The Effect of Precipitation Aging on the Mechanical Behavior and Microstructure of Aluminum-Lithium Alloys

The effect of variations in microstructure as a consequence of heat treating and aging on the mechanical properties of aluminum-lithium alloys was studied. The thermal treatments and composition were correlated to the microstructure and subsequent mechanical behavior of aluminum-lithium and aluminum-lithium-copper alloys that were solution heat treated and artificially aged for a series of aging times and temperatures. The underaged, peak-aged, and overaged thermal heat treatments were considered in determining the effect of the microstructure and processing on the mechanical properties. Standard ASTM tensile testing of the alloys was performed to determine mechanical properties such as yield strength, ductility, and ultimate tensile strength. Quantitative microscopy of the intermetallic precipitates was performed to related the measured deformation behavior to the microstructural features. Thus, the intermetallic precipitates in the microstructure which impede dislocation motion and control the precipitation strengthening response as a function of aging practice were measured by quantitative methods, and are the basis for controlling the mechanical behavior depending on their size distribution, average size, and interparticle spacing. The microstructure was studied, and measurements were made to determine the size, distribution, and morphology for the intermetallic strengthening precipitates as a function of the processing and composition. For the aluminum-lithium alloys studied, the primary strengthening was a direct consequence of ordered coherent Al3Li intermetallic precipitates which were uniformly distributed throughout the microstructure, which restricted the glide motion of dislocations during plastic deformation.