Showing papers on "Laves phase published in 2006"

Causes of breakdown of creep strength in 9Cr-1.8W-0.5Mo-VNb steel

Abstract: Premature breakdown of creep strength is a serious problem to be solved in long-term creep of advanced high Cr ferritic steels. The material studied was ASTM grade 92 steel crept at 550–650 °C for up to 63 151 h. Stress exponent for rupture life decreases from 17 in short-term creep to 8 in long-term creep, confirming the breakdown in the steel. The steel shows ductile to brittle transition with increasing rupture life, and the breakdown accords with the onset of brittle intergranular fracture. Creep cavities are nucleated at coarse precipitates of Laves phase along grain boundaries. These findings suggest the following story of the breakdown of creep strength. Laves phase precipitates and grows during creep exposure. Coarsening of Laves phase particles over a critical size triggers the cavity formation and the consequent brittle intergranular fracture. The brittle fracture causes the breakdown. The coarsening of Laves phase can be detected non-destructively by means of hardness testing of the steel exposed to elevated temperature without stress.

Effect of Al, Cr and Ta additions on the oxidation behaviour of Nb–Ti–Si in situ composites at 800 °C

Abstract: The oxidation behaviour of as-cast Nb–24Ti–18Si and heat-treated (1500 or 1400 °C/100 h) Nb–24Ti–18Si–5Al, Nb–24Ti–18Si–5Cr, Nb–24Ti–18Si–5Al–5Cr, Nb–24Ti–18Si–6Ta–5Al–5Cr, Nb–24Ti–18Si–8Cr–4Al and Nb–24Ti–18Si–6Ta–8Cr–4Al (at.%) alloys in static air at 800 °C for 100 h has been studied using TG, XRD and EPMA. All the alloys exhibited linear oxidation kinetics. The as-cast (Nb,Ti)3Si containing Nb–24Ti–18Si alloy suffered from catastrophic pest oxidation. The other alloys, in which the Nb5Si3 was present, did not pest. Al addition reduced the oxidation rate significantly. Higher oxidation rates and lower adherence of the oxide scale were observed with increase of Cr concentration and/or Ta addition in the Al-containing alloys. In the oxide scales, Si-containing and non-Si-containing oxides of the stoichiometry MO2 were present together with remnants of 5-3 silicide and/or Laves phase. The oxygen solubility in the latter two phases was not high and these phases did not oxidise easily compared to the (Nb,Ti)ss. Below the oxide scale of the Nb–24Ti–18Si–5Al alloy a thick (∼100 μm) internal oxidation zone was formed where the (Nb,Ti)ss contained up to 30 at.% dissolved oxygen. In the Nb–24Ti–18Si–5Cr alloy this zone was thinner and leaner in oxygen. In the Cr- and Al-containing alloys the internal oxidation zone was almost eliminated and the diffusivity of oxygen through the (Nb,Ti)ss was reduced owing to the synergy of the Ti, Cr and Al additions, which was not affected by the addition of Ta to the alloy. The presence of Cr-rich C14 silicide Laves phase in the microstructure of some alloys was not beneficial regarding their oxidation at 800 °C.

Precipitation of Fe2W laves phase and modeling of its direct influence on the strength of a 12Cr-2W steel

Abstract: Precipitation of Fe2W Laves phase in a 12Cr-2W power plant steel is investigated by using transmission electron microscopy (TEM). Fe2W Laves phase is found to be coherent with the matrix and has a stacking fault structure. The influence of formation of Fe2W Laves phase on the yield strength of the steel is quantitatively evaluated. The modeling result indicates that the strengthening effect from the formation of Laves phase particles is diminished by the loss of solid solution strengthening of alloying elements, and, as a result, the strength of the steel remains similar. The effect of clustering and coarsening of Laves precipitates on the strength of the steel is also studied. It is showed that clustering and coarsening decreases the strengthening effect of Laves phase. The limitation of the modeling approach currently adopted is also discussed.

High temperature wear and corrosion resistance of a Laves phase strengthened Co-Mo-Cr-Si alloy

Abstract: A new alloy designated as T-400C comprising of (in wt.%) 14.0Cr, 26.0Mo, 2.6Si, and balance Co with excellent wear resistance and corrosion resistance is developed based on average electron hole number and average d-electron density calculations, and thermal gravitational analysis. Cylinder-on-flat and disc-on-flat wear tests against nitrided 310 stainless steel flat specimens have been carried out at 482 °C using a Cameron–Plint test machine. Immersion corrosion test is conducted in 65% HNO 3 oxidizing acid at 66 °C, in 10% H 2 SO 4 reducing acid at 102 °C and in 5% HCl reducing acid at 66 °C to evaluate the general corrosion resistance of the developed alloy. Alloy T-400C demonstrated better wear resistance and excellent corrosion resistance as compared to a common commercial alloy T-400 with a nominal chemical composition of 8.5% Cr, 28.5% Mo, 2.6% Si and the balance Co.

