Microstrain in polycrystalline metals

Brown, Norman; Lukens, Karl F.

doi:10.1016/0001-6160(61)90053-0

Cited by 87 publications

(10 citation statements)

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“…Experimental studies on granular materials have predominantly been performed by precise mechanical testing using load-unload cycles accompanied by surface-visualization techniques. The first experimental investigations on the effect of grain size on microplasticity [8,9] ascribed strain-hardening predominantly to the ability of grain boundaries (GBs) to act as barriers to dislocation motion, where the strain for a given stress varied as the cube of the grain size: large grains showed more strain than smaller grains. However, it was soon recognized that the relationship between microplastic strain and stress was not so simple.…”

mentioning

confidence: 99%

From Micro‐ to Macroplasticity

Brandstetter¹,

Swygenhoven²,

Petegem³

et al. 2006

Advanced Materials

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Early results of experimental investigations on microplasticity have been gathered into collected works and treatises. [1][2][3] The plastic events taking place at lower strains are related to short-range dislocation motion [4] and are of different origin than the dislocation mechanism occurring in the macroplastic regime.[5] Therefore, macroplasticity theories based on a mechanism involving long-range dislocation motion cannot serve to predict micro-yielding. In terms of measured stressstrain behavior, the two regimes are poorly distinguished, especially when they are characterized by positive strain-hardening, i.e., an increase in strength when the microstructure is strained. To facilitate comparison of data, metallurgists have come up with the rather arbitrary 0.2 % definition of the macroscopic yield stress. [6,7] It is one of the most frequently used engineering materials parameters and very important for the precise design of functional components ranging from construction materials down to micrometer-sized microelectromechanical (MEMS) devices. In polycrystalline materials, it is taken for granted that the majority of grains are plastically deforming at the macroscopic yield stress. However, there is no easy and suitable method to verify this assumption, and the real amount of strain that should be assigned to microplasticity is unknown. Experimental studies on granular materials have predominantly been performed by precise mechanical testing using load-unload cycles accompanied by surface-visualization techniques. The first experimental investigations on the effect of grain size on microplasticity [8,9] ascribed strain-hardening predominantly to the ability of grain boundaries (GBs) to act as barriers to dislocation motion, where the strain for a given stress varied as the cube of the grain size: large grains showed more strain than smaller grains. However, it was soon recognized that the relationship between microplastic strain and stress was not so simple. When a polycrystal is subjected to an external force, an inhomogeneous state of internal stress is developed resulting from elastic anisotropy and plastic incompatibilities arising from different resolved shear stresses within the different grains. [10] In addition to the fact that plastic yielding does not start at the same time in all grains, it was recognized that internal stresses arising from the elastic interactions of neighboring grains provide additional stresses that activate non-favored slip systems in grain interiors [11] and/or change the maximum shear direction in the vicinity of a GB. The latter may result in GBs emitting dislocations [12,13] on slip systems in the GB region that are different from the primary slip system of the grain interior. [14,15] Several plasticity models have been developed to predict the mechanical behavior in terms of stress-strain and hardening as a function of grain size. Some theories are based on the postulate that geometrically necessary dislocations account for the inherent difference in the accumulation o...

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confidence: 99%

From Micro‐ to Macroplasticity

Brandstetter¹,

Swygenhoven²,

Petegem³

et al. 2006

Advanced Materials

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“…The flow stress behavior for the copper series appears to show three stages that we will reference as I, II, and III. Stage I falls into the microyield/microdeformation category, [3][4][5][6][7][8] and is primarily associated with total true strains ≤ 0.004. This region appears to be grain size dependent.…”

Section: Analysis Of Resultsmentioning

confidence: 99%

“…The deformation/strain resolution from the tensile tests of this study were not sufficient to provide the microstrain correlations with stress as reported by others. [3][4][5][6][7][8] For five-nines polycrystalline copper, Bilello and Metzger 6 observe discernable grain size dependence over a plastic range from 0 to 0.0016. However, their finest grain size corresponded to the largest used in our study.…”

