Stable Fcc Phase Research Articles

Magnetic Properties and Structural Transformation in Copper-304 Stainless Steel Multilayer Materials

ABSTRACTMultilayer foils of Cu-304 stainless steel (304SS) with equal layer thicknesses in the range t=5.0-500 Å and total thicknesses 10-20 μm have been synthesized using magnetron sputtering at ambient substrate temperature. The x-ray diffraction data of as-deposited films show two structural regimes: small thickness (t=5-10 Å) which is characterized by epitaxial FCC growth of 304SS on copper, and large thickness (t=13.5-500 Å) which shows epitaxial FCC 304SS growth near the interface and BCC 304SS growth away from the interface. FCC structured films show very small magnetic moments at room temperature similar to bulk 304SS stable FCC phase. However, a strong magnetic moment is observed for thicker samples due to ferromagnetic metastable 304SS BCC phase. Two opposing transformations occur in the 304 layers as the samples are heated. The first transformation is from the metastable BCC 304SS to the stable FCC phase. This transformation produces a strong drop in magnetic moment and is clearly visible in the large period multilayers which contain high volume fractions of BCC 304SS. The second transformation is from the original FCC phase to a new stable BCC phase in the 304SS near the copper-304SS interfaces.The transformation is produced by diffusion of nickel from the 304SS into the surroundingcopper and the chemical destabilization of the FCC phase which starts near 400 ºC.This transformation produces a sharp increase in magnetic moment. The magnetic signal drops to zero near 675 ºC which is the Curie temperature of ferromagnetic BCC Fe.75 Cr25..

A discussion on the formation of bainite and other precipitates in Cu-Zn and Ag-Zn alloys

Various transformation products appearing in Cu-Zn and Ag-Zn alloys upon aging at temperatures in the α + β and single α-phase range were examined by optical and electron microscopy. Upon aging at relatively low temperature, bainite plates with the 9R structure are first formed and then retransformed to face-centered cubic (fcc). At higher temperature, other transformation products with the fcc structure, such as rod-shaped α and massive α, are concurrently observed with the bainite. Measurements of solute concentration in bainite were performed by using analytical electron microscopy. Bainite plates were found to have lower solute concentration than that of stable α, not only in Cu-Zn but also in Ag-Zn alloys. These results were discussed in terms of the free-energy change in the transformation. The first appearance of the 9R bainite in place of the stable fcc phase in low-temperature aging should be attributed to the “shear” process which lowers the nonchemical energy and diminishes the necessary driving force for the transformation. The measured solute concentration of the bainite is well reproduced theoretically on the view that the bainite is a product of shear transformation combined with the concentration change.

Raman modes of the GeS-type orthorhombic phase of PbTe.

Raman scattering spectra of PbTe films, grown by laser-assisted deposition on a KCl substrate, containing the metastable GeS-type phase of PbTe, are investigated at different temperatures in the range 64--300 K. The Raman-active modes of the orthorhombic GeS-type phase of PbTe are detected in the low-temperature spectra of the films containing the predominantly GeS-type phase. The investigation of films with thick upper layers of the stable Raman-inactive PbTe fcc phase indicates that the Raman spectra of the films at RT are governed by the scattering on uncompensated Te atoms in the undoped samples and by second-order scattering in the doped films.

Pressure coefficient of the PbTe metastable CsCl-type-phase energy gap.

The transmittance and reflectance spectra of PbTe films grown on different substrates (KBr, KCl, ${\mathrm{BaF}}_{2}$) by the nonequilibrium growth technique of laser-assisted deposition are investigated. The films represent two-layer structures: the sublayer, adjacent to the substrate, consists of the high-pressure metastable CeCl-type phase while the upper sublayer is of the stable fcc phase. As the metastable-CeCl-type-phase lattice volume depends on the substrate nature, we investigate PbTe films with CsCl-type phase corresponding to different pressures. The optical energy gaps in the various films are determined from the maxima in the refractive index dispersions, calculated from the transmittance and reflectance spectra. Thus, the energy-gap pressure coefficient in the PbTe CsCl-type metastable phase is obtained.

Study of CoCr films deposited by magnetron sputtering under different nitrogen partial pressures

CoCr alloy thin films are considered to be an applicable perpendicular magnetic recording medium. The existence of nitrogen residual gas during sputtering has significant effects on the structure and properties of CoCr films. With the increasing of N 2, a phase transformation from hcp to fcc structure happens gradually when the partial pressure of N 2 ( P N 2) is in the range of 9.6 × 10 −3 −4.0 × 10 −2 Pa, and a transition from tensile to comprehensive stress takes place. The further increasing of P N 2 stabilizes the fcc phase. Finally, when P N 2 is greater than 0.23 Pa, a nitride appears with a possible form of (Co 1− x Cr x ) 2N (where 0 < x < 1). The existence of N 2 degrades the preferred orientation of crystallines of hcp phase, and has detrimental effects on the perpendicular magnetic properties. According to the experimental results, a mechanism for the effect of N 2 is proposed. The existence of N 2 lowers the surface mobility of deposited atoms, and causes the lattice distortion of grains. The transformation from hcp to fcc phase is the result of the nucleation and expansion of stacking faults. A metastable phase with bct (body-centered tetragonal) structure may exist during the formation of fcc phase because of the existence of lattice distortion. With the increasing of film thickness, the metastable phase transforms gradually to the stable fcc phase.

The PbSe metastable phase. I. The growth mechanism

PbSe and PbSe:Cd films, grown by two different non-equilibrium growth techniques, laser-assisted deposition and electron-beam evaporation, on two different types of substrate, KCl and BaF2, are investigated. The crystal structure of the films is studied by X-ray and reflecting high-energy electron diffraction. On the basis of the structural investigations it is found that the films grown by both non-equilibrium techniques represent heterophase junctions between the metastable CsCl-type phase and the stable FCC phase. The growth mechanism is semiquantitatively discussed in the framework of Sirota's examinations of the formation of the metastable phases. The substrate type is found to influence only the metastable-phase lattice constant, which in turn determines the thickness of the metastable-phase sublayer.

Computer simulation study of free energy barriers in crystal nucleation

We show how relatively standard Monte Carlo techniques can be used to probe the free-energy barrier that separates the crystalline phase from the supercooled liquid. As an illustration, we apply our approach to a system of soft, repulsive spheres [v(r)=ε(σ/r)12]. This system is known to have a stable face-centered-cubic (fcc) crystal structure up to the melting temperature. However, in our simulations, we find that there is a surprisingly low free-energy barrier for the formation of body-centered-cubic (bcc) crystallites from the melt. In contrast, there appears to be no ‘easy’ path from the melt to the (stable) fcc phase. These observations shed new light on the results of previous simulations that studied the dynamics of crystal nucleation in the r−12 system. We argue that the techniques developed in this paper can be used to gain insight in the process of homogeneous nucleation under conditions where direct, dynamical simulations are inconclusive or prohibitively expensive.

Reply to ‘‘Comment on ‘Elastic stiffness coefficients of nickel-iron single-crystal alloys at room temperature’ ’’

Ledbetter’s critical discussion on our work on the elastic stiffness coeffficients of nickel-iron single-crystal alloys at room temperature clearly shows that our conclusion regarding the linear dependence of the shear constants of this alloy crystal on the composition is, in general, not valid throughout the whole range of composition. The linearity is destroyed by the onset of instability of the fcc phase of the alloy crystal at about 45% Ni. However, so long as the alloy crystal remains in the stable fcc phase (from 100% to 45% Ni) shear constants do follow a linear dependence on composition.