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A novel Ti–2Al–1.3V (wt.%) alloy was prepared by thermomechanical consolidation of a composite powder produced by high-energy ball milling of a mixture of TiH2 granules with a low oxygen content (0.1wt.%) and Ti–6Al–4V machining chips with a high oxygen content (0.8wt.%) at a mass ratio of 2:1. The microstructures and mechanical properties of the alloy in as-extruded state and after different heat treatments were investigated. The as-extruded alloy exhibits a martensite microstructure consisting of ultrafine and nanometer sized α' laths and aciculae and domains of layered structure containing δ(TiH) layers. The alloy in this state is brittle and fractures prematurely before yielding due to the ultrafine martensitic microstructure and hydrogen embrittlement associated with the high hydrogen content of the alloy. After vacuum annealing at 700°C for 6 h followed by furnace cooling, the alloy is further dehydrogenated with the hydrogen content being reduced to 0.008wt.%, the microstructure of the alloy changes into a full α/β lamellar microstructure with the continuous and discontinuous β layers being only 80–90 nm thick and the lamellar colony sizes being 10–30μm. The fine structured alloy in this heat treatment condition shows an excellent combination of high tensile yield strength of 996 MPa and ductility of 13.9%. After a further heat treatment of 980°C for 1 h followed by air cooling, the microstructure of the alloy consists of equiaxed α grains and α/βt lamellar structured domains (βt = β transformed structure), and unfortunately, this microstructure leads to a significant decrease of both the tensile yield strength (to 876 MPa) and the tensile ductility (to 7.4%). The fracture behavior of the alloy in different states is also studied, and the correlation between the microstructure and mechanical properties is discussed. [ABSTRACT FROM AUTHOR]

Powder metallurgy (PM) Ti–6Al–2Sn–4Zr–2Mo–0.1Si–(0, 2, 4)W (wt%) alloys were fabricated by thermomechanical consolidation of TiH 2 -based powder compacts and subsequent heat treatments. Samples of the as-fabricated alloys were also heated at 650 °C for 200 h. The addition of 2 or 4 wt%W to the base alloy changed its microstructure of parallel α/β lamellar colonies and grain boundary α (α GB) to an interwoven α/β t microstructure consisting of α GB and a network of interpenetrating α plates with β transformed structure (β t) domains comprising variants of fine α laths and β matrix. The partition of the β stabilizing W between β and α phases and the low diffusivity of W atoms limited the growth of α plates/laths, decreasing the thickness of α plates/laths and increasing the volume fraction of β. The increased hardening of the β phase and enhanced α/β interface strengthening associated with the 4 wt%W addition led to a significant increase in the tensile strength of the alloy from 1281 ± 10–1411 ± 12 MPa. However, the high flow stress and the very fine microstructure caused significant strain localization in the weak α GB , resulting in premature fracture of the α GB (intergranular fracture) and the low ductility (1.4%). Here, premature fracture meant the fracture occurred prior to the alloy reaching its ultimate tensile strength. The heating caused the β interlaths in the W-free alloy to partially dissolve and become β particles distributed along the original lines of β interlaths accompanied by the precipitation of α 2 -Ti 3 Al in the α plates. The addition of W inhibited the dissolution of β interlaths and caused the precipitation of a higher volume fraction of α 2 precipitates during heating. The microstructural changes caused by heating resulted in a slight decrease in strength and a significant decrease in ductility for the W-free alloy, but a significant increase of the yield strength of the 2 W and 4 W alloys with some sacrifice of the tensile ductility. The microstructural reasons for the effects on mechanical properties were analyzed with the assistance of detailed characterization of the dislocations in the deformed specimens. • Effects of W alloying on a PM Ti–6Al–2Sn–4Zr–2Mo–0.1Si alloy are investigated. • W refines α plates and enhances formation of β transformed structure domains. • W enhances hardening of the β phase and α/β interface strengthening. • W increases the microstructural stability and prevents the strength reduction from heating. • High W content promotes α 2 precipitation strengthening of the alloy after heating. [ABSTRACT FROM AUTHOR]

