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along the grain boundaries as temperature and hold time increase. The principal grain-boundary carbide is a niobium-rich MC type. Boundaries containing MC carbides are often bordered by distinct regions lacking ?´´ and, to a lesser extent, ?' precipitates. These precipitate-free zones arise from local depletion of solute elements and vacancies near the grain boundary. At around 700-750 C, the fraction of carbide-covered boundaries grows with time; at higher temperatures, co-precipitation with the ? phase is frequently observed. Progressive

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