Design of High-Strength, High-Conductivity Alloys
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MRS BULLETIN/JUNE 1996
techniques. However, MMCs usually contain large volume fractions from particles of the dispersed phase that are large compared to precipitates formed by reaction in the solid state. The large size
gives inefficient hardening, and the large volume fraction of a phase of relatively low conductivity reduces the conductivity of the composite. This article will consider only alloys strengthened by precipitates. There are two avenues for precipitation reactions to occur, either by changing temperature or by changing atmosphere. Both have been used to make high-strength, high-conductivity alloys. Internal oxidation of copper powder containing aluminum and subsequent consolidation on the powder to produce an alumina-copper composite are the basis for a series of excellent alloys.1 Recently, internal nitridation of copper alloys containing titanium has shown promise for high-strength, highconductivity applications.2 In this article, we will consider design of alloys where precipitates are formed by lowering the temperature of a supersaturated solid solution—that is, the conventional precipitation hardening route. Precipitation hardening requires a decrease in solid solubility when the temperature decreases. For this case, the decrease in solid solubility should be
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Figure 1. Strength versus electrical conductivity of some commercial copper-based alloys.
13
Design of High-Strength, High-Conductivity Alloys
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very rapid. The heat treatment consists of heating to a high temperature where all the solute is in solution and then lowering the temperature where the precipitation reaction occurs. Alloys are required that have very low solid solubilities at the precipitation temperature. Thermodynamic principles and data can be used very effectively for the design of this feature to be incorporated into the alloy. The solid solubility in an alloy containing precipitates decreases as the precipitate size is increased due to a reduction in total surface energy. The yield stress first increases as the particle size increases. Then after the spacing between particles is large enough for dislocation bypass to occur, the yield strength decreases with particle size. In this regime, there is a tradeoff between high strength and high conductivity. The objective of this article is to show how all of these principles may be applied to the design of high-conductivity, high-strength alloys. Use of the computer is essen
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