Electron microscopy of internally faulted Cu-Zn-Al martensite

Abstract

Studies of the crystal structure of the β' parent and β' 1, martensite phases in a Cu 68Zn 15Al 17 alloy employing transmission electron microscopy show that a thermoelastic transformation from a DO 3 parent to an 18R martensite takes place. Further experiments revealed that the martensite structure is actually of the M18R type (modified 18R. monoclinic, β = 87.5°). The eighteen layer martensite structure ( AB'CB'CA'CA'BA'BC'BC'AC'AB') can form as a result of periodic shifts on the martensite basal plane in the [100] 18R direction. To effect these displacements partial dislocations with Burgers vector 1 3 [100] 18R are expected at the martensite-parent interface on every third (001) 18R plane. In some cases these shifts per se do not produce an exact invariant habit plane and additional “random” dislocations are required as a part of the inhomogeneous shear of the crystallography theory. The martensite-parent interface was studied in detail using martensite reflections, and these “extra” dislocations with a [100] 18R Burgers vector were observed. Their spacing was determined using a dark field, weak beam technique and the observed value (40 Å) is in good agreement with the magnitude of the inhomogeneous shear (ordered plus random faults) required to produce an invariant plane strain. The identification of interface dislocations with a [100] 18R Burgers vector supports the model proposed by Otsuka and Wayman to explain the thermoelastic nature of internally faulted martensites. However, regularly spaced dislocations, theoretically on every third (001) 18R plane (6.5 Å apart), were not observed.