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 "1/3" 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 "1/3" 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 "1/3" superlattice reflections are shifted to precise 1/3 positions. The rhombohedral distortion is effected by a homogeneous expansion along <111> directions, allowing the lattice and CDW's to lock-in and become commensurate.