Nature Of Structure Formation Research Articles

Modification of Al–Mg–Si casting aluminum alloys by liquid phase processing with nanosecond electromagnetic pulses

The AA 511 alloy of the Al–Mg–Si system was used as an example to demonstrate that aluminum melt irradiation with nanosecond electromagnetic pulses (NEPs) leads to a significant change in the nature of structure formation during crystallization. It was found that an increase in the frequency of melt irradiation with NEPs is accompanied by the refinement of the alloy structural components, while the greatest grain size reduction of the α-solid solution and intergranular inclusions of the eutectic Mg2Si phase is observed at a NEPs frequency f = 1000 Hz. An increase in the NEPs frequency leads to a significant increase in the concentration of magnesium in the α-solid solution and the fragmentation of Mg2Si phase intergranular inclusions, which is released in the form of compact isolated inclusions when the melt is irradiated at a frequency of 1000 Hz. It was shown that melt processing with NEPs leads to an increase in the Brinell hardness of as-cast specimens, as well as to a significant increase in the microhardness of α-solid solution grains (from 38.21 HV in the initial state to 61.85 HV after irradiation with a frequency of 1000 Hz). It was assumed that the effect of a pulsed electromagnetic field leads to a decrease in the critical values of the Gibbs free energy required to initiate nucleation processes, and to a decrease in the surface tension at the «growing crystal – molten metal» interface, which causes a modifying effect on the alloy structure due to a decrease in the critical size of crystal nuclei.

FEATURES OF THE STRUCTURE FORMATION OF SPODUMENS-CONTAINING GLASS-CRYSTAL MATERIALS AFTER HEAT TREATMENT

The promising areas of application of spodumene–containing glass–crystalline materials in various branches of science and technology, in particular, to increase the defense capability of the state, have been determined. The relevance of the development of lightweight high–strength sitalls based on them, taking into account the aspects of energy saving for obtaining armored elements, has been determined. It has been established that the use of IR spectroscopy is promising for studying the nanostructure of glass–crystalline materials. The compositions of lithium aluminum silicate glasses and the modes of their heat treatment have been developed. The features of the structure formation of spodumene–containing glass–crystalline materials obtained under the conditions of two–stage low–temperature heat treatment have been investigated. Taking into account the analysis of the nature of the IR spectra of chain silicates, the structure of glass–crystalline materials was investigated according to the IR spectra and the nature of structure formation was established depending on the initial composition of the glass. It has been established that the formation of a structural glass network with the participation of tetrahedra [AlO4], [BO4] and [SiO4] and the presence of cybotaxic groups [Si2O6] allows, under conditions of two–stage low–temperature treatment, to provide bulk crystallization of spodumene–containing glass–crystalline materials. It has been established that the formation of the vitrified structure of glass–crystalline materials allows to ensure their high values of Vickers hardness, microhardness and crack resistance and by the presence of a glass phase, which plays the role of a damper, to ensure their high armor resistance. This will increase the efficiency and make it possible to use them as an energy–destructive and energy–absorbing layer in the composition of the armor element «metal alloy – ceramics – sitall». The introduction of spodumene–containing glass–crystalline materials will increase the competitiveness of competitive domestic armored elements for personal protection.

Main process parameters and covers characteristics as criteria for evaluating the effectiveness of the of anodic micro-arc oxidation

The article is devoted to the evaluation of the feasibility and efficiency of modernization of the microarc oxidation process (MAO) by changing the conditions and parameters of sparking, as well as the selection of criteria for assessing the quality and predicting the service properties of formed ceramic-like coatings. The predominant role in changing the nature of structure formation and in ensuring the predicted quality and properties of the oxidized alloy is played by the intensity of sparking, which occurs in the process of electric discharges migrating along the treated surface of the object immersed in the electrolyte. Intensity of sparking determines the conditions for obtaining equidistant coatings: the less stochastic the nature of sparking around the oxidized part is, the more evenly and rapidly the thickness of the formed coating increases and its density is higher. These considerations allowed us offering the option of upgrading method of the anode MAO (AMAO), wherein only the anodic component of the current, causing thereby obtaining thin coatings with extensive porosity by stabilizing the process of arcing through the use of the system of quasi cathodes in the electrolysis bath. In this study, we consider the possibility of controlling the parameters of coatings obtained by the AMDO method in its standard and upgraded versions, by fixing in real time the nature of the distribution of spark discharges, indirect evidence of which is the change in the current density over time and the uniformity of the thickness of the coatings created. Taking into account the influence of micro-arc oxidation on the structural and qualitative features of formed layers, it was assumed that the surface of the coating obtained by the modernized method should have a higher quality, one of the indicators of which is a more smooth and uniform relief. Thus, another criterion for the efficiency of the modernized process can serve as amplitude and step parameters of roughness, which, without allowing an objective assessment of the microprofile of the formed coating, can give more extensive information about the nature of the increase in its thickness and quality changes, determining the level of operational capabilities of metal objects with such a coating.

