Acta Metallurgica最新文献

The strengthening contribution of δ' precipitates in Al-Li alloys was determined from low cycle fatigue tests under plastic strain control performed on single- and polycrystalline specimens. For underaged specimens the experiments show a plateau in the stress response for the first 10–100 cycles which is independent of the plastic strain amplitude. These stress levels were compared with theoretical estimates of particle strengthening due to internal order where the volume fraction ƒ and the mean particle radius r were the variables. Results of monotonic tests collected from the literature were also included in the analysis. Good agreement between theory and experimental results obtained from both cyclic and monotonic testing can be reached by assuming a specific antiphase boundary energy of 165 mJ/m² for the (111) planes

Using the embedded atom method, boundary energy calculations were done for (111)Ag//(001)Ni twist boundaries and interfaces between low-index planes of Ag and Ni. The E(θ) curve for (111)Ag//(001)Ni boundaries shows the existence of one deep cusp at θ = 0°, or at the variant θ = 30°. Experiments using the rotation of (111)Ag crystallites on (001)Ni film confirm this single cusp. Calculations for $\text{Ag}\text{Ni}$ interfaces containing low-index planes show that those containing the (111) plane have the lowest energy while those with the (110) are highest.

A phenomenological model of creep anelasticity is developed. It is applied to previously reported data for polycrystalline aluminum and copper, and single crystals of aluminum. The results demonstrate that an excellent description of the magnitude and kinetics of backflow can be obtained using two Voigt solids in series, one with a linear dashpot and the other incorporating a power law dashpot with an exponent of 2. This phenomenological model is combined with constant substructure forward creep data to demonstrate that stress reduction creep transients can be represented by a superposition of anelasticity and forward flow. While operationally successful, it is argued that this superposition is valid only if the two mechanisms operate independently. Neither the data nor the results of the model permit the mechanisms of backflow to be identified completely. However, several indirect observations indicate that the processes of backflow are directly related to the creep substructure. Further, the power 2 stress dependence suggests that the initial backflow processes are controlled by dislocation glide on noncompact planes. The linear behavior observed at longer times is then probably associated with relaxation of subgrain walls. Finally, it is demonstrated that forward flow and reverse flow under constant structure conditions cannot be represented by the same kinetic laws. This finding indicates that the two mechanisms are different, and supports the contention that creep transients can be described as a superposition of forward and reverse flow.

The concept of paraequilibrium, introduced a long time ago by Hultgren and analyzed thermodynamically by Hillert, has been analyzed in detail by means of a kinetic model. The concept applies to alloy systems where there is a large difference in mobility of the different components, e.g. in steels where substitutional elements diffuse many orders of magnitude slower than interstitial solutes like H, C and N. By definition paraequilibrium means that only the mobile elements are equilibrated while the sluggish ones behave as a single element. In the present work we have applied a phenomenological model that takes into account the diffusion in the matrix in front of a moving phase interface, a finite interface mobility, the effect of surface tension and the sluggishness of diffusion across the phase interface, i.e. the solute drag. We have studied theoretically the edgewise growth of a ferrite platelet in Fe-Mn-C alloys. In particular we have studied how the growth process depends on the Mn content. We have investigated under what conditions paraequilibrium or full thermodynamic equilibrium is established locally at a moving phase interface.

Thin films of copper deposited on the (11$\text{2}$0) plane of sapphire disintegrated into clusters when annealed at 650°C under nonoxidizing conditions. The breakdown was found to initiate at processing defects in the film; this is in contrast to earlier work with zirconia films on sapphire where cavities nucleated profusely at grain boundaries due to poor wetting between zirconia and alumina. A comparison between these two cases, as well as a theoretical model, suggest that processing defects are the source of instability when the contact angle is less than (α + π/2), where α is one half of the dihedral angle formed by a grain boundary with the free surface. The breakdown of the film eventually led to separated, single crystal beads of copper. The beads coarsened and changed their orientation by solid state diffusion, leading to a highly preferred orientation. The {l11} plane of copper was found to be parallel to the sapphire surface. But the texture was planar isotropic, that is, it was rotationally symmetric around the plane normal. This result is in agreement with other observations that copper forms {111} planar isotropic texture on (0001) sapphire, and also on polycrystalline α-alumina.

