Formation and kinds of boride on microstructure evolution and mechanical properties in Ti47.5Al2.0Cr2.0NbxB alloys

To adjust the kinds of borides and improve the mechanical properties, different concentrations of boron were added to the Ti47.5Al2.0Cr2.0Nb alloy by arc melting, and the phase composition, lamellar colony, elemental distribution and compressive properties were investigated. Results show that when the boron concentration increases to 2.0 at.%, dendritic crystals are transformed into equiaxed crystals. The size of the lamellar colony decreases from 232.9 to 92.3 mu m with the boron content increases from 0 to 3.0 at.%. After adding boron, borides with two distinct morphologies appear (curved and bar borides), and then the B2 phase disappears. With the increase of boron content, the percentages of curved boride and bar boride are changed, the former decreases and the latter increases. During the solidification process, the constitutional undercooling, at the solidification front and the nucleation particles of borides increase with increasing boron content. The refined microstructure and formation of boride improve the uniformity of the elemental distribution. Compressive results show that the maximum strength is 2228.2 MPa with 1.5 at.% B, and the maximum strain is 29.1% with 2.0 at.% B. The grain refinement strengthening, solid-solution strengthening of Nb and Cr, and the precipitation strengthening of borides are the main reasons for the improved compressive properties. The fracture morphology is a mixed fracture mode which contains both transbar fracture and inter-bar fracture.