MODELING OF PRECIPITATION REACTIONS IN INDUSTRIAL PROCESSING

The present investigation is concerned with modelling of diffusion-con trolled precipitation reactions in industrial processing, with particu lar emphasis on heat treatment. In the first part of the paper the com ponents of the model are outlined and constitutive equations presented which allow the fraction transformed to be calculated as a function o f time and temperature. The model uses a combination of chemical therm odynamics and kinetic theory to describe the microstructure evolution, with the particular feature of writing the Avrami equation in a diffe rential form. In general, the solution of the differential equation re quires stepwise integration in temperature-time space over a predeterm ined thermal cycle, but the mathematical treatment can largely be simp lified if the additivity condition pertaining to an isokinetic reactio n is satisfied. The theory is thus generic in the sense that it can be adopted to a wide spectrum of materials and heat treatment conditions , ranging from low and high alloy steels to aluminium alloys. In the s econd part of the paper this formalism has been applied to describe th e quench sensitivity of AA6082 extrusions. The process model takes int o account the thermal history of the base material and allows calculat ion of the peak strength following artificial ageing for a wide range of cooling conditions. The results show that the peak strength is both a function of the alloy composition, the homogenizing conditions and the cooling rate through the critical temperature range for beta'-Mg2S i precipitation, in agreement with general experience. It is concluded that the existing theoretical framework is sufficiently comprehensive to serve as a tool for alloy design and optimization of cooling sched ules for AlMgSi extrusions and an illustration of this is given toward s the end of the paper. Copyright (C) 1996 Acta Metallurgica Inc.