Laves phase crystal analysis (LaCA): Atomistic identification of lattice defects in C14 and C15 topologically close-packed phases

The identification of defects in crystal structures is crucial for the analysis of atomistic simulations. Many methods to characterize defects that are based on the classification of local atomic arrangement are available for simple crystalline structures. However, there is currently no method to identify both, the crystal structures and internal defects of topologically close-packed (TCP) phases such as Laves phases. We propose a new method, Laves phase crystal analysis (LaCA), to characterize the atomic arrangement in Laves crystals by interweaving existing structural analysis algorithms. The new method can identify the polytypes C14 and C15 of Laves phases, typical crystallographic defects in these phases, and common deformation mechanisms such as synchroshear and non-basal dislocations. Defects in the C36 Laves phase are detectable through deviations from the periodic arrangement of the C14 and C15 structures that make up this phase. LaCA is robust and extendable to other TCP phases. Graphic abstract

Prediction carbides composition in nickel-based superalloys directional crystallization

Objective. To study the specifics of the distribution of alloying elements in the carbides of the multicomponent system Ni-5Cr-9Co-6Al-1Ti-11.7W-1.1Mo-1.6Nb-0.15C by directional crystallization, using the calculated method of CALPHAD prediction. Research methods. To find regularities and calculate the distribution of alloying elements in the alloy, the latest CALPHAD method was chosen, and modeling of thermodynamic processes of phase crystallization was performed. The obtained results. The results of thermodynamic calculations of the chemical composition of carbides are given in the form of mathematical dependences. The equation of the influence of alloying elements on the dissolution (release) temperature of carbides is obtained. It is shown that the obtained dependences are closely correlated with the thermodynamic processes occurring in the system. Scientific novelty. It is shown that with increasing total concentration of carbide-forming elements, the chemical composition of carbides becomes more complicated. The titanium content of more than 2% leads to an increase in the temperature of the carbide liquidus, and at 4.5 % topologically densely packed phases is formed. When the concentration of molybdenum in the alloy is more than 4%, the probability of precipitating topologically close-packed phases in the structure increases markedly, which negatively affects the mechanical properties and heat resistance. Practical value. Based on an integrated approach for multicomponent heat-resistant nickel-based alloys, new regression models were obtained that allow adequately predict the chemical composition of carbides by the chemical composition of the alloy, which allowed to solve the problem of calculated prediction of carbide composition by chemical composition of the alloy.

Studies on the Oxidation Behavior of a Designed Nanocrystalline Coating on K38 Alloy at 1050 °C

A new framework for a nanocrystalline coating system is established and prepared to study the oxidation behavior with a significant difference in elemental composition. K38 superalloy is selected as a substrate alloy and the composition of the 2nd-generation single-crystal superalloy Rene N5 is used as the sputtered nanocrystalline coating. The oxidation behavior of the newly designed nanocrystalline coating is comparatively studied with the original K38 coating and its substrate alloy at 1050 °C for 500 h. Moreover, microstructure evolution on the interface is used for studying the influence of element interdiffusion behavior on the substrate alloy. Results show that the nanocrystalline coatings increase the oxidation performance of alloys at 1050 °C for 100 h. The sputtered SN-N5 nanocrystalline coating exhibits the best oxidation resistance among the three groups of specimens for 500 h. Interdiffusion occurred and is observed on the SN-N5 coating after long-term oxidation. However, no topologically close-packed phases participated in the substrate alloy.