Transformation behaviour of a Ti₅₀Ni₄₇Fe₃ alloy I. Premartensitic phenomena and the incommensurate phase

Abstract

The transformation behaviour 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’ behaviour of the Ti₅₀Ni₄₇Fe₃ alloys. The crystal structure of the high-temperature parent phase was confirmed to he the CsCl (B₂) structure with a₀, = 3·02 Å. On cooling from room temperature, 1/3(110) and 1/3(111) superlattice reflections appeared, coincident with the onset of the electrical resistance anomaly (increase). The ‘1/3’ superlattice reflections were observed to intensify with decreasing temperature, and close inspection showed that such reflections deviated slightly from the exact 1/3 positions relative to the parent CsCl structure, suggesting the formation of an incommensurate super-lattice over a 12°C temperature range. The appearance of the ‘1/3’ superlattice reflections is interpreted to be associated with the formation of CDWs. Antiphase-like microdomains ∼ 1500 Å in diameter were revealed in dark-field images obtained using the ‘1/3’ reflections, suggestive of discommensuration-type domains. The above behaviour is considered to correspond to a second-order ‘normal-to-incommensurate’ transition.