Section: Stage I-dislocation Source Activationmentioning

confidence: 99%

Equal Channel Angular Extrusion Progress Report for March 1998 - May 1999

Macheret¹,

Watkins²,

Korth³

et al. 1999

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Pure copper and Alloy 5083 aluminum were processed by equal channel angular extrusion (ECAE); their microstructural evolution and corresponding mechanical properties were investigated. Work also began on the possible use of ECAE to synthesize advanced materials or to consolidate metal powders or powder mixtures.The die tooling used for ECAE is described and selected microstructural and mechanical property results for ECAE-processed copper and cold-rolled (conventionally-processed) copper in the as-processed and annealed condition are compared. Results thus far show that the "pure" metal is prone to low temperature recrystallization after large strain hardening-more beneficial effects are expected in the dispersion-strengthened and precipitation-hardening alloys. The large range of tensile properties and grain sizes from the copper allowed a flow stress analysis to be performed. From this analysis, a new model for flow stress behavior is proposed.An evaluation of ECAE processing of material for spot welding electrodes began. Results to date include electrodes of ECAE-processed commercially pure copper (Alloy 101). Future work involving Glidcop® (Al 2 O 3 oxide dispersionstrengthened copper) and CuCrZr (Cr-Zr precipitation dispersion) materials will be required to fully investigate the benefits of ECAE for electrode life extension.Initial work on Aluminum Alloy 5083 showed that ECAE led to grain refinement as well as broke up and more uniformly dispersed the hardening precipitates. This is desirable for enhancing superplastic behavior.Study of ECAE for consolidating metal powder began. Early results with a Cu-Ag powder indicate that near 100% density was achieved with roomtemperature consolidation. iv SUMMARYThis program is evaluating the potential of a novel metals processing technique called equal channel angular extrusion (ECAE) for developing advanced industrial materials for the transportation industry. The program is a collaborative effort among three national laboratories, the Idaho National Engineering and Environmental Laboratory (INEEL), Pacific Northwest National Laboratory (PNNL), and Los Alamos National Laboratory (LANL). The program began in March 1998; this report describes progress through May 1999.INEEL is performing ECAE on a variety of materials and investigating their microstructural evolution and mechanical properties. Pure copper was chosen for the initial stage of the investigation because the extensive data on the microstructure and properties of copper in the literature allow a thorough comparative study of ECAE vs. conventional processing. Also, dispersionstrengthened copper alloys are widely used in industry. In particular, these alloys are used in the transportation industry for manufacturing spot welding electrodes. Evaluation of ECAE of copper and its alloys could, therefore, lead to developing improved materials for important industrial applications.PNNL is investigating the effect of ECAE on the grain size of aluminum Alloy 5083. It is hypothesized that ECAE will refine the ...

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“…AA -4.6 x 10-1 (Ats )2 5 Considering AA to represent the plastic flow which occurs during the com- Finally, it should be noted that for loads greater than L/LF -0.5 the "average" true stress for compression 0 a is lower than that in tension aT" This is due to the fact that during C, compression there occurs an increase in the area, whereas the opposite occurs during tension. At L/LF -1.0 the compression stress C is about O.…”

Section: -mentioning

confidence: 99%

A Basic Study of Cold Welding in Ultrahigh Vacuum

Conrad¹,

Rice²

1969

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The cohesion of the FCC metals Ag, Al, Cu, and Ni under ultrahigh -11 -9vacuum in the range of 10 to 10 torr was investigated using the technique of cold welding specimens previously fractured in the vacuum.The cohesion strength of the weld increased with compression load for all metals; all data fell on one curve of slight positive curvature when the cohesion load (stress) and the compression load (stress) were divided by the initial fracture load (stress) of the virgin specimen.The effect of compression load on the cohesion coefficient a for all metals could be described bywhere LC is the compression load and L is the initial fracture load. Ultra-F 0 sonic measurements on Cu indicated that the contact area was proportional to the ratio L F / Optical microscopy observations on the weld interface were 0 in accord with this; they also indicated the excellent matching of the two fractured surfaces possible with the present apparatus.The cohesion results are explained on the basis that the rupture of the weld occurs at a constant value of the "true" fracture stress.Limited studies were conducted with Cu to determine the effects of: (1) a subsequent heat treatment on the weld strength, (2) alloying with 30% Zn and (3) exposure of the freshly fractured surfaces to the gases N 2 , 02, CO, CO and air.The heat treatments reduced the strength of the weld; the 70-30 2 brass alloy exhibited a lower cohesion coefficient than unalloyed Cu; prolonged exposure to the gases 02, CO, CO and air caused an appreciable reduction in P 2 the cohesion coefficient, whereas no effectwas observed for N 2 . The adverse effects noted for the first two variables are not clearly understood at this time.The results for the exposure to the gases are in good agreement with predictions based on absorption theory and LEED observations reported in the I literature.

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Microstrain in polycrystalline metals

Cited by 87 publications

References 1 publication

From Micro‐ to Macroplasticity

From Micro‐ to Macroplasticity

Equal Channel Angular Extrusion Progress Report for March 1998 - May 1999

A Basic Study of Cold Welding in Ultrahigh Vacuum

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