Dissociation of grain boundaries induced by changes of composition,the ejection of dislocations from grain boundaries,and the nucleation of diffusion induced grain boundary migration

We present various observations of the dissociation of large and small angle grain boundaries in copper during the alloying of the specimens with zinc. In all large angle cases, this dissociation takes the form of the large angle boundary reacting to form another large angle boundary and a small angle boundary. This process may be of great importance in understanding the nucleation of diffusion induced grain boundary migration. Small angle boundaries can undergo complete disintegration upon the addition of zinc, and the magnitude of the stress involved in such a disintegration is estimated.     

含内部晶界位错的晶界处的马氏体成核

实验结果表明,含有内部晶界位错的晶界是马氏体相变的优先成核点。利用会聚束菊池花样分析详细研究了敏化AISI 304不锈钢中形成的BCC α'-马氏体的显微组织和结晶学。马氏体和奥氏体之间的取向关系确定为Kurdjumov-Sachs型。从准确的∑11重合位置点阵(CSL)晶界取向在晶界10.83°处观察到了内部晶界位错的存在,在该处,内部晶界位错的断层和扩展形成了马氏体核。马氏体核的结晶变体的选择既与成核处界面能的降低无关,也与奥氏体中易滑移造成的相变应变的协作无关。     

Interaction of dislocations with grain boundaries in Ni3Al

The interaction between slip dislocations and grain boundaries in hypo-stoichiometric Ni₃Al, with and without boron, has been investigated by using the in situ TEM deformation technique. In both alloys, the slip dislocations were incorporated into the grain boundaries and remained at the point of entry. The difference between the alloys was in the dominant response mode of the grain boundary to the stress concentration associated with a dislocation pileup. In the boron-free material, the stress was relieved primarily by the nucleation and propagation of a crack along the grain boundary. In contrast, in the boron-doped material, relief occurred by the emission of dislocations from the grain boundary. These results are consistent with boron increasing the cohesive energy of the grain boundary. The slip system activated at grain boundaries in the ductile ordered alloy was shown to satisfy the same slip transfer criteria that operate in f.c.c. disordered alloys.     

Hydrogen detrapping from grain boundaries and dislocations in high purity iron

Hydrogen detrapping in high purity iron was studied by measuring evolution rates of quenched-in hydrogen from 80 to 800 K using a quadrupole mass spectrometer in an ultra high vacuum system. The peak of the evolution rate was observed at 395 K in single crystal specimens and 415 K in polycrystalline specimens with a heating rate of 1 K min⁻¹. Effects of grain size and deformation on the evolution rate was also studied. It was shown that the results are consistent with the evolution rates calculated with the binding energy B = 0.51 ± 0.02 eV and the trap density term γC_{T = (4 ∼ 15) × 10⁻⁵} in polycrystalline iron, and B = 0.47 ± 0.02 eVand γC_T = (2 ∼ 13) × 10⁻⁵ in single crystal iron. The dominant traps are considered to be grain boundaries in polycrystalline specimens and dislocations in single crystal specimens.     

Chromium depletion and martensite formation at grain boundaries in sensitised austenitic stainless steel

A microstructural and compositional investigation of grain boundary precipitation and martensite formation in sensitised 304 stainless steel has been conducted. Grain boundary depletion of chromium has been quantified in terms of sensitisation time, temperature and boundary type by energy dispersive X-ray microanalysis in the transmission electron microscope. Chromium depleted profiles measured in grain boundary vicinities are sometimes asymmetrical and correlate with the expected profiles generated by growth of semicoherent and incoherent carbide interfaces. The depletion of chromium promotes martensite formation within near-grain boundary regions and this transformation has been directly studied by in situ cold stage microscopy down to − 150°C. Transformation occurs at the most severely depleted boundaries and initiation is favoured at slip band-boundary intersection points and along grain boundaries whose plane orientation matches that of the martensite habit plane. The preferential formation of grain boundary martensite could be an important factor in the stress corrosion and environment sensitive failure of this material.     

