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 & omega; phase in & beta;-Ti phase as well as & beta; + Ti2Ni type ultrafine eutectic structure. When the content of equiatomic (CoFeNi) alloying elements increased the & omega; phase formed in & beta;-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 & beta; + 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 & beta;-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.