A small-angle twist or second tilt component was introduced to tilt boundaries [001] in tin bicrystals with misorientation angles 28.3°. The method of triple junction was used to measure the temperature dependences of the relative surface tension of the boundaries obtained and to determine the temperature of the “special boundary-general boundary” phase transition. The dependence of the transition temperature on the twist component θ_k and on the second tilt κ_H was found. The temperature of the grain-boundary phase transition, T_c, decreases with growing θ_k and θ_H, but several times slower than under deviation of the misorientation angle on the coincidence misorientation. The tilt component diminishes T_c slower than the twist component does. The results confirm the dislocation model of the “special boundary-general boundary” phase transition, proposed earlier.

A continuum mechanics model based on the Eshelby method is used to predict the variation of elastic moduli and yield strengths of MMCs with loading orientation. These predictions are compared with mechanical data obtained both for aluminium reinforced with particulate SiC and for whisker-reinforced material. The modelled elastic properties correlate well with the data, and suggest a quick and accurate way of measuring effective inclusion volume fraction and aspect ratio. The model also accounts for the form, though not the magnitude, of the angular variation of the difference between compressive and tensile yield stresses. However, the relative magnitudes of the longitudinal and transverse yield stresses cannot be explained by considering only unrelaxed mean stresses.

Equations are developed for approximating the flow rate of mobile solutes and other point imperfections across a cylindrical surface of arbitrary radius r surrounding stationary defects such as straight edge dislocations, sharp tensile cracks and edge-dislocation pileups embedded in an infinite solid. The starting point for the calculations is Fick's generalized law which takes into account the interaction potential between the stress field of the stationary defect and the diffusing solute as well as the solute-concentration gradient. It is assumed that the initial solute concentration, c(time = 0), is uniform everywhere and that c (r = 0) = 0 for time > 0. The degree of accuracy of the approximations used in this analysis is discussed in detail. Comparisons of the calculated results with experimental data are very satisfactory.

Boron segregation to grain boundaries was investigated by means of particle tracking autoradiography (PTA) in a low carbon and two Mo-bearing steels and in an Fe-30% Ni alloy. Non-equilibrium segregation took place after rapid cooling to a degree which increased as the temperature difference between the austenitization and subsequent holding temperatures was increased. The amount of deformation during isothermal holding had a similar effect. The maximum segregation produced in this way can be many times higher than that associated with equilibrium segregation. The observations support the view that boron atoms are transported to the boundaries by forming complexes with vacancies which migrate to the boundaries prior to annihilation. During holding at different temperatures, the boron intensity curves produced by the PTA method exhibit three types of time dependence. Two of these involve the appearance of a segregation peak, after which there is either complete or partial disappearance of the segregation. The former is associated with the back diffusion of boron into the depleted zone, a process that takes place at the highest holding temperatures and in the absence of precipitation. The latter involves the conversion of temporary segregation into grain boundary precipitates and is observed at intermediate temperatures. The third type is observed at the lowest temperatures; in this case the development of non-equilibrium segregation is converted into precipitation prior to the appearance of segregation peak. The temperature range associated with each type of curve depends on the relation between the kinetics of non-equilibrium segregation on the one hand and that of precipitation on the other.

The hardening behavior of cyclically deformed α-β brass has been studied in the two-phase bicrystals having phase-interface planes parallel to the stress axis. Tension-compression tests at room temperature were performed under a constant total strain amplitude of ±0.4% to 10³ cycles. The hardening took place rapidly in the initial stages, and then gradually saturated, accompanying the establishment of stable lenticular stress-strain hysteresis loops. The main characteristics of the stable stress-strain curves of the bicrystals could be attributed to the contributions from the intrinsic slip properties of the constituent phase crystals, in particular from those of the β phase with marked dependency on the orientation of the stress axis. Also, the different hardening behavior observed between the specimens could be related to the difference in the slip behavior affected by the degree of strain compatibility across the interface.