Microstructure characterization and thermal stability of TC11/Ti2AlNb joints during thermal exposure

Abstract

TC11/Ti2AlNb welding joints undergoing hot deformation were thermally exposed at the range of 500–700 °C for 100, 200 and 500 h. In present research, typical microstructures of five different zones of the joints were investigated, fusion zone, two interfaces and two base alloys included. The microstructures of fusion zone, Ti2AlNb-based alloy and the interface between them were characterized by the coarsening of O phase during thermal exposure, while those of TC11 alloy and the interface between it and fusion zone featured the coarsening of αs phase. The transformation from B2 to β and O phase was stifled below 600 °C and Ti2AlNb-based alloy thus was verified to be stable at this temperature range. αs phase was stable during exposure at 500 °C. When exposed above 600 °C, αs phase coarsened by at least 52% with the extension of holding time. Due to the low thermal stability of αs phase compared with that of O phase, the fracture position of the samples tested at room temperature moved from fusion zone to TC11 alloy when temperature rose. In spite of different thermal exposure conditions, the samples tested at 300 and 500 °C all fractured at TC11 alloy for the severe coarsening of αs phase. The decrease of interfacial area resulting from the coarsening of O and αs phases facilitated the dislocation slipping and the strength of joints thus deteriorated. The mechanical properties of TC11/Ti2AlNb joints measured at room temperature, 300 and 500 °C were all correlated with the lamellar thickness of O and αs phases.