Microstructure of high-tensile strength brasses containing silicon and manganese

Abstract

The morphology, crystallography, chemistry, and distribution of the phases in commercial high-tensile strength brasses containing manganese and silicon with compositions conforming to U.S.A. Specifications C67300 (Cu-35Zn-2.5Mn-lSi) and C67400 (Cu-35Zn-2.5Mn-lSi-l.5Al) have been studied. The wrought and cast microstructures of both types of alloys consist of the copper-rich feea phase, ordered B2β’ phase, and a manganese silicide Mn₅Si₃, with the crystal structure D8₈. Particles of Mn₅Si₃ are distributed uniformly in the as-cast alloy C67300 but tend to concentrate at theβ′ boundaries in alloy C67400. Studies of the development of the microstructure show that Mn₅Si₃ particles form from the liquid and are also precipitated from solid solution. During cooling, the α phase precipitates at a higher temperature in alloy C67300 (800 °C) than in alloy C67400 (500 °C); nucleation of the α phase occurs on Mn₅Si₃ particles in alloy C67400. Tiny Mn₅Si₃ precipitates are formed in both alloys upon quenching from temperatures near the solidus. When the quenched specimens are tempered at temperatures between 400 °C and 500 CC, all of theβ′ phase transforms to α in alloy C67300, while in alloy C67400, α precipitation occurs at theβ′ boundaries and shows a Widmanstätten morphology.