Microstructure and mechanical properties of near α titanium alloy based composites prepared in situ by casting and subjected to multiple hot forging

Abstract

The work was devoted to study of microstructure and mechanical properties of discontinuously reinforced composite materials based on near-α Ti/TiB and near-α Ti/(TiB+TiC) fabricated in situ by casting. A near-α titanium alloy VT18U (Ti-6.8Al-4Zr-2.5Sn-1Nb-0.7Mo-0.15Si) was used as a matrix material. The boron addition corresponding to 6.5 vol.% of TiB and the carbon addition corresponding to 1.9 vol.% of TiC were used as additives to the titanium alloy. The as-cast materials were subjected to two-stage 3D forging in the α+β temperature field at T = 950 and 800 °C. This resulted in refinement of randomly oriented TiB whiskers while retaining near the same size of TiC particles. The forged workpieces were subjected to heat treatment, which included high temperature anneal in the β or upper part of the α+β phase field and provided similar matrix microstructures in the composites and the base alloy. The forged and heat treated composites demonstrated appreciably higher strength, creep resistance and lower ductility as compared with the base alloy. The load-bearing capacity of the reinforcements mainly contributed to the enhancement in strength and creep resistance. The carbon addition gave appreciable strengthening effect in VT18U/(TiB+TiC) at RT but at elevated temperatures it was not the case and only negligible positive influence on creep resistance was detected. At the same time, the carbon addition led to a strong decrease of the RT ductility. It was revealed that refined and randomly oriented TiB whiskers in VT18U/TiB provided the strengthening contribution and improvement in creep resistance comparable with those obtained in the same composite with aligned TiB whiskers having a higher aspect ratio. Presumably, more uniform distribution and shorter spacings between TiB whiskers in the case of refined borides promoted an increase in strength and creep resistance. Microstructure examination showed high adhesion strength of interfacial boundaries between the matrix and the reinforcements, which was retained up to T = 600–700 °C. The main failure mechanism of the composite materials was fracture of the reinforcements followed by ductile failure of the matrix.