Jiri Zyka - Academia.edu (original) (raw)

Papers by Jiri Zyka

Procedia structural integrity, 2019

Nickel-based superalloys have already found many different applications in industrial facilities,... more Nickel-based superalloys have already found many different applications in industrial facilities, especially those working at high temperatures, such as turbine blades in industrial and aircraft turbines, but also in the glass industry to construct glass shaping tools. During service, such tools are exposed to high temperature, high pressures and high-temperature corrosion in a severe corrosive medium like molten glass and thereby accentuate more exacting demands on high creep and corrosion resistance of nickel-based superalloys used in the glass industry. At present, the published data on microstructural changes and degradation processes in nickel-based superalloys used in the glass industry are sparse and often controversial. Therefore, the present study was initiated to perform a detailed evaluation of high temperature microstructural processes and damage in a carbide strengthened nickel-based superalloy which has been developed for an investment casting of spinner discs used in the glass industry for producing glass fibers. In this study, the relationship between the type, distribution and morphology of carbides, and the deformation and degradation processes in an alloy under investigation subjected to high temperature exposition was investigated. The paper aims to identify decisive factors governing high-temperature performance of the nickel-based superalloy under investigation.

Materials Science Forum, Apr 1, 2014

The aim of this work is to study the effect of aggregate type on the physico-mechanical propertie... more The aim of this work is to study the effect of aggregate type on the physico-mechanical properties and microstructure of bio-mortar (BM). Three different aggregates such as sand, dolomite and basalt were used. BM was prepared by mixing aggregates with bacterial cells (Sporosarcina Pasteurii) and one equimolar (1 M) of urea/CaCl 2 .2H 2 O. The results proved that the chemical composition and physical properties of aggregates play an important role in the microbial precipitation rate as well as size, morphology and crystallinity of the precipitated calcite, which strongly reflects on the properties of the prepared BM. The BM containing dolomite gave the highest compressive strength and lowest water absorption.

Metals, Dec 7, 2019

High entropy alloys (HEAs) have attracted researchers' interest in recent years. The aim of this ... more High entropy alloys (HEAs) have attracted researchers' interest in recent years. The aim of this work was to prepare the HfNbTaTiZr high entropy alloy via the powder metallurgy process and characterize its properties. The powder metallurgy process is a prospective solution for the synthesis of various alloys and has several advantages over arc melting (e.g., no dendritic structure, near net-shape, etc.). Cold isostatic pressing of blended elemental powders and subsequent sintering at 1400 • C for various time periods up to 64 h was used. Certain residual porosity, as well as bcc2 (Nb-and Ta-rich) and hcp (Zr-and Hf-rich) phases, remained in the bcc microstructure after sintering. The bcc2 phase was completely eliminated during annealing (1200 • C/1h) and subsequent water quenching. The hardness values of the sintered specimens ranged from 300 to 400 HV10. The grain coarsening during sintering was significantly limited and the maximum average grain diameter after 64 h of sintering was approximately 60 µm. The compression strength at 800 • C was 370 MPa and decreased to 47 MPa at 1200 • C. Porosity can be removed during the hot deformation process, leading to an increase in hardness to~450 HV10.

Manufacturing Technology, Oct 1, 2016

The objective of this work was to investigate and evaluate the effect of the composition and prod... more The objective of this work was to investigate and evaluate the effect of the composition and production on microstructure and tribological properties of cobalt alloys. The referece material was Co-28Cr-6Mo alloy, which is successfully used in the medicine for many years. The excellent corrosion resistance is the advantage, as well as better wear resistance than is offered by titanium or stainless steel implants. Standard Co-Cr-Mo alloy and also Co-Cr-Mo alloy with Ti in an amount 5 wt. %, were prepared by casting and also mechanical alloying followed by "Spark Plasma Sintering" consolidation. The influence of production route as well as influence of alloying elements on the microstructure and tribological properties was observed. Based on the obtained results, the Co-Cr-Mo-Ti alloy produced by casting seems to be most suitable, because the addition of titanium has greatly improved the wear resistance.

Manufacturing Technology, Dec 1, 2016

The aim of this work was studying the effect of the composition and production on mechanical and ... more The aim of this work was studying the effect of the composition and production on mechanical and tribological properties of cobalt alloys. Co-Cr-Mo alloy began to be used for manufacturing dental implants, but nowadays they are successfully used for the production of orthopaedic implants thanks to better wear resistance than is offered by titanium implants. However, there is still ongoing effort to even more improve the wear resistance and other properties of cobalt based alloy. The aim of this research was to find a suitable combination between composition and production, which would increase the wear resistance, keeping the other mechanical properties at least at the same level. Standard Co-Cr-Mo alloy and also Co-Cr-Mo alloy with various alloying elements, specifically Nb, Ti in an amount 5 wt%, were prepared by casting and also mechanical alloying followed by compacting method "Spark Plasma Sintering". The influence of production route as well as influence of alloying elements on the microstructure, mechanical and tribological properties were observed. Based on the obtained results, the Co-Cr-Mo-Ti alloy production by casting seems to be most suitable, because the addition of titanium has greatly improved the wear resistance. However, it is necessary to perform many other tests, especially tests of corrosion resistance and biocompatibility.