HOW TO MAKE MAGNESIUM ALLOYS BE RESISTANT TO OPERATIONAL RISKS

The paper studies regularities and mechanisms of structure and phase formation in the surface layers of magnesium alloys when they are processed by method of micro-arc oxidation [MAO]. It has been determined that the same specific features of structure formation, namely: existence of a thin dense inner sublayer and a thicker outer sublayer with developed porosity are common for all types of coatings on the surface of magnesium and aluminum alloys. Such structural state of a protective coating can not be considered as a guaranteed protection against operational impacts, taking into account the fields of their primary application that is aviation construction, automotive construction, instrumentation, building construction, etc. The paper has analyzed the effect of alkaline electrolytes with varying chemical composition due to additions of sodium fluoride or potassium on the structure and properties of these alloys as well as on the level of basic performance characteristics of the layers formed in such electrolytes. On the basis of the analysis a conclusion has been made that it is possible to extend their life-span under operational conditions. It has been revealed that the existing techniques and methods for process control of MAO aluminum and magnesium alloys, particularly processing modes and technological equipment capacity determine a nature of structure formation and changes in a phase composition of the formed coatings.

Halo mass function and the free streaming scale

The nature of structure formation around the particle free streaming scale is still far from understood. Many attempts to simulate hot, warm, and cold dark matter cosmologies with a free streaming cutoff have been performed with cosmological particle-based simulations, but they all suffer from spurious structure formation at scales below their respective free streaming scales -- i.e. where the physics of halo formation is most affected by free streaming. We perform a series of high resolution numerical simulations of different WDM models, and develop an approximate method to subtract artificial structures in the measured halo mass function. The corrected measurements are then used to construct and calibrate an extended Press-Schechter (EPS) model with sharp-$k$ window function and adequate mass assignment. The EPS model gives accurate predictions for the low redshift halo mass function of CDM and WDM models, but it significantly under-predicts the halo abundance at high redshifts. By taking into account the ellipticity of the initial patches and connecting the characteristic filter scale to the smallest ellipsoidal axis, we are able to eliminate this inconsistency and obtain an accurate mass function over all redshifts and all dark matter particle masses covered by the simulations. As an additional application we use our model to predict the microhalo abundance of the standard neutralino-CDM scenario and we give the first quantitative prediction of the mass function over the full range of scales of CDM structure formation.

Interaction between droplets in a ternary microemulsion evaluated by the relative form factor method

This paper describes the concentration dependence of the interaction between water droplets coated by a surfactant monolayer using the contrast variation small-angle neutron scattering technique. In the first part, we explain the idea of how to extract a relatively model free structure factor from the scattering data, which is called the relative form factor method. In the second part, the experimental results for the shape of the droplets (form factor) are described. In the third part the relatively model free structure factor is shown, and finally the concentration dependence of the interaction potential between droplets is discussed. The result indicates the validity of the relative form factor method, and the importance of the estimation of the model free structure factor to discuss the nature of structure formation in microemulsion systems.

Nature of structure formation in heat-resistant alloys �P975ID and �P741NP during hot deformation

1. After repeated deformation in the (γ+γ′)-region at 1100–1150°C, cast billets of alloys EP975ID and EP741NP retain the same type of structure as that which formed upon initial deformation. Repeated deformation of compacted, in distinction from cast, billets of alloy EP741NP produces a more rapid recrystallization with a decrease in the size of the γ-grains in the recrystallized structure. 2. Repeated deformation in the (γ+γ′)-region with an intermediate (γ+γ′)-anneal stimulates recrystallization, and with a γ-anneal retards it. 3. Upon repeated deformation in the γ-region at 1200°C a completely recrystallized structure is produced, independent of the initial structure and type of intermediate anneal. 4. The occurrence of recrystallization during repeated deformation leads to a decrease in the flow stress, σs. The minimum flow stress is found in specimens with a fully recrystallized structure, and the maximum in those in which a polygonized structure deformed during repeated deformation.

Nature of crystallization in the region near a crack with failure of crystallized glass

Results are presented for a study of features of the crystallization process for glass in a zone around a crack. It is shown that the stress field at the crack tip propagating stably in crystallized glass markedly changes the nature of structure formation in this zone. The thermal process for structure formation moves from the region of forming the maximum amount of crystallization centers to a region of maximum growth rate for crystals, which is accompanied by coarsening of the structure, an increase in structural defectiveness, and an increase in roughness of the glass ceramic fracture. A spontaneously propagating crack does not affect the nature of structure formation.