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THE NUCLEATION AND GROWTH OF S' PRECIPITATES IN AN ALUMINUM - 2.5% COPPER - 1.2% MAGNESIUM ALLOY.
ROYAL AIRCRAFT ESTABLISHMENT FARNBOROUGH (UNITED KINGDOM)
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The nucleation and growth of S precipitates in an aluminium-2.5 copper-1.2 magnesium alloy aged at 190 C were examined by transmission electron microscopy. The S precipitates were shown to be heterogeneously nucleated at dislocations, and to grow as laths on 210 planes in a 001 direction. The shape of the laths is consistent with the degree of mismatch of the matrix and precipitate planes at the precipitate-matrix interfaces. Precipitate sheets are formed at dislocations by the growth of the laths on 210 planes with a common 001 growth direction the growth planes conform to the strain energy requirement that the misfit vector of the precipitate should lie approximately parallel to the Burgers vector of the dislocation, and are those expected from consideration of the effect of the line tension of the dislocation. The possibility of refining the precipitate distribution is discussed, and an increase in the density of sites for heterogeneous nucleation e.g. by prestrain prior to ageing or a modification of the interfacial energy e.g. by addition of silicon are both shown to result in refinement of the S precipitate distribution. Author
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