Modeling of γ/γ′ phase equilibrium in the nickel-aluminum system

A theoretical model is proposed for the determination of phase equilibrium in alloys taking into consideration dissimilar lattice parameters. Volume dependent pair interactions are introduced by means of phenomenological Lennard-Jones potentials and the configurational entropy of the system is treated in the tetrahedron approximation of the cluster variation method. The model is applied to the superalloy relevant nickel-rich, γ/γ′ phase region of the Ni-Al phase diagram. The model predicts reasonable values for the lattice parameters and the enthalpy of formation as a function of composition, and the calculated phase diagram closely approximates the experimental diagram.     

Effect of the solidification and aging conditions on the mechanical properties of Pb–Ca–Sn battery alloys

The influence of the mold temperature on the mechanical properties and the structure of Pb–Ca–Sn battery alloys during their solidification and subsequent aging is studied.     

Study of K~+ doping on structure and properties of γ-Ce_2S_3

In this study,K~+-doped γ-Ce_2 S_3 was successfully prepared via a gas-solid reaction method using CeO_2,K_2 CO_3,and CS_2 as raw materials.The effects of the suitable sulfide system and different molar ratios of K to Ce(n_(K/Ce)=0-0.30) on the phase composition,crystal structure,chromaticity and thermal stability ofγ-Ce_2 S_3 were systematically investigated.Pure γ-Ce_2 S_3 was obtained by calcining the doped samples at840℃ for 150 min.After calcination at the same temperature the undoped K+samples exhibit a pure α-phase.Samples with a K/Ce molar ratio(n_(K/Ce)) of 0.10-0.25 comprise only the γ-phase;and when n_(K/Ce) exceeds 0.25,a new heterogeneous phase,KCeS_2,emerges.For values of n_(K/Ce) in the range of0-0.25,the γ-Ce_2 S_3 lattice parameters gradually increases with increasing K~+ content.When n_(K/Ce)exceedes 0.25,the lattice parameters remains unchanged.As n_(K/Ce) increased,the synthesized color gradually changes from red to orange—red and finally,to yellow.The redness value a~* reaches the maximum(L~*=33.86,a~*=36.68,b~*=38.15) when n_(K/Ce)=0.10,The n_(K/Ce)=0.10 composition continues to exhibit the y-phase after heat treatment at 420℃ for 10 min in air.The K+doping fills the internal vacancies of γ-Ce_2 S_3 and formed a solid solution,which is beneficial for the stability of its lattice,thus improving the thermal stability of γ-Ce_2 S_3(from 350 to 420℃).     

Influence of the solidification temperature–time parameters on the structure and mechanical properties of nickel aluminide–based alloys

The influence of the technological parameters of directional solidification, namely, the solidification rate and the temperature gradient, on the microstructure, the structure–phase parameters, and the mechanical properties of nickel aluminide–based intermetallic alloys is considered.     

Effects of the precursor size on the morphologies and properties of γ-Ce_2S_3 as a pigment

Ceria spheres with different sizes and sulfurized products with corresponding morphology were prepared by hydrothermal and gas-solid reaction method at 600–800 °C under CS2 atmosphere for a short time, respectively. Dimensional effect in preparation of γ-Ce2S3 was firstly investigated by means of techniques such as scanning electron microscopy(SEM), X-ray diffraction(XRD), thermal gravimetric analysis(TGA) and spectrophotometer. The results showed that when ceria nanoparticles with small size were used as precursors, the γ-Ce2S3 could be prepared at the lower temperature and the badly sintered products were obtained; when ceria nanoparticles with large size were employed as precursors, pure γ-Ce2S3 was difficultly obtained even if the temperature was up to 800 °C and the products tended to keep their original size. The heat-resistance property of the γ-Ce2S3 with large size was better than the smaller one, and the pure γ-Ce2S3 prepared from precursor with small size had a good pigmentary performance.     

Development of Cr and Al Pack Cementation Coatings on Co-Based γ/γ′ Superalloys

Metallurgical and Materials Transactions A - Co-based superalloys have been developed as candidate materials to replace Ni-based superalloys in hot sections of turbine engines, however, their...     

Microstructures and mechanical properties of rapidly solidified CuCr alloys

Rapidly-quenched CuCr alloys have been prepared by melt-spinning and the microstructures and mechanical properties examined as a function of alloy content and subsequent annealing treatment. Mechanical properties have been successfully measured by tensile testing on the ribbon samples and it is shown that this method is more suitable than hardness testing for a proper evaluation of the materials. An alloy containing 2% Cr has been prepared in the totally solid-solution state, whilst a 5% Cr alloy contains large chromium particles distributed uniformly throughout the ribbon and fine-grain-sized CuCr solution. These large chromium particles are deduced to arise by a uniform, primary-solidification mode prior to, and independent of, the nucleation and growth of the copper from the lower ribbon surface. The increased solid solubility obtained allows extensive precipitation and strengthening following ageing: the primary chromium particles play an important role both in strengthening and in restricting grain coarsening.     

