Elucidation of the formation process and shape determining factors of GP zones and nanoclusters in Al-Cu and Al-Mg-Si alloys

Shoji, Mizuki*; Kurihara, Kensuke*; Lobzenko, I.; Tsuru, Tomohito; Serizawa, Ai*

Keikinzoku, 74(12), p.535 - 545, 2024/12

https://doi.org/10.2464/jilm.74.535

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.

Intrinsic factors responsible for brittle versus ductile nature of refractory high-entropy alloys

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

https://doi.org/10.1038/s41467-024-45639-8

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.

First-principles atomic level stresses; Application to a metallic glass under shear

Lobzenko, I.; Tsuru, Tomohito; Shiihara, Yoshinori*; Iwashita, Takuya*

Materials Research Express (Internet), 10(8), p.085201_1 - 085201_12, 2023/08

https://doi.org/10.1088/2053-1591/acf2da

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.

Improved mechanical properties of Co-free high-entropy Cantor alloy; A First-principles study

Lobzenko, I.; Wei, D.*; Itakura, Mitsuhiro; Shiihara, Yoshinori*; Tsuru, Tomohito

Results in Materials (Internet), 17, p.100364_1 - 100364_7, 2023/03

https://doi.org/10.1016/j.rinma.2023.100364

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.

Interaction between solute atoms and vacancies in Al-Mg-X (X=Si, Ge) alloys

Kurihara, Kensuke*; Lobzenko, I.; Tsuru, Tomohito; Serizawa, Ai*

Keikinzoku, 72(7), p.427 - 429, 2022/07

https://doi.org/10.2464/jilm.72.427

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.

Synergetic effect of Si addition on mechanical properties in face-centered-cubic high entropy alloys; A First-principles study

Tsuru, Tomohito; Lobzenko, I.; Wei, D.*

Modelling and Simulation in Materials Science and Engineering, 30(2), p.024003_1 - 024003_11, 2022/03

https://doi.org/10.1088/1361-651X/ac455a

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.

Effects of local bonding between solute atoms and vacancy on formation of nanoclusters in Al-Mg-Si alloys

Kurihara, Kensuke*; Lobzenko, I.; Tsuru, Tomohito; Serizawa, Ai*

Keikinzoku, 72(2), p.47 - 53, 2022/02

https://doi.org/10.2464/jilm.72.47

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.

Dislocations shape and dynamics in BCC medium entropy alloys; Classical modelling with machine learning potentials

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.

First-principles study on dislocation core with short-range order in BCC high entropy alloys

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.

First-principles modeling of BCC to omega phase transition in ZrNbTaTiHf high-entropy alloy

Lobzenko, I.; Tsuru, Tomohito

no journal, ,