Premartensitic Behavior and Charge Density Waves in TiNi Alloys

Abstract

Variations of the intermetallic compound TiNi are well-known examples of shape-memory alloys. The addition of Fe, for example, is a method of changing the martensitic start temperature, and thus the shape memory effect, over a wide temperature range. A puzzling aspect of these materials has been the presence of precursive effects. Reports of the appearance of superlattice spots in x-ray studies, of resistivity and susceptibility anomalies, and of changes in crystal structure have appeared without a suitable explanation.

We have recently completed a detailed study of these premartensitic effects in TiNi(Fe) with Fe substituting up to 3.2 at.% for Ni. The premartensitic stage of theses alloys is found to be comprised of two distinct phases, one of which is a rare example of an incommensurate charge density wave (CDW) in a three dimensional metal. Two phase transitions occur, one separating the incommensurate phase from the normal, CsCl high temperature phase, and the other marking the onset of a rhombohedral distortion in which the lattice distorts to accommodate the periodicity of the charge density wave.

At the transition temperature, superlattice spots appear which are approximately 1% removed from the true 1/3 positions. Electron micrographs of a sample using a superlattice spot show antiphase-like domains of diameter ~ 1500 Å which are suggestive of discommensuration domains. However, these domains, once formed do not change size as expected from the theory of discommensurations. Accompanying the appearance of the superlattice spots is a kink in the susceptibility curve, indicating a change in the density of states at the Fermi energy.

Approximately 10 K below the onset of this incommensurate phase, a structural transition occurs to a rhombohedral phase. The nature of the distortion is such that the longer (111) axis now corresponds to exactly one wave length of the charge density wave. Thus, the lattice appears to distort to accommodate the CDW, rather than the more usual change in CDW periodicity. At this transition, a large peak in the specific heat is observed, which shows some hysteresis. The rhombohedral angle decreases abruptly at this temperature to 89.7°. At lower temerpatures, the rhombohedral distortion increases, with the CDW wavelength remaining locked-in. At this transition, electron micrographs show the appearance of needle-shaped domains, each of which has a single rhombohedral axis and which has a twin relationship with the matrix in which they appear. Examination of the superlattice spots within single needles indicates that a single (111) wavevector occurs along the rhombohedral axis and the (110) superlattice is oriented normal to this rhombohedral axis.