Papers by Author: Martin Petrenec

Abstract: The subject of the article is focused on the refining of the cast structure of gamma TiAl–2Nb alloy using cyclic heat treatment and on the analysis of the grain refining mechanism. Microstructure evolution after applied cycles of heat treatment was characterized using light, laser and electron microscopy and using microhardness tests. Application of five heat treatment cycles during which two phase transformations (eutectoid and alfa-recrystallization reactions) repeatedly took place resulted αin refining of the cast columnar structure. The mean grain-lamellar colony size 0.512 mm was transformed to fully lamellar structure containing gamma and alfa2 phases having the mean grain size 0.229 mm. Lamellae thickness of gamma was not changed while the thickness of alfa2 phase decreased to 78 nm. Refining of alfa2 phase resulted in the increase of the microhardness by 20 %. The recrystallized cast structure obtained by cyclic heat treatment and the knowledge on the mechanisms of the refining the structure were compared and discussed with the literature data.

Abstract: Effect of tensile dwell on low cycle fatigue of cast Inconel 792-5A is studied in symmetrical strain cycling at 800°C. Cyclic hardening/softening curves, cyclic stress-strain curves (CSSC) and fatigue life curves were obtained in continuous cycling and in cycling with tensile dwells. Dwells have slight effect on hardening/softening curves at high strain amplitudes. CSSC in cycling with dwells is shifted to lower stress amplitudes. No significant effect of dwells on Basquin curves is observed. Density of slip markings in continuous cycling is significantly higher in comparison with cycling with dwells. Samples cycled with dwells are typical of high density of secondary cracks, although sporadic slip markings were also found.

Abstract: Cyclic multiple step test in strain control have been performed on cylindrical specimens of cast polycrystalline Inconel 738LC and 792-5A superalloys at 800 °C in laboratory atmosphere. Hysteresis loops were analyzed according to the statistical theory of hysteresis loop. The effective and internal stress components were evaluated. The effective stress of γ´ precipitate has significant influence on the stress-strain response both materials. The stress amplitude in IN 792-5A is higher than in IN 738LC at approximately same total strain amplitude due to significantly higher effective stress of γ´ phase. Cyclic hardening/softening curves and cyclic stress-strain curves using short-cut procedure were obtained. Cyclic hardening/softening behavior depends both on temperature and strain amplitude. Low amplitude straining is characterized by the saturation of the stress amplitude. In high amplitude straining slight softening was found. The cyclic stress-strain curves for both materials can be fitted by power law. Cyclic stress-strain response in terms of internal and effective stress components is discussed in relation to microstructural parameters of the materials. The observation of surface relief revealed the presence of persistent slip markings.

Abstract: Cyclic strain controlled multiple step tests have been performed on cylindrical specimens of cast -TiAl based alloy with 2 at.% of Nb with nearly lamellar microstructure at 23 and 750 °C in laboratory atmosphere with the aim to study the effect of temperature on the internal and effective cyclic stress components. At these temperatures, the evolution of the effective and internal stress components and the effective elastic moduli were derived from the hysteresis loops analyzed according to the statistical theory of hysteresis loop. Cyclic hardening/softening curves and cyclic stress-strain curves were obtained at both temperatures. Cyclic stress–strain curves measured using short-cut procedure coincide with the basic cyclic stress-strain curve. They are shifted to lower stresses with increasing temperature. Cyclic stress-strain response at both temperatures was compared and discussed in relation to changes of internal and effective stress components and dislocation modes referred in literature concerning this class of the material.

Abstract: Smooth specimens made from austenitic-ferritic duplex steel were subjected to constant stress amplitude loading with positive mean stresses. Hysteresis loops were recorded during the fatigue life and plastic strain amplitude and cyclic creep rate were determined. Fatigue hardening/softening curves, cyclic creep curves and cyclic stress-strain curves for different positive mean stresses were evaluated. Typical dislocation structures developed in both phases of the duplex steel were identified using TEM, compared with the saturated plastic strain amplitude and correlated with the decrease of the cyclic creep rate during cycling and the slope of the cyclic stress-strain curve.

Abstract: The low-cycle fatigue properties of cast nearly lamellar TiAl alloys with diverse Nb content at room temperature and 750°C were compared. Monotonic tensile curves, cyclic stress-strain curves (CSSC) and combined fatigue life curves were obtained at both temperatures. After cyclic straining the surface relief and the fracture surfaces were observed using scanning electron microscopy. The increase in Nb content significantly improves tensile properties and CSSCs at both temperatures. The combined fatigue life curves especially at 750 °C are shifted to higher fatigue lives. The effect o Nb content on the tensile and cyclic strength is discussed in relation to the thickness of the lamellae. Persistent slip markings formed along interlamellar interfaces were predominant locations for fatigue cracks. At both temperatures and materials showed similar crack initiation and propagation behavior. The cracks initiated at surface or in sub-surface region creating smooth flat areas corresponding to the persistent slip bands.

Abstract: Fatigue properties of the new generation of TiAl alloys with 8at.% of Nb were studied and compared with those reported in the literature for previous generation with 2at.% of Nb. The high Nb content improves substantially the fatigue life time in the Basquin representation. Nevertheless, the fracture behaviour is still rather brittle even at 750°C. TEM study of the material after the fatigue testing was performed. The dislocation activity is concentrated in the  phase. The properties and behaviour of individual dislocations seem to be similar as those observed in single -phase TiAl alloys.

Abstract: The fatigue properties of ferritic-pearlitic-bainitic steel using specimens produced from massive forging were measured in stress controlled regime with positive mean stress. The cyclic creep curves and cyclic hardening/softening curves were evaluated. The fatigue life was plotted in dependence on the mean stress and on the plastic strain amplitude. The principal contribution to the drop of the fatigue life with the mean stress is due to the increase of the plastic strain amplitude in cycling with mean stress.

Abstract: Cylindrical specimens of cast polycrystalline nickel base superalloy Inconel 738LC were cyclically strained under total strain control at 23 and 800 °C to fracture. Cyclic hardening/softening curves, cyclic stress-strain curves, and fatigue life curves were obtained at both temperatures. Surface relief was studied in specimens fatigued to failure using scanning electron microscopy. Cyclic hardening/softening behaviour depends both on temperature and strain amplitude. Low amplitude straining was characterized by saturation of the stress amplitude. In high amplitude straining a pronounced hardening was found which was followed by saturation at room temperature and by cyclic softening at 800 °C. The cyclic stress-strain curves can be fitted by power law. They are shifted to lower stresses with increasing temperature. Fatigue life curves can be approximated by the Manson- Coffin and Basquin laws. The Manson-Coffin and Basquin curves are shifted to lower lives with increasing temperature. Slip markings were detected on specimen surface at all test temperatures. When temperature grows the density of slip markings is reduced.

Abstract: Dislocations structures were studied using transmission electron microscopy in specimens of cast polycrystalline nickel based superalloy Inconel 792-5A cyclically strained up to failure with constant total strain amplitudes at temperatures 23 and 700 °C. Spatial arrangement of dislocations in individual grains was determined using the technique of oriented foils. Planar dislocation arrangements in the form of bands parallel to the {111} planes were observed in specimens cycled at both temperatures. The bands showed up as thin slabs of high dislocation density cutting both the γ channels and γ´ precipitates. They correspond to areas of cyclic slip localization. Experimental results concerning the dislocation structure are used to discuss the effect of temperature on the cyclic stress-strain response and fatigue life.