A. I. Lotkov

Short-wavelength atomic-displacement modulation preceding the B2 → B19′ martensitic transformation in a TiNi-based alloy

Neutron diffraction studies of the fine structure of a Ti49Ni51 single crystal are performed before the onset of the B2 → B19′ martensitic transformation (temperature of the onset of the transformation Ms=200 K). Upon cooling below 460 K, extra reflections are found to form; their positions in the B2-phase reciprocal lattice correspond to the full non-Lifshitz star of the wave vector q=0.225〈112〉. At temperatures below 430 K, 0.45〈110〉 extra reflections appear, corresponding to second-order diffraction effects. In the temperature range 300–460 K, reversible and nonhysteretic changes in the intensities and positions of both types of extra reflections are observed. Analysis of the intensities and spatial distribution of the extra reflections in the B2-phase reciprocal lattice indicates that a displacive superstructure dominated by longitudinally polarized atomic-displacement waves with q=0.225〈112〉 arises in the single crystal in the pretransition temperature range.

On Certain Anharmonic Characteristics of B2-Phase Ti(Ni, Fe) Alloys under Hydrostatic Pressure

It is found that frequencies of the acoustic phonon mode in the B2-phase of Ti(Ni, Fe) alloys decrease prior to the B2 → R transformation as the isothermal hydrostatic pressure is increased. This is shown to be due to the anharmonic character of atomic-interaction forces in the B2 crystal lattice. The coefficient of volumetric expansion is predicted to have negative values in the pressure range studied.

Premartensitic and martensitic transformations in Ti49Ni51 single crystal: Ageing effect

The influence of isochronous annealing on the phase composition, long-range atomic-order parameter S in the B2 phase, premartensitic structure, and martensitic transformation sequence and temperatures in a Ti49Ni51 single crystal was studied by X-ray and neutron-diffraction. Ageing the test alloy was found to bring about concurrent changes in the type of the intermediate shear structure involved and in the sequence of martensitic transormations in the premartensitic temperature range studied. The increase in the order parameter for the B2 phase was most pronounced in passing from the zonal stage of ageing to the growth stage of the Ti3Ni4 precipitates, as revealed by X-ray diffraction. No apparent correlation between the type of the premartensitic structure, R-transformation temperature, and order parameter variations was observed. At the same time, the martensite start temperature into the B19′ phase was directly proportional to variations in S.

EXPERIMENTAL EQUATION OF STATE OF THE SHAPE MEMORY ALLOY TI50NI48FE2 AT 0-8 GPA AND 298 K

The influence of hydrostatic pressures (up to 8 GPa) at 298 K on the pressure curves of the volume and volume elastic modulus of the B2 phase of Ti50Ni48Fe2 single crystals is investigated by an ultrasonic pulse-phase technique. It is proposed on analysis of the results that the alloy undergoes a B2→R martensitic, transition at 4.0–5.4 GPa. It is shown that the universal equation of state accurately describes the pressure dependence of volume over the entire pressure range.

Anisotropy of Young's modulus, shear modulus, and Poisson's ratio for the B2-phase of a Ti50Ni48Fe2 single crystal under hydrostatic compression conditions up to 0.6 GPa

We present calculations of Youngs modulus, the shear modulus, and Poissons ratio as a function of direction in the (100) and (110) planes of the B2 phase of a Ti50Ni48Fe2 single crystal. The calculations were done on the basis of our measurements of the velocities of transverse and longitudinal ultrasonic waves propagating in the indicated alloy at atmospheric pressure and for a hydrostatic pressure of 0.6 GPa at a temperature of 298 K. We have shown that in contrast to the effect of temperature, as the pressure is reduced along the pathway to the martensitic transformation B2 ← R, application of pressure to the alloy found in the same pretransitional state increases the anisotropy of the crystal lattice.

Neutron diffraction studies of the premartensitic transformation B2 → B19′ in Ti49Ni51 single crystals

We have studied structural changes in the high-temperature B2-phase in a large single crystal at temperatures near the premartensitic transformation B2 → B19′. We are the first to observe an extra 1/2 (110) reflection in neutron diffraction patterns taken along the [110]B2 direction as the sample is cooled below 420 K, but still far from the martensite start temperature (Ms=180 K). This extra reflection heralds the formation of long-range order in atomic displacements with wave vectorq=(1/2±δ)[110]2Θ/a. Premartensitic diffraction effects (caused by the development and correlation of lattice waves of atomic displacements with wave vectorsq1′≊2Θ/a[1/3, 1/3, 0] andq1″≊2Θ/1[1/3, 1/3] that were clearly visible in this same single crystal before the martensitic transformation B2 → R, appeared at even lower temperatures with substantially lower intensities.