Determining the Effect of Microstructure and Heat Treatment on the Mechanical Strengthening Behavior of an Aluminum Alloy Containing Lithium Precipitation Hardened with the #delta#' Al_3Li Intermetallic Phase

摘要

The effect of the thermal treatment and composition on microstructure and subsequent mechanical behavior of an Al-2.6 wt. percent Li-O.09 wt. percent Zr alloy that was solution heat treated (SHT) and artificially aged for a series of aging times and temperatures was studied. The underaged, packaged, and overaged thermal heat treatments were studied to determine the effect of the microstructure and processing on the mechanical properties. The precipitates in the microstructure, which impede dislocation motion and control the precipitation strengthening response as a function of aging practice, were analyzed as the basis for controlling the strengthening depending on their size distribution, average size, and interparticle spacing. The average particle size, spacing, and size distribution were determined from the microstructure as a function of the thermal processing and composition. For the demonstration alloy, the primary strengthening was a direct consequence of ordered coherent Al_3Li (#delta#') intermetallic precipitates, which are uniformly distributed throughout the microstructure and restrict the glide motion of dislocations during plastic deformation. The Al_3Li average particle size, distribution, spacing, and volume fraction are closely related to the overall mechanical behavior and are a result of the heat treating practice and composition. Consequently, a micromechanical model was developed for predicting the precipitation hardening response in terms of the variation in polycrystalline strength with aging time, aging temperature, and composition. The overall micromechanical model, which was determined from the particle coarsening kinetics, dislocation mechanics, thermodynamics, resolved shear stress, as well as the dislocation particle shearing and bypassing mechanisms, accurately predicted the mechanical strength in the underaged, peak-aged, and overaged tempers of the demonstration alloy.