Robert Král | Charles University, Prague (original) (raw)

Papers by Robert Král

METAL Conference Proeedings, 2019

Materials Letters, 2019

Abstract Wet-milling of magnesium powder was performed at room temperature in ethanol covered by ... more Abstract Wet-milling of magnesium powder was performed at room temperature in ethanol covered by argon atmosphere. It is shown that the wet-milling in ethanol can produce highly deformed powder of flake-like morphology. Subsequently, bulk samples were consolidated by spark plasma sintering technique in the temperature range 400–550 °C. It is shown that milling caused severe microstructural refinement. The flake-like shape of the milled particles was transferred into the compacts and caused formation of a layer-like microstructure which was predominantly perpendicular to the external force applied during sintering. Grain growth was highly limited by powder particles’ interface and secondary phase particles present in the matrix. Formation of texture in the compacts was attributed to the combination of SPS thermo-mechanical effect and continuous static recrystallization acting during the sintering.

Metallurgical and Materials Transactions A, 2018

The microstructural and high-temperature mechanical properties of Fe-26Al-xNb (x = 3 and 5 at. pc... more The microstructural and high-temperature mechanical properties of Fe-26Al-xNb (x = 3 and 5 at. pct) are compared. The alloys were investigated ''as cast'' and after hot rolling at 1473 K (1200°C). Scanning electron microscopes equipped with EDS and EBSD were used for the microstructure and phase identification. The addition of 3 at. pct of Nb into the Fe 3 Al matrix leads to the formation of C14 k-Laves phase (Fe,Al) 2 Nb (LP) particles spread in the Fe 3 Al matrix, while an eutectic with thin lamellae of LP C14 k-Laves phase (Fe,Al) 2 Nb and matrix is also formed in the iron aluminide with 5 at. pct of Nb. The presence of incoherent precipitates is connected with the enhancement of the high-temperature strength and creep resistance.

Materials Letters, 2019

Abstract Magnesium alloy Mg-4Y-3Nd (WN43) was subjected to ECAP at T∼300 °C up to 8 passes, using... more Abstract Magnesium alloy Mg-4Y-3Nd (WN43) was subjected to ECAP at T∼300 °C up to 8 passes, using route BC. The initial as-extruded material with average grain size of ∼10 μm, was effectively refined down to sub-micrometer scale, reaching a grain size of ∼380 nm, after 8 passes. At the same time, secondary phase particle precipitation of β′-Mg12Nd2Y, and β-Mg14Nd2Y was induced. At 8 passes, fragmentation and coarsening of these phases occurred. Both grain refinement and β′/β precipitation strengthened the alloy. The strengthening was detected by nanoindentation hardness and accounted of up to 30% increment at 8 ECAP pass, compared to the initial extruded condition.

Corrosion Science, 2018

Corrosion behaviour of magnesium and WE43 magnesium alloy in 1 M NaCl aqueous solution was studie... more Corrosion behaviour of magnesium and WE43 magnesium alloy in 1 M NaCl aqueous solution was studied by potentiodynamic polarisation and advanced acoustic emission (AE) analyses. The AE parameters suitable for the monitoring of corrosion in magnesium were shown to be the AE counts, rise time, and duration. Moreover, the mechanisms of corrosion in these materials, i.e. the ion exchange, rupture of the protective film, and the pitting corrosion were identified and their evolution was followed by the novel Adaptive sequential k-means clustering technique. The presented methods can be also suitable for monitoring of different materials and for industrial applications.

International Journal of Electrochemical Science, 2016

The influence of Yb, Cr and Zr additives on electrochemical corrosion and stress cracking corrosi... more The influence of Yb, Cr and Zr additives on electrochemical corrosion and stress cracking corrosion behavior of Al-6.2Zn-2.3Mg-2.3Cu (mass fraction) alloys was investigated using open circuit potential, electrochemical impedance spectroscopy, cyclic polarization and U-type method. Better corrosion resistance of Al alloys was obtained through grain refinement effect by adding 0.16Zr and 0.16Zr-0.18Cr-0.3Yb. Both strength and corrosion resistance are inversely proportional to grain size, showing a Hall-Petch relation. The mechanism of grain refinement effect was revealed by TEM.

