As-Aged Structures of the Gamma Type Fe-Co-Cr-Ni Base Heat-Resisting Alloy and Correlation between the as-Aied Structures and the Mechanical Properties

Studies on tempering process of r-type heat-resisting alloys-V

Abstract

The as-aged structures of an Fe-Co-Cr-Ni base heat-resisting alloy LCN-155 were examined by metallographic methods-both optical and electronic. Mechanical properties at room temperature were then measured and correlated with the as-aged microstructure.
The experimental results obtained were as follows:
In a lower-temperature or shorter-time aging, first of all, M₂₃C₆ carbide precipitated at the grain boundaries, resulting in a marked decrease in ductility and there followed formation, of very fine nuclei of M₂₃C₆ along the cleavage planes within matrix, resulting in a progressiv increase of strength and hardness. But in a higher temperature or longer time aging, the dispersion state of the intragranular precipitates which correlated with the mechanicat properties was affected with the transition of precipitates. With an alloy which only onephase (M₂₃C₆) precipitated, the intragranular precipitates grew along the cleavage planes. and it showed a continuous localized structure. However, with an alloy in which two more phases (M₂₃C₆ and X phase) precipitated and transfered from one phase to another (M₂₃C₆→X phase) during aging, the cleavage precipitates was redistributed due to the carbide reaction and it showed a continuous (homogeneous) structure. It was concluded that this redistribution of precipitates due to carbide reaction would cause excellent mechanical properties.