Volume 94, Issue 9

Solute diffusion of Ag in polycrystalline γ-Fe-40 wt.% Ni alloy was studied by means of the radiotracer technique. For bulk and grain boundary diffusion, the following Arrhenius parameters were established: Dv0=1.2×10−3m2/s $ D_v^0 = 1.2 \times {10^{ - 3}}{m^2}/s $ and Q v = 279 kJ/mol and P 0 = 8:1 ⨯ 10 –14 m 3 /s and Q gb = 126 kJ/mol, respectively. The diffusion profiles reflect the very low Ag solid solubility which was estimated in the alloy via the specific activity of the applied 110m Ag radiotracer.

A pair interaction model applying the Bragg–Williams approach is presented. The formalism is given in a general form; ternary ordered compounds with different crystal structures and all possible point defect types are considered. Some applications are discussed. The Bragg –Williams approach is an easily applicable formalism that leads – proper use supposed – to accurate predictions concerning thermodynamic data as well as defect concentrations.

By means of a solid state electrochemical method, the activity of sodium oxide dissolved in various samples of ceramic Na-(β + β″)-alumina was measured as a function of the composition of the ceramics, with the independent variable being adjusted by the sodium activity in the ambience. Taking into account all the relevant findings, the lgaNa2O $\lg {\rm{ }}a_{{\rm{Na}}_2 {\rm{O}}} $ vs. lg⁡  a Na $\lg {\rm{ }}a_{{\rm{Na}}} $ relationship can quantitatively be described by a polynomial of third degree with respect to lg⁡  a Na . $\lg {\rm{ }}a_{{\rm{Na}}} .$ The gradual change of the sodium oxide activity indicates that the thermodynamic behaviour of the material is mainly determined by the occurrence of nonstoichiometric phases with easy transitions from one stoichiometry to the other one.

The thermodynamic properties of MgZn 2 and Mg 2 Zn 11 were investigated by calorimetry. The standard entropies of formation at 298 K, ΔfS298° $\Delta _{\rm{f}} S_{298}^{\rm{^\circ }} $were determined from measuring the heat capacities, C p , from near absolute zero (2 K) to 390 K by the relaxation method. The standard enthalpies of formation at 298 K, ΔfH298° $\Delta _{\rm{f}} H_{298}^{\rm{^\circ }} $were determined by solution calorimetry in hydrochloric acid solution. The standard Gibbs energies of formation at 298 K, ΔfG298° $\Delta _{\rm{f}} G_{298}^{\rm{^\circ }} $were determined from these data. The electronic contribution to the heat capacities of MgZn 2 and M 2 Zn 11 were found to be similar to pure Zn, indicating that the valence electrons of Mg are trapped by the density of states for Zn in the vicinity of the Fermi level.

The phase boundary separating the γ + γ′ and γ′ regions of the Ni –Al phase diagram (the γ solvus) was established over the temperature range 600– 1200 °C. The γ phase is the Ni –Al solid solution, and γ′ is (non-stoichiometric) Ni 3 Al. Cast alloys containing 22.0, 22.2, 22.4, 22.6, 22.8 and 23.0 at.% Al, all compositions being within ± 0.03 at.%, were used in the experiments. At temperatures from 1000 to 1200 °C, the γ solvus was determined using aging treatments designed to eliminate the dendritic structure in the as-received castings. At lower temperatures, from 600 to ≈ 900 °C, dissolution experiments were conducted. The method takes advantage of the absence of a barrier to the dissolution of a dispersed phase. Specimens of the supersaturated γ′ phase were aged at low temperatures, 600 or 650 °C, for long times (up to 633 h), to produce fine dispersions of the coherent γ phase precipitates. These specimens were then re-aged at 10 –20 °C intervals of temperature and re-examined to determine whether the γ phase precipitates had dissolved. The absence of the γ phase precipitates enabled the phase boundary to be located to within ± 10 °C for an alloy of known composition, or ± 0.03 at.% at a given temperature, from 600 to 900 °C. The phase boundary generally agrees with the lower end of Al concentrations established in previous research.

