Showing papers on "Lattice constant published in 1980"

Microscopic Theory of the Phase Transformation and Lattice Dynamics of Si

Abstract: An ab initio calculation fof the solid-solid phase transformation, static structural properties, and the lattice dynamics of Si is presented. A density-functional pseudopotential scheme is used with the atomic number as the only input. The detailed properties of the diamond to ..beta..-tin transition are accurately reproduced. The phonon frequencies and mode-Grueneisen parameters at GAMMA and X, along with the lattice constant, bulk modulus, and cohesive energy, are calculated and found to be in excellent agreement with experiment.

Thermal decomposition of human tooth enamel

Abstract: Further insight into human tooth enamel, dense fraction (TE), has been obtained by following the change and loss of CO3 2−, OH−, structurally incorporated H2O, Cl−, and, indirectly, HPO4 2− after TE had been heated in N2 or vacuum in the range 25–1000°C. Quantitative infrared spectroscopic, lattice parameter, and thermogravimetric measures were used. Loss of the CO3 2− components begins at much lower temperature (e.g., 100°C) than previously recognized, which has implications for treatments in vitro and possibly in vivo. CO3 2− in B sites is lost continuously from the outset; the amount in A sites first decreases and then increases above 200° to a maximum at ∼800°C (>10% of the possible A sites filled), where it is responsible for an increase ina lattice parameter. A substantial fraction of the CO3 2− in B sites moves to A sites before being evolved, apparently via a CO2 intermediary. This implies an interconnectedness of the A and B sites which may be significant in vivo. No loss of Cl− was observed at temperatures below 700–800°C. Structural OH− content increases ∼70% to a maximum near 400°C. Structurally incorporated water is lost continuously up to ∼800°C with a sharp loss at 250–300°C. The “sudden”a lattice parameter contraction, ∼0.014A, occurs at a kinetics-dependent temperature in the 250–300°C range and is accompanied by reordering and the “sharp” loss of ∼1/3 of the structurally incorporated H2O. The hypothesis that structurally incorporated H2O is the principal cause of the enlargement of thea lattice parameter of TE compared to hydroxyapatite (9.44 vs 9.42A) is thus allowed by these experimental results.

Self-consistent electronic structure of surfaces: Surface states and surface resonances on W(001)

Abstract: The low-temperature value of the tungsten bulk lattice parameter is 5.973 a.u. [N. E. Christensen and B. Feuerbacher, Phys. Rev. B 10, 2349 (1974)]. In our calculation, we used by mistake the value of 5.793 a.u. Our conclusions, however, are insensitive to this 3% difference, as confirmed by the following: (i) The same slab geometry, with a 6% surface layer contraction does not bring any significant difference in the surface electronic structure [M. Posternak, H. Krakauer, and A. J. Freeman, Phys. Rev. B 25, 755 (1982)](ii) Subsequent calculations by Ohnishi et al. [Phys. Rev. B 29, 5267 (1984)] and Mattheiss and Hamann [Phys. Rev. B 29, 5372 (1984)] give results in good agreement with ours; in particular, spectral properties and work function are insensitive to this lattice parameter difference.

Dependence of the phonon spectrum of InP on hydrostatic pressure

Abstract: The first- and second-order Raman spectra of InP have been measured in a diamond anvil cell as a function of hydrostatic pressure up to the phase transition (100 kbar). The Gr\"uneisen parameters of the LO and TO phonons at $\ensuremath{\Gamma}$, and also those of several optical and acoustical phonons corresponding to critical points at the edge of the zone, have been measured. Particular emphasis is placed in the determination of the dependence of the dynamical charge as a function of lattice constant: a nonlinear decrease with decreasing lattice constant is found. The results are interpreted in terms of the bond orbital theory and of a pseudopotential calculation of the dynamical charge as a function of lattice constant.

Characterization of Al-Si-alloys rapidly quenched from the melt

Abstract: Aluminium-silicon alloys with compositions in the range 0 at% to 33.9 at % Si were rapidly quenched from the melt at cooling rates between 106 and 107 K sec−1 using the melt-spinning technique. The resulting ribbons were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and X-ray diffraction methods. Metastable solid solubilities of silicon in aluminium were determined from lattice parameter and DSC data. The values found were strongly dependent on specimen thickness and a maximum of about 5 at % Si was reached for an alloy composition of 15 at % Sl (maximal equilibrium solid solubility of silicon in aluminium is 1.58 at % Si). Discrepancies between published values of metastable silicon solid solubities were related to the interpretation of the lattice parameter data. Alloy composition was shown to determine the lattice parameter of the silicon-rich phase. The crystallite sizes and the lattice distortions in the aluminium-rich and silicon-rich phases were determined by X-ray diffraction line profile analysis. From the aluminiumrich phase only strain broadening was observed whereas the silicon-rich phase gave rise to both size and strain broadening. The origin of the lattice strains was discussed. Changes in solidification behaviour are reflected in the structure parameters measured.

