Effect of hot forging on microstructure and mechanical properties of near α titanium alloy/TiB composites produced by casting

Abstract

In situ Ti/TiB discontinuously reinforced composites were fabricated using a casting route and near-α titanium alloy VT18U (Ti-6.8Al-4Zr-2.5Sn-1Nb-0.7Mo-0.15Si) as a matrix material. The boron addition in an amount of 1.2 wt.% corresponding to 6.5 vol.% TiB was found to be optimal for this matrix material. To obtain axially aligned TiB whiskers with a high aspect ratio and the most creep resistant matrix microstructure, isothermal 2D hot forging followed by annealing first in the β and then in the α+β temperature range was carried out. The mechanical properties of the novel VT18U/TiB composite material were established in tension parallel to the predominant orientation of the TiB whiskers. Along with tensile tests, creep tests were carried out and the obtained creep curves were compared with those obtained for the matrix alloy subjected to near the same forging and heat treatment. The composite material with aligned TiB whiskers demonstrated appreciably higher strength and creep resistance in comparison with those of the base alloy without significant reduction in ductility. SEM study of the fracture behavior showed that high adhesion strength of interfacial boundaries between the matrix and the TiB whiskers in the VT18U/TiB composites is retained up to T = 600–700 °C. The main failure mechanism of the composites at room and elevated temperatures is fracture of TiB whiskers followed by ductile failure of the matrix.