The ternary Al –Ca –Mg phase equlibria were investigated using X-ray diffraction methods, metallography, scanning electron microscopy with energy- and wave-length-dispersive X-ray microanalysis, and differential thermal analysis. The phase diagram was determined in the complete composition range. Large ternary solubilities were found for the binary phases CaMg 2 , Al 2 Ca and Al 3 Ca 8 . Microstructures of samples with compositions near the ternary monovariant eutectic L → CaMg 2 + Al 2 Ca show remarkable rod-like crystals that are identified as intergrowth between CaMg 2 and Al 2 Ca. A consistent thermodynamic model was developed using the Calphad method incorporating all experimental data. It was used to calculate all pertinent phase equilibria of the Al –Ca –Mg system.

Tensile tests were made on commercially pure Al (A1050), Al-4.4 wt.% Mg (A5182) and Al-4 wt.% Cu (A2017) alloys by varying the test temperature and the strain rate. Textures developed at various stages of the tensile deformation were investigated with the orientation distribution function analysis. During the tensile test of pure Al with a strain rate of 3 × 10 –4 s –1 at 20 °C, the tensile axis readily rotated toward the <111> stable end orientation. However, such a rotation occurred only at the later stage of the tensile deformation near the tensile stress, where the stress– strain curve became flat, and where work hardening was almost saturated. It is suggested that the size and the morphology of the dislocation cell, which had developed up to this stage, played a significant role in the subsequent texture development. In fact, the addition of Mg and Cu, which inhibited the development of the well-defined cell structure due to the Portevin –Le Chatelier effect, strongly suppressed the rotation of the tensile axis toward <111>. In all the materials investigated, textures were not affected by the strain rate and also by the temperature of the tensile test.

Combination of rapid solidification and internal oxidation was used for producing a fine dispersion of rare earth oxide particles in the copper matrix. An overall microstructural analysis has shown that the internal oxidation temperature, the rapidly solidified microstructure and its changing ahead of the internal oxidation front strongly influence the mechanism of the internal oxidation process and the resulting microstructure. The internal oxidation in a Cu –Yb alloy took place mainly by direct oxidation of intermetallic particles. Contrary to this, in a Cu – Er alloy two mechanisms of internal oxidation have been clearly observed: (i) Dissolution of intermetallic particles ahead of the internal oxidation front and oxidation of the erbium from the solid solution and (ii) direct oxidation of the Cu–Er intermetallic particles. While a reasonable optimum combination of processing conditions at internal oxidation in the solid state, yielding suitable oxide dispersions, seems to have been identified for the Cu–Er alloy, the same was not true for the Cu–Yb alloy. However, the internal oxidation of the Cu–Yb alloy in the semisolid state, which occurs by alternation of two processes – Yb oxides precipitation and Cu matrix solidification – led to a relatively uniform dispersion of Yb oxide particles.

A series of Zn– Al-based alloys, one group with 1% Cu and the other without Cu, was produced by using gravity casting. The effect of Al content on hardness, ultimate tensile strength (UTS), impact strength, and tensile creep behaviour at 120 °C and an applied stress of 40 MPa has been investigated. It has been found that the hardness and UTS of the alloys considerably increased with increasing Al content, but the creep strength decreased. The impact strength of the alloys improved with increasing Al content up to 7.5% and then reduced gradually with further increase in Al content. In addition, the effect of 1% Cu addition on the mentioned properties has been determined. While the addition of 1% Cu had a profound effect on increasing the hardness, UTS, and creep strength, it did not have a significant effect on the impact strength of the alloys. Metallographic examinations showed that an increasing Al content of the alloys increased the volume fraction of the Al-rich β dendrites in expense of a decreased volume fraction of the eutectic, which is in principle responsible for the change in mechanical properties of the alloys depending on the Al content

