Diffusion and electrotransport of carbon in γ-phase iron-nickel alloys

The diffusivity and electrotransport behavior of carbon in an iron-32.5 wt pct nickel alloy was investigated for the temperature range of 1220 to 1415 K for two carbon concentrations. The electrical resistivity was measured as a function of temperature between room temperature and 1625 K for carbon contents of 0.002, 0.10, and 0.21 wt pct. Values for the activation energy for diffusion of 32.0 ± 5.8 and 36.0 ± 5.2 kcal per mole were determined for alloys containing 0.10 and 0.21 wt pct carbon respectively, with slightly smaller values being found for the activation energy for electrotransport. The sign of the effective valence indicates that carbon migrates toward the cathode.     

Dislocation dissociation in some f.c.c. metals

The dissociation of a perfect screw dislocation into a stacking fault in an f.c.c. lattice is modeled by the modified lattice statics. The interatomic potentials are obtained from the work of Esterling and Swaroop and differ substantially from those empirical potentials usually employed in defect simulations. The calculated stacking fault widths for aluminum, copper, and silver are in good agreement with weak beam microscopy results.     

Relationship between rolling textures and shear textures in f.c.c. and b.c.c. metals

The rolling and shear textures of aluminium (f.c.c.) and Fe-16% Cr (b.c.c.) have been compared. First a 90° rotation relationship about the transverse direction was found experimentally between the stable orientations of the rolling textures and the shear textures. This was explained with the symmetry of the glide systems and the orientation relationship between both coordinate systems. Second for both kinds of deformation namely rolling and shear a 90° rotation relationship about the transverse direction was also found between the stable b.c.c. and f.c.c. orientations. This was explained with the Taylor theory and the Sachs model making use of the orientation relationship between f.c.c. and b.c.c. glide systems.     

Diffusion in transition metals and alloys

Metallurgical and Materials Transactions A - The activation energies for diffusion in transition metals is correlated against the localizedd electron population. Thed electron population is...     

Corrosion fatigue mechanisms in b.c.c. stainless steels

The influence of a 3.5% NaCl solution on the cyclic plastic deformation of a b.c.c. Fe-26Cr-1Mo alloy is analysed as a function of the applied electrochemical potential E, taking into account the dislocation behaviour, the formation of microcracks and the evolution of the cyclic corrosion current transients. Three different kinds of damage mechanisms have been pointed out: (i) in the cathodic region (E < −300 mV/SCE), the localization of the hydrogen reduction is favoured when microcracks are mechanically formed. This induces a very marked decrease of the fatigue life at high strain amplitudes, but this deleterious effect is reduced at low strain amplitudes for which the microcrack formation is delayed; (ii) in the passive region for −300 < E < + 500 mV/SCE, the damage mechanisms are related to the localization of the anodic dissolution due to a depassivation-repassivation process. A critical strain rate range ( ∼- 10⁻³ s⁻¹) for which this localization and the corresponding acceleration of the microcrack formation are maximum is encountered; (iii) in the passive region for E > + 500 mV/SCE, the cyclic strain enhances the formation of pits which induce an early formation of microcracks. The study of the microcracking process and its evolution is a key to specify the physical mechanisms by which an aqueous corrosive solution can affect the fatigue life of b.c.c. stainless steels according to the applied electrochemical potential.     

Structural bonding contributions and the elastic response of b.c.c. and f.c.c. crystals

The anisotropic elastic response of body centered cubic (b.c.c.) and face centered cubic (f.c.c.) crystals is reviewed within the framework of general elasticity and particular model dependent relations. It is found that important trends in the observed behavior of specific groups of crystals can be reproduced by models that include only structural contributions to crystal binding. In fact, a very simple model, consisting of only nearest and next nearest neighbor interactions, reproduces the salient behavior, including the algebraic signs of Poisson's ratio and the orderings of the shear moduli, Young's moduli, and Poisson's ratios associated with major crystallographic symmetries. The analysis provides insight into the differences between the b.c.c. transition metals and the b.c.c. alkali metals.     

Hardening by spinodally modulated structure in b.c.c. alloys

Hardening by composition modulation during the spinodal decomposition in b.c.c. alloys is discussed. It is found that two main factors determine the magnitude of incremental yield stress in modulated alloys: (il misfit effect by the coherent internal stress due to the decomposition and (ii) modulus effect by the spacial variation of elastic modulus in the modulated structure. The second factor becomes important if a wide-range decomposition is associated with a large variation in the shear modulus. The mechanism and the magnitude of each factor are theoretically discussed and compared with the limited experimental results available.     

f.c.c. to b.c.c. transformation in thin iron films deposited onto copper single crystals

The f.c.c. to b.c.c. transformation in thin iron films deposited onto (111), (101) and (001) copper substrates was investigated through transmission electron microscopy. Depending on the orientation of the substrates, specific α-iron variants of Kurdjumov-Sachs, Nishiyama and Pitsch orientations were found to be formed preferentially. Detailed discussions were made to clarify the origin of such preferential formation of variants and it has been concluded that in addition to the effect of misfit stress due to the lattice mismatch between γ-iron and copper crystals, the effect of directional matching between the two crystals at the interface plays an important role.     