Structure and Properties of the Cholesteric Blue Phases

Abstract: A study of Bragg reflections in both blue phases (BPI, BPII) of two mixtures of chiral and nonchiral biphenyls indicates both phases probably have the same translational symmetry and are either body-centered or simple cubic. The temperature dependence of several Bragg reflections indicates that the BPI-BPII transition may be either continuous or discontinuous and that, given the bcc lattice, the lattice parameter is commensurate with the pitch. These results are discussed relative to existing theories.

Tin(II) oxide: structure refinement and thermal expansion

Abstract: SnO, tetragonal, P4/nmm, a = 3.8029 (5), c = 4.8382 (8) A (neutrons, 293 K), Z = 2. Profile and integrated-intensity refinements at room temperature confirm the regular square-pyramidal coordination of Sn It, with S n O 2 .224(8) /k . Final R for the integrated-intensity refinement was 4 .3% for 28 measured intensities. The thermal expansion is less anisotropic than expected for a layer structure. Introduction. Various forms of tin monoxide, SnO, have been reported (Moore & Pauling, 1941; Donaldson, Moser & Simpson, 1963; Kwestroo & Vromans, 1967). The blue-black modification of SnO is isostructural with the tetragonal form of PbO but it is the only example of a regular square-pyramidal tin(II) coordination. The present neutron study, which was undertaken to check this tin(II) configuration, confirms the previous X-ray powder structure determination (Moore & Pauling, 1941). The sample material was powdered SnO obtained from OSI. High-temperature X-ray powder data were recorded on a C G R diffractometer with a hightemperature device (Barret, Gerard & Watelle-Marion, 1968) using pelleted samples and working under dry nitrogen. The unit-cell parameters were obtained by least-squares refinement from 20 lines (Cu K+~; 5 ° < 0 < 65 °) and were subsequently fitted by least squares by a polynomial of the form a(T) = a o + a~ T + a 2 T 2 where a denotes the cell constant a or c in /k and T the temperature in °C. Neutron diffraction patterns at 293 K were collected on the D1B diffractometer of the Institut Laue-Langevin with 2 = 1.282 A. The sample was inserted in a 10 mm diameter vanadium tube. Data were collected in two steps (2 ° < 20 < 82 ° and 65" < 20 < 145 ° ) of two and a half hours and reduced using conventional ILL programs (Wolfers, 1970). Integrated intensities were determined by fitting the shape of the Bragg peaks to Gaussians. The method of profile-fitting structure refinement (Rietveld, 1969; Hewat, 1973) was applied in the space group P 4 / n m m [tin in 2(c) and oxygen in 2(a)]. The refinement included the scale factor, three half-width parameters, the zero-point correction, the lattice constants, one positional parameter, two isotropic thermal parameters, the preferred-orientation parameter and the peak-asymmetry parameter for peaks below 20 = 30 °. The conventional integrated-intensity refinement used a local program (L'Helgoualch, Fonteneau & Pan0567-7408/80/112763-03501.00 © 1980 International Union of Crystallography 2764 T I N ( I I ) O X I D E netier, 1975) allowing for the refinement of a preferredorientation paramete r G (Rietveld, 1969); the refinement included the scale factor, one positional parameter, two isotropic thermal parameters and the preferredorientation paramete r and was based on 28 measured intensities (42 hkl) . The scattering amplitudes b(Sn) = 6 .2 and b(O) = 5.8 fm were used. The final pa ramete rs are given in Table 1;* there is fair agreement between the results of the two methods, both for the refined parameters and their s tandard deviations. The cell pa ramete rs in the range 273 K to 593 K are as follows (T in °C) : a (/k) = 3 .7984 (6) + 3.31 (36) x 10 -5 T + 7.9 (16) × 10 -8 T 2 c (A) = 4 . 8 3 6 8 (7) + 1 .25 (4) x 10 -4 T + 6.2 (19) x 10 -8 T 2. Discussion. The regular square-pyramidal coordinat ion of Sn II in te t ragonal SnO is confirmed; as expected (Shannon, 1976), the S n O distance (Table 1) is shorter for this regular coordinat ion than the average S n O distances observed in distorted square pyramids : 2 .302 A in NaESn(C204) 3 (Donaldson, Donoghue & Smith, 1976), 2 .312 A in SNC204 (Gleizes & Galy , 1979), 2 .288 A in , S n W O 4 (Jeitschko & Sleight, 1972). However , the most impor tant feature of the SnO structure does not lie in the S n O distance but rather in the O S n O angles: the coordinat ion of Sn n is usually described as being derived from an octahedron distorted by the lone pair (Brown, 1974) which leads, for the fourfold coordinat ion, to two sets of O S n O angles * A list of structure factors has been deposited with the British Library Lending Division as Supplementary Publication No. SUP 35531 (2 pp.). Copies may be obtained through The Executive Secretary, International Union of Crystallography, 5 Abbey Square, Chester CH 1 2HU, England. Table 1. Fina l ref ined p a r a m e t e r s Neutrons a Neutrons b a (A) 3.8036 (12) 3.8029 (5) c (/k) 4.8385 (15) 4.8382 (8) c/a 1.2721 (5) 1.2722 (2) z(Sn) 0.2384 (11) 0.2383 (8) B(Sn) (A z) 1.02 (14) 0.57 (7) B(O) (A 2) 1-06 (13) 0.99 (9) G c 0.063 (16) 0.023 (11) R v, Rw, ~, (%) 4.3, 4.3 4.4 R e , Rwe (%) 10.9, 11.6 d(Sn-O) (/k) 2.224 (8) 2.224 (6) X-ray 3. 804 (6) a 4.826 (8) a 1.269 0-2356 (19) e

