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Lobzenko, I.; Wei, D.*; Itakura, Mitsuhiro; Shiihara, Yoshinori*; Tsuru, Tomohito
Results in Materials (Internet), 17, p.100364_1 - 100364_7, 2023/03
High-entropy alloys (HEAs) have received attention for their excellent mechanical and thermodynamic properties. A recent study revealed that Co-free face-centered cubic HEAs carried a potential to improve strength and ductility, which is of high importance for nuclear materials. Here, we implemented first-principles calculations to explore the fundamental mechanism of improving mechanical properties in Co-free HEA. We found that the local lattice distortion of Co-free HEA is more significant than that of the well-known Cantor alloy. In addition, the short-range order formation in Co-free HEA caused highly fluctuated stacking fault energy. Thus, the significant local lattice distortion and the non-uniform solid solution states composed of low- and high-stacking fault regions contribute to improving strength and ductility.
Wei, D.*; Gong, W.; Tsuru, Tomohito; Lobzenko, I.; Li, X.*; Harjo, S.; Kawasaki, Takuro; Do, H.-S.*; Bae, J. W.*; Wagner, C.*; et al.
International Journal of Plasticity, 159, p.103443_1 - 103443_18, 2022/12
Times Cited Count：3 Percentile：80.2(Engineering, Mechanical)Kurihara, Kensuke*; Lobzenko, I.; Tsuru, Tomohito; Serizawa, Ai*
Keikinzoku, 72(7), p.427 - 429, 2022/07
Nanoclusters formed of the Al-Mg-Si alloy affect the aging behavior of the alloy depending on the formation temperature. Since Al, Mg and Si have adjacent atomic numbers, it is difficult to analyze them using the X-ray diffraction method. Therefore, in recent years, Al-Mg-Ge alloys in which Si is replaced with the homologous element Ge have been used. Attempts have been made to analyze the structure of the precipitate. In this study, we quantitatively evaluate the interaction between solute atoms and pores in Al-Mg-Si alloys and Al-Mg-Ge alloys using first-principles calculations based on the density general function theory, and solute atoms. From the viewpoint of bond stability between pores and pores, the precipitation behavior of both alloys was compared and examined.
Tsuru, Tomohito; Lobzenko, I.; Wei, D.*
Modelling and Simulation in Materials Science and Engineering, 30(2), p.024003_1 - 024003_11, 2022/03
Times Cited Count：1 Percentile：59.23(Materials Science, Multidisciplinary)High-entropy alloys (HEA) have been receiving increased attention for their excellent mechanical properties. Our recent study revealed that Si-doped face-centered cubic (FCC) HEAs have great potential to improve both strength and ductility. Here, we carried out first-principles calculations in cooperation with Monte Carlo simulation and structural factor analysis to explore the effect of Si addition on the macroscopic mechanical properties. As a result, Si addition increased the local lattice distortion and the stacking fault energy. Furthermore, the SRO formation in Si-doped alloy caused highly fluctuated SF energy. Thus, the heterogeneous solid solution states in which low and high SF regions are distributed into the matrix were nucleated. This unique feature in Si-doped FCC-HEA induces ultrafine twin formation in Si-doped alloys, which can be a dominant factor in improving both strength and ductility.
Wei, D.*; Wang, L.*; Zhang, Y.*; Gong, W.; Tsuru, Tomohito; Lobzenko, I.; Jiang, J.*; Harjo, S.; Kawasaki, Takuro; Bae, J. W.*; et al.
