Volume 98, Issue 6

The faceting of a cylindric tilt grain boundary (GB) in Cu and Cu-160 ppm Bi bicrystals was studied. The crystal lattices of both grains form a superlattice called a coincidence site lattice (CSL) with an inverse density of coincidence sites relating to the density of lattice sites Σ = 3 and Σ = 9. The Σ3 and Σ9 <110> tilt GBs became faceted upon annealing at 0.95 T m . The ratio of GB energy σ GB and surface energy σ sur of the specimen was measured, applying atomic force microscopy, from the profile of the GB thermal groove formed upon additional annealing. The 160 ppm Bi concentration in the Bi-containing bicrystal is between GB and bulk solidus lines in the Cu – Bi phase diagram. In this concentration region GBs contain a multilayer Bi-rich film which is supposed to be liquid-like. Wulff – Herring diagrams were constructed using the measured σ GB /σ sur values , revealing stable and metastable GB facets.

Viscosity data on Cu – Ni – Fe have been obtained using an oscillating cup viscometer. In this method, a liquid sample is suspended in a crucible which performs damped angular oscillations. Viscosities are calculated from the time period and the decay of the amplitude. The temperature dependence of the measured viscosities can be described by an exponential Arrhenius law, taking into account an activation energy for the viscous flow. The activation energies for the ternary alloys were found to be linear combinations of the corresponding activation energies of the pure elements Cu, Ni, and Fe. At constant temperature, a non ideal mixing behaviour was observed and the data were compared with several quantitative models. Close agreement of the experimental results was found with the predictions of two models by Kaptay and Hirai.

The oxidation behavior of a precipitation-hardened Pt-base alloy with 12 at.% Al, 6 at.% Cr and 5 at.% Ni was examined. At high temperatures in air this alloy forms an alumina scale and a precipitate-free layer underneath, both growing according to parabolic functions of time. Small amounts of Ni embedded in the oxide scale accelerate its growth in the initial phase of oxidation. With time, oxidation is retarded due to decreasing impurity concentration and grain coarsening in the oxide scale. The interdiffusion coefficients of the scale-forming element Al in the Pt-rich solid solution matrix are estimated.

Bismuth thermodynamic activities in Ni – Bi liquid solutions have been measured by the modified isothermal isopiestic method at 1723, 1753 and 1773 K. These results are coupled with thermochemical and topological data from literature, in order to achieve reliable re-optimization of the Ni – Bi system by the CALPHAD method. The model of disordered substitutional solutions has been applied to the liquid phase and to the face-centered cubic solid solutions (Ni). The compound NiBi 3 is stoichiometric and is modeled by a two-sublattice model corresponding to its formula. The nickel solubility in the solid bismuth is negligible, which is why the phase (Bi) is considered as consisting of bismuth only. The description Bi: Ni, Va has been used for the thermodynamic modeling of the nonstoichiometric NiBi phase.

The kinetics of the isothermal β → α + β transformation in a metastable β titanium alloy has been studied by dilatometry. Based on thermodynamic aspects, the volume diffusion of Mo in the titanium matrix ahead of the interface, the interface mobility, and the morphology of the primary α plates, a transformation model is developed, which incorporates mixed-mode behaviour of the transformation in the initial stage. Fraction curves are calculated based on the simulated growth of a typical α plate within the β phase having a volume determined from the nucleus density. For comparison, fraction curves are calculated using a three-dimensional model of the β grain in which a number of plates are simulated to grow with purely diffusion-controlled kinetics. It is shown that the calculated growth rate with the mixed-mode transformation model gives a better representation of the growth rate at the start of the transformation than the purely diffusion-controlled model.

Nanocrystalline vanadium powders have been produced by ball milling in a planetary mill. The morphology of the powder particles has been investigated by scanning electron microscopy. Crystallite size (size of coherently diffracting domains) and lattice-strain variation (microstrain) have been determined from the analysis of the X-ray diffraction-line broadening using the established integral breadth Williamson – Hall and Fourier Warren – Averbach methods. Results obtained from transmission electron microscopy analysis have been compared with the X-ray diffraction results. Ball milling causes an increase in the particle size and a decrease in the grain (crystallite) size with increasing milling time, a lattice-strain variation, due to deformation-induced dislocations, that increases with milling time and deformation-induced stacking faults of density increasing with milling time. The lattice parameter of the vanadium powders, as deduced from the diffraction-peak positions, decreases upon milling linearly with the inverse of the grain size, which has been attributed to grain (crystallite)-boundary stress.

Interfacial reactions between Sn – Ag – Cu lead-free solders and Au were investigated by the use of SAC/Au solid/solid reaction couples which were annealed at 150, 180 and 200 °C for various reaction times. Five intermetallic compounds, AuSn, AuSn 2 , AuSn 4 , (Au, Cu)Sn, and (Cu, Au) 5 Sn 6 , were formed at a higher temperature and lower Cu content. As the reaction temperature was dropped and Cu content was increased, only three intermetallic layers, AuSn, (Au, Cu)Sn, and (Cu, Au) 5 Sn 6 , could be observed. Meanwhile, when the Cu content in SAC alloys increased, the thickness of total reaction layers decreased, but that of (Au, Cu)Sn/(Cu, Au) 6 Sn 5 layers increased. It likely resulted from (Au, Cu)Sn/(Cu, Au) 6 Sn 5 layers acting as a diffusion barrier to resist Sn diffusing to Au – Sn intermetallic compounds and decrease the growth rate of Au – Sn intermetallic compounds. The growth and formation of intermetallic compounds are sensitive to the reaction temperature and Cu content in the SAC/Au couple systems.