Acta Polytechnica CTU Proceedings, Jun 11, 2020

High entropy alloy HfNbTaTiZr in as cast conditions and after high pressure torsion straining was... more High entropy alloy HfNbTaTiZr in as cast conditions and after high pressure torsion straining was characterized by nanoindentation. The length-scale dependent material response (indentation size effect) was characterized by indentation at various indentation depths. Hardness dependence on the characteristic length (depth of penetration) indicated decomposition of disordered high entropy alloy in the as cast sample, which probably occurred during slow cooling after casting. Subsequent severe plastic deformation by high pressure torsion led on the other hand to the short-range disorder of (originally partially decomposed as cast) structure. Further hardening was generated during high pressure torsion by the mechanisms of grain refinement and increasing dislocation density.

Entropy, Jan 26, 2019

Refractory high entropy alloys (HEA) are promising materials for high temperature applications. T... more Refractory high entropy alloys (HEA) are promising materials for high temperature applications. This work presents investigations of the room temperature tensile mechanical properties of selected 3 and 4 elements medium entropy alloys (MEA) derived from the HfNbTaTiZr system. Tensile testing was combined with fractographic and microstructure analysis, using scanning electron microscope (SEM), wavelength dispersive spectroscope (WDS) and X-Ray powder diffraction (XRD). The 5 element HEA alloy HfNbTaTiZr exhibits the best combination of strength and elongation while 4 and 3 element MEAs have lower strength. Some of them are ductile, some of them brittle, depending on microstructure. Simultaneous presence of Ta and Zr in the alloy resulted in a significant reduction of ductility caused by reduction of the BCC phase content. Precipitation of Ta rich particles on grain boundaries reduces further the maximum elongation to failure down to zero values.

Journal of Alloys and Compounds, Nov 1, 2018

Refractory metal high entropy alloy HfNbTaTiZr with ultrafine grained structure and grain size of... more Refractory metal high entropy alloy HfNbTaTiZr with ultrafine grained structure and grain size of z80 nm was processed by high pressure torsion. The development of microstructure, lattice defects and mechanical properties with increasing strain was examined. Grain refinement of HfNbTaTiZr alloy deformed up to the equivalent strain e z 50 resulted in a significant enhancement of strength while keeping sufficient ductility. However, further straining e > 100 led to a decrease of strength and the loss of ductility due to the decomposition of solid solution facilitated by vacancies introduced by severe plastic deformation.

IOP conference series, Feb 1, 2017

The new Al15Ti5Co35Ni25Fe20 high entropy alloy was obtained by the powder metallurgy from the ele... more The new Al15Ti5Co35Ni25Fe20 high entropy alloy was obtained by the powder metallurgy from the elemental powders (-325 mesh). The obtained alloy should be characterized by the dual BCC + FCC phase microstructure according to the valence electron concentration calculations (VEC=7.9). The main aim of the research was to determine the optimal conditions for sintering and heat treatment of Al15Ti5Co35Ni25Fe20 high entropy alloy. The alloy was sintered for 40 minutes at 600 °C for the preliminary consolidation of the powders and initial diffusion of the material. Then samples were annealed at various temperatures: 700, 800, 900, 1000 °C for 1 hour. Samples after annealing were characterized by the XRD and SEM. For the chosen temperature the annealing time was elongated to 2, 3, 5, 10 and 20 hours. The chemical homogeneity was determined.

MATEC web of conferences, 2014

When approving large castings for use, mechanical values are measured by room temperature tensile... more When approving large castings for use, mechanical values are measured by room temperature tensile test on test bodies made from the centres of the castings. These test bodies often exhibit lower values than required, in particular ultimate tensile strength and elongation. The presence of fragile niobium carbides in the structure of the alloy was identified as the main cause of this situation, which originated as a result of the slow speed cooling. Because of large grain size compared to gauge length diameter, crystal orientation of individual grains also plays an important role. Interdendritic carbides are oriented parallel with dendrites thus parallel with <100> directions in Ni superalloy crystal lattice. Grain oriented with <100> axis parallel to tensile direction has platelet carbides oriented perpendicular to the tensile axis. In that position carbides crack easily at low grain deformation. Longitudinal metallographic cuts of selected tested specimen gauge lengths were prepared and grain orientation of individual grains was investigated by EBSD, Specimens, where grain oriented with <100> axis parallel to tensile direction were found, performed poor elongation values 3%, compared to 7% where no such oriented grains were found.

