Tensile behaviour of alpha brasses at elevated temperature

The tensile deformation behaviour of three alpha (copper-zinc) brasses has been studied in the homologous temperature range of 0.4 to 0.9 and at strain rates of 10⁻⁴ and 10⁻²s⁻¹. Discontinuous yielding was observed at the lower strain rate, but only at temperatures up to 0.45 T[sub m]. In the full range of 0.4 to 0.6 T[sub m], there was a high rate of work hardening and virtually ho dependence of flow stress on strain rate or temperature. Behaviour in this region has been interpreted mainly using dynamic strain aging arguments, although an alternative explanation based on the possible existence of long range ordering in brasses at these temperatures has been offered. By contrast, in the 0.7 < T[sub H] < 0.9 range, there was a strong strain rate and temperature dependence of flow stress, and no work hardening occurred. Analysis of microstructure and of deformation kinetics were consistent with the occurrence of dynamic recrystall ization at the higher temperatures, which is in agreement with the observations of other workers with alpha brasses. A minimum in the tensile fracture ductility was observed at intermediate temperatures in the range 0.6 < T[sub H] < 0.75. Metallography revealed that the low ductility fracture involved the development and growth of microcracks (voids) at grain boundaries transverse to the tensile axis. Models are proposed for the mechanism of void nucleation and growth, and to account for the strong temperature dependence of the phenomenon. It is proposed that nucleation of the microcracks is due to migration and coalescence of vacancies at grain boundaries under certain conditions of temperature and stress, possibly assisted by grain boundary sliding. Validation of the theory was provided by experiments in which specimens were subjected to a "sensitizing" thermo-mechanical treatment.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat