Multiple Precipitates Research Articles

Coupling effect of multiple precipitates on the ductile fracture of aged Al–Mg–Si alloys

The coupling effect of multiple precipitates on the mechanical properties of an Al–Mg–Si alloy was investigated by experiment and modeling. A density-dependent superposition effect of multiple precipitates on yield strength was observed, differing from simple addition. Using a multiscale fracture model, the coupling effect of multiple precipitates on the strain to fracture was also revealed to be a density-dependent superposition of the individual types of precipitate, essentially similar to that for yield strength.

Microstructural Evolution in Orthotropic Elastic Media

We consider the problem of microstructural evolution in binary alloys in two dimensions. The microstructure consists of arbitrarily shaped precipitates embedded in a matrix. Both the precipitates and the matrix are taken to be elastically anisotropic, with different elastic constants. The interfacial energy at the precipitate–matrix interfaces is also taken to be anisotropic. This is an extension of the inhomogeneous isotrpic problem considered by H.-J. Jou et al. (1997, J. Comput. Phys.131, 109). Evolution occurs via diffusion among the precipitates such that the total (elastic plus interfacial) energy decreases; this is accounted for by a modified Gibbs–Thomson boundary condition at the interfaces. The coupled diffusion and elasticity equations are reformulated using boundary integrals. An efficient preconditioner for the elasticity problem is developed based on a small scale analysis of the equations. The solution to the coupled elasticity-diffusion problem is implemented in parallel. Precipitate evolution is tracked by special non-stiff time stepping algorithms that guarantee agreement between physical and numerical equilibria. Results show that small elastic inhomogeneities in cubic systems can have a strong effect on precipitate evolution. For example, in systems where the elastic constants of the precipitates are smaller than those of the matrix, the particles move toward each other, where the rate of approach depends on the degree of inhomogeneity. Anisotropic surface energy can either enhance or reduce this effect, depending on the relative orientations of the anisotropies. Simulations of the evolution of multiple precipitates indicate that the elastic constants and surface energy control precipitate morphology and strongly influence nearest neighbor interactions. However, for the parameter ranges considered, the overall evolution of systems with large numbers of precipitates is primarily driven by the overall reduction in surface energy. Finally, we consider a problem related to the microstructure of fully orthotropic geological materials.

Aging properties of vanadium-bearing high manganese stainless steel

Multiple precipitates have been utilized so as to improve the strength and strain hardening rate of austenite-based low density steel. Thus, a two-step heat treatment process including annealing and aging was designed for a V-containing Fe–20Mn–9Al-1.2C steel. Based on the analysis of microstructures and mechanical properties under different heat treatment processes, the precipitation behavior and strengthening mechanism were investigated. After annealing at 900 °C and aging at 550 °C, the microstructure of the steel consists of austenite matrix with V4C3 particles, κ-carbides and ferrite with DO3 particles. It is found that the steel exhibits an ultimate tensile strength (UTS) of 1339 MPa and specific strength (SS) of 199.3 MPa cm3/g. High strength and strain hardening ability result from the synthetic effect of shearing and Orowan mechanisms from multiple types of precipitates. Despite the low hardening ability, κ-carbides with shearing mechanism could enhance the yield strength significantly. Meanwhile, Orowan type V4C3 particles effectively improves the strain hardening rate as well as the strength. Fine planar slip bands with dispersive dislocation localization contribute to the strengthening of the steel.

The evolution and growth kinetics of precipitate plates growing by the ledge mechanism

The evolution and growth kinetics of precipitate plates growing by the ledge mechanism during solid-solid phase transformations have been studied by employing a previously developed finite-difference-based diffusional growth model. In particular, the effects of ledge density, ledge nucleation kinetics and the presence of multiple precipitates (in parallel groups) on solute buildup in the matrix, morphological evolution, and lengthening kinetics of precipitate plates are analyzed. For solute-poor precipitates, higher ledge densities along the broad faces of precipitate plates produce an increased solute buildup at the broad faces which causes ledge coalescence and can result in decreased plate lengthening rates due to diffusional interaction with the solute field at the plate tip. Two types of ledge nucleation kinetics were considered. One, in which the time interval between ledge nucleation events increases parabolically with time produces precipitate morphologies and plate lengthening kinetics similar to those observed experimentally for proeutectoid ferrite plates. The other, steady state ledge nucleation kinetics, yields growth behavior which, at high undercoolings, does not adequately match experimental observations. For groups of closely spaced precipitates arranged in a periodic array, small precipitate spacings (less than about 100 ledge heights) can lead to decreased plate lengthening kinetics at high supersaturations ( Ω > 0.55).

