Mechanical behavior of double-aged AA8090
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I.
INTRODUCTION
Low fracture toughness values have been consistently reported for S-L orientations in commercial aluminum-lithium alloy plate products, tl,21 Intergranular fracture is associated with a pancake-shaped grain structure which is characterized by an arrangement o f weak, high-angle boundaries stacked in the rolling plane o f plate products. Recrystallization during processing is completely suppressed by the precipitation o f fine, coherent A13Zr dispersoids. Consequently, the final grain structure relates directly to the cast structure, with constituent phases residing along the elongated, high-angle boundaries o f large unrecrystallized grains, t3,4~ The result is a low toughness orientation in which failure occurs by splitting or delamination similar to wood being cut with the grain. According to K1c measurements made by Rao and Ritchie,c21 short-transverse toughness o f A1-Li alloys may b e 65 pct l o w e r than L-T toughness. In an investigation o f the short-transverse tensile behavior o f AIZn-Mg alloys, Embury and Nest31 determined that intergranular failure o f an unrecrystallized microstructure was encouraged by the presence o f coarse constituents associated with the cast structure. Complete recrystallization was required to decouple the constituent particles from the as-cast grain boundaries. During the development o f aluminum-lithium alloys, intergranular fracture was not limited to short-transverse testing orientations. Experimental laboratory compositions and early commercial alloys were prone to intergranular fracture, and this behavior was the subject o f many investigations, tS-15J Since the topic is relevant to the work presented in this article, the subject will be discussed briefly. A more detailed discussion o f the progress in minimizing intergranular fractttre in aluminumlithium alloys is contained in a review article, tl6~ Intergranular fracture was attributed, in p a r t , to the presence o f tramp elements, such as sodium, potassium, calcium, hydrogen, o r sulfur, tSj These impurity atoms segregated to grain boundaries t6-~°1 w h e r e , by some C.P. BLANKENSHIP, Jr., formerly Graduate Research Assistant, University of Virginia, is Materials Scientist, General Electric Corporate Research and Development, Schenectady, NY 12301. E.A. STARKE, Jr., Dean, School of Engineering and Applied Science, and Oglesby Professor of Materials Science, is with the University o f Virginia, Charlottesville, VA 22903. Manuscript submitted June 1 6 , 1992. METALLURGICAL TRANSACTIONS A
mechanism, the boundary was weakened. Standard commercial practice involves minimizing the levels o f these impurities by using high-purity lithium metal and specialized casting processes. Strain localization by two different mechanisms has been shown to encourage intergranular fracttLre. Slip plane softening, associated with a decrease in resistance to shearing with the propagation o f each dislocation through a coherent 8' particle, causes intense slip bands to form w h i c h then impinge on grain boundaries and
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