Tube equal channel angular extrusion as a novel severe plastic deformation technique for Mg-3Al-1Zn alloy and pure niobium

Abstract

Severe plastic deformation (SPD) processed ultrafine grained (UFG) materials are finding wide application in many industries because of their superior properties. Plenty of work has been done on the SPD processing of bar, plate, and rod forms of materials. Unfortunately, not much study has been conducted on improving the fabrication technique of tubular forms of materials. The conventional tube manufacturing processes involve either a seamless tube production method using high temperature extrusion which results in a large, non-uniform grain size or production of tubes that are seam welded after extrusion through a porthole or spider die. Welded regions are often the weak part of the tube and are prone to early failure. An attractive substitute for conventional tube forming could be SPD processing that imparts high strength by reducing the grain size and not having a seam weld. To achieve improvement in the physical and mechanical properties of tubes, use of a novel SPD processing technique of tube equal channel angular extrusion (tECAE) is presented in this dissertation. Processing of magnesium (Mg) alloy and pure niobium (Nb) tubes were conducted in this study. Mg alloys tubes are promising candidates to be used in biomedical industries for stent application, in transportation, and lightweight engineering application. Mg alloys are difficult to work with and possess anisotropic properties. The tECAE technique successfully processed Mg-3Al-1Zn alloy tubes at temperatures as low as 120 °C. The role of starting texture on strength and deformation behavior of the Mg alloy was investigated by developing new processing routes. A viscoplastic self-consistent crystal plasticity model was used to supplement the experimental findings and improve the understanding of deformation modes in Mg alloy. Improved mechanical properties and unique textures were obtained after tECAE processing. Pure Nb tubes were also tECAE processed. Nb tubes are important for particle accelerators to make superconducting radio frequency cavities. The effect of accumulated strain during tECAE processing on the grain size, texture, and post-processing recrystallization temperature was investigated. This work shows that tECAE processing has the potential to reduce asymmetry in the mechanical properties and improve the strength and ductility by effective grain refinement.