The development of single crystal Ni-base superalloys for gas turbine applications has focused on creep behaviour in the high-temperature regime (above 700 °C). However, the study of deformation in the range of 20 °C to 700 °C is critical to understanding behaviour in the shanks of turbine blades in service, where the low-temperature properties of single crystal Ni-base superalloys have implications for fatigue and thermomechanical fatigue lifetimes. Changes in composition across superalloy generations have prioritised high-temperature properties. This led to a reduction in yield strength at low temperatures, particularly in the 4th-generation alloy TMS-138A.

In the present work, the commercial 2nd-generation CMSX-4, 3rd-generation RR3010 and 4th-generation TMS-138A alloys have been systematically studied across a range of temperatures below 650 °C using monotonic tensile testing. The difference in behaviour between alloy generations as a function of temperature has been studied, along with the effect of tensile axis orientations around [001] and the effect of strain rate. In situ characterisation techniques such as digital image correlation and synchrotron X-ray diffraction were used to study the response of the alloys during deformation. In addition, post mortem transmission electron microscopy was performed on the deformed materials to characterise the resulting defect structures.

A number of novel defect structures were observed, including planar faults, dislocation loops and dipoles. These defects were common to all alloys studied, but changed in an alloy-specific manner as a function of temperature. Large numbers of fine-scale dislocation loops were observed from 350 °C to 650 °C, with greater numbers in CMSX-4 and RR3010 than TMS-138A. These fine-scale loops were attributed to the collapse of edge and mixed superdislocation dipoles, and were hypothesised to form in a multi-step mechanism through vacancy-mediated short-range diffusion. g.b contrast analysis and comparison to simulated STEM images determined these loops had a Burgers vector of a2⟨101⟩. Screw dislocation dipoles were also observed, bounded by long sections of superdislocation in sessile Kear-Wilsdorf locks. These screw dipoles were determined to have a greater impact on the deformation behaviour as a function of temperature than the debris from edge/mixed dipoles. Shortrange ordered domains were observed within both the γ and γ′ phases in conditions consistent with the presence of inhomogeneous flow. This did not correspond to a measurable strain rate sensitivity at 450 °C within CMSX-4, unlike that observed at higher temperatures in prior literature.

Yield stresses decreased between alloy generations in line with reduced Ta and Ti content in the γ′ phase. Yield behaviour was also found to be highly orientation sensitive for small deviations in tensile axis. This work has highlighted the importance of studying the deformation behaviour within the low temperature regime and identified novel mechanisms for the creation of the observed debris.