The effect of grain size on the high-strain, high-strain-rate behavior of copper (original) (raw)
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Dynamic recrystallization in high-strain, high-strain-rate plastic deformation of copper
Acta Metallurgica et Materialia, 1994
When copper is deformed to high plastic strain (y ~ 34) at high strain rates (~ ~ I04 s -1) a microstructure with grain sizes of ~0.1 am can be produced. It is proposed that this microstructure develops by dynamic recrystallization, which is enabled by the adiabatic temperature rise. By shock-loading the material, and thereby increasing its flow stress, the propensity for dynamic recrystallization can be enhanced. The grain size-flow stress relationship observed after cessation of plastic deformation is consistent with the general formulation proposed by Derby [Acta metall, mater. 39, 955 (1991)]. The temperatures reached by the specimens during dynamic deformation are calculated from a constitutive equation and are found to be, for the shock-loaded material, in the 500-800 K range; these temperatures are consistent with static annealing experiments on shock-loaded specimens, that show the onset of static recrystallization at 523 K. A possible recrystallization mechanism is described and its effect on the mechanical response of copper is discussed.
Acta Materialia, 2007
The evolution of microstructure and the mechanical response of copper subjected to severe plastic deformation using equal channel angular pressing (ECAP) was investigated. Samples were subjected to ECAP under three different processing routes: B C , A and C. The microstructural refinement was dependent on processing with route B C being the most effective. The mechanical response is modeled by an equation containing two dislocation evolution terms: one for the cells/subgrain interiors and one for the cells/subgrain walls. The deformation structure evolves from elongated dislocation cells to subgrains to equiaxed grains with diameters of 200−500nm.Themisorientationbetweenadjacentregions,measuredbyelectronbackscatterdiffraction,graduallyincreases.ThemechanicalresponseiswellrepresentedbyaVoceequationwithasaturationstressof450MPa.Interestingly,themicrostructuresproducedthroughadiabaticshearlocalizationduringhighstrainratedeformationandECAPareverysimilar,leadingtothesamegrainsize.ItisshownthatbothprocesseshaveverycloseZener−Hollomonparameters(lnZ200-500 nm. The misorientation between adjacent regions, measured by electron backscatter diffraction, gradually increases. The mechanical response is well represented by a Voce equation with a saturation stress of 450 MPa. Interestingly, the microstructures produced through adiabatic shear localization during high strain rate deformation and ECAP are very similar, leading to the same grain size. It is shown that both processes have very close Zener-Hollomon parameters (ln Z 200−500nm.Themisorientationbetweenadjacentregions,measuredbyelectronbackscatterdiffraction,graduallyincreases.ThemechanicalresponseiswellrepresentedbyaVoceequationwithasaturationstressof450MPa.Interestingly,themicrostructuresproducedthroughadiabaticshearlocalizationduringhighstrainratedeformationandECAPareverysimilar,leadingtothesamegrainsize.ItisshownthatbothprocesseshaveverycloseZener−Hollomonparameters(lnZ 25). Calculations show that grain boundaries with size of 200 nm can rotate by $30°during ECAP, thereby generating and retaining a steady-state equiaxed structure. This is confirmed by a grain-boundary mobility calculation which shows that their velocity is 40 nm/s for a 200 nm grain size at 350 K, which is typical of an ECAP process. This can lead to the grain-boundary movement necessary to retain an equiaxed structure.
Strain-induced grain evolution in polycrystalline copper during warm deformation
Metallurgical and Materials Transactions A, 1998
The evolution mechanisms of dislocation microstructures and new grains at high strains of above 4 were studied by means of multiple compression of a polycrystalline copper (99.99 pct). Deformation was carried out by multipass compression with changing of the loading direction in 90 deg in each pass at temperatures of 473 K to 573 K (0.35 to 0.42 T m ) under a strain rate of 10 Ϫ3 s Ϫ1 . The flow stresses increase to a peak followed by a work softening accompanied mainly by dynamic recrystallization (DRX) at 523 K to 573 K. In contrast, the steady-state-like flow appears at 473K accompanied with the development of fine grains at strains as high as 4.2. The relationship of flow stress to the new grain size evolved can be expressed by a power law function with a grain size exponent of about Ϫ0.35, which is different from Ϫ0.75 for high-temperature DRX at above 0.5 T m . At 473 K, misorientations of deformation-induced dislocation subboundaries increase with increasing strain, finally leading to the evolution of new grains. It is concluded that the dynamic grain formation at 473 K cannot result from DRX, but from the evolution of deformation-induced dislocation subboundaries with high misorientations and, concurrently, the operation of dynamic recovery.
