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Fatigue life and fracture of cast gamma TiAl intermetallic alloy at room and elevated temperatures
Petrenec, Martin ; Šmíd, Miroslav ; Polák, Jaroslav
The low-cycle fatigue properties, surface relief and fracture surfaces of cast TiAl alloys with 2 at.% Nb having nearly lamellar microstructure were studied at room temperature and at 750 degrees C. Cyclic stress-strain curves (CSSC) and fatigue life curves were obtained at both temperatures. The surface relief and the fracture surfaces of fractured specimens were observed using scanning electron microscopy. At room temperature, significant cyclic hardening is observed whereas at 750 degrees C cyclic response was stable. Parameters of the CSSC, the Manson-Coffin law and the Basquin law were determined. Persistent slip markings formed along interlamellar interfaces were predominant locations for fatigue cracks at both temperatures. The cracks initiated at surface or in sub-surface region led to the formation of smooth flat areas on the fracture surface corresponding to the persistent slip bands.
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Impact of Heat Treatment Environment of Microstructure and Transformation Path in NiTi Shape Memory Alloy
Kuběnová, Monika ; Zálešák, Jakub ; Čermák, Jiří ; Barták, Tomáš ; Dlouhý, Antonín
We report results of differential scanning calorimetry (DSC) experiments in which heat flow (T) from and to Ti-50.9at%Ni shape memory samples was recorded during the temperature scan through a B2 B19’ transformation range. Prior to the DSC experiments, the samples were separately annealed in evacuated quartz capsules containing different hydrogen and helium mixtures with an overall filling pressure of 900 mbar. The quartz tubes containing the annealed samples were subsequently quenched into cold water. After quenching, the capsules were opened, martensitic transformations were investigated by DSC and the microstructure of the samples was assessed by transmission electron microscopy (TEM). Annealing in the mixtures with an increasing partial pressure of hydrogen led to a considerable drop in the latent heat associated with the B2 B19’ martensitic transformation. Results obtained using TEM suggest that hydrogen may function as a catalytic substance that accelerates the long range ordering of Ni atoms in early stages of Ni4Ti3-phase precipitation. The selected area diffraction study focused on patterns in <100>B2 and <110>B2 zones and provided evidence for diffuse scattering due to spatial modulations of the lattice constant. These variations in the background electron intensities might be related to a precursor of Ni4Ti3 phase in its early state of formation.
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Creep in NiAl-Mo Eutectics
Dudová, Marie ; Barták, Tomáš ; Kuchařová, Květa ; Dlouhý, Antonín
We report on creep in NiAl-Mo ternary eutectic and NiAl intermetallic, having respective nominal compositions Ni-45.5Al-9Mo and Ni-45.2Al (at.%). These alloys were directionally solidified in a high-temperature optical floating zone furnace. The eutectic alloy exhibited a well-aligned rod-like microstructure, consisting of NiAl matrix and 14% (by volume) continuous Mo-fibres oriented along a [001]-crystal growth direction. Cylindrical [001]-oriented specimens were loaded in compression at temperatures in a range 1073 – 1173 K. Formidable strengthening by regularly distributed fine fibres (typical diameter, 400 nm) was observed which resulted in minimum creep rates of the NiAl-Mo eutectic that were seven orders of magnitude lower than the corresponding minimum creep rates of the NiAl matrix. Preliminary microstructural investigations suggested that the strengthening effect is due to an interaction between Mo fibres and subgrain boundaries that form in the course of creep. This interaction leads to an increase of dislocation density in the vicinity of fibres and to an efficient redistribution of stresses in the microstructure.
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Influence of Heat Treatment on Microstructure and Transformation Characteristics of NiTi Shape Memory Wires
Zálešák, Jakub ; Kuběnová, Monika ; Čermák, Jiří ; Svoboda, Milan ; Dlouhý, Antonín
The present study shows how heat treatments affect microstructure and transformation behaviour of superelastic Ni-rich NiTi shape memory wires. Wire samples were sealed into quartz tubes, filled with hydrogen reduction atmosphere, heated for 10 minutes at temperatures increasing in a range from 500 to 600°C and subsequently quenched into cold water. Characteristic transformation temperatures MS, MF, AS, AF, thermal hysteresis and latent heats were investigated using differential scanning calorimetry (DSC). We observed significant changes in DSC charts which recorded B2 → R and R → B19‘ phase martensitic transformations. Increasing temperature of the heat treatment resulted in a convergence of the two transformation peaks. R-transformation commenced at systematically lower temperatures and the beginning of the B19‘ transformation was shifted to higher temperatures with increasing temperature of the heat treatment. In parallel, latent heat of the overall B2 → B19‘ transformation clearly increases. We rationalize these significant changes in the transformation behaviour on the basis of the microstructural development during the heat treatment which is characterized by means of SEM, TEM and EBSD techniques.
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Compressive Creep Tests of An Aluminium Powder Composite Reinforced by Ceramic Particles
Dvořák, Jiří ; Král, Petr ; Balog, M. ; Simančík, F. ; Sklenička, Václav
The aim of this work is to study of a composite mixture of aluminium powders, to analyze of composite microstructure and mostly to describe the creep behaviour of composite in compression. The experimental material was based on Al powder matrix (Al 99%) about the particle size of less than 200 µm. This powder was mixtured with silicon carbide particles (SiC) having the size about 10 µm. The Al composite powder was prepared via conventional direct hot-extrusion (DE) at temperature 450°C. The microstructural investigations were performed using OM and SEM. It was found that DE process exhibits some breaking of carbide particles to size of range from 0.5 to 5 µm. Creep tests in compression were conducted at temperatures 423 -623 K and at applied stresses in the range from 60 to 100 MPa. Consequently, for an evaluation of controlling creep mechanisms an activation energy of creep and the stress exponent of the creep rate were determined.
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