In alloys cluster expansions (CE) are increasingly used to combine first-principles electronicstructure calculations and Monte Carlo methods to predict thermodynamic properties. As a basis-set expansion in terms of lattice geometrical clusters and effective cluster interactions, the CE is exact if infinite, but is tractable only if truncated. Yet until now a truncation procedure was not well-defined and did not guarantee a reliable truncated CE. We present an optimal truncation procedure for CE basis sets that provides reliable thermodynamics. We then exemplify its importance in Ni3V, where the CE has failed unpredictably, and now show agreement to a range of measured values, predict new low-energy structures, and explain the cause of previous failures. [3,10] ordering. As a means for multiscaling based on density-functional theory (DFT) electronic-structure energetics, the CE is a basis-set expansion in n-body clusters (associated with n Bravais lattice points) and effective cluster interactions (ECI) that specify configurational energies. Except for implicit DFT errors in the energy database, the CE is exact for an infinite basis, but impractical if not vastly truncated [11,12]. Although there are many successes, a truncated CE can and has unpredictably failed.We present a new method for an optimal truncation of the basis set that gives reliable thermodynamics. We then detail its importance in face-centered-cubic (fcc) Ni 3 V, a system with order-disorder transition from disordered A1 phase to ordered DO 22 phase at T c of 1318 K [13]. Previous CE for Ni 3 V [14,15,16] had errors of 40 − 1000% for a range of thermodynamic properties, prompting a search for missing physics [16]. We show that our new method allows more reliable predictions, including that of key low-energy configurational excitations. As a synopsis, we compare in Table I our CE results, along with the previous ones, with experimental values of T c and ∆E L12−DO22 SRO , the energy difference between DO 22 and metastable L1 2 as assessed from the short-range order (SRO) measurements [17,18]. The new CE now agrees with a range of experimentally assessed values (more below). We find that prior failure in Ni 3 V is due to inappropriate truncation of the cluster basis set and overfitting to get the ECI -underscoring again the need for careful application of basis-set methods. We have tested this new CE method on a few cubic and noncubic binaries and ternaries and found it to be especially important when multibody ECI are significant.

We calculate the transition temperature versus concentration ͑T c vs c͒ phase diagrams of several phasesegregating alloys ͓fcc Ca-Sr, Au-Pt, and Rh-͑Pd,Cu,Ag,Au͔͒ using a multiscale method combining firstprinciples calculations and Monte Carlo via the cluster expansion ͑CE͒. We study Pd-Rh, with its well-known high-T miscibility gap, to verify the method's reliability. We predict that Ca-Sr segregates at low temperatures. We then show that a rapid estimate of T c is obtained from enthalpies analytically derived from a CE, and, using thermodynamic integration, we determine under what circumstances this mean-field estimate is accurate compared to Monte Carlo results. Also, we discuss how an electronegativity difference of the alloying elements quickly assess when vibrational entropy effects should be included in the estimate of T c .

The nudged-elastic band (NEB) method is modified with concomitant two climbing images (C2-NEB) to find a transition state (TS) in complex energy landscapes, such as those with a serpentine minimal energy path (MEP). If a single climbing image (C1-NEB) successfully finds the TS, then C2-NEB finds it too. However, improved stability of C2-NEB makes it suitable for more complex cases, where C1-NEB misses the TS because the MEP and NEB directions near the saddle point are different. Generally, C2-NEB not only finds the TS, but also guarantees, by construction, that the climbing images approach it from the opposite sides along the MEP. In addition, C2-NEB provides an accuracy estimate from the three images: the highestenergy one and its climbing neighbors. C2-NEB is suitable for fixed-cell NEB and the generalized solid-state NEB. KeywordsMaterials Science and Engineering, Density functional theory, Nickel, Shape memory effect, Optimization, Band structure DisciplinesEngineering Physics | Metallurgy The nudged-elastic band (NEB) method is modified with concomitant two climbing images (C2-NEB) to find a transition state (TS) in complex energy landscapes, such as those with a serpentine minimal energy path (MEP). If a single climbing image (C1-NEB) successfully finds the TS, then C2-NEB finds it too. However, improved stability of C2-NEB makes it suitable for more complex cases, where C1-NEB misses the TS because the MEP and NEB directions near the saddle point are different. Generally, C2-NEB not only finds the TS, but also guarantees, by construction, that the climbing images approach it from the opposite sides along the MEP. In addition, C2-NEB provides an accuracy estimate from the three images: the highest-energy one and its climbing neighbors. C2-NEB is suitable for fixed-cell NEB and the generalized solid-state NEB. C 2015 AIP Publishing LLC.[http://dx

