Enhanced Castability, Properties in Copper- Manganese Alloys
A near-congruent copper-manganese alloy produces high castability and good mechanical properties, which could potentially lead to cast applications. KEVIN CHAPUT AND KEVIN P. TRUMBLE, PURDUE UNIV., WEST LAFAYETTE, INDIANA
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opper and manganese alloys are well known for mechanical damping capacity, resiliency, and
magnetic behavior. With the exception of these specialty alloys, manganese is usually a secondary alloying element in copper. Te most common example is high-strength yellow brasses, also known as manganese bronzes, which are traditionally used in large pro- pellers and similar applications that require high strength as-cast. Tis helped manganese bronze gain a reputation as an ideal alloy in marine conditions, even though the extent to which manganese was responsible for the corrosion resistance is not clear. A recent study explored the con-
ventional casting of copper-manganese alloys based on the congruent com- position. An alloy of the congruent composition should exhibit partition- less solidification under equilibrium conditions at a melting point, with a planar solidification front and the (ideal) castability of a pure metal, but in an alloy of high solute concen- tration. However, only small tem- perature/composition ranges impose sufficient constitutional supercooling (CS) to drive dendritic solidification under typical casting conditions. Te intermediate case of cellular growth, in which the secondary and tertiary arms of dendrites do not develop,
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is observed for small constitutional undercooling in controlled directional solidification experiments, but rarely, if ever, in casting.
Understanding Cu-Mn Alloys
Copper casting alloys are commonly divided into three groups based on their freezing range, which governs the degree of solutal undercooling, associ- ated dendritic growth, and tendency
to form microporosity. Group 1 alloys, having freezing ranges less than 50K (50C), include high coppers, brasses and aluminum bronzes. Tese alloys are generally considered to have the highest castability. Group 2 alloys have solidification ranges from 50K to 110K (50C to 110C) and Group 3 alloys have larger ranges. Group 3 alloys include the leaded brasses and tin bronzes. Lead is essentially insoluble in
Fig. 1. This Cu-Mn binary phase diagram is from the Journal of Alloy Phase Equilibria.
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