Binary Rare Earth Laves Phases-an overview

Abstract: The rare earth metals form the largest subset of the AB 2 Laves phase compounds. The rare earth element (R) is always the larger A metal, and they form Laves phases with most of the Group 7-10 elements, plus Mg and Al. As with all Laves phases, the rare earth RB 2 compounds conform with the size rule which had been established in the mid 1930s. Electron concentration effects have also been observed in the RB 2 phases. A number of anomalies and unusual behaviors have been discovered over the years: the La deficiency in "LaNi 2 " and of R in other "RNi 2 " Laves phase compounds; the crystal structure sequence in the RMn 2 compound series; the relationship between the Debye temperature and the radius ratio; the Ce valence in the CeB 2 compounds; and the thermodynamic nature of the magnetic transition in the RCo 2 phases.

High-pressure synthesis of giant magnetostrictive PrxTb1−xFe1.9 alloys

Abstract: PrxTb1−xFe1.9 (0⩽x⩽1) magnetostrictive alloys with cubic Laves phase have been synthesized by a high-pressure synthesis method. Crystal structure, magnetic properties, magnetocrystalline anisotropy, and the magnetostriction of PrxTb1−xFe1.9 (0⩽x⩽1) alloys are investigated. Composition anisotropy compensation is realized in Pr0.9Tb0.1Fe1.9 alloy, which shows low magnetocrystalline anisotropy and a large magnetostriction value (λ‖−λ⊥=1497ppm) at 13kOe at room temperature. These characters suggest that Pr0.9Tb0.1Fe1.9 alloy may be a promising candidate for magnetostriction application.

Influence of magnetic arc oscillation and current pulsing on microstructure and high temperature tensile strength of alloy 718 TIG weldments

Abstract: The aim of the present work is to study the effect of magnetic arc oscillation and current pulsing on the microstructure and high temperature tensile strength of alloy 718 tungsten inert gas weldments. The magnetic arc oscillation technique resulted in refined Laves phase with lesser interconnectivity. The full benefits of current pulsing in breaking the dendrites could not be realized in the present study due to relatively higher heat input used in the welding process. In the direct aged condition weldments prepared using magnetic arc oscillation technique exhibited higher tensile strength due to the presence of refined and lesser-interconnected Laves particles. In the solution treated and aged condition, magnetic arc oscillated weldments exhibited lower tensile strength compared with the weldments made without arc oscillation due to the presence of large amounts of finer δ needles.

Detection of Ni magnetic moment in GdNi2 compound by magnetic Compton profile (MCP) method

Abstract: The Ni magnetic moment in the GdNi2 Laves phase compound is studied in detail by the use of the magnetic Compton profile (MCP) method. Analysis of the MCP, measured at 30 K, reveals that the spin component of Ni 3d electrons coupled antiparallel to that of Gd 4f electrons, and it was confirmed that the Ni retains a spin magnetic moment of 0.16 ± 0.08 μB. Furthermore, the total magnetic moment, including the angular momentum component, is found to be 0.23 ± 0.11 μB and this value that is obtained is in accord with that derived from direct measurement of the bulk magnetization. Our finding of the existence of the Ni magnetic moment contradicts the well-known fact that Ni loses its magnetic moment in the Laves phase of rare-earth (RE)–Ni compounds. In addition, the magnetic moment originating from the electrons with s- and p-character, which are donated by the Gd and Ni atoms, is also observed and quantified.

Site preference, thermodynamic, and magnetic properties of the ternary Laves phase Ti(Fe1–xAlx)2 with the crystal structure of the MgZn2-type

Abstract: Site preference and thermodynamic properties of the ternary Laves phase Ti(Fe1–xAlx)2 with MgZn2-type have been studied employing Rietveld refinement of X-ray and neutron powder diffraction data, X-ray single crystal data, and isoperibolic drop calorimetry techniques. A detailed relation between lattice parameters of Ti(Fe1–xAlx)2 and the Al content in the Laves phase has been presented. The Rietvelt refinement revealed that Ti atoms occupy the 4f site in the MgZn2-type, whilst Fe and Al atoms randomly share the 2a and 6h site. Heat of formation has been measured for Ti(Fe0.5Al0.5)2. For the ab initio density functional theory applications, a large number of structural models were investigated to calculate the concentration dependent (Al) heats of formation, magnetic structural stabilities, lattice parameters, and site occupancies, which are in good agreement with experiment.