The high-throughput search paradigm adopted by the newly established caloric materials consortium-CaloriCool®-with the goal to substantially accelerate discovery and design of novel caloric materials is briefly discussed. We begin with describing material selection criteria based on known properties, which are then followed by heuristic fast estimates, ab-initio calculations and measurements, all of which has been implemented in a set of automated computational tools. We also demonstrate how theoretical and computational methods serve as a guide for experimental efforts by considering a representative example from the field of magnetocaloric materials Disciplines

MnBi has attracted much attention in recent years due to its potential as a rare-earth-free permanent magnet material. It is unique because its coercivity increases with increasing temperature, which makes it a good hard phase material for exchange coupling nanocomposite magnets. MnBi phase is difficult to obtain, partly because the reaction between Mn and Bi is peritectic, and partly because Mn reacts readily with oxygen. MnO formation is irreversible and harmful to magnet performance. In this paper, we report our efforts toward developing MnBi permanent magnets. To date, high purity MnBi (>90%) can be routinely produced in large quantities. The produced powder exhibits 74.6?emu?g?1 saturation magnetization at room temperature with 9?T applied field. After proper alignment, the maximum energy product (BH)max of the powder reached 11.9?MGOe, and that of the sintered bulk magnet reached 7.8?MGOe at room temperature. A comprehensive study of thermal stability shows that MnBi powder is stable up to 473?K in air. RightsWorks produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. AbstractMnBi has attracted much attention in recent years due to its potential as a rare-earth-free permanent magnet material. It is unique because its coercivity increases with increasing temperature, which makes it a good hard phase material for exchange coupling nanocomposite magnets. MnBi phase is difficult to obtain, partly because the reaction between Mn and Bi is peritectic, and partly because Mn reacts readily with oxygen. MnO formation is irreversible and harmful to magnet performance. In this paper, we report our efforts toward developing MnBi permanent magnets. To date, high purity MnBi (>90%) can be routinely produced in large quantities. The produced powder exhibits 74.6 emu g −1 saturation magnetization at room temperature with 9 T applied field. After proper alignment, the maximum energy product (BH) max of the powder reached 11.9 MGOe, and that of the sintered bulk magnet reached 7.8 MGOe at room temperature. A comprehensive study of thermal stability shows that MnBi powder is stable up to 473 K in air.

Nitinol (NiTi), the most widely used shape-memory alloy, exhibits an austenite phase that has yet to be identified. The usually assumed austenitic structure is cubic B2, which has imaginary phonon modes, hence it is unstable. We suggest a stable austenitic structure that "on average" has B2 symmetry (observed by X-ray and neutron diffraction), but exhibits finite atomic displacements from the ideal B2 sites. The proposed structure has a phonon spectrum that agrees with that from neutron scattering, has diffraction spectra in agreement with XRD, and has an energy relative to the ground state that agrees with calorimetry data.

The metallic compound MnBi is a promising rare-earth-free permanent magnet material, unique among all candidates for its high intrinsic coercivity (Hci) and its large positive temperature coefficient. The Hci of MnBi in thin-film or powder form can exceed 12 and 26 kOe at 300 and 523 K, respectively. Such a steep rise in Hci with increasing temperature is unique to MnBi. Consequently, MnBi is a highly sought-after hard phase for exchange coupling nanocomposite magnets. However, the reaction between Mn and Bi is peritectic, and hence Mn tends to precipitate out of the MnBi liquid during the solidification process. As result, when the alloy is prepared using conventional induction or arc-melting casting methods, additional Mn is required to compensate the precipitation of Mn. In addition to composition, post-casting annealing plays an important role in obtaining a high content of MnBi low-temperature phase (LTP) because the annealing encourages the Mn precipitates and the unreacted Bi to react, forming the desired LTP phase. Here we report a systematic study of the effect of composition and heat treatments on the phase content and magnetic properties of Mn-Bi alloys. In this study, 14 compositions were prepared using conventional metallurgical methods, and the compositions, crystal structures, phase content and magnetic properties of the resulting alloys were analyzed. The results show that the composition with 55 at.% Mn exhibits both the highest LTP content (93 wt.%) and magnetization (74 emu g À1 with 9 T applied field at 300 K).

NiTi is the most used shape-memory alloy, nonetheless, a lack of understanding remains regarding the associated structures and transitions, including their barriers. Using a generalized solid-state nudge elastic band (GSSNEB) method implemented via density-functional theory, we detail the structural transformations in NiTi relevant to shape memory: those between body-centered orthorhombic (BCO) groundstate and a newly identified stable austenite ("glassy" B2-like) structure, including energy barriers (hysteresis) and intermediate structures (observed as a kinetically limited R-phase), and between martensite variants (BCO orientations). All results are in good agreement with available experiment. We contrast the austenite results to those from the often-assumed, but unstable B2. These high-and low-temperature structures and structural transformations provide much needed atomic-scale detail for transitions responsible for NiTi shape-memory effects.