A pathway to significantly improve the ductility of a high strength PM α+β titanium alloy by short GB α layers and refined αWGB colonies

Abstract

An approach combing ball milling (BM) and thermomechanical consolidation of a TiH2 based composite powder was used to fabricate powder metallurgy (PM) α+β Ti–4Al–4Mo–4Sn-0.5Si (wt%) alloy. Compared with the coarse-grained alloy with the same composition fabricated by the traditional blended elemental (BE-PM) method, the alloy exhibited a low oxygen content of 0.27 wt% and much-refined β grains with average size of 21 μm owing to the unpassivated and refined powder and fast consolidation process. The fine-grained alloy exhibited an α/βt lamellar microstructure in combination with short grain boundary α (GB α) layers and small GB Widmanstätten α (αWGB) colonies, while the coarse-grained alloy was composed of a similar α/βt lamellar structure in combination with long and continuous GB α plates and large-sized αWGB colonies. βt represented a transformed β structure consisting of fine secondary α laths and residual β layers. It was worth noting that the fine-grained alloy exhibited both higher strength and drastically higher ductility than the coarse-grained alloy (yield strength: 1238 vs 1156 MPa; elongation to fracture: 15.4% vs 4.1%). The enhanced strength was ascribed to the width refinement of α plates and increased fraction of βt domains. The excellent ductility was attributed the refined αWGB colonies and short GB α layers with increased variety of crystal orientations, which relieved strain concentration at GB α phase and thus suppressed the crack nucleation and propagation along it. Therefore, a high density of <a> and <c+a> dislocations were successfully activated in the thin α plates to accommodate plastic deformation, leading to an excellent ductility. This work provides a new strategy to fabricate high strength PM titanium alloys with excellent ductility.