Abstract
Dilute Pt-C alloys are prototypical for studying oversize carbide phase precipitation from interstitial solid solution. Earlier studies showed the essential function of quenched-in vacancies in the precipitation process. Vacancies play a dual, volume accommodation and structural, role in the transformation by modifying both the habit plane spacing and stacking sequence. It was also shown how the precipitation sequence in interstitial Pt-C alloys is analogous to that in substitutional Al-Cu alloys. Initially a “GP zone” consisting of a monolayer plate of carbon atoms and vacancies forms. Atomic resolution images of the socalled a precipitates have confirmed their structure. During subsequent coarsening of the precipitates, α’ platelets form. Schematic diagrams illustrating the α and α’ structures in <100> projection are given in Fig. 1. The single-layer a structure, or GP zone is identical to a ﹛100﹜ stacking fault stabilized by an intercalation of carbon. The two-layer α’ structure is the first true precipitate phase and has a crystal structure anti-isomorphous with calcium fluoride.
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