Preparation, phase stability and structure of the C36 Laves phase Nb1–xCo2+x

Abstract: The C36 Laves phase Nb 1-x Co 2+x has been reinvestigated in order to study phase stability and structure. Constitutional data have been obtained by investigating homogenized single- and two-phase samples and from diffusion couples. The C36 phase Nb 1-x Co 2+x crystallizes with hexagonal MgNi 2 structure type (Z=8, space group P6 3 /mmc, a = 4.7414(4) A and c = 15.458(1) A at x = 0.265(4)). The homogeneity range extends from 24.6(2) to 25.3(5) at% Nb. The temperature range of the phase field is limited by a eutectoid (C36 Nb 1-x Co 2+x = Nb 2 Co 7 + C15 NbCo 2 ) and a peritectic (L + C15 NbCo 2 = C36 Nb 1-x Co 2+x ) reaction at ≈1050°C and 1264°C, respectively. In addition, the title phase is involved in the peritectoid reaction Co(Nb) + C36 Nb 1-x Co 2+x = Nb 2 Co 7 at 1086 °C and in the eutectic reaction L = Co(Nb) + C36 Nb 1-x Co 2+x at 1239 °C. The C36 and C15 Laves phases of the Nb-Co system are separated by an extremely small two-phase field (<0.5 at%). The crystal structure exhibits pronounced deviations from ideal parameters obtained from a hard sphere model. The Co network displays a type of distortion known from many hexagonal Laves phases. Kagome layers display an elongation of the Co-Co edges of the basal triangles of Co 5 trigonal bipyramids and a contraction of Co-Co edges of the uncapped triangles. The Nb atoms are also displaced from their idealized sites. In the crystal structure of C36 Nb 1-x Co 2+x excess Co atoms randomly substitute Nb atoms - (Nb 1-x Co x )Co 2 . The excess Co atoms occupy preferentially the Nb2 site approximately twice as much as Nbl. These positions differ mainly in the conformation of the corresponding Nb 6 Nb 2 fragments (Nbl-Nbl eclipsed and Nb2-Nb2 staggered). In addition, Co atoms are displaced from the original Nb positions. The distortion of the Co and the Nb network is a consequence of the bonding situation of the defect-free crystal structure. The preferential site occupation of excess Co atoms is triggered by interactions with atoms beyond the first coordination shell. The C36 phase Nb 1-x Co 2+x exhibits Pauli-paramagnetic behavior (Χ P = +1.31 · 10 -3 emu mol -1 ). The temperature dependent part of the electrical resistivity q(300 K) - q 0 is only 17 μΩ cm whereas the residual resistivity is very large with q 0 = 140 μΩ cm indicating strong structural disorder.

The electronic structure and mechanical properties of MgCu2 Laves phase compound

Abstract: Ab initio calculations have been performed to calculate the density of states (DOS), charge density distribution and enthalpy of formation of MgCu2 Laves phase compound based on the method of augmented plane waves plus local orbitals (APW+lo) to reveal its electronic structure. The results show a covalent bonding between Cu–Cu and a metallic bonding between Mg–Cu in MgCu2 that are similar in TiCo2 and TiCr2. Hardness and reduced Young's modules of the three compounds are measured by nano-indenter. Differences in the mechanic properties can be explained by the calculated bonding characteristics.

First-principles calculation of lattice stability of C15–M2R and their hypothetical C15 variants (M=Al, Co, Ni; R=Ca, Ce, Nd, Y)

Abstract: When combining a stoichiometric Laves phase C15–A2B1 with a solid solution C15 phase(s) into a multicomponent system, a sublattice remodeling of the (A,B)2(A,B)1 compound is needed for the sake of database compatibility. This then requires a set of physically-grounded thermodynamic parameters for the hypothetical C15 variants (in the simplest case, A2A, B2B, and B2A), in order to avoid distortion of the phase field relating to the C15 phase in the A–B phase diagram due to the sublattice remodeling. For this purpose, the present investigation employed first-principles (FP) calculations to study the lattice stability of the stable binary C15–M2R (M=Al, Co, Ni; R=Ca, Ce, Nd, Y) and their hypothetical (unstable) C15 variants at T=0K. Our results demonstrated that use of the empirical parameters and energy constraint commonly used in the literature leads to a too large homogeneity range in some of the systems studied and, consequently, significant distortions of the phase diagram. In contrast, when enthalpies of formation based on FP calculations were used for the hypothetical C15 phases, such distortion of the phase diagram is minimized. The other advantage is that there is no need for re-optimization of the existing thermodynamic databases. Therefore, it is proposed that FP enthalpies of formation should be used for the thermodynamic descriptions of hypothetical C15 phases, at least when the empirical parameters fail to reproduce a reasonably accurate A–B binary phase diagram.