Showing papers by "Miroslav Karlík published in 2019"

Influence of Heat Treatment on Microstructure and Properties of NiTi46 Alloy Consolidated by Spark Plasma Sintering.

Abstract: Ni-Ti alloys are considered to be very important shape memory alloys with a wide application area including, e.g., biomaterials, actuators, couplings, and components in automotive, aerospace, and robotics industries. In this study, the NiTi46 (wt.%) alloy was prepared by a combination of self-propagating high-temperature synthesis, milling, and spark plasma sintering consolidation at three various temperatures. The compacted samples were subsequently heat-treated at temperatures between 400 °C and 900 °C with the following quenching in water or slow cooling in a closed furnace. The influence of the consolidation temperature and regime of heat treatment on the microstructure, mechanical properties, and temperatures of phase transformation was evaluated. The results demonstrate the brittle behaviour of the samples directly after spark plasma sintering at all temperatures by the compressive test and no transformation temperatures at differential scanning calorimetry curves. The biggest improvement of mechanical properties, which was mainly a ductility enhancement, was achieved by heat treatment at 700 °C. Slow cooling has to be recommended in order to obtain the shape memory properties.

Suppression of twinning mechanism on nanoscale: size effect in Cu–Ni–Al shape memory alloy

Abstract: In Cu–Ni–Al shape memory alloy, we observed a significant size effect on the twinning stress, i.e. the dependency of compression stress needed for twin-variant reorientation on sample size using in situ loading of micro- and nanoscale pillars in scanning and transmission electron microscopes. With decreasing dimensions of pillars, the twinning stress sharply increases following scaling power law with an exponent approximately n = − 2/3. For very small nanopillars, the projected twinning stress is so high that the nanopillars are deformed by plastic deformation instead of twinning. Our results shed light on some of the fundamental aspects of nanoscale behaviour of shape memory alloys which is important for applications in microelectromechanical systems.

Application of SPS consolidation and its influence on the properties of the FeAl20Si20 alloys prepared by mechanical alloying

Abstract: The FeAl20Si20 alloy was prepared by a combination of short-term mechanical alloying and spark plasma sintering. The processing parameters either of the mechanical alloying or spark plasma sintering were optimized to yield the maximal mechanical properties of the alloy. For the mechanical alloying, two amounts of powders batch (5 g or 20 g) were compared. The spark plasma sintering regimes combined pre-pressing prior heating and vice versa, direct and pulse current flow. The MA + SPS alloys exhibited ultrafine-grained microstructure composed of FeSi, Fe3Si and Fe3Al2Si3 phases (with an average crystallite size of approximately 30 nm) with a presence of randomly distributed Al2O3 particles (with diameters ranging from 5 to 100 nm). The FeSi, Fe3Si phases were supersaturated by Al, which resulted in an increase of lattice parameters. The hardness of the compact alloys reached up to approximately 1100 HV 0.1 for both the powder batches. The 20 g samples showed a standard deviation nearly half the of 5 g powder batches and the 20 g prepared by a regime combining pre-pressing prior heating up to consolidation temperature using pulse current flow resulted in the highest compressive strength of 2008 MPa. Combination of pre-pressing prior heating-up also reduced the increase of the Fe3Al2Si3 phase weight fraction especially in the 5 g alloys that otherwise had a tendency to microstructural coarsening.

Mechanical properties of FeAlSi powders prepared by mechanical alloying from different initial feedstock materials

Abstract: FeAl20Si20 (wt.%) powders prepared by mechanical alloying from different initial feedstock materials (Fe, Al, Si, FeAl27) were investigated in this study. Scanning electron microscopy, X-ray diffraction and nanoindentation techniques were used to analyze microstructure, phase composition and mechanical properties (hardness and Young’s modulus). Finite element model was developed to account for the decrease in measured values of mechanical properties of powder particles with increasing penetration depth caused by surrounding soft resin used for embedding powder particles. Progressive homogenization of the powders’ microstructure and an increase of hardness and Young’s modulus with milling time were observed and the time for complete homogenization was estimated.

Effect of Initial Powders on Properties of FeAlSi Intermetallics.

Abstract: FeAlSi intermetallics are materials with promising high-temperature mechanical properties and oxidation resistance. Nevertheless, their production by standard metallurgical processes is complicated. In this study, preparation of powders by mechanical alloying and properties of the samples compacted by spark plasma sintering was studied. Various initial feedstock materials were mixed to prepare the material with the same chemical composition. Time of mechanical alloying leading to complete homogenization of powders was estimated based on the microstructure observations, results of XRD and indentation tests. Microstructure, phase composition, hardness and fracture toughness of sintered samples was studied and compared with the properties of powders before the sintering process. It was found that independently of initial feedstock powder, the resulting phase composition was the same (Fe3Si + FeSi). The combination of hard initial powders required the longest milling time, but it led to the highest values of fracture toughness.

Mechanical Alloying of Fe-Al-Si Intermetallic Compounds Using Pure and Pre-Alloyed Powders

Abstract: In this paper, ternary FeAl20Si20 (wt.%) intermetallic alloy was prepared by mechanical alloying from different feedstock materials. Various combinations of elemental as well as pre-alloyed feedstock powders were milled in hard-soft, soft-hard and hard-hard initial mixing conditions with the aim to optimize the mechanical alloying process especially speed-up the processing time. The microstructure, phase composition and mechanical properties after different time of mechanical alloying were characterized. The effect of using the pre-alloyed powders on kinetics of mechanical alloying and final phase composition is compared with the results obtained on batches prepared from elemental powders. Using of appropriate pre-alloyed powders can slightly speed-up mechanical alloying process, nevertheless it has no significant effect on final microstructure, phase composition and mechanical properties.

Technological Aspects of Fe-Al-Si Intermetallic Alloy Preparation by Mechanical Alloying

Abstract: In this paper, recently developed ternary FeAl20Si20 (wt.%) alloy with promising high-temperature oxidation and wear resistance was prepared by mechanical alloying in a high-energy ball mill. The possibility to speed-up the mechanical alloying process by replacing aluminium (and partly silicon) elemental powder by the pre-alloyed powder (AlSi30) with relatively fine dispersion of Si in the Al-Si eutectic was examined. The microstructure, phase composition and mechanical properties after various time of mechanical alloying were characterized. The effect of using the pre-alloyed powders on kinetics of mechanical alloying is compared with the results obtained on batches prepared from elemental powders.

Characterization of mechanically alloyed FeAlSi intermetallic powders

Abstract: Jaroslav Čech, Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Czech Republic jaroslav.cech@fjfi.cvut.cz Petr Haušild, Czech Technical University in Prague, FNSPE, Dept. of Materials, CZE Miroslav Karlík, Czech Technical University in Prague, FNSPE, Dept. of Materials, CZE Filip Průša, University of Chemistry and Technology Prague, Dept. of Metals and Corrosion Engineering, CZE Pavel Novák, University of Chemistry and Technology Prague, Dept. of Metals and Corrosion Engineering, CZE Jaromír Kopeček, Institute of Physics, ASCR, v.v.i., Department of Functional Materials, CZE