High-resolution Electron Backscatter Diffraction Analysis Research Articles

Microstructure characterization of gradient structured surface layer in pure zirconium

A gradient nanostructure with thickness of 150 μm at surface layer was produced on pure zirconium (Zr) by ultrasonic shot peening (USSP). The microstructure of the gradient layer was characterized by electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). The results indicate that multiple gradient features that consist of grain size gradient, strain gradient (low angle grain boundaries (LAGBs) gradient and geometrically necessary dislocation (GND) density gradient) and deformation twinning gradient are observed. By high resolution EBSD analysis, it is found that 101¯2<101¯1>/112¯1<1¯1¯26> twinning and dislocation slip are observed in the highly deformed surface layer. The deformation twins are parallel or intersected with each other and subdivide the coarse grains into small blocks. The movement and accumulation of dislocations gradually transform into LAGBs which finally forms finely distributed equiaxed grains. The mutual promotion between slip and twinning accelerates the process of grain refinement. The combination of twin-twin intersections and twin-LAGBs interactions ultimately leads to the formation of nano grains in pure Zr during USSP process

Correlative EBSD and SKPFM characterisation of microstructure development to assist determination of corrosion propensity in grade 2205 duplex stainless steel

Correlative electron backscatter diffraction (EBSD) and scanning Kelvin probe force microscopy (SKPFM) analysis has been carried out to characterise microstructure development and associated corrosion behaviour of as-received and 750 °C heat-treated grade 2205 duplex stainless steel. High-resolution EBSD analysis revealed the presence of σ- and χ-phase, secondary austenite, Cr2N, and CrN after ageing treatment. SKPFM Volta potential measurements confirmed the formation of discrete reactive sites, indicating local corrosion propensity in the microstructure. Cr2N, σ-phase, and inter-granular χ-phase had the largest net cathodic activity, followed by CrN and intra-granular χ-phase showing medium electrochemical activity, with ferrite and austenite (including secondary austenite) showing net anodic activity. Corrosion screening confirmed selective corrosion of ferrite in the as-received and 750 °C-aged conditions with the corrosion propensity of secondary phases staying in-line with SKPFM observations. Stress corrosion micro-cracks were also observed and are discussed in light of microstructure corrosion propensity.

Crystal plasticity and high-resolution electron backscatter diffraction analysis of full-field polycrystal Ni superalloy strains and rotations under thermal loading

Electron backscattered diffraction (EBSD) has been employed to study a polycrystalline nickel superalloy containing a complex non-metallic agglomerate under thermal loading. Heterogeneous distributions of elastic strains are observed near the inclusion due to its complex geometry and these have been quantified. Lattice rotations were also related to geometrically necessary dislocation (GND) density (∼1014m-2), indicating the development of localized plasticity arising from the mismatch in thermal expansivity between the Ni polycrystal and the inclusion. A crystal plasticity finite-element (CPFE) model which explicitly represents the full detail of the complex microstructure was developed to interpret the experimental measurements, and good quantitative and qualitative agreement has been obtained. However, a limitation of the EBSD technique when investigating polycrystal systems is that full-field, transgranular strain measurement remains difficult due to the necessity to reference a lattice spacing within a grain for strain calculation. An inverse reference shifting methodology has been developed using CPFE modeling to overcome this problem, thereby allowing like-for-like and grain-by-grain strain comparisons to be made. The method, in conjunction with high-resolution EBSD, shows promise for the determination of full-field strains and rotations in polycrystalline materials, and provides key information for fatigue nucleation in these material systems.