HREM investigation of martensite precursor effects and stacking sequences in Ni-Mn-Ti alloys

Since the discovery of a long-period microtwinned martensite structure in Ni{sub 62.5}Al{sub 37.5} and the possible relationship between the stacking sequence of the close packed martensite and the precursor modulations of the B2 austenite, several attempts have been made to obtain new alloys with other martensite stackings than those found in Ni-Al. Recently a number of new stackings have been reported by Potapov and Udovenko et al. in the Ni-Mn-Ti system. In the present paper some of the alloys used by Udovenko et al. and Potapov et al. were examined by electron microscopy in which high resolution images immediately yield evidence for the exact stacking sequences and their local deviations. This is the case for those compositions in which the martensite transition start temperature (M{sub s}) is above room temperature. Also, since the M{sub s} is below room temperature. As magnetic interactions are found to be far less important than chemical and electronic ones during this transition, the present system is a good candidate for the investigation of the relation between precursors and final stacking sequences as in Ni-Al and the results will be compared with those of the latter.