Micropillar compression deformation of single crystals of the intermetallic compound Γ-Fe4Zn9

The plastic deformation behavior of single crystals of the Γ (Fe ₄Zn ₉ ) phase compound in the Fe−Zn system, which has been believed to be the most detrimental phase causing brittle failure in the coating layer (such as powdering) on galvannealed (GA) steels, has been investigated by micropillar compression tests at room temperature as a function of crystal orientation and specimen size. All micropillar specimens exhibit rich deformability in compression. Two distinct slip systems, { 1 ¯ 01}<111> and { 1 ¯ 01}<010>, are observed to operate. The critical resolved shear stress (CRSS) values for { 1 ¯ 01}<111> and { 1 ¯ 01}<010> both obey an inverse power-law scaling against the micropillar size with an exponent of -0.245 and -0.046, respectively. The bulk CRSS values for these two slip systems are estimated respectively to be 238 and 649 MPa by considering the specimen size effects of CRSS. The crystal orientation range for the operation of { 1 ¯ 01}<010> slip is limited to the vicinity of 〈111〉 because of the considerable difference in the CRSS values for these two slip systems. Dislocations with b (Burgers vector) = 〈010〉 dissociate into two partial dislocations, while those with b = 1/2<111> do not show any dissociation, both of which are consistent with the result of theoretical calculation of the generalized stacking fault energy on the { 1 ¯ 01} slip plane. Possible reasons for brittle failure related to the Γ phase occurring in the coating layer on GA steels are discussed based on the results obtained.