J.E. Epperson

Phase separation in a Ni-12.7 at. pct Al alloy at 550 °C

The phase separation at 550 °C in a Ni-12.7 at. pct Al alloy was investigated by means of small angle X-ray scattering, large angle X-ray scattering, electrical resistivity, and transmission electron microscopy. It was found that an abrupt change in the electrical resistivity and small angle scattering occurred within 15 seconds at 550 °C, following quenching from 1060 °C, and transmission electron microscopy confirmed the presence of small nuclei. However, after this period of rapid change, there was an incubation period of about ten hours in which there was minimal net growth of the nuclei. After the incubation period, growth was accelerated. High angle side bands were observed after a few minutes annealing, and their development closely paralleled the growth of the nuclei and precipitate particles at this temperature. For times beyond the incubation period, small angle X-ray scattering diagrams from single crystals became markedly anisotropic, indicating a quasi-regular spatial distribution of particles. The large and rapid change, in both electrical resistivity and in X-ray small angle scattering, observed after annealing at 550 °C of samples quenched from 1060 °C was attributed to the formation of tiny regions of short range ordered Ni3Al which constitute subcritical nuclei.

Lattice parameters and compositions of γ and γ′ during coarsening in the Ni-Al-Si system: A neutron powder diffraction study

Ternary Ni-base alloys are often used as model systems for studying coarsening in multi-component systems. Ni-Al-Ti, Ni-Al-Mo, Ni-Cr-Si and more recently, Ni-Al-Si alloys have been used for such studies. These alloys allow the coherent precipitation of L1{sub 2} phase (Ni{sub 3}X, designated as {gamma}{prime}) from an F.C.C.-solid solution and thus enable the study of coarsening of precipitates in coherent systems. With the improvement in theoretical understanding of coarsening in multi-component systems, phase composition information (composition of precipitates and matrix) has become essential for proper interpretation of coarsening data obtained from ternary systems. However, such information is not readily available for many ternary systems, including the Ni-Al-Si system. The aim of this paper is two-fold: (1) To present the lattice parameters, temperature dependence of the lattice parameters, and compositions of {gamma} and {gamma}{prime} as a function of alloy composition in the Ni-Al-Si system. (2) To demonstrate the application of neutron powder diffraction and Rietveld analysis to samples that are not ideally suited for powder diffraction. This situation often occurs when powder diffraction techniques are used for engineering applications.

Interaction of Si and Al during interdiffusion in NiAlSi alloys

Binary and ternary Ni-base alloys are often used as model alloys to study phenomena that occur in complex, multi-component Ni-base superalloys. Ni-Si, Ni-Al, Ni-Al-Mo, Ni-Cr-Si, Ni-Al-Ti and more recently Ni-Al-Si systems have been used to study coarsening of {gamma}{prime} L1{sub 2}-type precipitates in Ni-base alloys. One material parameter that is required to comparison of experimental results with theoretical predictions is the interdiffusion coefficient in the case of binary system and the matrix of interdiffusion coefficients in the case of ternary systems. Although some data are available for binary systems, very little information is available on diffusivities in the ternary systems. The current study was specifically initiated to obtain diffusion data in the {gamma}-phase (ternary solid solution Ni(Al, Si)) in the Ni-Al-Si system and to compare this with the diffusion data obtained from the individual binary systems. The interdiffusion coefficients obtained from the binary systems were compared to the effective interdiffusion coefficients obtained in the ternary systems. To facilitate the comparison, the interdiffusion coefficients in the ternary system were obtained along a constant Ni-line in the isotherm and hence only the relative amounts of Si and Al varied along the diffusion path.