Electrical resistivity measurements, differential thermal analysis, mechanical spectroscopy and small angle neutron scattering studies have been performed on commercial QE22 magnesium alloy. The aim of this work is to resolve the appearance of Guinier – Preston zones and also to survey the small angle neutron scattering technique for various temperatures in this alloy. The coupling of all the employed experimental techniques in this work is crucial for detecting the precipitation of the Guinier – Preston zones. This is due to the strong changes in the microstructure produced by the main precipitation process, which starts at about 440 K and ends at about 640 K. We found that the temperature at which Guinier – Preston zones start to grow is about 473 K and that the dissolution temperature is 537 K.

The as-quenched (solution-heat-treated) aluminium wrought alloy AA6110 (Al – Mg – Si – Cu) was mechanically surface treated (deep rolled) at room temperature. The deep rolled as-quenched AA6110 was cyclically deformed at room and elevated temperatures up to 250 °C using stress-controlled push-pull fatigue tests. Cyclic deformation behavior and Wöhler curves of the deep rolled as-quenched AA6110 have been investigated and compared to the polished condition as a reference. The effect of static/dynamic precipitation occurring during fatigue at elevated temperatures was analyzed and discussed. The stability of residual stresses as well as work hardening at elevated temperature was investigated using X-ray diffraction methods. Finally, from the S/N data, a borderline describing the effectiveness of deep rolling for different loading combinations in a stress amplitude – temperature diagram was established.

The effect of Ho additions on the microstructure and compressive properties of the alloy Nb-22Ti-16Si-7Cr-3Al-3Ta-2Hf (at.%) was investigated by using scanning electron microscopy, X-ray diffraction, transmission electron microscopy and by performing compression tests. The results show that Ho additions cause remarkable microstructural refinement. The addition of 0.1 at.% Ho to the alloy increases its strength and ductility. When the amount of Ho is increased to 0.2 at.%, the strength and ductility decreases. Ho addition inhibits the formation of the (Ti, Nb) 5 Si 3 phase and the amount of the (Ti, Nb) 5 Si 3 phase decreases, even disappears, with increasing Ho content. In addition, the fracture surfaces of Ho-free alloys deformed at room temperature in compression tests exhibit cleavage for both the (Nb, Ti) solid solution and the (Nb, Ti) 5 Si 3 intermetallic compound, while limited local plastic strain of the (Nb, Ti) solid solution is observed in Ho-doped alloys. The effect and mechanism of strengthening and plastic deformation of Ho additions to the alloy was discussed in view of the microstructure.

The present study was performed on the alloy AlSi9Cu3(Fe)(Zn) (EN 1706: 1998) to investigate the effect of different heat treatment conditions. High pressure die cast samples were quenched directly after casting and subsequently stored at room temperature or elevated temperatures. Combinations of natural and artificial ageing were also tested. The static mechanical properties of the alloy were measured at all stages. The results show that it is possible to attain improved mechanical properties by an optimised combination of both treatments.

The effects of quenching temperature on the microstructure and mechanical properties of a cast Fe – B – Ti alloy containing 2.0 wt.% of boron and 1.0 wt.% of titanium were investigated. No clear influence on microstructure has been observed unless the quenching temperature is above 900 °C. As the quenching temperature exceeds 950 °C, most of the boride networks are broken and a large quantity of irregular borides appears. When the quenching temperature reaches 1050 °C, the networks disappear completely. The mechanical properties of the cast Fe – B – Ti alloy begin to improve when the quenching temperature increases. The hardness, impact toughness and bend strength have no obvious change when the quenching temperature exceeds 1000 °C. The cast Fe – B – Ti alloy quenching at 1000 °C has excellent comprehensive mechanical properties, its hardness, bend strength and impact toughness are 58.3 HRC, 2.39 GPa, 17.5 J cm −2 respectively.

In the present work, the mechanism of nickel coating on BN particles by hydrazine reduction was studied. A nickel sulfate – ammonium sulfate – ammonia – hydrazine hydrate plating system was employed. It was found that Pd on the surface of BN particles acted as an active center for nickel deposition at the initial stage of the process. Thereafter, Ni itself would act as an active center to catch Ni from the solution through the reaction: Ni 2+ + 2H* ad ß Ni + 2H + . The rate of the process was found to be controlled by the reaction at the interface under the present experimental conditions. A kinetic model was proposed on the basis of the experimental results. The model predictions were found to be in agreement with the experimental data.

In the present work, thermal diffusivity measurements of three industrially significant alloys, viz. 90Ti.6Al.4V, stainless steel with 25Cr and 6Ni as well as plain carbon steel with 0.7 % carbon have been carried out as a function of temperature. The aim of this work is to study the effect of temperature and microstructure on the thermal diffusivities of these alloys. For the 90Ti.6Al.4V alloy, thermal diffusivity increases with temperature below 1225 K. Above this temperature, the values started decreasing probably due to the dissolution of Ti 3 Al intermediate phase in the matrix, which would result in an increase in the disorder of the structure. For 25Cr: 6Ni stainless steel, the thermal diffusivity is nearly constant up to about 700 K. Above this, there is an increase in the thermal diffusivities with temperature during the heating cycle, which was reproducible during thermal cycling. On the other hand, the slope of the curve increases above 950 K. In the case of the 0.7 % carbon steel, the thermal diffusivity shows a decreasing trend with temperature below the Curie point for the alloy, where the alloy consists of bcc + fcc phases. Above this point, only the fcc phase is prevalent and the thermal diffusivity was found to increase with temperature. Heat transfer is carried out by lattice vibration (phonons) as well as electrons. The contribution of electrons varies depending upon the type of alloy. In this study, the highest electron contribution was found in 0.7 % carbon steel, while the lowest was in stainless steel. The thermal conductivity values of these alloys are in good agreement with the calculated values using the model proposed by Mills.