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Shoji, Mizuki*; Kurihara, Kensuke*; Lobzenko, I.; Tsuru, Tomohito; Serizawa, Ai*
Keikinzoku, 74(12), p.535 - 545, 2024/12
While Plate-like Guinier-Preston (GP) zones are formed during aging process in Al-Cu alloys, spherical nanocluster formation occurs in the early stage of aging in Al-Mg-Si alloys. Unlike well-known GP (I) zone in Al-Cu, there is no specific configurations within the nanocluster. However, the solute concentration and local configuration should play decisive role in subsequent formation of precipitations. In the present study, the first-principles calculations were performed to investigate the factors determining the stable shape during the formation process of GP zones and clusters in Al-Cu and Al-Mg-Si alloys. As a result of formation energy of three-body bonds, the Cu-Cu-Cu triplet with the bond angle of 90deg was the most stable. Monte Carlo simulations with newly developed machine-learning potential were then performed, and consequently the segregation of Cu atom formed with bond angle of 90deg are observed more frequently. In contrast, three-body triplet in Al-Mg-Si alloy was most stable without any specific directional anisotropy, when the bond angle was 60deg, resulting in the formation of spherical nanoclusters. These results suggest that the intrinsic feature of the stability of local bonding dominates the shape of GP zones and nanoclusters, in which planar- or spherical-like cluster is formed.
Tsuru, Tomohito; Han, S.*; Chen, Z.*; Lobzenko, I.; Inui, Haruyuki*
Materia, 63(10), p.695 - 702, 2024/10
VNbMoTaW, a typical high-entropy alloy with the BCC phase, is composed of metals with high melting points, and is called a reflectory high-entropy alloy. TiZrNbTaHf, which are also known as typical high-entropy alloys with the BCC phase, are also single phase and have a melting point 500 C lower than that of VNbMoTaW, but are known to exhibit excellent ductility at low temperatures below room temperature. Understanding what properties govern the mechanical properties is essential for designing alloys such as high-temperature resistant alloys with excellent high-temperature strength and low-temperature ductility. Given that both VNbMoTaW and TiZrNbTaHf are single-phase alloys, the key lies in the relationship between the constituent elements and the properties underlying the deformation, such as dislocations. This paper presents the results of the differences in mechanical properties of these two refractory high-entropy alloys, using experimental, theoretical, and computer simulations to investigate the key factors controlling ductility and strength.
Tsuru, Tomohito; Han, S.*; Matsuura, Shutaro*; Chen, Z.*; Kishida, Kyosuke*; Lobzenko, I.; Rao, S.*; Woodward, C.*; George, E.*; Inui, Haruyuki*
Nature Communications (Internet), 15, p.1706_1 - 1706_10, 2024/02
Times Cited Count:13 Percentile:98.36(Multidisciplinary Sciences)Refractory high-entropy alloys (RHEAs) have attracted attention because of their potential for use in ultrahigh-temperature applications. Unfortunately, their body-centered-cubic (BCC) crystal structures make them more brittle than the ductile and fracture-resistant face-centered-cubic (FCC) HEAs. RHEAs also display significantly lower creep strengths than a leading Ni-base superalloy and its FCC matrix. To overcome these drawbacks and develop RHEAs into viable structural materials, improved fundamental understanding is needed of factors that control strength and ductility. Here we investigate two model RHEAs, TiZrHfNbTa and VNbMoTaW, and show that the former is plastically compressible down to 77 K, whereas the latter is not below 298 K. We find that hexagonal close-packed (HCP) elements in TiZrHfNbTa lower its dislocation core energy, increase its lattice distortion, and lower its shear modulus relative to VNbMoTaW whose elements are all BCC, leading to the formers higher ductility and modulus-normalized yield strength. Consistent with our yield strength models, primarily screw dislocations are present in TiZrHfNbTa after deformation, but equal numbers of edge and screw segments in VNbTaMoW. Dislocation cores are compact in VNbTaMoW and extended in TiZrHfNbTa, and different macroscopic slip planes are activated in the two RHEAs, which we attribute to the concentration of HCP elements. Our findings demonstrate how electronic structure changes related to the ratio of HCP to BCC elements can be used to control strength, ductility, and slip behavior to develop the next generation of high-temperature materials for more efficient power plants and transportation.
