LAUSR

Abstract In this work, a severe plastic deformation method, namely elliptical cross-section torsion extrusion (E-TE), was applied to achieve superplasticity of an in-situ TiB2/Al-Zn-Mg-Cu composite by obtaining the optimized microstructure of uniformly distributed particles and equiaxed-fine grains. The superplastic behavior and mechanisms of the E-TE processed composite were comprehensively investigated via tensile tests (at T = 350 °C–500 °C and e = 10−1 s−1-10−3 s−1) and microstructure characterizations. The tests demonstrated that the composite after a single pass of the E-TE gained a maximum elongation of 420% at 450 °C × 10−3 s−1. Even with the strain rate increasing to 10−2 s−1, the elongation still reached 360%, which implies a high strain rate superplasticity. Analysis based on the strain rate sensitivity (m ≈ 0.20–0.35) and microstructure characterizations showed that the deformation and failure mechanisms of the composite vary with the temperature. When the temperature is within 350 °C–400 °C, the composite realizes deformation mainly through intragranular dislocation slip and fails by intragranular fracture. With the temperature of 425 °C–450 °C, the deformation is attributed to cooperative grain boundary sliding (CGBS) accommodated by dislocation activity and liquid phase, where a small fraction of liquid phases observed at the triple junctions of GBs facilitates the composite's superplasticity by reducing the stress concentration and deformation incompatibility. The failure is ascribed to the intergranular fracture by cavity formation/coalescence. For temperature within 475 °C–500 °C, numerous liquid phases formed at GBs and significantly weakened the GBs' strength, deteriorating the composite's superplasticity. The composite's failure is resulted from the intergranular fracture by strength weakening of GBs.