Developments in deformation and recrystallization textures were studied in cold-rolled (50–90% reduction) ultra low carbon (ULC) steel using X-ray texture measurements and orientation imaging microscopy (OIM). During deformation, γ-fibre (ND//〈111〉) increased between 0 and 50% reduction but then did not change significantly, while α-fibre (RD//〈110〉) increased progressively from 0 to 90% reduction. After complete recrystallization, however, a steady increase in γ and almost no changes in α were observed with increasing strain. Developments in recrystallization textures were attributed to two parameters: (1) spacings (λi, as measured along the normal direction, ND, where i can be a specific component of α/γ-fibres) of the α/γ deformed bands; and (2) their relative ability to form recrystallized grains. While λi was determined by the deformation texture and the thicknesses of the deformed grains/bands and naturally decreased with increasing strain, estimations of parameter (2) were obtained from the so-called nucleation factors (Ni, defined as the number of recrystallized i grains per i band—as measured/estimated along the ND). At higher strains, noticeable drops in the Nis of α-fibre were observed. Two plausible causes for such drops were increased stored energy advantages for γ bands and orientation pinning in some of the α regions.
A study has been made of the spontaneous growth of tin whiskers from tin electrodeposits on phosphor bronze sheet. The driving force for the evolution of tin whiskers is a biaxial compressive stress of about 8 MPa developed in tin deposits by the formation of an intermetallic compound of Cu6Sn5, especially in grain boundaries of tin films. The biaxial compressive stress gives rise to strains normal to the film plane, which are dependent on the tin grain orientations. The shear stresses due to differences between strains of different grains along the thickness direction make the tin surface oxide film sheared approximately along boundaries of grains with particular orientations which are different from the major texture of the film. Tin extrudes from the grains, whose surface oxide films are sheared. The extrusion takes place continuously by expansion of the dislocation loops by climb and their subsequent glide toward the surface in the slip direction to form whiskers. (C) 1998 Acta Metallurgica Inc.
A micromechanical model describing “quasi-ductile” Hertzian contacts in otherwise brittle ceramics is developed. The elemental basis of the model is a discrete “fault” along an internal weak interface, constrained at its ends by an elastic matrix and subject to frictional sliding, in the subsurface zone of high shear stress in the Hertzian field. By summation over a prescribed density of shear faults within the active plastic zone, the analysis leads to a constitutive identation stress-strain function, with special provision for the incorporation of microstructural variables. Experimental data from a series of mica-containing glass-ceramics with contiguous platelet microstructures are used to confirm the essential predictions of the model. It is demonstrated that plasticity increases with volume fraction and aspect ratio, but not size, of the platelets. Parametric evaluations by curve fitting the indentation stress-strain data allow for predictions of intrinsic stress-strain responses for the glass-ceramics in conventional uniform stressing states.
An isothermal kinetics of non-equilibrium grain-boundary segregation was developed both for segregation processes and for desegregation processes within a phenomenological theory. An effective time concept of a cooling process was discussed. On these bases, a simple and accurate method for evaluation of the levels of non-equilibrium grain-boundary segregation during cooling was proposed. According to the method, we have calculated the levels of non-equilibrium segregation to austenite or prior austenite grain boundaries for boron in Fe-30%Ni alloy, aluminium in Inconel 600, chromium in 2.25%Cr1%Mo steel and tin in 2.25%Cr1%Mo0.08%Sn steel in different experimental conditions respectively. Results calculated from the kinetic model in this paper are in satisfactory agreement with the observed data of experimental measurements for all the above samples.
The superplastic Zn-22% Al eutectoid alloy was tested over a range of strain rates at 503 K using specimens machined with the rolling direction perpendicular to the tensile axis. It is shown that the mechanical properties of the alloy, including the elongations to failure, are essentially identical to those obtained when the rolling direction is parallel to the stress axis. Inspection after failure showed that cavities are formed in stringers and, as in the same alloy tested with the rolling direction parallel to the tensile axis, these stringers are aligned parallel to the stress axis. The results demonstrate that the cavities are not nucleated primarily at impurity or oxide particles, nor do they grow from pre-existing microvoids which may be introduced during thermomechanical processing. The cavities are located preferentially at the α β interfaces and at the associated triple points, and it is probable that they were nucleated at triple points and grain boundary ledges during bursts of grain boundary sliding.
