A nanomechanical testing framework yielding front&rear-sided, high-resolution, microstructure-correlated SEM-DIC strain fields

Abstract

The continuous development of new multiphase alloys with improved mechanical properties requires quantitative microstructure-resolved observation of the nanoscale deformation mechanisms at, e.g., multiphase interfaces. This calls for a combinatory approach beyond advanced testing methods such as microscale strain mapping on bulk material and micrometer sized deformation tests of single grains. We propose a nanomechanical testing framework that has been carefully designed to integrate several state-of-the-art testing and characterization methods, including: (i) well-defined nano-tensile testing of carefully selected and isolated multiphase specimens, (ii) front&rear-sided SEM-EBSD microstructural characterization combined with front&rear-sided in-situ SEM-DIC testing at very high resolution enabled by a recently developed InSn nano-DIC speckle pattern, (iii) optimized DIC strain mapping aided by application of SEM scanning artefact correction and DIC deconvolution for improved spatial resolution, (iv) a novel microstructure-to-strain alignment framework to deliver front&rear-sided, nanoscale, microstructure-resolved strain fields, and (v) direct comparison of microstructure, strain and SEM-BSE damage maps in the deformed configuration. Demonstration on a micrometer-sized dual-phase steel specimen, containing an incompatible ferrite-martensite interface, shows how the nanoscale deformation mechanisms can be unraveled. Discrete lath-boundaryaligned martensite strain localizations transit over the interface into diffuse ferrite plasticity, revealed by the nanoscale front&rear-sided microstructure-to-strain alignment and optimization of DIC correlations. The proposed testing and alignment framework yields front&rear-sided aligned microstructure and strain fields providing 3D interpretation of the deformation and opening new opportunities for unprecedented validation of advanced multiphase simulations.

Crystallographic slip activity fields identified automatically from DIC data, even for intersecting, diffuse and cross slip

“…The 4 single slip displacement fields are then superposed to form a combined displacement field. To simulate experimental conditions, Gaussian noise with a standard deviation of 0.05 pixel is added (which is relatively large considering that the noise in the DIC displacement data is often as low as 0.01 pixel) and subsequent Gaussian filtering is applied to emulate SEM-DIC conditions under which we recently showed to achieve high spatial strain resolutions [7,21]. See Table 1 for details on the generation of this virtual experiment.…”

“…For a polycrystalline or multi-phase microstructure proper alignment is required between the EBSD microstructure and SEM-DIC displacement field data to ensure the crystal orientation and its corresponding slip system kinematics is known for each grain. Here, we employ our own data alignment framework [7] to align these fields. We selected two grains that show more complex deformation (i.e.…”

“…In order to establish a well-defined loading state, an in-situ SEM-DIC micro-tensile test was performed on a single crystal Ferrite specimen (extracted from a Dual-Phase steel microstructure) according to the methodology described in Ref. [7]. See Figure 7a for a schematic drawing of the Focused Ion Beam (FIB) milling procedure to extract a tensile specimen from a ultra-thin wedge tip [26], with the resulting tensile specimen and the loading direction shown in Figure 7b.…”

“…in engineering alloys or at higher temperatures, additional slip families can be activated, which can enhance or deteriorate the mechanical properties of interest [1,2,3]. Moreover, complex dislocation activities and limited spatial resolutions of microscopy techniques can introduce (apparent) mechanisms such as cross-slip, curved slip and diffuse slip [4,5,6,7].…”