Size effect on the tensile strength of fine-grained copper (original) (raw)
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Strain path change effect on dislocation microstructure of multicrystalline copper sheets
Materials Chemistry and Physics, 2006
In this study, coarse-grained copper sheets were subjected to tension-rolling and rolling-tension strain path sequences. In both cases, two different types of strain path change were studied: the tensile and rolling directions were parallel and normal to each other. TEM observations of deformed samples showed the typical dislocation structures for the prestraining paths in tension and rolling. Special microband features, not observed during prestrain, were found during the second strain path, whatever the sequence and type of strain path change. The microstructure observed during reloading is discussed in terms of the sequence and type of strain path change, parallel or normal. The frequency of appearance of microbands is discussed in terms of the activity of new slip systems, i.e. not active during the prestrain path and connected with the number of the active slip systems after reloading. The results from this study, obtained for coarse-grained multicrystalline copper sheets, are compared with previous ones for fine and medium-grained copper.
Cold-worked and annealed copper powder are investigated in terms of realistic microstructural models based on the strain field model of dislocations and log-normal distribution of spherical crystallites. The dislocation type was observed to be either screw type or 100 type dipoles in cold-worked powder and edge type in annealed powder specimens, respectively. Dislocations are observed to be more correlated in the cold-worked state compared to the annealed state. Systematic differences exist in the mean square strain values for different crystallographic directions, indicating the importance of model-based approaches in analysing dislocation induced x-ray line broadening. For more correlated dislocation distribution model-based approaches should be adopted.
Effects of grain size and boundary structure on the dynamic tensile response of copper
2011
Plate impact experiments have been carried out to examine the influence of grain boundary characteristics on the dynamic tensile response of Cu samples with grain sizes of 30, 60, 100, and 200 lm. The peak compressive stress is 1.50GPaforallexperiments,lowenoughtocauseanearlystageofincipientspalldamagethatiscorrelatedtothesurroundingmicrostructureinmetallographicanalysis.Theexperimentalconfigurationusedinthisworkpermitsreal−timemeasurementsofthesamplefreesurfacevelocityhistories,soft−recovery,andpostimpactexaminationofthedamagedmicrostructure.Theresultingtensiledamageintherecoveredsamplesisexaminedusingopticalandelectronmicroscopyalongwithmicrox−raytomography.Thefreesurfacevelocitymeasurementsareusedtocalculatespallstrengthvaluesandshownosignificanteffectofthegrainsize.However,differencesareobservedinthefreesurfacevelocitybehaviorafterthepull−backminima,whenreaccelerationoccurs.Themagnitudeofthespallpeakanditsaccelerationratearedependentuponthegrainsize.Thequantitative,postimpact,metallographicanalysesofrecoveredsamplesshowthatforthematerialswithgrainsizeslargerthan30lm,thevoidvolumefractionandtheaveragevoidsizeincreasewithincreasinggrainsize.Inthe30and200lmsamples,voidcoalescenceisobservedtodominatethevoidgrowthbehavior,whereasin60and100lmsamples,voidgrowthisdominatedbythegrowthofisolatedvoids.Electronbackscatterdiffraction(EBSD)observationsshowthatvoidspreferentiallynucleateandgrowatgrainboundarieswithhighanglemisorientation.However,specialboundariescorrespondingtoRl(lowangle,<5)andR3(1.50 GPa for all experiments, low enough to cause an early stage of incipient spall damage that is correlated to the surrounding microstructure in metallographic analysis. The experimental configuration used in this work permits real-time measurements of the sample free surface velocity histories, soft-recovery, and postimpact examination of the damaged microstructure. The resulting tensile damage in the recovered samples is examined using optical and electron microscopy along with micro x-ray tomography. The free surface velocity measurements are used to calculate spall strength values and show no significant effect of the grain size. However, differences are observed in the free surface velocity behavior after the pull-back minima, when reacceleration occurs. The magnitude of the spall peak and its acceleration rate are dependent upon the grain size. The quantitative, postimpact, metallographic analyses of recovered samples show that for the materials with grain sizes larger than 30 lm, the void volume fraction and the average void size increase with increasing grain size. In the 30 and 200 lm samples, void coalescence is observed to dominate the void growth behavior, whereas in 60 and 100 lm samples, void growth is dominated by the growth of isolated voids. Electron backscatter diffraction (EBSD) observations show that voids preferentially nucleate and grow at grain boundaries with high angle misorientation. However, special boundaries corresponding to Rl (low angle, < 5 ) and R3 (1.50GPaforallexperiments,lowenoughtocauseanearlystageofincipientspalldamagethatiscorrelatedtothesurroundingmicrostructureinmetallographicanalysis.Theexperimentalconfigurationusedinthisworkpermitsreal−timemeasurementsofthesamplefreesurfacevelocityhistories,soft−recovery,andpostimpactexaminationofthedamagedmicrostructure.Theresultingtensiledamageintherecoveredsamplesisexaminedusingopticalandelectronmicroscopyalongwithmicrox−raytomography.Thefreesurfacevelocitymeasurementsareusedtocalculatespallstrengthvaluesandshownosignificanteffectofthegrainsize.However,differencesareobservedinthefreesurfacevelocitybehaviorafterthepull−backminima,whenreaccelerationoccurs.Themagnitudeofthespallpeakanditsaccelerationratearedependentuponthegrainsize.Thequantitative,postimpact,metallographicanalysesofrecoveredsamplesshowthatforthematerialswithgrainsizeslargerthan30lm,thevoidvolumefractionandtheaveragevoidsizeincreasewithincreasinggrainsize.Inthe30and200lmsamples,voidcoalescenceisobservedtodominatethevoidgrowthbehavior,whereasin60and100lmsamples,voidgrowthisdominatedbythegrowthofisolatedvoids.Electronbackscatterdiffraction(EBSD)observationsshowthatvoidspreferentiallynucleateandgrowatgrainboundarieswithhighanglemisorientation.However,specialboundariescorrespondingtoRl(lowangle,<5)andR3(60 <111> misorientation) types are more resistant to void formation. Finally, micro x-ray tomography results show three dimensional (3D) views of the damage fields consistent with the two dimensional (2D) surface observations. Based on these findings, mechanisms for the void growth and coalescence are proposed.
