Showing papers on "Laves phase published in 2009"

Composition, Microstructure, and Water Vapor Effects on Internal/External Oxidation of Alumina-Forming Austenitic Stainless Steels

Abstract: A family of creep-resistant austenitic stainless steels based on alumina (Al2O3) scale formation (AFA alloys) for superior high-temperature oxidation resistance was recently identified. Excellent oxidation behavior was observed at 650 and 700 °C in air with 10% water vapor. However, particularly at 800 °C, the presence of water vapor greatly increased the tendency for internal oxidation of Al. Water vapor also enhanced subscale Al depletion in some AFA alloys relative to dry air exposure. Increased levels of Nb additions were found to significantly improve oxidation resistance, as were reactive element additions of Hf and Y. Computational thermodynamic calculations of the austenitic matrix phase composition and the volume fraction of MC, B2-NiAl, and Fe2Nb base Laves phase precipitates were used to interpret oxidation behavior in terms of two-phase oxidation theory, reservoir effect, and the third-element effect of Cr. Of particular interest was the enrichment of Cr in the austenitic matrix phase by additions of Nb, which aided the establishment and maintenance of alumina. Higher levels of Nb additions also increased the volume fraction of B2-NiAl precipitates, which served as an Al reservoir during long-term oxidation. Implications of these findings for the design of AFA alloys with increased upper use temperature limits are discussed.

Thermodynamic modeling and optimization of the Fe–Ni–Ti system

Abstract: A thermodynamic assessment of the ternary Fe–Ni–Ti system together with a partial reassessment of the binary sub-systems Ni–Ti and Fe–Ti was made following the CALPHAD method and using the compound energy formalism (CEF). Two and four sublattices were used to model the bcc and fcc phases respectively. This allows describing the order–disorder transformations occurring not only in the ternary Fe–Ni–Ti system, but also in the quaternary Al–Fe–Ni–Ti system. The description of the C14 Laves phase TiFe2 was modified to three sublattices in order to be consistent with other Al–Fe–Ni–Ti sub-systems in which a three sublattice model is needed. Thermodynamic parameters were optimized using the available experimental data. On the basis of this optimization the stable phase diagram is calculated. Moreover stable and metastable equilibria between ordered and disordered phases based on fcc and bcc respectively are calculated.

Effect of Alloying Additions on Phase Equilibria and Creep Resistance of Alumina-Forming Austenitic Stainless Steels

Abstract: The high-temperature creep properties of a series of alumina-forming austenitic (AFA) stainless steels based on Fe-20Ni-(12-14)Cr-(2.5-4)Al-(0.2-3.3)Nb-0.1C (weight percent) were studied. Computational thermodynamics were used to aid in the interpretation of data on microstructural stability, phase equilibria, and creep resistance. Phases of MC (M: mainly Nb), M23C6 (M: mainly Cr), B2 [β-(Ni,Fe)Al], and Laves [Fe2(Mo,Nb)] were observed after creep-rupture testing at 750 °C and 170 MPa; this was generally consistent with the thermodynamic calculations. The creep resistance increased with increasing Nb additions up to 1 wt pct in the 2.5 and 3 Al wt pct alloy series, due to the stabilization of nanoscale MC particles relative to M23C6. Additions of Nb greater than 1 wt pct decreased creep resistance in the alloy series due to stabilization of the Laves phase and increased amounts of undissolved, coarse MC, which effectively reduced the precipitation of nanoscale MC particles. The additions of Al also increased the creep resistance moderately due to the increase in the volume fraction of B2 phase precipitates. Calculations suggested that optimum creep resistance would be achieved at approximately 1.5 wt pct Nb in the 4 wt pct Al alloy series.

Large, larger, largest--a family of cluster-based tantalum copper aluminides with giant unit cells. I. Structure solution and refinement

Abstract: This is the first of two parts, where we report the structure determination of a novel family of cluster-based intermetallic phases of unprecedented complexity: cF444-Al(63.6)Ta(36.4) (AT-19), a = 19.1663 (1) A, V = 7040 A3, cF(5928-x)-Al(56.6)Cu(3.9)Ta(39.5), x = 20 (ACT-45), a = 45.376 (1) A, V = 93,428 A(3) and cF(23,256-x)-Al(55.4)Cu(5.4)Ta(39.1), x = 122 (ACT-71), a = 71.490 (4) A, V = 365,372 A3. The space group is F43m in all three cases. These cluster-based structures are closely related to the class of Frank-Kasper phases. It is remarkable that all three structures show the same average structure that resembles the cubic Laves phase.

High-pressure phases, vibrational properties, and electronic structure of Ne ( He ) 2 and Ar ( He ) 2 : A first-principles study

