Temperature Effects on the Deformation Mechanisms in a Ni-Co-Based Superalloys

Abstract

The tensile properties of a Ni-Co-based superalloy were investigated from room temperature to 900 °C. From 25 to 650 °C, the yield strength and tensile strength of the alloy decreased slightly, while the elongation decreased sharply. From 760 °C to 900 °C, the yield strength and tensile strength were greatly reduced, while the elongation also had a low value. With the increase in temperature, the deformation mechanism transformed from anti-phase boundary shearing to stacking fault shearing, and then from deformation twinning to Orowan bypassing, respectively. Deformation twins were generated in the deformed alloy with high-density stacking faults and they can contribute to the high strength. The alloy in this study has good mechanical properties and hot working characteristics below 760 °C and can be used as a turbine disk, turbine blade, combustion chamber, and other aircraft structural parts.

Comments on the Intermediate-Temperature Embrittlement of Metals and Alloys: The Conditions for Transgranular and Intergranular Failure

“…Hu et al [60] showed that Ni-Co-based superalloys have very poor ductility, <5% elongation, at temperatures in this temperature range, as shown in Figure 12. The fracture surface gradually changed from ductile to intergranular brittle as the tensile testing temperature increased from room to 750 • C when decohesion took place at the interface between the γ' precipitates and the γ matrix on the fracture surface.…”

“…However, Ni-based superalloys are prone to high-temperature embrittlement in the intermediate-temperature range 600 to 900 • C unless precautions are taken [58,59]. Ni-based superalloys are used at temperatures of 650-700 • C and sometimes reach as high as 800 • C [60].…”

“…The fracture surface gradually changed from ductile to intergranular bri le as the tensile testing temperature increased from room to 750 °C when decohesion took place at the interface between the γ' precipitates and the γ matrix on the fracture surface. In this case, the poor ductility was ascribed to the prolific precipitation of γ' in it various forms and the low stacking fault energy, which produced a vast number of deformation twins when a change in the mode of deformation took place at high temperatures [60]. Trace elements were found to influence the creep cavitation process and AES was used to provide direct evidence of impurity segregation to the cavity surfaces at the boundaries.…”

“…Creep normally manifests itself when the temperature (T) > 0.3-0.4 (T m ), where T m is the melting point in • C [91]. Hence for steel, copper, aluminium, and nickel, the ratios are 0.6, with the centre of the hot-ductility troughs being at ~900, 700, 250, and 400 • C, respectively, as noted in Table 2, with the data being taken from the literature [11,27,34,35,51,56,60]. Although Al alloys can have a hot-ductility trough in the intermediate-temperature region, it is most likely that when failure occurs on tensile testing these alloys in the temperature range of the trough, it is due to one of the following: the weakening of the cohesive strength of the boundaries, the formation of liquid films, or oxygen infiltration to the grain surfaces, rather than creep cavitation.…”

Influence of Various Heat Treatments on Microstructures and Mechanical Properties of GH4099 Superalloy Produced by Laser Powder Bed Fusion

“…The change in the deformation mechanism is another cause behind the ductility drop at 900 °C. According to several pieces of research [ 61 , 62 , 63 ], once the deformation mechanism changes, the ductility might decrease simultaneously. The size of the γ′ phase and deformation temperature usually affect the deformation mechanism.…”