Powder metallurgy (PM) near α Ti−6Al−2Sn−4Zr−2Mo−0.1Si−0.5Y (wt.%) alloy with a low oxygen content and fabricated by thermomechanical powder consolidation of an elemental powder blend with a trace amount of Y addition was subjected to two heat treatments to obtain different microstructures which were free of grain boundary α (α GB) layers. One heat treatment (960°C/1h/AC, AC = air cooling) led to the formation of an α/β t microstructure consisting of 75.3 vol% primary α (α p) domains with globular and platelike morphologies, 21.5 vol% β transformed structure (β t) domains comprising ultrafine secondary α (α s) lamellae in a β matrix and 3.2 vol% thin β strips. The other heat treatment (960°C/1h/WQ+580°C/6h/AC, WQ = water quenching) led to the formation of a microstructure consisting of 38.2 vol% isolated α p plates, slabs, and globules dispersed in a β t matrix comprising nanometer-sized α s lamellae and residual β strips. The first microstructure rendered the alloy with high tensile strength of 1128 MPa and excellent tensile ductility of 15.6%, thanks to the activation of abundant and slips in all α grains and tensile twinning in the "hard" <0001> micro-textured α grains with the c-axis parallel to the tensile force direction which mitigated the strain localization and enhanced strain hardening. On the other hand, the harder nanostructure of the β t matrix in the second microstructure resulted in a significantly increased tensile strength of 1351 MPa while maintaining an excellent tensile ductility of 10.5%. This can be attributed to the activation of abundant and slips in thin α plates and a limited degree of tensile twinning in the "hard" α domains, as well as a large number of statistically stored dislocations (SSDs) within the micron-scaled α slabs/globules. Both microstructures rendered the alloy with favorable ductile fracture. [ABSTRACT FROM AUTHOR]

A method combining internal oxidation and thermomechanical consolidation of a composite powder with a trace amount of Y addition was used to fabricate powder metallurgy (PM) near-α Ti–6Al–2Sn–4Zr–2Mo−0.1Si−0.5Y (wt%) alloy, in which submicron Y 2 O 3 particles were in-situ formed and distributed uniformly in the matrix to achieve a low oxygen content and an excellent combination of high tensile strength of 1242 MPa and excellent tensile ductility of 16.2%. The in-situ reaction of uniformly dispersed Y particles in the composite powders prepared by high-energy ball milling with the oxygen in the matrix to form submicron Y 2 O 3 particles during powder consolidation can significantly scavenge oxygen in the matrix and eliminate the coarse grain boundary α layers. The Y 2 O 3 particles also work as nucleants for heterogeneous nucleation of α grains in the prior β grains during cooling, thus promoting the formation of α plates with a larger number of orientation variations and a decrease of their aspect ratios in the interwoven α/β t microstructure consisting of fine α plates and β transformed structure (β t) domains, which is beneficial to suppress the initiation and rapid propagation of cracks during tensile deformation. It is demonstrated that with the high flow stress rendered by the ultrafine interwoven α/β t microstructure and oxygen scavenging effect of Y on the matrix, twinning across α/β t interfaces and decohesion between the submicron Y 2 O 3 particles and matrix occur during tensile deformation. This further suppresses formation and propagation of microcracks often seen in the Y 2 O 3 particle free PM near-α titanium alloy of the same composition in the same situation. It is believed that the twinning and decohesion of the interfaces between Y 2 O 3 particles and matrix are responsible for the mitigation of strain localization needed for formation of microcracks, which is essential to maintain a good tensile ductility and allow the fracture to be totally ductilized. • A submicron Y 2 O 3 particle dispersed PM near α alloy with an interweaven α/β t microstructure. is fabricated. • The in-situ Y 2 O 3 particles cause a decrease of matrix O content and microstructural changes. • The microstructural changes cause clear decrease of strength and significant increase of tensile ductility. • The Y 2 O 3 /matrix decohesion and twinning lead to ductilization of the fracture. [ABSTRACT FROM AUTHOR]

An approach combing ball milling (BM) and thermomechanical consolidation of a TiH 2 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/β t lamellar microstructure in combination with short grain boundary α (GB α) layers and small GB Widmanstätten α (α WGB) colonies, while the coarse-grained alloy was composed of a similar α/β t lamellar structure in combination with long and continuous GB α plates and large-sized α WGB colonies. β t 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 β t domains. The excellent ductility was attributed the refined α WGB 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 and 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. [ABSTRACT FROM AUTHOR]