Multiscale Charaterization Techniques to Elucidate Mechanical Behavior of Materials
Deformation of materials, especially metal alloys, is a heterogenous multiscale phenomenon. This is due to the complex synergetic effects of several different factors: macroscopic boundary conditions for the applied forces, anisotropy in the microstructure/crystallographic orientation, crystal slipping due to multiaxial stress state, non-isotropic interactions of dislocations, etc. To elucidate the complex deformation mechanism of metals and alloys, a variety of multiscale characterization techniques have been developed and applied.
Defect density calculation is established based on the Nye dislocation density tensor that relates lattice curvature measured from electron backscatter diffraction to extract geometrically necessary dislocation density. Using this method, anisotropy effects in shear localization of metals is quantified using GND density calculation in 7039 aluminum alloy (grain morphological anisotropy) and high purity titanium (crystallographic anisotropy).
The GND density characterization technique has also been used to validate Ashby’s model on dislocation type evolution by characterizing deformed nickel samples. This is the first experimental validation of Ashby’s model, which proves that the SSDs are the dominant materials’ strength contributor at higher applied loads. GNDs are only present at a larger amount in the early stage of deformation.
To increase the sensitivity of the EBSD technique, reconstruction of 2D electron backscatter diffraction patterns into spherical Kikuchi map has been explored. It potentially enables more accurate orientation calculation for GND calculation as well as improved pattern center calibration and phase analysis.
To characterize local strain distributed across the entire sample, new digital image correlation based on computer vision algorithm has been developed to show very accurate surface strain mapping of homogeneous/heterogeneously deformed materials (with artificially speckle patterns) by comparing with results obtained from open-source software. Moreover, this method also has shown improved strain sensitivity in analyzing samples using its own pattern (natural pattern), in comparison to the cross-correlation based DIC method.
Microscopic residual stress and strain are more conveniently studies using HR-EBSD method. A new HR-EBSD method is developed by employing demons registration to remap reference pattern towards pattern to solve the phantom strain problem. Additionally, the rotation, stress, and strain sensitivity have been shown to be around 0.5×10-4, 35 MPa and 2×10-4, respectively.