TRANSFORMATION BEHAVIOR OF Ti50Ni47Fe3 ALLOY :I. INCOMMENSURATE AND COMMENSURATE PHASES

Abstract

The transformation behavior of TiNiFe alloys of nominal composition Ti50Ni47Fe3 has been studied by transmission electron microscopy and diffraction, and electrical resistance measurements, between room temperature and -196°C. Based on the present results and complementary neutron and x-ray diffraction data, charge density wave (CDW) phenomena and associated phase transitions were found to be involved in the premartensitic behavior of the Ti50Ni47Fe3 alloys. The crystal structure of the high temperature parent phase was confirmed to be the CsCl(B2) structure. Upon cooling below room temperature, 1/3(110) and 1/3(111) superlattice reflections appeared, coincident with the onset of an electrical resistance anomaly (increase). The superlattice reflections were observed to intensify with decreasing temperature, and close inspection showed that such reflections deviate slightly from the exact 1/3 positions relative to the parent CsCl structure, suggesting the formation of an incommensurate superlattice over a 12°C temperature range. The appearance of the superlattice reflections is interpreted to be associated with the formation of CDW's. A second premartensitic effect was found to occur approximately 12°C below the onset of the initial normal-to-incommendurate transition. This subsequent phase change involves a structural transition from a distorted CsCl parent to a rhombohedral product, during which the superlattice reflections are shifted to precise 1/3 positions. The rhombohedral distortion is effected by a homogeneous expansion along directions, allowing the lattice and CDW's to lock-in and become commensurate.