Displacive transformations in Au-18 wt pct Cu-6 wt pct Al
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is well known that an alloy of 75 wt pct Au and 25 wt pct Cu is face-centered cubic at elevated temperatures and that it orders to the compounds AuCuI and AuCuII at lower temperatures. It is less well known that the addition of aluminum to the AuCu binary alloy stabilizes a ternary “3/2” b electron compound that, at 500 8C, spans the 18 carat vertical section of the Au-Al-Cu system from an aluminum content of 2.4 to 7.9 wt pct, which is equivalent to electronto-atom (e/a) ratios of 1.22 and 1.63, respectively.[1] Of practical interest is that alloys with 76 wt pct Au and between 5 and 6 wt pct Al (e/a ' 1.5) can be thermally treated to induce a sparkling surface texture on a previously polished surface.[2–5] This surface texture was called “spangling” after its resemblance to the effect on galvanized steel, and the proprietary jewelry alloy is now generally known as “Spangold.” Experience with the manufacture of trial pieces of jewelry indicated that the phase transformation responsible for the surface texture was not a simple martensitic transformation of a quenched high-temperature phase. In particular, rapidly cooled as-cast or solution-annealed artifacts did not consistently undergo the spangling phase transformation to the degree desired. If, however, the sample was aged at a temperature between 100 8C and 150 8C after quenching, the phase transformation that produced the spangle was more easily obtained. Once “activated” by the aging treatment at 100 8C to 150 8C, the material exhibited a reversible displacive phase transformation, with a hysteresis of approximately 50 8C. This cycling produced a distortion of the surfaces, which was especially visible if they had been previously polished. The transformation products had laths, which crossed each other in a similar manner to the self-accommodating laths in thermoelastic martensitic
alloys.[6] These properties are characteristic of a shape-memory alloy (SMA). Further understanding of the transformation behavior of the 76 wt pct Au - 18 wt pct Cu - 6 wt pct Al alloy was sought, so that the manufacturing route could be optimized. Although the surface relief produced by the displacive transformation in the Spangold alloy resembled that of martensite, a phase transformation should not be labeled “martensitic” based on an examination of its product morphology alone, since the transformation mechanism is the determining criterion. A martensitic transformation is a diffusionless, “shear-dominant”, first-order transformation with resultant lattice distortion. The transformation proceeds by a nucleation and growth mechanism, and the product grows in a platelike or lathlike shape, as a result of an invariant-plane interface between the parent and product phases.[6,7,8] The kinetics and morphology of the transformation are dominated by strain energy rather than by shuffle displacements (of at least one atomic spacing).[6,7] Furthermore, the latticedistortive transformation has an “undistorted line” where the length of a vector in the original lattice remains unchanged wh
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