Refinement of αs phase and formation of nano-twins of Ti–7Mo–4Al–3Nb–2Cr alloyed by Zr element

Abstract

To refine the αs phase and form the nano twins, different contents of Zr were added to as-cast Ti–7Mo–4Al–3Nb–2Cr-xZr alloys. Phase and microstructure evolution, mechanical properties and related mechanisms were systematically studied. Results show that as the Zr increases from 0 to 8 wt%, only α and β phases exist and the peak of β (110) shifts from 39.5 to 39.1° by XRD. The volume fraction of the αs phase changes from 75.4 to 73.9%, and its length decreases from 0.62 to 0.22 μm. More dislocations appear in the α/β interface and the growth twin of {10 1¯ 1}α < 11 2¯ 0> type with a width of ∼20 nm forms in α phase as observed by TEM. The refinement of the αs phase is due to the addition of Zr to promote nucleation and inhibit the growth rate. With increasing Zr content from 0 to 8 wt%, the strength increases from 961 to 1303 MPa, while the toughness decreases from 77 to 62 MPa m1/2. The fracture toughness decreases by 19.5%, while the strength increases by 35.6%. The refinement of αs phase, the solubilization of the Zr in the matrix, the increase of dislocation in the α/β interface, and the formation of nano twins are the main reasons for improving the strength and toughness of the studied alloys.