Precipitation on low-angle grain boundaries in Al-Zn-Mg
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THE properties of a multicomponent crystalline solid are often influenced by the heterogeneous precipitation which occurs on the dislocations and grain boundaries. Thus, there is a basic interest in understanding how such defect structures affect the precipitation process. A1-Zn-Mg was selected for this study because the precipitation phenomenon plays a critical role in determining this alloy's mechanical properties. When AI-Zn-Mg is aged to gain its high strength, heterogeneous precipitation occurs on the grain boundaries, making the alloy into a useless, brittle material) The first step toward establishing a relationship between the defect and the precipitate in a heterogeneous precipitation process would be to examine the growth behavior of precipitates on a one-dimensional defect, e.g., a dislocation. Allen and Vander Sande 2-4 used a weak-beam microscipy to investigate the precipitation of the ~/phase, MgZn2, on dislocation lines. The precipitate growth was influenced by the angle between the dislocation line and the preferred growth direction of the lath. The next step in this general problem is to examine how a two-dimensional defect, the low angle grain boundary, affects the precipitate growth kinetics. Therefore, the basis of this study involves the precipitation phenomenon on low angle grain boundaries. Low angle boundaries, rather than high angle boundaries, were chosen for this analysis because these boundaries are D. M. VANDERWALKER, ResearchAssistant, and J. B. VANDER SANDE, Associate Professor,are with the Department of Materials Scienceand Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted November 6, 1979. METALLURGICALTRANSACTIONSA
easier to characterize from a fundamental point of view. It is essential that the boundary be completely characterized so that a relationship can be developed between the boundary structure and the precipitate growth kinetics. EXPERIMENTAL PROCEDURE Strips of A1-Zn-Mg were solutionized for two hours at 450 ~ and quenched in ice water. The material was then cold rolled on a hand roller, reducing the initial thickness by approximately 5 pct. 3 mm diameter discs were punched from the foil sheets. The samples were then reheated to 450 ~ to determine the degree to which the material should be recovered to acquire reasonably spaced dislocation nets. Microhardness measurements were made on specimens treated at 450 ~ for different lengths of time. The results are graphed in Fig. 1. The desired structure was produced by a 100 s heat treatment again followed by an ice water quench. Since the position of the hardness curve depends on the initial degree of deformation, for each new cold rolled sheet a few microhardness measurements were made to monitor the recovery behavior. Samples were then aged at 150 ~ in an oil bath for four different times, 15, 30, 45 and 90 rain to study various stages of precipitate growth. A Fischione jet polisher containing a 1/3 nitric acid, 2/3 methanol solution was used at - 35 ~ to prepare the specimens for
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