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 Ti80Cr20−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 + Ti2Ni 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 Ti80Cr14(CoFeNi)6 and Ti80Cr11(CoFeNi)9 alloys. Upon further increase in alloying elements, the β + Ti2Ni 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 %. Ti80(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 Ti80Cr20−x(CoFeNi)x alloys were systematically investigated. [Display omitted] •The addition of (CoFeNi) on Ti80Cr20−x(CoFeNi)x alloys destabilized β-Ti phase with precipitation of ω phase.•A large amount of (CoFeNi) induced ultrafine eutectic structure (UFES) composed of β-Ti and Ti2Ni-type Laves phases.•The nanoscale ω phase and UFES enhanced yield strength of the Ti80Cr20−x(CoFeNi)x alloys from 920 to 1922 MPa.•Optimum mechanical properties was achieved by controlling volume fraction of UFES the 4.6 ∼ 12.6 %.•The small amount of UFES (∼ 12.6%) and β-Ti phase were effective to induce large plasticity (∼ 27.5 %).