Papers by Keyword: High Temperature Deformation

Abstract: It has been widely accepted that the creep characteristics at high temperatures are mainly evaluated by a minimum creep rate and a time to fracture. Although, a shape of creep curve may vary depending on deformation conditions, the apparent minimum creep rates may become the same value. Thus, for detailed analysis and prediction of creep behavior, other values should be considered which reflects the shape of each creep curve. For the purpose, authors have proposed Satos Strain-Acceleration-Parameter (SAP) which reflects strain rate change during creep. Based on the concept of SAP, the whole creep curve can be represented by a set of small numbers of numerical parameters, and can be extrapolated from a part of creep curve [. It is also well accepted that the creep rates depend on microstructures, and microstructural changes cause strain rate change. The SAP would reflect stability and magnitude of microstructural change during deformation at high temperatures. In this paper, application of the concept of SAP to creep curves that show sigmoidal type primary creep is presented. The creep curve can be divided into two regime based on the SAP values. The sigmoidal creep curve is reasonably reproduced by the concept of Strain-Acceleration-Parameter, and reasonably agrees with experiment. Whole creep curve can be reasonably represented by a few numerical values which reflect shape of creep curve in each regime. The concept of SAP is applicable for quantitative evaluation of both normal and sigmoidal type of creep curves.

Abstract: This paper presents a 3D discrete dislocation dynamics (DDD) model describing dislocation processes in crystals subjected to loadings at high temperatures. Smooth dislocations are approximated by short straight segments. Every segment is acted upon by a Peach-Koehler force obtained by summing up forces from all dislocation segments and a force due to the applied stress. The model addresses interactions between individual dislocations and rigid precipitates. The model is applied to a migration of low angle tilt boundaries (LATBs) characterized by different initial dislocation density and constrained by precipitates of different sizes. The calculations showed that, for applied shear stresses σ_xz lower than a certain threshold σ_crit.(h), the LATB is inhibited by the precipitate field. For σ_xz above σ_crit.(h), the LATB passes through the precipitate field. Some combinations of σxz and h lead to a decomposition of the LATB. The LATBs thus may evolve in three distinct modes depending on the initial microstructure. The threshold stress behaviour is known from creep tests of dispersion-strengthened NiCr alloys [1]. Furthermore, the critical stresses obtained from our calculations are below Orowan stresses for corresponding particle distribution. This behaviour has been also reported in creep experiments [1].

Abstract: The paper presents the results of microstructure evolution studies of hard magnetic FeCr22Co15 alloy, destructed by tension and torsion at 800 and 850°C. The temperatures and deformation rates corresponded to the condition of superplasticity of Fe-Cr-Co alloys. Observations of longitudinal section of deformed samples in scanning electron microscope showed a formation of weak gradient microstructure with highest grain refinement in the surface layer of material. Precipitation of intermetallic σ-phase was also observed, with its maximum amount in zones of the highest deformation.

Abstract: Formation behavior of texture during high temperature compression of AZ80 is investigated on the specimens produced from the extruded bar and rolled plate. Special attention is paid on the formation of (0001) (compression plane) texture. Before deformation, (0001) is distributed frequently parallel to the compression plane in the specimen produced from the rolled plates, while (0001) is frequently normal to the compression plane in the specimen produced from the extruded bar. The deformation is performed at temperatures and strain rates ranging from 673K to 723K and 1.0×10^-4s^-1 to 5.0×10^-2s^-1, respectively. It is found that fiber textures are formed by the deformation and the main component of the texture varies depending on the deformation conditions. The sharpness of (0001) texture after deformation in the specimen produced from extruded bar is much weaker than that in the specimen produced from rolled plate. It is concluded that the growth of (0001) grains during deformation is the basic process for the (0001) texture evolution.

Abstract: A 0.02%C plain carbon and a 0.22%C TRIP steel were tested in compression in the temperature range 900°C to 1150°C and strain rate range 0.05s^-1 to 1s^-1. Thirty-two experimental flow curves were obtained in this way. The critical conditions for the initiation of dynamic recrystallization were determined by the double differentiation method. Using a dislocation density model to describe the austenite flow stress, the work hardening parameters r and h were derived and are used to model the flow curve in the absence of dynamic recrystallization. The latter was employed to calculate the fractional softening attributable to dynamic recrystallization. The kinetics of dynamic recrystallization are then described using Avrami kinetics. Finally, the dependences of the Avrami and work hardening parameters on Z, the Zener-Hollomon parameter, are used to model compression flow curves at strain rates an order of magnitude greater than the ones employed in the tests.

