Volume 94, Issue 11

Grain boundary (GB) solute diffusion of Ge in Cu polycrystals was systematically investigated by the radiotracer serial sectioning technique using 68 Ge and 71 Ge isotopes. The diffusion measurements were carried out under conditions of Harrison’s B- (923–626 K) and C- (576–451 K) type diffusion regimes. In the B-type diffusion regime, the triple product P = s δ D gb ( s is the segregation factor and δ the GB width) was found to follow the Arrhenius law P = 4:0 × 10 -16 × exp(-58.6 kJ mol -1 / RT Þ m 3 s -1 , where R is the universal gas constant and T the temperature. Measurements in the C-regime directly yielded the GB diffusion coefficient D gb , D gb = 2.8 × 10 -6 × exp(-84.8 kJ mol -1 / RT Þ m 2 s -1 . Combining the obtained P and D gb values and assuming δ ≌ 5 × 10 -10 m, the GB segregation factor for Ge in Cu was determined to follow an Arrhenius dependence s = 0.29 exp(26 kJmol -1 / RT Þ. The determined segregation coefficients s are discussed together with previous results on solute segregation in Cu and Ag, and it is concluded that the well-known Hondros relation, which relates segregation and bulk solute solubility, can generally be used only for a qualitative estimation of the segregation ability. Furthermore, the obtained results for the triple product s δ D gb and the directly measured D gb values are discussed in relation to previous GB diffusion measurements of some other solutes in Cu and Ag in the B- and C-regimes.

Fe 2 Mo powders have been produced from Fe 2 MoO 4 powders by gas – solid reduction using pure H 2 gas at 1023 and 1173 K. The thermal diffusivity of the cold-pressed Fe 2 Mo powders having a relative density between 0.52 and 0.72 has been measured at room temperature both in air and in vacuum using the laser-flash method. A correlation was observed between the thermal diffusivity and the relative density for the powders reduced at different temperatures, regardless of the difference in microstructure of the powders. In order to explain the porosity dependence of the effective thermal conductivity, a new simple method developed based on the Ohm’s law models was used. The model successfully simulates the experimental data, and the thermal conductivity of bulk Fe 2 Mo was estimated as 10.8 W/mK.

Ultrafine-grained titanium produced by equal channel angular pressing and subsequent cold rolling is studied by X-ray diffraction. It is found that a texture exists in which the hexagonal basal planes are parallel to the longitudinal axis of the billet. The crystallite size distribution and the dislocation structure are determined by X-ray diffraction peak profile analysis. The median and the variance of the crystallite size distribution are obtained as m = 39 nm and σ = 0.14, respectively. The dislocation model of anisotropic broadening of peak profiles enables to determine the active slip systems. It was found that all three possible hexagonal Burgers vectors of dislocations exist, with the composition of 66% < a > type, 33% < c > type and 1% < c + a > type.

Texture evolution was analyzed with the full-constraint Taylor model for an idealized perfectly plastic face-centered cubic material as well as for real, strain-hardening copper subjected to equal channel angular pressing (ECAP). For the idealized material, the stress in the plastically deformed part of the billet was shown to be uniform leading to complete filling of the die. Finite element simulations showed that plastic deformation is localized in a narrow shear zone and that the plastic strain and texture in the billet become uniform after ECAP. A simplified recipe for texture calculation akin to that proposed by Gholinia et al. was suggested: it reduces the deformation under ECAP to a combination of two rotations separated by tension-compression. For the case of copper, a strain hardening model based on dislocation density evolution was used. It was shown that due to significant strain hardening during the first ECAP pass, the flowing material does not fill the outside die corner and a strain and texture non-uniformity develops. A gradual decrease of the strain hardening in subsequent ECAP passes leads to a more uniform strain and texture across the billet. The simulated pole figures were shown to be in good agreement with the neutron diffraction data for copper deformed by ECAP (Routes A, C r , B γ and Bc r ) suggesting that the model used provides a reliable modeling tool for simulating texture evolution under ECAP.

High-energy synchrotron radiation has been used to study in situ annealing of cold-rolled Cu and Ti. The measurements were performed using a high-vacuum furnace in transmission geometry and an area detector. The diffraction images were subsequently processed to extract the orientation distribution. The recrystallization process could be followed with a time resolution of the order of 10 s, and good pole figures could be obtained from the very limited amount of data in single diffraction images. The pole figures compare favorably with pole figures of the same material measured ex situ with a conventional pole figure goniometer. For Cu, a rapid and complete change from a typical rolling texture to a cube texture was observed after annealing for 20 min at 650 °C. For Ti, changes were more subtle with a tendency for c -axes to diminish near the normal direction, as well as for a -axes to become aligned with the rolling direction. The method makes it feasible to study the kinetics of recrystallization with quantitative texture analysis.

In order to investigate the mechanism of the development of rolling textures in Al –Mg alloys, hot bands of high-purity Al, Al-3 wt. %Mg and Al-5 wt. %Mg alloys were solution-treated at 450 °C and cold-rolled by varying rolling reductions up to 97%. Rolling textures of these specimens were investigated with the orientation distribution function analysis. Comparisons of the results of the texture analysis with those of the hardness measurement and transmission electron microscopy observations revealed that, in all specimens, the texture development occurred mainly at the later half of work hardening, i. e., at rolling reductions above 80 %, where dislocation substructures were well developed. In pure Al, in which well defined, elongated cell structures were developed, {112}<111> and {123}<634> components developed remarkably, whereas in the Al-5% Mg alloys, in which only dense dislocation tangles and ill defined small cells were developed, the development of {112}<111> and {123}<634> components was strongly suppressed. All these observations thus suggested that the development of rolling textures was strongly affected by the development of dislocation substructures, which occurred concurrently during rolling, and that the formation of elongated cell structures was more favorable for the development of rolling textures. In cold rolling, elongated cell structures may be the final form of dislocation substructures, providing both mechanical stability and orientation stability.

