Equilibrium phases at the Ir-rich corner of the Ir–Nb–Ni–Al quaternary system

The equilibrium phases of three Ir–Nb–Ni–Al quaternary alloys at the Ir-rich corner were investigated in some detail. The phases were determined by X-ray profile as well as microstructure observation. Three phases of fcc/L1₂-Ir₃Nb/B2-IrAl were found in two samples, and four phases of fcc/L1₂-Ir₃Nb/L1₂-Ni₃Al/B2-IrAl were characterized in one sample. The electron probe microanalysis (EPMA) technique was applied for analyzing the phase composition of this quaternary system. A partial quaternary phase diagram at the Ir-rich side was plotted according to the EPMA results.     

Thermal conductivity mapping of the Ni–Al system and the beta-NiAl phase in the Ni–Al–Cr system

Micron-scale-resolution thermal conductivity mapping on graded compositions created in diffusion-multiple samples can be used to rapidly establish composition-phase-property relationships and to reveal the effects of solid-solutioning, order-disorder transition, compositional point defect, and site preference on thermal conductivity.

1. Download : Download high-res image (231KB)
2. Download : Download full-size image

     

Liquid–solid equilibria in the quasicrystalline regions of the Al–Pd–Mn phase diagram

Phase equilibria were investigated in the Al–Pd–Mn phase diagram in the region where quasicrystals and approximant phases form. With respect to previous thermodynamic studies, the extents of the liquidus phase fields of several approximant phases are either established or precisely determined. In particular, compositional profiles across interfaces show the ternary character of the T(AlPdMn) and ξ′ phases which are close to the binary Al–Mn and Al–Pd limits respectively. It is pointed out that the relative stability of some of the phases involves very small energy differences leading to very long transformation times and the solidification of metastable phases.     

Experimental Investigation of the Ti–V Side in the Ti–V–Sn Ternary System

Phase relations in the Ti–V–Sn system are of great importance for design of aerospace titanium alloys. However, reported Ti–V–Sn ternary phase diagrams present great differences. The isothermal section of the Ti–V side in the Ti–V–Sn system at 1073, 1173, and 1273 K was established using equilibria alloys. There are 11 two-phase equilibria and 3 three-phase equilibria, 9 two-phase equilibria and 3 three-phase equilibria, and 9 two-phase equilibria and 3 three-phase equilibria in the isothermal section at 1073, 1173, and 1273 K, respectively. In addition, remarkable ternary solubility in some binary compounds was detected, e.g., up to 21.18 and 22.23 at.% V in Ti₃Sn and Ti₂Sn, respectively, at 1273 K.     

An investigation on the mechanochemical behavior of the Ca–C–Cu2O–WO3 quaternary system to synthesize the Cu–WC nanocomposite powder

Fundamental aspects of the reaction path in the Ca–C–Cu₂O–WO₃ quaternary system to synthesize a copper matrix nanocomposite with reinforcement particles of tungsten carbide have been studied. The mechanism of reactions was specified through the analysis of the relevant sub-reactions. In the presence of carbon as a reducing agent (without Ca), the carbothermal reaction partially occurred even after 40 h of milling. On the other hand, calcium (without C) reduced both Cu₂O and WO₃ after 15 min of milling in a self-sustaining mode. In the simultaneous presence of Ca and C, the products included Cu and W₂C as well as a significant amount of remaining unreacted W. The Cu–WC nanopowder, with no trace of W₂C, was synthesized by the addition of excess carbon to the initial mixture. SEM observations indicated that the composite powders were agglomerated and the range of the particle size was within 100 nm. Elemental mapping spectra showed a relatively uniform distribution of WC in the Cu matrix.     

