copper, maintaining the pres- ence of liquid during solidifica- tion down to low temperatures. Tese alloys are notoriously prone to microporosity with poor soundness/tightness. Even with the use of gradient chills, pressure tightness can be prob- lematic in larger castings of the wide solidification range alloys. Microporosity often limits the mechanical properties of cast alloys, including strength, ductility, and especially fatigue resistance.
Te Cu-Mn binary phase
diagram (Figure 1) exhibits a congruent liquid-solid equi- librium, or congruent point, at 1146K (873C) and 34.6 wt% Mn. (All references to alloy percentages in this article are weight percentages, unless oth- erwise noted.) Liquid of this composition solidifies without change in composition or tem- perature (no freezing range), as in the case of a pure metal. Te vanishing conditions for
of the minimum. Te corresponding composition in weight percentage is 34.6 ± 1.2% Mn, which was the value used for the congruent composition in this study. Te as-cast structure was pretty
typical of an alloy cast in a conduc- tive mold. It was columnar to the centerline. Te alloy showed a deep pipe and centerline porosity, similar to a pure metal. Te alloy solidified as a single phase , (Cu,Mn). Interest- ingly, the (Mn) phase that is sup- posed to be stable at room tempera- ture was not present, thought to be due to the sluggish kinetics that have been reported in this system.
Cellular Solidification While the macrostructure of the
Fig. 2. Images of the etched Cu-35Mn alloy show the cellular structure to the center of the ingot (a) and axial section near the surface (b).
supercooling near the Cu-Mn congruent minimum suggest these alloys would exhibit high castabil- ity, which would be unusual among copper alloys, especially of such high concentration. Nondendritic solidifi- cation would similarly benefit casting fluidity by decreasing the resistance to liquid flow in narrow channels (thin sections). An additional contribution to high fluidity is expected from the lower liquidus temperature near the Cu-Mn congruent minimum com- pared to other copper casting alloys. For the same super- heat, the time for complete solidification is thus longer, due to the smaller metal- mold temperature difference driving the cooling rate. Te purpose of this paper is to explore these solidification and casting characteristics through systematic studies of microstructure development and mechanical properties of cast copper-manganese alloys based on the congruent
composition. By developing an alloy around the congruent composition it may be possible to obtain an alloy that has the mechanical response of a highly concentrated alloy, but casts like a pure metal. Te Cu-Mn phase diagram in
Figure 1 appears to be the most reliable for liquidus, solidus, and congruent point value. Te congruent point is 1146 ± 3 K and 38 ± 2 at% Mn, with a large uncertainty in the composition due to the shallowness
alloy looks similar to a conventional alloy cast in a conductive mold (Fig. 3), the microstructure is distinctly different. Figure 2 shows the forma- tion of a cellular solidification struc- ture. Tis solidification morphology is an intermediate of planar solidi- fication and the common dendritic morphology. Cellular solidification shows the formation of the primary arms but has a lack of secondary and tertiary arms commonly seen in a dendritic structure. Limiting the formation of secondary and tertiary arms allows for an increased perme- ability of the liquid during solidifi- cation. Tis limits the formation of fluid flow defects, especially micro- shrinkage porosity. In over 50 castings
Fig. 3. The macrograph of as-cast Cu-35Mn shows columnar grain structure.
there was no observation of microshrinkage poros- ity in these alloys. Tis means that the presence of the cellular solidification morphology in conjuncture with the narrow freez- ing range hindered their formation. Te benefit of casting around this con- gruent point was found to primarily be the formation of this beneficial structure that limits the formation of defects associated with dendritic solidification in such a concentrated alloy. To better understand
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