Papers by Keyword: High Temperature Fatigue

Abstract: Thermal barrier coatings are widely used to protect the substrate from high temperature and extremely aggressive environments in gas engines. In the present article, authors have been studied degradation of complex thermal barrier coating system deposited on polycrystalline nickel superalloy IN 713LC. The substrate material was grit blasted with alumina (Al₂O₃) particles prior to air plasma deposition of CoNiCrAlY bond coat. Top coat consists of conventional zirconia (ZrO₂) stabilized by yttria (Y₂O₃) -YSZ ceramic in combination with a eutectic nanocrystalline ceramic Eucor made of zirconia (ZrO₂), alumina (Al₂O₃) and silicia (SiO₂) –in the ratio of 50/50 in wt. %. The top coat was deposited using water stabilized plasma. Test specimens with the TBC coating system were fatigued under strain control condition in fully reversed symmetrical push-pull cycles at 900°C in air. The microstructure of TBC was characterized with scanning electron microscopy and energy dispersion X-ray analysis. The coating hardness and thickness were measured. Fracture surface and polished sections parallel to the specimen axis were examined to study damage mechanisms in coatings under cyclic loading at high temperature. TBC delamination was observed at the top coat/bond coat interface after cyclic loading at high temperature. Fatigue crack initiation sites are documented. Majority of fatigue cracks start from the surface and top coat/bond coat interface.

Abstract: The present work is focused on the study of low cycle fatigue behavior of grit blasted nickel-base superalloy Inconel 713LC (IN 713LC). Grit blasting parameters are obtained. Button end specimens of IN 713LC in as-received condition and with grit blasted surface were fatigued under strain control with constant total strain amplitude in symmetrical cycle at 900 °C in air. Hardening/softening curves, cyclic stress-strain curve and fatigue life data of both materials were obtained. Both materials exhibit the same stress-strain response. It has not been observed any improvement or reduction of low cycle fatigue life in representation of total strain amplitude versus number of cycles to failure of grit blasted material in comparison with as-received material. Surface relief and fracture surface were observed in SEM. The little effect of surface treatment on fatigue characteristics is discussed.

Abstract: The effect of thermal barrier coating (TBC) on low cycle fatigue behavior of cast superalloy Inconel 713 LC has been studied at 900 °C. The TBC consisting of a CoNiCrAlY bond coat and a zirconia (ZrO₂) top coat stabilized by 8% yttria (Y₂O₃) was deposited on the gauge section of cylindrical specimens using the atmospheric plasma spray technique. Cylindrical specimens of Inconel 713LC in as-received condition and with surface treatment were cyclically strained under strain control with constant total strain amplitude in symmetrical cycle at 900 °C in air. Hardening/softening curves, cyclic stress-strain curve and fatigue life data of coated and uncoated material were obtained. The stress response of the TBC coated specimens is lower in comparison with the uncoated specimens. Detrimental effect of surface treatment on the Basquin curve is documented. Specimen sectioning and fracture surface observations revealed fatigue damage mechanisms and help to discuss differences in fatigue behavior of the coated and uncoated superalloy.

Abstract: Ternary TiAl alloy with 8 at.% Nb and lamellar microstructure is subjected to low cycle fatigue tests at temperatures ranging from room temperature to 800 °C. The aim of the study is to find limit conditions when the microstructure is still stable and to study mechanisms of microstructural degradation when this limit is exceeded. Up to 750 °C, no cyclic softening or hardening is observed and cyclic stress-strain curve follows the tensile curve. Cyclic softening is characteristic for 800 °C. The TEM observation did not reveal any substantial changes in the microstructure due to the cycling up to 700°C. The lamellar structure is altered by cyclic straining at 750 °C and, to a higher extent, at 800°C. In specimens cycled to fracture at 800 °C, the domains without lamellar structure cover about 10% of volume and are almost dislocation free. The destruction of lamellar microstructure is the reason for the marked cyclic softening at 800 °C.

Abstract: The present paper is focused on scanning electron microscopy investigation of surface relief developed in cyclically strained specimens at high temperature. Symmetrical fatigue tests without hold times and with tensile or compressive hold times were used in strain control regime. The gauge length of specimens was analyzed after cyclic straining in a wide range of total strain amplitudes. Experimental temperature was 800 °C and selected hold time was 10 minutes.Surface relief is developed during the cyclic straining in localized plastic strain areas. Developed surface relief strongly depends on the type of fatigue test and a value of total strain amplitude. Well-developed surface persistent slip markings were observed after cyclic loading without hold times. Another group of observed slip markings with perpendicular orientation to stress axis exhibits irregular and wavy morphology. Similar markings were also observed on specimen surfaces after fatigue tests with tensile hold times as well as compressive hold times. Third group, named imperfect persistent slip markings, was observed after cyclic straining with tensile holds. In some suitably oriented grains, areas with high density of well-developed markings with irregular morphology were found.