Key Engineering Materials, 2018

Two cast NbC and TaC- strengthened cobalt-base superalloys have been developed for a precision ca... more Two cast NbC and TaC- strengthened cobalt-base superalloys have been developed for a precision casting of spinner discs for glass wool industry. In the present study, the relationships between the type and morphology of carbides and the degradation processes in both types of cast cobalt-based superalloys subjected to high temperature creep have been examined. It was found that the nature of carbides within the alloy microstructure plays a critical role in determining the creep damage processes and microstructure stability of the alloy system under high temperature creep. The morphology of the carbides is a strong function of their chemical composition. The interface decohesion between the complex carbides and the matrix and cracking of the brittle carbides homogeneously distributed in the crept NbC - strengthened alloy lead to brittle intergranular and/or interdendritic fracture. By contrast, Ta - strengthened alloy exhibited very small extent of isolated creep damage and the final ...

Acta Physica Polonica A, 2020

The properties of positrons delocalized in the HfNbTaTiZr complex concentrated alloy are investig... more The properties of positrons delocalized in the HfNbTaTiZr complex concentrated alloy are investigated theoretically and related to available experimental data. Positron lifetimes calculated for random atomic arrangements appear to be short compared to the experiment carried out on a vacancy-free sample. Contrarily, a configuration exhibiting a local atomic order provides a lifetime that is very close to the experimental value. These observations indicate that the local atomic order is occurring in the studied alloy, in agreement with other measurements. The analysis of calculated results shows that the positron lifetime and affinity are affected to some extent by the alloy substitutional disorder, lattice relaxations, and charge transfer among atomic species, which should be taken into account in precise calculations.

Materials Characterization, 2018

Degradation processes in high-temperature creep of two cast high-chromium cobalt-based superalloy... more Degradation processes in high-temperature creep of two cast high-chromium cobalt-based superalloys strengthened by niobium and tantalum for use in the glass industry were analysed via various experimental techniques. Constant load creep tests were conducted at 900, 950 and 1000°C in a tensile stress range from 40 to 80 MPa. It was found that the CoNb superalloy possesses considerably longer creep life compared to the CoTa superalloy under the same loading conditions. Conversely, the creep ductility of the fractured specimens shows the opposite order of the creep life. The creep behaviour of both superalloys obeys the Monkman-Grant formula indicating that the creep deformation mechanism and fracture processes are mutually interlinked. The homogeneously distributed creep damage of the CoNb superalloy is closely connected with primary carbides and is predominantly initiated either as interface decohesion between the carbide/matrix and carbide eutectics/matrix or by breakage of bulk M 23 C 6 carbides. The dominant type of creep damage in the CoTa superalloy is localized breakage of M 23 C 6 carbides in close proximity of the fracture path. The final brittle fracture in the CoNb superalloy occurs via relatively fast propagation of the longest cracks after the ultimate state of damage is reached. Due to premature fracture, the inherent creep ductility of the CoNb matrix is not exhausted. The final ductile transgranular creep fracture of the CoTa superalloy is caused by a local strain-induced instability of the dislocation microstructure leading to a loss of an external section of specimen (necking).

Acta Physica Polonica A, 2018

High entropy alloy HfNbTaTiZr was successfully processed by severe plastic deformation using high... more High entropy alloy HfNbTaTiZr was successfully processed by severe plastic deformation using high pressure torsion (HPT) and ultrafine grained microstructure was achieved. The microstructure of HPT-deformed HfNbTa-TiZr alloy was characterized by X-ray diffraction and compared with conventionally cast ingots. The lattice defects introduced by HPT processing were characterized by positron annihilation spectroscopy. The X-ray diffraction profiles of HTP-deformed samples were extremely broadened due to small sizes of coherently diffracting domains and a high microstrain introduced by severe plastic deformation.

$Research paper thumbnail of Alloy design for intrinsically ductile refractory high-entropy alloys$

Journal of Applied Physics, 2016

Refractory high-entropy alloys (RHEAs), comprising group IV (Ti, Zr, Hf), V (V, Nb, Ta), and VI (... more Refractory high-entropy alloys (RHEAs), comprising group IV (Ti, Zr, Hf), V (V, Nb, Ta), and VI (Cr, Mo, W) refractory elements, can be potentially new generation high-temperature materials. However, most existing RHEAs lack room-temperature ductility, similar to conventional refractory metals and alloys. Here, we propose an alloy design strategy to intrinsically ductilize RHEAs based on the electron theory and more specifically to decrease the number of valence electrons through controlled alloying. A new ductile RHEA, Hf0.5Nb0.5Ta0.5Ti1.5Zr, was developed as a proof of concept, with a fracture stress of close to 1 GPa and an elongation of near 20%. The findings here will shed light on the development of ductile RHEAs for ultrahigh-temperature applications in aerospace and power-generation industries.

