Integrated Analysis of Process-Structure-Property Relationships in Heterostructured Metallic Sheets Fabricated by Single-Roll Angular-Rolling

Abstract

The development trend of structural materials has recently been shifted towards a superior balance of strength and ductility, not just strengthening, as not only mechanical stability but also application to a variety in advanced engineering parts has emerged as the utmost importance. In accordance with the trend, materials engineering based on severe plastic deformation (SPD) has recently been regarded as a technology with high entry barriers for industrialization, especially in terms of productivity and toughness. A majority of conventional SPD processes are fairly discrete and require excessive equipment capacity compared to the sample scale, so that mass production and scaling-up are impossible. Above all, nanocrystallization by SPD is inevitably accompanied by a dramatic sacrifice in ductility. Therefore, the extremely low toughness of the SPD-processed materials is still a chronic bottleneck of industrialization to many advanced technologies. Recently, the design of heterostructured materials has attracted a great deal of attention as a state-of-the-art strategy to realize the superior combinations of strength and ductility. Heterostructured materials are a new class of materials with artificial microstructural heterogeneity. According to the geometry and distribution of soft and hard domains separated within materials, heterostructured materials can be tailored into various types. Interestingly, several heterostructures have demonstrated simultaneous enhancements of strength and ductility in heterostructured materials. Nevertheless, the research related to heterostructured materials is still in the beginning period of technology, and only a few hetero-structuring methods have been developed so far. Furthermore, the design field of heterostructured materials is trapped in a confined border in that hetero-structuring methods have quite limited controllable microstructural factors along with low productivity. In this regard, the present thesis aims at two goals in order to extend the research fields for SPD and heterostructured materials. The first goal is to invent an advanced continuous SPD process that can fabricate new types of heterostructures while ensuring high productivity. The second objective is to establish the design frame of new hetero-structuring to progress strength-ductility synergy a step further. To realize this, integrated analysis for process-structure-property relationships was carried out in heterostructured metallic materials fabricated by the newly developed SPD process. To achieve the first goal, the former part of this thesis has covered the development overview of a new continuous SPD process for metallic sheets called ‘single-roll angular-rolling (SRAR)’ and the hetero-structuring methodology using the process. The SRAR process was an advanced technique to concentrate much higher shear strain on the core region rather than both surface regions of a metallic sheet. The heterogeneous deformation resulted from the sequential deformation history in the SRAR process, including ‘circumferential shear deformation (CSD)’, ‘channel-angular shear deformation (CASD)’, and frictional deformation. The significant shear strain partitioning between the core and surface regions effectively split recrystallization and grain growth behaviors in the subsequent heat treatment, leading to the evolution of a unique heterostructure called ‘reverse gradient structure’. The heterostructured metallic sheet fabricated by the SRAR process consisted of the fine-grained (FG) core and the coarse-grained (CG) surfaces and exhibited simultaneous enhancements in strength and ductility, compared to the initial material. Based on a systematic comparison with rule-of-mixture, further investigation verified that the extraordinary mechanical performances were primarily attributed to extra work hardening of hetero-deformation induced (HDI) stress originating from plastic incompatibilities at the CG/FG interfaces under plastic deformation. In the gradient-structuring methodology based on the SRAR process, there were three major processing parameters: repetitive processing route, the number of processes, and the subsequent annealing condition. Therefore, the latter part of this thesis has dealt with the integrated analysis on process-structure-property relationships in the hetero-structuring depending on the key factors. Firstly, the finite element analysis and experimental verification revealed that route A without any sample rotation between consecutive passes was the most favorable processing route for the hetero-structuring. The primary reason was that the repetitive process in route A led to more prominent shear strain concentration and dislocation localization on the core region than that in route C with redundant strain. Secondly, to analyze the effect of subsequent annealing on the hetero-structuring, three kinds of reverse gradient-structured metallic sheets were manufactured by diversifying the post-annealing time at 400 °C to 10, 20, and 30 minutes after 6 passes of SRAR. The resultant materials involved different volume fractions of recrystallized fine grain, recrystallized coarse grain, and non-recrystallized grain. All these materials commonly exhibited outstanding combinations of strength and ductility, outperforming that of the initial material. According to a systematic comparison with rule-of-mixture, the post-annealed materials for 10 and 30 minutes manifested superiority in the further strengthening for the yield strength and the extra strain hardening for the flow stress, respectively. This investigation revealed the correlation between the HDI strengthening/strain hardening mechanisms and microstructural heterogeneities by the post-annealing conditions after the SRAR process. In the last chapter of this thesis, a new type of heterostructure called multi-layered gradient structure was fabricated by the multiple processing of two advanced hetero-structuring methods: SRAR and ultrasonic nanocrystalline surface modification. This metallic sheet with a multi-layered grain size gradient in the thickness direction boasted impressive strength-toughness synergy beyond the initial and reverse gradient-structured ones. The superiority was primarily ascribed to an excellent balance of the strengthening and strain hardening of HDI stress itself by maximized microstructural heterogeneity. Ultimately, this thesis proposes innovative hetero-structuring approaches to extend the confined design space in the research field related to SPD and heterostructured materials and advance a strength-ductility window saturated at a certain level a step further.