Microstructural and crystallographic response of shock-loaded pure copper
Journal of Materials Research, 2017
Microstructural and crystallographic aspects of high-velocity forming or "rapid" forming of rolled sheets of pure copper have been investigated in this work. Significant changes in crystallographic orientation and microstructure were observed when thin (0.5 mm) metal sheets of annealed copper were subjected to high strain rate deformation in a conventional shock tube at a very low impulse magnitude (;0.2 N s), which is inconceivable in conventional metal forming. Shock-loaded samples show characteristic texture evolution with a high brass {110}h112i component. A significant change in grain orientation spread was observed with increasing amount of effective strain without any drastic change in grain size. The texture after deformation was found to be strain-dependent. The path of texture evolution is dependent on the initial texture. Misorientation was limited to less than 5°. Deformation bands and deformation twins were observed. There was a decrease in twin [R3 coincidence site lattice (CSL)] boundary number fraction with increasing strain due to the change in twin boundary character to high-angle random boundary (HARB) as a result of dislocation pile up. The study shows the probability of a high-velocity shock wave forming pure Cu.
Shock compression of monocrystalline copper: Experiments, characterization, and analysis
Materials Science and Engineering: A, 2010
Monocrystalline copper samples with [0 0 1] and [2 2 1] orientations were subjected to shock/recovery experiments at 30 and 57 GPa and 90 K. The slip system activity and the microstructural evolution were investigated. Different defect structures, including dislocations, stacking faults, twins, microbands, and recrystallized grains were observed in the specimens. The residual microstructures were dependent on crystalline orientation and pressure. The differences with crystalline orientations are most likely due to different resolved shear stresses on specific crystalline planes. The geometric relationships between the shock propagation direction and crystalline orientation are presented under uniaxial strain. It is shown that the [2 2 1] orientation, by virtue of having fewer highly activated slip systems, exhibits greater concentration of deformation with more intense shear on the primary system. This, in turn leads to greater local temperature rise and full recrystallization, in spite of the thermodynamic residual temperature of ∼500 K and rapid cooling (within 20 s) to ambient temperature. The profuse observation of microbands is interpreted in terms of the mechanism proposed by Huang and Gray [J.C. Huang, G.T. Gray III, Acta Metallurgica 37 (1989) 3335-3347].
Effect of grain size on high strain rate deformation of copper
Metallurgical Transactions A, 1991
Experiments were performed to observe the deformation characteristics of oxygen-free high-conductivity (OFHC) copper at high strain rates (up to 40,000 s-1) and to relate differences in grain size with differences in deformation behavior. The rod impact and torsional Hopkinson bar test methods were used in these experiments. Results show that grain size reductions substantially reduce surface irregularities that develop during deformation. The effect of grain size on the yield stress and on the strain-hardening behavior of copper is small and is similar to the effect of grain size in copper at quasistatic strain rates. The observation that grain size has a substantial effect on surface irregularities may have important implications for applications in which stable deformation of thin sections is of concern.
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...
Metallurgical and Materials Transactions A, 2013
Shear localizations with different strains in annealed copper were obtained by a modified split Hopkinson pressure bar. Microstructure and microtexture evolution of the shear localization regions were examined using optical microscopy, electron backscatter diffraction technique, and transmission electron microscopy. The results show that both the mechanical response and deformation behavior are correlated closely to the shear strains. The elongated dislocation cells, stretched subgrains, and the refinement of subgrains are observed within shear localizations during dynamic deformation. Ultrafine grains of 100 to 300 nm with high-angle-boundaries are produced within the shear band with the shear strain of 5.8. Microtexture characterization reveals that a stable orientation, in which h110i directions of the crystals tend to align with the shear direction, develops both in the deformation and recrystallization areas. The {111} planes of the crystals tend to parallel to the shear plane in the deformation area, whereas the aggregated extent of this orientation becomes weak in the recrystallization area. In addition, some grains exist with the {100} planes parallel to the shear plane in the deformation and recrystallization areas. The rotational dynamic recrystallization is a reasonable mechanism for the microstructure evolution. The effects of cooling stage on the growth of grains and the change of dislocation density are estimated as a complementarity to this mechanism.
Microstructural Effects on Damage Nucleation in Shock-Loaded Polycrystalline Copper
Metallurgical and Materials Transactions A, 2014
Polycrystalline copper samples with varying thermomechanical histories were shock loaded to induce spall via laser-driven plate impacts at low shock stress (<6 GPa). Electron backscattering diffraction was used to obtain statistics on grain boundary (GB) misorientations within the spall plane and at all GBs that contained damage. Specimens with pre-existing plastic deformation showed dominant intergranular damage at boundaries in the 25 to 50 deg misorientation range, while heat-treated samples had mixed trans-and intergranular damage with a lessened misorientation influence at damaged GBs. 3-D X-ray tomography data were used to analyze global volume statistics and qualitatively inspect the shape of voids present in samples of varying thermomechanical histories. It was found that annealed samples had a mixed mode of sphericaland sheet-like voids, indicative of trans-and intergranular damage, respectively, and the microstructure with the highest number of R3 twin boundaries had the highest concentration of spherical voids. Data from a plastically pre-strained sample showed a dominance of needle-and sheet-like voids, indicating primarily intergranular damage due to the higher strength of the bulk material forcing the damage to nucleate at weaker defects, in this case GBs.