Abstract The microstructures formed in the early stage of deformation of Cu, Al, and TiAl in single slip and at low temperatures are analysed. While dislocations are highly mobile in Cu and Al, they are subject to a significant lattice friction in TiAl. Also, they are dissociated over a fewnm in Cu, not in Al and TiAl. These various properties are manifested in the self-organization of dislocations in bundles. The role of nanometric prismatic loops in producing specific debris is emphasized in the context of the so-called collinear interaction. Possible artefacts arising from microstructural relaxation after unloading are discussed and limitations of weak-beam analyses assessed.
Abstract An improved method is presented for determination of solidification curves, i.e., solid fraction versus temperature, for commercial Mg alloys using heat-transfer modeled differential thermal analysis (DTA) curves. A better simulation of the measured DTA signal is attained through an independent measurement of the time constant as function of temperature for the applied equipment. This enables a better desmearing of the DTA signal. Challenging Mg alloys could be appropriately handled by redesigning the tantalum encapsulation. Due to high oxygen affinity and vapor pressure of the investigated magnesium alloys, this special adaptation of the DTA setup using sealed Ta capsules was indispensable for generation of reproducible and reliable data.
Abstract Elevated temperature tensile properties of AlSi7Mg (Al-7 %Si– 0.7 %Mg) and its composites with 10 and 20 vol.% SiCp in the as-extruded form were investigated. The cast ingots of the matrix alloy and the composites were produced by the permanent die casting technique and extruded at 480°C using an extrusion ratio of 10 : 1. The high-temperature tensile tests were carried out over the temperature range from 25 to 430°C. Mechanical properties after extrusion show that the composite samples have strength values superior to that of the matrix alloy at ambient temperature. At elevated temperatures the results indicate that the composites exhibit good strength retention up to 300°C, above which the strengthening effect of SiCp disappears as the temperature is increased. The strain rate sensitivity “m” was observed to be 0.11 for the matrix alloy, 0.13 and 0.07 for the composites with 10 and 20 vol.% SiCp, respectively, at a test temperature of 430°C in the strain rate range from 4 × 10–5 to 4 × 10–2 s–1.
Abstract In present work, reduction kinetics of Cu2O by hydrogen gas was studied by thermogravimetric analyses (TGA). The TGA experiments were carried out both isothermally and non-isothermally on shallow powder beds. It was established that additions of Ni or NiO did not have any serious effect on the kinetics of reduction of Cu2O. The composition and microstructures of the reaction products were analyzed after each experiment by X-ray diffraction (XRD) as well as by scanning electron microscopy (SEM). The activation energy for the reaction was evaluated from isothermal as well as non-isothermal reduction experiments and was found to be in good agreement. The impact of the stability of the oxide on the activation energy for hydrogen reduction is also discussed.
The texture evolution in equiaxed polycrystalline L1(0)-FePd during the coarsening regime of annealing at 600 degrees C after cold-rolling has been investigated, using X-ray diffraction and scanning electron microscopy. A change in dominant texture was observed. Using the nomenclature established for texture description of face-centered cubic metals, the texture of the L1(0)-FePd develops from a brass-type texture into a strong cube-type texture during coarsening. Also, a large density of special high-angle grain boundaries emerged during coarsening, which can be described as coherent Sigma 2-boundaries and are associated with (90 +/- 5)degrees-rotations about a common cube-axis between the neighboring grains. The emergence of Sigma 2-boundaries may play an important role for previously reported abnormalities in the grain growth kinetics.
Data are presented on the kinetics of coarsening of gamma'-type Ni3Ti precipitates (L1(2) crystal structure) in three binary Ni-Ti alloys containing 10.31, 11.84, and 13.72 at.% Ti aged at 720 degrees C for times up to 64 h. Data on the distributions of particle sizes (PSDs) are also presented. These data, as well as previously published data, are analyzed in light of a new theory of coarsening in which diffusion is controlled by transport through the non-sharp interface between the matrix and precipitate phases. The new theory, called the trans-interface diffusion-control led (TIDC) theory of coarsening, predicts time (t)-dependent behavior of the type < r >(n) proportional to t for the growth of precipitates of average radius < r > and X-Ti proportional to t(-1/n) for the depletion of the solute concentration of the matrix, X-Ti. The exponent n is intimately related to the width of the interface between the precipitate and matrix phases, delta, which is assumed to depend on the particle radius as delta proportional to r(m), where n = m + 2. The shape of the scaled distribution of particle sizes (PSD) depends on n and the thermo-physical kinetic constants are independent of volume fraction. The data on kinetics are evaluated and compared for n = 2.375, determined from analyses of the PSDs, and for n = 3, which is the traditionally accepted value. The agreement between the data on kinetics and predictions of the TIDC theory is acceptable, and the TIDC theory is the only one capable of explaining the experimentally observed absence of an effect of volume fraction on the kinetics at larger volume fractions, and the shapes of the PSDs.
