Effects of W alloying and heating on microstructure and mechanical properties of a PM Ti-6Al-2Sn-4Zr-2Mo-0.1Si alloy for high temperature applications

Powder metallurgy (PM) Ti-6Al-2Sn-4Zr-2Mo-0.1Si-(0, 2, 4)W (wt%) alloys were fabricated by thermo-mechanical consolidation of TiH2-based powder compacts and subsequent heat treatments. Samples of the as-fabricated alloys were also heated at 650 degrees C for 200 h. The addition of 2 or 4 wt%W to the base alloy changed its microstructure of parallel alpha/beta lamellar colonies and grain boundary alpha (alpha GB) to an interwoven alpha/beta t microstructure consisting of alpha GB and a network of interpenetrating alpha plates with beta transformed structure (beta t) domains comprising variants of fine alpha laths and beta matrix. The partition of the beta stabilizing W between beta and alpha phases and the low diffusivity of W atoms limited the growth of alpha plates/laths, decreasing the thickness of alpha plates/laths and increasing the volume fraction of beta. The increased hardening of the beta phase and enhanced alpha/beta 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 alpha GB, resulting in premature fracture of the alpha 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 beta interlaths in the W-free alloy to partially dissolve and become beta particles distributed along the original lines of beta interlaths accompanied by the precipitation of alpha 2-Ti3Al in the alpha plates. The addition of W inhibited the dissolution of beta interlaths and caused the precipitation of a higher volume fraction of alpha 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.(c) 2023 Elsevier B.V. All rights reserved.