Discontinuous precipitation in CuCo alloys

This contribution deals with the cooperative discontinuous growth of coherent rod-like Co particles from supersaturated copper-rich CuCo solid solutions, and the subsequent spheroidization of these rods into columnar arrays. Electron microscopy has been used to determine the precipitate morphology, size, and distribution. A model is presented for the discontinuous reaction in which a local equilibrium is assumed between the product rods and a uniformly depleted matrix. It is concluded that two major kinds of forces act on the grain boundary during rod growth : A chemical force, predominant at lower temperatures, and a precipitate traction, thought to be available at higher temperatures, where the chemical force is dissipated by volume diffusion. The apparent mobility of the grain boundary, as inferred from this work is low; this is attributed to the presence of a general coherent precipitate, which forms ahead of the reaction front. The kinetics of the rod spheroidization process are also considered. and shown to be consistent with the present observations.     

Morphology and chemical nanoanalysis of discontinuous precipitation in MgAl alloys—I. Regular growth

Discontinuous precipitation in MgAl alloys has been studied both by conventional TEM (morphology) and STEM with local chemical analysis (concentration profiles). Numerous growth defects have been identified. In the case of regular growth, a detailed analysis of concentration profiles using Cahn's solution has been performed, proving the necessity to introduce three velocity scales in order to describe the overall kinetics. The thermodynamical balance of the phenomenon has been derived by applying Hillert's model.     

Experiments on the aging of Sn–Ag–Cu solder alloys

Abstract

The microstructural stability of the Sn–3·8%Ag–0·7%Cu solder alloy was investigated by studying microstructural changes caused by heating small samples for various times, up to 1000 h, at 150°C. The first change, evident at high magnification after heating for 1 h, occurred from the as cast lamellar plus fibrous form of the Ag₃Sn and Cu₆Sn₅ interdendritic eutectic phases to a particulate form. With further heating, coarsening of the two compound phases occurred, gradually rendering the Sn dendrite pattern less distinct. Due to the very rapid diffusion of Cu in solid Sn, the Cu₆Sn₅ phase coarsened most rapidly, growing from its originally finely divided (200 nm) size in the ternary eutectic to form many particles up to 3 m m or more in size in a time of 100 h. At that time, nearly 50% of the total Cu was contained in these particles. The Ag₃Sn phase coarsened more slowly. Approximate measurements of average particle size as a function of time suggested that coarsening occurs by Ostwald ripening, controlled by diffusion in the Sn phase.     

Coherent precipitation effect on thermo-power of AlCu alloys

In situ thermoelectric measurements of AlCu alloys are described for several types of heat treatments. The thermoelectric power shows deviations from the solid solution matrix during the sequence of precipitations occurring in these alloys (i.e. GP zones, θ′',θ′ and θ). The sign and the amplitude of these deviations can be modeled on the basis of the HEW theory for the resistivity maximum. Extension to the diffusion thermopower displays a size and morphology (isotropic, platelike) effect in the electron Bragg scattering by the coherent precipitates, corresponding rather well with the experimental data.     

Kinetics of cellular dissolution in an AlZn alloy

Dissolution of the lamellar precipitate by cell boundary migration has been studied in an Al-18.9at.% Zn alloy in the temperature range 554–637 K. Microstructural observations have revealed that the process of dissolution in this alloy is cellular mode of transformation in the early stages. The boundary diffusivities were calculated by using the theory of Petermann and Hornbogen modified for cellular dissolution. The diffusivities calculated from the experimental data are four orders of magnitude higher than the volume diffusivities. From the Arrhenius plots, activation energies of 51.5 ± 2 and 47.5 ± kJ mol⁻¹ were obtained from the temperature dependence of diffusivity and mobility respectively. These values are about half the activation energy required for volume diffusion of Zn in Al and compare very well with the activation energy of 60 kJ mol⁻¹ for the cellular precipitation in this alloy system. There exists a range of temperature between 502 and 532 K where the two kinetic processes, cellular precipitation and dissolution, are equally probable. The forward migration of the grain boundary during cellular precipitation is acted upon by the back pull of dissolution and the migrating grain boundary remains motionless.     

Mechanisms and kinetics of θ' dissolution in Al-3% Cu

In situ hot stage studies of θ' dissolution at 370°C in Al-3% Cu, bulk aged before thinning, have been carried out with the aim of determining the mechanisms and kinetics of θ' dissolution. An important observation concerns the presence of dislocations linking semicoherent θ' precipitates to both other θ' precipitates and to the growing incoherent θ phase. The dislocation distribution changes continuously during dissolution, with climb excursions taking place as the θ' fragments and dissolves. In situ studies on specimens having undergone deformation prior to thinning reveal that the dissolution rate of θ' was reduced, suggesting that dissolution is controlled by a vacancy flux to accommodate the large local volume change accompanying the transformation. This mechanism is in accord with the observed linear relationship between precipitate radius and dissolution time. The addition of 0.05% Cd substantially reduces the θ' dissolution rate, and possible reasons to account for this observation are discussed.     