Shock-wave processing of a nickel-based superalloy (Pyromet 718) powder is explored and compared to conventional hot-isostatic processing. The shock consolidated powder has extensively deformed particle interiors, A fine microcrystalline phase was observed at interparticle regions, and is attributed to rapid solidification of melt formed during the consolidation process. The shock consolidated material has a yield strength of 1 GPa prior to heat treatment, which is at least 10% higher than that of the hot isostatically pressed alloy. A high density of very fine disk-shaped γ″ precipitates nucleate in the shock consolidated material after solution treatment and upon aging at 620°C for 8–40 h. A lower density of coarser precipitates nucleate in the hot-isostatically pressed alloy subjected to similar thermal aging conditions. The shocked and heat treated alloy exhibits a 20% higher yield strength and a 40% higher UTS than the hot isostatically pressed and heat treated alloy, with comparable ductility. The improvement is attributed to a finer intraparticle grain size, very fine grained interparticle material, and a higher density of very fine and uniformly dispersed y″ precipitates resulting from faster nucleation kinetics in the shock consolidated material.
Plastic deformation of ferritic stainless steel alloy AL 29-4-2 has been found to severely decrease the diffusivity and permeation of hydrogen and to increase the solubility somewhat. These changes are compared to the effects of deformation on these quantities in austenitic alloys. The differences in behavior of these classes of stainless steels are interpreted in terms of trapping and phase transformation behavior. The uniformly deformed material is studied in an effort to simulate the state of material at the tip of a hydrogen embrittlement crack. The observed differences in crack propagation behavior in the alloys can be rationalized in terms of hydrogen transport and solubility in the deformed state.
Dislocations are observed in many shape memory alloys after thermal or stress cycling. The amount of dislocations (with Burgers vector bβ = a0〈010〉 and 〈1̄11〉 line direction in the β phase) increases with the number of cycles. The dislocations are accumulated in the sample and are incorporated in the corresponding growing phase. The relative energy of the dislocations when embedded in the parent phase or in one or another variant of martensite is evaluated in this work. The crystallographic changes of the dislocations provide a primary selection rule for those martensite variants in which the dislocations have the lowest energy. In order to proceed more quantitatively a full calculation of the dislocation energies has to be performed using the anisotropic theory. In this work these calculations have been made on the basis of measured elastic constants of the β and 2H phases of a Cu-Al-Ni alloy. It is found that a given dislocation could indeed have different energies depending whether it is embedded in the β phase or one or another oriented variant of martensite. In particular, those dislocations with Burgers vector lying on the basal plane of the 2H martensite show the lowest energy as compared to their self energy when embedded in other oriented variants (Burgers vector out of the basal plane) or in the β phase.
A grain-bridging model of crack-resistance or toughness (R-curve, or T-curve) properties of nontransforming ceramics is developed. A key new feature of the fracture mechanics treatment is the inclusion of internal residual (thermal expansion mismatch) stresses in the constitutive stress-separation relation for pullout of interlocking grains from an embedding matrix. These internal stresses play a controlling role in the toughness properties by determining the scale of frictional tractions at the sliding grain-matrix interface. By providing a physical account of the underlying micromechanics of the bridging process the analysis allows for predetermination of the material factors in the constitutive relation, thereby reducing parametric adjustments necessary in fitting the theoretical toughness curve to experimental data. The applicability of the model is illustrated in a case study on indentation-strength data for a “reference” polycrystalline alumina with particularly strong T-curve characteristics. From theoretical fits to these data the constitutive relation, and thence the entire T-curve, can be deconvolved. This “parametric calibration”, apart from demonstrating the plausibility of the model, allows for quantitative predictions as to how the toughness and strength characteristics of ceramics depend on such microstructural variables as grain size and shape, grain boundary energy, level of internal stress and sliding friction coefficient. An indication of this predictive capacity is provided by a preliminary calculation of the grain-size dependence of strength, using some existing data for other aluminas as a basis for comparison.
A theory is developed to predict the long range order parameter, composition and temperature at the interface of a chemically ordered phase as a function of interface velocity and liquid composition during rapid crystal growth. It extends the solute trapping theory of Aziz to a solid phase consisting of two sublattices. The engulfment of atoms randomly on the two sublattices by the rapidly moving liquid-solid interface is balanced against the interdiffusion across the interface that attempts to restore local equilibrium. With increasing interface velocity the theory predicts a progression from the solidification of a phase with equilibrium long range order parameter and with equilibrium solute partitioning to the solidification of a disordered crystalline phase with the same composition as the liqiud. Predictions for solids with free energy functions in which the order disorder transition is first or second order show that the decrease of order parameter to zero with increasing interface velocity will be discontinuous or continuous respectively. Also solute trapping can occur at either a higher or a lower growth rate than disorder trapping depending on the free energy function.
