Effect of trade addition of tin on the isothermal and di- isothermal geing characteristic of anlminium- magnesium 0.26 silicon alloy

Abstract

Pre?precipitation and precipitation processes in quenched supersaturated solid solutions of age?hardenable alloy systems are known to be vacancy?aided phenomena. The clustering kinetics would be significantly altered when the availability of free vacancies for solute transport is reduced due to the presence of trace elements which strongly interact with the vacancies. In two?step ageing, the clusters formed during the initial isothermal treatment can influence the behaviour at the subsequent ageing temperature. This thesis embodies a study of the effect of trace addition of Sn to the Al–Mg–Si system on the initial clustering kinetics, both during single and double ageing treatments. It also covers the formation and growth of zones and intermediate precipitates. The experimental techniques adopted include resistometry, optical and electron microscopy, and hardness measurement. Data on clustering kinetics were obtained by resistometry. Zone formation was studied by resistometry and electron microscopy. Resistivity readings at liquid nitrogen temperature on wire specimens were recorded during isochronal, isothermal, and di?isothermal ageing experiments. During isothermal ageing, a transition from ‘fast’ to ‘slow’ reaction was observed, and the process was attributed to vacancy trapping by G.P. zones. The time at which this transition takes place, t*, was found to obey an Arrhenius?type equation. Activation energies of 31.8 and 39.6 kJ/mol were obtained for the alloy without Sn and with Sn, respectively, from such an analysis. The initial ageing rate method for the same process yielded 35.7 and 40.2 kJ/mol for the two alloys, respectively. The delay in clustering kinetics observed for the Sn?bearing alloy was attributed to the process involving Sn–vacancy interaction. The zones exhibit a high degree of stability even when they are small in size. In di?isothermal ageing experiments, activation energies of 48.3 kJ/mol for the base alloy and 41.5 kJ/mol for the Sn?bearing alloy were obtained using the concept of t*. The enhanced kinetics during two?step ageing in the Sn?bearing alloy were explained in terms of (i) the smaller cluster size that could be attained during pre?ageing and its dissolution later, and (ii) the possibility of Sn atoms behaving as a reversible sink for vacancies. The electron?microscopic observations were also found to fit this view of cluster sizes being different in the two alloys. Hardness measurements, light and electron microscopy provide information mainly about the ageing process occurring at elevated temperatures. The study points out that the growth of intermediate precipitate(s) was hastened when atoms of Sn were present. The structural details when final ageing was done at high temperature (300?°C), following pre?ageing at low temperature, have also been investigated. The study reveals the pronounced influence of tin on precipitate formation and growth at these temperatures. A qualitative study of the width of the precipitate?free zone in single? and double?aged conditions for both alloys was undertaken using the light microscope, and some general deductions were made. Chapter I gives an introduction to the nature of vacancies in pure metals and alloys, the role of trace elements, the effect of pre?ageing, and mechanisms of the slow reaction. Chapter II gives details of materials used and the experiments carried out. Chapter III presents the results and general observations. Chapter IV includes the discussion of the data obtained. The conclusions derived from the present investigations are summarised in Chapter V.