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PREDICTION OF FRACTURE TOUGHNESS TRANSITION FROM TENSILE TEST DATA APPLYING NEURAL NETWORKS
Dlouhý, I. ; Hadraba, Hynek ; Chlup, Zdeněk ; Válka, Libor ; Žák, L.
Reference temperature localizing the fracture toughness temperature diagram on temperature axis was predicted based on tensile test data. Regularization artificial neural network (ANN) was adjusted to solve the interrelation of these properties. For analyses, 29 data sets from low-alloy steels were applied. The fracture toughness transition dependence was quantified by means of master curve concept enabling to represent it using one parameter - reference temperature. Different strength and deformation characteristics from standard tensile specimens and notched specimens, instrumented ball indentation test etc. have been applied. A very promising correlation of predicted and experimentally determined values of reference temperature was found.
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Variational Concepts with Applications to Microstructural Evolution
Fischer, F D. ; Svoboda, Jiří ; Hackl, K.
In systems at elevated temperature the development of the microstructure of a material is controlled by diffusional and interface migration processes. As first step the description of the microstructure is reduced to a finite number of time-dependent characteristic parameters (CPs). Then the Thermodynamic Extremal Principle (TEP) is engaged to develop the evolution equations for these characteristic parameters. This treatment is demonstrated on a bamboo-structured material system predicting the spatial and time distribution of chemical composition as well as the deformation state.
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AN INTERPLAY BETWEEN HEAT TREATMENT CONDITIONS AND B2 <-> B19 ' TRANSFORMATION IN Ni-Ti SHAPE MEMORY ALLOYS
Kuběnová, Monika ; Zálešák, Jakub ; Čermák, Jiří ; Dlouhý, Antonín
This study investigates the influence of a heat treatment atmosphere on multistage martensitic transformations in a Ti-50.9at% Ni shape memory alloy. Evacuated quartz tubes were filled with hydrogen while the hydrogen partial pressure was carefully controlled in each experiment. The encapsulated specimens were subjected to heat treatments consisting of annealing and aging. After the heat treatment, the path of martensitic transformation was investigated using differential scanning calorimetry (DSC). On cooling, martensite initial temperatures decrease with the increasing partial pressure of hydrogen applied during the heat treatment. Moreover, the formation of martensite phase may not take place for partial pressures that exceed a certain limit. On the other hand, two-and multiple-step B2/R/B19' martensitic transformations are observed in specimens heat treated at lower hydrogen pressures. Results obtained using transmission electron microscopy (TEM) clearly show that the size of Ni4Ti3 precipitates becomes smaller in alloys that were heat treated at higher hydrogen pressures. The results are discussed in terms of a chain of events. It is suggested that diffusion of hydrogen into the specimen during the annealing step first influences the nucleation of the Ni4Ti3 phase. The sizes and distribution of the Ni4Ti3 precipitates than control the characteristics of B2/B19' transformation.
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Microstructure and hardness of TiB2
Halasová, Martina ; Bača, L. ; Šajgalík, P. ; Chlup, Zdeněk ; Dlouhý, Ivo
MICROSTRUCTURE AND HARDNESS OF TIB2 TiB2 in its pure form is extremely hard material with high melting point. This behaviour predetermines it to be used as e.g. nozzles, armour or wear parts. It is also very brittle and the effort is to decrease brittleness trough addition of various dopants. This contribution deals with the TiB2 dopped with Ta and Ni in various ratio. For observing the microstructure was used SEM and Vickers hardness was calculated by measuring the diagonals of indents. For comparison the values obtained by machine were shown in graph, too.
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Microstructure Characterization and Faigue Behavior of Beta Phase Containing Gama-TiAl Alloy
Petrenec, Martin ; Beran, Přemysl ; Šmíd, Miroslav ; Chlupová, Alice ; Kuběnová, Monika
The microstructure and fatigue properties of Ti–44Al–7.8Nb – 0.2Ni (at.%) alloy after graded cooling heat treatment were investigated. Different techniques as SEM, TEM and FIB were used for characterization of microstructure. Quantification of phase volume fraction was measured by neutron diffraction method. After graded cooling heat treatment the structure consists of fully lamellar uniform grains which contain lamellar phases /2 and cubic phases (ordered B2 and disordered phases) at grain boundary and interlamellar spaces. The foils for TEM were prepared using FIB technique and observed in TEM in order to identify B2 phase by selected electron diffraction. Low cycle fatigue behaviour at 750 °C shows stable fatigue crack propagation due the presence of plastic beta and B2 phases.
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Vliv metody ECAP na mikrostrukturu a creepové chování slitiny AL-3Mg-0,2Sc
Dvořák, Jiří ; Král, Petr ; Kvapilová, Marie ; Sklenička, Václav
The coarse-grained material (Al-3Mg-0.2Sc alloy) was subjected to ECAP at room temperature. Compression creep tests were conducted on both states of material at the applied stresses from 16 to 80 MPa and temperature of 473 K. The results demonstrate that the minimum creep rate of the ultrafine-grained (UFG) material is up to two order at magnitude higher than that of the unpressed coarse-grained material. A microstructural investigation of this alloy was performed using electron microscopy (TEM and EBSD). The experiments show that ECAP reduced the average grain size to ~ 300 nm and led to the higher occurrence of high-angle boundaries. It should be stressed that the application of eight ECAP passes causes a substantial decrease in the creep resistance of the alloy. Based upon the results it may be possible to suggest, that the creep behaviour of UFG alloy is influenced by grain boundary sliding accommodated by diffusion-controlled movement of intragranular mobile dislocations.
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Creep behaviour of Al-3Mg-0,2Sc alloy processed by ECAP method
Dvořák, Jiří ; Král, Petr ; Sklenička, Václav
Ternary aluminium alloy Al-3Mg-0.2Sc was subjected to equal-channel angular pressing (ECAP) at room temperature to refine its original coarse grain size. For this purpose the ECAP die was used which had a 90° angle between the die channels. By application of this method an ultrafine-grained microstructure was achieved with the grain size ~ 0.3mm after 8 ECAP passes by route BC. Subsequent compression constant stress creep tests were conducted at 473 K and in the applied stress range of 16–40 MPa. It was found, that resulting microstructure of this alloy leads to a decrease of minimum creep rate by an order of magnitude in comparison with creep behaviour of an ultrafine – grained ECAP pure aluminium. Further, the creep behaviour of Al-3Mg-0.2Sc alloy at lower stresses indicates a possible occurrence of the threshold stress for creep.
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