Forming mechanism of growth twins and microstructure evolution on improvement of strength and toughness properties by β-eutectoid element in Ti-7Mo-4Al-4Zr-3Nb-2Cr-xFe alloys

Abstract

To form growth twins and simultaneously improve strength and toughness by using a β-eutectoid element, Ti-7Mo-4Al-4Zr-3Nb-2Cr-xFe alloys were prepared. The phase composition, morphology of growth twins, tensile properties and fracture toughness were tested. Also, the formation of growth twins and the strength-toughness mechanism were studied. The results show that α and β phases exist after heat treatment. The relative content of β phase increases from 6.1 % to 12.5 % with increasing Fe content. Dislocation entanglements and dislocation walls appear at the α/β interface. There are two kinds of α phases, which are αp and αs phases. The growth twin with a width of ∼20 nm is found in αs phase due to the mismatch with the matrix lattice during solidification process. The smallest size of αp phase is at 1.0 wt% Fe. As the stable element of the β phase, Fe increases the phase transformation resistance from β phase to α phase to limit the growth of α phase. When Fe content increases from 0 to 1.5 wt%, tensile strength increases from 1107 to 1314 MPa. By increasing Fe content from 0 to 1.0 wt%, fracture toughness and elongation increase from 64 to 80 MPa·m1/2 and 5.4–7.1 %, respectively. The strength and toughness of the alloys increase simultaneously within 0–1.0 wt%Fe. The solution strengthening of Fe, the refinement of αp phase, and the formation of growth twins and dislocation walls are the main reasons for the improvement in strength and toughness.