Fcc-hcp Transformation Research Articles

Epitaxial growth of TiN films by N-implantation into evaporated Ti films

Nitrogen ions (N2+) with 62 keV have been implanted into 100-nm-thick Ti films prepared by the evaporation on thermally cleaned NaCl substrates held at room temperature. The epitaxial growth process of resultant TiN films has been studied by transmission electron microscopy, Rutherford backscattering spectrometry, and elastic recoil detection analysis. It has been revealed that the (110)-oriented TiNy is formed by nitriding the (110)-oriented TiHx in the as-deposited Ti film without change of the orientation of the fcc-Ti sublattice, and that the (001)-oriented TiNy and the “rotated” (110)-oriented TiNy rotated by ∼9° with respect to the (110)-oriented TiNy, respectively, are epitaxially formed by the transformation of (03⋅5)-oriented hcp-Ti to (001)-oriented fcc-Ti and by the transformation of (2̄1⋅0)-oriented hcp-Ti to rotated (110)-oriented fcc-Ti. Then, the inheritance of the square atomic arrangement and parallelogram atomic arrangement of hcp-Ti plays a very prominent role in the epitaxy of the (001)-oriented and the rotated (110)-oriented TiNy, as well as the occupation of N in octahedral sites of the fcc-Ti. The increase in the lattice constants of hcp-Ti, especially, the steeper increase of the lattice constant, c, by the N occupation can be considered as one of the driving forces for the hcp-fcc transformation. The growth mechanism of epitaxial TiNy films is discussed.

Epitaxial growth of (001)-oriented titanium nitride thin films by N implantation

The epitaxial growth process of titanium nitride (TiN) films, formed by implanting nitrogen ions (N2+) with 62 keV into 100-nm-thick Ti films grown on NaCl substrates held at 250 °C, has been studied by transmission electron microscopy, Rutherford backscattering spectrometry, and elastic recoil detection analysis. It has been revealed that the (001)-oriented TiNy is epitaxially grown by N implantation into the as-grown (03⋅5)-oriented hcp Ti. The TiNy is formed by the transformation of the hcp Ti to (001)-oriented fcc Ti during the N implantation, partially inheriting the atomic arrangement of the square and/or the octahedron of the hcp Ti, as well as the occupation of N in octahedral sites of the fcc Ti. Strain due to the expansion of the lattice and/or the volume of hcp Ti by N implantation can be considered as one of the driving forces for the hcp–fcc transformation of the Ti lattice. The nitriding mechanism of epitaxial Ti thin films is discussed.

Early nitriding stage of evaporated-Ti thin films by N-ion implantation

The early growth stage of epitaxial titanium nitride (TiN) films, formed by implanting nitrogen ions (N2+) with 62 keV into 100-nm-thick evaporated-Ti films, was studied by transmission electron microscopy, Rutherford backscattering spectrometry and elastic recoil detection analysis. Evaporated-Ti films spontaneously absorb hydrogen (H) from the interior of the NaCl substrate, and then TiHx partially grows in addition to the hcp-Ti. The implantation of N into evaporated-Ti films expands the hcp-Ti lattice and reduces the H concentration in the evaporated-Ti film. The former induces the hcp–fcc transformation and then leads to the growth of (001)-oriented TiNy by the occupation of N in octahedral (O) sites in the fcc-Ti sublattice. The latter induces contraction of the fcc-Ti sublattice by the escape of H from (110)-oriented TiHx and then leads to the growth of (110)-oriented TiNy by the occupation in O sites of the H-escaped metastable fcc-Ti lattice by N. The nitriding mechanism of epitaxial Ti thin films is discussed.

Martensitic phase transformation and lattice dynamics of fcc cobalt.

The phonon dispersion of the high-temperature fcc phase of pure cobalt has been measured by inelastic neutron scattering at 833 K. Transverse phonon branches were measured above and below the Martensitic hcp-fcc transformation in order to learn about an influence of lattice dynamics, if any. No new anomalies could be observed, except those confirming an anomalous decrease of the ${c}_{44}$ (hcp) shear constant when approaching the transformation from below and those confirming a transverse $T{[00\ensuremath{\xi}]}_{\mathrm{hcp}}$ and $T{[\ensuremath{\xi}\ensuremath{\xi}\ensuremath{\xi}]}_{\mathrm{fcc}}$ branch of relative low energy. The small shear constant and results from an analysis of elastic diffuse and Bragg data can be understood in the framework of a dislocation-driven transformation mechanism proposed by Seeger.