Materials Science Forum, 2016

The grain refinement after thermo-mechanical treatment (TMT) was investigated in AZ91, AE42, und ... more The grain refinement after thermo-mechanical treatment (TMT) was investigated in AZ91, AE42, und QE22 magnesium alloys. The optimal over-aging temperature was determined to be 300 °C in the case of AZ91 and AE42 alloys and 350 °C for QE22 alloy. After optimized TMT, the average grain sizes were 13.5 µm (AE42), 11.1 µm (AZ91) and 1.9 µm (QE22). The QE22 alloy exhibited the superior superplastic properties, with maximum elongation to failure 750 % and strain rate sensitivity parameter m=0.73. The Friction Stir Welding showed that the original base material grain structure of the alloys AZ31 and AZ91 replaced by ultrafine grains in the stir zone. The purpose of the present paper is to present the results of the grain refinement in magnesium alloys by thermo mechanical treatment and stir welding.

Physica Status Solidi (a), 1993

Materials Science and Engineering: A, 2007

The effect of deformation conditions on plastic deformation and acoustic emission (AE) in hot-rol... more The effect of deformation conditions on plastic deformation and acoustic emission (AE) in hot-rolled magnesium alloy AZ31 has been investigated in the temperature range of 20-200 • C by constant strain rate tensile tests. Two sets of samples differing in the preheating temperature before individual passes of hot rolling have been studied. Both the yield stress and the tensile strength decrease with increasing temperature of deformation. The ductility was found to increase significantly with increasing temperature of deformation in both specimens. Unstable plastic deformation (Portevin-Le Chatelier effect) has been observed for all used strain rates both at room and elevated temperatures. Plastic instabilities were accompanied by a pronounced AE activity. The AE bursts were correlated with the individual regions of plastic instabilities on the deformation curve. Mechanisms controlling plastic instabilities are suggested respecting the microstructure evolution as observed by optical and transmission electron microscopy.

Journal of Materials Science, 2010

Extruded AZ31 alloy was processed by equal channel angular pressing (ECAP) up to 12 passes at 180... more Extruded AZ31 alloy was processed by equal channel angular pressing (ECAP) up to 12 passes at 180 °C following route Bc, i.e. rotating the sample 90° between individual passes. Microstructure evolution was investigated using EBSD and TEM, as a function of strain imposed by ECAP. The first ECAP pass resulted in the formation of a new texture component which relates to

International Journal of Materials Research, 2009

The influence of equal channel angular pressing (ECAP) on the microstructure changes of magnesium... more The influence of equal channel angular pressing (ECAP) on the microstructure changes of magnesium alloy AZ31 was investigated. The microstructure changes were correlated with electrochemical characteristics of the surface. Eight passes of ECAP resulted in significant grain refinement (factor 100) of the initial squeeze-cast (SC) alloy. The influence of microstructure changes on electrochemical properties of the surface was evaluated using electrochemical impedance spectroscopy. The variation of electrochemical characteristics of the surface of the SC and ECAP alloy was determined after 3 and 7-day exposure in the corrosion solution of 0.1 M NaCl. The fine-grained deformed structure after ECAP was found to have significantly higher charge transfer resistance as compared to the squeeze-cast material.

Acta Materialia, 2010

Anisotropy of elastic properties of ultrafine-grained polycrystalline copper after one, two and f... more Anisotropy of elastic properties of ultrafine-grained polycrystalline copper after one, two and four passes of equal-channel angular pressing (ECAP) is investigated by means of ultrasonic methods. For each material, Young’s and shear moduli in the principal processing directions are evaluated and the symmetry and orientation of the anisotropy are identified. The relation between the determined symmetry and the processing mechanisms

Materials Science Forum, 2008

Technical purity Cu (99.95 wt%) polycrystals have been processed at room temperature by equal cha... more 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 ECA...

The magnesium alloy ZK60 received from Dead Sea Magnesium in direct-chill cast condition was extr... more The magnesium alloy ZK60 received from Dead Sea Magnesium in direct-chill cast condition was extruded at T = 350 ◦C using an extrusion ratio of ER = 12. The extruded bars were severely plastically deformed at 250 ◦C by either equal channel angular pressing or swaging. While swaging led to similar grain re nements 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 gure 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 ...

