Milan Dopita

EBSD investigation of the grain boundary distributions in ultrafine-grained Cu and Cu–Zr polycrystals prepared by equal-channel angular pressing

Abstract Ultrafine-grained copper and cooper–zirconium polycrystals prepared by equal-channel angular pressing following the route Bc to various strain (1, 2, 4 and 8 passes) were investigated using electron back-scatter diffraction. Equal-channel angular pressing resulted in significant grain-size reduction. The original course-grained structure evolved from prolate bands of cells/subgrains enclosed by lamellar nonequilibrium grain boundaries (after the first two passes) towards an equiaxed homogeneous microstructure with equilibrium grain boundaries (after 8 passes). Significant changes in the volume fraction and the character of grain boundaries were observed both in Cu and in Cu – Zr alloy. Pronounced evolution of twin-related boundaries (3n grain boundaries) with strain (number of ECAP passes) was found in Cu while only a weak increase of 3 and 9 grain boundaries was observed in Cu – Zr alloys.

Refining bimodal microstructure of materials with MSTRUCT

The possibilities of modelling the diffraction profiles from bimodal microstructure in computer program MSTRUCT are demonstrated on two examples. A special “Double Component” profile effect can be utilized for such problems. At first it was applied to an analysis of a mixture of two nanocrystalline anatase powders with different crystallite sizes and the relative ratio of both components was determined from X-ray diffraction data. In the second case study, diffraction peaks from a pure polycrystalline copper sample treated by equal channel angular pressing were fitted using a two-phase model of large recrystallized defect-free grains and ultrafine crystallites with high dislocation density. The method is shown to be suitable for determination of the relative fraction of the microstructural components as well as other parameters (e.g. dislocation density).

Mechanical Properties and Microstructure Development of Ultrafine-Grained Cu Processed by ECAP

Technical purity Cu (99.95 wt%) polycrystals have been processed at room temperature by equal channel angular pressing. The results of mechanical tests and the microstructure characterization by various experimental techniques are presented. The yield stress as well as the strength were shown to increase with increasing strain and exceed the respective values of a coarsegrained material. The microstructure development and its fragmentation after ECAP was investigated by the TEM and EBSD. The proportion of high angle grain boundaries was found to increase with increasing strain reaching the value of 90% after 8 ECAP passes. Two kinds of defects were identified in ECAP specimens by positron annihilation spectrometry (PAS): (a) dislocations which represent the dominant kind of defects, and (b) small vacancy clusters (so called microvoids). The main increase of defect density was found to occur during the first ECAP pass. PAS analysis indicated that in the specimens subjected to one ECAP pass the mean dislocation density ρD and the concentration of microvoids cν exceeded the values of 1014 m-2 and 10-4 at.-1, respectively. After 4 passes, the number of defects becomes saturated and practically does not change with increasing strain.

Phase composition and surface properties of nylon-6 nanofibers prepared by nanospider technology at various electrode distances

Phase composition, morphology and surface properties of nylon-6 nanofibers prepared by Nanospider technology have been studied for dependence on spinning distance using a combination of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electrokinetic analysis, and scanning electron and transmission electron microscopy (SEM, TEM). The effect of the electric field strength on the nanofiber phase composition was investigated via the variable distance of the electrodes. Quantitative XRD phase analysis revealed the dependence of the phase composition on the electrode distance, which in the case of roller electrospinning, differs from that by melt spinning. A combination of XRD, XPS, and TEM suggested a core-shell structure model of the nanofibers. The XPS and electrokinetic analysis revealed the difference in surface chemistry and zeta potential at the face and reverse side of the nanofiber textile adjacent to a polypropylene (PP) antistatic spunbond, which may be important in subsequent chemical modification of nanofiber textiles and in its use for tissue engineering.

Capability of X-ray diffraction for the study of microstructure of metastable thin films

The capability of X-ray diffraction for the microstructure investigations of metastable systems is illustrated on supersaturated and partially decomposed thin films of titanium aluminium nitrides with high aluminium content. The anisotropy of the elastic constants and their role in these investigations is discussed.

