Microstructural evolution and grain refinement in HCP-Zr shear bands

Abstract
The mechanics of shear band formation has been studied in HCP-Zr to examine the microstructural evolution of the finite volume of material that comprises the shear band at several levels of deformation. Commercial grade HCP-Zr alloy is employed in the study for its relative ease in forming shear bands owing to its significant work hardening rate and significant plastic anisotropy. Hat shaped specimens are subjected to large plastic shear strains (of the order of 25-100) at strain rates of 10⁴s^-1 in a split-Hopkinson bar experimental setup. The extent of deformation is controlled using different hat heights (0.75mm, 1.0mm and 2.0mm) that produce discrete levels of shear strain implicit in the shear band formed. Results suggest that despite the extreme constraint of the hat-shape specimen multiple shear bands occur in the confined region, which coalesce upon large deformations. Electron Microscopy examinations of the narrow shear band regions reveal a microstructure dominated by ultra-fine grains of the order of 200 nm. Such observations of fine grain size are consistent across the range of deformation studied here despite the vast differences in diffraction signature. Thus while the occurrence of a shear band ensures fine-grain size, subsequent reorganizations most likely occur as deformation is incremented. The microstructural evolution exhibits a characteristic path including a radical alteration of the grain orientation spectrum in the shear band. Experimental evidence suggests that this process is sub-divided in two parts i) formation of fine grains, with spatial textural relationships, which upon continuing deformation create ii) completely randomized structures. The criteria and bounds of such reorganizations are evaluated.