Microstructure and mechanical properties of α+β titanium alloy based composites fabricated in situ by casting and subjected to hot forging

摘要

The work was devoted to study of microstructure and mechanical properties of discontinuously reinforced composite materials based on Ti/TiB and Ti/(TiB+TiC) fabricated in situ by casting. A two-phase α+β titanium alloy VT25U (Ti-6.8Al-2.1Sn-2Zr-3.5Mo-0.8W-0.2Si) was used as a matrix material. The boron and carbon additions corresponding to 8 vol.% of TiB and 2 vol.% of TiC were used as additives to the titanium alloy. 2D forging in the α+β temperature field was used to obtain aligned TiB whiskers with a higher aspect ratio in VT25U/TiB. Two-stage 3D forging was applied to refine the reinforcements in VT25U/TiB and VT25U/(TiB+TiC). To obtain the most creep resistant and the same matrix conditions, the forged composites were subjected to the same heat treatment including anneal in the β temperature field. The produced composites demonstrated appreciably higher yield strength and creep resistance in comparison with those of the matrix alloy. The load-bearing capacity of the reinforcements mainly contributed to the enhancement in strength and creep resistance. The carbon addition led to coarsening of borides and reducing the β phase content. Therefore, the carbon addition did not give improvements in strength and creep resistance as compared with VT25U/TiB, whereas the RT ductility of VT25U/(TiB+TiC) was found rather low. Refined and randomly oriented TiB whiskers provided the mechanical properties comparable with those obtained in the case of aligned TiB whiskers. Microstructural examination confirmed high adhesion strength of interfacial boundaries between the matrix and the reinforcements, which was retained up to T=700°C.