IN 792-5A, a variant within the IN 792 alloy series, is a high-temperature cast nickel alloy stre... more IN 792-5A, a variant within the IN 792 alloy series, is a high-temperature cast nickel alloy strengthened by the presence of precipitates of the γ’ phase and carbides. This alloy is currently used, for instance, as the material of cast blades for jet engines. The companies UJP PRAHA a.s and PBS Velká Bíteš a.s. have joined their research capacities in a project aimed to identify the properties of the alloy with focus on its heat treatment. Within the project, creep tests of the IN792-5A alloy as cast and following three-step heat treatment were performed in cooperation with the Institute of Physics of Materials, Academy of Sciences of the Czech Republic. Such heat treatment was found to extend appreciably the alloy’s creep life. In the as-cast state the alloy fails to meet the requirements for its lifetime. The present contribution discusses the causes of the differences in the creep behaviour of the IN792-5A alloy between the two structure states. The structure of the alloy and the...

Materials, 2019

High entropy alloys (HEA) have been one of the most attractive groups of materials for researcher... more High entropy alloys (HEA) have been one of the most attractive groups of materials for researchers in the last several years. Since HEAs are potential candidates for many (e.g., refractory, cryogenic, medical) applications, their properties are studied intensively. The most frequent method of HEA synthesis is arc or induction melting. Powder metallurgy is a perspective technique of alloy synthesis and therefore in this work the possibilities of synthesis of HfNbTaTiZr HEA from powders were studied. Blended elemental powders were sintered, hot isostatically pressed, and subsequently swaged using a special technique of swaging where the sample is enveloped by a titanium alloy. This method does not result in a full density alloy due to cracking during swaging. Spark plasma sintering (SPS) of mechanically alloyed powders resulted in a fully dense but brittle specimen. The most promising result was obtained by SPS treatment of gas atomized powder with low oxygen content. The microstructu...

Journal of Materials Research, 2018

A homogeneous HfNbTaTiZr high-entropy alloy was successfully processed via powder metallurgy rout... more A homogeneous HfNbTaTiZr high-entropy alloy was successfully processed via powder metallurgy route. For the initial powder feedstock material fabrication, the electrode induction-melting gas atomization procedure was used, resulting in a spherical powder morphology and dual bcc phase composition distinguishable within the individual particles. Spark plasma sintering was then used for the powder compaction at sintering temperatures ranging from 800 to 1600 °C. By the characterization of the compact microstructures, lattice defects (microscopic porosity and vacancy-like misfit defects), and mechanical properties (hardness and three-point bending strength), the sintering conditions were optimized to obtain a fully dense, homogeneous, single-phase bcc material. It was found that such properties are achieved when sintering at 80 MPa pressure for 2 min at temperatures above 1200 °C, where the single bcc phase structure exhibited ductile behavior with considerable flexural strength and ductility at ambient temperature. Positron annihilation spectroscopy was used to characterize the evolution of atomic and mesoscale defects during optimization of the sintering process.

a UJP PRAHA a.s., Nad Kamínkou 1345, 15600, Praha 5 – Zbraslav, zyka@ujp.cz b PBS Velká Bíteš a.s... more a UJP PRAHA a.s., Nad Kamínkou 1345, 15600, Praha 5 – Zbraslav, zyka@ujp.cz b PBS Velká Bíteš a.s, Vlkovská 279, 595 12 Velká Bíteš, hrbacek.karel@pbsvb.cz c Ústav fyziky materiálů AV ČR, v.v.i., Žižkova 513/22, 616 00 Brno-Veveří, sklen@ipm.cz Abstrakt IN 792-5A (varianta slitiny řady IN 792) je niklová slévárenská žárupevná slitina vytvrzená fází γá karbidy. V současné době se používá například pro odlévání lopatek tryskového motoru. V posledních letech probíhá ve spolupráci UJP PRAHA a.s a PBS Velká Bíteš a.s. a výzkum vlastností této slitiny se zaměřením na rozbor tepelného zpracování. V rámci tohoto úkolu byly ve spolupráci s ÚFM AV ČR provedeny zkoušky odolnosti proti tečení slitiny IN792-5A ve stavu po odlití a ve stavu po třístupňovém tepelném zpracování. Bylo zjištěno, že toto tepelné zpracování vede k podstatnému zvýšení životnosti při tečení. Životnost slitiny ve stavu po odlití nesplňuje kladené požadavky. Příspěvek se zabývá rozborem příčin takto rozdílného creepového c...