Plastic deformation of single crystals of the δ_1pand δ_1kintermetallic compounds in the Fe-Zn system by micropillar compression

The plastic deformation behavior of single crystals of the δ1p and δ1k phase compounds in the Fe-Zn system, which are the major constituent phases in the coating layer of galvannealed (GA) steels, has been investigated by micropillar compression tests at room temperature as a function of crystal orientation and specimen size. (0001)[112¯0] basal slip and (101¯0)[1¯21¯0] prism slip are observed to operate in the δ1p phase compound while (0001)[112¯0] basal slip is observed in the δ1k phase compound. For all cases, a significant strain burst occurs immediately after yielding, forming giant steps on the slip planes along the slip direction and leading in many cases to instantaneous 'slip plane (shear) failure'. The extent of such strain burst as well as instantaneous 'slip plane failure' is reduced by introducing dislocations (pre-straining) prior to micropillar testing, accompanied by the reduction in the stress at which such strain burst occurs as well as that in the extent of strain burst. The critical resolved shear stress (CRSS) for basal and prism slip in the δ1p and δ1k phase compounds all show an inverse power-law scaling against the specimen size with an exponent in the range of 0.02-0.06. The bulk CRSS values are estimated respectively to be approximately 320, 430, and 765 MPa for the basal slip in the δ1p and δ1k compounds and the prism slip in the δ1p compound by taking into account the specimen size effects of CRSS.