Kink Band Boundaries Research Articles

Development of axial-plane slaty cleavage and schistosity in experimental and natural materials

An experiment is described in which folds and an axial-plane foliation were produced in an artificial layered specimen made from salt and mica. The foliation differs on a micro-scale from natural foliations due to the unnatural experimental conditions. Taking results of annealing experiments into account a model is presented for the development of natural foliations. The model involves kinking of an earlier foliation and later or concurrent migration of kink-band boundaries and growth of new micas parallel to old micas and to kink-band boundaries. Four examples of natural schists are described and the possibility of the mechanism operating in the development of axial-plane slaty cleavage and schistosity is discussed. It is concluded that the mechanism probably operates in both cases but alone it cannot explain the domainal fabric commonly found in slates.

Experimentally induced kinking and annealing of single crystals of galena

Single crystals of galena have been experimentally compressed at 10° to the (001) plane at temperatures from 20°C to 300°C at a fixed confining pressure of 1.5 Kbars. A fluid medium, Heard-type apparatus was used for testing at strain rates of 10 −5 and 10 −7 sec −1. Stress-strain curves rose steeply until strains of approximately 2 per-cent were reached. At higher strains the curves often became flat or showed a slight negative slope. Deformed specimens were examined by reflected light and scanning electron microscopy, by Laue X-ray diffraction and were tested for Vickers hardness. Deformation at temperatures of 20°C to 200°C resulted in the formation of two well developed sets of kink bands, complex slip on the {110} planes and further slip on {100} planes. Deformation at 300°C was characterised by polygonisation and recrystallisation in kink bands, thus indicating that recovery processes (dislocation climb) were operating. Specimens deformed at a strain rate of 10 −5 sec −1 at temperatures of 200°C and 300°C were annealed at these temperatures for 10 days under confining pressure. At 200°C kink band boundary migration had begun and at 300°C polygonisation and recrystallisation away from kink zones were observed. Rapid dynamic recrystallisation and annealing recrystallisation of naturally deformed galena can be expected to occur even at temperatures below 200°C during tectonic deformation at low strain rates (10 −10 to 10 −14 sec −1).

面心立方晶の加工硬化曲線における段階IからくIIへの遷移機構

The transition from stage I to II in the deformation of Cu, Cu-1 at%Ge, and Ag single crystals has experimentally been studied with particular attention to the formation of deformation bands (kink bands) through ‘bend gliding’. The bend gliding originates from the local inhomogeneity of primary main slip in a specimen deformed under the uniaxial tension, and the slip inhomogeneity is enhanced by the presence of grown-in sub-boundaries. It is shown that stage I terminates with the formation of kink bands through the stabilization process of the local bend-gliding regions. At the kink-band boundary dislocations are blocked from further movement and rapidly accumulated. Consequently the flow stress is increased and also secondary slip systems are activated by the stress concentration, resulting in a swift transition from stage I to II. The temperature dependence of the extent of stage I, which has generally been observed but not well understood, can also be interpreted in terms of the formation of dislocation walls or kink bands which becomes easier as the deformation temperature is raised. In this connection, a phenomenon named ‘annealing-hardening in pure metals’ found in stage I may provide evidence in support of this view.

Deformation mechanisms in experimentally and naturally deformed amphiboles

Experimental deformation of an igneous hornblendite at 600–750°C, 10 kbars confining pressure and a strain rate of 10 −5 sec −1 results in kink bands whose normals cluster about [001] and axes of external rotation subparallel to [010]. This is consistent with glide on the system T = (100), t = [001]. At temperatures of 800°C to the breakdown temperature of the hornblende, external rotation axes spread along the (100) plane although kink band boundary poles remain subparallel to [001]. This indicates glide on the (100) plane in variable directions. Analysis of bend zones in a naturally deformed actinolite indicates glide on the (100), [001] system. Mechanical (101) twinning was not observed in any of the samples, and we suggest that in these amphiboles the critical resolved shear stress for glide is lower than that for twinning.

