Abstract
Precipitation hardening involves solutionising, quenching and annealing steps, the latter often at various temperatures. The phenomena observed in Al-Mg-Si alloys are very complicated and partially not well understood. During and after quenching, solute atoms diffuse through the lattice assisted by vacancies and form atom clusters that gradually grow. These act back onto vacancies, which complicates the situation. We apply positron annihilation techniques in addition to traditional hardness, resistivity and thermal measurements to clarify what happens in various stages of thermal treatment: The quenching process can be divided into a stage of vacancy loss and of precipitation. Very short artificial ageing treatments after heating at different rates show that there is a competition between vacancy losses and cluster formation as the temperature increases. The difference between natural ageing and artificial ageing can be defined based on the importance of excess vacancies. Based on such results the behaviour of “invisible” objects such as vacancies and small clusters can be better understood but some open question remain such as the kinetics of secondary ageing or the details of the negative effect of natural ageing on artificial ageing.
Highlights
Precipitation strengthening in aluminium alloys involves dissolving solute atoms in a matrix at high temperatures, preserving the solid solution by quenching, and ageing the alloy after, during which solute atoms diffuse through the matrix and form clusters, zones and later precipitates – terms that are not always defined in the same way in the literature – and increase strength
2.1 General overview We give a short overview of methods suitable for investigating the small atomic clusters that form during natural ageing (NA) or short artificial ageing (AA)
It is experimentally known that the precipitated volume is proportional to hardness or strength during NA [44, 51] and we extend this to AA as we are only interested in trends, i.e. write
Summary
Precipitation strengthening in aluminium alloys involves dissolving solute atoms in a matrix at high temperatures, preserving the solid solution by quenching, and ageing the alloy after, during which solute atoms diffuse through the matrix and form clusters, zones and later precipitates – terms that are not always defined in the same way in the literature – and increase strength (or hardness). Where the function φφ expresses the driving force given by the decreasing solute supersaturation, K the temperature dependence of precipitation and xxvv the non-equilibrium vacancy fraction. As precipitation starts with the smallest possible objects, solute-vacancy and solute-solute pairs, knowledge of the earliest stages is crucial to explain subsequent stages in which the final hardened microstructure is formed. Both vacancies and atomic clusters in such early ageing stages are elusive and experiments meant to gain information are difficult. We start with an overview of the available methods with an emphasis on PAS and discuss a problem associated with age hardening of Al-Mg-Si alloys. Two attempts to extend currently available methods and two problems associated to Al-Mg-Si alloys, which are waiting to be solved, are presented
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