Papers by Keyword: Coarsening

Abstract: The Al-Cu-Mg alloys currently used at elevated temperature for aerospace applications, such as 2618 and 2219, were developed in the 1950s. Since then, not only have property requirements evolved significantly with the widespread introduction of damage tolerant design, but also the understanding and modelling capacity of the alloys' property-composition-processing relationships have developed beyond recognition. Moreover there is a renewed need for higher strength/toughness, higher temperature solutions in many aircraft's hot areas.A kinetic model has been developed to predict the strengthening capability and the thermal stability of hardening phases. It is based on a homogeneous nucleation, growth and coarsening model applied to S' (Al₂CuMg) and θ' (Al₂Cu); the yield strength is then calculated from the precipitates' size distribution. It suggests two areas of interest in the Al-Cu-Mg diagram.Three targeted compositions were then explored inside and outside the areas of interest and their thermal stability assessed up to 250°C. Different behaviours were observed and are explained by the strengthening potential and the coarsening resistance of S' and θ'. The two interesting areas for thermally stability are confirmed. An area of poorer thermal stability was also identified, associated with a high Cu content in solid solution which accelerates precipitate coarsening kinetics.

Abstract: The coarsening behavior of Cu₆Sn₅ grains formed at the Sn37Pb/Polycrystalline Cu interface is investigated in terms of their orientation evolution during solid-state aging. The results show that the coarsening behavior of the interfacial Cu₆Sn₅ grains during solid-state aging are quite different from that during liquid-state soldering. The occurrence of the coarsening in the solid-sate reaction is caused by the different surface energies between two adjacent grains. In addition, the texture morphologies of the interfacial grains affect their coarsening rates. In particular, the interfacial Cu₆Sn₅ grains formed at 200 oC exhibit a texture with the [0001] direction normal to the interface after 16 days of aging at 150 oC, and the formation of this texture accelerates the coarsening process of the interfacial Cu₆Sn₅ grains.

Abstract: Significantly reducing the minimum temperature while maintaining maximum temperature of thermomechanical fatigue (TMF) cycles can reduce the life even when mechanical strain ranges are similar. This applies to in-phase (IP) and out-of-phase (OP) TMF cycles. This reduction in life has generally been attributed to a combination of changes in microstructure arising from aging and increases in the cyclic inelastic strain promoted by increases in the elastic modulus as the minimum cycle temperature is reduced. TMF cycles under both IP and OP conditions were conducted with maximum cycle temperatures within the 750-950C range and with minimum cycle temperatures of either 100 or 500C. A reduction in minimum temperature was observed to promote a decrease in TMF life by as much as a factor of ten for all TMF experiments. The reduction in TMF life is primarily controlled by increases in the inelastic strain range associated with increases in the elastic modulus that arise when the minimum temperature is reduced.

Abstract: Deformation structures and annealing behaviour have been analysed in the centre layer of two AA1050 samples cold-rolled to von Mises strains of 3.6 and 6.4. During annealing at 270-300°C structural coarsening and discontinuous recrystallization occurred in both samples. In the coarsened microstructure, the fraction of high angle boundaries was slightly lower than that in the as-rolled conditions. Recrystallization textures of both samples contained significant fractions of the rolling texture components. The fraction of the retained rolling texture was however greater in the strain-6.4 sample. The {001}<310> and {110}<566> components were also pronounced in this sample. The size of recrystallized grains having orientations of the rolling texture was considerably smaller than the size of grains having other crystallographic orientations. This may be attributed to orientation pinning that hinders growth of grains with orientations of the rolling texture.