Acta Materialia, 225, p.117571_1 - 117571_16, 2022/02
Times Cited Count：22 Percentile：99.69(Materials Science, Multidisciplinary)Kurihara, Kensuke*; Lobzenko, I.; Tsuru, Tomohito; Serizawa, Ai*
Keikinzoku, 72(2), p.47 - 53, 2022/02
Nanoclusters formed in Al-Mg-Si alloys affect the aging behavior of the alloys depending on the formation temperature. In the present study, first-principles calculations were carried out to evaluate the two- and three-body interactions between Mg, Si atoms and vacancies in the Al matrix and to estimate the effect of local bond structures on the formation of nanoclusters. Monte Carlo simulations were subsequently performed to investigate the stable structure of nanocluster formed in Al-Mg-Si alloy. We found that Mg-Si bond and Si-Vac bond were stable in Al matrix. The result showed that the solute atoms are easy to aggregate with another type of atoms and that Si atom had a strong attractive interaction with a vacancy. Mg-Si-vacancy three-body bond were more stable than Mg-Si two-body bond and Si-vacancy two-body bond in Al matrix. Therefore, vacancies were strongly trapped within the cluster region due to the stable local bonds composed of Mg and Si atoms which indicates that the nanoclusters in Al matrix were thermally stabilized by the stable bonds between solute atoms and vacancy. In addition, these results suggested that inner bonds of nanocluster played a significant role in not only the thermal stability but also the formation and growth behavior of nanoclusters during aging at low temperatures.
Shiihara, Yoshinori*; Kanazawa, Ryosuke*; Matsunaka, Daisuke*; Lobzenko, I.; Tsuru, Tomohito; Koyama, Masanori*; Mori, Hideki*
Scripta Materialia, 207, p.114268_1 - 114268_4, 2022/01
Times Cited Count：2 Percentile：33.62(Nanoscience & Nanotechnology)This study reports grain boundary (GB) energy calculations for 46 symmetric-tilt GBs in 
-iron using molecular mechanics based on an artificial neural network (ANN) potential and compares the results with calculations based on the density functional theory (DFT), the embedded atom method (EAM), and the modified EAM (MEAM). The results by the ANN potential are in excellent agreement with those of the DFT (5% on average), while the EAM and MEAM significantly differ from the DFT results (about 27% on average). In a uniaxial tensile calculation of GB, the ANN potential reproduced the brittle fracture tendency of the GB observed in the DFT while the EAM and MEAM mistakenly showed ductile behaviors. These results demonstrate the effectiveness of the ANN potential in calculating grain boundaries of iron, which is in high demand in modern industry.
Tsuru, Tomohito; Lobzenko, I.; Yuge, Koretaka*; Aoyagi, Yoshiteru*; Shimokawa, Tomotsugu*; Kubo, Momoji*; Ogata, Shigenobu*
no journal, ,
In the present study, we investigated the short-range order and the core structure of dislocations in body centered cubic (BCC) HEAs using density functional theory (DFT) calculations. Special quasirandom structures (SQS) scheme was employed to mimic randomly-distributed five-component BCC-HEAs with equiatomic fraction. According to the phase stability based on DFT calculations, MoNbTaVW and ZrNbTaTiHf HEAs are taken as the typical cases of energetically stable and unstable BCC-HEAs.
Tsuru, Tomohito; Lobzenko, I.; Shiihara, Yoshinori*; Wei, D.*; Yamashita, Shinichiro; Itakura, Mitsuhiro; 10 of others*
no journal, ,
High entropy alloys (HEAs) are chemically complex single- or multi-phase alloys with crystal structures. There are no major components but five or more elements are included with near equiatomic fraction. In such a situation, deformation behavior can no longer be described by conventional solid solution strengthening model. Some HEAs, indeed, show higher strengthening behavior and anomalous slip. However, the mechanisms of these features have yet to be understood. Dislocation structure and motion should be the key to identify the unique feature of mechanical properties of HEAs. In the present study, we investigated the core structure of dislocations in body centered cubic (BCC) HEAs using density functional theory (DFT) calculations. The Random structure and ZrNbTaTiHf and the SRO structure obtained from the 800 K MC calculation in two BCC-HEA MoNbTaVW was prepared. Then, the energy distribution when the dislocation dipoles were introduced at 135 sites were calculated. We found that the dislocation formation energy is smaller in ZrNbTaTiHf, which has a large difference in MSAD and a large lattice distortion.
