Jitka Stráská

Ordered array of ω particles in β-Ti matrix studied by small-angle X-ray scattering

Abstract Nanosized particles of ω phase in a β -Ti alloy were investigated by small-angle X-ray scattering using synchrotron radiation. We demonstrated that the particles are spontaneously weakly ordered in a three-dimensional cubic array along the 〈 1 0 0 〉 -directions in the β -Ti matrix. The small-angle scattering data fit well to a three-dimensional short-range-order model; from the fit we determined the evolution of the mean particle size and mean distance between particles during ageing. The self-ordering of the particles is explained by elastic interaction between the particles, since the relative positions of the particles coincide with local minima of the interaction energy. We performed numerical Monte Carlo simulation of the particle ordering and we obtained a good agreement with the experimental data.

An ultrasonic internal friction study of ultrafine-grained AZ31 magnesium alloy

Internal friction in ultrafine-grained AZ31 magnesium alloy is investigated by resonant ultrasound spectroscopy. It is shown that the internal friction significantly increases at elevated temperatures (

Micro-Tensile Behavior of Mg-Al-Zn Alloy Processed by Equal Channel Angular Pressing (ECAP)

{\gtrsim }100

Microstructure Evolution in Ultrafine-grained Magnesium Alloy AZ31 Processed by Severe Plastic Deformation

≳100 °C), and that this increase can be attributed to grain boundary sliding (GBS). The evolution of this phenomenon with grain refinement is studied by comparing the results obtained for an extruded material and for materials after additional one, two, and four passes of equal channel angular pressing. It is observed that the activation energy for diffusive GBS significantly decreases with decreasing grain size, and so does also the threshold temperature above which this internal friction mechanism is dominant. The results prove that the ultrafine-grained AZ31 alloys exhibit diffusive GBS at temperatures close to the ambient temperature, which is an interesting finding with respect to the possible applications of these alloys in superplastic forming technologies.

Microstructure and Defect Structure Evolution in Ultra-Fine Grained MgAlZn Alloy

Resonant ultrasound spectroscopy (RUS) was applied to monitor the micro-cracking process occurring during cooling at polished surfaces of an ultrafine-grained AZ31 magnesium alloy. It was observed that although the net of micro-cracks covered only narrow regions along the edges of the sample, its appearance resulted in a strong increase of the attenuation of the free elastic vibrations, and was, thus, sensitively detectable from the evolution of the RUS resonant spectra with temperature. This approach enabled a reliable determination of the threshold temperature for micro-cracking.

Microstructure evolution in ultrafine-grained interstitial free steel processed by high pressure torsion

Commercially available AZ31 magnesium alloy was four times extruded in an equal rectangular channel using three different routes (A, B, and C). Micro tensile deformation tests were performed at room temperature with the aim to reveal any plastic anisotropy developed during the extrusion. Samples for micro tensile experiments were cut from extruded billets in different orientations with respect to the pressing direction. Information about the microstructure of samples was obtained using the electron back-scatter diffraction (EBSD) technique. Deformation characteristics (yield stress, ultimate tensile stress and uniform elongation) exhibited significant anisotropy as a consequence of different orientations between the stress direction and texture and thus different deformation mechanisms.

Study of the loading mode dependence of the twinning in random textured cast magnesium by acoustic emission and neutron diffraction methods

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.

Microstructure stability of ultra-fine grained magnesium alloy AZ31 processed by extrusion and equal-channel angular pressing (EX–ECAP)

Commercial MgAlZn alloy AZ31 was processed by two techniques of severe plastic de‐ formation (SPD): equal channel angular pressing and high pressure torsion. Several mi‐ croscopic techniques, namely light, scanning and transmission electron microscopy, electron backscatter diffraction, and automated crystallographic orientation mapping were employed to characterize the details of microstructure evolution and grain fragmen‐ tation of the alloy as a function of strain imposed to the material using these SPD techni‐ ques. The advantages and drawbacks of these techniques, as well as the limits of their resolution, are discussed in detail. The results of microstructure observations indicate the effectiveness of grain refinement by severe plastic deformation in this alloy. The thermal stability of ultrafine-grained structure that is important for practical applications is also discussed.