Bulk ferromagnetic glasses in the Fe–Ni–P–B System

The ferromagnetic metallic glass Fe₄₀Ni₄₀P₁₄B₆, available only as 30–50 μm thick ribbons, has been extensively studied over the last three decades. We used a flux-melting and water-quenching technique to prepare bulk glassy Fe₄₀Ni₄₀P₁₄B₆ alloys in the form of 2-mm diameter spheres and 1-mm diameter rods. The Curie temperature for the bulk glasses is higher than the average value of Curie temperatures reported for the rapidly quenched ribbons. The glass-transition temperature and the crystallization temperature of the bulk glasses are lower and higher, respectively, than the average values reported for rapidly quenched ribbons, making the supercooled-liquid region as wide as 42 K. The bulk glasses crystallize by a homogeneous nucleation followed by a growth at a constant rate. The nucleation rate in the bulk glasses is four orders of magnitude lower than in the rapidly quenched ribbons, suggesting that the previous thickness limitation was due to impurities in the melt (heterogeneous nucleation).     

Phase equilibria in the system Al–Co–Si

The Al–Co–Si system was studied for three isothermal sections at 600 °C (equilibria with Si), 800 °C (alloys up to 50 at.% Co) and 900 °C (alloys with more than 50 at.% Co). A total number of seven ternary compounds were characterized in the ternary system and the homogeneity ranges of the various ternary solid solutions of binary Co–Al and Co–Si compounds were studied. X-ray powder diffraction and optical microscopy was used for initial sample characterization and electron probe microanalysis of the annealed samples was used to determine the phase compositions within the ternary system. Lattice parameters have been determined for all ternary compounds and the change of lattice parameters with the composition is given for the solid solution phases.     

On some limitations of the Johnson–Mehl–Avrami–Kolmogorov equation

Some limitations of the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation used widely for describing kinetics of phase transformation are demonstrated using probabilistic analysis and Monte Carlo simulations. The JMAK equation predicts correctly the real transformed fraction only if the number of the growing nuclei in the controlled volume is large. If the number of growing nuclei is small, the transformed fraction predicted by the JMAK equation deviates significantly from the real transformed fraction, no matter how large the volume of the controlled volume is. As an alternative, another equation should be applied, which for any number of the growing nuclei predicts correctly the true amount of transformed fraction.     

Rhenium distribution in the matrix and near the particle–matrix interface in a model Ni–Al–Ta–Re superalloy

The distribution of Ni, Al, Ta, and Re in a model Ni-base superalloy is investigated in the as-heat treated alloy by energy dispersive X-ray analysis and three-dimensional atom probe. Re was found to partition mainly to the γ phase in which it forms clusters with sizes of about 1 nm and mutual average distances of 20 nm.     

Liquidus projection for the Mo-rich portion of the Mo–Si–B ternary system

The solidification behavior in the Mo-rich portion of the Mo–Si–B ternary system has been examined based on the microstructures of arc-cast alloys. Several solidification characteristics in the Mo-rich portion of the system have been identified using XRD, SEM and TEM. The liquidus projection in the Mo-rich portion includes six primary solidification reactions; five reactions originating from the Mo–Si and Mo–B binary sections (Mo, Mo₂B, βMoB, Mo₃Si and Mo₅Si₃) and one from the ternary-based (Mo₅SiB₂) T₂ phase. The liquidus surface in general descends from the high melting temperature Mo–B binary side to the lower melting temperature Mo–Si binary side. The solidification path of alloys with compositions in the T₂ phase region is always preceded by the peritectic reaction of βMoB+L⇒T₂. In addition, four Class II reactions (four-phase reactions) and one Class I reaction (invariant ternary eutectic reaction) have been identified. The extent of solidification segregation in alloys with compositions in the Mo(ss)+T₂ two-phase field is discussed as it pertains to materials processing.     

On the origin of the high coarsening resistance of Ω plates in Al–Cu–Mg–Ag Alloys

