Formation of stacking faults in polycrystalline silicon bronze under tensile deformation

Abstract

The changes in the x-ray Debye–Scherrer diffraction pattern, as a function of plastic strain, have been measured for a silicon bronze (Cu–6·6 at. % Si–1·2 at. % Mn) tensile specimen. Analysis of line positions and line breadths gave the following results. The stacking-fault probability increased with extension, and near fracture (70–75% extension and about 70 Kg/mm² stress) its value was around 0·008. The extrapolated, or ‘true’ lattice parameter, measured in the radial direction, decreased, but not smoothly. The longitudinal stress calculated, using known elastic constants and theory, from the changes in lattice parameters accompanying the plastic deformation, compared favourably with that of the stress–strain curve from a standard mechanical test. This was also true when the stress was calculated from the changes in separation of the 111–200 peaks, provided a correction was made for the effect of stacking faults. If this correction was not made, then the calculated stresses fell below the values of the mechanical stress–strain curve. Evaluation of all the results, including those from line broadening, obtained for the Cu–Si–Mn alloy suggest that it has a stacking-fault energy of 2–6 ergs/cm², which places it in the intermediate range of values observed for other similar alloys. Examination by x-ray diffraction of the alloy in the filled or drawn condition showed a much higher stacking-fault density, although analysis of the data still yielded the same stacking-fault energy. The influence of the mode of deformation on the extent of faulting is thus clearly demonstrated.