| 1. |
- Li, Changle, 1992-, et al.
(författare)
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Critical assessment of Co-Cu phase diagram from first-principles calculations
- 2020
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 102:18
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Tidskriftsartikel (refereegranskat)abstract
- Using first-principles alloy theory, we perform a systematic study of the Co-Cu phase diagram. Calculations are carried out for ferromagnetic and paramagnetic Co1-xCux solid solutions with face-centered-cubic (fcc) crystal structure. We find that the equilibrium volumes and magnetic states are crucial for a quantitative description of the thermodynamics of the Co-Cu system at temperatures up to 1400 K. In particular, the paramagnetic state of Cu-rich alloys with persisting local magnetic moments is shown to be responsible for the solubility of a small amount of Co in fcc Cu whereas the excess entropy in the ferromagnetic Co-rich region critically depends on the adopted lattice parameters. None of the common local or semilocal density functional theory approximations have the necessary accuracy for the lattice parameters when compared to the experimental data. The predicted ab initio Co-Cu phase diagram is in good agreement with the measurements and CALPHAD data, making it possible to gain a deep insight into the various contributions to the Gibbs free energy. The present study provides an atomic-level description of the thermodynamic quantities controlling the limited mutual solubility of Co and Cu and highlights the importance of high-temperature magnetism.
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| 2. |
- Li, Changle, 1992-, et al.
(författare)
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Effects of composition and magnetism on interfacial energy in Cu-Co alloys
- 2022
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Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 6:5
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Tidskriftsartikel (refereegranskat)abstract
- The composition and magnetic dependent interfacial energy in Cu-Co immiscible alloys is investigated within a coherent interface model using ab initio calculations. We translate the composition dependence of the interfacial energy to the temperature dependence considering the variations of the equilibrium compositions of precipitate and matrix with respect to temperature. The obtained results are in reasonable agreement with those obtained by experiments and thermodynamic calculations. Reviewing the experimental methods for determining the interfacial energy based on kinetic models for precipitate nucleation and coarsening, as well the thermodynamic models based on broken-bond models, we point out that the temperature effect on the interfacial energy in the above models is primarily due to the composition change of the interface. The present work emphasizes the effort to understand the meaning of the speciously same quantity in different methods.
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| 3. |
- Tang, Yingchun, et al.
(författare)
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First-Principles Calculations of Elastic and Thermodynamic Properties for Multi-component Co-based Superalloys
- 2023
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Ingår i: Metallurgical and Materials Transactions. A. - : Springer. - 1073-5623 .- 1543-1940. ; 54:5, s. 1635-1648
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Tidskriftsartikel (refereegranskat)abstract
- First-principles calculations were performed to investigate the elastic and thermodynamic properties for multi-component Co-based superalloy systems and explored the effect of alloying on stabilizing the γ′ phase. First, the comparisons were carried out for the γ′ phase in Co3(Al,TM) (TM being transition metals) and Ni3Al systems between the present computational results using the EMTO-CPA method and other available DFT calculations as well as experimental data. The lattice parameters, elastic constants, and Debye temperatures are consistent with experimental results and other calculations. The predicted thermodynamic properties, e.g., the Gibbs free energy, excess entropy, and linear thermal expansion coefficient, agree well with CALPHAD results, experimental results, and other available first-principles calculations. A combination of EMTO-CPA method and Debye–Grüneisen model is utilized in this work to ensure that the alloying effect on the stability of the γ′ phase in a multi-component Co-based system is captured efficiently. This could open the path for designing novel multi-component Co-based alloys based on first-principles calculation. To demonstrate this, predictions for the properties of multicomponent systems were undertaken. Our results show that Ni aids in the stabilization of the (CoNi)3(Al, Mo, Nb) phase. Graphical Abstract: [Figure not available: see fulltext.]
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| 4. |
- El-Tahawy, M., et al.
