Hierarchical microstructure and strain partition of a PM plus titanium alloy with an ultrahigh strength-ductility synergy

Powder metallurgy (PM) alpha + beta Ti-4Al-4Mo-4Sn-0.5Si alloys were fabricated through thermomechanical powder consolidation of TiH2-based powder compacts with two different processing routes: one involving blending the elemental powders (BE route) and the other involving mechanical milling a blend of elemental powders (MM route). With the MM + annealing + solution and aging (STA) route, a hierarchical microstructure consisting of necklace-like micron-scaled grain boundary alpha (alpha GB) segments and isolated submicron-scaled primary alpha (alpha(p)) rods dispersed in a beta transformed structure (beta t) matrix containing a high number density of nano-scaled secondary a (as) laths was produced in the alloy. The alloy with the hierarchical microstructure achieved an excellent combination of a very high yield strength of 1339 MPa,an ultimate tensile strength of 1467 MPa and an excellent ductility of 10.4%. The increased strength was mainly due to the high fraction of beta t matrix in which the fine-dispersed as precipitates with different orientations efficiently blocked the dislocation motion during deformation. The excellent ductility achieved despite of the high flow stress can be attributed to the elimination of strain concentration in the alpha GB segments which prevented their cracking prior to the material reaching the necking point during plastic deformation. This was in contrast with the typical alpha/beta t lamellar microstructure of the same alloy composition obtained with the BE + same heat treatment route where strain concentration in the aGB plates occurred due to the direct slip transmission from a colonies to adjacent parent alpha GB grains during plastic deformation, thus resulting in intergranular premature cracking. TEM observation demonstrated that the strain gradient from the alpha/beta t interfaces to the domain interiors of both alpha GB segments and ap rods activated a high density of geometrically necessary dislocations near the interfaces, and caused an obvious synergistic strengthening effect which also contributed to the slow drop of strain hardening rate and thus was beneficial to improving the ductility.