Precipitation processes in aluminum-lithium aluminum-lithium - magnesium alloys
Abstract
A number of precipitation processes occur during the artificial aging of Al-Li alloys. Along with the coarsening of $\delta\sp\prime$ precipitates, the stable $\delta$ precipitates form at the grain boundaries and within the matrix. The $\delta\sp\prime$ precipitate free zones (PFZ) are observed at the grain boundaries and around the $\delta$ precipitates within the matrix. The formation and growth of grain boundary PFZs in Al-Li alloys has been studied as a function of aging time, alloy composition and the grain morphology. The PFZ growth is diffusion controlled and a model describing the diffusion controlled PFZ growth kinetics has been proposed. The PFZ growth rate increases with increasing Li content of the Al-Li alloys, and it is proposed that the diffusivity of Li increases with the Li concentration. In Al-Li-Mg alloys containing small amounts of Mg, so that the ternary Al$\sb2$LiMg precipitates do not form, the PFZ growth kinetics can be explained on the basis of the model developed for Al-Li alloys. Increasing Mg additions enhance the PFZ growth rate and the $\delta\sp\prime$ coarsening rate. Mg additions increase the volume fraction of $\delta\sp\prime$ precipitates and also cause an increase in the diffusivity of Li. Mg additions do not effect the morphology and lattice parameter of the $\delta\sp\prime$ precipitates. The $\delta\sp\prime$ coarsening kinetics and the particle size distributions in Al-Li-Mg alloys can be predicted from the empirical relationships developed for binary Al-Li alloys. The response of Al-Li alloys to differential scanning calorimetric (DSC) experiments have been evaluated by performing DSC experiments in conjunction with transmission electron microscopy. The state of the microstructure at various temperatures during a DSC run has been studied. The thermoanalytic data representing the $\delta$ precipitation have been subjected to certain non-isothermal kinetic analysis, to understand the mechanisms of precipitation of the $\delta$ phase. Isothermal kinetics of $\delta$ precipitation at 225$\sp\circ$C in Al-3.9 w%Li alloy has been monitored through DSC. Approximate kinetic parameters describing the non-isothermal precipitation kinetics during a DSC run have also been evaluated.
Degree
Ph.D.
Advisors
Sanders, Purdue University.
Subject Area
Metallurgy
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