(författare)
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Metastable Phase Formation in Electrodeposited Co-Rich Co-Cu and Co-Ni Alloys
- 2023
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 170:6
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Tidskriftsartikel (refereegranskat)abstract
- In a previous work [El-Tahawy et al., J. Magn. Magn. Mater. 560, 169660 (2022)], we reported that from a sulfate type bath, hcp-Co can be electrodeposited at high pH and low current density and investigated the structure and magnetoresistance (MR) characteristics of such hcp-Co electrodeposits. Based on this earlier work, Co-rich Co-Cu and Co-Ni alloy electrodeposits were prepared under the same conditions by adding varying amounts of CuSO4 and NiSO4, respectively, to the CoSO4 bath. According to the results of detailed structural studies by various X-ray diffraction (XRD) geometries, in both the Co-Cu and Co-Ni systems an hcp phase formed exclusively up to about 2 at% of the alloying element. Above this concentration, a significant fcc phase fraction appeared in Co-Cu and a minor fcc fraction in Co-Ni up to about 8 at%. This means that the destabilization effect of Cu on hcp-Co is higher than that of Ni. Although the reduction of the stability of hcp-Co with increasing Cu and Ni content is a well-known phenomenon, a quantitative comparison of this effect in Co-Cu and Co-Ni alloys is missing from the literature. The measured lattice constants are analyzed in comparison with Vegard's law for the Co-Cu and Co-Ni element pairs deduced from data previously reported for the hcp and fcc phases of all three pure elements. For Co-rich Co-Ni alloys, the concentration dependence of the lattice parameters was found to follow Vegard's law for both the hcp and fcc phases due to the miscibility of the two components. For the Co-rich Co-Cu alloys, the data indicate a positive deviation from Vegard's law for both the hcp and fcc phases in agreement with the known similar behavior of fcc Co-Cu alloys for the whole composition range. The positive deviation from Vegard's law in the Co-Cu system is due to the excess mixing volume required for solid solution alloy formation of these immiscible elements in either phases. The MR data are discussed in the light of the observed phases and of the MR parameters reported in our previous work on the hcp and fcc phases of pure Co.
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| 5. |
- Li, Changle, 1992-
(författare)
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Ab Initio Investigation of Interfacial and Grain Boundary Properties of Metals and Alloys
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Phase interface (IF) and grain boundary (GB) are both common yet critical planar defects influencing the mechanical and physical properties of polycrystalline materials. Due to the complex nature of IFs and GBs in terms of structure and chemistry, determining the accurate excess energies associated with these defects is challenging for both experimental measurements and theoretical simulations. In this thesis, using first-principles methods, I make efforts towards establishing an efficient and robust model for predicting the IF and GB properties in elemental metals and complex multi-component alloys including the temperature and composition dependences.First I focus on the temperature dependent interfacial energy (IFE) for Cu-Co alloys. IFE plays a critical role in determining the nucleation and precipitate coarsening thermodynamics and kinetics. I start with assessing the phase diagram of Cu-Co alloys by ab initio calculations, which is used for establishing the composition-temperature relationship of precipitates and matrix. Calculations of the physical and thermodynamic properties for the ferromagnetic (FM) and paramagnetic (PM) $\ce{Cu}_{1-x}\ce{Co}_{x}$ solid solutions are performed using the exact muffin-tin orbital (EMTO) method in combination with the coherent potential approximation (CPA) for dealing chemical and magnetic randomness. This study demonstrates that the equilibrium volumes and