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Effects of dendritic orientation and of aging treatment on the thermal expansion of Ni-based and Co-based cast alloys

Cedric Heil, Patrice Berthod, Pierric Lemoine

Thermal expansion is of great importance for alloys working at high temperature. Both dendrite orientation and morphologies of carbides can influence this property in cast alloys. Two cast alloys, Ni-30Cr-1.0Cand Co- 8Ni-30Cr-0.5C-7.5Ta (in wt.%), containing high quantities of chromium carbides and of tantalumcarbides respectively, were heat treated at 1200°C for durations up to 100 hours. Their thermal expansion behaviours were characterized between roomtemperature and 1200°C and studied with respect to the dendrite orientation and the heat treatment duration. The thermal expansion is almost linear up to temperatures higher than 1000°C but, in some cases, a contraction applied by carbides on the matrix weakened because of high temperature, occurs before reaching 1200°C and during the isothermal dwell before cooling. The average thermal coefficient is slightly influenced by dendrite orientation and by a more or less long treatment at high temperature before test. The negative deformation of matrix due to carbides, and the permanent contraction kept at the end of experiment, seem being favoured in the dendrite direction in the case of the cobalt alloy. It is less true for the nickel alloy. The coarsening of the Cr7C3 carbides of the Nibased alloy and the fragmentation of the TaC carbides of the Co-based alloy, lower both the contraction at high temperature and the resulting permanent deformation.