Crystalline phase transition in the copolymer of vinylidenefluoride and trifluoroethylene

Abstract: The measurement of the temperature dependence of elastic, dielectric, and piezoelectric constants for the film of 55/45% VDF‐TrFE copolymer suggests that the Curie temperature exists at about 70 °C. The lattice spacing sharply changes at a temperature range from 55 to 75 °C. The remanent polarization is also determined from the D versus E hysteresis loop below 75 °C. It is found that the spontaneous strain in crystal lattice is linearly related to the square of remanent polarization. The transition is ascribed to electrostriction due to dipolar orientation.

Electronic magnetic and cohesive properties of some nickel-aluminium compounds

Abstract: A method developed recently by Williams and co-workers (1979) for the calculation of electronic structure and cohesive properties is applied to Ni3Al, NiAl and Al3Ni and is generalised to make possible the performance of self-consistent spin-polarised calculations which are applied to Ni3Al. Results are given for the densities of states, the magnetic moment and its pressure derivative, lattice constants, bulk modulus and the heats of formation, and are compared with experimental data. In an attempt to isolate dominating binding forces, the heats of formation are decomposed into site- and angular-momentum contributions.

Crystal Structure of the High-Pressure Phase of Tellurium

Abstract: The pressure-induced structural change of tellurium has been studied by powder X-ray diffraction using a diamond-anvil and a cubic-anvil apparatus over the pressure range of 40–100 kbar at room temperature. The diffraction pattern for the high-pressure metallic phase at 45 kbar is interpreted as a monoclinic lattice with lattice constants a =3.104 A, b =7.513 A, c =4.760 A, β=92.71° and V =111.0 A 3 . The coordinates of the four atoms in a unit cell are given by x 1 =0.25, y 1 =0.23, z 1 =0.48; x 2 =0.48, y 2 =0.0, z 2 =0.02, according to the space group C 2 2 , 2a+2a. This atomic arrangement can be viewed as a layer structure. No further structural change was observed up to 100 kbar.

Self-consistent linearized augmented-plane-wave study of the electronic structure and superconductivity of fcc lanthanum under pressure

Abstract: We report the results of a linearized augmented-plane-wave calculation of the electronic structure of fcc La at three lattice constants corresponding to ambient pressure, 50, and 120 kbars. The Kohn-Sham-Gaspar approximation for exchange and correlation is used and the potential is allowed a fully non-muffin-tin form. The f bands lie approx.2--2.5 eV above the Fermi level and are approx.1 eV wide, resulting in a very small (0.05 electrons) localized f occupation. Under pressure the f bands rise and broaden appreciably, resulting in only a slight increase in f occupation. The rigid-muffin-tin approximation for the electron-phonon interaction lambda overestimates the superconducting transition temperature T/sub c/ by 40%, but we find that the drastic increase in T/sub c/ under pressure can be attributed primarily to changes in the electronic stiffness eta. Structural transitions which occur at 25 and 53 kbars may be related to changes in Fermi-surface topology which we find to occur approximately at these pressures.

