G. Kostorz

Shape and internal structure of Guinier-Preston zones in Ag ☆

Abstract The early stages of decomposition of Al Ag single crystals with 0.3, 3 and 5 at.% Ag were followed by small-angle X-ray scattering (SAXS). The composition, form and structure of the Guinier-Preston (GP) zones were characterized for ageing temperatures ranging from 20 to 250°C. The composition of the GP zones (∼Ag2Al) was found to be nearly independent of the ageing temperature. Below 170°C homogeneous (η-)zones are formed. Above 170°C the (ϵ-)zones have a silver depleted core and a shell of nearly pure silver. The GP zones are spheres faceted mainly along {111} planes. Model calculations allowed the degree of faceting to be determined from the experimental scattering cross sections. For higher ageing temperatures, the result agrees well with predictions based on the modified regular solution model of Alexander et al.

Shape and growth anomalies of γ' precipitates in NiAlMo alloys induced by elastic interaction

In order to explore the possibility of obtaining a system where strong elastic interactions may be observed, two model alloys were produced containing a bimodal size distribution of precipitates. One of the classes is vastly larger than the other, and the values of lattice mismatch relative to the matrix are different for the two classes. The evolution of the microstructure during the decomposition process was studied combining transmission electron microscopy as a rather local probe with small-angle neutron scattering. With these two complementary techniques a statistically reliable determination of the particle size distributions, shape and space correlations of the precipitate ensemble was possible. The influence of the elastic strains on the morphology, on the kinetics of coarsening and on the size distribution of [gamma][prime] precipitates is reported.

Transition between planar and wavy slip in cyclically deformed short-range ordered alloys

Abstract Fatigue experiments in Ni20 at.%Cr and Cu17 at.%Mn single crystals are reported. Both alloys show short-range order but have different solid-solution hardening parameters. A transition from planar to wavy glide is observed in the late stages of fatigue life. In Cu17 at.%Mn extrusions occur. Transmission electron microscopy shows that this is due to the development of three-dimensional dislocation arrays from the planar arrays observed in earlier stages. A similar tendency is found for Ni20 at.%Cr, but the development of extrusions is only seen in the vicinity of cracks. The planarity of the glide and the transition from planar to wavy glide are discussed on the basis of short-range order and the parameters characterizing solid-solution hardening.

Small-angle neutron scattering of precipitates in Ni-rich NiTi alloys—I. Metastable states in poly- and single crystals

Abstract Two successive metastable regimes of decomposition are revealed in Ni(10.8–11.9) at.% Ti single crystals and polycrystals by small-angle neutron scattering (SANS) after various aging steps at 853 K. During the first two hours of aging, the Ti concentration in Ti-rich regions grows linearly with time, while their size remains unchanged. After a few hours, an intermediate plateau in the integrated SANS intensity is reached and indicates an intermediate metastable state. This γ″ state, fully developed after about 12 h, corresponds to coherent non-spherical precipitates containing (18.4 ± 2.5) at.% Ti. After 50–100 h, a further increase in Ti concentration in the precipitates sets in, while they grow and tend to form clusters. After 400–1000 h, a second metastable state, γ′, with (22 ± 2) at.% Ti in the precipitates, is reached. The growth rate in this range is higher than for classical coarsening, and very pronounced spatial correlations of precipitates appear along cubic directions.

Phase separation in Ni-rich Ni–Ti: the metastable states

Abstract The microstructure of Ni-11.3 at.% Ti single crystals was investigated after quenching from the solid solution at 1440 K and after ageing at 873 K for various periods of time by using transmission electron microscopy and X-ray scattering near fundamental reflections and superstructure reflections of the ordered precipitates. The two-stage decomposition sequence with the initial formation of metastable γ″ precipitates followed by metastable γ′ precipitates as found by small-angle neutron scattering is confirmed. The long-range order parameter of these precipitates is always close to the maximum value achievable at a given composition. The presence of the first metastable state (γ″) is related to a rearrangement of the precipitates on a macro-lattice in order to minimize the elastic energy due to coherency strains.

