The development of dislocation substructure in hydrogen embrittled niobium-8 to 10 at % vanadium alloy

The development of dislocation substructure in niobium-8 to 10 at % vanadium alloy single and polycrystals is presented to emphasize the effect of interstitial hydrogen and substitutional vanadium. The formation of cross grid of straight long screw dislocations in the initial stages of deformation is associated with a high lattice frictional stress caused by the substitutional vanadium. Similarly, the presence of non-screw segments, edge dipoles and loop debris at higher strains is also considered to arise from the high lattice frictional stress. On the other hand, the formation of cell structure at 195 K in both single and polycrystals tested in tension is related to the presence of hydrogen. In addition, the nucleation of cracks along the active {1 0 1} or {1 1 2} type slip planes is related to the accumulation of hydrogen atoms along cell walls. Furthermore, the stress concentration due to the array of long straight dislocations is believed responsible for twinning observed at 77 K in the presence of hydrogen. The dislocation substructure associated with deformation twins indicates that hydrogen embrittlement through crack nucleation cannot be linked with deformation twinning in this alloy.     

Deformation and fracture behavior of niobium-10 at % vanadium alloy with hydrogen

The results of e systematic study of deformation and fracture behaviour of niobium-10 at% vanadium alloy with or without o nominal 200 p.p.m. wt% hydrogen and deformed in tension are presented as function of orientation and two testing temperatures; 195 and 295 K. Single crystals, large and small grain size polycrystals are used to determine the mechanisms of hydrogen embrittlement. Metallographic observations using both optical and scanning electron microscopy of the surface features associated with deformation and fracture behaviour of this alloy containing hydrogen are illustrated. In particular, the relationship between slip, twinning and cracks is described Furthermore, the reasons for the relatively large ductility associated with niobium rich alloys in comparison with other Nb-V alloys are put forward. Discrete dislocation analysis of the models of twinning is used to substantiate the ideas presented on the effect of hydrogen on the nucleation of twins and cracks in this alloy,     

Precipitation of vanadium carbide in an Ni-38 at.% Fe 3 at.%V-2.4 at.% C alloy

An electron microscopy investigation of the precipitation of vanadium carbide in a nickel-iron base alloy is reported. Three types of precipitate occur, matrix-dot, stacking fault and secondary precipitate depending upon ageing time and temperature. The morphology identity, and orientation relationships of the secondary precipitate have been determined. The features of matrix-dot precipitate free and enhanced zones have been studied, and a mechanism is proposed to explain the latter.     

Breakdown of the power-law creep in a Class I Al-10 at % Zn alloy

The creep behaviour of Al-10 at% Zn at 573 K is divisible into three deformation regions; low stress region, intermediate stress region and high stress region. The creep characteristics of the low stress region and intermediate stress region are consistent with dislocation climb and viscous glide, respectively. In the high stress region, the stress exponent,n increases with stress, the activation energy is higher than those observed in the other two regions, the activation area is slightly decreasing with stress and the internal stress is almost negligible. Present analysis shows that these characteristics are consistent with the thermally-activated glide motion of dislocations as a rate controlling mechanism at high stresses.[/p]     

Ageing of a Pd-35 at % Ag-25 at % Cu solid solution alloy

A palladium-35 at % silver-25 at % copper alloy has been prepared to simulate a succesful, commercial alloy. The commercial alloy is widely used because of its good corrosion resistance and formability and the fact that it develops good wear resistance and low electrical resistivity after ageing. However, little is known about the transformations which result from heat treatment of this alloy. Thus, the laboratory prepared material was used for determination of ageing transformations by means of resistance measurements, metallography, transmission microscopy, and X-ray diffraction. The following sequence of reaction was found to occur: $$\begin{gathered} Supersaturated \to Spinodal \to Ordering and discontinuous \hfill \\ solid solution decomposition precipitation \hfill \\ \end{gathered} $$ The transformations are quite similar to those occurring in several other solid solution systems whose initial spinodal decompositions are followed by discontinuous transformations with one or more ordered phases.     

Effect of 0.1% vanadium addition on precipitation behavior and mechanical properties of Al-6063 commercial alloy

The effect of 0.1 wt% vanadium additions on the precipitation behavior and the mechanical properties of a commercial Al-6063 alloy were studied. A master alloy containing 3 wt% V was added during casting. The cast ingot was homogenized, extruded and cooled employing two different cooling modes: forced air and water. Further aging was carried out following the standard T5 and T6 treatments for alloys with and without vanadium. The microstructural characterization, mechanical properties, and fractografic study were carried out. The addition of 0.1% vanadium to Al-6063 alloy under T5 treatment, accelerates the precipitation kinetics of β″ and β′ phases. The alloys with and without vanadium under T6 show a similar behavior, the co-existence of β′ and β″ precipitation is observed in both alloys. In general, vanadium additions to Al 6063 have a detrimental effect on the mechanical properties, showing only a beneficial effect for certain specific conditions.     