Observations of the effect of an applied stress on the morphology of discontinuous precipitation in a CuCd alloy

The influence of an applied tensile stress on the morphology and growth rate of the discontinuous precipitation product in a Cu−3.8 wt% Cd alloy was first reported by Sulonen. In this contribution, we investigate and further quantify a number of factors associated with the role of applied stress in this alloy, including absolute and relative growth rates of interfaces with normals parallel to and perpendicular to the tensile axis, and the morphological stability of discontinuous precipitation front as a function of the applied stress. Once morphological instability has occurred, the amplitude and wavelength of the protuberences formed depends on the value of the applied stress, and on the angle between the average interface normal and the tensile axis. It is suggested that the process is best viewed as one of transformation-assisted viscoelastic deformation.     

A study of precipitation in interstitial alloys—IV. The precipitation sequence in Pt-C

A systematic transmission electron microscopy study of the entire carbide precipitate nucleation and growth process in quenched, aged platinum has been made. Five distinct stages have been identified in the growth sequence, four coincide with changes in the Burgers and displacement vectors and the fifth in the configuration of the precipitates. Diffraction contrast analysis has shown that all the precipitate platelets lie in {001} matrix planes, are vacancy in character and are semi-coherent. Precipitates form initially by co-precipitation of vacancies and carbon atoms and the growth sequence follows a ripening type process. At intermediate aging temperatures voids become the preferred growth sites presumably acting as vacancy sources. Various aspects of the precipitation reaction have been explained from the standpoint of the role of vacancies (V), and the system can be treated either as a Pt-C binary or a Pt-C-V ternary alloy.     

Kinetics of the amorphous to icosahedral phase transformation in AlCuV alloys

Single phase icosahedral samples are obtained by annealing melt-spun Al₇₅Cu₁₅V₁₀ amorphous alloys. The kinetics of this amorphous to icosahedral phase transformation were measured isothermally and nonisotheramally by differential scanning calorimetry and from changes in the electrical resistivity. Transmission electron microscopy and energy dispersive X-ray analysis indicate that the transformation proceeds polymorphically by nucleation and growth, ruling out a “micro-quasicrystal” model of the glass in this system. A standard Johnson-Mehl-Avrami analysis of the isothermal, transformation data yields Avrami exponents in the range 2–2.5, which are inconsistent with a polymorphic transformation. These anomalous Avrami exponent arise from an inhomogeneous distribution of quenched-in nuclei. Fits are made to a kinetic model assuming a constant nucleation rate and growth on these quenched-in nuclei. An analysis of the nucleation rates obtained from these fits gives an estimate for the interfacial energy between the icosahedral phase and the glass of 0.002 J/m² ⩽ α ⩽ 0.015 J/m², demonstrating that the short range order must be similar on both sides of the interface.     

Cold Sintering of Fe–Ag and Fe–Cu Nanocomposites by Consolidation in the High-Pressure Gradient

Russian Journal of Non-Ferrous Metals - The results of fabricating dense Fe–Ag and Fe–Cu nanocomposites from mixtures of powders consolidated by high-pressure cold sintering and from...     

Density and Dynamic Viscosity of Sn,Sn–Ag,and Sn–Ag–Cu Liquid Lead-Free Solder Alloys

Powder Metallurgy and Metal Ceramics - Traditionally, the soldering process was carried out, applying mainly lead-based solder materials. However, the prohibition against using lead (Pb) in...     

Mechanisms of formation of hardening precipitates and hardening in aging of Al–Li–Cu–Mg model alloys with silver additions

The mechanisms of the influence of silver additions on the phase transformations that occur in aging are revealed. The contribution of Ω'-phase particles to the deformation stress in Al alloys is estimated. The mechanisms of the effect of low (up to 0.5 wt %) silver additions and the copper content on the structure of the Ω'-phase precipitates in Al alloys are found. According to the proposed model, silver atoms remain immobile during the decomposition of a solid solution and nucleation centers of the Ω' phase form near them in low-temperature aging. Upon hardening aging, fragmented Ω'-phase particles intersect with each other, and the contribution of the intersection regions to the hardening of alloys by Ω'-phase particles is principal.     

Misoriented dislocation substructures and the fracture of polycrystalline Cu–Al alloys

The evolution of the dislocation substructure in polycrystalline Cu–Al alloys with various grain sizes is studied during deformation to failure. A relation between the fracture of the alloys and the forming misorientation dislocation substructures is revealed. Microcracks in the alloy are found to form along grain boundaries and the boundaries of misoriented dislocation cells and microtwins.     

Grain boundary migration with precipitation and dissolution of a liquid phase in MoNi alloy

When 85Mo15Ni (by weight) alloys prepared by liquid phase sintering at 1380°C are heat-treated at 1520°C, the grain boundaries and liquid films between the grains migrate, leaving behind them a new MoNi solid solution with Ni content higher than that in the initial solid formed during the liquid phase sintering treatment. The grain boundaries migrating during this discontinuous dissolution of the liquid phase have a flat shape. When the temperature change is reversed by first sintering at 1520°C and subsequently heat-treating at 1380°C, the grain boundaries and liquid films again migrate, with the composition change of the solid reversed. Liquid precipitates form at grain boundaries and migrate with them, their size and total volume increasing during the migration. The grain boundaries have a curved shape and their initial migration rate is considerably higher than that during the discontinuous dissolution. This contrasting migration behaviour between the discontinuous dissolution and precipitation is attributed to the coherency strain energy operating locally on both grain boundary segments and liquid droplets, and to the high mobility of fine liquid droplets.