COARSENING KINETICS OF ALUMINUM-3-LITHIUM (DELTA') PRECIPITATES IN ALUMINUM-LITHIUM ALLOYS (ENCOUNTER, COALESCENCE, DISTRIBUTION, SIZE)
Abstract
Quantitative transmission electron microscopy (TEM) techniques were applied to investigate the coarsening behavior of (delta)' in a variety of Al-Li alloys. The alloys investigated include: Al-2.8Li-0.3Mn; Al-2.8Li-0.14Zr, and a series of binary alloys whoes compositions range from Al-2.4Li to Al-4.5Li. For all the alloy systems investigated, the rate of growth of (delta)' was found to obey Ostwald ripening kinetics. Increasing the lithium content increased the aging kinetics. For alloys which contained zirconium with the same lithium content, the addition of zirconium accelerated the aging kinetics. The Al(,3)Zr dispersoids acted as nucleation and growth sites for the (delta)' particles. The growth of (delta)' on the pre-existing Al(,3)Zr particles affects the initial or "critical size" of the composite particles. The particle size distributions (PSD's) in the binary alloys varied with lithium content. The negatively skewed PSD predicted by many ripening theories was only observed in the more dilute alloy with small (delta)' volume fractions (0.12). However, progressively increasing the (delta)' volume fraction to approximately 0.55 resulted in a PSD which became positively skewed. The shape of the PSD was volume fraction dependent changing from negatively to positively skewed. The PSD's were modeled using the Weibull distribution function. The adjustable parameters a and b of the Weibull function systematically decreased with increasing lithium content. The linear regression equations relating lithium content in weight percent of (delta)' volume fraction to the value of respective parameters were determined. Using the Weibull function one is able to predict the (delta)' PSD simply by knowing composition or volume fraction. The PSD in the zirconium containing alloy was more complex. Rather than a simple distribution function it was actually a combined or bimodal distribution. One distribution constructed from (delta)' particles which contained Al(,3)Zr particles has PSD's shifted to larger particle sizes compared to the distribution constructed from particles which did not contain Al(,3)Zr. The shift in the combined PSD was determined to be responsible for the accelerated aging kinetics of zirconium containing Al-Li alloys. The accelerated aging kinetics could be seen in both growth rate curves and in the yield strength-aging behavior.
Degree
Ph.D.
Subject Area
Materials science
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