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1
Content available remote

Structural Changes in Deformed Soft Magnetic Ni-Based Metallic Glass

100%
Juríková A., Csach K., Miškuf J., Ocelík V.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
393-395
EN
The effects of intensive plastic deformation of the soft magnetic metallic glass Ni-Si-B on the structural relaxation were examined. The enthalpy changes studied by differential scanning calorimetry revealed that the intensive plastic deformation was associated with the partial structural relaxation of the amorphous structure.
2
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Influence of Plastic Deformation on Magnetic Transition in Soft Magnetic Amorphous Alloys

100%
Miškuf J., Csach K., Juríková A.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1273-1275
EN
The influence of the intensive plastic deformation on the structure of ferromagnetic amorphous alloys has been studied by means of thermoanalytical methods. It was shown that the work of plastic deformation led to the local heating in thin shear band layers and the dissipation of the energy caused structural changes in the region outside these bands. The plastic deformation influences the structure of the alloys and their structural sensitive magnetic properties.
3
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Influence of Plastic Deformation on Nanocrystallization of Finemet Alloy

100%
Csach K., Miškuf J., Juríková A., Janák G.
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issue 5
820-822
EN
The Finemet alloy prepared by the crystallization of an amorphous precursor is a basic model material for study of magnetic nanoparticle structures. Intensive plastic deformation localized in adiabatic shear bands causes the intensive heating and structural changes in the amorphous alloy. These changes influence the magnetic properties of the amorphous precursor for the Finemet crystallization.
4
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Creep Behaviour of Co-Based Amorphous Alloys

100%
Csach K., Miškuf J., Juríková A., Vojtaník P.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
91-94
EN
The detailed study of the temperature dependence of the viscosity of (Co, Fe)-(Si, B) amorphous metallic alloys up to crystallization temperatures was carried out. The kinetics of the non-equilibrium viscosity was described using Vogel-Fulcher-Tammann law below the glass transition temperature.
5
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Structural Relaxation of Ni-Si-B Amorphous Ribbon

100%
Juríková A., Csach K., Miškuf J., Ocelík V.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
99-102
EN
The structural relaxation of the Ni-Si-B amorphous ribbon was studied by means of differential scanning calorimetry and thermomechanical analysis. It was shown that considerable length changes associated with reversible structural relaxation were revealed after a previous creep applied at higher temperatures.
6
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Influence of Plastic Deformation on Devitrification of Fe_{73.5}Nb_{3}Cu_{1}Si_{13.5}B_{9} Amorphous Alloy

100%
Csach K., Miškuf J., Juríková A.
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vol. 126
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issue 1
98-99
EN
The method of thermogravimetry in external magnetic field was modified using square wave alternate magnetic field for an estimation of the temperature dependence of the magnetization. This method was used for the study of influence of plastic deformation of the amorphous alloy Fe_{73.5}Nb_{3}Cu_{1}Si_{13.5}B_{9} on the structural changes.
7
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Microstructure Mechanisms Governing the Creep Life of Ultrafine-Grained Cu-0.2 wt%Zr Alloy

80%
Kral P., Svoboda M., Dvorak J., Kvapilova M., Sklenicka V.
Acta Physica Polonica A
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2012
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vol. 122
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issue 3
457-460
EN
Equal-channel angular pressing was conducted at room temperature and extrusion was performed up to 12 passes using route where the billets were rotated by 90° in the same sense between consecutive passes. Tensile creep tests were performed at 473, 573 and 673 K at different constant applied stresses. It was observed that the original coarse grain size of unprocessed alloy was reduced to 0.3 μm after 8 equal-channel angular pressing passes and the grain growth during creep was restricted by precipitates with the mean diameter ≈ 4.0 nm. No significant effect on creep resistance was found after one equal-channel angular pressing pass at 473 and 573 K. However, the longest time to fracture was exhibited by alloy after 2 equal-channel angular pressing passes at 573 and 673 K but with further increasing number of equal-channel angular pressing passes a decrease in the time to fracture was observed. Nevertheless, the beneficial effect of equal-channel angular pressing on creep resistance was still documented after 8 passes for temperatures of 473 and 573 K. By contrast, creep tests performed at 673 K showed that the time to fracture of ultrafine-grained material is shorter as compared with that for as-received state. The 3D laser measurement of surface showed that the creep fracture process is accelerated by formation of vertical surface step relief and cavitation at the intersection of the shear bands during creep.
8
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3D Discrete Dislocation Dynamics Applied to Interactions between Dislocation Walls and Particles

80%
Záležák T., Dlouhý A.
Acta Physica Polonica A
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2012
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vol. 122
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issue 3
450-452
EN
A 3D discrete dislocation dynamics model is presented that describes dislocation processes in crystals subjected to mechanical loadings at high temperatures. Smooth and curved dislocations are approximated by a set of short straight line segments. A Peach-Koehler force acting upon each segment involves all segment-to-segment interactions and externally applied stress. The segment velocity is a product of a corresponding mobility and the glide or climb component of the Peach-Koehler force. The model addresses interactions between dislocations and rigid spherical precipitates. A migration of low angle tilt boundaries situated in a field of precipitates is simulated as an example. The numerical implementation exploits symmetries of the model that yield an optimized and highly efficient numerical code. Results provide detailed insight into how dislocation arrangements surmount particle fields in 3D crystals.
9
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3D Discrete Dislocation Dynamics: Influence of Segment Mobility on Critical Shear Stress

