Hydrogen sorption properties of 90 wt% MgH2–10 wt% MeSi2 (Me = Ti, Cr)

The hydriding/dehydriding characteristics of 90 wt% MgH₂–10 wt% MeSi₂ (Me = Ti, Cr) composites, synthesized by ball milling under argon, were studied by Sievert’s type apparatus and by high-pressure differential scanning calorimetry (HPDSC). The composites have demonstrated similar absorption kinetics and capacity at temperature of 300 °C and a pressure of 1 MPa. They showed fast kinetics and achieved absorption capacity higher than 6 wt%. Slightly higher values for the enthalpy of hydriding and dehydriding were obtained at scanning conditions in the HPDSC for the composite 90 wt% MgH₂–10 wt% CrSi₂ compared to those for the composite with addition of TiSi₂.     

Shear stress measurement in nickel and nickel–60 wt% cobalt during one-dimensional shock loading

The shear strength of pure nickel (Ni), and its alloy, Ni–60Co (by wt%), has been determined during one-dimensional shock loading in the impact stress range 0–10 GPa. The influence of the reduced stacking fault energy (SFE) for the Ni–60Co has been investigated. The shear strength (τ) and the lateral stress (σ _y ) both increase with the impact stress for each material. The shear stress has been found to be higher in the nickel than in the alloy. The progressive decrease of the lateral stress behind the shock front indicates an increase of the shear strength. A more complex mechanism of deformation has been found for the alloy since twin formation has been observed in the microstructure, while none has been seen in nickel. It is thought that mechanical twinning plays a predominant role in the deformation mechanism of the alloy resulting in the reduction of the material strength.     

Microstructure and properties of Sip/Al–20 wt% Si composite prepared by hot-pressed sintering technology

In the present work, 50 vol% Sip/Al–20Si composite was prepared by hot-pressed sintering technology. Si particles were uniformly distributed in the Sip/Al–20Si composite, and only the presence of Si and Al phases were detected by XRD analysis. Dislocations, twins, and stacking faults were found in the Si particles. Several Si phases were found to be precipitated between Al matrix and Si particles. Si/Al interface was clean, smooth, and free from interfacial product. HRTEM indicated that the Si/Al interface was well bonded. The average CTE and thermal conductivity (TC) of Sip/Al–20Si composite were 11.7 × 10^?6/°C and 118 W/(m K), respectively. Sip/Al–20Si composite also demonstrated high mechanical properties (bending strength of 386 MPa). Thus, the comprehensive performance (low density and CTE, high TC, and mechanical properties) makes the Sip/Al–20Si composite very attractive for application in electron packaging.     

Transient and steady state creep of age-hardenable Al–5 wt% Mg alloy during superimposed torsional oscillations

An experimental study has been conducted to assess the effect of imposed small torsional oscillations on the creep behavior of Al–5 wt% Mg alloy during phase transformation. A series of tensile tests were conducted on the material with different levels of torsional oscillations and with different aging and matrix temperatures. The carefully designed experiments prove that increasing the shear strain amplitude of torsional oscillations and/or testing temperatures resulted in an increase of both transient and steady state creep parameters. The changes in work-hardening behavior of the samples with aging temperatures were rationalized in view of type, size, and distributions of phases existing in the original matrix. The results clearly show that the mechanism operating in the creep process was the precipitate–dislocation intersection. The microstructure of the samples studied was investigated by transmission electron microscopy.     

Influence of solidification variables on the microporosity formation on Al–Cu (4.5 wt%) alloy with axial heat processing

The effects of solidification rate, hydrogen concentration, and level of convection on porosity formation in Al–Cu (4.5 wt%) alloys were investigated using Axial Heat Processing (AHP). This processing technique is similar to the conventional directional solidification (DS) technique, except that it utilizes a graphite baffle immersed near the solidification interface to control the shape of the interface and impart an axial temperature gradient. It was found that the samples produced by AHP contained 20–40% less microporosity than similar samples produced by conventional DS. The reduction was also more pronounced with decreasing a cooling rate and increasing an initial hydrogen concentration in the melt. These differences are attributed to the solute accumulation that is due to the confinement of the liquid below the baffle and the concomitant reduction in the convection level near the interface.     

Microstructure of vapour quenched Ti–29 wt% Mg alloy solid solution

A Ti– wt% Mg (Ti–45 at% Mg) alloy produced by vapour quenching has been studied using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, imaging parallel electron energy-loss spectroscopy and electron diffraction. The alloy is shown to be composed of highly supersaturated solid solutions between two elements hitherto believed to be immiscible. It has a microstructure consisting of columnar units, columnar grains, columnar subgrains, herring-bone patterns and parallel stripes ranging in size from 15 m to 2 nm. Superimposed on the microstructure are periodic compositional bands with a period of 0.5 m. The origins of the periodic compositional bands and the herring-bone patterns are discussed.     

