Crystal-Defect-Associated Transformation Strain Paths of Electric Current Pulse (ECP) Induced α to β Transformation in Cu-40％Zn

A systematic study has been made on a Cu-40%Zn alloy treated by an electric current pulse (ECP) and by the examination of the microstructure and the crystallographic features of both the parent and the product phases. The β precipitates under ECP show a Kurdjumov Sachs Orientation Relation (K-S OR) in the vicinity of the grain boundaries (GBs), but a Nishiyama Wasserman (N-W) OR within the grains. Along the GBs the {111}_α /<110>_α dislocation arrays were spotted, whereas the {111}_α /<112>_α stacking faults were observed in the grain interiors. A closer examination of the lattice strain required for the phase transformation revealed that the maximum lattice deformation under the K-S OR is a shear on the {111}_α plane in the <110>_α direction. The dislocations arrays existing along the GBs offer the pre-strain that favors the precipitation of β particles obeying the K-S OR. Oppositely, the stacking faults within the grains provide pre-stains for the formation of the β precipitates respecting the N-W OR. This study sheds some light on the mechanisms by which crystal defects initiate phase transformation in a Cu-40%Zn alloy.