Solid State Phenomena Vol. 186

Abstract: Ni/Si multilayer contact structures to 4H-SiC after subsequent annealing steps are investigated with electron microscopy methods. After high temperature annealing step, specific defects in the contact structures are observed. The influence of phase transformations during annealings on the morphology on the contacts is discussed and the explanation of formation mechanism of voids within contact layer is proposed.

Abstract: The microstructure of Co_0.75Ni_0.25Si₂ solid solution, obtained by two different techniques was studied. The solidification processes were conducted using Bridgman and Czochralski methods. The processes were conducted under atmospheric pressure and in the helium atmosphere. Various pulling down (Bridgman method) and pulling up (Czochralski method) rates were applied for ingots and boules preparation. The obtained Co_0.75Ni_0.25Si₂ ingots and boules were subjected to the metallographic observations and chemical microanalysis, mainly used Scanning Electron Microscopy techniques. They were studied using X-ray phase analysis too. Comparing the investigation results it was found that the ingots obtained by the Bridgman method and boules obtained by the Czochralski method were single crystalline. The boules, obtained using the Czochralski method, possess better structural quality than ingots obtained by the Bridgman method.

Abstract: In this paper a specific method of severe plastic deformation (SPD) applied to the NiTi shape memory alloy and the obtained structure and the alloy properties are presented. Cold rolling combined with transverse movement of the rolls was the method. The maximal strain obtained was ε_c ≈ 10.20 for the relatively large specimens. Then the alloy was annealed in order to obtain samples of a given grain size. In the paper the structure of the alloy annealed at 350°C was studied with the use of TEM equipped with Nanomegas attachment ASTAR allowing determination of the orientation of grains in nanoareas.

Abstract: Commercial brass Ms36, 2mm thick was annealed and deformed in 6 passes in dual rolls equipment with attached equal channel equipment (DRECE). Then, material was deformed again using constrained groove pressing (CGP) by pressing of grooves 4.2 mm thick, and the groove angle of 45 deg. The experiment was performed 8 times (pressing out grooves and straightening at room temperature). Both methods allowed deformation without changing of the thickness of the sample, which was almost constant near 2 mm. The tensile experiment have shown the Yield Strength YS after 8x groove pressing of 210 MPa and Ultimate Tensile Strength UTS increased 27% up to 430 MPa. At the same time total elongation decreased from 34 to 15 %. The structure of the material after DRECE 6 passes was investigated using conventional TEM and have shown only rather uniform distribution of dislocations. After additional 8 groove pressing experiment, frequent, narrow deformation twins were observed accompanied by the formation of subgrains. Orientation imaging microscopy performed have shown average grain size after DRECE process near 5 μm, which decreased after 8 processes of groove pressing down to 2.9 μm. The fraction of low angle boundaries (below 5 deg) decreased after groove pressing down to 73% from 85% after DRECE process and annealing, while the fraction of high angle grain boundaries (>15 deg) increased after groove pressing up to 24% from 14%, however the total length of high angle boundaries increased more than 2 times since grain size decreased. The structure studies have shown rather mild effect on the grain refinement of both methods and they have to be modified to obtain material approaching nanosize range.

Abstract: The objective of this study was to determine the effect of ECAP-processing on the texture and the microstructure development of the deformed and recrystallized states. The commercial purity AA3104 aluminum alloy was deformed via route A up to 7 passes and then annealed to obtain the state of partial recrystallization. The shear bands formation and the texture transformation in annealing were investigated with the use of high resolution transmission and scanning electron microscopes equipped with the facilitates for the local orientation measurements. It was found that the instability of the layered structure of the flat grains within some narrow areas led to the formation of the kink-type bands, which were precursors of the shear bands. The orientations of the new grains occurring in the recrystallized samples were not random, i.e. only specific groups of orientations were observed. In most of the observed cases, the growth of the recrystallized grains led to nearly homogeneous equiaxed grains of a similar size.

