Characterisation of plastic and creep strains from lattice orientation measurements

Abstract

Electron backscatter diffraction (EBSD) is a powerful technique for measuring crystallographic orientation in polycrystalline materials. This thesis explores the potential of EBSD for characterising localised inelastic strain from lattice orientation measurements. A systematic study under uniaxial isothermal loading conditions was performed to examine the influence of microstructure and deformation conditions on strain-induced lattice orientation changes (misorientation). The study was conducted on both service-aged and un-aged Type 316H stainless steels through a series of monotonic tests in tension, compression and in constant load creep. The study demonstrates that the development of misorientation depends on many factors which need consideration before EBSD can be applied for strain assessment. It is shown that the measured evolution of misorientations is a function of microstructure and grain size. A misorientation-based strain assessment method is proposed which is relatively insensitive to microstructure and grain size. In service-aged steel, the measured evolution of misorientations is shown to be independent of the deformation temperature (between 24°C and 550°C) and deformation mode (tension vs. compression) for strain rates down to about 10-6s-1. Empirical correlations between the accumulated plastic strain and different misorientation metrics are developed for true strains up to 0.23. However, at 550°C the evolution of measured misorientations is shown to be strain rate dependent below 10-6s-1. The potential of EBSD to distinguish plastic strain from creep strain is demonstrated. Misorientation development is shown to occur at a faster rate with increasing strain in plastic than in creep deformation. Similarly, the proportion of twin boundaries in service-aged steel is shown to reduce with increasing strain at a faster rate in creep than in plastic deformation. Two novel methods for creep strain estimation are proposed which utilise the disparities in the misorientation development and twin boundary reduction under the two different deformation regimes. A good correspondence is established between the strain estimates from the proposed methods and those derived from hardness measurements and digital image correlation. The methods are shown to be applicable to real power plant components through successful mapping of plastic and creep strain distributions in weldments after different periods in service.