Koretaka Yuge

Atomic displacement in the CrMnFeCoNi high-entropy alloy – A scaling factor to predict solid solution strengthening

Although metals strengthened by alloying have been used for millennia, models to quantify solid solution strengthening (SSS) were first proposed scarcely seventy years ago. Early models could predict the strengths of only simple alloys such as dilute binaries and not those of compositionally complex alloys because of the difficulty of calculating dislocation-solute interaction energies. Recently, models and theories of SSS have been proposed to tackle complex high-entropy alloys (HEAs). Here we show that the strength at 0 K of a prototypical HEA, CrMnFeCoNi, can be scaled and predicted using the root-mean-square atomic displacement, which can be deduced from X-ray diffraction and first-principles calculations as the isotropic atomic displacement parameter, that is, the average displacements of the constituent atoms from regular lattice positions. We show that our approach can be applied successfully to rationalize SSS in FeCoNi, MnFeCoNi, MnCoNi, MnFeNi, CrCoNi, CrFeCoNi, and CrMnCoNi, which are all medium-entropy subsets of the CrMnFeCoNi HEA.

Estimation of Macroscopic Physical Property in Disordered States: Special Microscopic States Approach

Based on classical statistical thermodynamics, we develop a theoretical approach that provides new insight into how macroscopic and microscopic physical properties are bridged via crystal lattice for condensed mat- ters. We find that in order to determine macroscopic physical properties and their temperature dependence in equilibrium disordered state, information about a few specially selected microscopic states, established from geometrical characteristics of the crystal lattice, is sufficient. These special states are found to be independent of constitument elements as well as of temperature, which is in contrast to the standard conception in statistical thermodyanamics where a set of microscopic states mainly contributing to determining macroscopic physical properties depend on temperature and constituent elements. Validity and applicability of the theoretical ap- proach is confirmed through prediction of macroscopic physical properties in practical alloys, compared with prediction by full thermodynamic simulation. The present findings provide efficient and systematic prediction of macroscopic physical properties for equilibrium disordered states based on those for special microscopic states without any information of interactions for given system.Based on classical statistical thermodynamics, we develop a theoretical approach that enables estimation of macroscopic physical properties and their temperature dependence for equilibrium disordered states in crystalline materials, using information about a few specially selected microscopic states. These special states are established from geometrical characteristics of the crystal lattice, which means that they are independent of constituent elements as well as of temperature. The present approach therefore provides efficient and systematic prediction of macroscopic physical properties for disordered states, without any information of interactions for given system. Validity and applicability of the present approach is confirmed through prediction of macroscopic physical properties in practical alloys, compared with prediction by full thermodynamic simulation.

Equilibrium Macroscopic Structure Revisited from Spatial Constraint

In classical systems, we reexamine how macroscopic structures in equilibrium state connect with spatial constraint on the systems. For example, volume and density as the constraint for liquids in rigid box, and crystal lattice as the constraint for crystalline solids. We find that in disordered states, equilibrium macroscopic structure, depending on temperature and on multibody interactions in the system, can be well characterized by a single special microscopic structure independent of temperature and of interactions. The special microscopic structure depends only on the spatial constraint. We demonstrate the present findings providing (i) significantly efficient and systematic prediction of macroscopic structures for possible combination of constituents in multicomponent systems using first-principles calculations, and (ii) unique and accurate prediction of multibody interactions in given system from measured macroscopic structure, without performing trial-and-error simulation.

Surface design of alloy protection against CO-poisoning from first principles

Pt-based alloy catalysts are of significant importance in fuel cells due to enhanced electrode reactivity and selectivity. Designing alloy surfaces suitable for catalyst via first-principles predictions has long played a central role in identifying promising candidates. We propose surface design for polymer electrolyte fuel cell (PEFC) based on the use of thermodynamically stable alloy surfaces. Using first-principles calculation, we have explored stable Pt alloy surfaces that possess superior catalytic properties for CO-despoisoning in hydrogen-related reactions of fuel cells. The stable Pt(25)M(75) (M = Rh, Cu) alloy surfaces both exhibited weaker CO and stronger H binding compared to the pure Pt surface, which yielded a significant increase in H coverage by around one order. These modifications of molecular adsorption are attributed to the deeper band centre of surface d electrons. Understanding the adsorption properties of the stable atomic structure at surfaces will help us to design suitable alloy surfaces with high-catalytic activity and prolonged actuation in fuel cells.