The β - 3C to α - 4H transformation in SiC with Al, B, and C additions: Kinetics dominated by growth akin to ostwald ripening

The cubic-to-hexagonal phase transformation upon heating SiC has been a focus and/or precondition of hundreds of research studies. In single (or large) crystal SiC, partial dislocation motion propagates stacking faults to invoke the transformation, analogous to the classic FCC-HCP transformation in Co. The hexagonal phase can exist in >200 different stacking modulations of planes of Si-C tetrahedra, with the α-6H polytype regarded as the high temperature equilibrium phase. (“6” is the repeat stacking in a unit cell.) The propensity of stacking faults in all SiC materials provide the nuclei for the transformation. Most transformation studies of polycrystalline SiC cite the work of Heuer, et al., which describes a transformation similar to that in single crystals. The Heuer model incorporates grain growth by observing the nucleated alpha phase as sandwiched between “sheaths” of beta phase, with the sheaths continuing to grow and “protect” the growing alpha phase throughout the transformation. Grain growth incurred during processing is typically treated as kinetically independent of the transformation.

Nitriding of Evaporated-Ti Thin Films by Ion Implantation

Nitrogen ions ( N2+) with 62 keV have been implanted into 100-nm-thick Ti films evaporated on thermally cleaned NaCl substrates. Unimplanted and N-implanted Ti films have been examined by transmission electron microscopy, Rutherford backscattering spectrometry and elastic recoil detection analysis. The analysis has provided evidence that N-implantation results in the epitaxial formation of NaCl-type TiNy and simultaneously induces the release of H from evaporated-Ti films containing TiHx. The nitriding of evaporated-Ti films is mainly divided into two elemental processes. One is accompanied by the hcp-fcc transformation and the other is not. The formation mechanism for TiNy is discussed.

Theory of the phase diagram of iron and thallium: The Burgers and Bain deformation mechanisms revised.

A theoretical model is proposed for the pressure-temperature phase diagram of iron and thallium. A unified phenomenological and atomistic description is given for the Burgers and Bain deformation mechanisms. The bcc-hcp and bcc-fcc transformations are associated, respectively, with six- and two-dimensional order parameters, which are expressed as periodic functions of the atomic displacements. The fcc-hcp transformation is interpreted as a displacive transition between two low-symmetry phases. A thermodynamic potential describing the three preceding transformations is constructed, and the corresponding theoretical phase diagram, including the triple point, is worked out. It is compared to the experimental phase diagrams of iron and thallium.

Surface relief and the mechanism of a phase transformation

Surface relief of the invariant plane strain type is invariably observed during martensitic transformations. However, some diffusion-controlled transformations also exhibit this effect. The observation of a surface relief effect is therefore no proof for a martensitic transformation mechanism. The same reasoning applies to the formation of groups of variants that accomodate each other's strain. Self-accommodating groups of four variants of martensite plates have been observed in several martensites but they are also found in precipitation reactions. While these observations cannot be used as proof for a martensitic mechanism, neither does the presence of structural ledges in a reaction interface necessitate diffusion. Both these arguments have been used by different sides in a recent controversy (1,2), and the present paper is an attempt to clarify the situation. Starting with the simple example of AlAg, the mechanism of surface relief in a diffusional fcc-hcp transformation and its relation to ledge growth will be examined. Applying the same mechanism to increasingly complex cases the relation to structural ledges, fcc-bcc transformations and finally a general martensite interface will be developed.

Reversible structural transformation in cobalt-base amorphous alloys

A reversible, displacive transformation in the short-range order of cobalt-rich amorphous alloys has been inferred from magnetic measurements. The issues of reversibility and possible phase separation are discussed with new analysis. Comparison is made with magnetic effects accompanying the hcp-fcc transformation in crystalline cobalt and cobalt-rich alloys. An electronic model for the transformation is proposed.

Phase transitions in ytterbium under pressure (semimetal-semiconductor transition)

The semimetal-semiconductor transition in ytterbium has been studied in the pressure range 0.50 kbar and up to 600 degrees C using the Seebeck coefficient as a tool. In the FCC phase the thermoelectric power exhibits a marked increase with pressure and the semimetal-semiconductor transition is observed as a change in the slope. In the semiconducting phase the thermopower initially increases very rapidly with pressure followed by a steep decrease at higher pressures. The temperature coefficient of thermopower in the semiconducting phase increase is large and positive in contrast to the semimetallic phase. The FCC-HCP transformation at high temperatures is reported. A semimetal-semiconductor transition in the HCP phase has also been observed. The experimental results are discussed qualitatively on the basis of a simplified two-band model.