The basal plane stacking faults in 18R martensite of copper base alloys

Abstract

Stacking faults present within the M18R martensite plates in copper base alloys have been studied by transmission electron microscopy and analysed using the ¯g· ¯R= N invisibility criterion. It is concluded that the basal plane stacking faults, contrary to what is generally reported in the literature, are not of the shear type with displacement vectors±1/3[100]. Several defect sequences are analysed to show that the displacement vectors of the faults do not lie on the fault planes and are of ± 1/18 [691] or 1/18[¯691] type. It is also shown that while the invisibility criterion can be effectively applied to distinguish the sequence from the shear faults in M18R martensite, the same criterion is insufficient to distinguish between the two types of faults in N18R martensite. The sequence faults are considered to be formed during the matrix ( β 1) to martensite ( β′ 1) transformation and this possibility is discussed using different mechanisms proposed in the literature for the martensitic transformation.