Crystallization kinetics of Zr-33 at % Ni amorphous alloy

The amorphous to crystalline transition in Zr-33 at% Ni amorphous alloy has been found to occur by polymorphic crystallization. The product of the cyrstallization process has been identified as the equilibrium Zr₂Ni intermetallic phase. The kinetics of crystallization have been studied independently using differential scanning calorimetry (DSC) and transmission electron microscopy. The activation energy of crystallization has been evaluated by isothermal and continuous heating in DSC. The isothermal anneals have revealed that the crystallization follows the Johnson-Mehl-Avrami kinetics with an Avrami exponent close to 4. The microstructural changes accompanying crystallization have been studied for an interpretation of the Avrami exponent. The nucleation and growth rates of crystals have been estimated at different temperatures in order to determine the activation energies of the two processes. It has been found that nucleation is thermally activated and growth is interface controlled.     

Superconductivity of heat-treated Nb-65 at.% Ti alloy

Brief reviews are given of the effect of heat-treatment on the microstructure of cold-worked bcc metals and the superconducting properties of niobium alloys. Particular attention is paid to the influence of interstitial impurities in these processes. The annealing effects in microstructure and superconducting properties of a cold-worked Nb-65 at.% Ti alloy, containing oxygen as a major impurity, have been studied. The precipitation process takes the form TiNb + O₂ various Ti oxides Ti + TiO. Differences in precipitation sequence are described for vacuum-annealing and annealing in impure argon. Flux-pinning is related to the microstructural observations. At least three pinning mechanisms appear to operate; dislocation pinning and two types of precipitate pinning. These observations are in accord with previously proposed pinning models.     

High-temperature deformation characteristics of Ti-15at.% Al alloy

The deformation characteristics of Ti-15 at.% Al alloy have been investigated by compression tests in the temperature range 873 to 1273 K (0.44 to 0.64T _m) and by extensive transmission electron microscopy. Two types of deformation patterns were identified depending on the temperature: at lower temperatures below about 1073 K, the yield stress of the sample showed inverse temperature dependence, and serrations were found on the flow curves, whereas the normal dependences of the yield stress on temperature and strain rate were found at higher temperatures above about 1073 K. Corresponding dislocation substructures were composed of coarse bands of localized slip at 1023 K, and of rather uniformly distributed dislocations at 1123 K, and sub-boundaries as well as free dislocations at 1273 K. The main operating mechanisms in these temperature regimes were assumed to be the co-operative movement of numerous dislocations under the condition of the dynamic strain ageing, viscous glide of dislocations and dynamic recovery, respectively.     

The optical properties of Al-8 at.% Ce alloy in the liquid, amorphous, and crystalline states

The method of spectral ellipsometry is used in the spectral range of 0.52–2.7 eV for measuring the optical constants of liquid, amorphous, and polycrystalline alloys of Al-8 at.% Ce. The results are used to calculate the dispersion dependences of light conduction, of reflectivity, and of functions of characteristic loss of electron energy of these alloys. It is found that the band-to-band transitions have a significant effect on the optical properties of alloy in the amorphous and polycrystalline states. Two absorption bands are observed in the spectra of light conduction of these alloys, namely, in the amorphous alloy at photon energy of 1.72 eV and 0.69 eV, and in the polycrystalline alloy at 1.55 eV and 1.03 eV. The reflectance spectra of liquid and amorphous alloys almost coincide, which is due to the similarity of their atomic structures.     

Kinetics of discontinuous precipitation in a Zn-2.5 at % Cu alloy

The morphology and growth kinetics of discontinuous precipitation in a Zn-2.5 at % Cu alloy have been studied in the temperature range 383–583 K by optical and scanning electron microscopy. The precipitate phase has a lamellar morphology, and maintains a statistically constant interlamellar spacing under isothermal growth conditions. The interlamellar spacing increases with an increase in temperature. The isothermal growth kinetics in terms of reaction front migration rate is maximum at 523 K. The upper temperature limit for the occurrence of reaction in this alloy has been predicted to be 643 K. A detailed kinetic analysis of the experimental data using several analytical models has confirmed discontinuous precipitation in this system to be a boundary diffusion controlled reaction, and enabled the determination of the grain boundary chemical diffusivity of Cu in a Zn-rich Zn-Cu alloy in the temperature range studied. The corresponding activation energy values determined in this study, range between 65 to 86 kJ/mol^–1, which compare well with the relevant data in the literature.