The Growth Kinetics and Shape Evolution of Precipitates Growing by the Ledge Mechanism

ABSTRACTThe characteristics of ledge growth of precipitates are investigated by a previously developed finite-difference computer model. The plate lengthening simulation results which incorporate measured ledge heights and spacings are in reasonable agreement with the observed lengthening behavior of plates in Fe-C alloys. The simulation which takes into account the diffusion field interaction among multiple precipitates shows that a variety of precipitate morphologies actually observed can possibly be formed solely by the change in the extent of diffusion field overlap among ledges and/or neighboring precipitates. This overlap is in turn caused by changes in the solute supersaturation, distribution of ledge nucleation sites and ledge nucleation rates. A brief discussion is made of the relative growth kinetics of ledged and disordered interfaces.

Electron-dense precipitates in glomus cells of rat carotid body after fixation in glutaraldehyde and pyroantimonate-osmium tetroxide mixture as possible indicators of calcium localization.

An attempt was made to study the subcellular localization of calcium in carotid body glomus cells of adult rats using fixation with glutaraldehyde followed by treatment with a mixture of pyroantimonate and osmium tetroxide. Precipitates were seen as electron-dense particles (EDP) in the glomus cells, mostly within membrane-bound organelles, such as dense-cored vesicles, mitochondria, small clear vesicles, multivesicular bodies, and especially in lysosomes. However, EDP were also seen in the nuclei and in the free cytoplasm of the glomus cells and even outside them. Preincubation of carotid bodies in media containing calcium and either high potassium or calcium-ionophore A 23187 resulted in a marked increase in the general precipitation pattern, there being an increased amount of EDP both in the glomus cell nuclei and in the cytoplasm. Dense-cored vesicles more often showed precipitates than those in the controls. Some dense-cored vesicles contained multiple precipitates, typically located in the electron-lucent area between core and vesicle membrane. Extensive diffusion of ions probably occurred during fixation before precipitation, making the localization of calcium and other precipitating cations unreliable. However, it is possible that precipitates, which were regularly seen in the dense-cored vesicles, may reflect the content of bound calcium. The possible significance of calcium in glomus cell function is discussed, and the need for more adequate methods is emphasized.

Chondrocyte involvement in condylar cartilage calcification utilizing potassium pyroantimonate-osmium tetroxide

The mandibular condylar cartilage of neonatal mice was reacted with K-pyroantimonate to follow the sequence of caiclum transfer through chondrocytes involved in calcification and mineralization. Using this ultrastructural technique, proliferating cells did not show intracellular accumulations of Ca-pyroantimonate whereas chondroblasts revealed a very moderate reaction in the form of multiple precipitates widely disperesed throughout the cells' cytoplasm. The intensity of the reaction significantly increased in young hypertrophic chondrocytes which appeared to localize their intracellular calcium aggregates within their mitochonddral cristae and along the plasmalemma. As chondrocytes became more mature they concentrated the antimonate-calcium complex along the tips of their peripheral processes and subsequently appeared to release these precipitates to an extracellular, mainly matrix vesicles location. Concurrent with the first signs of extra-cellular crystal formation (always in close association with calcified matrix vesicles) hypertrophic chondrocytes consistently became depleted of Capyroantimonate precipitates. Despite the fact that the above histochemical technique did not further elucidate the mechanism involved in the transfer of mitoclondrial calcium to the plasmalemma and subsequently to the matrix vesicles, the present data support the hypothesis that mitochondria of premineralizing hypertrophic chondrocytes play a key role in the calcification of young condylar cartilage undergoing endochondral ossification.

The growth of cadmium mercury thiocyanate and zinc mercury thiocyanate crystals in gels

Experiments relating to the gel growth of cadmium mercury thiocyanate (CHT) and zinc mercury thiocyanate (ZHT) in single crystal form are described. Single CHT crystals of 2–3 mm in size have been successfully grown in silica gels at near ambient temperatures, within 7–10 days. Similar growth conditions have yielded ZHT crystals of needle form 10 mm long, 1 mm wide, and ∼ 0.2 mm thick. The behavior of the system was indicative of a continuous precipitation dissolution cycle. The nature of the multiple precipitates observed in certain experiments is discussed. The aqueous solubility of CHT at 33.6°C has been determined as 1.065 × 10 −2 g/cm 3. Characterization of the crystals in terms of their structure (as determined by X-ray diffraction techniques), morphologies, thermal stability, and chemical purity is described.