In the investigations on plastic forming of Cu, Al and Pb, under conditions of plane state of strain, priority has been given to variants with a change in the loading scheme (modified channel die test). It has been found that a direct consequence of the application of such a procedure is a temporary, considerable decrease of the flow stress and its subsequent increase to a value exceeding that attained in a standard test. The reasons for such a behavior have been analysed.

A nonlinear finite element method is presented for analysing the thermo-mechanical fatigue behavior of the DD8 single crystal nickel-based superalloy. The specimens had axial orientations along the [001] direction. In-phase and out-of-phase thermo-mechanical fatigue were simulated under mechanical strain control with a temperature variation from 450 to 900 °C. The cyclic mechanical behavior, the equivalent stress and plastic strain were investigated in both γ' phase and γ matrix.

The extent of brittle crack as a function of hydrogen charging conditions was studied for a HSLA steel using circumferentially notched cylindrical tensile samples. Two different notch depths were used. The effect of hydrogen could be well represented by an effective hydrogen potential which was defined using a representative hydrogen concentration and a diffusive time parameter, for relatively faster strain rates. The high triaxiality in deep-notched samples led to the initiation of ductile failure mechanisms overwhelming the brittle cracking process.

Samples of titanium-stabilized interstitial-free steel were coiled at 600 and 700 °C. The effect of the coiling temperature on the formation of textures and precipitates during coiling, cold rolling and recrystallization annealing was studied by means of neutron texture measurements, small angle neutron scattering analysis and transmission electron microscopy observations. Whereas coiling at the lower temperature resulted in an insufficient scavenging of carbon in solid solution, coiling at the higher temperature promoted scavenging of carbon in solid solution, and consequently the formation of a large number of precipitates. The large amount of carbon in solid solution caused the retardation of softening in the cold-rolled sample during annealing below 700 °C. Upon annealing above 700 °C the impact of the coiling temperature on grain size and precipitates was not observed. The samples coiled at different temperatures exhibited a nearly identical cold rolling texture. However, the cold-rolled sample having a large amount of carbon in solid solution displayed a weak recrystallization texture.

The Ostwald ripening of spherical and rod-shaped Cu precipitate particles in an Fe-1.5 mass% Cu alloy aged between 873 and 1023 K has been examined by measuring both the particle size by electron microscopy and the Cu concentration in the α-Fe matrix by electrical resistivity. The average radius of spherical and rod-shaped particles increases with ageing time t as t 1/3 , in agreement with the growth by diffusion-controlled coarsening. The kinetics of the depletion of supersaturation with t obey the predicted t – 1/3 time law. The Cu/α-Fe interfacial energy and the diffusivity of Cu in α-Fe have been independently derived from the data on coarsening. The interfacial energies are measured as 0.57 and 1.1 Jm –2 for the spherical and rod-shaped particles respectively. The pre-exponential factor and the activation energy for bulk diffusion are found to be 4.54α10 –5 m 2 s – 1 and 241 kJmol – 1 .

The morphology and growth kinetics of the discontinuous precipitation of liquid and depleted solid phases from a supersaturated solid solution has been studied. The transformation was carried out in the two-phase (liquid + solid α) field in the temperature range 996–1050 K in two Cu– In alloys containing 8.9 and 9.5 at.% In. A lamellar structure consisting of alternate lamellae of α and liquid phases has been observed to grow from the grain boundaries of the supersaturated α solid solution. The nucleation of the liquid droplets occurred at the grain boundary and a thin layer of liquid formed between the droplets. The liquid film started migrating resulting in the development of a lamellar structure. In both alloys the growth rate increased with increasing temperature of transformation but the interlamellar spacing decreased with increasing transformation temperature. The growth kinetics was analyzed using the models of Cahn and Petermann and Hornbogen and from a mass balance using Fick’s first law. From the diffusivity and activation energy values it can be concluded that the growth of the cellular structure occurs by solute transport through the liquid film.

Processing maps have been developed for hot deformation of Mg–2Zn– 1Mn alloy in as-cast condition and after homogenization with a view to evaluate the influence of homogenization. Hot compression data in the temperature range 300 – 500 °C and strain rate range 0.001 – 100 s – 1 were used for generating the processing map. In the map for the as-cast alloy the domain of dynamic recrystallization occurring at 450 °C and 0.1 s –1 has merged with another domain occurring at 500 °C and 0.001 s – 1 representing grain boundary cracking. The latter domain is eliminated by homogenization and the dynamic recrystallization domain expanded with a higher peak efficiency occurring at 500 °C and 0.05 s –1 . The flow localization occurring at strain rates higher than 5 s –1 is unaffected by homogenization.

The paper deals with some characteristics of electrospark deposition. A relevant device and the process are described, the material transfer is shown schematically, and the average droplet mass, the thickness of deposited layer, and the layer roughness are determined. Two types of substrate (tool steel, austenitic stainless steel), two types of shielding gas, (Ar, He), and three types of filler material, (WC, TiC, Stellite 6) were used. With some deposits, chemical analyses of deposit surfaces were performed and with some others through-thickness chemical analyses. Among the final conclusions the most important one is that the addition of a shielding gas results in a considerable increase in deposit quality. The device manufacturer, however, recommends deposition without the addition of a shielding gas.