In-situ electron microscopy observations of the martensitic transformation and the martensitic phase in thin foils of a Ti50Ni47Fe3 alloy have been made. The Ms temperature in such thin foils is about -98°C. Below the Ms temperature, martensite plates nucleate and grow, and "consume" the previously formed needle-like domains and their matrix. The crystal structure of the martensite was determined to be of the B19' type (monoclinic). In addition to (111) internal transformation twins, stacking faluts on the (001) basal plane and antiphase domain boundaries (APB's) were observed within the martensite plates. The APB's in the martensite, which are a consequence of atomic ordering rather than electron ordering (CDW's), are different from those observed in the premartensitic phases. The "1/3" superlattice reflections characteristic of the prior charge density wave formation are not found in the martensite. The orientation relationships between the martensite, needle domains and matrix rhombohedral phase were also determined. The "premartensitic" effects found in the Ti50Ni47Fe3 alloy are suggested to be separate electronically driven structural changes and not precursory effects insofar as the martensitic transformation per se is concerned. The sequence of transformation events (upon cooling) in the Ti50Ni47Fe3 alloy is as follows : parent phase (B2) incommensurate phase (distorted cubic) commensurate phase (rhombohedral) martensitic phase (monoclinic, B19')