AE21 magnesium alloy was prepared by extrusion followed by equal channel angular pressing (EX ECA... more AE21 magnesium alloy was prepared by extrusion followed by equal channel angular pressing (EX ECAP). The microstructure evolution, the mechanical properties evolution and corrosion resistance evolution was studied. The microstructure was observed by transmission electron microscopy (TEM). The corrosion resistance was evaluated by electrochemical impedance spectroscopy (EIS). The thermomechanical processing by ECAP led to substantial grain refinement that affected both the mechanical and corrosion properties. The yield strength was enhanced by ECAP up to two passes (2P) t, while for higher number of passes it declined. The corrosion resistance decreased between 0-2P and then saturated. The microstructural changes between each pass resulted in the evolution of the mechanical and corrosion properties.

The e ect of thermal processing prior to severe plastic deformation by extrusion and swaging on g... more The e ect of thermal processing prior to severe plastic deformation by extrusion and swaging on grain re nement in ZK60 and AZ80 alloys was studied. The alloys received in direct-chill cast condition were homogenized at 415 ◦C and annealed (over-aged) at temperatures from 200 to 380 ◦C. Both the direct-hill cast and over-aged conditions were extruded at T = 300 ◦C using an extrusion ratio of ER = 19. Hot extrusion resulted in substantial grain re nement in both cases, the e ect being stronger in the over-aged material. The extruded bars were severe plastically deformed at 250 ◦C by swaging. Swaging re ned further the grain size depending on the deformation ratio (number of passes). At optimal processing conditions the grain size was 450 nm and 570 nm for the AZ80 and ZK60 alloys, respectively.

Journal of Magnesium and Alloys

Scripta Materialia

Abstract This letter presents a systematic study of serrated flow of the FeAlCrMo medium-entropy ... more Abstract This letter presents a systematic study of serrated flow of the FeAlCrMo medium-entropy alloy in compression, first observed at 400 °C and a strain rate of ~10−4 s−1. In this work, these parameters were varied in order to evaluate how they affect the deformation behavior. Compression curves together with the acoustic emission data suggest that dynamic strain aging (the Portevin-Le Châtelier effect) is responsible for the jerky flow. Furthermore, at certain conditions, peculiar periodic serrations appeared, which possessed bimodal distribution of drop magnitudes. In this multicomponent material, this effect likely originates in complex dislocation-solutes interactions at the atomic scale.

Metallurgical and Materials Transactions A

Eight iron aluminide alloys with different contents of V and Cr were prepared up to 25 at. pct of... more Eight iron aluminide alloys with different contents of V and Cr were prepared up to 25 at. pct of both elements. The effect of the V and Cr concentration on the microstructure and mechanical properties was investigated by several complementary techniques. This investigation revealed that the microstructure of all the investigated alloys was comparable regardless of their chemical composition. All the alloys were in a solid solution condition without any major chemical inhomogeneity. For all alloys, a comparable grain size and D03 crystallographic structure was observed. In situ X-ray diffraction measurement revealed that the crystallographic structure was stable up to 1073 K (800 °C) regardless of the chemical composition. Mechanical testing showed that the compressive yield stress significantly increased with the increasing total sum of V plus Cr. Much higher values of yield stress were measured for symmetric concentrations of V and Cr when compared to non-symmetric ones. Eventually, it was shown that the formation of at least a rough system of the lattice positions occupation by four types of atoms in four sub-lattices derived from D03 is the most probable strengthening factor for alloys with symmetric concentrations of V and Cr.

Acta Physica Polonica A

The eect of thermal processing prior to severe plastic deformation by extrusion and swaging on gr... more The eect of thermal processing prior to severe plastic deformation by extrusion and swaging on grain renement in ZK60 and AZ80 alloys was studied. The alloys received in direct-chill cast condition were homogenized at 415 • C and annealed (over-aged) at temperatures from 200 to 380 • C. Both the direct-hill cast and over-aged conditions were extruded at T = 300 • C using an extrusion ratio of ER = 19. Hot extrusion resulted in substantial grain renement in both cases, the eect being stronger in the over-aged material. The extruded bars were severe plastically deformed at 250 • C by swaging. Swaging rened further the grain size depending on the deformation ratio (number of passes). At optimal processing conditions the grain size was 450 nm and 570 nm for the AZ80 and ZK60 alloys, respectively.

