Effect of High Pressure Torsion Processing on Precipitate Containing Alloys

Abstract

Severe plastic deformation (SPD) processes yield excellent grain refinement in metallic materials by application of very large shear strains while retaining the initial sample dimensions. Its effects on simultaneously enhancing the strength and ductility in pure metals and single-phase alloys has been studied in great details since the past three decades. The focus of the recent studies has been on the metallurgical alchemy achieved during SPD processing that yields good structure – property correlations. Most of the studies investigating the compositional modulation achieved during SPD have been performed in binary alloys using the high-pressure torsion (HPT) processing technique. However, detailed investigations on the effect of large strains on multicomponent alloy systems, containing precipitate phases, have rarely been performed till date. Hence, a systematic study on the microstructure evolution with dissolution of precipitate phases and their reprecipitation during severe plastic deformation processing is highly desirable.

The work carried out in the present thesis focuses on the effect of severe plastic strains applied by HPT technique on the state of precipitates in two ductile cubic alloy systems, namely the niobium alloy (BCC) system and the aluminium alloy (FCC) system. The first part of the thesis focusses on the niobium alloys, pertaining to the composition of the dissolved and reprecipitated carbide phases and the corresponding change in its strength. The results obtained from multiscale characterization techniques have been used to further assess the additivity of strengthening mechanisms in niobium alloys. The plasticity of the HPT deformed niobium alloys has also been investigated and correlated with the microstructures to determine the optimum applied strain. The second part of the thesis aimed at increasing the plasticity and irradiation damage tolerance of Nb – 1Zr alloy, which is popularly used in the nuclear industry, by increasing the sites for absorption of the irradiation induced defects by HPT processing. In the last part of the thesis, the effect of Sc addition in the strengthening of AA-2195 Al alloy deformed by HPT was examined. The microstructural features and metallurgical alchemy yielded by large plastic strains were investigated at the nanoscale to understand the post HPT mechanical response. The role of excess vacancies, dislocations and non-equilibrium grain boundaries were investigated by theoretical calculations and corroborated experimentally by atom probe tomography to understand the contributors to compositional modulations of various alloying additions.