Tensile properties of Ni-based superalloy 720Li: temperature and strain rate effects

Abstract

Tensile properties, deformation, and fracture behavior of a wrought nickel-base superalloy 720Li have been studied in standard solutionized and two-stage-aged condition in the temperature range of 25° C to 750 °C. Effect of strain rate on tensile behavior was assessed at 25 °C, 400 °C, and 750°C at five strain rates that range between 10⁻⁵ s⁻¹ and 10⁻¹ s⁻¹. The yield strength and ultimate tensile strength of the alloy remained unaffected by temperature until about 600 °C and 500 °C, respectively, typical of superalloys strengthened by fine and coherent intermetallic Ni₃Al-based precipitates. The flow stress of the alloy was found to be insensitive to the strain rates studied at 25 °C and 400 °C. However, at 750 °C, the flow stresses showed strain rate sensitivity at strain rates <10⁻³ s⁻¹. The strain hardening behavior at 25 °C and 400 °C were similar. At 750 °C, stain hardening was observed only at strain rates >10⁻³ s⁻¹, and at lower strain rates, tensile instability was seen to set in immediately after yielding. The alloy exhibited ductile dimple fracture at all the temperatures and strain rates studied. Microstructural investigations indicate that in regimes where flow stresses are insensitive to strain rate, deformation occurs through heterogeneous planar slip, whereas in strain rate sensitive regimes, thermally activated diffusion processes promote homogeneous deformation.