Study of the h.c.p.-f.c.c. phase transition in cobalt by acoustic measurements

Acoustic measurements have been performed in the vicinity of the h.c.p.-f.c.c. phase transition in cobalt. The main results are the following: At low frequency (~ 1 Hz), a high transitory internal friction peak is observed. It is shown that this internal friction is a function of T/vT temperature rate, v vibration frequency) but is not proportional to T/v. This behavior is explained in the framework of a recent model based on a Clausius-Clapeyron-like equation expressing the coupling between stress and phase transition; At medium (~5 KHz) and high (~ 20 MHz) frequencies, the transitory internal friction becomes negligible. However, a small peak appears accompanied by an elastic modulus decrease. It is shown that these anomalies are related to the shear modes parallel to the transformation plane. Thus, they might be related to the motion of the partial dislocations composing the h.c.p.-f.c.c. interface.     

The fine structure f.c.c./b.c.c. boundaries in a Cu-0.3% Cr alloy

The interfacial structures of overaged b.c.c. precipitates in a Cu-0.3%Cr alloy have been studied with transmission electron microscopy using weak-beam and lattice imaging techniques and the results have been compared with theoretical structures obtained with the “O”-lattice calculation. Lattice imaging showed that one set of close-packed planes in the structures were accurately parallel and using normal diffraction and Kikuchi line measurements the precipitates were observed to have a range of orientation relationships varying from Nishiyama-Wasserman to Kurdjumov-Sachs and beyond. All precipitates showed at least one fine dislocation array with spacings < 2.5 nm and several examples of arrays with two sets of dislocations were also observed. For three precipitates investigated in detail reasonably good agreement between the theoretical and observed values of the spacings, directions of dislocations and overall habit planes of the interfaces was obtained. Confirmation of the (533)_f.c.c. habit plane of the lath precipitate was also obtained.     

Ground state structures in ordered b.c.c. ternary alloys with second-neighbor interactions

Ground state structures in ordered b.c.c. ternary alloys are investigated using the cluster method of Allen and Cahn. It is shown that when a tetrahedral motif which includes first and second nearest-neighbor pairwise energy interactions is used, the solutions consist of either a mixture of three phases or a single-phase state with the same energy, depending on the range of interaction ratios and compositions. The method is used to determine the ground state structures in the b.c.c.-based NiAlTi alloys, using pair interactions suggested by Tso and Sanchez.     

Tetrahedron approximation of the cluster variation method for b.c.c. alloys

The tetrahedron approximation of the cluster variation method is formulated for b.c.c. alloys for any number of components. This includes the possibility of treating ferromagnetic alloys. First and second nearest neighbour interactions are taken into account. For a representative selection of values of these interactions the phase diagrams are presented. Tricritical points have been determined accurately by an analysis of the second Hessian determinant. With the same energy parameters Monte Carlo simulations have also been performed to test the quality of tetrahedron approximation for the calculation of phase diagrams. Good agreement was obtained between the phase diagrams calculated by both methods. The tetrahedron treatment of the cluster variation method is thus a good approximation for the calculation of phase equilibria in b.c.c. alloys.     

HCl leaching behaviour of a bastnasite ore in terms of thorium and rare earth elements

Today, nuclear energy is considered as one of the most important energy sources. Thorium (Th) is also very important for nuclear energy because it can exist in nature spontaneously just as uranium (U) without depending on presence of another radioactive element. As plutonium and other trans-uranium elements are produced in lesser amounts compared to U in Th fuel cycle, Th is considered as the cleanest nuclear plant fuel. Bastnasite is one of the minerals which Th can be economically obtained. Bastnasite (Ce, La)FCO3, which is a rare earth fluorocarbonate, contains approximately 0.2–0.3% Th and 75% rare earth oxides. Within the scope of this study, dissolution behaviour of Th and rare earth element (REE) contents present in the bastnasite ores which is obtained from Eskisehir region of Turkey, with HCl leaching method was examined. In this context, the effects of different leaching parameters such as leaching time, HCl concentration and pulp temperature on Th and REE (Ce, Nd, La) dissolution efficiency and the best results have been obtained in the conditions of 5.48 M of HCl dosage, 220 min. of leach time and 70°C pulp temperature. In these optimum conditions, Th, Ce, Nd and La were obtained with dissolution efficiencies of 80.12, 77.12, 70.12 and 71.13%, respectively.     

Overview no. 50: Cleavage-limited maximum strength of work-hardened b.c.c. polycrystals

b.c.c. metals and alloys maintain a non-decaying work-hardening rate up to very high plastic strains when deformed monotonically at low temperatures (T < 0.3 T_M), specially by axisymmetric elongation (wire drawing). This fact seems to offer the possibility of production of strong metallic filaments which are tougher than other fibre reinforcing materials. However, b.c.c. metals being prone to cleave, their rising stress-strain curves could be trimmed by brittle failure beyond some point. This paper studies such contingency and shows that two failure modes are possible: a microscopic-type failure, when the flow stress meets the (strain-dependent) tensile cleavage stress, σ_f^T, and, for large strains, a macroscopic-type failure when the critical stress intensity factor (also strain-dependent) is reached for cracks associated to inclusions or for crack-like surface defects. From the rather limited information gathered on the evolution of the brittle failure stress with strain it is deduced that σ_f^T is mainly controlled by the instantaneous—strain-induced—grain size and that it will rarely limit the b.c.c. wire strength directly. On the contrary, macroscopic-type failures can constitute the absolute strength limit for alloys with very high workhardening rate. Guidelines to further improve such strength limit arise from the present overview.