The thickening kinetics of Ω plates in an Al–4Cu–0.3Mg–0.2Ag (wt. %) alloy have been measured at 200, 250 and 300°C using conventional transmission electron microscopy techniques. At all temperatures examined the thickening showed a linear dependence on time. At 200°C the plates remained less than 6 nm in thickness after 1000 h exposure. At temperatures above 200°C the thickening kinetics are greatly increased. Atomic resolution Z-contrast microscopy has been used to examine the structure and chemistry of the (001)_Ω6(111)_α interphase boundary in samples treated at each temperature. In all cases, two atomic layers of Ag and Mg segregation were found at the broad face of the plate. The risers of the thickening ledges and the ends of the plates were free of Ag segregation. The necessary redistribution of Ag and Mg accompanying a migrating thickening ledge occurs at all temperatures and is not considered to play a decisive role in the excellent coarsening resistance exhibited by the Ω plates at temperatures up to 200°C. Plates transformed at 200°C rarely contained ledges and usually exhibited a strong vacancy misfit normal to the plate. A large increase in ledge density was observed on plates transformed at 300°C, concomitant with accelerated plate thickening kinetics. The high resistance to plate coarsening exhibited by Ω plates at temperatures up to 200°C, is due to a prohibitively high barrier to ledge nucleation in the strong vacancy field normal to the broad face of the plate. Results also suggest that accommodation of the large misfit that exists normal to the broad face of the plate is unlikely to provide the driving force for Ag and Mg segregation.     

Microstructures and phase equilibria of the transition metal corner in the Rh–Al–C and Ir–Al–C ternary systems

The isotherms of the Rh–Al–C and Ir–Al–C ternary systems were determined at 1373 K for the transition metal rich corner. It has been revealed that the three-phase field consisting of primary solid solution of a transition metal, B2 type intermetallic compound and graphite extends over a very wide compositional range in these two alloy systems. There exists no carbide in related transition metal rich regions. The phase constitution in the Rh–Al–C and Ir–Al–C ternary alloys prepared by arc melting is basically the same as that by spark plasma sintering (SPS) except the presence of Al₂O₃ particles. The reaction scheme and liquidus surface projection in the transition metal corner of the Rh–Al–C and Ir–Al–C ternary systems were estimated by deliberating on the solidification sequence, as-cast microstructures and invariant reactions on the binary edges.     

Softening produced by the microstructural instability of the intermetallic alloy Ti–46.5Al–2W–0.5Si

The influence of heat treatments on the softening process of a TiAl intermetallic alloy containing three phases (namely γ, α₂ and B2) has been examined. At the annealing temperatures studied, the lamellar microstructure exhibits faster softening kinetics than the globular one due to the transformation that takes place between the α₂ and B2 phases while the globular structure softens by a coarsening/dissolution process of the respective B2 and α₂ globules initially present.     

Influence of the thermomechanical treatment on the microplastic behaviour of a wrought Al–Zn–Mg–Cu alloy

For plastic deformations smaller than the conventional limit of 0.2% for the yield stress, the so-called microplastic behaviour of a rolled Al–Zn–Mg–Cu alloy is investigated experimentally. Tension and compression responses are compared along the rolling and the transverse directions. The alloy shows an asymmetric response in tension and compression (i.e., compressive stress minus tensile stress for a given absolute plastic strain is non-zero). Moreover, this asymmetry changes sign between the rolling and the transverse directions. The difference between tension to compression is observed to decrease as the conventional limit is approached.The influence of the heterogeneous microstructure of the alloy and the fabrication process on these asymmetries is discussed. Modelling of the material response based on a self-consistent scheme is used to estimate the internal stresses resulting from the thermomechanical treatment, and also to investigate the influence of the heterogeneous elastoplastic behaviour of two types of constitutive grains (recrystallised and unrecrystallised). For a plastic strain smaller than 0.02%, the microplastic behaviour is not well described with the adopted model since the underlying assumption of uniform stress and strain fields per phase is questionable in this initial level of plasticity. However, the model shows that the asymmetries observed at plastic strains ranging from 0.02% to 0.2% are consistent with the intragranular stresses developed during the stretching step.     

Bulk metallic glass formation in the Mg–Cu–Zn–Y system

The element Cu in the bulk glass-forming alloy Mg₆₅Cu₂₅Y₁₀ was substituted with the element Zn to form a Mg₆₅Cu₂₀Zn₅Y₁₀ alloy, which caused a significant improvement of the glass-forming ability of Mg₆₅Cu₂₅Y₁₀ alloy. For the Mg₆₅Cu₂₀Zn₅Y₁₀ alloy, fully glassy rod with a 6-mm diameter can be obtained by copper mold casting.