Exploring the Grain Refinement Mechanisms Induced by High-Pressure Torsion Processing

Abstract

Severe plastic deformation (SPD) techniques have been used to produce bulk nanostructured metallic materials with superior mechanical properties. Among all SPD techniques, high-pressure torsion (HPT) has been proven the most effective in achieving nanostructures. Therefore, extensive efforts have been devoted to studying HPT processing and its effect on the microstructures and properties of materials. This thesis project is part of an ongoing effort to study HPT and, at the same time, expand our knowledge of SPD. In Chapter 1, extensive literature reviews of SPD, HPT and deformation mechanisms under SPD are provided, and three major issues regarding SPD processing materials are identified. Chapter 2 discusses the choice of a duplex stainless steel as a model material to be processed by HPT. A brief review of duplex stainless steel is given in this chapter. After the choice of material, a review of all the materials characterisation techniques used in this thesis project is presented, as well as details of all the relevant experimental procedures. Chapter 3 describes the plan-views and cross-sectional views of shear strain imposed on austenite/ferrite duplex stainless steel discs at different stages of HPT processing. The effect of the shear strain was correlated to the hardness evolution of the discs. Chapter 4 presents the systematic investigation that was carried out into the microstructure and hardness evolutions of the duplex stainless steel. It was found that the microstructures of both phases of the duplex stainless steel evolved concurrently under continuously increasing shear strain imposed by HPT. Chapter 5 presents a detailed investigation of both cold rolling and HPT caused significant grain refinements to a duplex stainless steel. It was found that while conventional cold rolling of a face-centred cubic structure produces a microstructure with a high density of extended dislocations, increasing the applied stress using HPT gives a nanotwinned coarse-grained structure. In Chapter 6, major conclusions are drawn from this thesis project. Some possible future work is proposed as an extension of the project.