Deformation of HCP Metals and Alloys under extreme strain rates: Effect of shock waves

Abstract

The plastic deformation of materials under extreme conditions is of immense importance due to their different responses under various conditions during their service life. The shock or blast response (dynamic deformation), which is essentially a high-strain-rate phenomenon, is one such extreme condition. Studies on the microstructural and textural changes resulting from shock loading in hcp materials remain limited. Therefore, this thesis aims to bridge the gaps in the literature by addressing various aspects of shock deformation in hcp metals and alloys, specifically titanium (Ti), magnesium (Mg), and zinc (Zn). In most cases, microstructural changes are primarily governed by the peak pressure of the shock wave. This study includes the experimental evaluation of microstructure and texture developed in hexagonal closed-packed (HCP) materials on shock loading as a function of axial ratio (c/a). Further the response of material during deformation at extreme strain rate along different orientations has been examined for the evolution of microstructure and texture. To answer these questions, different experiments and simulation methodologies have been adopted. The sheets of three pure hcp metals, namely, Ti (c/a<1.633), Mg (c/a ~1.633), and Zn (c/a >1.633), were subjected to shock loading along their normal direction. Deformed materials indicate a significant reduction in grain size and a weakening of texture in each case. The differences observed in the microstructure of Ti, Mg, and Zn are because of the dominant deformation slip systems and twins. Activation of twinning depends on c/a ratio. In the second part, investigation on dome-shaped commercially pure (CP) titanium and the α-titanium alloy Ti5Al2.5Sn at relatively high shock pressure has been performed. Microstructural features in CP titanium during shock deformation show the presence of both contraction twins (CT1) and extension twins (ET1) form, former being dominant. Shock loading generates different deformation zones like compression, neutral, tension, and thinning across the sample thickness, each region characterized by unique microstructural features, including different types of deformation twins and grain boundary structures, resulting in a pronounced gradient in texture. The investigations on Ti5Al2.5Sn also indicate leads to microstructural changes, such as grain elongation, heterogeneity within grains, the development of dense dislocation networks, and formation of extension twins. In the third part, the formation of gradient microstructures in pure magnesium and the magnesium alloy AZ31 mas been investigated as a result of shock wave deformation. Both the materials develop a gradient microstructure composed of four distinct features: gradient grain size, gradient texture transition, gradient twinning, and gradient dislocation structures. The deformation weakens the initial basal texture and develops new texture components. In the last part, CP magnesium with four distinct initial orientations was investigated under quasi-static (1*10⁻³ s⁻¹) and dynamic (1*10³ s⁻¹) loading conditions. The initial orientation was found to influence strain hardening and twining activity. Despite differing initial textures, all samples evolved toward a basal texture. A visco-plastic self-consistent (VPSC) model has been applied to predicted texture evolution, stress-strain response, and slip/twin activity, closely matching experimental results. Deformation caused by shock/blast wave is a complex process involving multiple mechanisms within the material. The final microstructure after shock deformation is dependent on the residual strain and macroscopic plastic strain examined by the material. The macroscopic strain primarily governs the development of texture in the material, which is a function of material parameters and deformation strain path.