Precipitation hardening
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I.
INTRODUCTION
A N alloy is said to be precipitation hardenable when its hardness or yield strength increases with time at a constant temperature (the aging temperature), after rapid cooling from a much higher temperature (the solution treatment temperature). The phenomenon of precipitation hardening was discovered in the first decade of this century by Wilm,l who observed that the hardness of aluminum alloys containing small amounts of Cu, Mg, Si, and Fe increased with time at room temperature, after having been quenched from a temperature slightly below its melting point. The first plausible explanation for this "aging" process was presented by Merica e t a l . 2 who postulated that age hardening occurred in alloys in which the solid solubility increased with increasing temperature, thereby enabling a new phase to form at the lower temperature by precipitation from an initially supersaturated solid solution. Wilm's discovery and the explanation of Merica e t a l . gave birth to an entirely new area of research in physical metallurgy. The main focus of the research throughout the decades of the 20's and 30's was with the mechanism of precipitation, or aging, rather than with the mechanism of strengthening p e r s e . Unraveling the mysterious nature of the decomposition process was quite an undertaking, because the precipitates were too small to be observed directly using instrumentation of that epoch. A history of the progression of thought on the mechanism of precipitation from solid solution through the late 30's is contained in a comprehensive review article by Mehl and Jetter. 3 It is perhaps a measure of the preoccupation with the mechanism of precipitation, rather than concern for the manner in which precipitates strengthen the matrix in which they reside, that the article by Mehl and Jetter makes no reference to the A.J. ARDELL is Professor and Chairman, Department of Materials Science and Engineering, University of California, Los Angeles, CA 90024. This paper is based on a presentation made at the symposium "50th Anniversary of the Introduction of Dislocations" held at the fall meeting of the TMS-AIME in Detroit, Michigan in October 1984 under the TMSAIME Mechanical Metallurgy and Physical Metallurgy Committees. METALLURGICAL TRANSACTIONS A
possible role of dislocations, which had been discovered in 1934 by Orowan, 4 Taylor, 5 and Polanyi. 6 In fact, no mention is made of dislocations in any of the papers in the symposium volume in which the paper of Mehl and Jetter is published. The earliest attempted explanation of age hardening using the concept of the dislocation was that of Mott and Nabarro,7 who suggested that strengthening arose through the interaction between dislocations and the internal stresses produced by misfitting coherent precipitates. Several years later Orowan 8 derived the famous equation relating the strength of an alloy containing hard particles to the ratio of the shear modulus of the dislocation and the average planar spacing of the particles. While the theory of Mott and Nabarr
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