Effect of Silicon Addition on Microstructure and Mechanical Properties of a High Strength Ti-4Al-4Mo-4Sn Alloy Prepared by Powder Metallurgy

Ti-4Al-4Mo-4Sn-xSi (wt%) (x = 0, 0.2, 0.5) alloys were fabricated by hot extrusion of compacts of blends of TiH2, AlMo60 master alloy and other required elemental powders followed by heat treatments involving vacuum annealing, solution treatment, air cooling and aging. The microstructure of the alloys consists of alpha plates, lamellae of alpha and transformed beta structure (beta(t)), beta(t) blocks as well as grain boundary alpha layers. With the addition of 0.2 wt%Si, the Si atoms exist in equal concentration in both alpha and beta(t) regions as solute atoms, and they retard the growth of alpha phase, causing a decrease of the average thickness of alpha plates and lamellae and slight increase of the volume fraction of beta(t) lamellae and blocks. The solid solution strengthening and enhanced alpha/beta interface hardening brought by the 0.2 wt%Si cause the yield strength of the Ti-4Al-4Mo-45n alloy to increase significantly from 1030 to 1130 MPa with a slight decrease of the tensile ductility (7.4% vs 6.8%) and without changing the fracture behavior. With the addition of 0.5 wt%Si, the extra Si causes nanometer sized silicide (Ti5Si3) precipitates to form in both alpha and beta(t) regions and further microstructural refinement. The additional alpha/beta interface hardening and precipitation hardening cause the yield strength of the alloy to increase further to 1177 MPa, but the tensile ductility deteriorates clearly with the elongation to fracture decreasing to 4.9%. The main reason for this ductility decrease is the premature fracture of grain boundary alpha layers caused by the higher flow stress which can induce more strain localization at the grain boundary alpha layers. (C) 2021 Elsevier B.V. All rights reserved.