase titanium alloys, composition formula, Microstructure model, mechanical properties