Understanding the confined TiB fiber-like structure for strength-ductility combination of discontinuous-reinforced titanium matrix composites

Abstract

Titanium matrix composites (TMCs) with homogeneous structure (HS), fiber-like structural composite region (FLSCR) and fiber-like structural Ti region (FLSTR) were respectively designed and fabricated by using powder metallurgy, hot extrusion and rolling process. Confined TiB whiskers within the newly fiber-like structure play an important role in the microstructure, mechanical behavior and special fractography. The fracture elongation of FLSCR-10% composites is up to 19.1% which is much higher than that of HS-4% composites (14.6%) with the total 4 vol % TiB, while the strength decreases from 811 MPa (HS-4%) to 778 MPa (FLSCR-10%). Similarly, FLSCR-15% composites possess 60 vol % ductile Ti region (15.5%, 797.8 MPa) exhibits higher fracture elongation but lower strength than that of FLSTR-10% composites (12.4%, 890.8 MPa) with the same total 6 vol % TiB. It is inferred that the TMCs with FLSCR exhibit relatively lower strength but much higher ductility compared with the TMCs with HS and FLSTR while fixing their same total TiB volume fractions (4 vol % or 6 vol %). The strength is directly related to the TiB content of the composites region resulting from the grain refinement and load-bearing effect of TiB whiskers. Larger Ti region indirectly reduces the volume ratio of the composite region, leading to a weaker strengthening effect. However, a larger Ti region enhances the strain hardening behavior of the TMCs, thus increasing its ductility. In addition, the static toughness of the TMCs with FLSCR can be notably improved by continuous Ti region, which prevents the rapid fracture of the composites by efficiently blunting or reflecting their crack propagation.