Materials Science Forum Vols. 522-523

Abstract: In situ observation of the mechanical failure behavior was conducted for different kinds of the plasma sprayed thermal barrier coating (TBC) systems by means of an optical microscopy under the static loadings at room and elevated temperatures; as the fundamental aspect, in order to clarify the thermomechanical failure mechanism of TBC system in connection with various coating characteristics. Mechanical tensile or compressive loading was applied progressively to the TBC specimen by an axial loading mode. It was found that the failure behavior of TBC system depends strongly on the testing temperature under both the tensile and compressive loadings. At the elevated temperature which is higher than the ductile-brittle transition temperature (DBTT) of metallic bond-coat (BC), in particular, the ceramic top-coat (TC) spallation can be prevented by virtue of the stress relief induced by the enhanced plastic flow in the BC layer. At the room temperature which is lower than the DBTT of BC, on the contrary, the TC spalling was inevitably induced, but the initiation site of TC spalling is closely related with the magnitude of local plastic deformation in the alloy substrate. Furthermore, an influence of thermally grown oxides (TGO) layer developed at the TC / BC interface on the crack initiation and propagation behavior was investigated in some detail.

Abstract: In order to clarify the failure behavior of plasma sprayed thermal barrier coating (TBC) systems under the complicated modes of thermal-mechanical-chemical loadings, the stress rupture property evaluation and failure analysis were conducted for Y2O3-ZrO2 (YSZ) and CaO-SiO2-ZrO2 (C2S-CZ) TBC systems in air and two kinds of high-temperature corrosive environments. Static creep loading was found to bring about the typical creep failure for TBC systems even in the aggressive environment so called hot corrosion almost in similar manner to the case in air. On the contrary, it was revealed that the dynamic fatigue loading tends to cause a significant failure life reduction of TBC systems both in air and in corrosive environments. For YSZ TBC system, the penetration crack preexisting through the top-coat layer tends to provide a nucleation site for the fatigue crack even in air, and more significantly a short circuit path for the corrosive species in hot corrosive environment. For C2S-CZ system, on the contrary, the top-coat / bond-coat interface tends to provide easily the nucleation site for a main crack to propagate thereafter toward both the alloy interior and outer surface. Under lower stress level at 950°C, however, the oxide-induced crack closure together with crack tip blunting attributed mainly to the high reactivity of Ca compounds as a major constituent of the TC is effective to suppress substantially the crack propagation, so as to cause the prolonged failure life as compared to YSZ system even in aggressive gaseous environment.

Abstract: Ni-base single crystal (SC) superalloys containing high concentrations of refractory elements are prone to generate a diffusion layer called Secondary Reaction Zone (SRZ) beneath their bond coating during exposure at high temperatures. SRZ causes a reduction of the load bearing cross section and is detrimental to the creep properties of thin-wall turbine airfoils. In this study, a new coating system – “EQ coating”, which is in thermodynamic equilibrium with the substrate, has been proposed and the formation behavior of SRZ beneath bond coat materials was investigated on the 5th generation Ni-base SC superalloy developed by NIMS. Diffusion couples of several alloys were made and were heat treated at 1100°C for 300 h, 1000 h. The concentration profiles were analyzed by EPMA. Also, cyclic oxidation tests were carried out at 1100°C in air.

Abstract: A coating with duplex structure of a outer β-NiAl and an inner α-Cr layer, was formed on a Ni-40Cr-3Re (in at%) alloy with or without Zr addition, and the coated alloys were oxidized under thermal cycling in air for up to 2300ks. The coated alloys containing Zr showed a two-step parabolic oxidation, the kp1st in the early stage of oxidation was 2.6~5.4×10-11 kg2
m-4
s-1 for four alloys tested, and kp 2nd for the longer oxidation increased with increasing Zr content from 2.6~5.4×10-11 for the alloy with 0.1at%Zr to 9.6×10-11 kg2
m-4
s-1 for the alloy with 1.0at%Zr. The rapid oxidation for the alloy with 1.0at%Zr is due to the formation of ZrO2 as an internal oxide. The oxide scale in the 1st stage consisted of both α- and θ- Al2O3 with whiskers, and with further oxidation the α-Al2O3 became the major product in the 2nd stage. After the oxidation for 2300ks the as-prepared, outer β-NiAl was changed into a mixture of β-NiAl and γ’-Ni3Al for the Ni-40Cr-3Re alloy containing Zr, while in the coated Ni-40Cr-3Re alloy the outer layer became a mixture of γ’-Ni3Al and γ-Ni(Al,Cr). It was concluded that the addition of Zr into the coated Ni-40Cr-3Re alloy helps maintain high Al contents in the outer Ni-aluminide layer by forming a protective Al2O3 layer.

