Grains In Polycrystalline Aggregate Research Articles

Effect of elastic reaction stress on plastic behaviors of grains in polycrystalline aggregate during tensile deformation

A deformation model with the consideration on reaction stress between grains in polycrystalline aggregates is proposed, in order to solve the problems of strain and stress incompatibility appearing in previous Sachs- and Taylor-type models. The corresponding simulation for tensile deformation indicates that the model predicts orientation evolutions similar to that made by Taylor assumption, while the attention to both of the stress and strain compatibility has been given without cryptic fitting parameters. A ‘hard” area, where no track of tensile direction could run through, was predicted near 〈1 1 1〉 according to the simulation, which agrees with some experimental observations.

K-0415 多結晶金属の塑性変形に伴う微視的変形挙動の観察(G03-4 静的強度)(G03 材料力学部門一般講演)

The surface roughening is closely related to the inhomogeneity of polycrystalline metals, that is, to the inhomogeneous plastic deformation of respective grains. In the present study, inhomogeneous deformation on the free surface of polycrystalline aluminum during uniaxial tension is studied. The inhomogeneous deformation of grains, which is represented by the area of the grain and the grain boundary length in the free surface, is observed by means of the laser-scanning microscope as a nondestructive method. Discussions are made mainly on the evaluation of inhomogeneous deformation of respective grains in polycrystalline aggregate.

Origin of broken phenocrysts in ash-flow tuffs

Surprisingly little attention has been devoted to the textural nature of phenocrysts of feldspar and quartz in tuff. Although many geologists have briefly alluded to “broken” phenocrysts, none have addressed their origin in any detail. Petrographic study of 117 cooling units in the middle Tertiary ash-flow province of the Great Basin, United States, provides a basis for characterization of the shapes and for interpretation of the origin of felsic phenocrysts in ash-flow tuffs. Although not proven to be wholly ineffective, breakage of phenocrysts by mutual impact in the erupting magma and pyroclastic flow is doubtful for at least four reasons. First, the statistical probability of mutual collision between phenocrysts diminishes exponentially as their proportion to vitroclasts diminishes (e.g., only 1% probability for 10% phenocrysts); collision is less likely if pyroclasts move by laminar rather than turbulent flow. Second, the coating of glass and/or melt on the phenocrysts provides a cushion that absorbs the impact force. Third, plagioclases broken by impact in the laboratory have unusual shapes unlike those seen in Great Basin tuffs. Fourth, euhedral phenocrysts of feldspar are commonplace in many Great Basin tuffs, and in some they constitute a significant proportion of the phenocrysts, indicating that mutual impact does not modify all intratelluric crystals during explosive eruption. The two most populated categories of phenocryst shape in Great Basin tuffs probably correspond to what has been previously called “broken” phenocrysts. Somewhat less than half of the plagioclase and many sanidine phenocrysts are subhedral to anhedral. These are similar in shape, size, and composition to grains in polycrystalline aggregates within the same thin section. Kindred aggregates and discrete phenocrysts could have been derived from holocrystalline to partly crystalline material in the magma chamber that was disaggregated to varying extents during explosive eruption. More than half of the plagioclase and all of the quartz phenocrysts in Great Basin tuffs consist of irregularly shaped fragments with cuspate, embayed outlines, resembling pieces of a jigsaw puzzle, which we call phenoclasts. Inclusions of glass are common and are especially evident in larger, more or less whole crystals. Textural features of some phenocrysts in cognate pumice clasts in the tuffs reveal that they broke apart while still in the vesiculating but unfragmented magma. As the erupting magma decompressed, vesiculation of the melt that was entrapped at higher pressures as inclusions within the phenocrysts blew them apart, forming the phenoclasts. Shapes of felsic phenocrysts in volcanic rocks provide insight into their mode of emplacement. Euhedral phenocrysts are common in ash-flow tuffs as well as lava flows. Phenoclasts, however, are diagnostic of ash-flow tuffs, because they do not occur in Plinian ash-fall deposits and are rare in lava flows. These textural contrasts are useful for interpretation of generally older, but in any case altered and recrystallized, volcanic rocks. In such rocks, critical groundmass features and field relations that could provide clues to their origin have been obscured, but the shapes of relict phenocrysts are commonly well preserved.

Investigation of the iterative series expansion method by means of standard functions

Three-dimensional orientation distributions of grains in polycrystalline aggregates are referred to as crystallographic textures. Commonly, they are computed from two-dimensional centro-symmetric pole figures by employment of series expansion techniques or so-called direct inversion methods. Both types of approach lead to inaccuracies which are due to the absence of the odd coefficients and by truncation errors in the first case and to the under-determination of the set of linear equations combining cells in the pole figures and in the three-dimensional orientation space in the second case. For both types of calculation methods various correction procedures have been suggested. In case of the series expansion methods, the introduction of the non-negativity condition was reported to considerably improve the obtained solution. However, before large series of experimental data can be processed by such a method, its reliability has to be checked by use of analytical tools. Hence, in the present study a recently introduced iterative series expansion method which accounts for the non-negativity condition is examined by use of standard functions.

Investigation of the ADC Method for Direct ODF Approximation by Means of Standard Functions

AbstractThree‐dimensional orientation distributions of grains in polycrystalline aggregates are referred to as crystallographic textures. They can be calculated from centrosymmetric two‐dimensional pole figures by means of series expansion methods or by direct methods. Both techniques lead to inaccuracies, caused by the absence of the odd coefficients and by truncation errors in the first case and by the underdetermination of the set of linear equations combining discrete cells in the pole figures and in the three‐dimensional orientation space in the second case. The direct method of “arbitrary defined cells” (ADC) is investigated by means of the component fit method and by use of standard functions, respectively. The maximum value of the orientation density achieved by the ADC method represents a good approximation. However, weak negative and positive ghost adulterations are identified.