Effects of scandium addition in wrought aluminium alloys

Abstract

A series of seven papers is presented in this thesis. Paper 1 is a comrehensive review of a considerable part of the available literature on Scandium in Aluminium alloys. Paper 2 and Paper 3 is a study of isothermal precipitation kineetics and mechanisms in a binary Al-Sc slloy. Precipitaion of the equilibrium phase A13Sc was found to occur either continuously or disontinuously, depending on teh temperature. Measurements of electrical conductivity were used to estimate the solvus line of SC in A1, and for estimating the interface energy between A13Sc particles and the A1 matrix.

Paper 4 and Paper 5 are studies of alloys in the ternary A1-Mg-Sc and A1-Sc-Si systems, respectively. It was found that Mg had no marked influence on the precipitaion behavior of A13Sc, and the A13Sc particles were found to prevent recrystallisation of a deformed A1-Mg-Sc alloy even when it was heated to temperatures close to the solidus. Si, however, was found to have a strong influence on the precipitation hardening potential of A1-Sc-Si alloys. At 500°C the A13Sc particles were found to disappear with increasing Si level.

In Paper 6 and Paper 7 the effects of adding Sc and Zr to an A1-Mg-Si alloy are explored. A13Sc particles could be formed during heat treatments at 550°C when at least 0.2 wt.% Sc was added by to the slloy. However, the highest number density of particles was achieved by adding both 0.2 wt.%Sc and 0.1wt.%Zr to the alloy. The A13Sc particles did not act as nucleation sites for Mg2Si, and the Sc addition to the alloy did not have any influence on the preciptation hardening of the alloy. In hot rolled material, one did not manage to identify A13Sc particles. In specimens that were hot rolled material, solutionised and then age hardened, the ductility was found to increase with increasing Sc level, while 0.4wt.%Sc was found to decrease the strength of the alloy somewhat.