Abstract: Eutectic Al-Si alloys are typically used for the production of internal combustion engine pistons. A high-cycle, high-temperature fatigue characterization of AlSi12 alloy performed using specimens extracted from actual pistons is presented and discussed. Fatigue strength at 10⁷ cycles were obtained at test temperatures of 250 °C, 300 °C and 350 °C. The fatigue strength reduction was quantified. The micro structural features were quantified by quantitative metallography and fatigue fracture surfaces inspected to identify the initiation causes.

Abstract: The work is focused on the study of degradation of ZrO₂ stabilized by Y₂O₃ (YSZ) thermal barrier-coating system with CoNiCrAlY bond coat applied on cast polycrystalline nickel-based superalloy Inconel 713LC. Cylindrical specimens in as-coated conditions were cyclically strained under strain control with constant total strain amplitude in symmetrical cycle at high temperature (900 °C) in air. Coating system YSZ with CoNiCrAlY bond coat were prepared by APS method on blasted surface. The microstructure of TBC was characterized with scanning electron microscopy and energy dispersion X-ray analysis. The coating thickness and hardness profile was measured. Fracture surface, surface relief and polished sections parallel to the specimen axis were examined to study damage mechanisms in coatings under cyclic loading at high temperature. It was find that initiation of the fatigue crack usually occurs on interface YSZ-CoNiCrAlY and the trajectory of the further crack propagation was documented.

Abstract: The influence of the cooling condition after solution treatment on the high temperature fatigue resistance of 23Cr-26Ni heat resistant steel was investigated. Two different cooling conditions were applied to the steel after solution treatment at 1230oC for 3 hours. One specimen was water quenched immediately after the solution treatment. The other one was furnace cooled at a rate of 0.5oC/min down to 750oC after the solution treatment. Then, both specimens were aged at 750oC for 5 hours. The low cycle fatigue (LCF) test was conducted to investigate the influence of high temperature on the LCF behaviors of the heat-resistant 23Cr26Ni alloy. Under two different heat treatment conditions, the LCF test was performed at total strain amplitudes ranging from ±0.4~0.9% at room temperature (RT) and 600°C. During the test, initial cyclic hardening occurred at both experimental temperatures. This phenomenon was attributed to the increase in the dislocation density due to cyclic deformation, which resulted in the interaction between the newly created dislocations and precipitates. Cyclic softening was observed in the later stages of the LCF test at RT. The formation of precipitates and increase in the dislocation density were observed using TEM. Also, the XRD and EDS techniques were used to verify the type and composition of the precipitates.

Abstract: Cylindrical specimens of Inconel 713LC in as-cast condition and with Al diffusion coating by the CVD technique were cyclically strained under total strain control at 800 °C. Hardening/softening curves, cyclic stress-strain curves, and fatigue life curves are obtained. The coating has a beneficial effect on the Manson-Coffin curve while the fatigue life is reduced in the Basquin representation. The stress response of the coated material is lower in comparison with the uncoated one. Sections parallel to the specimen axis have been examined to study fatigue damage mechanisms.

Abstract: Superplastic forming of titanium alloy sheets requests long time operating conditions in the range of 900-950°C. Moreover, in a classical press-furnace process environment, die surface temperature drops during sheet unloading and induces high temperature thermo-mechanical fatigue. In order to withstand such extreme conditions in oxidative atmosphere, cast heat resistant nickel chromium steel grades have been developed. The high chromium content (close to 25%) aims to protect against the oxidizing environment, whereas the nickel content is selected with respect to the expected in service loads. The 50% nickel grades are in general used for heating plates, huge casings and cover-plates; whereas 40% nickel grades are selected for inserts and medium size self-standing dies. Cost considerations (Nickel and machining) are also taken into account by the end users for making their choice. An extensive testing program has been performed, in the range of 20 to 950°C, to understand the high temperature fatigue behaviour of these grades and to identify material behaviour models for simulation purposes. This paper presents the major results of these research works and highlights the impact of the nickel content in terms of stress level and life time. Nevertheless, when looking on behaviour, test results show that a unified elasto-visco-plastic cyclic behaviour model is well suited for thermo-mechanical cyclic modelling whatever the grade is. Isothermal identification strategy and out of phase SPF die representative anisothermal fatigue validation are presented.