Ústav materiálových věd a inženýrství

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Backstress shift modelling concept for improving uniaxial ratcheting predictions for wrought 304 stainless steel and additively manufactured Inconel 718
(Elsevier, 2025-11-01) Adamec, Tomáš; Hassan, Tasnim; Zapletal, Josef; Kondepati, Sudhir Kumar; Šebek, František
The Chaboche model is one of the widely used models, but it still shows limitations in predicting various complex responses. For example, issues in predicting ratcheting responses of metals and alloys under stress-controlled loading, especially under uniaxial cyclic loading, have been demonstrated. Therefore, this study evaluates the performance of the Chaboche model under uniaxial cyclic loading with an emphasis given to the simulation of uniaxial ratcheting responses. A modification to the model is proposed to enhance its prediction of the uniaxial ratcheting response for a wide range of ratcheting rates. The modification technique is called the backstress shift model, developed on the basis of experimental observations of the similarity between the strain- and stress-controlled hysteresis loops. A backstress memory surface is introduced and its material parameters are calibrated using responses of stainless steel 304 and Inconel 718 superalloy. For this study, data for steel are collected from the literature and experiments are performed on superalloy to acquire a set of data for development and validation of the proposed model. The modified model demonstrates better predictability of the uniaxial ratcheting responses compared to the Chaboche model with the threshold, especially for the additively manufactured nickel-based superalloy. The modified model also works well for the wrought 304 stainless steel.
Erosion development in AISI 316L stainless steel under pulsating water jet treatment
(Elsevier, 2024-01-26) Hloch, Sergej; Poloprudský, Jakub; Šiška, Filip; Babinský, Tomáš; NAGH, Akash; Chlupová, Alice; Kruml, Tomáš
Erosion of solids by liquid droplets is a phenomenon which is a compromise between mechanical properties of the material and droplet hydrodynamic parameters. While a number of studies deal with the deformation of drops, the deformation evolution inside the material has not yet been revealed, mainly from the point of view of the time action of the impinging drops The mechanical response of AISI 316L was investigated under gradually increasing numbers of impingements of liquid droplets, with a droplet volume of Vd approximately equal to 0.9 mm3, generated by an ultrasonic pulsating water jet with the frequency f = 40 kHz from 1 to 20 s. The surface roughness and the wear rates were determined using a laser profilometer. The cross-section of the selected samples was subjected to microhardness measurement with a load of 0.150 N in a 2D grid, which included the entire perimeter of the deformed area. The minimal microhardness measurement grid under the groove had dimensions of 15 x 15 indents, equal to an area of approximately 450 x 600 mu m. A maximum hardness increase was observed at the lowest measured depth of 30 mu m. An increase in hardness was observed at 300 mu m below the surface. The hardening in the deeper subsurface area was most likely caused by shear stress. This shows the high degree of similitude between the solid and liquid droplet impingements. The results indicate that the currently accepted theory on the development of erosion over time has shortcomings, as demonstrated in this work by the ratio between the utilised droplet diameter and the grain size of the material.
Superflux of an organic adlayer towards its local reactive immobilization
(NATURE PORTFOLIO, 2023-10-18) Salamon, David; Bukvišová, Kristýna; Jan, Vít; Potoček, Michal; Čechal, Jan
On-surface mass transport is the key process determining the kinetics and dynamics of on-surface reactions, including the formation of nanostructures, catalysis, or surface cleaning. Volatile organic compounds (VOC) localized on a majority of surfaces dramatically change their properties and act as reactants in many surface reactions. However, the fundamental question "How far and how fast can the molecules travel on the surface to react?" remains open. Here we show that isoprene, the natural VOC, can travel similar to 1 mu m s(-1), i.e., centimeters per day, quickly filling low-concentration areas if they become locally depleted. We show that VOC have high surface adhesion on ceramic surfaces and simultaneously high mobility providing a steady flow of resource material for focused electron beam synthesis, which is applicable also on rough or porous surfaces. Our work established the mass transport of reactants on solid surfaces and explored a route for nanofabrication using the natural VOC layer.
Role of In-flight Temperature and Velocity of Powder Particles on Plasma Sprayed Hydroxyapatite Coating Characteristics
(Elsevier, 2012-01-15) Čížek, Jan; Khor, Khiam Aik
This paper presents a systematic research on the process of thermal spraying of HA encompassing all stages of layer deposition: powder production and characterization (optimized production led to spherical 29.29 - 50.51 um powder with 0.0% content of tri-calcium phosphate [TCP] or tetra-calcium phosphate [TTCP] phases), plasma jet properties influence on the in-flight powder properties (major influence of spray distance factor), the influence of the in-flight temperature and velocity of hydroxyapatite powder particles (ranges of 2294 K–2708 K and 152 m/s – 291 m/s) on the final characteristics of the deposited coatings. Six combinations of the system parameters leading the low-medium-high in-flight properties were selected and the respective coatings were investigated, with some data in contradiction to previously published results. It was found that the temperature of the particles plays a critical role for the development of detrimental CaO (content increase of up to 14.6%) and metastable TTCP (up to 49.5%) phases in the coatings. Particle in-flight velocity was found to influence the open porosity of the coatings (8.8%–27.9%) and has an impact on the micro-hardness and moduli of the coatings (relative differences of up to 2.8 times). Joint incidence of both in-flight properties was found to significantly influence the microstructure of the coatings and its respective surface roughness levels (Ra = 7.4–19.4 um). The morphology of impinging splats was found to be influenced by both the in-flight velocity (dominant factor) and temperature (secondary factor) of the HA particles.
Effects of annealing temperature on microstructure and mechanical properties of cold sprayed AA7075
(Elsevier, 2023-01-01) Judas, Jakub; Zapletal, Josef; Řehořek, Lukáš; Jan, Vít
This paper examines the softening behaviour of cold sprayed 7075 aluminum alloy after isothermal annealing. The as-received powder was deposited by a high-pressure cold spray device using heated nitrogen as propellant gas. To investigate the effects of post-deposition heat treatment, the excised samples were isochronally annealed in the temperature range of 200 to 400 °C. The feedstock powder and the free-standing coatings were initially characterized through various electron microscopy techniques (SEM, EDS, EBSD) and relevant microstructural features were determined. Mechanical properties of the as-sprayed and heat-treated samples were evaluated by quasi-static tensile and microhardness testing. It has been shown that extensive plastic deformation during cold spraying resulted in a dense coating with relatively low internal porosity. Specimens in an as-deposited state possessed a high level of strain hardening and exhibited a brittle rupture associated with intersplat cracking. When subjected to heat treatment, the cold spray deposits showed a general trend of microhardness reduction and progressive sintering of the microstructure with increasing annealing temperature. Furthermore, post-mortem observation revealed a gradual transition in fracture mechanism, manifested by improving material ductility and the occurrence of typical dimple morphology. The aspects responsible for the softening of the cold sprayed 7075 alloys are discussed further.

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