Effect of coherent strain energy on γ/γ′ phase equilibria in NiAlTi alloys

The characteristic influences of elastic strain energy on the phase equilibria of the γ + γ′ two phases in NiAlTi alloys have experimentally been investigated by means of analytical transmission electron microscopy. The results obtained are as follows; the tie-line ends of phase decomposition do not always coincide with the equilibrium phase boundaries and move inside the miscibility gap when the precipitates are coherent with the matrix, but move both to the right hand for the incoherent precipitates and are finally fixed for the large incoherent precipitates on the equilibrium compositions of the phase diagram. These experimental results are theoretically rationalized on the basis of the free energy of the microstructure proposed by us.     

Effects of strain aging on the structure and mechanical properties of PM 92·5W–5Ni–2·5Fe heavy alloys

Abstract

This paper describes the effects of strain aging on the mechanical properties and the microstructure of forged 92·5W–5Ni–2·5Fe and its heavy alloys microalloyed with cobalt. The investigation was performed on cold rotary forged rods deformed 15, 20 and 30% and strain aged at temperatures from 673 to 1273 K for 1·8–32·4 ks. The results show that for these alloys, there is a temperature range from 773 to 873 K in which maximum ultimate strength and hardness can be attained. Furthermore, the strain aged alloys have shown strength and hardness increase at a temperature of 973 K in a time period of 10·8 ks. The fracture analysis has shown the presence of predominant transgranular fracture of the tungsten phase and γ-phase in the strain-aged alloys in comparison with the forged alloys. The results indicate that interface and tungsten phase strengthening are predominant mechanisms of strain aging.     

The effect of FeS film on the surface of iron powder particles on mechanical properties and fracture of Fe–Cu–C alloys

This article provides a new kind of P/M processing by designing thin-layered FeS film coated on the surface of iron powder particles for preparing high density Fe–Cu–C materials. Experimental results showed that FeS lubricating coating on the surface of iron powder particles was significant as a means of reducing friction in the pressing process of Fe–Cu–C alloys. After being pressed, the green density increased from 7.18 to 7.42?g?cm^?3. The sintered density increased from 7.10 to 7.37?g?cm^?3. In the sintering process, the FeS was liquid, and useful in purifying particle surface and strengthening grain boundaries to improve the mechanical properties. The hardness and tensile strength of the Fe–2.0Cu–0.9C–0.5FeS₂ material were 80.5 HRB and 590?MPa. Analysis of fracture showed the main fracture was transgranular fracture. And FeS spherical particles gathered in the pores of the Fe–Cu–C alloys.     

Microstructure and mechanical properties of RS NiCrAl melt-spun alloys

The microstructure and mechanical properties of three melt-spun NiCrAl alloy ribbons have been studied in the as-cast condition as well as after thermal treatments. The microstructure of the alloys is dendritic-microcellular in as-cast condition and phases present for 10 at.% Al and 30 at.% Al alloys are as is predicted by the equilibrium phase diagram. In the 20 at.% Al alloy, γ' has frozen in metastable form and partial ordering takes place during cooling in the solid state. After thermal treatments the ribbons generally maintain a refined microstructure; α phase precipitates are always found in β and γ' phases in 20 and 30 at.% Al alloys. The hardness of the alloys increases with aluminum content. The tensile strength at room temperature is related to the phases present in the material for each state of treatment. The alloys are brittle, a higher ductility always being obtained in the as-cast condition.     

Effects of microstructure on the short fatigue crack initiation and propagation characteristics of biomedical α/β titanium alloys

This article presents the results of a study of the effects of microstructure on the fatigue strength and the short fatigue crack initiation and propagation characteristics of a biomedical α/β titanium alloy, Ti-6Al-7Nb. The results are compared to those obtained from a Ti-6Al-4V extra-low interstitial (ELI) alloy. Fatigue crack initiation occurs mainly at primary α grain boundaries in an equiaxed α structure, whereas, in a Widmanst?tten α structure, initiation occurs within the α colonies and prior β grains, where α plates are inclined at around 45 deg to the stress-axis direction. In an equiaxed α structure, the short fatigue crack initiation and propagation life, where the length of the crack (a) is in a microstructurally short fatigue-crack regime (2a < 50 μm), occupies around 50 pct of the total fatigue life. On the other hand, the fatigue crack in a Widmanst?tten α structure initiates at very early stages of fatigue, and, therefore, the fatigue crack-initiation life occupies a few percentages of the total fatigue life in an α structure. Then, the short fatigue crack propagates rapidly and is arrested at the grain boundaries of α colonies or prior β grains for a relatively long period, until the short crack passes through the boundaries to specimen failure. Therefore, the short fatigue crack-arrest life occupies more than 90 pct of the total fatigue life in a Widmanst?tten α structure. These trends are similar between the Ti-6Al-7Nb and Ti-6Al-4V ELI alloys and biomedical α/β titanium alloys. The total fatigue life for the Ti-6Al-7Nb alloy with an equiaxed α structure is changed by the volume fraction of primary α phase and the cooling rate after solution treatment. By increasing the volume fraction of the primary α phase from 0 to 70 pct, the fatigue limit of the Ti-6Al-7Nb alloy is raised. Changing the cooling rate after solution treatment by switching from air cooling to water quenching improves the fatigue limit of the Ti-6Al-7Nb alloy significantly.     

