The {100} and (110) cleavage energies of body-centered cubic TiZrNbHf high-entropy alloy are calculated using two alloy models: special quasi-random structures (SQSs) and the coherent potential approximation (CPA). The projector augmented wave method, as implemented in the Vienna ab initio simulation package (VASP), in combination with SQSs is adopted to evaluate the impact of local lattice distortions, whereas the exact muffin-tin orbitals (EMTO) method is used in combination with both SQSs and CPA to study the effect of chemical disorder using rigid underlying lattices. The variations of the cleavage energy as a function of surface chemistry and structure from the EMTO and VASP calculations are consistent with each other. Furthermore, the cleavage energies from CPA are in good agreement with those from SQSs, confirming that an averaged supercell approach reproduces well the mean-field CPA results. The alloy's cleavage energies estimated by the rule of mixtures compare well with those from the direct calculations, and the surface chemistry dependence of the cleavage energies is mainly controlled by the number of Nb atoms in the surface terminal layers owing to the large cleavage energy of Nb metal.

Developing high-strength and ductile face-centered cubic (fcc) high-entropy alloys (HEAs) has attracted significant attention. The generalized stacking fault energy (GSFE) is a very useful concept to describe stable and unstable planar defects and their energies on a slip plane. It plays an essential role in designing high performance fcc HEAs and understanding the nanoscale plasticity phenomena. In this work, using first-principles simulations, we investigate the configuration-averaged GSFEs of 29 single-phase fcc HEAs and identify indicators that can be used to tune stacking fault energies. First we determine the equilibrium structural parameters for all considered alloys and compare them with available experimental data. With the obtained GSFEs, we analyze the relationship between the stacking fault energies and materials properties, and investigate scaling relations between planar fault energies and the tendencies to exhibit deformation twinning and transformation to hexagonal close-packed martensite. We find that unstable SFE and shear modulus correlates strongly. Moreover, we reveal that the ratio of intrinsic SFE to unstable SFE, gamma isf/gamma usf, is a characteristic materials measure, and the tendencies to twinning and martensitic transformation rank with it. Our results are expected to be useful for an efficient alloy design and selection of solutes in fcc HEAs.

Understanding the mechanisms of phase formation and their influence on the mechanical behavior is crucial for materials used in structural applications. Here, the phase decomposition under heat treatment in the Cr7Mn25Co9Ni23Cu36 (atomic percentage) high-entropy alloy and how secondary phases formed affect its tensile mechanical response are reported. The microstructural analysis shows that heat treatment at 800 degrees C /2 h and 600 degrees C /8 h led to the formation of sigma phase, but the sigma phase was not observed for 2 h heat treatment at 600 degrees C and below. The experimentally observed thermal stability and phases are compared to the calculated phase diagram and rationalized by recourse to thermodynamics and kinetics. The mechanism of phase decomposition is discussed based on ab initio calculations, indicating that decomposition into two solid solution phases is energetically preferred over a single solid solution phase with nominal composition.

A first-principles based modeling approach to the effect of temperature on the isothermal single-crystal and polycrystalline elastic parameters of Fe-rich solid solutions is reported. The approach integrates alloy theory for chemical and magnetic disorders with accessible experimental data for the equilibrium volume and ferromagnetic phase transition, and is adopted to predict the temperature-dependent elastic parameters of the body-centered cubic phase of three reduced activation steels, CLAM/CLF-1, F82H, EUROFER97, considered as high-temperature material in power reactors. The predictions are assessed based on available experimental data for a reduced activation steel and both experimental and theoretical data for pure Fe. Alloying effects on the elastic constants relative to pure Fe are found to differ in the magnetically ordered and disordered phases. Contributions due to loss of long-range magnetic order, volume expansion, and entropy are important in determining the temperature dependence of the elastic parameters in all investigated materials. A previously reported, peculiar magneto-volume phenomenon on the equation of state in pure Fe is gradually removed by alloying and magnetic disordering, which requires particular attention when describing the thermo-chemical effects derived from the equation of state in Fe-rich solid solutions.

We used density-functional theory to assess the electronic structure, elastic properties and planar fault energies of the B2 Ti(AlxNb1-x) (0.2 <= x <= 0.8) phase in relation to the composition and chemical ordering. We found that the covalent bonding becomes stronger for B2 Ti(AlxNb1-x) with higher Al concentration and long range order (LRO) parameter. Based on a universal ductile-to-brittle criterion by integrating Pettifor's Cauchy pressure with Pugh's modulus ratio, the deformability becomes less for Ti(AlxNb1-x) with higher Al concentration and LRO parameter, which is well correlated with the bonding character. Rice's ratio has an anti-correlation with Pugh's modulus ratio for Ti(AlxNb1-x). According to Rice's criterion, Ti(AlxNb1-x) with various Al concentration and LRO parameter are brittle in pure mode I loading, however, Nb-enriched disordered and low-ordered Ti(AlxNb1-x) may satisfy Rice's criterion for nucleation of dislocation and thus, are ductile in mode II or III loading. The hardness increases but the fracture toughness decreases obviously with increasing the degree of covalent bonding in Ti(AlxNb1-x).

To widen the applications of new materials in additive manufacturing (AM), the traditional method of printing using pre-alloyed powders should be improved because the pre-alloying process is expensive and makes it difficult to adjust the composition of new materials. This study investigates the synthesis of a FeCoCrNi high-entropy alloy (HEA) containing 1.5 at.% Si in situ using selective laser melting (SLM). A remelting strategy and process optimization based on polynomial regression modeling allowed for the printing of almost fully dense (99.78 %) samples. The samples comprised columnar grains, each containing numerous subgrains of a single-phase face-centered cubic solid solution. No precipitation or segregation were observed. The room temperature tensile properties of the samples were excellent, with yields and tensile strengths reaching 701 +/- 14 and 907 +/- 25 MPa, respectively, and an elongation at fracture of 30.8 +/- 2%. These properties were attributed to solid solution strengthening and novel dislocation loop strengthening mechanism. These findings demonstrate that HEAs with a high relative density and good mechanical properties can be directly synthesized by SLM using inexpensive pure metal powders, thereby extending the application potential of AM to manufacture new materials.

A density-functional theory investigation of the (100) and (110) surfaces of the body-centered cubic (bcc) Fe1-xbCrxb binary alloys, x(b) <= 15 at.%, is reported. The energies and segregation energies of these surfaces were calculated for chemically homogeneous concentration profiles and for Cr surface contents deviating from the nominal one of the bulk. The implications of these results for the surface alloy phase diagram are discussed. The surface chemistry of Fe-Cr(100) is characterized by a transition from Cr depletion to Cr enrichment in a critical bulk Cr composition window of 6 < x(b) < 9 at.%. In contrast, such threshold behavior of the surface Cr content is absent for Fe-Cr(110) and a nearly homogeneous Cr concentration profile is energetically favorable. The strongly suppressed surface-layer relaxation at both surfaces is shown to be of magnetic origin. The compressive, magnetic contribution to the surface relaxation stress is found to correlate well with the surface magnetic moment squared at both surface terminations. The stability of the Cr surface magnetic moments against bulk Cr content is clarified based on the surface electronic structure.