Microstructural evolution of equal-channel angular pressed interstitial-free steel

Abstract Equal-channel angular pressing (ECAP) belongs to advanced technologies for improving mechanical properties of materials. In the present work, the influence of the number of ECAP passes using route BC on both grain size and mechanical properties of interstitial-free steel was investigated by means of transmission electron microscopy, electron back-scattering diffraction, and microhardness testing. It was found that the grain size decreases with the increasing number of passes. At the same time, an increase in microhardness was observed. The evolution of microstructure with increasing strain imposed through ECAP, in particular the process of grain fragmentation and the formation of high-angle boundaries were also examined.

Antiferroelectricity in lanthanum doped zirconia without metallic capping layers and post-deposition/-metallization anneals

We report the effects of lanthanum doping/alloying on antiferroelectric (AFE) properties of ZrO2. Starting with pure ZrO2, an increase in La doping leads to the narrowing of the AFE double hysteresis loops and an increase in the critical voltage/electric field for AFE → ferroelectric transition. At higher La contents, the polarization-voltage characteristics of doped/alloyed ZrO2 resemble that of a non-linear dielectric without any discernible AFE-type hysteresis. X-ray diffraction based analysis indicates that the increased La content while preserving the non-polar, parent AFE, tetragonal P42/nmc phase leads to a decrease in tetragonality and the (nano-)crystallite size and an increase in the unit cell volume. Furthermore, antiferroelectric behavior is obtained in the as-deposited thin films without requiring any capping metallic layers and post-deposition/-metallization anneals due to which our specific atomic layer deposition system configuration crystallizes and stabilizes the AFE tetragonal phase during growth.We report the effects of lanthanum doping/alloying on antiferroelectric (AFE) properties of ZrO2. Starting with pure ZrO2, an increase in La doping leads to the narrowing of the AFE double hysteresis loops and an increase in the critical voltage/electric field for AFE → ferroelectric transition. At higher La contents, the polarization-voltage characteristics of doped/alloyed ZrO2 resemble that of a non-linear dielectric without any discernible AFE-type hysteresis. X-ray diffraction based analysis indicates that the increased La content while preserving the non-polar, parent AFE, tetragonal P42/nmc phase leads to a decrease in tetragonality and the (nano-)crystallite size and an increase in the unit cell volume. Furthermore, antiferroelectric behavior is obtained in the as-deposited thin films without requiring any capping metallic layers and post-deposition/-metallization anneals due to which our specific atomic layer deposition system configuration crystallizes and stabilizes the AFE tetragonal phase dur...

Temperature evolution of microstructure of deformed submicrocrystalline Cu–Zr samples

The addition of zirconium to copper improves the microstructural stability from the region of only slightly above 100 °C to about 400 oC. This also decreases with the number of ECAP passes. With annealing temperature the diffraction profiles became of anomalous shape with a sharp peak and long tails. Simple 2D diffraction patterns indicated the presence of multimodal microstructure leading to wide and narrow components of diffraction profiles. Therefore, bimodal microstructure model was applied for the evaluation and whole measured diffraction patterns were fitted by our own software MSTRUCT [1] as a sum of two Cu phases with different microstructures. This software combines different procedures and algorithms known from the software created by M. Leoni and P. Scardi or by G. Ribárik with some features of MAUD by L. Lutterotti. It appeared that two factors had main influence on the diffraction profiles and, consequently, could be reliably determined from the experiment – microstrain (and/or dislocation density) in the deformed component and the ratio of deformed and recovered fractions.