METAL Conference Proeedings, 2019

Materials Letters, 2019

Abstract Wet-milling of magnesium powder was performed at room temperature in ethanol covered by ... more Abstract Wet-milling of magnesium powder was performed at room temperature in ethanol covered by argon atmosphere. It is shown that the wet-milling in ethanol can produce highly deformed powder of flake-like morphology. Subsequently, bulk samples were consolidated by spark plasma sintering technique in the temperature range 400–550 °C. It is shown that milling caused severe microstructural refinement. The flake-like shape of the milled particles was transferred into the compacts and caused formation of a layer-like microstructure which was predominantly perpendicular to the external force applied during sintering. Grain growth was highly limited by powder particles’ interface and secondary phase particles present in the matrix. Formation of texture in the compacts was attributed to the combination of SPS thermo-mechanical effect and continuous static recrystallization acting during the sintering.

Metallurgical and Materials Transactions A, 2018

The microstructural and high-temperature mechanical properties of Fe-26Al-xNb (x = 3 and 5 at. pc... more The microstructural and high-temperature mechanical properties of Fe-26Al-xNb (x = 3 and 5 at. pct) are compared. The alloys were investigated ''as cast'' and after hot rolling at 1473 K (1200°C). Scanning electron microscopes equipped with EDS and EBSD were used for the microstructure and phase identification. The addition of 3 at. pct of Nb into the Fe 3 Al matrix leads to the formation of C14 k-Laves phase (Fe,Al) 2 Nb (LP) particles spread in the Fe 3 Al matrix, while an eutectic with thin lamellae of LP C14 k-Laves phase (Fe,Al) 2 Nb and matrix is also formed in the iron aluminide with 5 at. pct of Nb. The presence of incoherent precipitates is connected with the enhancement of the high-temperature strength and creep resistance.

Materials Letters, 2019

Abstract Magnesium alloy Mg-4Y-3Nd (WN43) was subjected to ECAP at T∼300 °C up to 8 passes, using... more Abstract Magnesium alloy Mg-4Y-3Nd (WN43) was subjected to ECAP at T∼300 °C up to 8 passes, using route BC. The initial as-extruded material with average grain size of ∼10 μm, was effectively refined down to sub-micrometer scale, reaching a grain size of ∼380 nm, after 8 passes. At the same time, secondary phase particle precipitation of β′-Mg12Nd2Y, and β-Mg14Nd2Y was induced. At 8 passes, fragmentation and coarsening of these phases occurred. Both grain refinement and β′/β precipitation strengthened the alloy. The strengthening was detected by nanoindentation hardness and accounted of up to 30% increment at 8 ECAP pass, compared to the initial extruded condition.

Corrosion Science, 2018

Corrosion behaviour of magnesium and WE43 magnesium alloy in 1 M NaCl aqueous solution was studie... more Corrosion behaviour of magnesium and WE43 magnesium alloy in 1 M NaCl aqueous solution was studied by potentiodynamic polarisation and advanced acoustic emission (AE) analyses. The AE parameters suitable for the monitoring of corrosion in magnesium were shown to be the AE counts, rise time, and duration. Moreover, the mechanisms of corrosion in these materials, i.e. the ion exchange, rupture of the protective film, and the pitting corrosion were identified and their evolution was followed by the novel Adaptive sequential k-means clustering technique. The presented methods can be also suitable for monitoring of different materials and for industrial applications.

International Journal of Electrochemical Science, 2016

The influence of Yb, Cr and Zr additives on electrochemical corrosion and stress cracking corrosi... more The influence of Yb, Cr and Zr additives on electrochemical corrosion and stress cracking corrosion behavior of Al-6.2Zn-2.3Mg-2.3Cu (mass fraction) alloys was investigated using open circuit potential, electrochemical impedance spectroscopy, cyclic polarization and U-type method. Better corrosion resistance of Al alloys was obtained through grain refinement effect by adding 0.16Zr and 0.16Zr-0.18Cr-0.3Yb. Both strength and corrosion resistance are inversely proportional to grain size, showing a Hall-Petch relation. The mechanism of grain refinement effect was revealed by TEM.