Abstract: Some methods for quantitative characterization of the microstructures deformed to large plastic strains both before and after annealing are discussed and illustrated using examples of samples after equal channel angular extrusion and cold-rolling. It is emphasized that the microstructures in such deformed samples exhibit a heterogeneity in the microstructural refinement by high angle boundaries. Based on this, a new parameter describing the fraction of regions containing predominantly low angle boundaries is introduced. This parameter has some advantages over the simpler high angle boundary fraction parameter, in particular with regard to data collected from electron-backscatter diffraction investigations, where boundaries with very low misorientation angles cannot be reliably detected. It is shown how this parameter can be related to the recrystallization behavior. Another parameter, based on mode of the distribution of dislocation cell sizes is outlined, and it is demonstrated how this parameter can be used to investigate the uniformity, or otherwise, of the restoration processes occurring during annealing of metals deformed to large plastic strains.

Abstract: The as-cast and friction stir processed (FSP) hypereutectic AlSi29Fe3 were reheated to the semi-solid state, the coarsening dynamics of the Fe-rich phase during the process of remelting were investigated by the means of quantitatively metallurgical analysis. The results indicated that the coarsening of the Fe-rich phase in semi-solid state for both microstructures followed the Lifshitz-Slyozov-Wagner (LSW) theory:. However, the coarsening exponent n of the Fe-rich phase in the as-cast microstructure was 3, in the FSP microstructure was 2. Therefore, the growth rate of the Fe-rich phase in the FSP microstructure was larger than in the as-cast microstructure. In the meantime, the morphology of the Fe-rich phase changed to short bar from granular in the FSP microstructure.

Abstract: Formation of the patterns of catalytically active metal containing droplets on substrates is an important process for the growing Si wire-like crystals because it predetermines the main crystal parameters. Understanding physical mechanisms of droplet evolution during thermal treatments and derivation of associated parameters are the clue to the controlled formation of droplet ensembles and thus to the predicted growing Si wire-like crystals. In this work, the kinetics of the evolution of droplet ensembles on the surface of substrate as a result of coalescence and atom evaporation is studied theoretically. Obtained theoretical results are compared to the experimental data on the evolution of the ensembles of Au/Si droplets on Si substrate formed by rapid thermal anneals of thin gold film in the temperature range from 900 to 1050°C. The activation energy of droplet diffusion on the surface of substrates is estimated.

Abstract: Phase field modeling of precipitation kinetics in Al – Zr – Sc and Al – Zr – Ti ternary alloys has been performed. The free energy was evaluated using the Thermo-calc data. Our simulations showed that L1₂ precipitates in Al – Zr – Sc alloy consists of Sc rich zone of in core and Zirconium rich zone at the precipitate / matrix interface. In Al – Zr – Ti system, Al₃ (Zr-Ti) precipitates are homogeneous and no segregation is observed. Phase-field simulation results are compared with 3D APT data.

Abstract: We compare three models of 2D precipitation kinetics that give access to different time-space scales. Kinetic Monte Carlo simulations (KMC), cluster dynamics (CD) and nucleation-growth-coalescence model (NGCM), based on a same atomic model, lead to an excellent agreement as long as the interfacial free energy is evaluated accurately and the interaction between diffusion fields is taken into account in the CD. The NGCM model noticeably improves the previous approaches of the same kind by using a constrained-equilibrium hypothesis to describe the solid solution. Moreover, in the coalescence regime, we show that CD leads to cluster distributions that are wider and more symmetric than the LSW distribution due to the probabilistic feature of the growth law of a cluster, that makes it differ from the purely deterministic NGCM approach.

Abstract: In the present paper TiO2 (anatase) nanoparticles were synthesized by ultrasonic radiation proving the potential of this method. The synthesized anatase is heat treated at a temperature of 500°C in open air to coarse it using times from 1 h to 72 h. The heat treatment conditions were selected to prevent phase transformation and to solely sponsor coarsening of anatase from 6.2 nm to 28.3 nm. The synthesized and heat treated anatase were characterized using Electron Microscopy (Transmission and Scanning), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) method, UV-vis, Raman and Infrared spectroscopy. In the preset work is presented an analysis of anatase that is complemented by the agreement in the different characterization methods. This helps in the understanding of anatase coarsening as a function of heat treatment time and grain size, being the late a more suitable approach. This work opens up new perspectives to produce synthetic nanoparticulate anatase with potential for various applications.