Machine Learning-Based Strength Prediction for Refractory High-Entropy Alloys of the Al-Cr-Nb-Ti-V-Zr System

The aim of this work was to provide a guidance to the prediction and design of high-entropy alloys with good performance. New promising compositions of refractory high-entropy alloys with the desired phase composition and mechanical properties (yield strength) have been predicted using a combination of machine learning, phenomenological rules and CALPHAD modeling. The yield strength prediction in a wide range of temperatures (20–800 °C) was made using a surrogate model based on a support-vector machine algorithm. The yield strength at 20 °C and 600 °C was predicted quite precisely (the average prediction error was 11% and 13.5%, respectively) with a decrease in the precision to slightly higher than 20% at 800 °C. An Al13Cr12Nb20Ti20V35 alloy with an excellent combination of ductility and yield strength at 20 °C (16.6% and 1295 MPa, respectively) and at 800 °C (more 50% and 898 MPa, respectively) was produced based on the prediction.

Effect of Solidification Rates at Sand Casting on the Mechanical Joinability of a Cast Aluminium Alloy

Implementing the concept of mixed construction in modern automotive engineering requires the joining of sheet metal or extruded profiles with cast components made from different materials. As weight reduction is desired, these cast components are usually made from high-strength aluminium alloys of the Al-Si (Mn, Mg) system, which have limited weldability. The mechanical joinability of the cast components depends on their ductility, which is influenced by the microstructure. High-strength cast aluminium alloys have relatively low ductility, which leads to cracking of the joints. This limits the range of applications for cast aluminium alloys. In this study, an aluminium alloy of the Al-Si system AlSi9 is used to investigate relationships between solidification conditions during the sand casting process, microstructure, mechanical properties, and joinability. The demonstrator is a stepped plate with a minimum thickness of 2.0 mm and a maximum thickness of 4.0 mm, whereas the thickness difference between neighbour steps amounts to 0.5 mm. During casting trials, the solidification rates for different plate steps were measured. The microscopic investigations reveal a correlation between solidification rates and microstructure parameters such as secondary dendrite arm spacing. Furthermore, mechanical properties and the mechanical joinability are investigated.

Influence of magnesium on high-temperature structural-phase stability of Al-Ni-La system alloys

The paper considers a relevance of the Al-Ni-La system cast alloys development as promising materials for application at elevated temperatures. The influence of magnesium on the structural-phase characteristics of alloys-representatives with a nickel content of about 2% wt. and lanthanum - about 5,5 and 11,5% wt. were studied in the cast condition and after annealing at 425 ° C for 5 hours. It is shown, that the addition of magnesium in the amount of 0,6 wt%. to alloys with a lanthanum content of 5,5 % wt. helps to increase the size of the lanthanum-containing eutectic component in the cast state, but stimulates its grinding after annealing. Since doubling the lanthanum content, magnesium has almost no effect on the structure of the eutectic in the cast state, but intensifies the process of changing its structure during annealing. In this case, the size of the eutectic components is almost unchanged and can be compared with an undoped alloy. Increasing the magnesium content twice to 1,2% wt. in the alloy with a lanthanum content of 11% wt. leads to a noticeable enlargement of Al11La3 intermetallics. After annealing, this structural component retains the characteristics of a fibrous structure and at the same time increases in size by about half. The magnesium content in the eutectic zones and in the solid solution hardly changes after annealing. The obtained data indicate the possibility of using magnesium as an additional alloying element of cast heat-resistant alloys of the Al-Ni-La system, which is able to simultaneously contribute to their strengthening both under normal conditions and at elevated temperatures. In this case, magnesium, in the amount of about 0,6% wt., also helps to preserve the fine structure of the eutectic components at high temperatures. Keywords: Al-Ni-La, Al-Ni-La-Mg, alloying, structural stability, heat resistance.

PROSPECTS FOR IMPROVING THE PROPERTIES OF SECONDARY FOUNDRY ALLOYS OF THE AL-SI SYSTEM USING THE MODIFICATION PROCESS

The results of analytical studies of the use of modern modifiers for secondary aluminum alloys, which affect the structure of the metal of castings and allow to obtain the necessary physical and mechanical characteristics. It is shown that modifiers influencing the size of the primary grain and the shape of eutectic silicon inclusions are of the greatest interest for the production of castings from secondary silumins. It is shown that according to modern ideas the structure of the metal melt is not homogeneous. In some temperature range, complete mixing of atoms does not occur, and microregions with a short-range structure characteristic of the crystalline phase appear. These formations are called differently: atomic groups, clusters, clots, islands, complexes of atoms, clusters, etc. In the last decade, ultrafine powders of chemical compounds (nanopowders), which act as additional crystallization centers during primary crystallization, have become increasingly used as modifiers of cast alloys.

Increasing the ductility of the Al-Si alloy with low silicon content by deformation-heat treatment

A method of deformation-heat treatment is considered for Al-Si alloy with 4.5 % silicon, which consists in a series of small hot deformations with intermediate annealing. The proposed method allows one to achieve grinding and spheroidization of silicon inclusions, which in the cast state have the form of lamellar excreta at the grain boundaries and significantly reduce the plasticity of the material. Spheroidization, grinding and mixing of inclusions that are achieved during this deformation-heat treatment lead to significantly increase the ductility of the alloy without loss of hardness. The processes of structure change during the deformation of aluminum alloys with low Si content are insufficiently studied. In particular, of interest was the possibility of increasing the ductility of such materials by grinding and mixing silicon inclusions during hot deformation. In this case, to prevent a decrease in ductility due appearance of microconcentrators of stress in the form of acute angles of the crushed silicon inclusions, the deformation was carried out as a series of stages with intermediate annealing. In addition, it was assumed that the cyclic change of temperature in this mode will contribute to the spheroidization of fragments of crushed silicon inclusions by changing the solubility of silicon in solid solution from temperature. It is shown that the proposed mode of deformation-heat treatment of these alloys of the Al-Si system allows to significantly increase their ductility – the critical degree of deposition (deposition before cracking) from 67.8 % in the cast state to – 92.1 %. The hardness of the material can be increased by hardening under cold plastic deformation. In this case, since the material after deformation-heat treatment is more plastic, it has greater reserves for hardening in this way. It is shown that owing to hard plastic deformation, the hardness of the material of samples with 4.5 % Si, which has undergone deformation-heat treatment, can increase to values of 95 ± 17 HV, which is significantly higher than in the cast state. At the same time, the hardness (and, probably, strength) of a similar cast material can also be increased due to hardening, but to lower values – 67 ± 12 HV. Key words: aluminum, aluminum-based alloy, deformation-heat treatment, silicon inclusions, plasticity, hardness.