Optimization of (α + β) microstructure and trade-off between strength and toughness: Based on Mo[eq] and d electron theory in β-Ti alloy

Abstract

To optimize the (α + β) microstructure and find a trade-off between strength and toughness, Ti-xMo-4Al-4Zr-3Nb-2Cr-1Fe alloys were prepared according to Mo[eq] and d electron theory. Microstructure of α phase and dislocation was observed, and the related mechanisms were determined. Results show that the relative content of β phase increases by adjusting Mo content. Length-width ratios of αp and αs phases decrease from 8.8 to 6 and 10.8 to 9.1 as Mo increases from 5 to 6 wt%. When the Mo content increases further, length–width ratio increases. The dislocation density reaches its maximum at 6Mo. The low diffusion rate of Mo and refined β grains causes the refinement of the α phase. The increase of grain boundary and the appearance of lattice distortion increase the dislocation density, but the formation of twins consumes partial dislocation. The tensile strength first increases and then decreases, reaching a maximum of 1326 MPa at 6Mo. The toughness of 6Mo alloy is 85 MPa·m1/2. The strength increases by 12% while the toughness only decreases by 4%. The precipitation strengthening caused by the optimization of the (α + β) microstructure and the dislocation strengthening caused by the increased dislocations are the determining mechanism of the trade-off between strength and toughness.