Characterisation and Plasticity in Extruded Al-Mg-Si Profiles engaging In-situ EBSD

Abstract

A comprehensive characterisation and study of plasticity in two extruded Al-Mg-Si profiles has been carried out. The primary objective of the experimental work has been directed towards obtaining an improved understanding of the operating deformation mechanisms and mechanical anisotropy observed on all length scales during plastic deformation. In-situ deformation in the SEM combined with EBSD investigations has been an important tool in order to obtain this objective. The experimental results have been divided into two separate parts. Part A covers the characterisation and mechanical anisotropy investigations, while Part B covers the more detailed in-situ investigations.

Two alloys, one with a recrystallized microstructure and the other with a nonrecrystallized (fibrous) microstructure, have been subjected to a detailed characterisation concerned with mechanical anisotropy, through-thickness variations and effects of various heat-treatments. The experimental investigations showed that both alloys possess highly anisotropic properties. The effects of temper designation, directional dependency and position through the thickness were studied.

The in-situ deformation studies gave new insights into the fundamental reasons for the observed mechanical anisotropy and the related deformation mechanisms. Detailed investigations of the slip traces in combination with calculated Schmid value distributions provided information on potential slip activity for the various slip systems. It was found that the number of slip systems activated was very heterogeneous and this number can even vary from region to region within one individual grain. In other words, the strain distribution seemed very heterogeneous. Further, the actual number of activated slip systems was in general less than predicted by the widely used Taylor model. Consequently, if the accuracy of texture-based calculations should be improved, more advanced models like the GIA (Grain Inter-Action) and the LAMEL models should be applied.

It was also found that crystallographic orientations having a [100] or a [111] parallel to the deformation direction (DD) were more stable during simple tension deformation. Moreover, crystallographic orientations not having this configuration rotated in order to align the DD to one of the above directions. Also the rotation of individual grains seemed to have a strong relationship to the actual activation of slip systems.

The mechanical anisotropy and shape tolerances could be explained in terms of crystallographic texture, i.e. variations in the actual activation of slip. As a result, the macroscopic properties (e.g. mechanical anisotropy) were to a large extent controlled by the mechanisms operating at the microscopic length scale. A full understanding of the operating mechanisms should therefore involve exact information from all length scales.