Effect of Cr content on the microstructure and mechanical properties of Ti–Cr alloys manufactured by laser directed energy deposition

Abstract

Titanium (Ti) and titanium alloys have been increasingly used in fixed and removable prostheses and aerospace bearing materials because of its excellent biocompatibility, corrosion resistance and mechanical properties. Cr element is often added to titanium alloys as a solid solution strengthening element. However, the content of Cr was restricted in the commercial titanium alloy to avoid β-flecks and deterioration of mechanical performance because of the strong segregation of Cr. A series of homogeneous Ti-xCr (x = 6, 8.5, 10, 13.4, 15 wt%) binary alloys near the eutectoid point were manufactured by laser directed energy deposition. The phase composition and morphology, mechanical properties and deformation mechanism were investigated. It was demonstrated that the eutectoid reaction of additive manufacturing Ti–Cr alloy has been suppressed. Hypoeutectoid Ti-xCr alloys (x = 6, 8.5, 10 wt%) obtained microstructures consisting of α and β phase. The Ti-xCr alloy with eutectoid compositions (x = 13.4 wt%) obtained a microstructure mainly composed of ω and β phase. And hypereutectoid Ti-xCr alloy (x = 15 wt%) exhibited a fully β microstructure. For α+β type Ti–Cr alloys, and the average width of α lath decreased from 868 nm to 157 nm with Cr content increased from 6 wt% to 10 wt%. Correspondingly, the α+β type Ti–10Cr alloy presented a good combination of strength (an ultimate tensile strength of 1042 MPa) and ductility (an elongation of 7.1 %). Then, a high density of ω phase was found in the Ti-13.4Cr alloy with eutectoid compositions, which leads to its brittleness. Due to the solid solution of high concentration of Cr, β-type Ti–15Cr alloy obtained a high ultimate tensile strength of 941 MPa. Furthermore, stress-induced ω phase and {112}<111> twinning formed during the deformation, which helps to accommodate stress concentration, thereby the β-type Ti–15Cr alloy can maintain high ductility (20.3 %).