Microstructure and Mechanical Properties of a Near-α-Titanium-Alloy/TiB Composite Prepared in situ by Casting and Subjected to Deformation and Heat Treatment

Abstract

This paper presents the results of our study of the microstructure and mechanical properties of a short-fiber composite material based on Ti/TiB prepared in situ by casting. We used a two-phase titanium alloy VT18U (Ti–6.8Al–4Zr–2.5Sn–1Nb–0.7Mo–0.15Si) as the matrix material for this study. The addition of boron and pure titanium into the titanium alloy led to the formation of 6.5 vol % TiB fibers. Two deformation treatments were used in this research. The first was isothermal forging in two directions (2D) at temperatures of the upper part of the α + β phase field to provide an elongation of TiB fibers along one direction; the second treatment was 3D forging at temperatures of the α + β phase field to ensure the refinement and random orientation of borides for fabricating material with isotropic properties as far as possible. The deformed semifinished samples of the composite materials and of the matrix alloy were annealed. The composite materials demonstrated noticeably higher strength and creep resistance compared to the matrix alloy and retained an acceptable plasticity. The microstructural studies of the fractured samples showed a high adhesion strength of boundaries between the matrix and the TiB fibers, which is retained even with increasing test temperature irrespective of the orientation and morphology of the borides. The failure of the composites begins with the breaking of borides and is followed by the ductile fracture of the matrix material.