Sintering of injection-moulded WC–7 wt% Ni in a hydrogen atmosphere

Sintering injection-moulded WC–7 wt% Ni in a hydrogen atmosphere at temperatures higher than 900°C resulted in an inhomogeneous microstructure, as Ni2W4C ( phase) preferentially developed near the surface of specimens. Subsequent to sintering at temperatures higher than the liquid-phase formation temperature, the surface of specimens was virtually composed of phase. This arose from the excessive decarburization occurring near the surface of specimens, and the exudation of the Ni-based metal binder towards the surface of specimens. Primarily caused by the dissolution of W into the Ni binder phase, the saturation magnetization of WC–7 wt% Ni decreased at temperatures lower than 1000°C.     

High temperature oxidation behavior of a Mo–3Si–1B(wt%) alloy

Abstract

Molybdenum base alloys with the addition of small amounts of silicon (2–4.5 wt.%) and boron (～1 wt.%) can form a passivating layer protecting the alloy from further rapid oxidation. When such molybdenum base alloys are exposed to oxidizing environments at high temperatures, a borosilicate glass layer can form that will reduce the transport of oxygen to the alloy to limit further oxidation. Oxidation is then controlled by diffusion through the borosilicate glass layer. The focus of this research was to study the mechanisms and kinetics of high temperature oxidation of a Mo–Si–B alloy. The base alloy has a composition of Mo–3Si–1B (wt.%) and was studied in a variety of gas environments over a range of temperatures in order to elucidate the critical factors that allow it to develop a protective borosilicate glass layer. The borosilicate glass layer is protective when no continuous channels exist in the layer extending from the gas interface to the alloy interface. The borosilicate layer is believed to contain channels in the early stages of development and the elimination of the channels is obtained by appropriate control of the temperature and gas flow conditions whereby MoO₃ is removed via vaporization while the borosilicate viscosity is not increased due to loss of B₂O₃. Once the borosilicate layer is continuous and free of channels, subsequent oxidation occurs by inward diffusion of oxygen and the outward diffusion of molybdenum through this layer with vaporization of MoO₃ occurring at the gas/borosilicate layer interface, and MoO₂ and additional borosilicate forming at the alloy/MoO₂ interface.     

Microstructure development in the directionally solidified Al–4.0 wt% Cu alloy system

The effect of convection on microstructure formation is examined experimentally and theoretically for the vertically upwards-directional solidification of Al–4.0 wt% Cu alloys. In this alloy system, the rejected solute is heavier than the solvent so that fluid flow occurs due to the presence of radial gradients in temperature and composition. A numerical model is presented which shows that convection effects cause the composition to vary along the interface such that the composition increases from the center to the periphery of the sample. This composition variation causes the macroscopic interface to become convex, and give rise to a systematic variation in microstructure along the interface. Critical experiments have been carried out to examine planar to cellular (and cellular to dendritic) transition in a given sample due to the increase in concentration along the interface, and the experimental results are analyzed through the measurements of interface composition and thermal gradient. In addition, the variation in local primary spacing with interface composition is also characterized and compared with the results of the numerical model. It is shown that microstructure transitions and microstructural scales can be correlated quantitatively with the theoretical results based on interface composition and on temperature and solute gradients at the interface.     

Corrosion characteristics of anodized Ti–(10–40wt%)Hf alloys for metallic biomaterials use

The effect of anodizing on corrosion resistance of Ti–xHf alloys has been investigated. Ti–xHf alloys were prepared and anodized at 120, 170 and 220 V in 1 M H₃PO₄ solution, and crystallized at 300 and 500°C. Corrosion experiments were carried out using a potentiostat in 0.15 M NaCl solution at 36.5 ± 1°C. The Ti–xHf alloys exhibited the α′ and anatase phases. The pore size on the anodized surface increases as the applied voltage is increased, whereas the pore size decreases as the Hf content is increased. The anodized Ti–xHf alloys exhibited better corrosion resistance than non-anodized Ti–xHf alloys.     

Synthesis of Fe–4.6?wt% B alloy via electro-deoxidation of mixed oxides

Fe–4.6 wt% B alloy was synthesized via electro-deoxidation of the mixed oxide precursor. The oxides, Fe₂O₃ and B₂O₃, mixed in suitable proportions were sintered at 900 °C yielding pellets with a two-phase structure; Fe₂O₃ and Fe₃BO₆. The sintered pellets, connected as cathode, were then electro-deoxidized in molten CaCl₂ or in CaCl₂–NaCl eutectic, against a graphite anode at 3.1 V. The electrolysis at 850 °C has successfully yielded a powder mixture of Fe and Fe₂B. Sequence of changes during the electrolysis was followed by interrupted experiments conducted at 850 °C. This has shown that iron is extracted quite early during the electrolysis through the depletion of oxygen from the starting oxide; Fe₂O₃, forming the other iron oxides in the process. Boron follows a more complicated route. Fe₃BO₆, the initial boron-bearing phase, was depleted in the early stages due to its reaction with molten salt. This gave rise to the formation of calcium borate. Boron was extracted from calcium borate in later stages of electrolysis, which appeared to have reacted in situ with the iron forming compound Fe₂B. An erratum to this article can be found at     

Structure and Properties of Nanostructured Vacuum-Deposited Foils of Invar Fe–(35–38 wt%)Ni Alloys