The crystal structure of phase IV of poly(vinylidene fluoride)

Abstract: The crystal structure of phase IV (polar α or δ phase) poly(vinylidene fluoride) was studied by x‐ray diffraction. Oriented phase IV was obtained by orienting phase II (α phase) and then corona poling at approximately 2 MV/cm. The resulting material is a mixture of phases I, II, and IV. Infrared spectra indicate that phase IV has the same chain conformation (TGTG′) as phase II. The reflections of phase IV can be indexed to an orthorhombic unit cell with lattice constants similar to the phase II lattice constants, a=0.496 nm, b=0.964 nm, and c (chain axis)=0.462 nm. There are two chains in this unit cell. The two chains pack with the chain dipole moments parallel. Three different ways of packing the chains were investigated: both chains up, one up and one down, and a statistical up‐down packing. It was found that to within a 97% confidence level, the chains pack with a statistical up‐down packing.

Space group and lattice constants of Ca10(PO4)6CO3

Abstract: Ca10(PO4)6CO3 is an `apatite' with pseudo-hexagonal symmetry with monoclinic space group Pb and lattice constants a = 9557 (3) A, b ≃ 2a, c = 6872 (2) A and γ = 12036±004°

Energy Band Structure and Lattice Constant Chart of III-V Mixed Semiconductors, and AlGaSb/AlGaAsSb Semiconductor Lasers on GaSb Substrates

Abstract: Variations in energy gap, band structure, and relative dielectric constants were examined in 18 ternary and 15 quaternary III-V mixed semiconductor materials for optical devices. A semiconductor laser constructed with AlGaSb/AlGaAsSb on GaSb emitting a wavelength in the range from 1.3 to 1.7 µm is described. Uniform layers of mixed semiconductors, AlGaSb and AlGaAsSb, were successfully grown on a (111)B-oriented GaSb substrate. Lasing oscillation near 1.2 µm was observed at 77 K, slightly beyond 4.0 kA/cm2, in a hetero-isolation stripe laser with an active layer 1.6 µm thick.

Lindemann melting criterion and the Gaussian core model

Abstract: A series of molecular-dynamics calculations has been carried out to examine melting in the classical Gaussian core model at reduced density ${\ensuremath{\rho}}^{*}=0.2$. The stable crystal form at this density is body-centered cubic. Thermal equilibrium values of mean-square particle displacement have been evaluated over the entire temperature range below melting. As a result the Lindemann ratio $f$ (of rms displacement to lattice spacing at the melting point) was found to be 0.160\ifmmode\pm\else\textpm\fi{}0.005, considerably larger than Shapiro's estimate for several bcc metals. This finding further undermines the Lindemann hypothesis that $f$ should be constant for all solids of a given crystal structure at their melting points.

Observation of a Strongly Enhanced Magnetic Susceptibility of Pd in Au-Pd-Au Sandwiches

Abstract: Exceptionally large increases in the magnetic susceptibility (indicating nearly magnetic ordering) of thin films of Pd sandwiched between thicker Au films have been observed at low temperatures - presumably due to the expansion of the Pd average lattice constant by the Au. The large resultant Stoner factors and the modified paramagnon model of Levin and Valls indicate the possibility of observing p-wave superconductivity in Pd structures with reduced proximity effects.

Martensite lattice changes during tempering

Abstract: An X-ray diffractometer study of martensite formed in an 18 wt pct nickel, 0.98 wt pct carbon austenite single crystal yields the shapes, positions, and integrated intensities of 200, 020, and 002 peaks. Martensite, which forms below − 60 °C, was tempered at successively higher temperatures from −45 to 450 °C. The results show that after subambient aging, during which C atoms in c-oriented octahedral sites have clustered, carbide precipitation starts and small regions (∼30A in the [001]) with negative tetragonality appear. Upon subsequent tempering these are augmented by larger regions which have small positive tetragonality. In this process the “c” lattice parameter changes markedly but the “a” and “b” lattice parameter increase very little. These results indicate the formation of carbon depleted martensite which is coherently strained by the carbide particles. At and just above 100 °C the 200, 020, and 002 peaks all become doublets as the martensite matrix discontinuously breaks free of coherency and becomes highly imperfect ferrite. This change also occurs during the so-called “first stage of tempering.” Further tempering decreases the defect content of this ferrite. The lattice of the martensite is extensively reoriented during tempering just above room temperature. These reorientations probably accommodate the lattice parameter changes described above and may be carried out by movement of twin boundaries.