Cavity nucleation at high temperatures involving pile-ups of grain boundary dislocations

The current concept of sliding-induced cavitation is first reviewed, with due consideration to the respective role of the remote and internal stresses. The results show that the transient stresses produced by sliding play only a secondary role in cavity nucleation and that at high temperatures, the effect of sliding is eliminated in less than 1 millisecond. It is thus concluded that sliding cannot be the cause for cavity nucleation. Next, a model involving pile-ups of grain boundary dislocations (GBDs) is proposed. Unlike the sliding mechanism, the high strain energy ahead of the pile-up is a steady state phenomenon during secondary creep. It helps to compensate the large capillarity forces in the formation of sub-micron sized cavities, thereby rendering cavity nucleation barrierless. However, a threshold stress exists below which the cavities cannot grow to effect fracture. The present model suggests that cavity nucleation is feasible in single phase metals and alloys at the intersections of cell and grain boundaries. Predominant cavity formation after the onset of steady state creep and an intermediate temperature ductility trough during hot tensile tests are also features of the model. Good agreements are found between the model's predictions and the experiments.     

A statistical analysis of cavity nucleation at particles in grain boundaries

Cavity nucleation on grain boundary particles during creep has been examined using a classical thermodynamic method. The particle sizes and spacings are assumed to obey a log-normal distribution. It is found that a threshold shear stress is needed for cavity nucleation to occur. When the resolved shear stress on a grain boundary segment reaches the threshold stress, a critical normal stress for cavity nucleation is produced on the panicle-matrix interface by grain boundary sliding and cavity nucleation occurs. The threshold stress is determined mainly by the concentration and distribution of grain boundary particles and falls in the stress range of engineering applications. A program has been developed to calculate the fraction of particles which can serve as nucleation sites. The model is used to predict the onset of cavitation in the oxide dispersion strengthened alloy Inconel MA 754. Implications for avoiding the nucleation of cavities in engineering alloys are discussed.     

The kinetics of ferrite nucleation at austenite grain boundaries in Fe-C alloys

The nucleation kinetics of grain boundary allotriomorphs of proeutectoid ferrite at austenite grain faces have been measured in three high purity Fe-C alloys as a function of isothermal reaction time and temperature. Several correction techniques, including discrimination between different nucleation sites and the effect of carbon diffusion fields on further nucleation of ferrite, were incorporated into a stereological procedure utilizing the SchwartzJSaltykov size distribution analysis. This analysis enabled the number of ferrite particles per unit unreacted grain boundary area to be obtained as a function of isothermal reaction time, and thus the time-dependent nucleation kinetics to be obtained as a function of temperature and carbon concentration. These rates were then compared with those predicted by classical heterogeneous nucleation theory using various models for the critical nucleus. It was concluded that viable critical nuclei must have predominately low energy interphase boundaries. Only a very small fraction of the austenite grain face area appears to be capable of supporting nucleation. Formerly Graduate Student, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931 Formerly Professor at Michigan Technological University     

The nucleation of martensite in steel

Calculations of the total energy for transforming austenite to martensite in the form of thin ellipsoidal plates, fully coherent with the austenite, show that the process may be spontaneous in the presence of pre-existing dislocations. It is found that dislocations, or groups of dislocations, in the austenite are suitable sites for martensite nucleation in that their strain fields may interact favourably with the strain field associated with the Bain deformation thereby eliminating the energy barrier to nucleation. The driving force for twinning to occur virtually simultaneously with nucleation is large and when this happens energy is released for thickening and growth of the nucleus. It is also found that the strain energy of coherent plates of martensite, whether twinned or untwinned, is a function of their orientation in the austenite, although the lowest strain energy cases occur nevertheless over a relatively wide range of orientations.The proposed theory of dislocation-assisted nucleation of martensite is qualitatively able to account for the majority of experimental observations pertaining to martensite nucleation.     

A model for creep based on the climb of dislocations at grain boundaries

A model for creep based on the climb of dislocations at grain boundaries is presented. It is shown that when a sliding interface or slip band intersects a grain boundary, a traction distribution is established on the boundary. The diffusional flow induced by these tractions results in a steady state creep process. This slip band model predicts an activation energy corresponding to grain boundary self diffusion, with a stress exponent and a grain size dependence which increase and decrease, respectively, as the applied stress increases. The theoretically determined creep rates are in good agreement with the data for metals and alloys which deform by grain boundary sliding and exhibit superplastic flow properties. Other models for creep controlled by grain boundary diffusion are briefly reviewed and compared with the present model. It is concluded that superplastic deformation involves both purely diffusional flow and dislocation motion.     

