STUDIES IN PRECIPITATION HARDENING OF SOME ALUMINIUM BASE ALLOY

Abstract

The experimental investigations presented in this thesis, were undertaken to study the effect of quenched in vacancies, lattice distortions and dislocations on the ageing characteristics of some binary and ternary Aluminium base precipitation hardening alloys. Activation energies of migration and formation of vacancies Were also estimated on the basis of study of Portevin»LaChatelier phenomenon. Effeotsof trapped thermal vacancies on the process of prectpitat- ion in ternary Al,iCu4Ig alloys were studied. A comparative, study was made on binary Al-Cu alloy, The results 'Indicate that in ternary alloys$ rate of quenching has a definite effect on the kinetics of hardening process and the p.aksbardness values, It is suggested, that in contrast to Al.Cu binary alloys . not only is the concentrat» ion of trapped thermal vacancies higher, but also it is retained for longer period during ageing of the ternary alloys due to existence of strong solute atom (Magnesiva).vacancy pairs. This, coupled with possible condensation of vacancies to form voids and Collapse of clusters of vacancies to form dislocation loops (each of which Is a potential site for precipitation of intermediate phase) Can account for the dependence of rate of hardening on quenching rate. The observation of maximum peak-hardness corresponding to an optima rate of quenching may be explained in terms of sine of platelets of intermediate precipitate associated with the peak.