Key Engineering Materials, 2009
This article reports on a study of the microstructure and mechanical response of copper polycrystals with grain sizes in the micrometer range. Three-dimensional dislocation dynamics simulations are used for the first time to investigate grain boundary strengthening and the Hall-Petch law. The methodology, which involves constructing a microcrystalline representative volume element with periodic boundary conditions, is briefly presented. Simulation results show that the initial density of dislocation sources and the cross-slip mechanism are two key factors controlling the heterogeneity of plastic deformation within the grains. At yield, the smaller the grains size, the more plastic deformation is heterogeneously distributed between grains and homogeneously distributed inside the grains. A size effect is reproduced and it is shown that the Hall-Petch exponent decreases from the very beginning of plastic flow and may reach a stable value at strains larger than the conventional proof st...
Impact of dislocation cell elastic strain variations on line profiles from deformed copper
Zeitschrift für Kristallographie Supplements, 2008
Energy scanned, sub-micrometer X-ray beams were used to obtain diffraction line profiles from individual dislocation cells in copper single crystals deformed in compression. Sub-micrometer depth resolution was provided by translating a wire through the diffracted beams and using triangulation to determine the depths of the diffracting volumes. Connection to classic volume-averaged results was made by adding the line profiles from 52 spatially resolved dislocation cell measurements. The resulting sub profile is smooth and symmetric, in agreement with early assumptions; the mean strain and full width half maximum are consistent with the average of the parameters extracted from the more exact individual dislocation cell measurements.
Journal of the …, 2005
A further development of the mechanism-based strain gradient plasticity model well established in literature is reported. The major new element is the inclusion of the cell size effect in dislocation cell forming materials. It is based on a 'phase mixture' approach in which the dislocation cell interiors and dislocation cell walls are treated as separate 'phases'. The model was applied to indentation testing of copper severely pre-strained by equal channel angular pressing. The deformation behaviour and the intrinsic length scale parameter of the gradient plasticity model were related to the micro-structural characteristics, notably the dislocation cell size, resulting from the deformation history of the material. r
The effect of grain size on the high-strain, high-strain-rate behavior of copper
Metallurgical and Materials Transactions A, 1995
Copper with four widely differing grain sizes was subjected to high-strain-rate plastic deformation in a special experimental arrangement in which high shear strains of approximately 2 to 7 were generated. The adiabatic plastic deformation produced temperature rises in excess of 300 K, creating conditions favorable for dynamic recrystallization, with an attendant change in the mechanical response. Preshocking of the specimens to an amplitude of 50 GPa generated a high dislocation density; twinning was highly dependent on grain size, being profuse for the 117-and 315-/xm grain-size specimens and virtually absent for the 9.5-~m grain-size specimens. This has a profound effect on the subsequent mechanical response of the specimens, with the smaller grain-size material undergoing considerably more hardening than the larger grain-size material. A rationale is proposed which leads to a prediction of the shock threshold stress for twinning as a function of grain size. The strain required for localization of plastic deformation was dependent on the combined grain size/shockinduced microstructure, with the large grain-size specimens localizing more readily. The experimental results obtained are rationalized in terms of dynamic recrystallization, and a constitutive equation is applied to the experimental results; it correctly predicts the earlier onset of localization for the large grain-size specimens. It is suggested that the grain-size dependence of shock response can significantly affect the performance of shaped charges.
Microstructural Features of Ultrafine Grained Copper under Severe Deformation
In this work the microstructural features of pure copper were studied using two methods of severe plastic deformation: equal-channel angular pressing (ECAP) and hard cyclic viscoplastic (HCV) deformation. During the first step the metal was severely deformed up to 10 B c routes of ECAP. The ultrafine grained microstructure was received. The elongated laminar substructure has low angle and diffuse grain boundaries, but high dislocation density. Metal shows high hardness and strength but low ductility at tension straining. During the second step – HCV deformation – the strain amplitude of tension-compression cycles was stepwise increased from 0.2 % up to 2.5 % for 30 cycles and for five series. The results show, that under HCV deformation the ultrafine grained microstructure with high-angle grain boundaries was formed. The mechanism of microstructure evolution contains the elongated (ECAP processed) subgrains fracture under shear stresses by atomic layers of crystals and new microstru...