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Although qualitative relationships between fatigue lives and the sizes of the microstructural features, such as pores and particles, are well known, the quantitative models are lacking because of the unavailability of the required detailed microstructural data. The purpose of this work was to obtain such data for the high porosity (HP) and reduced porosity (RP) variants of the aluminum 7050-T7451 thick-plate alloys. Both alloys had similar tensile and fracture properties, but the reduced porosity variant showed superior fatigue performance attributed to the smaller sizes of the fatigue crack initiating particles and pores. Those size differences, as well as the differences in the throughthickness size gradients, have been characterized in this work. The sizes, shapes, and orientations of particles and pores were analyzed first on the plane sections and then converted to the true threedimensional (3-D) characteristics using the moment method. In the conversions, the particle and pore shapes have been assumed as triaxial ellipsoids and their size distributions as lognormal. The spatial distributions were quantified using the nearest neighbor spacing method. Results confirmed that the reduced porosity alloy had smaller particles and pores than the high porosity variant. The size distributions in the former were also more confined. In both alloys, the largest particles and pores were at the plate centers and the smallest at the surface. Their spatial distributions could be categorized as random with clusters.

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The purpose of the fatigue life prediction program at UCLA is to characterize the accumulation of the microstructural damage during fatigue of the aluminum alloys and to use obtained data in the formulation of the microstructure based stochastic life prediction models. Emphasis during the first year of the program was on the implementation of all required quantitative microscopy techniques and on the characterization of the starting microstructures in the "high" and "low" porosity 7050-T7451 commercial plate alloys.The first assembled set of data includes characteristics of the grain structures, constituent particles and pores on the sections parallel to the plate surface as a function of the distance from the surface. The results show that both alloys have partially recrystallized structure with the recrystallization levels changing from zero at the surface to about 200/ in the center. The unrecrystallized grains are 85 to 240 pm in size, while for the recrystallized ones the sizes are in the 45 to 85 pm range. The average pore size is 7.74 PM for the high porosity alloy and 7.15 pm for the low porosity material with the area fractions of 0.074% and 0.072% respectively. The sizes of the constituent particles are 8.38 and 8.21 Pm and the area fractions 0.88/ and 0.65% for the high and low porosity alloys respectively. Preliminary results from the tessellation analysis indicate that the pores have spatial distribution which can be categorized as regular with clusters. For the constituent particles the distributions are clustered. A number of the constituent particles have cracks and the measurements of their frequency are in progress. The most common precipitate phases present in the alloy are il', 111, 11 2 and 14. The precipitates inside the grains are small ranging from 5 to 45 nm in diameters while precipitates on the grain boundaries are generally larger and their sizes vary between 20 -250 nm. Precipitate free zones have been also observed and the measurements of their characteristics are in progress. The theoretical efforts during the first year of the program have been limited to the literature survey of the available stochastic life prediction models and to the development of a preliminary Markov chain model. Plans for the 1992 include finishing "' characterization of the 7050 alloys, formulation of the preliminary life : L prediction models and start of the characterizations of the 8090 alloys.