Premartensitic phases of Ti50Ni47Fe3

Abstract

The addition of small amounts of iron to the shape-memory alloy Ti-Ni suppresses the martensitic transition, permitting a detailed study of ‘‘premartensitic’’ effects. Using x-ray and neutron-diffraction data, specific-heat, resistivity, and susceptibility measurements, we show that two distinct phases appear. The first occurs at $T_I$=232 K in a sample containing 3.2 at. % Fe and is characterized by superlattice reflections near ((1/3),(1/3),(1/3)) and ((1/3),(1/3),0) positions. These are incommensurate, but are located at different relative positions in different zones. We show that these can be indexed as commensurate reflections on a rhombohedral ‘‘ghost’’ lattice, but with the main Bragg peaks centered at the original cubic positions. At $T_r$=224 K for this alloy, a rhombohedral phase appears such that the Bragg peak in the unique direction lies on the ‘‘ghost’’ reciprocal lattice. Thus, the host lattice distorts to accommodate the wave vector of the modulation that produces the superlattice reflections. A model is proposed that involves discommensurations in the lattice strain, analogous to discommensurations in the charge-density-wave phase, to explain the ‘‘ghost’’ lattice effect.