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.
Highlights
Plastic deformation in metals occurs by intermittent motion of dislocations as has been shown in many experimental settings
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
Intermittent plasticity in diffraction spot evolution. For both Ti-7Al and AZ31, it was possible to connect diffraction spots showing isolated transients to grains indexed by ffHEDM
Summary
Plastic deformation in metals occurs by intermittent motion of dislocations as has been shown in many experimental settings. The motion of dislocations progresses in “bursts” that vary in size (both spatial extent and the number of dislocations participating) and temporal frequency. Accompanying this dislocation motion is a release of elastic energy (relaxation) producing drops in the local stress level and the driving forces for dislocation motion which may be associated with corresponding changes (both increases and decreases) in the stress states of neighboring grains in a polycrystalline aggregate.. The present work makes an advance in linking the stress state and intermittent slip, critical for advancing the constitutive models used for metal plasticity, through utilization of newly available high rate x-ray data
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