Abstract: The article presents the results of microstructure evolution studies of the FeCr30Co8 hard magnetic alloy, subjected to deformation by tension and torsion at 700 and 720 °C. The observations in the longitudinal section of the samples in the scanning electron microscope (SEM) showed a formation of gradient microstructure with the maximum grain refinement in the surface layer of the material. The EBSD examination confirmed the refinement of structure in the surface layer and the presence of sub-grained structure of the material. A little larger refinement of α phase grains was observed at temperature of deformation 700 °C than at 720 °C. However, the deformation was inhomogeneous along the whole longitudinal section of the sample. The highest deformation degree resulted from the torsion.

Abstract: According to compressive test date, relationship between flow stress, temperature, strain rate and strain is studied through single and multiple regression, and constitutive equation is researched. The results show the relationship between flow stress and stain is applied to hyperbolic sine equation in the hot compressive deformation of Ti-6Al-4V-rare earth titanium alloy. Relationship between logarithmic hyperbolic sine of its flow stress and reciprocal of absolute temperatures is liner, and hot compressive deformation is controlled by activation energy.

Abstract: A way to overcome the low deformability of magnesium alloys at room temperature is toincrease the temperature of forming operations. The stress exponent n, which is known to be a keyparameter in the control of plastic stability, generally decreases when temperature increases.Nevertheless, low n-values are not enough to ensure large capacity of deformation since fracturecan also result from strain induced cavitation. In the present investigation, both the mechanisms ofhigh temperature deformation and damage were studied in selected Mg alloys. Since damage datacan also give information on the deformation mechanisms, the strain induce cavitation behaviourwas mainly studied thanks to X-ray micro tomography which provides 3D information like thecavity shapes or the variation with strain of the number of cavities. Moreover, additionally toconventional post mortem analyses, it was attempted to perform the 3D damage characterisation inin situ conditions, namely directly during high temperature deformation tests.

Abstract: Magnetically hard Fe-Cr-Co-based alloys are distinguished by their good ductility, excellent magnetic properties and low cost. Their superior magnetic properties are obtained by magnetic treatment and multistage tempering, which results in spinodal decomposition of the solid solution into the isomorphous α1 and α2 phases. However, the α1+α2 microstructure causes a reduction in the plasticity and strength of the material. It can often be advantageous for permanent magnets to maintain fine magnetic properties throughout their volume along while retaining good mechanical properties only in the subsurface layer. To improve the mechanical properties of the latter, FeCr30Co8 samples were deformed in tension combined with torsion. Loading was applied at 750°C, which ensured that the conditions for superplastic deformation were fulfilled. Here, we present the results of microstructure investigations of the samples treated in the aforementioned manner. Observations of the longitudinal section of the samples showed the formation of a gradient microstructure with the maximum grain refinement in the surface layer and the characteristic rotation of the elongated α phase grains from positions nearly perpendicular to the tension axis at the surface to positions tilted at approximately 45º to the tension axis inside the material. Deformation at superplastic conditions also activated precipitation of the σ intermetallic phase, particularly in the areas of highest deformation. The refinement of the microstructure and precipitation of the σ-phase resulted in a significant increase in hardness at the surface of the FeCr30Co8 samples.

Abstract: Texture formation during uniaxial compression at elevated temperatures in Fe-3mass%Si and 430 stainless steel is studied. The behavior is analyzed in relating to the understanding on the texture development in Al-Mg solid solution alloy at high temperatures. It is found that the fraction of {001} (compression plane) is much higher than that of {111} both in Fe-3%Si and 430 steel, which is different from the deformation at room temperature. In Fe-3%Si, the fraction of {001} becomes higher at higher temperatures and with lower strain rates. EBSD measurement suggests that the development of {001} is attributed to the preferential growth of {001} oriented grains. In the case of 430 steel, no obvious deformation condition dependence of {001} development is seen, probably due to the fine particles pinning the grain boundaries.