J. E. Hilliard

Enhanced elastic modulus in composition‐modulated gold‐nickel and copper‐palladium foils

The biaxial elastic modulus Y[111] has been measured by bulge testing in Au‐Ni and Cu‐Pd foils containing short‐wavelength one‐dimensional composition modulations produced by vapor deposition. As compared with homogeneous foils of the same average composition, the modulated foils exhibited an appreciable increase in modulus—from 0.21 to 0.46 TPa for Au‐Ni and from 0.27 to 1.31 TPa for Cu‐Pd. For the latter system, the increase was found to be proportional to the square of the amplitude of the modulation. The enhancement of the modulus decreased with increasing wavelength and for wavelengths greater than 3 nm the modulus was the same as that for homogeneous foils. It was also observed that the deformation was non‐Hookian; the slope of the stress‐strain curves decreased with increasing strain.

Elastic modulus in composition‐modulated copper‐nickel foils

The biaxial elastic modulus Y[111] was measured by bulge testing on Cu‐Ni thin foils containing short‐wavelength composition modulations produced by vapor deposition. In the wavelength range of 1.3–3.0 nm, we observed a twofold increase in the modulus as compared with homogeneous foils. The increase was proportional to the square of the composition amplitude. The maximum modulus occurred at a wavelength of 1.7 nm and an average composition of 45 at. % Cu. The stress‐strain curves of the modulated foils were reversible but non‐Hookean, while homogeneous foils displayed a Hookean behavior with moduli in good agreement with those calculated from the single‐crystal elastic constants of bulk Cu‐Ni alloys.

Elastic modulus in composition‐modulated silver‐palladium and copper‐gold foils

The biaxial elastic modulus Y[111] has been measured by bulge testing in Ag‐Pd and Cu‐Au thin foils containing one‐dimensional composition modulations with wavelengths ranging from 0.9 to 5.5 nm. For the Ag‐Pd foils having wavelengths of 2.0–3.0 nm, the modulus was up to 2.3 times greater than that of homogeneous foils of the same average composition. The increase was found to be proportional to the square of the amplitude of the modulation. In foils containing modulations of wavelengths less than 2.0 nm or greater than 3.0 nm, the modulus was the same as that of homogeneous foils. It was observed that the deformation was elastic but non‐Hookean. For the Cu‐Au modulated foils no modulus enhancement was observed: The modulus for any wavelength was in good agreement with that calculated from the bulk elastic constants.

Effect of Coherency Strains on Diffusion in Copper‐Palladium Alloys

Interdiffusion coefficients have been measured in Cu–Pd alloys containing 70–90 at.% Pd at temperatures from 355° to 440°C using films containing short wavelength composition modulations produced by evaporation. From the relative intensities of the high‐ and low‐angle satellites about the 111 Bragg peak, it was established that the composition modulations were coherent for λ 38 A. Over the range 28 to 38 A, there was a progressive loss in coherency and this was accompanied by a twofold decrease in the diffusivity. This decrease confirms quantitatively the effect of coherency strains on diffusion predicted by Cahn. The dependence of the effective diffusion coefficient on wavelength in the coherent modulations yielded K=−10.3×10−6 erg·cm−1 for the gradient‐energy coefficient.

Mechanical properties of composition modulated Cu‐Ni foils

Measurements have been made of the Young’s, flexural, and torsional moduli of compositionally modulated Cu‐Ni foils. The average composition of the foils was 50‐at. % Ni and the wavelength of the modulation ranged from 8 to 80 A. At a wavelength of 20 A, the flexural modulus exhibited a maximum and had a value two times greater than that measured at shorter or longer wavelengths or that calculated from the bulk single‐crystal elastic constants of a homogeneous alloy. The Young’s and torsion moduli exhibited two maxima at 12 and 28 A. These results are consistent with earlier measurements of the biaxial modulus using a bulge tester. It was also observed that at a wavelength of 20 A there was a threefold increase in the breaking stress of the foils as compared with homogeneous bulk alloys.

Superconducting properties of V/Fe superlattices

Measurements are reported on the superconducting properties of V/Fe superlatitices showing the interplay between ferromagnetism and superconductivity. When the V layer thickness is on the order of the BCS coherence length and the Fe layer is only a few atomic planes thick, a 2D–3D crossover is observed in the temperature dependence of the parallel upper critical field Hc2‖. This implies the coexistence of superconductivity and ferromagnetism in the Fe layers. Three-dimensional behavior for thinner Fe layers is observed (∼1 atomic plane) and 2D behavior for thicker Fe layers (>10 atomic planes).

Determination of the primary elastic constants from thin foils having a strong texture

A new method is described for determining the primary elastic constants (stiffnesses cij’s and compliances sij’s) with respect to the crystallographic axes. Although we will consider only cubic materials the method can be applied to materials of lower symmetry. The method utilizes the symmetry present in a textured (preferred growth) foil and can be used for cases when bulk single crystals are difficult to prepare for ultrasonic experiments. The method involves the use of expressions, that have not been previously derived, relating the biaxial, flexural, Young’s and shear modulus to the primary elastic constants for textured foils. The agreement of the experimentally observed moduli with ones calculated from the expressions to be presented demonstrates that the accuracy is comparable with existing methods.

Interdiffusion in composition‐modulated copper‐gold thin films

Vapor‐deposited Cu‐Au thin films were produced containing composition modulations with wavelengths between 8 and 26 A. The modulations produced satellite peaks in the x‐ray diffraction patterns. Amplification factors and the corresponding diffusion coefficients were obtained by measuring the decay rate of the satellite intensities. The amplification factor reached a maximum at 17 A and decreased at both shorter and longer wavelengths. Interdiffusion coefficients between 10−21 and 10−19 cm2/sec were measured over the temperature range 200–260 °C. The effective diffusion coefficient is linearly dependent on the function B2(λ) for λ≳10 A. From the wavelength dependence of the measured diffusivities, a gradient‐energy coefficient of −4.7×10−6 erg/cm was obtained and is in good agreement with theoretical estimates. These experimental results were compared with the predictions from a proposed model for diffusion on cubic lattices.

Magnetic properties of compositionally modulated thin films

The magnetization density per cm3 of Ni‐content of compositionally modulated Cu–Ni thin films at 5 K was found to depend on the modulation wavelength (λ) and amplitude (A) and reached ∼90% of that of pure Ni. Short‐λ and low‐T studies on Cu–Ni showed an inter‐wavelength coupling of the moments of Ni‐rich regions that may depend on the thickness of the latter relative to that of the Cu‐rich regions. Evidence for xy‐like two‐dimensional ferromagnetism was also observed in Cu–Ni with λ containing only a few Ni‐rich planes. Coercivity and anisotropy were found to depend strongly on the modulation characteristics; the former increased sharply with decreasing λ while the latter had an oscillatory dependence.

A note on compositionally modulated Cu‐Ni films with lattice‐commensurate wavelengths

It has been found that the number and sharpness of the x‐ray satellite peaks from Cu‐Ni films containing short‐wavelength (0.6–11.4 nm) composition modulations (produced by vapor deposition) were strongly dependent on the degree to which the number of monolayers of the components in each period approached integer values. It is demonstrated that this is an important factor in accounting for the difference that has previously been observed between the x‐ray spectra from modulated metallic and semi‐conductor films.