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Study of the thermal expansion and the dimensional stability at high temperature of carbides-containing cobalt alloys

Author(s): Saidou Kane, Lionel Aranda, Grégory Michel, Patrice Berthod

Two cobalt-based alloys, a ternary one (Co-30Cr-0.3C) and a quaternary one (Co-30Cr-0.3C-4.5Ta)were elaborated by foundry and their thermal expansion behaviour tested by dilatometry. Three cycles {heating from room temperature – two hours – stage at 1050, 11150, 1200 or 1250°C – cooling to room temperature} were applied to samples cut in the ingots. The thermal expansion of the ternary alloy (containing lowfraction of chromiumcarbides) was almost linear and reproductive while curious phenomena occurred for the quaternary alloy (containing high fraction of tantalumcarbides). During the high-temperature 2-hours stage of each cycle the samples continued to deform. The amounts of these isothermal deformations were studied versus the number of the considered cycle, versus temperature and versus the type of alloy.After examination of the changes involved by the thermal cycles on the microstructures of the alloys, it appears that the accumulation of stresses in thematrix and in the carbides during the heating because of their interaction due to their difference of thermal expansion coefficient was not the single cause. Indeed carbides have significantly evolved: decreased volume fractions of chromium carbides and fragmentation of tantalum carbides.

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