New Wang-Landau approach to obtain phase diagrams for multicomponent alloys

We develop an approach to apply Wang-Landau algorithm to multicomponent alloys in semi-grand-canonical ensemble. Although the Wang-Landau algorithm has great advantages over conventional sampling methods, there are few applications to alloys. This is because calculating compositions in semi-grand-canonical ensemble using the Wang-Landau algorithm requires a multi-dimensional density of states in terms of total energy and compositions. However, constructing the multi-dimensional density of states is difficult. In this study, we develop a simple approach to calculate the alloy phase diagram using Wang-Landau algorithm, and show that compositions in semi-grand-canonical ensemble require just some one-dimensional densities of states. Finally, we applied the present method to Cu-Au and Pd-Rh alloys and confirmed that the present method successfully describes the phase diagram with high validity and accuracy.

Microstructure and Mechanical Properties of NbSi2/MoSi2 Crystals with Lamellar Structure

NbSi2/MoSi2 duplex silicide crystals are potentially a new-class of ultra-high temperature structural materials. Improvement in the thermal stability of their lamellar microstructure was accomplished by the addition of a minute amount of either Cr or Zr. The mechanical properties of the duplex silicide, such as fracture toughness and high temperature strength, show strong orientation dependence, thereby suggesting the importance of the control of microstructure to improve their properties.

Interface Migration with Segregation in MoSi2-Based Lamellar Alloy Simulated by Phase-Field Method

MoSi2–based alloys are attracting attention as ultra-high temperature structural material for super-high efficiency gas turbine power generation systems. In this study, the effects of Cr-and Zr-addition on interface migration in MoSi2/NbSi2 lamellar silicide were examined by phase field simulations employing the segregation energies evaluated by the first principles calculation in addition to thermodynamic free energy in order to take into account the chemically-driven interfacial segregation. The simulation results indicate that both Cr and Zr can segregate at the lamellar interface to suppress its migration, and the Zr-addition is more effective to lower the interface migration rate than the Cr-addition owing to its higher segregation energy.

Free Energy Calculations of Precipitate Nucleation

Our recently proposed calculating method reliably predicts the nucleation free energy barrier of the homogeneous and coherent precipitations. Helmholtz free energy change is clearly defined and calculated by the purely enthalpic and entropic contributions between the initial state of the isolated solute atoms scattering around the matrix and the final state of the cluster of size n traveling around the matrix. The enthalpic term is calculated by the reliable first principles method and the entropic term is estimated by the ideal solution model. The vibrational free energy is also included by the quasi-harmonic approximation. The model calculation was performed on bcc Cu precipitations in the Fe-Cu system. The predicted values of the critical number of 12 atoms and the critical free energy barrier of 0.6eV show good agreement with the experimentally estimated ones for the annealing temperature of 773K and the initial concentration of 1.4at%Cu.

Accurate estimation of a phase diagram from a single STM image

We propose a new approach to constructing a phase diagram using the effective Hamiltonian derived only from a single real-space image produced by scanning tunneling microscopy (STM). Currently, there have been two main methods to construct phase diagrams in material science: ab initio calculations and CALPHAD with thermodynamic information obtained by experiments and/or theoretical calculations. Although the two methods have successfully revealed a number of unsettled phase diagrams, their results sometimes contradicted when it is difficult to construct an appropriate Hamiltonian that captures the characteristics of materials, e.g., for a system consisting of multiple-scale objects whose interactions are essential to the system’s characteristics. Meanwhile, the advantage of our approach over existing methods is that it can directly and uniquely determine the effective Hamiltonian without any thermodynamic information. The validity of our approach is demonstrated through an Mg–Zn–Y long-period stacking-ordered structure, which is a challenging system for existing methods, leading to contradictory results. Our result successfully reproduces the ordering tendency seen in STM images that previous theoretical study failed to reproduce and clarifies its previously unknown phase diagram. Thus, our approach can be used to clear up contradictions shown by existing methods.

Short-Range-Order for fcc-based Binary Alloys Revisited from Microscopic Geometry

Short-range order (SRO) in disordered alloys is typically interpreted as competition between chemical effect of negative (or positive) energy gain by mixing constituent elements and geometric effects comes from difference in effective atomic radius. Although we have a number of theoretical approaches to quantitatively estimate SRO at given temperatures, it is still unclear to systematically understand trends in SRO for binary alloys in terms of geometric character, e.g., effective atomic radius for constituents. Since chemical effect plays significant role on SRO, it has been believed that purely geometric character cannot quantitatively explain the SRO trends. Despite these considerations, based on the density functional theory (DFT) calculations on fcc-based 28 equiatomic binary alloys, we find that while convensional Goldschmidt or DFT-based atomic radius for constituents have no significant correlation with SRO, atomic radius for specially selected structure, constructed purely from information about underlying lattice, can successfully capture the magnitude of SRO. These facts strongly indicate that purely geometric information of the system plays central role to determine characteristic disordered structure.