Understanding plasticity and deformation mechanisms in nickel base superalloys through in-situ loading experiments using high resolution strain pattern analysis

Abstract

Nickel-base superalloys are widely used in high-temperature and demanding environments as structural materials for their high strength at high temperatures and good creep/oxidation resistances. The way that some of these alloys are strengthened means that they can also be used as model systems to study the plasticity, deformation mechanism, and strain hardening behaviours, the understand of which is essential for microstructural optimisation and future alloys design The present work combines the development and utilisation of the in-situ High-Resolution Digital Image Correlation (HRDIC) technique, combined other characterisation techniques such as Electron Backscattered Diffraction (EBSD), Electron Channelling Contrast Imaging (ECCI) and Relative Displacement Ratio (RDR), to investigate the development of nano-scale strain patterns at a sub-grain scale during deformation, especially at the onset of plastic deformation. The evolution of the strain pattern and slip system activity were analysed during the uniaxial tensile testing, from below the yield up to a strain of several precent. The data was then used to study the evolution of the plasticity, deformation mechanisms and strain hardening behaviour of the material. Crystal Plasticity (CP) modelling using DAMASK was performed to simulate the microstructural response of the region of interest (ROI) captured by HRDIC. Simulated results were directly compared with the experimental to study the influence of inter/intra-granular stress on strain heterogeneity and slip activity. To understand the strain patterning behaviour, in-situ testing provided a clear advantage over ex-situ testing as it enables the strain maps to be calculated at multiple strain steps. From this it was possible to show that the development of the strain patterns within individual grain was less complex than the strain maps relative to the original state suggested. This work has also demonstrated that the early stage of apparent strain hardening rate during tensile testing is not only affected by the rapid dislocation generation but also significantly by the elasto-plasticity transition, kinematic hardening and inter/intra-granular stresses. A further significant observation was that slip bands often contain more than a single Burgers vector, particularly during the onset of plasticity. The consequences of such complex shear within slip traces have not been explored yet but it is likely to be relevant to slip transfer and also the degree of strain hardening. The CP modelling failed to predict the deformation behaviour at sub-grains levels which should be attribute to the lack of information on grain neighbourhood. It was also found that the CP modelling cannot fully represent the inter/intra granular stress condition which leads to a poor prediction on slip compatibility.

Date of Award	21 Sept 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Michael Preuss (Main Supervisor) & Joao Quinta Da Fonseca (Co Supervisor)