Temperature dependence of the samarium oxidation state in SmB6 and Sm1-xLaxB6

Abstract: 2014 The cubic lattice parameter temperature dependence of SmB6 between 300 K and 4.2 K as well as the X-ray absorption at the LIII edge measured in the same temperature range, give direct evidence of the average samarium valence change which goes from 2.60 at 300 K to 2.53 at 4.2 K. Previous work claimed that the Sm2+ : Sm3+ ratio was temperature independent in SmB6. As for SmB6 a samarium valence change has been observed with decreasing temperature below 300 K in the Sm0.75La0.25B6 solid solution. J. Physique 41 (1980) 1141-1145 OCTOBRE 1980, Classification Physics Abstracts 78 . 70D Samarium hexaboride 5mB6 has attracted much experimental and theoretical attention in the last few years. SmB6 is a homogeneous mixed valent compound in which the Sm2+ : Sm3+ ratio at room temperature has been estimated to be about 4 : 6 from magnetic susceptibility, Mossbauer resonance measurements, LIII X-ray absorption, and X-ray photoelectron spectroscopy (XPS) [1-10]. Furthermore previous Mossbauer resonance and X-ray absorption experiments performed respectively down to l.l K and 150 K had not indicated significant variations of the SM2, : Sm 3 + ratio [3, 4, 5]. On the other hand, the lattice-parameter measurements carried out previously by us between 300 K and 4.2 K suggested a samarium valence change in SmB6 [5, 11]. In the dynamic (homogeneous) mixed valence state, each rare earth ion can be viewed as fluctuating between two configurations with the 4f shell occupation number differing by one. The charge fluctuation time ip is such that fast measurements like X-ray absorption or XPS detect both configurations separately, while slow measurements such as Mossbauer isomer shift detect only an average time, in the homogeneous mixed valent compounds. In order to determine quantitatively the samarium valence change in SmB6 as a function of temperature, X-ray absorption measurements have been carried out. The results obtained by direct observation will be compared with those deduced from lattice parameter temperature dependence. 1. Experimental. SmB6 has been prepared according to the reaction Density measurements, X-ray and chemical analysis indicate an atomic ratio B/Sm = 6. The cubic lattice parameter temperature dependence of SmB6 and SMO.7.5LaO.2.5B6 have been studied between 300 K and 4.2 K with a X-ray powder diffractometer using a monochromatic CoKa X-ray radiation [ 13]. The X-ray absorption experiments have been performed at the French synchrotron in Orsay (LURE). The X-ray beam is emitted by 1.72 GeV electrons in the ring D.C.I. The radiation was monochromatized with the help of a (220) silicon crystal. The (*) Department of Physics, Tohoku University, Sendai, Japan. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198000410100114100

Lattice constant, bandgap, thickness, and surface morphology of InGaAsP-InP layers grown by step-cooling, equilibrium-cooling, supercooling and two-phase-solution growth techniques

Abstract: Constant composition InGaAsP epitaxial layers can be grown on (100) InP substrates at a constant temperature using the diffusion-limited step-cooling growth technique, and in general, compositionally graded layers result when the diffusion-limited equilibrium-cooling, supercooling, and two-phase-solution growth techniques are used. The lattice constant and energy gap of the epitaxial layers grown using the step-cooling technique are nearly independent of small variations of X p l and the amount of step cooling, but are dependent on growth temperature. The dependence of lattice constant and energy gap of the epitaxial layers on X Ga l and X As l has been determined for the step-cooling and supercool ing techniques.

Interaction between major outer membrane protein (O-8) and lipopolysaccharide in Escherichia coli K12.

