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Substructure of directionally deformed single domain ZnS polycrystals

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  • Save American Journal of M aterials Science 2013, 3(2): 36-40 DOI: 10.5923/j.materials.20130302.03 Substructure of Polysynthetic Crystals of Zinc Sulfide Mono-Domained by Oriented Deformation U. K. Ernazarov1,*, Zh. Zhanabergenov2 1Karakalpak State University, Berdahstr 1, 742015 Nukus, Uzbekistan 2Karakalpak State Pedagogik Institute, Nukus, Karakalpakstan, Uzbekistan Abstract Th is work p resents a study of polysynthetic crystals of zinc sulfide (ZnS) consisting of thin do mains of sphalerite-type of single oriented substructure by X-ray diffraction (XRD) and electron microscopy. XRD studies have shown that upon plastic deformat ion in a certain direction thickness of domains of one direction increases in the main part of polysynthetic crystal ZnS. Based on the XRD and electron microscopy studies we found that the mono-doma in substructure of the polysynthetic ZnS crystals consists of dangling oblique cross-domain boundary of the original crystals generated by inhibited dislocations in the crystal. We identify the sequential processes of formation of the substructure, which are accumulat ions of the inhibited d islocations in certain reg ions of the deformed crystals, of the slip part ial dislocations stretched in the direction perpendicular to the slip plane of the dislocation co mplexes divid ing the crystal into "fan-shaped" disoriented sub-blocs, which in macroscopic scale imitate “S”-shaped bending of the shear plane. Keywords PolysyntheticZns Crystals, Dislocations, Polysynthetic Zns Crystals 1. Introduction Crystals of zinc sulfide (ZnS) grown fro m the melt at a temperatu re b elow 1000°C h ave po lysynthet ic structu re consisting of thin do mains with sphalerite structure of t wo orientations. As a result of the oriented plastic deformation they tu rn into monodo main crystal o f sphalerite with a specific substructure[1], which is single-oriented. In Refs[2, 3] substru ctu re o f th e mon o-do main crystals h as been presented as a "fan-like" set of crystalline sub grains creating “S”-shaped bend of the slip plane around the direction [110] . Nanometer size of the elements forming the substructure and low resolution of the methods used in these studies did not allow to establish structure of the o riginal and deformed polysynt het ic crystals. Fo r th ese reasons th e o rig in of formation of such substructure of the mono-oriented crystal h as remain ed un clear. Lat er, it h as b een est ab lish ed experimentally[4] structure of the crystal not subjected to orien ted p last ic defo rmat ion. It cons ists o f micro t win domains of sphalerite with average th ickness of 30−40 Å passing through the whole crystal and forming a coherent plane-parallel b oundaries .A lso it cont ains a nu mber of randomly distributed doma ins with limited size along the slip direct ions[112]. It is well-known that the limited size of these domains is consequence of inco mp lete shifts of the * Corresponding author: ernas (U. K. Ernazarov) Published online at Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved crystallographic planes that are spread out from different centers during the martensitic type phase transitions[5, 6]. This type of cross-domain boundaries are called as inclined boundary that are broken inside the crystal. Partial dislocations are located a long perimeter of such doma ins that are inhib ited by various obstacles.Also, they create obstacles to the dislocationsgliding along the plane of stacking faults. Formation of substructures arising as a result of interaction of the partial dislocations with those inhibited at the inclined boundaries gliding under the action o f applied mechanical stress has been studied theoretically in Refs.[7-9]. Properties of substructure of the init ial polysynthetic crystals not subjected to plastic deformation has been studied experimentally[4] and theoretically[7-9]. These studies have led us to the idea of this work about investigation of deformed crystals of ZnS by XRD and electron microscope to establish the origin of format ion of substructure of the mono-oriented crystal. 2. Experimental Methods ZnS crystals have been grown fro m the melt by the Bridg man method in the argon atmosphere under the pressure of 150 at m. Samp les of size 2×2×2 mm3 have been cut in such a way that the gliding planes, whichare active under deformation,are located under the angle 45° with respect to uniaxial pressure. Upon oriented pressing, gliding of the comp lete and partial dislocations in the crystal can take place only in one of the planes (111) selected by twinning. Method of deformation of crystals had been described in detail in Ref.