magnetic states are crucial for a quantitative description of the thermodynamics of the Cu-Co system at temperatures up to 1400 K. The predicted ab initio Cu-Co phase diagram is in good agreement with the measurements and CALPHAD data. Then, the composition and magnetic dependent IFEs for a coherent $\ce{Cu}_{1-x}\ce{Co}_{x}$/$\ce{Cu}_{x}\ce{Co}_{1-x}$ interface are investigated at various magnetic states including FM, PM, and the mixed PM+FM states to account for the magnetic state change at different temperatures. Then, I translate the composition dependence of the IFE to the temperature dependence. The obtained results are in reasonable agreement with those obtained by experiments and thermodynamic calculations. The first part of the thesis provides an ab initio database for the IFEs in Cu-Co system and emphasizes the importance of understanding and properly describing various physical and thermodynamic quantities in different materials modeling approaches.The second focus of this thesis is on grain boundary energy (GBE). We calculate the GBEs for ten face-centered cubic (fcc) and seven body-centered cubic (bcc) metals. Various types of symmetric tilt GB structures ranging from twin boundary up to $\Sigma$19 coincident site lattice (CSL) boundaries are studied using the Vienna Ab initio Simulation Package (VASP). Ab initio results show a correlation between the GBEs of the same grain boundary structure in different fcc and bcc metals. Importantly, I show that the correlation factor is best determined by the ratio of the low-index surface energy. By using this correlation, the general GBEs of fcc and bcc metals are predicted at 0 K. Furthermore, using the Foiles's method, which assumes that the general GBE has a similar temperature dependence as the elastic modulus $c_{44}$, the general GBEs at elevated temperatures are predicted. The so obtained theoretical results show a good agreement with the available experimental data. Finally, the proposed method for predicting the general GBEs is applied to complex multicomponent alloys (austinite Fe-Cr-Ni and ferritic Fe-Cr alloys), yielding a parameterized prediction of the composition and temperature dependent GBE. After examining two common experimental methods used for determining the general GBEs, it is concluded that the two sets of experimental GBEs for fcc metals agree well with each other, while for bcc metals they correspond to different grain boundary structures and differ by a factor of 2. This part of the thesis introduces an effective and robust model for predicting the general GBEs of metals and alloys, facilitating grain boundary engineering for advanced alloy design. The third focus is on alloying GB segregation in complex alloys. Manganese (Mn) and Nickel (Ni) segregation behaviors at bcc Fe-Cr grain boundaries are investigated. In this segregation study, three GB structures, namely, $\Sigma$3(111), $\Sigma$9(114), and $\Sigma$11(332), are considered. First, a systematic comparison of the theoretical segregation energies for Mn and Ni solutes in pure Fe GBs is conducted between VASP and EMTO calculations. The EMTO results agree reasonably with VASP and previous theoretical data, indicating a reliable potential for capturing the solute segregation behaviors. Next, the Mn and Ni segregation energies at bcc Fe-Cr solid solution GBs are determined at various concentrations of the matrix and at the FM and PM states to account for the temperature effects on the magnetic state using the EMTO-CPA method. Strong magnetic effects on the segregation energy are observed. Particularly, it is found that the magnetic states of Mn atoms depend strongly on local chemical and structural environment, which has a remarkable effect on the segregation energy. It is found that Mn and Ni show different segregation tendencies at FM and PM states. This part of the thesis puts forward an attempt to investigate the solute segregation properties in complex solid solutions as compared to pure metal or dilute alloys, and improves our understanding of GB segregation in engineering alloys, like steels.
