M.W. Grabski

Rate of energy storage and microstructure evolution during the tensile deformation of austenitic steel

Abstract The results of investigations of the energy storage process in austenitic stainless steel deformed in tension are reported. Conclusive evidence is presented for the existence at the initial stage of deformation of the maximum in the instantaneous rate of energy storage defined as des/dew, where es is the stored energy and ew is the mechanical energy expended in plastic deformation. Its occurrence is interpreted in terms of the evolution of the microstructure during deformation.

Energy storage during the tensile deformation of Armco iron and austenitic steel

Abstract A modification of the single-step method based on continuous detection of IR radiation emitted directly from a strained sample was employed to study energy storage during the initial stage of plastic deformation of Armco iron and austenitic stainless steel. The existence of a maximum in the dependence of the ratio of stored energy to expended energy on strain was confirmed for both materials in the range of homogeneous deformation and was interpreted in terms of variations in the contributions of endo-energetic and exo-energetic processes; the contribution of endo-energetic processes is related to the increase in the dislocation density due to the generation of dislocations and the contribution of exo-energetic processes is related to energy dissipation due to movement, rearrangement and annihilation of dislocations.

Effect of the grain size on the rate of energy storage during the tensile deformation of an austenitic steel

Abstract The effect of the grain size on the energy storage process in a low carbon austenitic steel deformed in tension is studied. The energy conversion at each instant of the deformation process is characterized by the instantaneous rate of energy storage, d e s d e w , where e s is the stored energy and e w is the mechanical energy expended on the plastic deformation. It has been shown experimentally that, in the initial stage of plastic deformation in this austenitic steel, the dependence of the rate d e s d e w on e w exhibits a maximum. The location of the maximum depends on the grain size of the material. In fine-grained samples, the maximum appears at smaller strains. After reaching a certain degree of deformation, plots of d e s d e w vs. the strain for the samples of both groups are practically the same. These results are interpreted in terms of the microstructural evolution during deformation. It has been shown that the grain boundaries favour the formation and affect the evolution of low energy dislocation structures.

The effect of the spreading of grain boundary dislocations on the tensile behaviour of a fine-grained austenitic steel at high temperatures

Abstract High temperature stress-strain data were obtained for type 310 austenitic steel with grain sizes of 1 and 63 μm. The data were fitted to the equation σ = σ y + A ϵ 1 2 where σy is defined by the Hall-Petch relation. When shear modulus changes had been taken into account the work-hardening parameter A was found to be temperature independent up to 720 K for recrystallized 1 μm samples and up to 870 K for well-annealed 63 μm samples. These temperatures correspond exactly to the spreading temperatures Ts of extraneous grain boundary dislocations. We confirmed that grain boundaries can become non-equilibrium structures when they are created during recrystallization. The equilibration process occurs at about 0.7Tm. When the test temperature is raised above Ts a crossing of the stress-strain curves for samples of different grain sizes occurs. To explain the observed dependence of A on T a model is proposed which is based on (1) the assumption that annihilation of lattice dislocations by spreading in grain boundaries is an important dynamic recovery mechanism and (2) the application of Ashbys concept. The results indicate the necessity of reinterpreting many studies on the grain size dependence of properties.

Mode of deformation and the rate of energy storage during uniaxial tensile deformation of austenitic steel

The mechanism of slip and its consequence in the process of energy storage during uniaxial tension of austenitic steel was studied. The interpretation of the energy storage process in terms of the slip development and microscopic shear band formation is presented.

Effect of distribution of grain boundary diffusivity on plastic flow of austenitic steel I: Characterization of microstructure. Determining the distribution of grain boundary diffusivity

Abstract Measurements of the kinetics of extrinsic grain boundary dislocations (EGBDs) when spreading and investigations of the microstructure of 0.12Cue5f822Crue5f817Ni austenitic steel subjected to different variants of thermal treatment are presented. Diffusional properties of grain boundaries (GB) were studied by the statistical distribution of activation energy for each state of the material. It was found that the distribution shifts towards higher values when the annealing temperature rises. The discussion deals with the structural and chemical factors responsible for the effects observed. The processes of grain growth, recovery of GB structure, secregation and precipitate dissolution have been taken into consideration.

Slip behaviour and energy storage process during uniaxial tensile deformation of austenitic steel

The mechanism of slip and its consequence in the process of energy storage during uniaxial tension of austenitic steel were studied. Interpretation of the energy storage process in terms of slip development and microscopic shear band formation is presented.

The effect of carbon and sulphur on the character of the grain boundary population in α-iron

Abstract The grain boundary misorientations and CSL frequencies of a population of about 250 boundaries in ultra high purity α-iron doped with small quantities of carbon and sulphur have been determined by a SEM microdiffraction BKD (Backscattered Kikuchi Diffraction) technique. The aim is to examine the effect of these elements on the character of the grain boundary population (CGBP). Samples containing about 75 wt. ppm of sulphur and/or 150 (200) wt. ppm carbon were recrystallized at 725 °C after cold hydrostatic extrusion to ϵ = 1.1 It was found that the addition of sulphur and carbon modifies the CGBP of α-iron. Sulphur clearly increases the density of low angle grain boundaries (LANGB), but only in the pure Feue5f8S binary alloy. The fractions of the LANGB and coincidence site lattice grain boundaries (CSLGB) in the Feue5f8Cue5f8S alloy are rather similar to those of Feue5f8C and thus the CGBP could be controlled by carbon. As a consequence, the lowest fraction of high energy random grain boundaries (RANGB) appears in the Feue5f8S alloy at about two-thirds of the total population. The effect of carbon and sulphur on the CGBP is interpreted in terms of grain boundary selection as a consequence of an impurity-specific dragging effect or a precipitation pinning effect on the migration of boundaries during recrystallization and subsequent grain growth.

Effect of distribution of grain boundary diffusivity on plastic flow of austenitic steel II: The role of dislocation annihilation in grain boundaries in the plastic flow of polycrystalline materials

Abstract Analysis of the effect of temperature and grain size on the hardening process of polycrystalline material has been carried out, assuming that grain boundaries (GB) are the most important dislocations sinks, that the kinetics of dislocation annihilation are controlled by the mobility of GB dislocations and hence by GB diffusion, and that the GB diffusivity is characterized by statistical distribution. To verify the equations derived, tensile tests have been performed within the wide range of temperatures on 0.12Cue5f822Crue5f817Ni austenitic steel subjected to different variants of thermal treatment. It has been found that the course of the changes of the strain-hardening coefficient depends, on the one hand, on the diffusional properties of GB in a given state of material, and on the other hand on grain size and the strain rate applied. The direction and extent of the changes are more adequately described when the distribution of GB diffusivity is taken into account instead of the mean value of GB diffusivity.