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At fixed temperature and composition, the Brinell hardness of alpha brass is directly proportional to the area of grain boundary in a unit volume of metal. The resistance to deformation in the Brinell test is shown to be the sum of three distinguishable parts. These are, first, the elastic resistance, measured by the Harris strainless indentation technique and found to be independent of the presence of grain boundary. Second is the resistance to plastic deformation of the bodies of the grains, measured as the hardness at zero grain boundary area and found to be constant for grains of all sizes. Third is the resistance of the grain boundaries to the passage of shear through the metal, measured as the ratio of hardness to grain boundary area. The grain boundary contribution to hardness exhibits a sharp maximum in the neighborhood of 25% zinc, where the stacking fault energy of alpha brass is at a minimum. For shear to pass through the grain boundary without causing rupture, it is necessary that the slip be homogeneously distributed. This requires the intervention of cross-slip, which represents an energy requirement beyond that for slip within the grains. The formation of stacking faults opposes cross-slip and greatly increases the energy for the deformation of the boundary. Hardening diminishes with rising temperature. At 600°C, at ordinary speeds of testing, grain boundary hardening is negative, that is, softening occurs. Negative hardening is absent at high speeds of testing, showing that the softening effect is related to diffusion. At high temperature, shearing across the grain boundary gives way to shearing parallel to the grain boundary.
quantitative metallography, hardness, Brinell hardness, elastic hardness, grain boundary hardness, impact hardness, plastic hardness, high-temperature hardness, low-temperature hardness, grain boundary, grain boundary deformation, grain boundary energy, grain boundary sliding, grain boundary softening, grain boundary area measurement, grain boundaries cross-slip, grain boundary brittleness, alpha copper/zinc, alpha silver/zinc, alpha brass
Distinguished Service Professor Emeritus, University of Florida, Gainesville, Fla.
Professor and chairman, University of Alabama at Birmingham, Birmingham, Ala.