Bimodal titanium alloys with ultrafine lamellar eutectic structure fabricated by semi-solid sintering

Abstract We report on a novel approach to synthesize (Ti 100- x - y Fe x Co y ) 82 Nb 12.2 Al 5.8 (at.%) bimodal alloys and provide fundamental insight into their underlying microstructural evolution and mechanical behavior. In our work, a bimodal microstructure is attained via selection of phases and composition in a eutectic reaction followed by semi-solid sintering. Specifically, if one selects an atomic ratio of Ti/Fe corresponding to the eutectic composition, the resultant (Ti 63.5 Fe 26.5 Co 10 ) 82 Nb 12.2 Al 5.8 alloy shows a bimodal microstructure of micron-sized fcc Ti 2 (Co, Fe) embedded in an ultrafine lamellar eutectic matrix containing ultrafine bcc β-Ti and bcc B2 superstructured Ti(Fe, Co) lamellae. This structure forms from the complete eutectic reaction between β-Ti and Ti(Fe, Co). The phase boundary of β-Ti and Ti(Fe, Co) lamellae consists of a coherent interface with the orientational relationships: (110) β-Ti //(110) Ti(Fe, Co) , (200) β-Ti //(100) Ti(Fe, Co) and ( 1 1 ¯ 0 ) β - Ti // ( 1 1 ¯ 0 ) Ti ( Fe, Co ) . Such bimodal alloy exhibits ultra-high compressive yield strength of 2050 MPa with a compressive plasticity of 19.7%, which exceed published values of equivalent materials. These unusual mechanical properties are attributed to a mechanism that involves blocking, branching and multiplication of β-Ti lamellae, dislocation interactions in Ti(Fe, Co) lamellae, and the stability of coherent interfaces. In addition, unusual phenomenon of introduced high-density dislocations in B2 superstructured Ti(Fe, Co) lamellae, other than β-Ti lamellae, can be rationalized based on the formation and decomposition of superlattice dislocations according to classic crystallographic strengthening theory.