Thermocyclic treatment (tcт) of metals - way for getting optimum structures and properties

The aim of this work is to study the basic principles of thermal cyclic processing (TCТ) of metals to obtain structures that determine the optimal complex of mechanical properties. The basic provisions of metal heating centers using periodically repeated heating and cooling cycles are given. The TCТ method, as a heat treatment method, is based on constant accumulation from cycle to cycle of positive changes in the structure of metals. Studies have shown that with rapid heating, the growth of austenitic grain occurs slowly and, therefore, heating to high temperatures (up to 10000C) does not lead to an intensive increase in grain. It has been established that grain size increases at a variable heating temperature 3 times slower than under isothermal conditions at the corresponding temperature. Provided that the growth rate of the new phase (austenite) is small and the nucleation rate of grains is significant, it turns out that by the end of the a®g transformation, a fine-grained structure is retained. Further heating or holding at a constant temperature leads to a rapid coarsening of austenite grains. If cooling (for example, in air) of rapidly heated steel is performed 10–150C higher than the temperature of the Ас1 point, then fine perlite grain is formed due to reverse recrystallization. With one thermal cycle, ferrite in subeutectoid steels almost does not undergo changes. But if several such heating and cooling are performed, then the entire ferrite-pearlite structure undergoes a change. It has been established that the higher the heating rate during heating and heating and the less overheating above Ас1, the finer the grain in carbon structural steel. However, this increases the need to increase the number of heat treatment cycles. The mechanism of structure formation explaining these phenomena and practical recommendations on the implementation of the process of the technical and economic process are presented. This approach makes it possible to form the optimal metal structure. At the same time, opportunities can be significantly expanded in terms of obtaining materials with desired properties and improving on this basis machines, structures, individual units and parts. All this puts TCТ in the category of promising areas in metalworking.

Microstructural Evolution during Partial Remelting of 6061 Aluminum Bulk Alloy Prepared by Cold-Pressing of Alloy Powder

The microstructural evolution was investigated during partial remelting of 6061 aluminum bulk alloy prepared by cold-pressing of atomized alloy powders. Meanwhile, the effect of heating temperature on semisolid microstructure was also studied. It was found that after partial remelted, a semisolid microstructure with small and nearly spherical particles can be obtained. The microstructural evolution can be divided into three stages: the rapid coarsening of grains and powders, the structure separation and spheroidization of powders, and the final coarsening behavior of primary particles. For most of the primary particles (larger than 10 μm) in the semisolid state, one particle originates from one original powder in the cold-pressed bulk alloy. Furthermore, proper elevated the heating temperature is beneficial to obtain ideal semisolid microstructure.

Evolution of dendritic nanosheets into durable holey sheets: a lattice gas simulation study

Monte Carlo lattice gas simulations are performed to study sintering in a realistic dendritic platinum nanosheet. The morphological and topological transformations observed in the simulations are in good agreement with sintering experiments. Employing an intuitive method of quantifying surface area, the stability of the surface area of the dendritic nanosheets is analyzed. The surface area is found to have a double exponential decay, one decay corresponding to rapid coarsening of dendritic features into pores and the other decay corresponding to a slow disappearance of unstable pores. Long duration simulations indicate that the thickness of the dendritic nanosheet remains fairly stable. Stability simulations of a single model pore in a sheet establish that there exists a narrow range of sheet thickness and pore size combinations that produces stable holey sheets. Outside this parameter range pores either rapidly close or expand without bound. The thickness of the engineered dendritic platinum nanosheet and the size of the crevices between dendritic arms put the Pt sheet into this stable range, further corroborating the detailed simulations and explaining the persistence of pores observed in actual dendritic platinum nanosheets.

Observation and Mearsurement of Concentration Moments in Rapid Coarsening, Self-Stressed, Au-Ni Thin Plates

The focus of our study is to demonstrate experimentally how elastic stress effects diffusion behavior and coarsening kinetics in a two-phase binary alloy. This work, based on the theory of Cahn and Kobayashi, focuses on elastic stresses in a thin plate. For the case of phase separation with lattice misfit between solute-rich and solute-poor phases, the diffusion of solute distorts the host lattice and causes elastic stress in the matrix. If the plate is sufficiently thin, the stress can cause the plate to buckle. The buckling stress biases the direction of diffusion, which increases the bending stress even further. Thus, the diffusion and the buckling stress are coupled; each affects the other. The interplay between the two is easiest to observe in a solution in which the solute and solvent have high misfit.

Goss Texture Development in Fe–Si Steels

Goss texture development in silicon steels has been studied through EBSP measurements and various computer simulations and calculations. The results of these studies suggest the possible role of high energy grain boundaries (HEGB) in the abnormal growth of Goss grains. The Goss orientation has a fraction of HEGBs that is higher than any other commonly observed orientations in the primary recrystallized silicon steels. The HEGBs have high GB diffusion coefficients which cause rapid coarsening of precipitates on these HEGBs and release them earlier, at the time when other GBs are still pinned. A difference in the mobility between the HEGBs and the other GBs favours the abnormal growth of Goss grains. The Monte-Carlo methods that have been developed and used to validate this assumption have generated abnormally growing Goss grains. The experimentally observed grain boundary character distributions (GBCD) around the growing Goss grains have been reproduced in simulation by assuming high mobility to HEGBs. Apart from the high mobility differences between different GBs, the importance of the fraction of GBs with high mobility around growing Goss grains is realized.