Abstract It is shown that Arrhenius plots of diffusion coefficients of Cr in β-Ti can be represented under each pressure by two straight lines which are fitted in the temperature regions above and below about 1390 K. The activation enthalpies for high temperature are 167 kJ/mol under 0.1 MPa and 191 kJ/mol under 2.1 GPa, and those for low temperature are 147 kJ/mol under 0.1 MPa and 153 kJ/mol under 2.1 GPa. The ratios of activation volumes for impurity diffusion to the molar matrix volume are in the range of 0.38 to 0.45. When the phonon-softening model is applied, the activation enthalpy for the impurity diffusion in the absence of any phonon-induced lattice instabilities is 226 kJ/mol under 0.1 MPa and increases with pressure. Its pressure dependence indicates that the activation volume for the diffusion with no assistance from phonons is 66% of the molar matrix volume, and this result agrees with the activation volume for the diffusion in “normal” bcc metals.
Abstract In this paper, we attempt to apply the modified hardening theory to estimate the creep resistance, which is considered as a key factor for controlling the creep deformation mechanism in cast nickel-base superalloys. It is suggested that, when the applied stress is high enough for the dislocations to cut into the γ′ particles, the creep resistance is almost a constant and independent of applied stress. At low applied stress, creep deformation is mainly controlled by dislocations climb, where the creep resistance has two components of a threshold stress and a friction stress. The model is verified for two cast nickel-base superalloys DZ17G and IN738. The results of detailed calculations are in good agreement with the experimental data.
Abstract The Pd–B system in the composition range of the solid solution PdBy has been the subject of a number of investigations as it provides an attractive model system for interstitial compounds. No consensus about the crystal structures and phase boundaries, particularly in the low-temperature regime, has been reached in the earlier works. In the light of very recent results on the crystal structure and composition ranges of the low-temperature phases, on the basis of X-ray, electron, and neutron diffraction analyses, a tentative phase diagram is presented which is compatible with practically all previous results.
Abstract In high-purity 4 N (99.99 wt.%) Al containing 50 wt.ppm Cu, very strong {100}<001> recrystallization textures are developed after 98% cold rolling and annealing at 500 °C. They are about three times stronger than those observed in standard high-purity 4 N Al without Cu addition. In this paper, the mechanism of the formation of such strong {100}<001> recrystallization textures were investigated in detail by using the EBSP (electron backscatter pattern) analysis. It was found that, at the earliest stage of recrystallization, {100} <001>-recrystallized grains were nucleated by forming a row lying parallel to the rolling direction and growing preferentially within the elongated deformed grain in which they were nucleated. Most of these {100}<001>-recrystallized grains had orientations very near to the exact cube orientations. {100}<001>-recrystallized grains in this materials were characterized by their rapid growth, which was observed at the later stages of recrystallization. Since they were surrounded by deformed regions having very strong β-fiber rolling textures formed by heavy cold rolling, their grain boundaries were high-angle grain boundaries with high mobility. Assisted further by high stored energy introduced by heavy rolling reductions, {100}<001>-recrystallized grains could grow very rapidly, traversing readily several deformed grains. In this material, {100}<001>-recrystallized grains were not nucleated so abundantly. However, {100}<001>-recrystallized grains were nucleated much earlier and grew much faster than recrystallized grains with other orientations. They were, therefore, always larger than recrystallized grains with other orientations. Due to such size advantages, {100}<001>-recrystallized grains could rapidly consume fine-recrystallized grains with other orientations during the subsequent grain growth process. As a result, microstructures observed after annealing at 500 °C for 2 h consisted mostly of coarse {100}<001>-recrystallized grains slightly misoriented with each other. It is, thus, rapid growth of few {100}<001>-recrystallized grains during recrystallization, and their preferential grain growth, that enhance the development of very strong {100}<001> recrystallization textures in this high-purity Al. Cu seems to suppress nucleation of recrystallized grains with other orientations.
Abstract This paper addresses the decomposition kinetics of synthesized homogeneous expanded austenite formed by gaseous nitriding of stainless steel AISI 304L and AISI 316L with nitrogen contents up to 38 at.% nitrogen. Isochronal annealing experiments were carried out in both inert (N2) and reducing (H2) atmospheres. Differential thermal analysis (DTA) and thermogravimetry were applied for identification of the decomposition reactions and X-ray diffraction analysis was applied for phase analysis. CrN precipitated upon annealing; the activation energies are 187 kJ/mol and 128 kJ/mol for AISI 316L and AISI 304L, respectively. Isothermal stability plots for expanded austenite developed from AISI 304L and AISI 316 were obtained.
Abstract Details of phase separation in the microstructure of amorphous Si–(B)–C–N ceramics derived from polymers have been resolved using the recent results of structural investigations. The formation of an amorphous phase built of atomic compounds SiC i/4N(4–i)/3 and consequently located along the composition line between SiC and Si3N4 in the ternary Si–C–N phase diagram demonstrates a generic feature of phase separation in all these materials. The amorphous carbon phase separates as a counterpart in the microstructure of Si–C–N ceramics, and in the case of Si–B–C–N ceramics such counterpart represents B–N–C domains of the composition (BN) c C y located along the tie line C–BN in the ternary B–C–N phase diagram. The effect of phase separation has been also pondered as a source of exceptional material properties.