Abstract: In order to improve the cyclic-oxidation resistance of Ni and SUS304 steel, the coating of a Ni aluminide containing La on both samples were carried out by the electrodeposition of Al and La in a molten salt. The electrodepositions of Al and La were conducted using a potentiostatic polarization method at 1023 K in an equimolar NaCl-KCl melt containing 3.5 mol%AlF3, and that containing 4
mol%La2O3-24
mol%NH4Cl, or 3.5 mol%LaF3. Observation and analysis of the cross-section of the specimen after polarization showed that a deposited layer consisting of a thick inner Ni aluminide layer and a thin outer layer containing a large amount of La was formed on both samples. The cyclic-oxidation resistance of the Ni specimen covered with the deposit layer having the La layer as the outer layer was higher than those of the untreated Ni specimen and the Ni specimen coated with the deposit layer without La. The stainless steel covered with the Ni aluminide showed a high cyclic-oxidation resistance in the atmosphere containing water vapor at 1273 K in comparison with untreated stainless steel, without depending on the existence of La in the deposited layer.

Abstract: The thermal sprayed coatings are widely used in waste incineration boilers and fossil fuel-fired boilers. However, the defects, such as porosity, cracks and unmelted particles, in these coatings are detrimental to corrosion performance. In this study, the nickel based self fluxing alloy coating was fused by YAG laser to improve hot corrosion resistance of the coating. Under appropriate laser parameters, the nonporous, crack-free coating was produced. Hot corrosion test, conducted in the presence of a mixed salt of Na2SO4/NaCl/KCl at 550
Гshowed that the modified coating exhibited excellent corrosion resistance compared with the as sprayed and gas fused coatings.

Abstract: In the high temperature oxidation of metallic alloys oxide scale strains and in particular critical scale strains play a key role with regard to scale adherence. Scale spallation/exfoliation is a vital issue for not only long term alloy performance but also steel sheet production. In many cases materials selection is based on thermodynamic considerations and short term laboratory data, not taking into account changes in the oxidation mechanisms resulting from stresses induced by thermal cycles, oxide growth, specimen or component geometry, or other operational factors. This paper presents a very concise summary of the present knowledge in the form of an approach to a comprehensive scale failure model which is based on a number of microscopic and macroscopic system parameters. This failure model is part of an on-going work which aims at a computer-assisted assessment of oxide scale mechanical reliability.

Abstract: The mechanical behavior of oxidized low alloyed steel has been studied at temperatures between 700°C and 900°C. Such a work is carried out to obtain information about the damages which occur in the oxide scales developed on steels during hot rolling process. Actually the mechanical behavior is tested with a four point bending apparatus at high temperature under controlled atmosphere. Some information about mechanical damages have been collected during the mechanical loading thanks to recording of the sample acoustic emission. During this study, two procedures have been used which differ in the thermal scheme used to build the oxide scales. The results obviously show that the procedure has a great influence on the mechanical behavior of the oxidized low alloyed steel. For instance a marked decrease of the stress is observed at constant displacement between the 900°C and 800°C tests of one procedure, as the opposite behavior is observed with the other procedure. Different points are considered to explain such results, as the thermal strains which are only present in one of the two procedures, the phase transformations in the metal and in the oxide.

Abstract: For industrial purposes, the adhesion control of secondary scale on hot rolled steel sheet is important. A basic study was carried out to clarify the effect of scale microstructure on the scale adhesion of low carbon steel (0.03%C-0.2%Mn). When scale of FeO (about 8μm thickness) was generated at 800°C and transformed by continuous cooling from 250~600°C to 200°C, the scale transformed from 400°C showed good adhesion. The scale consisted of magnetite seam from the steel substrate, lamellar structure of magnetite and α-Fe, and magnetite layer from the scale surface. The orientation analysis by TEM showed the relationship {110}Fe // {100}Fe3O4, <110>Fe // <100>Fe3O4, and the lattice strain was calculated as 4%. On the other hand, FeO/Fe substrate showed the relationship {100}Fe // {110}FeO, <110>Fe // <110>FeO, and 25% lattice strain was calculated. It is considered that the adhesion of scale should be affected by the lattice strain, thus Fe3O4/Fe substrate showed better adhesion than FeO/Fe substrate. The temperature of FeO formation also affects the scale adhesion through the extent of Fe super saturation in FeO.

Abstract: It is very difficult to obtain mechanical properties of oxide films formed on a material in high temperature environments despite its importance of estimating material degradation caused by such as thermal stress. Corrosion/oxidation tests were conducted for pure titanium and titanium alloy in high temperature corrosive environments of wet air and water vapor with hydrogen chloride at temperatures from 673 K to 973 K to look into basic behavior of degradation and the growth of titanium oxide films. It was found that oxide films were usually formed on the specimen surface and the growth was accelerated by the corrosiveness of the environment. In order to examine mechanical properties and exfoliation of corrosion products or oxide films formed on titanium and its alloy, tests of single particle impact on the specimen surface with a glass bead were performed in high temperature corrosive environments. The piling-up surfaces around impact craters were formed and plastically strained. The oxide film formed on the metal surface was detached in a wide range of the circumference and fractured a little far from the rim of the crater. Then fracture and exfoliation stress of the oxide film were estimated by the calculation of impact energy and fractured and detached areas. It was found that both the fracture and exfoliation stress of the oxide films were different depending on the corrosive environment and chemical composition of titanium alloy.