Filip Siska

{101¯2} Twinning in magnesium-based lamellar microstructures

The magnesium-based alloy Mg–9Al–1Zn has been extruded and heat treated to produce a dense population of lamellar plate-shaped particles. In compression with a testing orientation well aligned for prolific { 1 0 1 ¯ 2 } twinning, precipitation resulted in a significant increase in the yield point, but there was no change in the volume fraction of twins that were produced. It is proposed that the larger number of smaller twins observed in the aged condition is the result of inhibition of twin growth by the particles.

Influence of Cold-Sprayed, Warm-Sprayed and Plasma Sprayed Layers Deposition on Fatigue Properties of Steel Specimens

Titanium powder was deposited onto steel specimens using four thermal spray technologies: plasma spray, low-pressure cold spray, portable cold spray, and warm spray. The specimens were then subjected to strain-controlled cyclic bending test in a dedicated in-house built device. The crack propagation was monitored by observing the changes in the resonance frequency of the samples. For each series, the number of cycles corresponding to a pre-defined specimen cross-section damage was used as a performance indicator. It was found that the grit-blasting procedure did not alter the fatigue properties of the steel specimens (1% increase as compared to as-received set), while the deposition of coatings via all four thermal spray technologies significantly increased the measured fatigue lives. The three high-velocity technologies led to an increase of relative lives to 234% (low-pressure cold spray), 210% (portable cold spray), and 355% (warm spray) and the deposition using plasma spray led to an increase of relative lives to 303%. The observed increase of high-velocity technologies (cold and warm spray) could be attributed to a combination of homogeneous fatigue-resistant coatings and induction of peening stresses into the substrates via the impingement of the high-kinetic energy particles. Given the intrinsic character of the plasma jet (low-velocity impact of semi/molten particles) and the mostly ceramic character of the coating (oxides, nitrides), a hypothesis based on non-linear coatings behavior is provided in the paper.

Twinning in magnesium-based lamellar microstructures

The magnesium-based alloy Mg–9Al–1Zn has been extruded and heat treated to produce a dense population of lamellar plate-shaped particles. In compression with a testing orientation well aligned for prolific { 1 0 1 ¯ 2 } twinning, precipitation resulted in a significant increase in the yield point, but there was no change in the volume fraction of twins that were produced. It is proposed that the larger number of smaller twins observed in the aged condition is the result of inhibition of twin growth by the particles.

The Prediction of Size Effect on J-R Curve for Eurofer97 Steel by Simplified Mechanical Model

The possibilities to derive fracture toughness from small specimens are naturally limited due to constraint requirements which are especially restrictive in toughness testing. The loss of constraint at the crack tip is more likely to occur as specimen size decreases. Application of miniature specimens in fracture toughness testing thus requires a suitable methodology or correction procedure to deal with phenomenon of the constraint loss. Schindler et al. have proposed a simplified mechanical model that can be used to scale-up the key test characteristics from miniature specimen to the larger one. The model is applied to the miniature bending specimens to describe size effect on J-R curve of the Eurofer97 steel. The examined steel exhibits quite high toughness values at upper shelf region of fracture toughness. As a result, experimentally determined J-R curves of three different sizes of pre-cracked bending specimens showed high values of J-integral, which were significantly different each other. Using semi-empirical definition of the exponent of the power law function of J-R curve the performance of the Schindler’s model was quite successful. It was shown that the model is able to handle with size effect of tested pre-cracked three-point-bend specimens.Copyright

Textural Fractography of Fatigue Failures under Variable Cycle Loading

A new concept of counting time at fatigue processes is proposed, aimed to reach fractographic compatibility in cases of different loading sequences. Values of cycle effectivity are summarized to give the new reference time. The improvement is shown in application - textural fractography of three specimens loaded by constant cycle, constant cycle with periodic overloading, and a random block, respectively. In contrast to the conventional crack growth rate, the reference crack growth rate is related to common morphologic features of all fracture surfaces.