80%
Záležák T., Dlouhý A.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
654-656
EN
We use 3D discrete dislocation dynamics technique to study a low-angle tilt boundary migration subjected to applied shear stress at high temperatures, where diffusion significantly contributes to the dislocation motion. The model considers Peach-Koehler forces due to interactions between individual straight dislocation segments. The model also addresses dislocation plasticity in a field of impenetrable incoherent spherical precipitates. Velocities of the individual dislocation segments are calculated in relation to the crystallography of the material. Several calculation series have been carried out for different velocity and driving force relations. The results show that there exists a critical applied shear stress, below which the low angle dislocation boundary cannot surpass the rigid precipitates and remains in an equilibrium configuration. This agrees with experimental results obtained in creep tests of dispersion strengthened alloys. The critical stresses have been calculated also for situations where the applied stress was decreased during the interaction between the low-angle tilt boundary and the precipitates.
10
Content available remote

Influence of Long-term Annealing and Hot Bending on Creep of P92 Pipe

80%
Kral P., Dvorak J., Kvapilova M., Kucharova K., Sklenicka V., Svobodova M., Cmakal J.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
543-547
EN
This work is focused on the creep behaviour of the thick-walled hot bended P92 pipe, with an outer diameter of 350 mm and wall thickness of 39 mm, in its different parts, namely in straight parts and bends. Selected creep specimens machined from axial section of pipe were thermally aged at 650°C to simulate microstructure degradation typical for long-term service conditions. Subsequent tensile creep tests at constant load were performed at 600°C under 140 MPa and creep behaviour of various structure states was compared. Microstructure was investigated by scanning electron microscopy equipped with electron backscatter diffraction. It was found that ageing at 650°C for 10⁴ h caused the significant reduction of creep resistance down to about 10% of initial state and an additional slight reduction of creep resistance after longer ageing for 2×10⁴ h. Further, creep behaviour was significantly influenced by specimen position in the extrados and intrados parts of bends. Microstructure investigation revealed that long-term annealing has negligible effect on high-angle grain boundary spacing and misorientation. By contrast, creep deformation of long-term annealed specimens led to significant decreasing in high-angle grain boundary spacing and caused a change in the misorientation distribution of boundaries.
11
Content available remote

Creep Behaviour of a Zr-1 wt% Nb Alloy at Elevated Temperatures

80%
Kvapilová M., Kuchařová K., Kloc L., Sklenička V.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
548-551
EN
This paper presents experimental data regarding creep behaviour of a Zr-1 wt% Nb alloy at elevated (623 K) and at high temperatures (873-1123 K) corresponding to loss-of-cooling situation of fuel cladding tubes for nuclear reactors. For an elaboration of methodological procedure and comparison purposes, the tensile creep tests were conducted using both constant stress and constant load over a wide range of applied stress. The substantial differences in the acquired creep data between constant stress and constant load creep testing were found especially at high stresses and large creep strain levels.
12
Content available remote

Critical View on the Creep Modelling Procedures

80%
Kloc L.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
540-542
EN
Process of creep deformation is rather complex, consisting of many interconnected subprocesses, mainly: (i) the creep strain itself, based on dislocation mobility as well as grain boundary activity, (ii) development of dislocation substructure (work strengthening and dynamic recovery), (iii) development of phase structure (phase transformations, precipitation, particle coarsening, etc.), and (iv) nucleation and development of voids and microcracks, i.e. creep damage. The creep experiments are time consuming and expensive, moreover, it is not possible to make experiments under the service conditions of particular materials due to very slow creep strain, the process seems to be ideal field for computer modelling. The experimental data are obviously available for the steady conditions only, so the effects of varying conditions during startup or shutdown of the components can be described by modelling. The model of creep deformation is obviously based on the so-called "creep constitutive equation", which should describe the strain rate dependence on stress, temperature and some other variables. Nevertheless, the comprehensive physical description of all the above mentioned processes is still missing. This paper should illustrate the shortcomings of most "creep constitutive equations", confronting them to some experimental results on common structural materials under non-steady loading conditions.
13
Content available remote

Structural Stability of Amorphous Alloy of Modified Finemet Type

80%
Huráková M., Csach K., Juríková A., Miškuf J., Rajňák M., Ďurišin M., Kvačkaj T.
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EN
The Finemet type amorphous alloys are well known as high frequency soft magnetic materials. They have good soft magnetic properties which are characterized by low coercive force and high permeability because of the lack of crystalline anisotropy. The structural stability of the amorphous ribbon of Finemet type modified by Mn, Al and Cr prepared by melt-spinning process was studied using differential scanning calorimetry and dynamical mechanical analysis. With increase of the crystalline portion in the sample, the Curie transition is shifted to the higher temperatures. The magnetic relaxation processes at frequencies above 10⁴ Hz were detected by mass magnetic susceptibility measurement.
14
Content available remote

Creep Behavior of a Zirconium Alloy Processed by Equal-Channel Angular Pressing

70%
Sklenicka V., Dvorak J., Kral P., Svoboda M., Kvapilova M., Kopylov V., Nikulin S., Dobatkin S.
Acta Physica Polonica A
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2012
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vol. 122
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issue 3
485-489
EN
A Zr-2.5 wt%Nb alloy was processed by equal-channel angular pressing and then tested under creep conditions at 623 K using a tensile stress within the range from 120 to 300 MPa. The results show conventional power-law creep with a stress exponent of n > 3 which is consistent with an intragranular dislocation process involving the glide and climb of dislocations. It is demonstrated that diffusion creep is not important in these tests. For comparison purposes, the experiments were conducted using both the unprocessed alloy and after processing by equal-channel angular pressing. It was found that under same testing conditions the measured minimum creep rates in the pressed alloy with ultrafine grain sizes were faster than in the same alloy in a coarse-grained unprocessed condition.
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