Modelling of liquid film migration in Al-Cu alloys

Microstructural evolution in the presence of liquid film migration (LFM) is simulated for Al-Cu alloys using a cellular automaton (CA) model. Simulations are performed for the microstructural evolution and concentration distribution in an Al-4 wt.%Cu alloy with initially equiaxed grain structures holding in a temperature gradient. A slight deviation from local equilibrium, estimated from experimental data, is considered to be the driving force for LFM. The direction of LFM is triggered by concentration fluctuations setting a concentration gradient as a further driving force. The simulation successfully reproduces the experimentally observed microstructures generated by LFM accompanied by a particle free zone behind the liquid film. The solid concentration in the particle free zone is found to be the equilibrium solid concentration. The simulated concentration profile across the migrating liquid film agrees well with experimental measurements. The simulated grain structure becomes coarser and highly elongated after holding in the temperature gradient. The results reveal that the increase in transversal grain width is mainly controlled by LFM, while the grain elongation in longitudinal direction is attributed to both LFM and temperature gradient zone melting. The solid concentration decreases from the initial (supersaturated) composition to the local equilibrium solid concentration corresponding to the local temperature. This article is part of the theme issue 'Transport phenomena in complex systems (part 2)'.

Thermodynamic description of metastable fcc/liquid phase equilibria and solidification kinetics in Al-Cu alloys

The thermodynamic description of the fcc phase in the Al-Cu system has been revised, allowing for the prediction of metastable fcc/liquid phase equilibria to undercoolings of Δ T = 421 K below the eutectic temperature. Hypoeutectic Al-Cu alloys that are prone to pronounced microsegregation were solidified containerlessly in electromagnetic levitation. Solidus and liquidus concentrations were experimentally determined from highly undercooled samples employing energy-dispersive X-ray analysis. Solid concentrations at a rapidly propagating solid/liquid interface were additionally calculated using a sharp interface model that considers all undercoolings and is based on solvability theory. Modelling results (front velocity versus undercooling) were also corroborated by in situ observation with a high-speed camera. A newly established thermodynamic description of the fcc phase in Al-Cu is compatible with existing CALPHAD-type databases. Inconsistencies of previous descriptions such as a miscibility gap between Al-fcc and Cu-fcc on the Al-rich side, an unrealistic curvature of the solidus line in the same composition range or an azeotropic point near the melting point of Cu, are amended in the new description. The procedure to establish the description of phase equilibria at high undercoolings can be transferred to other alloy systems and is of a general nature. This article is part of the theme issue 'Transport phenomena in complex systems (part 2)'.

Plastic straining and concomitant microstructure recrystallization of Ni-Cu alloy in the undercooled condition

Microstructure and microtexture of rapidly solidified undercooled Ni-Cu alloys were investigated. The characteristic undercooling of Ni80Cu20 alloy was determined as 45K, 90K and 160K. Dendrite deformation due to rapid solidification led to strong deformation microtexture. Due to recrystallization upon annealing after recalescence, many subgrains were formed in the microstructure. Further, annealing the quenched alloy at 900°, new microtextures and subgrains were formed, which was due to recrystallization and dislocation network rearrangement. The results of comparative experiment proved the recrystallization mechanism of the microstructure refinement in the non-equilibrium solidification structure of the undercooled binary alloy

SPECIFIC STRUCTURE OF EFFECTIVE MEMBRANE ALLOYS BASED ON NIOBIUM, VANADIUM AND ZIRCONIUM

Аморфные, нанокристаллические мембранные сплавы на основе элементов V группы с уникальными механическими и функциональными свойствами и с матричной дуплексной микроструктурой активно способствуют развитию водородной энергетики. Имеются еще не вполне разрешенные проблемы для этих новых сплавов -их низкая термическая стабильность, недосточная механическая прочность (пластичность, твердость), а также охрупчивание интерметаллидное и гидридное. Для эффективного применения разрабатываются сплавы с тройным составом - в которые помимо элементов V группы входят и легирующие металлы никель и титан. Получают не только аморфные и нанокристаллические сплавы, применимые в электронике и электроэнергетике, а также мембранные сплавы с дуплексной матричной структурой, объединяющей аморфные, так нано- и квазикристаллические дендритно упрочняющие фазы, как упрочняющие аморфную матрицу. В специализируемых мембранных тройных сплавах формируются соединения NiTi и NiTi, стабилизирующие и предохраняющие нано- и кристаллические мембраны от хрупкого разрушения. Установлено, что интенсивное образование гидридов в этих альтернативных мембранных сплавах столь же не желательно, как и для традиционных сплавов на основе палладия. Рассматриваемые сплавы действительно позволяют получить газообразный водород высокой чистоты с применением новых составов взамен дорогостоящих мембран на основе сплавов Pd - Au / Ag / Cu. With unique mechanical and functional properties, amorphous, nanocrystalline and matrix duplex microstructure membrane alloys based on group V elements actively contribute to the development of hydrogen energy. There are still not completely resolved problems for these new alloys - their low thermal stability, insufficient mechanical strength (plasticity, hardness), and intermetallic and hydride embrittlement. For effective use, alloys with a triple composition are being developed - which, in addition to the elements of group V, also include nickel and titanium as alloying metals. Not only amorphous and nanocrystalline alloys are obtained that are applicable in electronics and power engineering, as well as membrane alloys with a duplex matrix structure that combines amorphous, nano-and quasicrystalline dendritic-hardening phases strengthening the amorphous matrix. In specialized membrane ternary alloys, NiTi and NiTi compounds are formed, which stabilize and protect nano-and crystalline membranes from brittle destruction. It has been found that the intense formation of hydrides in these alternative membrane alloys is as undesirable as for palladium-based compounds. The alloys under consideration actually make it possible to obtain high-purity gaseous hydrogen using new compositions instead of expensive membranes based on Pd - Au / Ag / Cu alloys.

Verification of Possibility of Molten Steels Decopperization with ZnAl2O4

In the previous research works, ZnAl2O4 material was considered as one of the solutions for the decopperization process of molten steels; up to 33% of decopperization efficiency was reported by utilising the ZnAl2O4 filter. In order to verify the decopperization possibility of ZnAl2O4 materials, iron-based alloys with various copper and carbon contents were interacted with ZnAl2O4 substrates in a heating microscope under an argon gas atmosphere at 1600 °C. Fe-Cu alloys were found to react with the ZnAl2O4 substrate during the interaction process, and a reaction layer with a complex composition around the alloy droplet was formed; however, Cu was not detected in the reaction layer. Cu was later found diffused inside of the ZnAl2O4 substrates. Furthermore, the Cu-Zn compounds were detected when the copper content in Fe-Cu alloys was 10 wt% Cu. After interaction experiments, copper was decreased in all cases. Thereby, the copper evaporation and infiltration into the ZnAl2O4 substrate were considered as the reasons for copper loss. Moreover, oxygen dissolved in melt was found to have a great effect on the copper evaporation process.

Effect of Short Solution Heat Treatment Time on the Microstructure and Properties of Al-Si-Mg-Cu Alloys

In this work, a solution heat treatment of Al-Si-Mg-Cu casting alloy was analyzed. A new short solution heat treatment (SHT) with only 60 min has been allowed. The results revealed that this short SHT enables the improvement of the dendritic structure and the spheroidization of the eutectic silicon particles. Furthermore, the alloy showed improved mechanical properties when compared to the same alloy subjected to a longer SHT of 4 h. It was observed that increasing the SHT temperature can accelerate the dissolution and homogenization of the silicon particles and intermetallic precipitates in the matrix.