Net or near net shape products can be manufactured by technologies involving solidification processing, metal forming, paniculate processing, and droplet consolidation. One example of droplet consolidation is spray deposition in the Ospreytm mode. In this process, a stream of liquid metal is atomized by an inert gas to form a spray of molten droplets; these are accelerated towards a substrate where they impinge and consolidate. An integral model for the Ospreytm spray deposition process has been developed using established theoretical principles. Mathematical models describe the interconnected processes of droplet-gas interactions in flight and subsequent droplet consolidation on the substrate. The models predict droplet velocity and temperature as a function of flight distance, the extent of droplet solidification on arrival at the substrate, and temperature distribution in the consolidated material during deposition. This approach demonstrates the utility of modeling studies in order to establish quantitative guidelines for optimization of the process in terms of the evolution of microstructure in droplet consolidation.
The experimental data of Lobo and Geiger on the activity of carbon in ferrite have been re-analyzed in detail using the statistical thermodynamic relationships of Darken-Smith/Kaufman-Radcliffe-Cohen, Lacher-Fowler-Guggenheim and McLellan-Dunn. Indirect regression analyses had usually to be employed in order to extract a value for the interaction energy between nearest neighboring carbon atoms, ωα. However, the Lobo-Geiger data reported at only two temperatures fulfilled the requirements for performance of a valid indirect analysis using the three expressions correlating activity with composition and ωα. At both temperatures, all three analyses yielded ωα < 0, i.e. an attractive interaction between adjacent carbon atoms; ωα values obtained ranged from −27 to −42 kJ/mol. Published reports that ωα > 0 were shown to have resulted from failure to fulfill the requirements for a valid indirect regression analysis. Using a relationship for the electrostatic interaction energy between carbon atoms due to Blandin, Deplante, Friedel and Machlin, and an equation for elastic strain energy interaction due to Eshelby, it was concluded that the former energy is the more important contributor to both ωα < 0 and ωγ > 0; in both ferrite and austenite, the elastic interaction and electrostatic interaction energies are of opposite sign.
A theoretical analysis of rate sensitive pencil glide is carried out. It can be applied to pencil glide with any set of slip directions, not only 〈111〉, and in any reference frame. As a first application, the behaviour of an isotropic polycrystal deforming by rate sensitive 〈111〉 pencil glide is studied. The present approach is also applied to the prediction of rolling textures with the Taylor model and with the relaxed constraint model. It is shown that the latter is in better agreement with experimental results. The influence of the rate sensitivity index is also examined, and qualitative agreement is obtained with an experimental result for rolling at warm temperatures.
Existing theories of Harper-Dorn creep rely on the assumption that, under conditions where Harper-Dorn creep is observed, the dislocation density rapidly achieves a characteristic value which is independent of the method of preparation of the sample, of the applied stress, and of the extent of creep strain, but they do not attempt to justify this assumption. We perform a dimensional analysis in which it is assumed that the ratio of strain rate ϵ to applied stress σ, which is experimentally observed to be characteristic of the material and independent of the history of the material and of the applied stress, can depend only on the Burgers vector, the shear modulus μ, and the drag coefficients Bc for dislocation climb and Bg for dislocation glide. If we also accept the experimental observation that ϵ/σ has the activation energy of self-diffusion, the resulting relation indicates a value of ϵ/σ which is 1011 times too large. The only other large number which can enter the theory is the ratio of μ to the Peierls stress σp, and this indicates that the Peierls stress must play a significant role in the mechanism. We obtain a numerically reasonable theory by assuming that the equilibrium dislocation density is that at which the stress exerted by a dislocation on its neighbour is equal to the Peierls stress, while the motion of the resulting dislocation array under the applied stress is controlled by climb. The internal stresses in the Harper-Dorn régime are less than the applied stress.
In directional solidification of NiAlMo alloys solidifying along the monovariant eutectic trough L→γ + α, the morphology of α-Mo fibers and their crystallographic orientation relationship with the γ-NiAlCr matrix depend on growth rate and misorientation between the crystallographic [001] growth axis of the matrix and the local heat flow direction. At high growth rates and misorientations of more than 6 degrees, fibers can adopt a ribbon like shape but fibers and ribbons generally remain faceted, preserving the standard orientation relationship with the matrix. At low growth rates (< 2 cm/h) and misorientations by more than about 10 degrees fibers become non-faceted and follow the Pitsch orientation relationship. Spacing of faceted fibers increases with increasing misorientation. If it is assumed that interface undercooling in eutectic growth is associated with maximum interface curvature and that eutectic growth occurs at minimum interface undercooling, spacing and morphological variations in faceted growth can be explained in terms of misorientation and interface energy. Simple structural criteria do not allow to identify the boundaries between matrix and fibers as well developed low-energy interfaces. There is evidence, however, that eutectic α fibers crystallize at the limit between faceted and non-faceted growth, and that fiber morphology is controlled by nucleation of new growth layers at the triple points between the γ-, α- and liquid phases. Solid state transformations change the volume fraction of fibers without much influencing fiber morphology. Concerning the phase diagram reported by Wakashima et al. [Acta metall. 31, 1937 (1983)], displacement of phase boundaries in the vicinity of the ternary γ + γ' + α eutectic is proposed.