Tsuru, Tomohito; Lobzenko, I.; Ogata, Shigenobu*; Han, W.-Z.*
Journal of Materials Research and Technology, 28, p.1013 - 1021, 2024/01
Times Cited Count:2 Percentile:33.88(Materials Science, Multidisciplinary)Some solute atoms induce hardening and embrittlement in body-centered-cubic refractory metals. Especially interstitial oxygen has a dramatic hardening effect in Nb, where the yield stress of oxygen-doped Nb alloys becomes more than twice as high as that of pure Nb. Conventional mechanisms cannot explain the oxygen-induced dramatic hardening since the interaction between dislocation and oxygen is relatively weak. Here, we focused on the three-body interaction of a screw dislocation with oxygen and vacancy. Our first-principles calculations revealed that the formation of vacancy-oxygen pair enhances the attractive interaction with a screw dislocation though the interaction between oxygen and dislocation is repulsive. Furthermore, this feature was found to be a unique nature of oxygen in Nb. The vacancy-oxygen pair increases the energy barrier for dislocation motion more significantly than an isolated vacancy and oxygen interstitial. We have discovered a new oxygen-induced mechanism: a unique octahedral-tetrahedral shuffling process of oxygen dominantly contributes to the dramatic hardening. Thus, the widely distributed vacancy-oxygen pairs behave as strong obstacles for dislocation motion that causes damage accumulation and successive hardening in oxygen-doped BCC alloys.
Lobzenko, I.; Tsuru, Tomohito; Shiihara, Yoshinori*; Iwashita, Takuya*
Materials Research Express (Internet), 10(8), p.085201_1 - 085201_12, 2023/08
Times Cited Count:0 Percentile:0.00(Materials Science, Multidisciplinary)Unlike alloys with a crystal lattice, metallic glasses (MG) do not possess distinctive defects but demonstrate a highly heterogeneous response to mechanical deformation, even in near-elastic regimes. The difficulties in describing such non-uniform behavior hamper the prediction of the mechanical properties of MGs. We apply first-principles calculations of atomic stress in CuZr MG to reveal its response to shear strain. That approach allows one to probe such parameters as displacement vector, charge transfer, or change in chemical bonds on the lowest atomic level. We find correlations between the mentioned parameters and show the importance of atomic von Mises stress in the comprehensive description of the mechanical state of a glassy system.
Kurihara, Kensuke*; Lobzenko, I.; Tsuru, Tomohito; Serizawa, Ai*
Materials Transactions, 64(8), p.1930 - 1936, 2023/08
Times Cited Count:1 Percentile:0.00(Materials Science, Multidisciplinary)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 estimate the effect of local bonding on the formation of nanoclusters. Monte Carlo simulations were subsequently performed to investigate the stable structure of the nanocluster formed in Al-0.95 mass pct Mg-0.81 mass pct Si alloy. We found that the Mg-Si and Si-Vac pairs are stable in the Al matrix. The result shows that the solute atoms easily aggregate with different types of solute atoms and that the Si atom has a strong attractive interaction with a vacancy. Furthermore, Mg-Si-vacancy three-body clusters is more stable than Mg-Si and Si-vacancy pairs in the Al matrix. The nanoclusters in the Al matrix were thermally stabilized by the stable configurations between solute atoms and vacancy. Thus, the electronic structure calculations suggested that the local bondings within a nanocluster play a significant role in not only the thermal stability but also the formation and growth behavior of nanoclusters during aging at low temperatures.
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:86 Percentile:99.65(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.