Abstract: We have carried out a comprehensive first-principles study of the energetic, structural, and electronic properties of solid rare-gas RG-helium binary compounds, in particular, NeHe2 and ArHe2, under pressure and at temperatures within the range of 0T2000 K. Our approach is based on density-functional theory and the generalized gradient approximation for the exchange-correlation energy; we rely on total Helmholtz freeenergy calculations performed within the quasiharmonic approximation for most of our analysis. In NeHe2, we find that at pressures of around 20 GPa the system stabilizes in the MgZn2 Laves structure, in accordance to what was suggested in previous experimental investigations. In the same compound, we predict a solid-solid phase transition among structures of the Laves family of the type MgZn2 →MgCu2, at a pressure of Pt =1201 GPa. In ArHe2, we find that the system stabilizes in the MgCu2 Laves phase at low pressures but it transitates toward the AlB2-type structure by effect of compression at Pt=13.84 GPa. The phonon spectra of the NeHe2 crystal in the MgZn2 and MgCu2 Laves structures, and that of ArHe2 in the AlB2-type phase, are reported. We observe that the compressibility of RG-RG and He-He bond distances in RGHe2 crystals is practically identical to that found in respective RG and He pure solids. This behavior emulates that of a system of noninteracting hard spheres in closed-packed configuration and comes to show the relevance of short-range interactions on this type of mixtures. Based on size-ratio arguments and empirical observations, we construct a generalized phase diagram for all RGHe2 crystals up to a pressure of 200 GPa where we map out systematic structural trends. Excellent qualitative agreement between such generalized phase diagram and accurate ab initio calculations is proved. A similar construction is done for RGH22 crystals; we find that the MgCu2 Laves structure, which has been ignored in all RG-H2 works so far, might turn out to be competitive with respect to the MgZn2 and AlB2-type structures. Furthermore, we explore the pressure evolution of the energy-band gap in RGHe2 solids and elaborate an argument based on electronic-band theory which explains the observed trends.

Thermodynamic assessment of the Fe–Al–Zr phase diagram

Abstract: The thermodynamic assessment of the Fe–Al–Zr ternary system was performed using the CALPHAD technique. The four published isothermal sections at 1073, 1173, 1273 and 1423 K, together with the proposed liquidus surface and the corresponding invariant reaction scheme, were taken into account. The reliability of the previous binary assessments was checked for the systems of interest, namely, for the Fe–Zr, Zr–Al and Fe–Al systems. Reassessment of the C15 Laves phase of the Fe–Zr system has been done to obtain a better description of the phases in the ternary system. The solution phases, including liquid, α -Fe, γ -Fe, α -Zr, β -Zr and γ -Al, were modelled as substitutional solutions, while the new ternary phases ( τ 1 , τ 2 and τ 3 ) were modelled as linear and stoichiometric compounds, respectively. The C14 and C15 Laves phases have been assessed using a two-sublattice model. Any ternary solubility of the other binary phases was not taken into account in the ternary assessment. The liquidus projection and several isothermal sections were calculated, and they agreed reasonably well with the reported experimental data.

Study on Laves phase in an advanced heat-resistant steel

Abstract: The Laves phase is one of the most significant precipitates in ferritic/martensitic heat-resistant steels. Laves phase precipitates in the creep rupture specimens with different rupture life were studied on a 10 wt.% Cr heat-resistant steel. JMatPro thermodynamic and kinetic calculations were carried out to simulate and predict the precipitation behavior of the Laves phase in the steel at the equilibrium state. The morphologies of the Laves phase developed with creep time were characterized under both scanning electron microscope (SEM) and transmission electron microscope (TEM). Effects of Co on the growth behavior of Laves phase and the corresponding fracture mode were analyzed. It was found that the Laves phase in the steel grew to 200 nm in size after only 1598 h at 600°C, indicating that the addition of Co in the steel could accelerate the growth of Laves phase, and the coalescence of large Laves phase would lead to the brittle intergranular fracture.

Ferromagnetism and Magnetocaloric Effect around 95 K in the Laves Phase EuRh1.2Zn0.8

Abstract: Samples of the solid solution EuRh2−xZnx with x ≈ 1 were synthesized by high-frequency melting of the elements in sealed tantalum tubes. They crystallize with the structure of the cubic Laves phase MgCu2. The structure of one crystal was refined from single-crystal X-ray diffractometer data: space group Fd3m, a = 768.58(6) pm, wR2 = 0.0121, 99 F2 values, 6 variables. The rhodium and zinc atoms statistically occupy the 16d site of the three-dimensional tetrahedral network. The europium atoms have coordination number 16 by 12 Rh/Zn and 4 Eu neighbors. The sample with nominal composition EuRh1.2Zn0.8 was studied with respect to its physical properties. EuRh1.2Zn0.8 shows stable divalent europium (8.00 μB/Eu atom) and orders ferromagnetically at TC = 95 K. Magnetization isotherms characterize EuRh1.2Zn0.8 as a soft ferromagnet. 151Eu Mossbauer spectroscopic data confirm the europium valence state (δ = −7.44 mm/s at 77 K) and the magnetic ordering. According to magnetocaloric investigations, EuRh1.2Zn0.8 has ...

Effect of Al additions on the oxidation behavior of Laves phase NbCr2 alloys at 1373 K and 1473 K

Abstract: Effect of Al on oxidation behavior of the Laves phase NbCr 2 based alloys, which are fabricated by hot pressing and mechanical alloying, has been investigated at 1373 K and 1473 K, respectively. The oxidation resistance of the alloyed NbCr 2 alloys increases with Al concentration increasing from 2 to 12 at.%. However, the alloyed NbCr 2 has higher oxidation rate than unalloyed NbCr 2 at 1473 K. When they are oxidized at 1373 K, NbCr 2 –12 at.% Al alloy shows lower oxidation rate than unalloyed NbCr 2 . The mechanism of oxidation has been analyzed using thermodynamic and kinetic.

Hydrogen storage behavior in C15 Laves phase compound TiCr2 by first principles

Abstract: Hydrogen storage in TiCr2 alloy of AB(2) type with C15 Laves phase structure has been studied using density functional theory and plane-wave pseudopotential technique. The H atom interacts stronger ...