Effects of trace ytterbium addition on microstructure,mechanical and thermal properties of hypoeutectic Al–5Ni alloy

Over the past decade, the cast aluminum alloys with excellent mechanical and conductivity properties have emerged as potential materials for thermal management. However, the traditional Al–Si based alloys are difficult to make significant breakthrough in conductivity performance. The hypoeutectic Al–5Ni alloy also possesses sound castability and is expected to be applied in thermal management applications. In this study, the effects of ytterbium (Yb element) at 0–0.5 wt% on the microstructures as well as the electrical/thermal conductivity and mechanical properties of the Al–5Ni alloy were systematically investigated. The experimental results indicate that the addition of Yb at a relatively low amount not only reduces the secondary dendrite arm spacing of the α-Al grains, but also modifies the morphology and distribution of eutectic boundary phase. Moreover, it is found that the dosage of Yb at 0.3 wt% in the Al–5Ni alloy can simultaneously improve the yield strength, ultimate tensile strength and electrical/thermal conductivity. The strengthening and toughening of the Al–5Ni alloy are mainly attributed to the decrease of secondary dendrite arm spacing and the improvement of eutectic phases. The transmission electron microscopy/selected area electron diffraction (TEM/SAED) analysis indicates that the ytterbium in Al–5Ni alloy will form Al₃Yb phase, which mainly agglomerates in the Al₃Ni phase region. This phase is helpful to decrease the solubility of impurity elements (e.g., Fe and Si) in the α-Al matrix, which is beneficial to electrical/thermal conductivity. The value of this study lays foundation for manufacturing Al–Ni alloys with high thermal conductivity and acceptable mechanical properties.     

Effects of la addition on microstructure and mechanical properties of SN–58BI solders joints with osp pads

Abstract

Rapid whisker growth was significantly alleviated in a Sn–58Bi alloy doped with 0·5 wt-%La. Experimental results showed that many thin, plate form intermetallic phases appeared in the solder matrix. The absence of tin whiskers was correlated to the shape effect of RE containing intermetallic plates. After reflowing, Cu₆Sn₅ intermetallic compounds appeared at the solder/pad interfaces of Sn–58Bi and Sn–58Bi–0·5La packages with OSP pads, which grow linearly during the aging at 75 and 100°C for various times ranging from 100 to 1000 h. In addition, the interfacial intermetallics layers in Sn–58Bi–0·5La solder joints were also observed thinner than those in undoped Sn–58Bi joints. However, RE containing interfacial intermetallic compounds in Sn–58Bi–0·5La solder joints led to lower bonding strengths in both ball shear tests (0·4 mm s^?1) and high speed ball shear tests (2000 mm s^?1). All the reflowed and aged solder joints of both packages were ruptured through the solder balls indicating brittle characteristic.

On a réduit significativement la croissance rapide de barbe dans un alliage de Sn–58Bi dopé avec 0·5% en poids de La. Les résultats expérimentaux ont montré que plusieurs phases intermétalliques fines en forme de plaque apparaissaient dans la matrice du métal d’apport. On a corrélé l’absence de barbes d’étain aux effets de la forme des plaques intermétalliques contenant de la terre rare. Après la refusion, les composés intermétalliques de Cu₆Sn₅ apparaissaient à l’interface du métal d’apport et des pastilles des paquets de Sn–58Bi et de Sn–58Bi–0·5La avec pastilles d’OSP, lesquels s’accroissent linéairement lors du vieillissement à 75 et à 100°C pour des durées variées allant de 100 à 1000 heures. De plus, on a observé que les couches intermétalliques de l’interface dans les joints à brasure tendre de Sn–58Bi–0·5La étaient plus minces que celles des joints non dopés de Sn–58Bi. Cependant, les composés intermétalliques de l’interface contenant de la terre rare dans les joints à brasure tendre de Sn–58Bi–0·5La menaient à des résistances de cohésion plus faibles tant dans les épreuves de cisaillement de bille (0·4 mm s^?1) que dans les épreuves de cisaillement de bille à haute vitesse (2000 mm s^?1). Tous les joints à brasure tendre avec refusion et vieillissement des deux paquets se fracturaient à travers les billes de soudure, indiquant une caractéristique fragile.