Near-congruent solidification of castings
A study on the microstructure development of as-cast Cu-Mn alloys based around the congruent minimum at 34.6 wt % Mn and 873 °C was performed. Initially, this was to evaluate the alloy as an alternative to wide freezing range Pb and Sn bronzes that are plagued with porosity. The shallow minimum and associated narrow freezing ranges around the congruent point result in a completely cellular (non-dendritic) solidification morphology for a composition range ~3 wt % Mn about the congruent composition (C c). The degree of cellular solidification was found to depend on the mold material. Increased mold conductivity lead to a narrower composition range of complete cellular solidification. By casting alloys of different compositions into a composite mold, the effect of the mold conductivity allowed an evaluation of the congruent point reported by Gokcen. These results fit well with the constitutional supercooling criterion. While solidification at a point ideally would be planar, this was not observed even with minor deviations from the Cc. An additional study of the microstructure development along the minimum trough in the liquidus surface between the Cu-Mn and Ni-Mn binary congruent points of the Cu-Mn-Ni ternary system was conducted. This study revealed that alloys near the binary congruent minima were more cellular than alloys near the middle of the phase diagram, along the trough. As the composition approached the center of the Cu-Mn-Ni diagram, the morphology became more dendritic, characteristic of an isomorphous system. Even though these alloys did not solidify in a completely cellular manner, they were free of any microshrinkage porosity. The alloys in this study (Cu-Mn and Cu-Mn-Ni) show promise for use in structural applications due to the lack of microshrinkage porosity, potent solution strengthening of manganese and strong aging response.
Trumble, Purdue University.
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