Abstract Experimental data for phase equilibria and thermodynamic properties have been used for thermodynamic analysis of the high-temperature heazlewoodite. A two-sublattice model in the framework of the Compound Energy Formalism is proposed for the Gibbs energy of a second high-temperature heazlewoodite phase around the composition Ni3S4. Optimized model parameters have been obtained which reproduce all data simultaneously within experimental error limits. The Gibbs energy modeling is carried out consistently with the recent assessment by the author of the nickel – sulfur system at 1 bar pressure over the entire composition range for temperatures from 25 °C to above the liquidus.
Abstract Data are presented on the kinetics of coarsening of γ′-type Ni3Ti precipitates (L12 crystal structure) in three binary Ni –Ti alloys containing 10.31, 11.84, and 13.72 at.% Ti aged at 720 °C for times up to 64 h. Data on the distributions of particle sizes (PSDs) are also presented. These data, as well as previously published data, are analyzed in light of a new theory of coarsening in which diffusion is controlled by transport through the non-sharp interface between the matrix and precipitate phases. The new theory, called the trans-interface diffusion-controlled (TIDC) theory of coarsening, predicts time (t)-dependent behavior of the type 〈r〉 n ∝ t for the growth of precipitates of average radius 〈r〉 and X Ti ∝ t–1/n for the depletion of the solute concentration of the matrix, X Ti. The exponent n is intimately related to the width of the interface between the precipitate and matrix phases, δ, which is assumed to depend on the particle radius as δ ∝ rm , where n = m + 2. The shape of the scaled distribution of particle sizes (PSD) depends on n and the thermo-physical kinetic constants are independent of volume fraction. The data on kinetics are evaluated and compared for n = 2.375, determined from analyses of the PSDs, and for n = 3, which is the traditionally accepted value. The agreement between the data on kinetics and predictions of the TIDC theory is acceptable, and the TIDC theory is the only one capable of explaining the experimentally observed absence of an effect of volume fraction on the kinetics at larger volume fractions, and the shapes of the PSDs.
Abstract Phase decomposition in rapidly solidified Co–Al – Fe B2 alloys during isothermal ageing at 923 K has been investigated by means of X-ray diffraction and transmission electron microscopy. At the early stage of ageing, A2 disordered particles precipitate inside B2 matrix grains, being aligned along the <100> direction of the matrix, and equilibrium A1 phase (α-Co) is also formed mostly on grain boundaries. With further ageing, the plate-like A2 particles increase in size. On the other hand, the A1 phase on grain boundaries increases in size and also spreads into the B2 grain like tree branches as so-called Widmanstätten precipitate. Finally, the A2 phase disappears. The A1 phase remains on grain boundaries and also inside the B2 grains as spherical particles. Using the composition gradient method, the metastable precipitation limit of the A2 phase, that is the metastable phase boundary B2/(A2 + B2), was experimentally determined at 923 K. It extends smoothly into the stable A2 + B2 two-phase field on the Co–Fe side of the ternary Co–Al –Fe system.
Abstract An earlier study of the creep behaviour of Elektron 21 alloy has been extended to 290 °C (0.61T m; T m: melting temperature). The combined results confirm a stress exponent of creep rate close to 6, but with an activation energy for creep of (310 ± 20) kJ/mol for 240 to 290 °C and 40 to 90 MPa. Possible mechanisms giving rise to this behaviour are discussed.
Abstract We describe four criteria for the selection of alloying elements capable of producing castable, precipitation-strengthened Al alloys with high-temperature stability and strength: these alloying elements must (i) be capable of forming a suitable strengthening phase, (ii) show low solid solubility in Al, (iii) low diffusivity in Al, and (iv) retain the ability for the alloy to be conventionally solidified.With regard to criterion (i), we consider those systems forming Al3M trialuminide compounds with a cubic L12 crystal structure, which are chemically and structurally analogous to Ni3Al in the Ni-based superalloys. Eight elements, clustered in the same region of the periodic table, fulfill criterion (i): the first Group 3 transition metal (Sc), the three Group 4 transition metals (Ti, Zr, Hf) and the four latest lanthanide elements (Er, Tm, Yb, Lu). Based on a review of the existing literature, these elements are assessed in terms of criteria (ii) and (iii), which satisfy the need for a dispersion in Al with slow coarsening kinetics, and criterion (iv), which is discussed based on the binary phase diagrams.
Abstract Density and surface tension of liquid Ni –Cu –Fe alloys have been measured over a wide temperature range, including the undercooled regime. A non-contact technique was used, consisting of an electromagnetic levitator, an optical densitometer, and an oscillating drop tensiometer. At temperatures above and below the liquidus point, density and surface tension are linear functions of temperature. The concentration dependence of the density is significantly influenced by a third-order (ternary) parameter in the volume, while the surface tensions can be derived from the thermodynamic potentials E G of the binary phases alone.
