Intermittent plasticity in individual grains: A study using high energy x-ray diffraction

Abstract

Long-standing evidence suggests that plasticity in metals may proceed in an intermittent fashion. While the documentation of intermittency in plastically deforming materials has been achieved in several experimental settings, efforts to draw connections from dislocation motion and structure development to stress relaxation have been limited, especially in the bulk of deforming polycrystals. This work uses high energy x-ray diffraction measurements to build these links by characterizing plastic deformation events inside individual deforming grains in both the titanium alloy, Ti-7Al, and the magnesium alloy, AZ31. This analysis is performed by combining macroscopic stress relaxation data, complete grain stress states found using far-field high energy diffraction microscopy, and rapid x-ray diffraction spot measurements made using a Mixed-Mode Pixel Array Detector. Changes in the dislocation content within the deforming grains are monitored using the evolution of the full 3-D shapes of the diffraction spot intensity distributions in reciprocal space. The results for the Ti-7Al alloy show the presence of large stress fluctuations in contrast to AZ31, which shows a lesser degree of intermittent plastic flow.

High dynamic range CdTe mixed-mode pixel array detector (MM-PAD) for kilohertz imaging of hard x-rays

“…The MM-PAD ASIC combines a charge integrating frontend with threshold driven charge dumps to achieve within each image both the single x-ray detection capability performance expected of photon counters [2] and an instantaneous count-rate performance which exceeds that of photon counting imagers [3], all while framing at a kHz rate. These characteristics make the detector ideally suited for many difficult time resolved experiments, such as the temporal response of individual metal grains to periodic stress [9,10], and ptychography, both in the sparse and high flux regimes [2][4].…”

“…This detector has been used in several high-energy x-ray experiments at the APS and CHESS. One experiment [10], collected 4 dimensional data sets, tracking the dynamic behavior of individual grains in a sample of titanium and magnesium allows under stress at the 1-ID-E beamline at APS. Within these data sets, much of the area of the recorded diffraction in each image had << 1 photon/pixel, allowing a spectral analysis of the recorded signal from the incident 80 keV x-rays, shown as a logarithmic plot in Each module has a dedicated printed circuit board mounted to a secondary ADC board at right angles which in turn mates to a backplane board.…”

“…Cadmium telluride (CdTe), when compared to silicon, has the huge advantage of offering high-efficiency direct detection of high energy x-rays with an absorption depth of approximately 1 mm at 100 keV. A plot of efficiencies for selected materials and sensor thicknesses is shown in Figure 1.This extended x-ray energy range is important for studies in material science where properties of physically large samples and/or high atomic number materials are investigated [9,10]. CdTe has been extensively studied for x-ray and gamma-ray detection.…”

Mild-to-wild plastic transition is governed by athermal screw dislocation slip in bcc Nb

“…This type of correlated dislocation activity, referred to as dislocation avalanches, is found to be scale-free and consequently cannot be characterized by average quantities. Long-range elastic coupling at a scale much beyond the minimum structural length-scale, the lattice parameter in a metal, controls these collective defect physics and leads to statistical signatures of slip sizes that follow fractal behavior with sizes up to the micro-meter scale 11 – 13 and that may propagate across grain boundaries 14 .…”

Microstructural signatures of dislocation avalanches in a high-entropy alloy

“…Such homogenization schemes are fundamentally based on Gaussian-like statistics with well-defined mean values and stand in stark contrast to a few selected bulk * robert.maass@bam.de deformation studies that report scale-free deformation [7][8][9]. The signature of scale-free and thus non-Gaussian dislocation activity has been revealed with acoustic-emission sensing on bulk hexagonal closest packed (hcp) crystals [10,11] and a plethora of recent small-scale deformation experiments across numerous material systems [12][13][14][15][16][17][18][19][20][21]. One of the dominant findings was that a lot of experimental data showed agreement with analytical statistical physics frameworks, notably a pinning-depinning model for avalanches near the depinning transition [22].…”