Structural properties of ordered high-melting-temperature intermetallic alloys from first-principles total-energy calculations

Abstract

Intermetallic compounds which are ductile at high temperatures are of great technologica1 interest; however, purely experimenta1 searches for improved intermeta11ic materials are time consuming and expensive. Theoretical studies can shorten the experimental search by focusing on compounds with the desired properties. While current ab initio density-functional calculations cannot adequately determine materia1 properties at high temperature, it is possible to compute the staticlattice equation of state and elastic moduli of ordered binary compounds. Known correlations between equilibrium properties and high-temperature properties such as the melting temperature can then be used to point the way for experiments. We demonstrate the power of this approach by applying the linear augmented-plane-wave method to the calculation of the equation of state and all of the zero-pressure elastic moduli for SbY in the B1 (NaC1) phase, CoAl and RuZr in the B2 (CsC1) phase, and NbIr in the L 10 (Au-Cu I) phase. The calculated equilibrium lattice constants are a11 within 2% of the experimentally determined values. The only experimentally known elastic moduli in these systems are the bulk and shear moduli for polycrysta11ine SbY, CoA1, and NbIr. The predicted bulk moduli are with 7%%uo of experiment. Theory enables us to place limits on the experimen-ta1 po1ycrystalline shear modulus. The experimenta1 shear moduli of SbY and CoA1 are within our theoretica1 bounds, but the experimenta1 shear modulus of NbIr is 35% smaller than our lower bound. We stress that in the case of CoAl our calculations provided a prediction for the bulk and shear moduli that were subsequently confirmed by the experiments of Fleischer. Wt; also discuss the band structures and electronic density of states for these materials.

An Interpretation of Martensitic Transformation in L1<SUB>2</SUB>-Type Fe<SUB>3</SUB>Pt from Its Electronic Structure

“…From the c=a dependence of total energy shown in Fig. 2, the elastic modulus c 0 (¼ 1=2ðc 11 {c 12 Þ) is calculated to be 11.6 GPa by using the method reported by Mehl et al 13) This value of c 0 is significantly small compared with that of normal metals (for example 48 GPa 14) for BCCFe and 52 GPa 15) for FCC-Pt). In order to understand the origin of the instability for the tetragonal distortion observed experimentally in Fe 3 Pt, we have calculated the density of states (DOS) of the perfectly ordered Fe 3 Pt, and the result for the cubic phase (c=a ¼ 1) is shown in Fig.…”

Site preference and effect of alloying on elastic properties of ternaryB2 NiAl-based alloys

“…35 The first two constants describe the crystal response to tension, while C 44 describes the response to shear strain. In order to determine the elastic constants C = (C 11 − C 12 )/2 and C 44 , we applied volume-conserving orthorhombic and monoclinic distortions, respectively, and calculated the internal energy response 36 to these small distortions. Elastic constants C 11 and C 12 can be obtained by the combination of C and bulk modulus B = (C 11 + 2C 12 )/3.…”

Ab initio calculations of elastic properties of Ru1−xNixAl superalloys

“…11 Elastic constants are evaluated from the total energy E tot of crystals to which volume-conserving orthorhombic ͓CЈ = ͑C 11 − C 12 ͒ / 2͔ and monoclinic ͑C 44 ͒ distortions have been applied. 12 The two cubic elastic constants C 11 and C 12 are decoupled using the relation B 0 = ͑C 11 +2C 12 ͒ / 3. Upper-bound ͑G H ͒ and lower-bound ͑G S ͒ estimates of the shear modulus G for polycrystals are found according to the Hashin and Shtrikman formalism.…”