Materials Science Forum, 2016

The grain refinement after thermo-mechanical treatment (TMT) was investigated in AZ91, AE42, und ... more The grain refinement after thermo-mechanical treatment (TMT) was investigated in AZ91, AE42, und QE22 magnesium alloys. The optimal over-aging temperature was determined to be 300 °C in the case of AZ91 and AE42 alloys and 350 °C for QE22 alloy. After optimized TMT, the average grain sizes were 13.5 µm (AE42), 11.1 µm (AZ91) and 1.9 µm (QE22). The QE22 alloy exhibited the superior superplastic properties, with maximum elongation to failure 750 % and strain rate sensitivity parameter m=0.73. The Friction Stir Welding showed that the original base material grain structure of the alloys AZ31 and AZ91 replaced by ultrafine grains in the stir zone. The purpose of the present paper is to present the results of the grain refinement in magnesium alloys by thermo mechanical treatment and stir welding.

Physica Status Solidi (a), 1993

Materials Science and Engineering: A, 2007

The effect of deformation conditions on plastic deformation and acoustic emission (AE) in hot-rol... more The effect of deformation conditions on plastic deformation and acoustic emission (AE) in hot-rolled magnesium alloy AZ31 has been investigated in the temperature range of 20-200 • C by constant strain rate tensile tests. Two sets of samples differing in the preheating temperature before individual passes of hot rolling have been studied. Both the yield stress and the tensile strength decrease with increasing temperature of deformation. The ductility was found to increase significantly with increasing temperature of deformation in both specimens. Unstable plastic deformation (Portevin-Le Chatelier effect) has been observed for all used strain rates both at room and elevated temperatures. Plastic instabilities were accompanied by a pronounced AE activity. The AE bursts were correlated with the individual regions of plastic instabilities on the deformation curve. Mechanisms controlling plastic instabilities are suggested respecting the microstructure evolution as observed by optical and transmission electron microscopy.

Journal of Materials Science, 2010

Extruded AZ31 alloy was processed by equal channel angular pressing (ECAP) up to 12 passes at 180... more Extruded AZ31 alloy was processed by equal channel angular pressing (ECAP) up to 12 passes at 180 °C following route Bc, i.e. rotating the sample 90° between individual passes. Microstructure evolution was investigated using EBSD and TEM, as a function of strain imposed by ECAP. The first ECAP pass resulted in the formation of a new texture component which relates to

International Journal of Materials Research, 2009

The influence of equal channel angular pressing (ECAP) on the microstructure changes of magnesium... more The influence of equal channel angular pressing (ECAP) on the microstructure changes of magnesium alloy AZ31 was investigated. The microstructure changes were correlated with electrochemical characteristics of the surface. Eight passes of ECAP resulted in significant grain refinement (factor 100) of the initial squeeze-cast (SC) alloy. The influence of microstructure changes on electrochemical properties of the surface was evaluated using electrochemical impedance spectroscopy. The variation of electrochemical characteristics of the surface of the SC and ECAP alloy was determined after 3 and 7-day exposure in the corrosion solution of 0.1 M NaCl. The fine-grained deformed structure after ECAP was found to have significantly higher charge transfer resistance as compared to the squeeze-cast material.

Acta Materialia, 2010

Anisotropy of elastic properties of ultrafine-grained polycrystalline copper after one, two and f... more Anisotropy of elastic properties of ultrafine-grained polycrystalline copper after one, two and four passes of equal-channel angular pressing (ECAP) is investigated by means of ultrasonic methods. For each material, Young’s and shear moduli in the principal processing directions are evaluated and the symmetry and orientation of the anisotropy are identified. The relation between the determined symmetry and the processing mechanisms

Materials Science Forum, 2008

Technical purity Cu (99.95 wt%) polycrystals have been processed at room temperature by equal cha... more 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 ECA...

The magnesium alloy ZK60 received from Dead Sea Magnesium in direct-chill cast condition was extr... more The magnesium alloy ZK60 received from Dead Sea Magnesium in direct-chill cast condition was extruded at T = 350 ◦C using an extrusion ratio of ER = 12. The extruded bars were severely plastically deformed at 250 ◦C by either equal channel angular pressing or swaging. While swaging led to similar grain re nements 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 gure 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 ...

