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.
Single-Atom Colloidal Nanorobotics Enhanced Stem Cell Therapy for Corneal Injury Repair
(American Chemical Society, 2025-05-13) Ju, Xiaohui; Javorková, Eliška; Michalička, Jan; Pumera, Martin
Corneal repair using mesenchymal stem cell therapy faces challenges due to long-term cell survival issues. Here, we design cerium oxide with gold single-atom-based nanorobots (CeSAN-bots) for treating corneal damage in a synergistic combination with stem cells. Powered by glucose, CeSAN-bots exhibit enhanced diffusion and active motion due to the cascade reaction catalyzed by gold and cerium oxide. CeSAN-bots demonstrate a two-fold increase in cellular uptake efficiency into mesenchymal stem cells compared to passive uptake. CeSAN-bots possess intrinsic antioxidant and immunomodulatory properties, promoting corneal regeneration. Validation in a mouse corneal alkali burn model reveals an improvement in corneal clarity restoration when stem cells are incorporated with CeSAN-bots. This work presents a strategy for developing glucose-driven, enzyme-free, single-atom-based ultrasmall nanorobots with promising applications in targeted intracellular delivery in diverse biological environments.
Effect of microstructure on machinability of extruded and conventional H13 tool steel
(ELSEVIER SCI LTD, 2025-06-09) Kolomý, Štěpán; Malý, Martin; Doubrava, Marek; Sedlák, Josef; Zouhar, Jan; Čupera, Jan
H13 tool steel samples were fabricated using material extrusion to explore their machinability, offering a promising alternative to laser powder bed fusion for producing complex parts like moulds and cores. Three material states were studied: as-built (AB), heat-treated additively manufactured (HTAM), and heat-treated wrought (HTW). Machining tests focused on cutting speed, feed per tooth, and cooling conditions (dry/flood), while tracking their effect on cutting forces, surface roughness, hardness, microstructure, and residual stresses. Heat treatment significantly reduced porosity (similar to 45 % decrease between AB and HTAM) and transformed the microstructure to full martensite, increasing hardness and cutting forces. Interestingly, the HTAM sample showed lower cutting forces than HTW-by 23.7 % in dry and 24.5 % under flood cooling. HTW generally produced smoother surfaces at lower cutting parameters, but its roughness increased at higher conditions compared to HTAM. The softest AB sample experienced the highest surface hardening (similar to 12 %) when machined at low cutting speeds, while the HTW sample showed most uniform plastic deformation, extending up to similar to 50 mu m below the surface. Dominantly tensile residual stresses were measured in HTW, while AB and HTAM showed mainly compressive residual stresses under dry conditions. This study highlights viability of extruded H13 for industrial use, particularly in mould applications.
Cryogenic Treatment of Martensitic Steels: Microstructural Fundamentals and Implications for Mechanical Properties and Wear and Corrosion Performance
(MDPI, 2024-01-23) Jurči, Peter; Dlouhý, Ivo
Conventional heat treatment is not capable of converting a sufficient amount of retained austenite into martensite in high-carbon or high-carbon and high-alloyed iron alloys. Cryogenic treatment induces the following alterations in the microstructures: (i) a considerable reduction in the retained austenite amount, (ii) formation of refined martensite coupled with an increased number of lattice defects, such as dislocations and twins, (iii) changes in the precipitation kinetics of nano-sized transient carbides during tempering, and (iv) an increase in the number of small globular carbides. These microstructural alterations are reflected in mechanical property improvements and better dimensional stability. A common consequence of cryogenic treatment is a significant increase in the wear resistance of steels. The current review deals with all of the mentioned microstructural changes as well as the variations in strength, toughness, wear performance, and corrosion resistance for a variety of iron alloys, such as carburising steels, hot work tool steels, bearing and eutectoid steels, and high-carbon and high-alloyed ledeburitic cold work tool steels.
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.

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