The process of electron beam vacuum deposition of the Fe–(35–38 wt%)Ni alloys at substrate temperatures Ts from 300 to700 °C were used to produce vacuum-deposited foils with the FCC structure, differing by the size of characteristic microstructural elements(grains and subgrains). It was shown that refinement of foil microstructural elements to nanoscale is accompanied by their microhardness increase up to 4–5 GPa. The change of the thermal expansion coefficient(TEC) of the nanostructured(NS) foil of the Fe–35.1Ni alloy within the temperature range from-50 to 150 °C has some deviation from that observed for cast Invar alloy of the same composition. It has been found that the main factors affecting the peculiarities of thermal expansion of the NS foil can be related to the presence of small fraction of BCCphase in them, high level of crystalline lattice microstrains and inhomogeneous magnetic order in FCCphase. It was shown that as a result of additional thermal treatment of NS foils their invar properties become similar to that observed for cast Invar alloy but mechanical properties remain on the same level.     

Effects of Co additions on electromigration behaviors in Sn–3.0 Ag–0.5 Cu-based solder joint

As the miniaturization trend of electronic packing industry, electromigration (EM) has become a critical issue for fine pitch packaging. The EM effects on microstructure evolution of intermetallic compound layer (IMC) in Sn–3.0 Ag–0.5 Cu + XCo (X = 0, 0.05, 0.2 wt%) solder joint was investigated. Findings of this study indicated that current stressing of Sn–3.0 Ag–0.5 Cu–0.2 Co solder joint with 10⁴ A/cm² at 50 °C for 16 days, no remarkable EM damages exhibited in solder matrix. Whereas, after current stressing at 150 °C for 1 and 3 days, Sn–3.0 Ag–0.5 Cu specimens showed obvious polarity effect between cathode and anode. Different morphology changes were also observed at both sides. After current stressing for 1 day, two IMC layers, Cu₆Sn₅ and Cu₃Sn, with wave type morphology formed at cathode. Sn phases were also observed inside in the IMC layer. However, only Cu₆Sn₅ formed in anode. Three days later, Sn phases were found in anode. Besides, Co additions, aging treatment, Ag₃Sn, and other IMCs improved the resistance of EM by the evidence of retarding polarity effect.     

Microstructural stability, microhardness and oxidation behaviour of in situ reinforced Ti–8.5Al–1B–1Si (wt%)

Microstructural stability, microhardness and oxidation behaviour of an in situ reinforced Ti–8.5Al–1B–1Si (wt%) alloy was examined in both air and argon environments. When exposed for up to 5760 min at temperatures below the /+2 transius, hardening occurred in both air and argon environments. The increase in hardness in the air heat-treated samples is attributed to a combination of solid-solution strengthening due to the oxygen and the precipitation of the 2 phase, while the increase in hardness in the argon heat-treated samples is primarily due to the precipitation of the 2 phase. On the other hand, when heat treated above the /+2 transius, after an initial increase in hardness there is a drop in hardness which is attributed due to elimination of the 2 phase and a decreased contribution of boron and silicon in the matrix towards the solid-solution strengthening by virtue of coarsening of the TiSi2 and TiB reinforcements. Oxidation of the alloys follows a parabolic oxidation law when oxidized both in an environment of flowing air and static air with the primary oxidation product being TiO2. The activation energy for oxidation is 200 kJ mol-1 in an environment of flowing air and 303 kJ mol-1 in static air. The difference in activation energy arises from the difference in the availability of oxygen at the reaction front in the two cases. © 1998 Chapman & Hall     

Microstructure development in the directionally solidified Al–4.0 wt% Cu alloy system

The effect of convection on microstructure formation is examined experimentally and theoretically for the vertically upwards-directional solidification of Al–4.0 wt% Cu alloys. In this alloy system, the rejected solute is heavier than the solvent so that fluid flow occurs due to the presence of radial gradients in temperature and composition. A numerical model is presented which shows that convection effects cause the composition to vary along the interface such that the composition increases from the center to the periphery of the sample. This composition variation causes the macroscopic interface to become convex, and give rise to a systematic variation in microstructure along the interface. Critical experiments have been carried out to examine planar to cellular (and cellular to dendritic) transition in a given sample due to the increase in concentration along the interface, and the experimental results are analyzed through the measurements of interface composition and thermal gradient. In addition, the variation in local primary spacing with interface composition is also characterized and compared with the results of the numerical model. It is shown that microstructure transitions and microstructural scales can be correlated quantitatively with the theoretical results based on interface composition and on temperature and solute gradients at the interface.     

The enhancement of creep in Al–22?wt% Ag alloy by cyclic stressing

The plastic deformation behavior of Al–22 wt% Ag alloy during phase transformation was investigated by studying the creep behavior under cyclic stress reduction of low frequencies. The cyclic creep curves obtained describe clearly the cyclic stress acceleration behavior. Increasing frequency of cyclic stress reduction enhanced the creep deformation depending upon the combination of the experimental variables as testing temperature, aging temperature and static creep rate. The irregularity in the creep parameters, n, β and ε_st with increasing aging temperatures, has been explained on the basis of structure transformations occurring in Al–Ag system and their mode of interaction with mobile dislocations.