Abstract: An ordered hexagonal lattice structure with a lattice constant of about 7nm was reconstituted from outer membrane protein O-8 and/or O-9 and lipopolysaccharide [H. Yamada and S. Mizushima, J. Bacteriol. 135, 1024–1031 (1978)]. Using this reconstitution system, the interaction between O-8 and lipopolysaccharide was studied and the following results were obtained. At least one lipopolysaccharide molecule per one O-8 trimer was required for the hexagonal lattice formation. The lattice constant of the hexagonal lattice formed under the conditions was about 6.5 nm. In the presence of a larger amount of lipopolysaccharide, the lattice constant increased to 7.5 nm, being almost the same as that observed in the dodecylsulfate-treated cell envelope. The lattice constant observed with the heptoseless lipopolysaccharide was also 6.5 nm when the lipopolysaccharide/O-8 ratio was low. Although the increase of the lattice constant was observed upon further addition of the lipopolysaccharide, it was appreciably smaller than that with the wild-type lipopolysaccharide. A hexagonal lattice was also formed when lipopolysaccharide was replaced by an equivalent amount of lipid A or even fatty acids. The lattice constant was the same as that with the wild-type lipopolysaccharide when the lipid/protein ratio was low. However, contrary to the case of lipopolysaccharide, the lattice constant remained almost unchanged even when a larger amount of either lipid A or fatty acid was added to the reconstitution system. A slight increase in the lattice constant was observed only when an extremely large amount of lipid A was added to the reconstitution system. These results suggest that (1) both the lipid A moiety and the polysaccharide moiety of lipopolysaccharide participate in the interaction with the O-8 trimer, (2) in the lipid A moiety the fatty acid region primarily participates in the hexagonal arrangement of the O-8 trimer and the minimum requirement of lipopolysaccharide for the lattice formation is most probably one lipopolysaccharide molecule per O-8 trimer and (3) the polysaccharide moiety, probably both the 3-deoxy-d-manno-octulosonate region and the distal end region including heptose, are involved in an interaction of lipopolysaccharide with the O-8 trimer to give a proper lattice conformation.

Phase Equilibria in the System Fe-Al-O

Abstract: The composition limits for the spinel solid solution Fe(Fe1-x Alx)2O4 were determined experimentally as a function of oxygen pressure at 1280°, 1380°, and 1500°C, using precise lattice parameter measurements. The results were combined with information from the literature to develop self-consistent stability diagrams of P02 vs cation fraction at constant temperatures for the iron-aluminum oxide system.

A New Low Temperature Phase (fct) of Fe–Pd Invar

Abstract: A new fct phase of disordered Fe–Pd Invar alloys was found at low temperatures. The low temperature phase diagram was obtained from X-ray diffraction experiments (4.2 K to 300 K). The fct phase ranges from 28% to 33.5% Pd at 0 K and the temperature dependence of lattice constants was determined. A decrease of the X-ray Debye temperature associated with a lattice softening was observed for 34% Pd in the low temperature region.

InGaAsP quaternary alloys: Composition, refractive index and lattice mismatch

Abstract: Extrapolation schemes for the calculation of InGaAsP material parameters are described. Experimental data on alloy composition, bandgap, lattice parameter and refractive index are presented and compared with calculated values. Refractive index steps between InGaAsP and InP were found to vary significantly with lattice mismatch. Errors in alloy composition of mismatched epitaxial layers, deduced from bandgap and lattice parameter measurements, can be caused by a tetragonal distortion of the crystal lattice.

The addition of Mn and Al to the hydriding compound FeTi: Range of homogeneity and lattice parameters

Abstract: Isothermal sections, at 1000 °C, of the Ti-rich corners of the Mn-Fe-Ti and Al-Fe-Ti ternary phase diagrams have been determined. Particular attention has been paid to the FeTi hydriding compound. Mn and Al both substitute for Fe in the compound; the former up to 27 at. pct, the latter up to 24 at. pct. Analysis of the lattice parameter data suggest that effective atomic radii, in the B2 structure, are Al: 1.38A, Fe: 1.18A, Mn: 1.22A, and Ti: 1.40A.

The Effect of Zirconium on the Isothermal Oxidation of Nominal Ni ‐ 14Cr ‐ 24Al Alloys

Abstract: The isothermal oxidation of Ni-14Cr-24Al-xZr-type alloys was performed in still air at 1100, 1150, and 1200 C for times up to 200 hr. The zirconium content of the alloys varied from 0-0.63 atom percent (a/o). The oxidized surfaces were studied by optical microscopy, X-ray diffraction, and scanning electron microscopy. The base alloy was an alumina former with the zirconium-containing alloys also developing some ZrO2. The addition of zirconium above 0.066 a/o increased the rate of weight gain relative to the base alloy. Due to oxide penetratio, the weight gain increased with Zr content; however, the scale thickness did not increase. The Zr did increase the adherence of the oxide, particularly at 1200 C. The delta W/A vs. time data fit the parabolic model of oxidation. The specific diffusion mechanism operative could not be identified by analysis of the calculated activation energies. Measurements of the Al2O3 scale lattice constants yielded the same values for all alloys.