Abstract: The Ni-4at%W alloys was prepared with cold crucible levitation, subsequent levitation re-melting and high pressure torsion (HPT) intensive deformation. In samples after levitation precipitates of Ni4W phase as well as grain boundary continuous phase were formed. Levitation re-melting resulted in partial dissolution of the precipitates, increasing W content in the matrix and lead to the microstructure refinement. The deformation by HPT, in the range of 300-400%, lead to the lead to the homogenization of the solid solution and partial decomposition of the alloys into pure Ni and W. In the homogenous majority of the sample the microstructure transformed from dendrite microstructure to faceted grains. The analysis of the grains sizes and shapes showed that the average grain size in the sample re-melted by levitation was twice the grain size of the sample only prepared with CCLM. Also, the grains in this sample were elongated by 50-60% in one direction, while in the sample prepared by CCLM they were equiaxial. As the nominal composition of the alloys in both cases was the same, differences observed in the microstructure after re-melting and HPT processing must result only from the differences in the cooling rate leading to small differences in W content in solid solution and phase composition after solidification. High rate of cooling in the levitation methods resulted in Ni4W metastable phase precipitation as well as in the refinement of the microstructure, stronger after additional re-melting of the alloy by levitation.

Abstract: The paper describes the mechanism of deformation at 77 K of pure aluminum bicrystals of different grain orientations. The following orientations were selected: {100}/{110} (cube/Goss) and - {100}/{100} (cube/shear) to represent the unstable vs. stable and the unstable vs. unstable behaviours, respectively. The bicrystalline samples were deformed in the plane strain conditions with the use of a channel-die immersed inside a reservoir with liquid nitrogen. The low temperature deformation increases the tendency to form plain strain inhomogeneities of the deformation in the grains with an unstable orientation. In both sets of crystallite compositions, the grain boundary was situated perpendicularly to the compression plane. A particular interest was paid to the analysis of the tendencies of the crystal lattice rotations near the grain boundary and the description of the deformation behaviour of the material in the macro- scale (hardening behaviour). A detailed analysis of the crystal lattice rotations was possible with the application of the local orientation measurements by means of scanning and transmission electron microscopes, equipped with the electron backscattered diffraction and convergent beam electron diffraction facility, respectively. The experimental results of the local orientation measurements were used to evaluate the accuracy of the numerical prediction of the macro-scale behaviour of bi-crystalline samples by a single Cristal Plasticity Model. The investigation shows that the crystallites behave essentially as single crystals in the same deformation conditions. Due to the similar hardening behaviour of the investigated crystallites (similar values of the Taylor factors) the grain boundary remains unchanged. The calculated lattice rotations are similar to those observed experimentally. Key words: aluminium bi-crystals, texture, microstructure, single crystal plasticity model

Abstract: The results of studies carried out on AA1200 aluminum alloy deformed by Accumulative Roll Bonding (ARB) are presented in this paper. The commercial purity material was deformed up to 10 cycles (equivalent plastic strain of 8) at room temperature. The deformed microstructures and the crystallographic textures were characterized by transmission (TEM) and scanning (SEM) electron microscopes and high resolution orientation mapping. It was found that increased deformation leads to a strong increase of quantity of high angle (>15°) grain boundaries and strong grain refinement (up to 200-300 nm). The microstructure observations and TEM and SEM local orientation measurements allowed identifying fine and strongly disoriented planar dislocation structure of nanolayers described by strong texture components close to two nearly complementary positions of {112} orientation.

Abstract: The article presents the results of microstructure evolution studies of the FeCr30Co8 hard magnetic alloy, subjected to deformation by tension and torsion at 700 and 720 °C. The observations in the longitudinal section of the samples in the scanning electron microscope (SEM) showed a formation of gradient microstructure with the maximum grain refinement in the surface layer of the material. The EBSD examination confirmed the refinement of structure in the surface layer and the presence of sub-grained structure of the material. A little larger refinement of α phase grains was observed at temperature of deformation 700 °C than at 720 °C. However, the deformation was inhomogeneous along the whole longitudinal section of the sample. The highest deformation degree resulted from the torsion.

Abstract: The present paper reports an experimental investigation of rapid solidification (RS) influence on the structure and mechanical properties of commercial AZ91 magnesium alloy. In order to obtain RS material melt spinning process was applied in protective atmosphere, resulting in formation of 50 to 100 μm thickness RS ribbons. Application of plastic consolidation (PC) by hot extrusion to the highly fragmented magnesium strips allowed to obtain high bulk strength material. It was found that yield strength (YS) and ultimate tensile strength (UTS) of RS+PC material with comparison to the cast and extruded samples were increased from 220 MPa to 303 MPa and from 287 MPa to 385 MPa, respectively, while plasticity of the RS material was slightly decreased. It was noticed that the grain size of both materials was at the same level of 2 μm, thus higher mechanical properties of RS material was ascribed to dispersion strengthening caused by the high amount of fine (below 50 nm in diameter) Mg₁₇Al₁₂ phases evenly distributed in the material structure.