Deformation mechanisms in a polycrystalline Ni-based superalloy

In order to reduce emissions from aviation, the most straightforward approach is to increase jet engine service temperatures. However, this raised temperature window extends into a new deformation mechanism regime that needs to be explored and understood to be able to design and optimise new and existing Ni-based disc superalloys. For this purpose, this work investigates the deformation behaviour of the established coarse-grained RR1000 alloy between around 650°C and 750°C within the stacking fault (SF) shear regime in order to identify the active deformation mechanisms and to establish their influence on mechanical properties. Two microstructures were used to assess the influence of the γ′ precipitate size on SF shear: one representing a typical disc γ′ size distribution, another with finer secondary and tertiary γ′ precipitates generated through an additional heat treatment. Mechanical testing included monotonic tensile tests, high stress creep and low-cycle fatigue tests. Test pieces were investigated primarily via (scanning) transmission electron microscopy (S/TEM) to identify dislocation and fault structures. The tensile test matrix included three different strain rates tested at 650°C, 700°C and 750°C, with specimens run to failure and various low strains. It revealed an onset of the SF shear regime above 650°C with the SF density increasing at higher temperatures and slower strain rates. A multitude of SF mechanisms were often found to be active simultaneously, particularly at very low strain levels. However, a key deformation mechanism throughout was coupled dislocations forming planar slip bands. Of the SF types, dislocation ribbons and microtwinning contribute the most to the strain. It is the first time that ribbons have been found under tensile test conditions in a polycrystalline alloy. Microtwinning became active at higher strains showing that the dominant deformation mechanism can change at different stages of the deformation process, e.g. from simultaneous SF and antiphase boundary (APB)-coupled shear to predominantly APB-coupled shear to microtwinning. In the finer γ′ microstructure, yield strength increased at higher temperatures. As this microstructure was more prone to SF shear, the elevated strength was attributed to an increased frequency of γ′ cutting, enabled by thermally activated processes such as SF formation. Little evidence of Kear-Wilsdorf locking was found and was only seen within the typical γ′ microstructure. Microtwinning appeared to be linked to softening and increased the ductility in the fine microstructure, which otherwise suffered from very localised deformation; whereas microtwinning led to a ductility loss in the more homogeneously deforming typical microstructure. To compare deformation modes, some high stress creep at 700°C and low-cycle fatigue tests, at room temperature and 700°C were carried out additionally. The deformation mechanisms active under high stress creep conditions were very similar to those observed after slow strain rate tensile testing. In both test modes, a number of dislocation sources were identified including specific grain boundaries, triple points and, in particular, intragranular carbides – releasing both full and partial dislocations. The fatigue tests of the typical microstructure showed deformation is dominated by very planar slip bands formed of coupled dislocations, which were more localised at room temperature; double cross-slip at the elevated temperature leads to more homogeneous deformation and broader slip bands. The active deformation mechanisms were very similar to those in fast strain rate tensile testing, which emphasises the suitability of variable strain rate testing in assessing deformation mechanisms active under low-cycle fatigue and high stress creep conditions if strain rates are comparable and deformation levels low. Lastly, heat treated samples were further probed via high-resolution TEM (HR-TEM), energy-dispersive X-ray (STEM-EDX) and electron energy loss (STEM-EELS) spectroscopy to study SFs generated during the alloy’s heat treatment, which have not been described as such before. These extended SFs were found in significant densities on up to four slip systems in the majority of test pieces. They were found to form during ageing and, therefore, are termed heat-treatment SFs. As they can be mistaken for deformation features and occasionally act as semi-permeable membranes to dislocation motion, they demand specific attention. Co segregations were found at these SFs. With regards to structure, they appear to be mostly intrinsic in nature, but results were not particularly consistent. It is suggested that residual stresses (or other local stress variations) in combination with local phase transformations might play a role in the formation of these faults, but further investigations are required. This study presents a complex overview and map of deformation mechanisms active within the SF shear regime for two different microstructures of the Ni-base disc alloy RR1000. These results could in the future be incorporated in yield strength, creep or fatigue models and the insights about variable tensile testing might reduce future mechanical testing efforts.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2023

Author dc:creator

• Schluetter, Regina

Advisor dc:contributor.advisor

• Rae, Catherine

• Deformation
• Deformation mechanisms
• Materials Science
• Mechanical properties
• Ni-based superalloy
• Physical metallurgy
• Stacking faults

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/ab86a5d4-cb9a-43ba-93a5-4a7bfaa512f2/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.112298

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/374112