Synchronous time dependent X-ray scattering and anelastic stress relaxation experiments were carried out to investigate the dynamic premartensitic properties of an In-24 at.% Tl alloy. The relaxation of the X-ray intensity, 0.2 degrees off the 220 Bragg diffraction peak, has been observed under a uniaxial [001] tensile strain of the order of 10−4 as the temperature approaches the martensitic transformation temperature. The activation energy calculated from the Arrhenius plot of the relaxation times obtained from the X-ray diffraction is 0.78 eV which is equal to the one found previously in the course of an internal friction experiment. However, the activation energy obtained from the anelastic stress relaxation is 0.11 eV. It is proposed that the high activation energy of 0.78 eV represents the energy of formation of a premartensitic strain embryo. The much smaller activation energy of 0.11 eV is interpreted as the activation energy of stress assisted growth of this embryo.
Le modèle géométrique de la structure des joints de grains a été appliqué à l'étude des macles de déformation des métaux de structure hexagonale compacte. Ce modèle géométrique permet de décrire les macles, dans une certaine approximation, comme des joints en coïncidence d'indice faible. Cette description est très générale et permet à partir du cas simple ca = √3 de traiter la macle (10–12) dans tous les métaux de structure hexagonale compacte. Une méthode originale permet de retrouver tous les complexes bicolores associés à cette orientation de macle et plus particulièrement, une configuration de symétrie maximale a été mise en évidence. A partir de cette configuration il est possible de proposer une description géométrique de la structure de la macle (10–12). En outre, un mécanisme de dislocations permet de mieux comprendre la germination et la croissance de cette macle.
During continuous cooling performed from 950°C at a rate between 1 and 10°C s−1, the high strength low alloyed steel 300M acquires a mixed structure composed of plate martensite, lower bainite and retained austenite. The formation of lower bainite has been interpreted in terms of phenomenological theory developed in the case of martensite formation. Indeed, the habit plane of lower bainite experimentally determined well agrees with the one calculated from matrix algebra or graphical method, when the shear elements of the austenite lattice are as follows: (1̄11)γ, [211]γ and (1̄11)γ, [121̄]γ. The PITSCH's orientation relationship was evidenced between cementite and ferrite of lower bainite, while between ferrite of lower bainite and austenite, an orientation relationship closer to Nishiyama-Wassermann's than Kurdjumov-Sach's was found. For appropriate thermal treatments, upper bainite and austenite under form of “S” where also revealed. Moreover, silicon does not seem to induce the formation of silicocarbides and would not play a role in the mechanism of the bainite transformation; it only contributes to delay this transformation.
The microstructure of Ho-substituted sintered Fe-Nd-B magnets, prepared by two different routes, has been characterized in order to understand the changes in intrinsic coercivity with Ho content. Spherical inclusions in the size range of 500 Å-l μm have been observed inside the matrix Re2Fe14B grains. Characterization by X-ray diffraction, electron microdiffraction and energy dispersive X-ray microanalysis, shows that these inclusions are f.c.c. rate earth oxides. The volume fraction of these inclusions increases with Ho content. Ho partitions preferentially into the matrix phase and no segregation of Ho to the interphase interfaces is observed. Lorentz electron microscopy experiments suggest that the inclusions in the lower end of the size distribution are capable of pinning the magnetic domain walls and hence these non-magnetic inclusions must contribute to the increase in intrinsic coercivity.
Fracture mechanics and quantitative fractography were used to characterize fracture origins and analyze the fracture mechanism in a TiB2-AlN composite. During fracturing in air at room temperature, the composite did not show typically brittle behavior. Thermoelastic mismatch between the unlike phases caused microcracking, and crack branching (mirror formation) during fracture was not observed. A rising resistance curve is presumably operating in the composite. A stress-induced microcrack toughening mechanism was identified which induced the greatest toughness at cryogenic temperatures. As temperature increased, thermoelastic mismatch and fracture toughness decreased until at 1071 K the composite showed typically brittle behavior accompanied by crack branching.