Deformation and recrystallisation of orthopyroxene from the giles complex, Central Australia

The microstructures and fabrics of naturally deformed orthopyroxenites from the Giles Complex, central Australia are described in some detail. Coarse grained enstatite is deformed and recrystallised where it is incorporated in planar gneissic (mylonite) zones which show a gradation in strain from their margins inwards. Deformation takes place by slip on (100) [001] to produce regular lattice bending and kinking, and recrystallisation takes place preferentially along grain boundaries and kink band boundaries (KBB's). The microstructures and preferred orientation of recrystallised grains along KBB's are interpreted in terms of possible nucleation mechanisms, and both bulge nucleation (Bailey and Hirsch, 1962) and subgrain coalescence (Hu, 1963) are likely contributors. Electron microprobe analyses have indicated a small compositional difference between new (recrystallised) and host (deformed) grains, which is related to the nucleation mechanism. The total preferred orientation patterns for host and new grains are discussed with special reference to previous measurements and interpretations.

About three sets of kink bands near Herbeumont (Belgian Ardennes)

In the area studied, three differently oriented sets of kink bands occur. The geometry of these structures is described. One particular set with mean kink angles of about 70‡ shows statistically that the interlimb angles are bisected by the kink band boundary planes. In the author's opinion it reflects the mechanism of formation.

The geometry of kink bands in crystals ? a simple model

A crystallographic explanation for the geometry of kink bands in crystals is proposed and three possible types of kinking are discussed, 1. simple kinking, 2. kinking associated with a phase change and 3. kinking associated with twin formation. The angular relationships at kink band boundaries have been calculated for a number of minerals and the results compared with observations in the literature. It is proposed that the crystallography of a mineral may be the dominant control of kink band geometry rather than external stress conditions as is generally assumed, this is probably particularly true of kink bands formed under natural conditons.

Experimental Evidence of Basal Slip in Quartz

Polished crystals of quartz, deformed at high temperature and confining pressure with high shear stress on the basal plane, develop slip bands on their surfaces parallel to the trace of the basal planes, indicating slip on this plane. The distribution of the slip markings on cylindrical surfaces shows that the a-axes are the slip directions. Thin sections of the polished crystals contain deformation lamellae parallel to the base. The deformation lamellae are therefore parallel to slip zones in these crystals. Optical studies of basal deformation lamellae with phase-contrast illumination indicate that they are sharp boundaries, less than 0.2 micron thick, separating regions with higher and lower refractive indices than the quartz at some distance from the lamellae. Compensator measurements also reveal birefringences higher and lower than normal on opposite sides of these boundaries. Changes of indices and birefringence are consistent with the photoelastic effects which would be expected from an array of basal edge dislocations with Burgers vector parallel to an a-axis. Calculations of the stress field and resultant stress-optical effects due to such an array require a density of dislocations of per centimeter in average lamellae. The validity of this model is supported by electron micrographs of etched polished surfaces of crystals containing experimentally produced lamellae. The lamellae appear as rows of pyramidal etch pits which are exactly linear within 50 Å. The density of pits ranges from 5 to per centimeter. The geometrical characteristics of deformation bands parallel to the c-axis indicate that they are kink bands originating by basal slip; their orientations relative to the a-axes support the conclusion that the a-axes are the slip directions. The band boundaries are presumed to consist of "walls" of locked basal edge dislocations. This model is consistent with the lack of measurable optical changes in the vicinity of most band boundaries; changes of indices and birefringence along some band boundaries, qualitatively similar to lamellae, result from residual elastic stresses at asymmetrical boundaries. These elastic stresses are commonly relieved by tensile fracture in the undeformed host crystal and may be partially relieved by the development of prismatic dislocations at the kink-band boundary. Most natural lamellae so far studied exhibit the appearance in phase-contrast illumination and the changes in birefringence which have been found in experimental lamellae. Electron microscopy of etched polished thin sections of natural quartzite containing lamellae shows bands of etch pits coincident with the lamellae. The distribution of etch pits within these lamellar bands is much less regular than in experimental lamellae, but the density of etch pits is of the order required to give the observed optical effect if the etch pits represent dislocations predominantly of the correct sign. It is concluded that natural lamellae are typically comprised of irregular arrays of locked-in dislocations. Differences of orientation between natural and experimental near-basal lamellae are discussed; and it is concluded that natural lamellae may be initially parallel to the base, with subsequent internal rotation by other slip systems, or they may originate at an angle to the base as en échelon arrays of basal dislocations.