Small-Angle Neutron Scattering of Precipitates in Ni-Rich Ni–Ti Alloys. II. Methods for Analyzing Anisotropic Scattering Data

Methods for analyzing small-angle neutron scattering data for single crystals with anisometric precipitates and anisotropic arrangement are described. A method for projecting the measured scattering intensity in a (110) plane and obtaining the anisotropic correlation function is described. The method uses an indirect Fourier transformation with Hermitian functions as the basis. The correlation function gives model-independent information on the precipitate form and arrangement. A modified paracrystal cluster model is also introduced and size polydispersity is included in a local monodisperse approximation. The size distribution is obtained by least-squares fitting. The correlation function and the cluster model give consistent results for the morphology of precipitates in Ni-rich Ni–Ti single crystals.

Bimodal size distributions of γ′ precipitates in Ni-Al-Mo—II. Transmission electron microscopy

Abstract A double step aging procedure was used to produce bimodal size distributions of precipitates in two Ni -Al-Mo alloys. The evolution of the microstructure during the decomposition was studied using transmission electron microscopy. Shape and changes in the spatial correlations of the precipitate ensemble as well as precipitate size distributions were determined for a very wide time scale. Particle shape transitions during aging were monitored by the determination of aspect-ratio distributions. The results are compared with small-angle neutron scattering analysis in the preceding paper and good agreement is found between the size distributions determined by the two techniques. In the alloy with high coherency strains peculiar phenomena were observed during aging, e.g. coalescence among the small precipitates, preferential growth of small precipitates in the presence of larger ones, and a dramatic narrowing of the size distribution of large precipitates indicating a uniformization in size towards late stages of aging.

Strengthening of cobalt-tungsten alloys upon discontinuous precipitation

Abstract Precipitation and hardening were investigated in cobalt-tungsten alloys containing 5–13 at.% W. A significant hardening up to 860 HV in alloys aged at 600°C is caused by discontinuous precipitation. Transmission electron microscopy shows that as-quenched alloys consist of f.c.c. α-Co grains with frequent twins and martensitic plates. During ageing L12-type order develops within α-Co and simultaneously discontinuous precipitation starts at a grain boundaries. The microstructure consists of hexagonal e-Co and Co3W lamellae as found during in situ high temperature studies in the electron microscope. High resolution electron microscopy reveals coherency strains within a few atomic layers at the e-Co/Co3W interface and nonhomogeneous D19 ordering within the Co3W lamellae, resembling in some places nonstoichiometric B19 ordering.

Bimodal size distributions of γ′ precipitates in Ni-Al-Mo—I. Small-angle neutron scattering

Abstract The evolution of the microstructure during the decomposition of two Ni -Al-Mo alloys with a bimodal size distribution of precipitates was studied using small-angle neutron scattering. Precipitate size distributions and space correlations of the bimodal precipitate ensemble were determined by model fitting of the scattering curves. The correlation effects between the precipitates were taken into account by the locally-monodisperse (LMD) hard-sphere model. This model assumes that the ensemble of precipitates have their positions correlated with their sizes, being locally monodisperse. The form factors were used according to the shape of the precipitates, spherical in the case of alloy I, Ni-8.8Al-9.6Mo (at.%) (isotropic system), and cuboidal for alloy II, Ni-9.5Al-5.4Mo (anistropic system). To characterize the microstructure of the two alloys a set of characteristic parameters (e.g. volume fraction, average radii, precipitate number density, etc.) were obtained using relations between the moments r¯n of the size distribution N(r). The model proved particularly suited to characterize the isotropic system. In the case of the anisotropic system it was insensitive to changes of the size distribution N(r) during aging.

Distortion scattering due to guinier-preston zones in AlAg

Abstract For a decomposing Al-3 at.%Ag alloy the mismatch of Guinier-Preston zones was determined from high-resolution measurements of the diffuse X-ray scattering near the 222 Bragg reflection. It was found that the mismatch of the Guinier-Preston zones is largest directly after quenching and decreases monotonically with ageing time. In the temperature range from 110 to 200°C the rate of mismatch reduction increases with increasing temperature.