The relations between thermodynamics, nanostructure, hardening and stability of an Al-Cu-Mg alloy processed by high pressure torsion

Abstract

This thesis presents a study on microstructural evolutions of an ultrafine-grained (UFG) Al-Cu-Mg alloy processed by high-pressure torsion (HPT). This work aims to develop a physically based hardening model to predict the strength of cluster strengthened UFG ternary alloys, and to reveal the relation between thermodynamics and high strain in severe plastic deformation (SPD). Experiments by means of Vickers hardness, differential scanning calorimetry (DSC), Xray diffraction (XRD), transmission electron microscopy (TEM) and atom probe microscopy (APM) have been carried out to provide the relevant information for the calibration and validation of the models. Analysis of XRD line profile broadening using the Rietveld method and Williamson-Hall method shows that the dislocation density increases significantly due to severe plastic deformation, which contributes to the increase of strength. APM reveals the presence of nanoscale co-clusters and defect-solute clustering. The relation between peak temperature for S phase formation and the equivalent strain for HPT was studied with the aid of a diffusion model. The model suggests that on increasing strain, the exothermic peaks correlated to S precipitation shift to lower temperatures. The model is consistent with the data from DSC thermographs of samples after different number of HPT rotations. In both the strengthening model and the stored energy model, strengthening due to dislocations, grain refinement, co clusters (due to short range order and modulus strengthening) and solute segregation are all incorporated to explain the multiple mechanisms. The models show good correspondences with measured microstructure data, measured hardness and measured enthalpy in DSC. The thermal stability of nanostructures in the Al-Cu-Mg alloy obtained by HPT has been studied during DSC heating processes. A significant increase of crystallite size and a significant decrease of dislocation density are revealed from XRD profile broadening when heat treated up to 210 °C, which correlates with an exothermic peak in DSC thermographs. Clusters are thought to act as obstacles that hinder the movement of dislocations, stabilize the ultrafine microstructures. In single reversal (SR) HPT, the hardness slightly decreases after 1/4 reversal turn; and increases again when the reversal rotations continue to increase. This phenomenon is thought to be due to the geometrically necessary dislocation (GND) density which decreases during the inverse straining. This study introduces concepts of the solute-defect complexes and the multiple local interaction energies between solute and dislocations to explain the strengthening mechanisms. The understanding of the HPT processing and microstructural modification has been enhanced through construction of models.

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