Precipitate nucleation on dislocations in Fe-N

Observations of the nucleation of coherent, disc-shaped α precipitates at dislocations in an Fe-N alloy have been made using an HVEM equipped with an accurately calibrated heating stage. This approach allows the effects of dislocation character and undercooling on precipitate nucleation to be studied systematically, and to be related to the predictions of existing theoretical models. It is found that at small undercoolings the influence of dislocation character is significant; at larger undercoolings, nucleation occurs in the matrix, and all dislocation sites are equally favoured. A classification of the nucleation potency of dislocations has been established, in which the most favoured conditions occur when the maximum misfit vector of the precipitate lies parallel to the Burgers vector of the dislocation, and the dislocation line lies in the habit plane of the precipitate.     

The effects of hydrogen and segregation on fatigue crack nucleation at defined grain boundaries in nickel bicrystals

In order to explore the role of impurity segregation in intergranular fatigue crack initiation and propagation, tests have been made on nickel bicrystals, variously heat-treated to induce increasingly severe degrees of sulfur segregation, and using air, vacuum and hydrogen as the environment. The grains of the bicrystals were oriented to yield two types of boundaries: type I, with strong elastic-plastic incompatibility and type II, a compatible tilt boundary. If the boundaries were clean, persistent slip band cracking in the grains occurred in preference to intergranular cracking. Although equal degrees of sulfur segregation, as measured by Auger-spectroscopy, could be produced at the two types of boundaries by the heat treatments, the incompatible one was much more susceptible to intergranular cracking than the other, which could be made to crack intergranularly only by high partial pressures of hydrogen. The results show that high stresses associated with incompatibility coupled with a lowering of cohesive forces at the boundary produced by the segregant are the main factors controlling intergranular fracture.     

Cavity nucleation at particles on sliding grain boundaries. A shear crack model for grain boundary sliding in creeping polycrystals

The paper summarizes the results of classical nucleation theory applied to the nucleation of creep cavities at hard second-phase particles. Stress concentrations at particles in sliding grain boundaries are analysed for the cases that the particles can be circumvented by diffusion and power-law creep and for purely elastic material response. The shear stress which is effectively transmitted through a sliding grain boundary in a polycrystal is calculated using a shear crack model. Comparison of the results with finite element calculations for a planar, hexagonal array of grains shows that the accuracy of the model is satisfactory. The duration of elastic transients is estimated and it is found that, in the numerical examples considered, stress relaxation times are shorter than the incubation time for cavity nucleation. From the calculated stress concentration factors it is concluded that cavities cannot be nucleated by the considered mechanism of vacancy condensation unless the nuclei have a narrow, rather than spherical, shape.     

Dislocation-disclination model of heterogeneous martensite nucleation in transformation-induced-plasticity steels

A dislocation-disclination model is proposed, describing the heterogeneous nucleation of an embryo of hcp martensite at a tilt grain-boundary segment containing some extrinsic dislocations. The total energy gain due to hcp embryo nucleation is analyzed in detail, and the existence of both the equilibrium and critical embryo sizes under varying external conditions (temperature and shear stress) is shown. Depending on the external conditions, these characteristic embryo sizes may vary in wide ranges. So, the equilibrium size increases while the critical size decreases as the external shear stress increases and the temperature decreases. It is also demonstrated that a critical external stress exists which induces athermal embryo nucleation when the nucleation-energy barrier disappears and the terms of equilibrium and critical embryo sizes lose their significance. The critical external stress has been studied, depending on the temperature and characteristic parameters of the grain boundary where the fcc-to-hcp martensite transformation takes place. We have shown, in particular, that the critical external stress increases in direct proportion to both the grain-boundary misorientation angle and temperature.     

Thermotransport in grain boundaries

Thermotransport effects in grain boundaries in lead were studied by means of a temperature gradient (≤60 K cm⁻¹) lying parallel to the planes of the boundaries. The temperature gradient was observed to result in grain boundary sliding, migration and enhanced boundary diffusion. These effects are interpreted in terms of climb and/or glide motion of grain boundary dislocations induced by the gradient. The behaviour of boundaries in temperature gradients seems to differ from the behaviour reported in concentration gradients (diffusion induced boundary migration) suggesting different mechanisms to operate in both cases.     

Structure of grain boundaries

The present discussion is concerned with the more relevant experimental studies, performed since about 1952, that provide evidence on the structure of large angle boundaries. These experiments include diffusion, mobility, energy, and field ion studies of grain boundaries. It is shown that the results of these experiments provide support for a model that can be described quite accurately in terms of coincidence in the boundary plane, a dislocation array, and a ledge structure.