The flow behavior and hot deformation mechanisms of Ti-6Al-4Vin both β and (α + β) phase regions have been studied by compression tests in the temperature range 750 – 1100 °C and strain rate range 0.05 –15 s –1 . The flow curves displayed discontinuous yielding at temperatures above 850 °C and high strain rates, typically 5 and 15 s –1 . Flow oscillations were also observed under such conditions. The relative discontinuous flow stress drop increased with increasing temperature in the (α + β) phase region and reached a steady state in the β phase region. The discontinuous yielding is attributed to the sudden increase in the mobile dislocation density occurring due to the activation of β grain boundary sources during hot deformation. The apparent activation energy estimated using the standard kinetic analysis is about 246 kJ/mol in the β region and 621 kJ/ mol in the (α + β) region while the strain rate sensitivity values are 0.34 and 0.18, respectively. The mechanism in the β region has been suggested to involve grain boundary sliding and dynamic recovery of the β phase while in the (α + β) region a transient mechanism, involving dynamic recovery followed by post deformation recrystallization, has been proposed.

The dependence of stacking fault energy (SFE) on the carbon and nitrogen content in iron-based high-alloyed austenite has been thermodynamically calculated and compared with their experimentally measured values. It is shown that such calculations are confined to dilute solid solutions where the content of interstitials does not exceed 1.5 at.%. In more concentrated solid solutions a significant change in their electronic structure renders such a calculation impossible. The influence of the electronic structure on the SFE and the segregation effect in the considered systems are discussed.

The influence of hydrogen charged cathodically on the tensile behavior of pressure vessel steel SA508 Cl.3 has been investigated in the temperature range 200– 350 °C. It was found that the charged hydrogen induced a marked softening and a decrease in ductility at these temperatures, especially in the dynamic strain aging (DSA) region. Correspondingly, some flat or brittle-like areas appeared on the fracture surfaces locally and the dimples became smaller and shallower. Moreover, the presence of hydrogen moved the critical temperature at which DSA takes place to a higher temperature. The possible interactions between hydrogen and DSA at relatively high temperatures and the related mechanisms are also discussed according to the above experimental results.

In order to utilize the shape memory alloys (SMAs) for prospective damping applications, a near equiatomic NiTi alloy was investigated on a dynamic mechanical analyzer in the infrasonic frequency range. A high damping capacity stable to the variation of temperature can be obtained during a two-stage phase transformation and in the martensite state approaching to the martensite-parent phase transformation temperature in the NiTi SMA. The transient damping peaks associated with the phase transformation in the NiTi SMA shows a strong frequency dependence in both solutionized and annealed specimens. The frequency dependence parameter, n , was found to be in the range 0.3 – 0.6. For the applied strain amplitudes from 8.5 × 10 –6 to 5.0 × 10 –5 a clear strain amplitude dependence was not observed in the damping peaks, except for the solutionized specimens. While the damping capacity of the martensite state, characterized at 300 K, showed obvious frequency dependences in both the solutionized and annealed specimens, only the annealed specimens manifested the strain amplitude dependence. The different damping behavior was attributed to the internal structure of martensite, including dislocations, the twinning types and variant interfaces, changeable with the prior thermomechanical treatments.

Resonance frequency of quartz tuning fork crystal for use in chips of code division multiple access, personal communication system, and a global system for mobile communication was analyzed by an analytical method and the finite element method (FEM). Resonance frequency values obtained by the analytical solution of the equation of motion and FEM analysis were compared and reasonable agreement showed validity of the FEM results. A resonance frequency close to 31.964 kHz was aimed following FEM analysis results and a general scheme of commercially available 32.768 kHz tuning fork resonators was followed in designing tuning fork geometry, tine electrode pattern and thickness. Based on FEM design, actual tuning fork crystals were fabricated using photolithography and oblique evaporation by a stencil mask. The average resonance frequency of the fabricated tuning fork samples at a vacuum level of 3 × 10 –2 torr was 31.228 to 31.462 kHz. The discrepancy between FEM modeling and observed values of resonance frequency is attributed to error in exactly manufacturing tuning fork tine width by photolithography. However, design parameters affecting other crystal parameters have to be adjusted comprehensively to optimize the tuning fork performance.

The miscibility gap of the Ga–Pb system has been investigated by different experimental methods and techniques. Viscosity, acoustic and electrical conductivity measurements were carried out through the entire immiscibility region. Possible reasons of revealed discrepancies in the absolute values of the phase separation temperatures are discussed in relation with peculiarities of the experimental methods employed. The shape of the miscibility gap has been analysed.