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Activation Energy for Grain Growth of the Isochronally Annealed Ultrafine Grained Magnesium Alloy after Hot Extrusion and Equal-Channel Angular Pressing (EX-ECAP)

100%
Stráská J., Stráský J., Janeček M.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
578-581
EN
Magnesium alloy AZ31 prepared by hot extrusion and 4 passes of equal-channel angular pressing (EX-ECAP) has ultra-fine grained microstructure with an average grain size of 900 nm. Grain growth is analysed using a general equation for the grain growth and an Arrhenius equation. The calculated value of the activation energy for grain growth differs with the annealing temperature. The fitted value of activation energy for grain growth in the intermediate temperature range (210-400°C) is in accordance with the results of other authors, but it is shown in this study that such value is abnormally low and physically meaningless. More real values of apparent activation energy in this temperature range were calculated from the model assuming a linear increase of activation energy with increasing annealing temperature. Result of this linear model of evolution of activation energy in the temperature range between 210-400°C is expressed by the interval estimation of apparent activation energy values. It is concluded that the evolution of apparent activation energy can be explained by a change in the mechanism underlying the grain boundary migration. In the low temperature range, the grain boundary diffusion is dominant since the material is ultra-fine grained, whereas at higher temperatures, the lattice self-diffusion is more important.
2
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Mechanisms of Plastic Deformation in Ti-Nb-Zr-Ta Based Biomedical Alloys with Fe and Si Content

100%
Stráský J., Harcuba P., Horváth K., Janeček M.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
574-577
EN
Specialized beta titanium alloys containing biocompatible elements (Nb, Zr, Ta) are increasingly considered as a material for orthopaedic implants. In this study, small additions of Fe and Si are used to increase the strength of commercial Ti-35Nb-7Zr-5Ta (TNZT) alloy. Six different advanced alloys with iron content up to 2 wt% and silicon content up to 1 wt% were manufactured by arc melting and hot forging. Flow curves were determined from tensile tests carried out at room temperature. The yield stress is increased from 450 MPa to 700 MPa due to small Fe and Si additions. Fe causes solid solution strengthening exhibited by sharp yield point and significant work hardening. (Ti,Zr)₅Si₃ intermetallic particles further increase the strength via precipitation hardening. An unusual serrated yielding behaviour of benchmark TNZT alloy is caused by twinning as shown by acoustic emission measurement and electron backscattered diffraction analysis.
3
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Annealing Effects after Various Thermo-Mechanical Treatments on Microstructure and Mechanical Properties of Wrought Magnesium Alloy ZK60

100%
Oo M., Janeček M., Kral R., Wagner L.
Acta Physica Polonica A
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2012
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vol. 122
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issue 3
606-609
EN
The magnesium alloy ZK60 received from Dead Sea Magnesium in direct-chill cast condition was extruded at T = 350C using an extrusion ratio of ER = 12. The extruded bars were severely plastically deformed at 250C by either equal channel angular pressing or swaging. While swaging led to similar grain refinements as observed after equal channel angular pressing, yield stress and tensile strength values were by far superior in the swaged material. This result is explained by the unfavourable 45° texture component of the (0002) pole figure that develops during the massive shear deformation in equal channel angular pressing. Unlike equal channel angular pressing, the grain orientation after swaging is similar as in the as-extruded condition. Annealing at elevated temperatures indicates very low thermal stability of microstructure and mechanical properties in the equal channel angular pressing material while both properties in the swaged microstructure are as stable as in the as-extruded reference.
4
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Evolution of Corrosion Resistance in the LAE442 Magnesium Alloy Processed by ECAP

88%
Minárik P., Král R., Janeček M., Chmelík F., Hadzima B.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
772-774
EN
The influence of processing by equal channel angular pressing on the corrosion resistance was investigated in the extruded LAE442 magnesium alloy by the linear polarization method. Continuous increase of the corrosion resistance with the increasing number of passes was measured. The polarization resistance of the 12P sample was substantially higher as compared to the extruded state. The results were discussed in terms of the corrosion layers formed on the surface after seven days of immersion. It was concluded that the substantial increase of polarization resistance was caused by the combined effect of grain refinement and aluminum and lithium oxides formation within the corrosion layer.
5
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Microstructure and Mechanical Properties of Severely Deformed AX41 Magnesium Alloy

88%
Krajňák T., Máthis K., Gubicza J., Stráská J., Janeček M.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
768-771
EN
The object of the present paper is the study of mechanical properties and microstructural evolution of AX41 magnesium alloy, severely deformed using a combination of hot extrusion and equal channel angular pressing. Equal channel angular pressing processing was performed at 250°C following route Bc. Mechanical properties of the ultrafine-grained alloy were investigated in tension at a constant strain rate of 10^{-4} s^{-1} at room temperature and 100°C. The dislocation density was determined by X-ray line profiles analysis. Microstructural observations performed by electron backscattering diffraction after 8 passes of equal channel angular pressing revealed very fine and homogeneous microstructure with a grain size of 0.3-6 μm. It has been found that the room temperature mechanical properties such as yield stress and tensile strength reach their maximum value even after the first pass which is in good agreement with the evolution of the dislocation density. Further processing by equal channel angular pressing led to the decrease in both the yield strength and the dislocation density, despite the slight grain size refinement.
6
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Alpha Variant Selection Determined from Grain Misorientations in Ti-6Al-7Nb Alloy with a Duplex Microstructure