[1]. Construction of cross sections American Journal of M aterials Science 2013, 3(2): 36-40 37 of the points of the reciprocal lattice is done by scanning the reciprocal space of the c rystal with a DRON-UM1. Prima ry beam has been formed by Si (111) monochromator and point collimator with output gap size of 0.05 mm.Distribution of intensity along 004 and 33 1 orientations of domains has been registered by scintillation detector with a gap of 0.1 mm width in the θ-2θ scanning mode with step Δ2θ = 0.02°. Before each of the angular intervals, c rystal orientation with respect to the primary beam along the horizontal plane of the X-ray goniometer has been changed to Δθ = 0.1°. Fro m these data obtained for the distribution of the scattering intensity two-dimensional sections reciprocal lattices have been built. In these sections scales along the coordinate axes have been selected as a fraction of the length of the reciprocal lattice vector g111. Images of thin sections of the crystal have been obtained by the electron microscope UEM V-100K. The advantage of this mode is that the resulting diffraction pattern clearly shows the area of the crystal, which directly contributes to the diffracted beam. Thin samples used in these studies were obtained by ultramicrotoming along the cleavage planes (110). In these studies diffract ion peaks of sphalerite have been used, for wh ich the requirement h + k + l = 3n ± 1 is fulfilled. Recip rocal lattice with such Miller indices are spatially separated, which allows obtaining the averaged sub structural characteristics of indiv idual orientations of domains. Usage of the XRD method for constructingthe sections of the recip rocal lattice together with the diffraction pattern fro m the electron microscope provides an unambiguous determination of the real structure of crystals with nanoscale inhomogeneities. 3. Results 3.1. X-ray di ffractometry Fig. 1a shows a fragment of the reciprocal space of the polysynthetic crystal and the directions of some reciprocal lattice vectors of domains of one orientation needed to describe the experimental conditions and diffraction characteristics of the experimentally established distribution of scattering intensity. One of the characteristic features of the diffractive cross section of the reciprocal lattice of the non-deformed and deformed up to ε ≈ 8 %samp les[Figs. 1, b and c] is presence of the bands of large intensity.On the section of the reciprocal lattice of the original crystal the bands are extended along the direction of the g111 and are connected with each other thus confirming the conclusion of Ref.[4] about average thickness of the domains of sphalerite. The intensity ratio of nodes 004 and 33 1 defined by summing of symmet ric parts of the cross sections taking into account the structural factors of the planes (F2331/F2400=1/3) was approximately equal to 1:3. Th is result indicates that thin domains of each orientation of the orig inal polysynthetic ZnS occupy the volumes, which are appro ximately equal to each other. On sections of nodes 004 and 33 1 deformed up to ε ≈ 8÷12%, direct ions of the "strands" are perpendicular to the corresponding diffraction vectors and are located between the directions [112] and [115]. Then the ratio of the intensity of nodes 004 and 33 1 was found to be approximately 1:7, which confirms the increase in the net volume of do mains of one orientation due to fibrewise reorientation of the layers of the do mains of the other orientations. The symmetry of the lengths of the "strands" of nodes 004 and 33 1 along the directions [112] and [115] shows that in some parts of the crystal, crystallographically disoriented subunits and net change of directions of the respective reciprocal lattice vectors have been formed that reach 5-7 angular degrees. Then the main part of the intensity of the diffracted rays is localized in the head sections of the reciprocal lattice. The features described in the distribution of the scattering intensity of the reciprocal-lattice shows that the origin of the boundaries of blocks (sub-blocks) is related to dislocations and that the net volume occupied by them is not significant.Consequently, format ion of new systems of dislocation structures begins at these levels of plastic deformation and goes along with the reorientation of d o main s . 