Exploring the mechanism of room-temperature superplasticity of Al-Zn alloys

Abstract

In the aerospace and automobile industries, many workpieces are required to have complex shapes and almost defect-free internal structures. These requirements are difficult to meet using traditional cold plastic processing or hot processing. Therefore, the superplastic forming (SPF) process was developed to meet the manufacturing requirements for these workpieces. However, superplastic deformation has long been considered a phenomenon that only occurs in high-temperature conditions. The denaturation temperature will mostly exceed that of half the melting point of the material (temperature in Kelvin). Under these conditions, the SPF process will consume significant energy, which will obviously limit the application of this effective process. Therefore, how to reduce the temperature of superplastic deformation to achieve low temperature superplasticity has become one of the most important issues in this field. In 2018, a special Al-Zn alloy was reported by Russian and Japanese scientists to have room-temperature superplasticity, but the explanation given for this outstanding property was not satisfactory. Therefore, this project will employ advanced in-situ scanning electron microscopy (SEM) testing and high-resolution transmission electron microscopy (TEM) technology to conduct a comprehensive study of the mechanism behind this material’s room-temperature superplasticity. These advanced methods have facilitated exploring the whole process of room-temperature superplasticity. This first chapter of this thesis summarises the SPF process and its development over the last 50 years. It also introduces the mechanisms and requirements for high-temperature superplasticity and the research process for room-temperature superplasticity. The second chapter describes the methods used in this project: X-ray diffraction (XRD), SEM, TEM, high-pressure torsion (HPT) and in-situ tensile test. It also explains the details of the study procedure. The third chapter lists most of the study’s experimental results achieved using the above-mentioned methods. The results of the in-situ SEM tensile test show the process of superplastic deformation at different stages; the results of the high-resolution transmission microscopy illustrate the changing process of Zn atoms during this deformation. Zn atoms will move in the Al matrix to continuously support superplasticity. These data are used to fully explore this material at different stages, from macroscale to microscale. The fourth chapter addresses the reasons for these uncommon results, discussing the movement of Zn atoms and the extraordinary effect of Zn atoms at grain boundaries. Most importantly, the chapter summarises the mechanisms of room-temperature superplasticity based on all these data. The last chapter states the major observations and conclusions of this project and notes some questions that require further investigation.