Lobzenko, I.; Tsuru, Tomohito
no journal, ,
Lobzenko, I.; Tsuru, Tomohito
no journal, ,
Tsuru, Tomohito; Lobzenko, I.; Han, S.*; Chen, Z.*; Kishida, Kyosuke; Inui, Haruyuki*
no journal, ,
In this study, the effect of the constituent elements on the mechanical properties of the ternary BCC medium entropy alloy (MEA) model was investigated by the first-principles calculation. For NbTiZr with different composition as BCC-MEA, we construct an atomic model with random solid solution and short-range ordered (SRO) structure obtained from Monte Carlo analysis. For each of the Random structure and SRO structure, various bulk properties related to the formation of the SRO, mean square atomic displacement (MSAD), elastic properties, stacking defects, twins, etc. are investigated. The formation energy and distribution of dislocation dipoles were evaluated by first-principles calculation of the dislocation structure, and the influence of Group 4 elements was examined.
Tsuru, Tomohito; Lobzenko, I.
no journal, ,
High-entropy alloys (HEAs) are defined as alloys with a crystal structure consisting of a mixture of five or more elements in high concentration. However, the mechanism of the observed simultaneous increase in strength and ductility and the excellent mechanical properties of some HEAs has not been understood. In the present study, the origin of the mechanical properties of HEA was investigated by atomistic simulations based on first-principles calculations. As a result, the local lattice distortion and the formation of short-range order (SRO) structure were reproduced in alloy systems with FCC structure. It was found that in some alloy systems, a significant change in stacking fault energy for SRO formation contributes to the improvement of macroscopic mechanical properties.
Tsuru, Tomohito; Lobzenko, I.
no journal, ,
Dislocation has been regarded as the essential lattice defect in plastic deformation, especially in metallic materials. The fundamental properties of the dislocation core have a dominant influence on the intrinsic ductility or brittleness of materials. The interaction between dislocations and other crystal defects plays a critical role in determining the mechanical properties of metals. Especially, plastic deformation in BCC metals is achieved by a fundamental motion of screw dislocations through a kink mechanism. In the present study, we evaluated softening/strengthening behavior of dilute and highly-concentrated BCC alloys by the first-principles calculations. The introduction of dislocations within our periodic cell was accomplished by applying a continuum linear elastic theory solution for the periodic dislocation dipole array. Then, we proposed new analytical models describing the kink process of screw dislocations, in which the fundamental properties are evaluated by the electronic structure calculations. The analytical models based on the solid solution and the line-tension model were applied efficiently to predict the fundamental mechanical properties.
Lobzenko, I.; Tsuru, Tomohito
no journal, ,
Excellent mechanical properties of high-entropy alloys (HEA), such as increased strength and high ductility, have recently became a subject of extensive studies. First-principles modeling of HEA is complicated by the essential randomness of the atomic structure, which requires large systems. Therefore, classical molecular dynamics is one of the best tools for studying mechanical properties of HEAs. On the other hand, there is a lack of interatomic potentials due to rather low accuracy of embedded atom type potentials in the case of alloying of many elements in close concentrations. That is why a relatively new approach based on artificial neural networks should be employed to build interatomic potentials for such materials as HEA. Current work discusses two new potentials for medium entropy ternary alloys MoNbTa and ZrNbTa. The technique of machine learning allows effective fitting of the data set calculated using the quantum mechanics approach. We have verified the quality of our potentials by comparing elastic constants values with results of first-principles modeling. Comparing two alloys we found that bulk modulus and elastic constants become smaller if Mo is substituted with Zr. Also, the change in C11 and C12 components show that the material comes closer to the elastic instability region.