3.2. Electron Microscope Original polysynthetic crystals of ZnS show the diffraction pattern with parallel to each other maxima along the direction[111] in the elongated reciprocal lattice. On some electron diffraction patterns weak stretched maxima are observed with rods of a length of g111/3 and sharp boundaries along the nodal line[111] at the distance (ghhh±g111/2) fro m the main peaks of the structures. Estimat ions showed that these points correspond to the to the interplanar distance of 6.25 Å of the reciprocal lattice. They correspond to hexagonal packing of defects, and to thickness 9.37 Å of the finest domains, which is equal to three layers d111(d111=a/3, a = 5.4092 Å). Fig. 2a shows the diffract ion pattern of the state of the crystal obtained with the use of the maximu m 33 1 of the domains of one orientation. Despite blurring of the boundary of the image o f the do main related to large increase of the diffract ion image, it is possible to see the features discussed in Ref.[3]about extents of the domains and inter-do main boundaries. Analysis of the diffraction patterns of the samples deformed at ε ≈ 3÷5% show that the diffraction peaks of only those doma ins which are oriented in one direction preserve the elongated form, which is parallel to the direction[111]. Peaks at positions ghhh±g111/2 disappear and the sharp boundaries of the rods have been blurred. Thickness of the domains estimated fro m diffraction images of the state of the crystal reaches 80÷100 Å. Analysis of diffraction patters of some regions of the crystal shows that at plastic strains ε≈6÷12% direction of elongation of ma xima of only one orientation of the domains will be changed in the direction [112] . Figures 2b and c display diffraction patterns characteristic to this plasticdeformation. The regions extended in the direction perpendicular to the slip planes are seen in Fig. 2b withenhanced intensity and low contrast. More organized forms of the dislocation structure is seen in the diffraction pattern[Fig. 2c ] formed at such level of plastic deformation. XRD spectra of the central part of crystal deformed at ε≈18-20% has a maximu m corresponding to 38 U. K. Ernazarov et al.: Substructure of Polysynthetic Crystals of Zinc Sulfide M ono-Domained by Oriented Deformation only one orientation of the do mains stretched parallel to the axis of oscillat ion[112]. Figure 1. Diagram illustratingthe experimental conditions andthe main directions of the reciprocal lattice (a), the cross section of the reciprocal lattices of the init ial (b) and polysynt het ic cryst als of ZnS deformed up to 12% (c) a b c Fi gure 2. Diffract ion patterns of the (a) init ial and (b,c) deformed samples of ZnS up to 8÷12% American Journal of M aterials Science 2013, 3(2): 36-40 39 4. Discussion of the Results According to Ref.[3], the lamellar reorientation of the domains of the sphaleriteZnS, which are unstable to uniaxial strain, takes place due to the movement o f part ial dislocations with the lattice parameters a||[1 1 0], b= 1[112] 6 and the slip plane (111). Twinning dislocations are generated at the boundary between the twins.During the movement the plane of t winning shiftsinto one interplanar spacing d111 in the direction[111].The observed increase of the thickness of the domains of one orientation as a function of the plastic deformation indicates to correlated motion of part ial dislocations. Assuming that they pass only once through every plane of stacking faults and that both orientations of the domains have approximately the same average thicknesses we find that consistent shift of about half of the (111) planes is required for format ion ofa single do main. Consequently, magnitude of the plastic deformat ion required for obtaining the mono-oriented state of sphalerite equals to ε≈18 ÷ 22%, which is in good agreement with theoretical estimations of Refs.[1-3]. The above described mechanism of transformation of polysynthetic structure into single oriented one is achieved only if the do mains form a flat-parallel-sphalerite coherent boundary. The results of this paper show that the declined cross-domain boundaries formed by a small group of hindered dislocations strongly influence on the processes of plastic deformation at small thicknesses of the do mains. Consequently, upon constructing the real sequence of the processes of format ion of the substructures, it is necessary to take into account the collective effect of the interaction of the group of inhibited dislocations with glid ing partial dislocations occurring by de-twinning up to co mplete re-orientation through the volume of the deformed crystal. As is known, the reason of inhibition of g lid ing dislocations during the martensitic type phase transition might be different [5, 6]. In the in itial polysynthetic crystals ZnS the inhibited dislocations are distributed at the locations where front planes of the growth of sphalerite phases face each other. This dislocation restricts the shear plane fro m one side and forms the stable and unstable slopes of the boundaries broken inside the crystal. The results of the diffraction studies show that the significant portion of the sloping boundaries lost stability at small plastic deformations (ε≈3÷5 %) and together with de-twinning partial d islocations take part in the processes of plastic deformation of micro twins. On the contrary, the declined borders, which are stable at such plastic deformations, resist to the motion of the de-twinned dislocations with elastic fields. At the locations of the inclined boundaries dislocation precipitations will be formed that divides the crystal into non-reoriented subunits. On the diffraction patterns this region of the crystal is shown with high intensity and low contrast [Fig. 2 b]. Analysis of Fig. 2b shows that the area where the dislocation precipitates are located is stretched perpendicular to the active gliding planes and size of the area is of the same magnitude as thickness of the domains. At larger plastic deformat ions ε≈6÷12% densification of dislocation precipitates will take place. As a result of that immob ile dislocations will be located closer to each other to allo w overlapping of their elastic fields. Relaxation of the elastic energy stored in the dislocation precipitates occurs by partial annihilation and rearrangement of dislocations in the configuration creating plastic bending of the glid ing planes [7-9]. For larger p lastic deformations the regions of plastic bending will be expanded in the direction perpendicular to that of the gliding plane due to absorbing the de-twinning partial d islocations. In macroscopic scale the regions of the plastic bending possess small thicknesses and divide the crystal to “fan-shaped” reoriented subunits. Angle of disorientation of the subunits (5 ÷ 7 angular degrees) obtained from analysis of XRD results is the net magnitude of plastic bending creating several areas of plastic bending. Co mplet ion of reorientation of the do mains of unstable orientation through the volume of the crystal obtained by analysis of several XRD spectra for the samples deformed by ε ≈ 18 ÷ 22% [Fig. 3] shows to passage of the de-twinned partial dislocations through the border of sub units or their absorption at the borders. Figure 3. XRD pattern for ZnS deformed under 18÷22%.Swivel axis is[112] with oscillation Interval 15º 5. Conclusions Thus, we have perfo rmed X-ray d iffraction and electron microscopy studies of substructure of polysynthetic ZnS crystals. XRD studies have shown that upon plastic deformation in a certain direction thickness of domains of one direction increases in the main part of polysynthetic crystal ZnS. This process is accompanied by formation of dislocation precipitates in certain areas of the crystal, which at larger plastic deformat ions are rearranged together with mobile part ial dislocations into more stable configurations determining the substructure of the uni-oriented ZnS cry s tals . Plastic deformat ion at ε≈18÷22% co mpletes the transition of the crystal into the uni-oriented state. The rearrangement 40 U. K. Ernazarov et al.: Substructure of Polysynthetic Crystals of Zinc Sulfide M ono-Domained by Oriented Deformation of the broken and inclined boundaries in the original Crystals At Plastic-Deformation. Fizika Tverdogo Tela. polysynthetic crystal, capture and absorption by them the 1984;26(7):2033-42. mobile partia l dislocations are the sequence of the processes [4] Abdikamalov BA, Erezhepov M T, Ernazarov UK. of formation of "fan-shaped" disoriented sub-units.In in Observation of a Substructure in Polysynthetic Zinc-Sulfide macroscopic scale they imitate “S”-shaped bending of the And Selenide Crystals. Fizika Tverdogo Tela. 1992; gliding plane around the direction [1 1 0] perpendicular to 34(5):1425-8. [112] of the forces and smallest gliding. [5] Vishnyakov YD. Packing Defects in a Crystalline Structure. M oscow, Russia: M etallurgya; 1970. 215 p. [6] Friedel J. Dislocations. Oxford : New York, Paris: Pergamon press; 1964. 491 p. REFERENCES [7] M alashenko VV. Dynamic drag of dislocations in crystal with structural imperfections. Technical Physics. 2009;54(4): [1] Abdikamalov BA, Bredikhin SI, Kulakov M P, Shekhtman 590-3. VS, Shmurak SZ. Phase-Transition AT Plastic-Deformation Of Sulfurous Zinc-Crystals. Fizika Tverdogo Tela. 1976; [8] M alygin GA. Analysis of the parameters of a brittle-ductile 18(8):2468-70. transition during impact loading of neutron-irradiated BCC metals and alloys. Physics of the Solid State. 2006; [2] Shekhtman VS, Shmytko IM , Aristov VV, Abdikamalov BA. 48(9):1716-23. Structure Changes At Uniaxial Pressure of Polysynthetic Crystals of Zns Sphalerite. Fizika Tverdogo Tela. 1976;18(5): [9] Rusin NM , Borisova SD. Simulation of stresss fields of 1358-61. dislocation pile-ups. Physical mesomechanics. 2008;12(2): 51-8. [3] Shmytko IM , M atveeva LA, Bredikhin SI, Shekhtman VS, Shmurak SZ. on Twinning M echanism of Zns Polysynthetical

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