Structural studies of M-type ferrites used as template layers for the growth of oriented Y-type ferrites through chemical solution deposition method

Thin films of trigonal Ba2Zn2Fe12O22 (Y-type) ferrite were prepared by the chemical solution deposition using SrTiO3(111) substrates covered with hexagonal magnetoplumbite (M-Type) seed layers. Seven M phases with different chemical composition, magnetic character and lattice misfit (-0.8 % to -7.0 %) values were investigated in their use as template and buffer layers. The films were studied by X-ray diffraction, atomic force microscopy and electron back-scatter diffraction. X-ray diffraction analysis in parallel beam setup with Eulerian cradle was concentrated on the precise determination of lattice parameters, studies of preferred orientation by symmetric and asymmetric scans (w and j) and studies of possible stresses for all the layers by measurement of reflections at different inclinations j and y. For this, correct reference values of lattice parameters are required in particular for non-cubic phases. For most of the phases tested, the values are either completely missing in PDF4+ database or often a number of values can be found there for a single phase. Therefore some data analysis of PDF4+ with respect to the statistics and/or also to the stoichiometry was performed. For all the seed layers strong out-of-plane (000l) orientation of Mand Y-films was observed but with somewhat different degree. The in-plane orientation depended on misfit between the M-interlayer and substrate and also M-interlayer and the top Y-film. The optimum growth was reached using SrTiO3(111)/(BaSr)(GaAl)12O19, SrTiO3(111)/Ba(FeAl)12O19, and SrTiO3(111)/SrGa12O19 substrate/seed layer architectures. In general, the best results were achieved when the misfit values between the seed layer and substrate, and between the seed layer and top Y-layer are approximately equal and when the surface of seed layers are formed by hexagons. Then single domain perfect hexagon-on-hexagon orientation was observed for M-film. For Y-layer also the in-plane orientation (001)Y // (111)STO // and [100]Y // STO was observed but six maxima detected in j -scans instead of three confirmed in-plane obverse/reverse twinning. The results demonstrated the possibilities of the intelligent material design (namely anisotropy manipulation) for this important class of magnetic materials by proper design of key constituents: lattice misfits and surface topography of seeding structures.

Formation and high-temperature stability of metastable (Cr,Zr)2O3/(Zr,Cr)O2 nanocomposites

Successive crystallization of amorphous Cr-Zr-O thin films, formation of the (Cr,Zr)2O3/(Zr,Cr)O2 nanocomposites and thermally induced changes in the hexagonal crystal structure of metastable (Cr,Zr)2O3 were investigated by means of in situ high-temperature synchrotron diffraction experiments up to 1100°C. The thin films of Cr-Zr-O were deposited at room temperature using reactive ion beam sputtering from zonal Cr-Zr targets under oxygen flow. The resulting amorphous Cr-Zr-O solid solutions contained up to 15 at.% Zr. During the annealing in vacuum, the Cr-Zr-O solid solutions decomposed into two metastable phases, Cr-rich (Cr,Zr)2O3 and Zr-rich (Zr,Cr)O2, which crystallized in hexagonal and tetragonal structure, respectively. With increasing Zr content in amorphous Cr-Zr-O, the start of the phase segregation and crystallization was shifted from 600°C at 3 at.% Zr to 1000°C at 15 at.% Zr. With the aid of the in situ high-temperature synchrotron powder diffraction experiments, it was found that the metastable Cr2-2xZrxO3-x can accommodate up to approx. 3 at.% Zr. The zirconium atoms occupy partially the Wyckoff positions 6b in the corundum-like crystal structure of Cr2O3 that are empty in the stoichiometric chromium oxide. The incorporation of Zr into the crystal structure of Cr2O3 inflated the elementary cell and modified the thermal expansion of Cr2-2xZrxO3-x. The tetragonal structure of zirconia was stabilized by chromium. The phase segregation during the crystallization led to the formation of (Cr,Zr)2O3/(Zr,Cr)O2 nanocomposites. The size of crystallites in these nanocomposites decreased with increasing Zr content from 60 nm to 30 nm and increased only slightly at the highest annealing temperatures. In summary, this contribution illustrates the microstructure design in nanocomposites on the example of metastable chromium and zirconium oxides.