Shiihara, Yoshinori*; Itai, Yuki*; Lobzenko, I.; Tsuru, Tomohito
Frontiers in Materials (Internet), 9, p.895626_1 - 895626_10, 2022/05
Times Cited Count:3 Percentile:24.31(Materials Science, Multidisciplinary)The stress state at an atomic level and its governing physics inside a random alloy are essential elements in developing a model for solid solution strengthening in random alloys, which is one of the primary strengthening mechanisms of high-entropy alloys (HEAs). Through first-principles calculation, we investigated the atomic stress in fcc and bcc random alloys that were subsets of CrMnFeCoNi and VNbMoTaW HEAs, respectively. By focusing on the charge transfer and volume change with respect to a bulk crystal, we examined whether the internal stress fields in the fcc and bcc alloys could be interpreted from a unified viewpoint in terms of these physical quantities. Regression analyses using the random forest method revealed that the charge transfer and volume change simultaneously govern the stress state inside an alloy, albeit with varying degrees of intensity.
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:14 Percentile:81.67(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:88 Percentile:99.68(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:22 Percentile:81.98(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.
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.
Lobzenko, I.; Tsuru, Tomohito; Shiihara, Yoshinori*; Iwashita, Takuya*
no journal, ,
Revealing the origin of the mechanical properties of metallic glasses (MG) is a long-standing problem. MGs respond to the external strain with the activation of collective atomic motion, but the triggers of such motions are not revealed yet, in contrast to the well-defined dislocations in crystals. To study this collective atomic response in detail, we use the atomic stress calculations in the first-principles framework. Four small random Cu50%Zr50% structures were prepared and put under the strain from 0.5 to 8.0%. The stress response is shown separately for Cu and Zr. We analyze the system's transformation between the affine and relaxed states and find a significant deviation from elastic behavior. As the atomic von Mises stress change indicates, the xy shear strain invokes atomic stress response in other shear components. Other local parameters, such as charge transfer, atomic displacements, and atomic strain, are also discussed.
Lobzenko, I.; Shiihara, Yoshinori*; Mori, Hideki*; Matsunaka, Daisuke*; Tsuru, Tomohito
no journal, ,
Refractory multi-component alloys (MCA) form an important class of materials with high potential for use in severe conditions. One of the main problems hindering the application of these alloys is the low ductility inherited from the body-centred cubic (BCC) crystal structure. Dislocation motion is the factor significantly influencing the ductility of the material, so a comprehensive understanding of the dislocation dynamics in refractory MCAs should be achieved to pave the way for designing refractory alloys with increased ductility. To achieve high accuracy in classical molecular dynamics simulations of dislocation motion, we apply the technique of machine learning (ML) for interatomic potential development. It is known that alloys with hexagonal closed-packed (HCP) elements such as Zr exhibit higher ductility, which is why two medium-entropy alloys, MoNbTa and ZrNbTa, were chosen to study the influence of elements' constitution on dislocations dynamics. The inter-atomic potentials for MCAs built using ML need a specific dataset. In the process of the potential development, we identify which structures contribute to a better quality of materials' mechanical properties prediction by the potentials. Results of the simulations have shown qualitative and quantitative differences between the two alloys under study. One example of that difference can be seen in the shapes of the screw dislocation core. In contrast to MoNbTa, ZrNbTa demonstrates a non-compact core with an extension on a (110) plane.
Tsuru, Tomohito; Lobzenko, I.
no journal, ,
High-entropy alloys (HEAs) have been receiving increased attention for their excellent mechanical properties. Especially, some body-centered cubic (BCC) HEAs exhibit excellent mechanical properties which shows a good balance between high strength and high elongation. In addition, BCC-HEAs containing certain elements cause peculiar slip traces. However, the mechanisms of these characteristics are not fully understood. To elucidate the mechanism of the excellent mechanical properties of HEA, it is essential to understand the atomic-level depiction of the defect structure, taking into account the influence of the constituent elements. Dislocation structure and dislocation motion should be key to characterizing the mechanical properties of HEA. It is worthwhile to consider the differences between two types of BCC-HEA, VNbMoTaW and TiZrNbHfTa. We believe that the origin of the significant lattice distortion and the peculiar dislocation core structure lies in the presence of sufficient concentrations of group IV elements. In this study, we investigated the dislocation core structure in BCC-HEAs using first-principles calculations. The results show that the mean square atomic displacement of TiZrNbHfTa is very large. We also developed a new method to capture the dynamics of dislocations in 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.