M. Sluiter

Nano-Indentation Hardness and Elastic Moduli of Bulk Single-Crystal AlN

The hardness of 0.5-mm-thick single-crystal aluminum nitride (α-AlN) was measured by the nano-indentation method at room temperature. The hardness was 18 GPa while the bulk, Youngs and shear moduli, were evaluated to be 220, 374 and 154 GPa, respectively.

A lead-free high-TC ferroelectric BaTi2O5: A first-principles study

Previous experimental indications of ferroelectricity with Curie temperature (TC) of 747 K in BaTi2O5 are confirmed through assessment of its structural stability and estimation of its spontaneous polarization using first-principles density functional theory calculations. The present work establishes that the polarization is observed along b axis only and determines its structural origin with calculations of the Born effective charges. We find its piezoelectric response is comparable to that of PbTiO3, which with its high TC and nonperovskite structure should open up possibilities for new lead-free ferroelectric materials.

A first-principles study of phase stability in NiAl and NiTi alloys

Abstract The thermodynamic stability of the b.c.c.- and f.c.c.-based ordered phases of Niue5f8Ti and Niue5f8Al has been studied with a highly accurate first-principles electronic structure method. The occurrence of a martensitic transformation in Niue5f8Al B2 ordered intermetallic alloys will be discussed and related to the existence of intermediate structures between b.c.c.- and f.c.c.-based phases. It will be shown that closely related ordered structures can exist on f.c.c. and b.c.c. lattices in the composition range where the transformation occurs. A variety of antiphase boundaries in the B2 and L12 structures will be examined and the pertinent energies will be compared with experimental data. In addition, the rather unusual appearance of the Niue5f8Ti B2 ordered structure in the phase diagram will be discussed.

Theoretical study of phase stability in Ni-Al and Ni-Ti alloys

The thermodynamic stability of the bcc and fcc based ordered phases in Ni-Ti and Ni-AI has been studied with a highly accurate first-principles electronic structure method. The occurrence of a martensitic transformation in Ni-AlB2 ordered intermetallic alloys is discussed with relation to the existence of intermediate structures between bcc and fcc based phases. It is shown that closely related ordered structures can exist on fcc and bcc lattices in the composition range where the transformation occurs. The Ni-rich side of the Ni-Al phase diagram has been computed, and a comparison with a recent assessment is made. In addition, the rather unusual appearance of the NiTi B2 ordered structure in the phase diagram is discussed.

Using first-principles results to calculate finite-temperature thermodynamic properties of the Nb-Ni μ phase in the Bragg-Williams approximation

Results of first-principles (FP) total energy calculations for 32 different configurations of the μ phase in the binary system Nb–Ni are used in the compound energy formalism (CEF) to model finite-temperature thermodynamic properties. A comparison with Cluster Expansion Hamiltonian–Cluster Variation Method (CEH-CVM) calculations indicates that the CEF describes temperature-dependent site occupancies as well as the CEH-CVM within the temperature range of interest for applications. This suggests that the Bragg–Williams–Gorsky approximation (BWGA) used in the CEF is sufficient to describe site occupancies and thermodynamics of the μ phase. A phase diagram is calculated using the μ phase description derived in the present work together with a previous Calphad description for the other phases of this system. The FP-CEF approach significantly improves the description of the thermodynamic properties as a function of composition compared to the Calphad procedure generally used up to now.

Ordering effects in the Re-W and Re-Ta sigma phases

The site occupancy in the Re–W and Re–Ta sigma phases is studied using a first-principles statistical thermodynamics approach. The factors which govern the stability of the sigma phase were analysed using selected supercells for total energy calculations. Cluster variation calculations in the tetrahedron approximation were performed to study the effect of composition and of finite temperature on the ordering phenomena in the sigma phase. The difference between the site occupancy of both Re–W and Re–Ta sigma phases is discussed.

Theoretical approach of phase selection in refractory metals and alloys

Abstract First-principles total energy calculations are used to examine the phase stability of the Frank–Kasper (FK) structures. The sequence of FK phases observed as a function of composition in many transition metal binary systems is shown to have a band-filling origin. We show that the alloying effects can be treated using a first-principles statistical thermodynamics approach and first results are given for Re–Ta and Re–W sigma alloys.

A first-principles study of phase stability in Ni-Al and Ni-Ti alloys

Abstract The thermodynamic stability of the b.c.c- and f.c.c.-based ordered phases of Ni–Ti and Ni–Al has been studied with a highly accurate first-principles electronic structure method. The occurrence of a martensitic transformation in Ni–Al B2 ordered intermetallic alloys will be discussed and related to the existence of intermediate structures between b.c.c- and f.c.c.-based phases. It will be shown that closely related ordered structures can exist on f.c.c. and b.c.c. lattices in the composition range where the transformation occurs. A variety of antiphase boundaries in the B2 and L12 structures will be examined and the pertinent energies will be compared with experimental data. In addition, the rather unusual appearance of the Ni–Ti B2 ordered structure in the phase diagram will be discussed.

The effect of segregation and partial order on the thermodynamics of (1 1 1) antiphase boundaries in Ni3Al

Thermodynamic properties of thermal and a-thermal (1 1 1) antiphase boundaries (APB) in Ni3Al are computed from first-principles. The eAect of oA-stoichiometry, partial disordering and segregation are evaluated and a rough estimate of the vibrational contribution to the antiphase boundary energy is given. Although the vibrational eAect is found to be small, the configurational eAects are large, so that at non-zero temperature and in oA-stoichiometric Ni3Al the antiphase boundary energy (APBE) may be only half of that in perfectly ordered stoichiometric Ni3Al at zero temperature. This result points to a discrepancy between electronic structure calculations and experimental measurements. ” 1999 Elsevier Science B.V All rights reserved.

A study of the thermodynamics of segregation and partial order at (111) antiphase boundaries in Ni3Al

Abstract The thermodynamic properties of thermal and athermal (111) antiphase-boundaries in Ni3Al are computed from first principles. The effects of off-stoichiometry, partial disordering and segregation are evaluated and a rough estimate of the vibrational contribution to the antiphase-boundary energy is given. Although the vibrational effect is found to be small, the configurational effects are large, so that at non-zero temperature and in off-stoichiometric Ni3Al the antiphase-boundary energy may be reduced to only half of that in perfectly ordered stoichiometric Ni3Al at zero temperature. This suggests a discrepancy between electronic structure calculations and experimental measurements.