Browsing by Subject "HCP"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Investigation and modeling of processing-microstructure-property relations in ultra-fine grained hexagonal close packed materials under strain path changes(2009-05-15) Yapici, Guney GuvenUltra-fine grained (UFG) materials have attracted considerable interest due to the possibility of achieving simultaneous increase in strength and ductility. Effective use of these materials in engineering applications requires investigating the processing-microstructure-property inter-relations leading to a comprehensive understanding of the material behavior. Research efforts on producing UFG hexagonal close packed (hcp) materials have been limited in spite of their envisaged utilization in various technologies. The present study explores multiple UFG hcp materials to identify the general trends in their deformation behaviors, microstructural features, crystallographic texture evolutions and mechanical responses under strain path changes. UFG hcp materials, including commercial purity Ti, Ti-6Al-4V alloy and high purity Zr, were fabricated using equal channel angular extrusion (ECAE) as a severe plastic deformation (SPD) technique following various processing schedules. Several characterization methods and a polycrystal plasticity model were utilized in synergy to impart the relationships between the UFG microstructure, the texture and the post-ECAE flow behavior. Pure UFG hcp materials exhibited enhanced strength properties, making them potential substitutes for coarse-grained high strength expensive alloys. Incorporation of post-ECAE thermo-mechanical treatments was effective in further improvement of the strength and ductility levels. Strong anisotropy of the post-ECAE flow response was evident in all the materials studied. The underlying mechanisms for anisotropy were identified as texture and processing-induced microstructure. Depending on the ECAE route, the applied strain level and the specific material, the relative importance of these two mechanisms on plastic flow anisotropy varied. A viscoplastic self-consistent approach is presented as a reliable model for predicting the texture evolutions and flow behaviors of UFG hcp materials in cases where texture governs the plastic anisotropy. Regardless of the material, the initial billet texture and the extrusion conditions, ECAE of all hcp materials revealed similar texture evolutions. Accurate texture and flow behavior predictions showed that basal slip is the responsible mechanism for such texture evolution in all hcp materials independent of their axial ratio. High strength of the UFG microstructure was presented as a triggering mechanism for the activation of unexpected deformation systems, such as high temperature deformation twinning in Ti-6Al-4V and room temperature basal slip in pure Zr.Item Texture Control by Selective Deformation Mechanism Activation in Magnesium Alloy(2014-07-01) Foley, David ChristopherThe need for high strength, light weight structures in automotive and aerospace applications has driven a resurgence of interest in magnesium and its alloys. Unlike aluminum, wrought magnesium typically has a high degree of mechanical anisotropy because of its hexagonal close packed structure. Our objectives were to develop high strength (>350MPa yield) in a bulk magnesium alloy using grain refinement and to control the mechanical anisotropy by controlling crystallographic texture. This dissertation covers the development of thermomechanical processing methods used to tailor the strength and anisotropy of a magnesium alloy with 3%Zn and 1%Zr. The areas of focus in this study were as follows. First, we developed severe deformation processing strategies that increase strength in single-phase Mg alloy via grain refinement to submicron average grain size. We also established the achievable crystallographic textures in Mg alloy using 90o equal channel angular extrusion. In support of these first two goals, we determined the deformation mechanisms activated by differing strain paths and temperatures in single phase Mg alloy. Then, we established the effectiveness of these severe deformation processing strategies on both bar and plate workpiece geometries. We generated a wide range of crystallographic textures using thermomechanical processing. Using this knowledge, we established the effect of grain sizes down to submicron levels on room temperature deformation mechanism activity in single phase Mg alloy. We accomplished these goals through the use of equal channel angular extrusion, rolling, and heat treatment coupled with microscopy, diffraction, and mechanical testing. Notable achievements include demonstration of tensile twin suppression by grain refinement, the development of quasi-single-crystal textures, and the capacity to generate material with nearly identical texture but a range of grain sizes spanning almost two orders of magnitude. The experiments also supported (and were supported by) the development of visco-plastic self-consistent crystal plasticity modeling predictions thanks to the efforts of my colleagues. This work will further the development of advanced manufacturing and design using wrought Mg alloys.