Precipitation behavior of delta prime in a binary aluminum-lithium alloy
Abstract
The precipitation of $\delta\sp\prime$ in a binary Al-3.2wt%Li alloy was studied by X-ray diffraction. The growth kinetics of the $\delta\sp\prime$ phrase was examined after isothermal aging treatments at 433K, 453K and 473K. It is observed that the decomposition of the solid solution is a two stage process, leading to the two phase $\alpha$-$\delta\sp\prime$ structure. The first stage is identified as a precoarsening process, wherein all the $\delta\sp\prime$ particles grow by the diffusion of Li atoms from the supersaturated $\alpha$ matrix. The second stage is a true precipitate coarsening process characterized by the classical Ostwald ripening behavior. The activation energy determined from the experimental data in the precoarsening stage is 130 KJ/mole, and is in good agreement with the values for the activation energy for the diffusion of Li in Al reported in literature. The coarsening rate constant determined from the experimental data in the second stage is in good agreement with values obtained from TEM investigations. A structural study of the $\delta\sp\prime$ performed by X-ray diffuse scattering experiments. This study was conducted on a single crystal sample of the same alloy which was aged at 473K for 12 hours. Short range order parameters and the average atomic displacements were determined. The measured values of the displacements are very small, indicating that the strain associated with the precipitation is very small. The measured values of the short range order parameters indicate that the $\delta\sp\prime$ phase has a very high degree of order. A simple model is proposed to examine the implications of the experimental observations. The short range order parameters determined from the model are in good agreement with the experimentally determined short range order parameters. This model, when applied to earlier stages of aging, produces results that are consistent with the observations made in the growth kinetic study. Based on the predictions of this model it is believed that the $\delta\sp\prime$ particles possess a fairly high degree of order even in the as-quenched state.
Degree
Ph.D.
Advisors
Liedl, Purdue University.
Subject Area
Metallurgy
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