A small-angle neutron scattering study of the kinetics of phase separation in a supersaturated Ni-12.5 at. pct Si alloy

The kinetic behavior of precipitation in a supersaturated Ni-12.5 at. pct Si alloy single crystal has been studied by the small-angle neutron scattering (SANS) technique to supplement earlier transmission electron microscopy (TEM) and wide-angle X-ray diffraction (XRD) work. The SANS measurements performed at room temperature on quenched specimens subjected to isothermal anneals at 400, 450, 505, and 550 °C for various amounts of time have revealed the presence of an interference peak in the scattering function. The particle size, determined according to the Guinier approximation, is found to grow in accordance with the diffusion controlled model put forth by Lifshitz and Slyozov, and independently by Wagner. The activation energy for solute diffusion is determined using the rate constants governing the growth of particle size and the variation of the mean interparticle distance. Results are in agreement with the values given in the literature. Transition from an earlier growth stage has been observed, and enhanced diffusion is noted at temperatures below 505 °C; both observations are consistent with the previous X-ray results. The dynamical scaling law appears to be followed by the data obtained in the coarsening stage. A disruption of scaling occurs at the point when the particle growth changes from a parabolic rate behavior to a cubic coarsening rate. Dynamical scaling offers the potential for projecting the service lifetimes for components from experimental measurements carried out over a much shorter time interval. Discrepancies in the size parameters determined by different techniques are discussed.

The short-range order structure of a water-quenched Ni-12.5 At. Pct Si alloy―a synchrotron X-ray diffuse scattering study

A wide-angle X-ray diffuse scattering measurement of a water-quenched Ni-12.5 at. pct Si alloy single crystal was performed using synchrotron radiation. The Cowley-Warren short-range order (SRO) parameters and static displacement terms between atom pairs were obtained. The first seven SRO parameters were fitted in an 8000-atom model, from which it was deduced that no orderedγ′ domains existed in the water-quenched structure, but an enhancement of the C16 and C17 nearest-neighbor configurations did occur as compared to a completely random model. The C16 configuration is the basic structural unit of the L12 ordered structure, and the C17 may be thought of as a faulted variant thereof. These results were confirmed by performing equivalent SRO modeling in a 140,608-atom model. Silicon-silicon nearest neighbors show a repulsive strain of approximately 12 pct, while Ni-Ni pairs contract by less than 1 pct.

The diffuse-scattering method for investigating locally ordered binary solid solutions

Diffuse-scattering investigations comprise a series of maturing methods for detailed characterization of the local-order structure and atomic displacements of binary alloy systems. The distribution of coherent diffuse scattering is determined by the local atomic ordering, and analytical techniques are available for extracting the relevant structural information. An extension of such structural investigations, for locally ordered alloys at equilibrium, allows one to obtain pairwise interaction energies. Having experimental pairwise interaction energies for the various coordination shells offers one the potential for more realistic kinetic Ising modeling of alloy systems as they relax toward equilibrium. Although the modeling of atomic displacements in conjunction with more conventional studies of chemical ordering is in its infancy, the method appears to offer considerable promise for revealing additional information about the strain fields in locally ordered and clustered alloys. The diffuse-scattering methods for structural characterization and for the recovery of interaction energies are reviewed, and some preliminary results are used to demonstrate the potential of the kinetic Ising modeling technique to follow the evolution of ordering or phase separation in an alloy system.

Dynamical Scaling in a Ni-Si Alloy Undergoing Phase Separation

The dynamics of the decomposition processes in a Ni-12.5 at.% Si alloy has been studied by the small-angle scattering (SANS) method in the temperature range between 400°C and 550°C. The mode of phase separation, as well as the dynamical scaling behavior, was examined in accordance with existing theories and with computer modelling studies.

A small angle X-ray scattering study of voids formed in oxygen-doped niobium due to 58Ni+ bombardment☆

Abstract The small angle X-ray scattering was measured from a series of oxygen-doped Nb single crystals which had been irradiated to 50 displacements per atom at 1050 and 1125 K with 4.0 MeV 58 Ni + ions. The diffraction results indicated less perfect ordering of the voids than would have been inferred from earlier transmission electron microscopy observations and this was attributed to a micro-domain structure for the void array. In addition, there was qualitative evidence that a significant portion of the observed small angle scattering resulted from a nearly random component of the microstructure; this was tentatively attributed to intrinsic voids and/or void-oxide complexes in the Nb matrix beyond the near-surface zone damaged by the heavy ion bombardment. This proposal is consistent with the observation of weak small angle scattering from unirradiated Nb-0.006% O single crystals. The mean void size and number density of voids were in fair agreement with published transmission electron microscopy results for Nb-0.006% O, but not in the case of Nb-0.5% O. For the higher oxygen concentration, void number densities lower by a factor of 40–70 were determined from the small angle scattering experiment.