Formation of a bimodal structure in ultrafine Ti-Fe-Nb alloys with high-strength and enhanced ductility

Bulk (Ti70.5Fe29.5)100−xNbx (x=0, 3, 5 and 7 at%) alloys were prepared by cold crucible levitation melting, and their mechanical properties were tested in compression at room temperature. A (Ti70.5Fe29.5)97Nb3 alloy specimen in compression exhibited an ultimate compressive strength of 2.53 GPa and a compressive plastic strain of 15%. Electron microscope observations indicated that lamellar structures present in the eutectic Ti70.5Fe29.5 alloy could be modified by the addition of Nb to obtain a bimodal structure. The improvement of the mechanical properties is attributed to two factors: (1) the bimodal phase size distribution with micrometer-sized primary β-Ti dendrites embedded inside a matrix of refined ultrafine eutectics (β-Ti+TiFe), and (2) the larger lattice mismatches between the β-Ti and TiFe phases in Nb-modified eutectic Ti-Fe alloys that introduce coherency strain at the interface. The orientation relationship of A2 β-Ti with B2 TiFe in binary and Nb-modified Ti-Fe alloys is TiFe (110)[001] || β-Ti (110)[001].