Composition dependence of the microstructure and the mechanical properties of nano/ultrafine-structured Ti–Cu–Ni–Sn–Nb alloys Original Research Article

The composition dependence of the microstructure and the mechanical properties of nano/ultrafine-structured Ti–Cu–Ni–Sn–Nb alloys were investigated. At 16.8 at.% Cu + 14.4 at.% Ni and 8 at.% Nb + 4.8 at.% Sn, the alloy contains about 20 vol% dendritic bcc-β-Ti solid solution and a nanostructured matrix with an average grain size of about 50 nm. The nanostructured matrix consists of a hcp-α-Ti solid solution and Ti2Cu and Ti2Ni intermetallics. The alloy exhibits 1313 MPa yield strength and 8.9% plastic strain. The intermetallics in the matrix lead to intrinsic brittleness and result in a brittle fracture surface. With decreasing Cu + Ni content and increasing Nb + Sn content, the volume fraction of the dendrites significantly increases and the grain size of the matrix becomes coarser. As a result, the yield strength decreases and the plastic strain increases. For 14 at.% Cu + 12 at.% Ni and 10 at.% Nb + 4 at.% Sn, the matrix forms an eutectic structure with very few intermetallics; the alloy exhibits 1052 MPa yield strength and 21.1% plastic strain. The alloys with few or without intermetallics exhibit a typical shear fracture mode with obvious viscous flow on the fracture surface.