Kinetic study of static recrystallization in an Fe–Al–O ultra-fine-grained nanocomposite

Abstract A nearly abrupt coarsening of grains is observed in a newly developed Fe–Al–O ultra-fine-grained nanocomposite with a significant volume fraction (4%) of alumina nano-precipitates. The microstructure of the alloy was analysed in different states (as-received and annealed) by means of scanning electron microscopy, transmission electron microscopy (TEM) and hardness. The initial grain size 150–200 nm increases up to 50 μm during annealing 1000 °C/8 h and thereafter demonstrates saturation. A linear correlation between volume fraction of coarse grains and hardness was found. It was identified by TEM that alumina nano-precipitates stabilize the dislocation microstructure against recovery very effectively and the grain coarsening is due to fast growth of very few dislocation free grains. Thus, the observed grain coarsening has the attributes of static recrystallization.

Subsized Specimens for Fracture Resistance Characterisation Including Transferability Issues

The contribution is focused on characterization of methods enabling to apply small/subsized specimens for fracture resistance characterization. The applied methods are divided into transition region and upper shelf region. The approaches used in the upper shelf region represent at the same time methods applicable for ductile materials without transition. Relating to subsized samples two basic approaches are applicable: (i) miniaturized samples based on common standard ones and (ii) specific specimens/methods, e.g. small punch test etc. The results described in the paper belong to the first group. For interpretation of data generated under low constraint conditions toughness scaling models and master curve approached are commented. In ductile region, either the sample used generate valid toughness characteristics, or, if not, there is no way how to correct measured data except damage quantification through micromechanical models.

A Numerical Analysis of Deformation Processes in Oxide Dispersion-Strengthened Materials - Influence of Dislocation-Particle Interactions

A recently developed 3D discrete dislocation dynamics (DDD) model is employed to study kinetics of dislocation ensembles subjected to high temperature creep in microstructures of metal matrix composites. We particularly focus on a migration of low angle tilt boundaries in a field of rigid impenetrable particles. This type of dislocation boundaries represents a typical microstructural feature mediating plastic deformation during the high temperature loadings. The article compares results of numerical studies that considered distinct dislocation-particle in-teractions in order to describe the response of dislocation structure to the applied stress. The resultssuggest that, regardless the details related to the dislocation-particle interactions, a critical applied stress always exists, below which the boundary migration process ceases [1,2]. The existence of crit-ical threshold is confirmed by creep tests of ODS materials. This critical threshold, contrary to theclassical Orowan stress, is proportional to the dislocation density. The displacements of individual dislocation segments on the micro-scale level reflect the changes in the dislocation-particle interactions quite sensitively. Atthemacro-scale level, the overall strain rate, which averages out velocities of all the individual dislocation segments, is also significantly influenced by the changes in dislocation-particle interaction

The Application of Miniaturized Three-Point-Bend Specimens for Determination of the Reference Temperature of JRQ Steel

This contribution deals with determination of the reference temperature of JRQ steel using miniaturized specimens. The dimensions of used miniaturized specimens were 3 × 4 × 27 mm (thickness × width × length). This specimen type offers the utilization of limited amount of test material or broken halves of precracked Charpy and larger specimens. The test material comes from the broken halves of 0.5T SEB specimens previously tested for purposes of the reference temperature determination in Ciemat, Madrid. The fracture toughness tests of specimens were performed in the transition region of the steel according to the recommendations of standard ASTM E1921 and according Wallin’s recommended temperature range for miniaturized specimens. The determined reference temperature of the Master Curve was very similar to the determined ones from three-point-bend specimen of sizes 0.2T, 0.4T and 0.5T. The obtained results confirm a necessity of conduct of tests at low temperatures and testing sufficient number of specimens in order to generate enough valid data for determination of the reference temperature.Copyright

{10(1)over-bar2} Twinning in magnesium-based lamellar microstructures

The magnesium-based alloy Mg–9Al–1Zn has been extruded and heat treated to produce a dense population of lamellar plate-shaped particles. In compression with a testing orientation well aligned for prolific { 1 0 1 ¯ 2 } twinning, precipitation resulted in a significant increase in the yield point, but there was no change in the volume fraction of twins that were produced. It is proposed that the larger number of smaller twins observed in the aged condition is the result of inhibition of twin growth by the particles.