76%
Harcuba P., Václavová K., Čapek J., Stráský J., Janeček M., Janovská M.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
570-573
EN
Titanium occurs in two structures; a high temperature body-centered cubic structure which is known as β phase and an ambient temperature α phase which has the hexagonal closed-packed structure. In the present study a biomedical Ti-6Al-7Nb alloy was investigated. The so-called duplex structure consisting of α lamellae and equiaxed primary α-grains was prepared by a thermal treatment. The α lamellae are created during cooling from a β-field according to the Burgers relation. This relation allows the formation of the α lamellae with different crystallographic orientations - so-called variants. The preferential misorientation between α lamellae was studied by a detailed electron backscattered diffraction analysis. The misorientation of grains in the duplex structure was modelled by a sum of random Mackenzie distribution and Gaussian peaks related to the preferred misorientations according to the Burgers relation. The preferred misorientations based on the Burgers relationship were identified in the biomedical Ti-6Al-7Nb alloy with duplex structure. It is confirmed that the variant selection of α lamellae is not random.
7
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The Very High Cycle Fatigue Behaviour of Ti-6Al-4V Alloy

76%
Janeček M., Nový F., Harcuba P., Stráský J., Trško L., Mhaede M., Wagner L.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
497-502
EN
The high cycle and very high cycle fatigue properties of the titanium alloy Ti-6Al-4V with a duplex microstructure were investigated at room temperature. High cycle fatigue tests were performed in the range from 10⁴ to 10⁷ cycles by rotating bending at the frequency of 30 Hz. The very high cycle fatigue tests were carried out in the range from 10⁷ to 10¹⁰ cycles in tension-compression on an ultrasonic fatigue testing machine at the frequency of 20 kHz. The stress amplitude was found to decrease with increasing number of cycles in the whole range from 10⁴ up to 10⁹ cycles and only at the highest number of cycles (N_{F}=10⁹) the alloy exhibits the fatigue limit of 460 MPa. The detail fractographic analysis was performed to characterize the fatigue failure mechanisms. Both subsurface and surface crack initiation were observed in very high cycle fatigue region. No inclusions, but only local chemical inhomogeneity in microstructure was observed at the locations of subsurface fatigue crack initiation in alpha-grains.
8
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Phase Transformations in Ti-15Mo Investigated by in situ Electrical Resistance

76%
Zháňal P., Harcuba P., Šmilauerová J., Stráský J., Janeček M., Smola B., Hájek M.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
779-782
EN
In this study phase transformations in metastable beta Ti-15Mo alloy were investigated by an in situ electrical resistance measurement in a wide range of temperatures from -196°C to 850°C. Different temperature ranges of the evolution of electrical resistance were correlated with underlying phase transformations. In the low temperature range, stage I (from -196°C to 220°C) the decrease of electrical resistance with increasing temperature is caused by the dissolution of ω_{ath} (formed during quenching by athermal shuffle transformation) which is accompanied by the relaxation of lattice strain, while the diffusional assisted growth of ω_{iso} in the range from 220°C to 380°C (stage II) is the main mechanism causing the increase of resistance. Another decrease of the resistance in the range from 380°C to 550°C (stage III) is explained by the dissolution or transformation of ω_{iso}. The increase of resistance above 550°C (stage IV) is related to the growth of α-phase particles. The fully reversible character of ω_{ath} growth and dissolution during heating and cooling in the stage I up to 100°C was confirmed by temperature cycling during repeated in situ resistance runs from RT. Pre-ageing of samples at 300°C promotes the formation of ω_{iso} particles. Subsequently, ω_{ath} particles are not created, which is fully consistent with electrical resistance measurements. The presence of ω_{ath} and the orientation relationship between ω and β were identified by the electron diffraction.
9
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Interrelation of Microstructure and Corrosion Resistance in Biodegradable Magnesium Alloys with Aluminum, Lithium and Rare Earth Additions

64%
Minárik P., Landa M., Lesná I., Zemková M., Jablonská E., Hadzima B., Janeček M., Král R.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
491-496
EN
The influence of equal channel angular pressing and rotary swaging on the microstructure and corrosion resistance was investigated in three magnesium alloys with the addition of aluminum, lithium and rare earth elements - AE21, AE42 and LAE442. The processing resulted in grain refinement in all cases; nevertheless, the effect on the corrosion resistance was ambiguous. A continuous increase of the polarization resistance during the gradual equal channel angular pressing was observed in the AE42 and LAE442 alloys, whereas there was almost no effect in the AE21 alloy. The rotary swaging of AE42 resulted in a decrease of polarization resistance. The increase of polarization resistance in the alloys with the higher concentration of alloying elements was caused by the combined effect of grain refinement and better dispersion of particles in the matrix thanks to shear deformation during equal channel angular pressing. In the AE42 alloy, the increase of the corrosion resistance after equal channel angular pressing was also proven by chemical analysis of the solution and acoustic emission detection. This beneficial effect was not observed when the concentration of alloying elements was lower or when the deformation mode was different from shearing.
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