Papers by Author: Petr Lukáš

Abstract: Fatigue life of a cast Ni-base superalloy IN 713LC under combined cycle loading consisting of a superposition of low- and high-cycle fatigue at 800 °C was experimentally determined. No measurable effect of combined cycle was found for studied loading conditions. High scatter of fatigue life related to the initiation of cracks on casting defects was observed. Size of the largest defect in a specimen was predicted by the largest extreme value distribution method. The predicted size was compared with fractographic observation of defects resulting in final fatigue failure.

Abstract: The lifetime of superalloy single crystals CMSX-4 and CM186LC subjected to tensile mean stress with vibrations at high temperatures has been experimentally studied. Both beneficial and detrimental effect of cyclic stress component has been observed. An increase of lifetime due to superposition of cyclic component on the mean stress is explained by reduction of creep rate due to vibrations. The onset of decrease of lifetime with increasing stress amplitude is observed when the fatigue damage due to initiation and propagation of fatigue crack overbalances the beneficial influence of high frequency cycling.

Abstract: Fatigue behaviour of ultrafine-grained copper of purity 99.9 % produced by ECAP technique was studied in a broad region of stress amplitudes. Fatigue strength is by a factor of about 2 higher than that of conventional-grain-size copper in the broad region of fatigue lives from 6x103 to 2x1010 cycles. The grain structure is stable and undergoes only very marginal changes during cycling. Fatigue slip markings on specimen surface follow the trace of the shear plane of the last ECAP pass. Fatigue notch sensitivity is also higher than that of conventional-grain-size copper, but not dramatically. The cyclic stress-strain curve of studied copper is temperature insensitive, while its S-N curve is temperature dependent.

Abstract: This paper reports about a combined neutron and X-ray diffraction study on the residual stresses in the ferrite matrix of cold-rolled fully pearlitic steel sheet. Neutron diffraction revealed compressive residual phase microstresses of about – 500 MPa in rolling direction. However, even in normal direction there are significant tensile residual microstresses, indicating that the morphology of the lamellar microstructure cannot be properly described as a “sandwich structure”. Neutron diffraction was also used during an in-situ tensile test to estimate the microstress level in the cementite phase. The combination of neutron and X-ray diffraction allows to separate, near the surface, the residual phase microstresses from the macrostresses. The latter are also important in rolling direction and imply some risk of undesirable shape changes after forming operations.

Abstract: The performance of an unconventional high-resolution neutron diffractometer using Bragg diffraction optics that can be efficiently employed in strain/stress diffractometers are reported. Presented results demonstrate their experimental abilities for powder diffraction, namely for residual strain/stress measurements. In addition to the macrostrain scanning capability, the device can also be used for microstrain/stress studies by suitable analysis of the diffraction profiles. Two examples of the strain measurements are presented.

Abstract: The precise characterization of the multiphase microstructure of low alloyed TRIP steels is of great importance for the interpretation and optimisation of their mechanical properties. In-situ neutron diffraction experiment was employed for monitoring of conditioned austenite transformation to ferrite, and also for retained austenite stability evaluation during subsequent mechanical loading. The progress in austenite decomposition to ferrite is monitored at different transformation temperatures. The relevant information on the course of transformation is extracted from neutron diffraction spectra. The integrated intensities of austenite and ferrite neutron diffraction profiles over the time of transformation are then assumed as a measure of the volume fractions of both phases in dependence on transformation temperature. Useful information was also obtained on retained austenite stability in TRIP steel during mechanical testing. The in-situ neutron diffraction experiments were conducted at two different diffractometers to assess the reliability of neutron diffraction technique in monitoring the transformation of retained austenite during room temperature tensile test. In both experiments the neutron investigation was focused on the volume fraction quantification of retained austenite as well as on internal stresses rising in structure phases due to retained austenite transformation.

Abstract: New functionally graded Al2O3/Y-ZrO2 ceramics are developed as a new material for production of hip prosthesis, especially all-ceramic hip bearings. The used concept of graded ceramics is expected to provide better long-term performance based on improved mechanical properties and lower clinical risks. Mechanical properties are strongly dependent on the distribution of residual stresses resulting mainly from phase specific stresses after cooling from the sintering temperature. However, these stresses could be optimized by an appropriate selection of a concentration profile of constituent phases. The non-destructive neutron diffraction mapping of residual stresses has been used to investigate this problem experimentally. In addition, the smallangle neutron scattering technique was applied to study the porosity in the ceramics as function of the production parameters.

Abstract: High strength and ductility of the TRIP steels is often attributed to the transformation induced plasticity effect resulting from the strain induced martensitic transformation of the retained austenite in the bainite microstructure. The present work reports results of in-situ neutron diffraction experiments focused on monitoring the phase evolution in two TRIP steel samples (two different thermomechanical treatments) subjected to tensile loading at room temperature. Comparison of the single lineprofile analysis of reactor data (TKSN-400 at NPI Rez) and multi lineprofile analysis of data obtained at spallation neutron source (diffractometer ENGIN-X at ISIS RAL Chilton) suggests that the former can be used in the first approximation for in-situ monitoring of the phase evolution in TRIP steels subjected to mechanical loads.

Abstract: Creep behaviour of two types of superalloy single crystals of the orientation <001> was studied at 850 °C in air to assess their relative suitability for turbine blade applications: CMSX-4 and its potential low cost alternative, CM186LC. The chemical composition of these two superalloys is similar, their microstructure, especially g/g’ distribution, differs substantially. At the same applied stress, the time to failure of CM186LC is shorter than that of CMSX-4. Simultaneously, the creep rate of CM186LC is higher than that of CMSX-4 for the whole lifetime. This is attributed to easier activation of dislocation sources within large g’particles present in CM186LC crystals.