Large enhancement of yield strength in Ti80Cr20−x(CoFeNi)x alloys by modulating phase stability and introducing ultrafine eutectic structure

The microstructural evolution, mechanical properties, and deformation behavior of Ti ₈₀Cr _20−x(CoFeNi) _x (x = 0, 3, 6, 9, 12, 15, 17, 20 at%) alloys have been investigated with an aim to enhance the strength. The addition of equiatomic (CoFeNi) was effective in introducing ω phase in β-Ti phase as well as β + Ti ₂Ni type ultrafine eutectic structure. When the content of equiatomic (CoFeNi) alloying elements increased the ω phase formed in β-Ti phase, which enhanced the yield strength from 920 MPa to 1255 MPa with maintaining large plasticity above 40% in Ti ₈₀Cr ₁₄(CoFeNi) ₆ and Ti ₈₀Cr ₁₁(CoFeNi) ₉ alloys. Upon further increase in alloying elements, the β + Ti ₂Ni type ultrafine eutectic structure was introduced and gradually increased with increasing (CoFeNi) content up to 15 at%, which enhanced yield strength up to 1550 MPa with reasonable plasticity of 27 %. Ti ₈₀(CoFeNi) ₂₀ UFECs show highest yield strength with brittle manner due to the higher volume fraction of ultrafine eutectic matrix and absence of strong interaction between slip bands in β-Ti dendrites and shear bands in ultrafine eutectic matrix. Based on these results, the phase stability, microstructural evolutions and mechanical properties on the Ti ₈₀Cr _20−x(CoFeNi) _x alloys were systematically investigated.