Lobzenko, I.; Shiihara, Yoshinori*; Tsuru, Tomohito
no journal, ,
High-entropy alloys (HEA) are excellent structural materials due to their promising mechanical properties. Works on body-centered cubic (BCC) HEAs show increased ductility if group 4 elements are present in the composition. Theoretical studies of that effect by first-principles modeling are complicated by the essential randomness of HEA atomic structure, which requires large systems. To achieve high accuracy in classical molecular dynamics we have developed interatomic potentials using machine learning of artificial neural networks (we refer to them as ANN potentials). We present in the current work results for two medium-entropy alloys (MEA): MoNbTa and ZrNbTa. Comparison of basic mechanical properties show decrease of bulk modulus and elastic constants if Mo is substituted with group 4 element Zr. Edge and screw dislocations are studied. Classical modelling allows construction of big calculation cells, that prevents self-interaction of the dislocation core due to long-range stress field. Moreover, big cells ensures better randomness of alloys, which is vital in simulations of HEA and MEA mechanical properties. Screw dislocation movement is induced by applying shear strain. In case of edge dislocation the shape and energy is studied in the process of migration of the dislocation core between two adjacent easy core configurations. In this way the Peierls barrier is calculated. Results for two MEA are compared to elucidate the role of group 4 element. Finally, to understand the stress field of dislocations we employ atomic stress calculation scheme in the framework of ANN potentials. Atomic stress calculations is possible based on virial stress definition due to the fact that atomic energy in the ANN scheme ultimately depends on pair distances between atoms.
Tsuru, Tomohito; Lobzenko, I.
no journal, ,
High-entropy alloys (HEAs) have received attention for their excellent mechanical and thermodynamic properties. Especially some body-centered cubic (BCC)-HEAs have an excellent balance between high strength and elongation. BCC-HEAs containing specific elements show unusual slip traces. It is essential to understand the atomic-level depiction of defect structures considering the effects of the constituent elements to elucidate the origin of excellent mechanical properties of HEA. The core structure and motion of dislocations should be the key to identifying the unique feature of the mechanical properties of HEAs. It is worthwhile considering the difference between the two BCC-HEAs, VNbMoTaW and TiZrNbHfTa. We believe that the origin of significant lattice distortion, unusual dislocation core structure, and slip trace is derived from the existence of the group IV elements such as Ti and Zr with a sufficient concentration. In the present study, we investigate the dislocation score of in BCC-HEAs using the first-principles calculations. We modeled randomly-distributed ternary- and quinary alloys with equiatomic fractions. We explored the effect of local lattice distortion and chemical composition on mechanical properties. As a result, the mean square displacement (MSAD) of TiZrNbHfTa was quite large, corresponding to over 6% of the Burgers vector. This extremely large lattice distortion must strongly influence the mechanical properties since the friction stress of dislocation motion definitely increases.
Lobzenko, I.; Tsuru, Tomohito; Shiihara, Yoshinori*; Mori, Hideki*
no journal, ,
High-entropy alloys (HEA) exhibit excellent mechanical properties, which makes them good candidates for structural materials. Works on body-centered cubic (BCC) HEAs show increased ductility if HCP elements are present in the composition. Origins of that effect could be studied in computer experiments, but first-principles modeling is complicated by the essential randomness of HEA atomic structure, which requires large systems. To achieve high accuracy in classical molecular dynamics we apply the technique of machine learning for interatomic potentials development. Current work focuses on the mechanical properties of two medium-entropy alloys (MEA): MoNbTa and ZrNbTa. Interatomic potentials were built for these two alloys using artificial neural networks (we refer to them as ANN potentials). Our study reveals a drastic change in the basic properties of the material when Mo is substituted with HCP element Zr. In particular, the alloy with Zr has decreased bulk modulus and elastic constants. The change in C11 and C12 elastic constants shows that the material comes closer to the elastic instability region. We have also studied the shape of the screw dislocation core in the two MEA. Results show that the non-compact core shape in the ZrNbTa alloy has a larger width.