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| 6. |
- Li, Changle, 1992-
(författare)
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First-Principles Investigation of Bulk and Interfacial Properties of Cu-Co Binary System
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Due to the complex nature of phase interfaces, acquiring precise interfacial energies is usually a big challenge for both experimental measurements and computational modelings. In this thesis, we put forward an efficient route for assessing the temperature dependence of the interfacial energy using density functional theory (DFT). For our investigations, we select the Cu-Co binary system as a model with large miscibility gap. Most of the first-principles calculations presented here are carried out using the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA), but other alternative DFT methods are also included in the various stages of the project.The first step is to acquire an accurate thermodynamical description of the Cu-Co binary system. We assess the quality of the predicted thermodynamic properties by an effort to reproduce the phase diagram for the entire range of composition using first-principles calculations and alloy theory. The calculations are performed for the random Cu-Co alloys with face-centered cubic (fcc) structure at both ferromagnetic (FM) and paramagnetic (PM) states, depending on the composition. We demonstrate that the equilibrium volumes and magnetic states are crucial for the proper description of the magnetic entropy of the Cu-Co system at elevated temperatures. More specifically, the contribution of magnetic entropy to the free energy in the Cu-rich region obtained for the PM state turns out to be critical. Furthermore, the adopted equilibrium volumes strongly affect the contribution of the vibrational entropy to the free energy. When all effects are properly accounted for, we find that the ab initio phase diagram of the Cu-Co system agrees well with the Thermo-Calc phase diagram and the experimental observations.The Cu-Co system has a large miscibility gap. The interface between the decomposed Cu-rich and Co-rich phases plays critical roles in the precipitation nucleation and growth, therefore having huge effects on the physical and mechanical properties of the alloys. Therefore, adopting the thermodynamical properties of the bulk Cu-Co alloys successfully obtained by our ab initio calculations, we go further and investigate the interfacial properties of the Cu-Co alloys using a coherent interface model. The chemical, magnetic, and strain energy contributions to the formation energy of the interfaces are analyzed separately. We find that the chemical interfacial energies generally decrease with increasing concentrations, namely when the compositions accross the interface become more homogenous. We identify a sizable contribution to the interfacial energies from the magnetic effects. The temperature dependence of the interfacial energy is estimated, to the first-order approximation, through considering how the equilibrium compositions of the two phases vary at different temperatures. Our results show that the temperature dependence of the interfacial energy originates primarily from the temperature-induced increase of the mutual solubility of the alloy constituents and the loss of the magnetic long range order near the Curie temperature. Our ab initio results are compared with the experimental data as well as with those extracted from Thermo-Calc modeling. The present thesis provides an atomic-level description of the bulk and interfacial properties of the Cu-Co binary system using quantum mechanics simulations. This approach is believed to be useful for a complete thermodynamical description of other similar immiscible alloy systems as well from first-principles.
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| 7. |
- Li, Changle, 1992-, et al.
(författare)
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Theoretical and experimental grain boundary energies in body-centered cubic metals
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Grain boundary energy (GBE) and its temperature dependence in body-centered cubic (bcc) metals are investigated using abinitio calculations. We reveal a scaling relationship between the GBEs of the same grain boundary structure in different bccmetals and find that the scaling factor can be best estimated by the ratio of the low-index surface energy. Appling the scalingrelationship, the general GBEs of bcc metals at 0 K are predicted. Furthermore, adopting the Foiles’s method which assumesthat the general GBE has the same temperature dependence as the elastic modulus c44 [Scr. Mater., 62 (2010) 231–234], thepredicted general GBEs at elevated temperatures are found in good agreement with available experimental data. Reviewing twoexperimental methods for determining the general GBEs, we conclude that the two sets of experimental GBEs for bcc metalscorrespond to different GB structural spaces and differ by approximately a factor of 2. The present work puts forward an efficientmethodology for predicting the general GBEs of metals and alloys, facilitating GB engineering for advanced alloy design.
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| 8. |
- Li, Changle, 1992-, et al.
(författare)
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Theoretical and experimental grain boundary energies in body-centered cubic metals
- 2023
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 255
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Tidskriftsartikel (refereegranskat)abstract
- Grain boundary energy (GBE) and its temperature dependence in body-centered cubic (bcc) metals are investigated using ab initio calculations. We reveal a scaling relationship between the GBEs of the same grain boundary structure in different bcc metals and find that the scaling factor can be best estimated by the ratio of the low-index surface energy. Applying the scaling relationship, the general GBEs of bcc metals at 0 K are predicted. Furthermore, adopting the Foiles's method which assumes that the general GBE has the same temperature dependence as the elastic modulus c44 [Scr. Mater., 62 (2010) 231–234], the predicted general GBEs at elevated temperatures are found in good agreement with available experimental data. Reviewing two experimental methods for determining the general GBEs, we conclude that the two sets of experimental GBEs for bcc metals correspond to different GB structural spaces and differ by approximately a factor of 2. The present work puts forward an efficient methodology for predicting the general GBEs of metals, which has the potential to extend its application for homogeneous alloys without strong segregation of the alloying element and facilitates GB engineering for advanced alloy design.
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