AE21 magnesium alloy was prepared by extrusion followed by equal channel angular pressing (EX ECA... more AE21 magnesium alloy was prepared by extrusion followed by equal channel angular pressing (EX ECAP). The microstructure evolution, the mechanical properties evolution and corrosion resistance evolution was studied. The microstructure was observed by transmission electron microscopy (TEM). The corrosion resistance was evaluated by electrochemical impedance spectroscopy (EIS). The thermomechanical processing by ECAP led to substantial grain refinement that affected both the mechanical and corrosion properties. The yield strength was enhanced by ECAP up to two passes (2P) t, while for higher number of passes it declined. The corrosion resistance decreased between 0-2P and then saturated. The microstructural changes between each pass resulted in the evolution of the mechanical and corrosion properties.

The e ect of thermal processing prior to severe plastic deformation by extrusion and swaging on g... more The e ect of thermal processing prior to severe plastic deformation by extrusion and swaging on grain re nement in ZK60 and AZ80 alloys was studied. The alloys received in direct-chill cast condition were homogenized at 415 ◦C and annealed (over-aged) at temperatures from 200 to 380 ◦C. Both the direct-hill cast and over-aged conditions were extruded at T = 300 ◦C using an extrusion ratio of ER = 19. Hot extrusion resulted in substantial grain re nement in both cases, the e ect being stronger in the over-aged material. The extruded bars were severe plastically deformed at 250 ◦C by swaging. Swaging re ned further the grain size depending on the deformation ratio (number of passes). At optimal processing conditions the grain size was 450 nm and 570 nm for the AZ80 and ZK60 alloys, respectively.

Journal of Magnesium and Alloys

Scripta Materialia

Abstract This letter presents a systematic study of serrated flow of the FeAlCrMo medium-entropy ... more Abstract This letter presents a systematic study of serrated flow of the FeAlCrMo medium-entropy alloy in compression, first observed at 400 °C and a strain rate of ~10−4 s−1. In this work, these parameters were varied in order to evaluate how they affect the deformation behavior. Compression curves together with the acoustic emission data suggest that dynamic strain aging (the Portevin-Le Châtelier effect) is responsible for the jerky flow. Furthermore, at certain conditions, peculiar periodic serrations appeared, which possessed bimodal distribution of drop magnitudes. In this multicomponent material, this effect likely originates in complex dislocation-solutes interactions at the atomic scale.

Metallurgical and Materials Transactions A

Eight iron aluminide alloys with different contents of V and Cr were prepared up to 25 at. pct of... more Eight iron aluminide alloys with different contents of V and Cr were prepared up to 25 at. pct of both elements. The effect of the V and Cr concentration on the microstructure and mechanical properties was investigated by several complementary techniques. This investigation revealed that the microstructure of all the investigated alloys was comparable regardless of their chemical composition. All the alloys were in a solid solution condition without any major chemical inhomogeneity. For all alloys, a comparable grain size and D03 crystallographic structure was observed. In situ X-ray diffraction measurement revealed that the crystallographic structure was stable up to 1073 K (800 °C) regardless of the chemical composition. Mechanical testing showed that the compressive yield stress significantly increased with the increasing total sum of V plus Cr. Much higher values of yield stress were measured for symmetric concentrations of V and Cr when compared to non-symmetric ones. Eventually, it was shown that the formation of at least a rough system of the lattice positions occupation by four types of atoms in four sub-lattices derived from D03 is the most probable strengthening factor for alloys with symmetric concentrations of V and Cr.

Acta Physica Polonica A

The eect of thermal processing prior to severe plastic deformation by extrusion and swaging on gr... more The eect of thermal processing prior to severe plastic deformation by extrusion and swaging on grain renement in ZK60 and AZ80 alloys was studied. The alloys received in direct-chill cast condition were homogenized at 415 • C and annealed (over-aged) at temperatures from 200 to 380 • C. Both the direct-hill cast and over-aged conditions were extruded at T = 300 • C using an extrusion ratio of ER = 19. Hot extrusion resulted in substantial grain renement in both cases, the eect being stronger in the over-aged material. The extruded bars were severe plastically deformed at 250 • C by swaging. Swaging rened further the grain size depending on the deformation ratio (number of passes). At optimal processing conditions the grain size was 450 nm and 570 nm for the AZ80 and ZK60 alloys, respectively.