Key Engineering Materials Vols. 345-346

Abstract: Tensile test of TiAl alloy was carried out in 700oC gas environment(H2, pure air, Ar, vacuum) at three strain rates, 2.8x10-4, 2.8x10-5, and 2.8x10-6s-1. With decreasing strain rate, the ultimate tensile strength decreased in all environments similarly. However, elongation to fracture differed from each environment. With decreasing strain rate, the elongation to fracture in vacuum and Ar increased slightly, that in H2 increased much and that in pure air significantly. The reasons of such behaviour are not known. One of the reasons for high elongation to fracture would be formation of multiple microcracks in H2 and pure air on the gauge surface of the specimen during tensile deformation.

Abstract: Oxide dispersion strengthened (ODS) materials is leading candidates for blanket/first-wall structures of the fusion reactor. ODS materials for structure application in fusion rector would allow to increase the operating temperature to approximately 650
. Therefore, this work focused on the optimization of metallurgical features to improve high temperature strength and elongation through understanding of contents of Cr and Al. In the study, the three kinds of ODS steels such as 19Cr-ODS (K1), 13Cr-Al-ODS (K2) and 19Cr-Al-ODS (K4) with Y2O3 content of 0.37wt% have been produced. And tensile test were performed on three ODS ferritic/martensitic steel between RT, 300, 400 and 600
Dispersion hardening represents an interesting approach to improve the mechanical properties at elevated temperature, as they are foreseen in the future fusion reactor It has been successfully demonstrated that it is possible to expanse the temperature range for the application of fusion reactor.

Abstract: In this study, Nano Infiltration Transient Eutectic Phase SiC ceramics (NITE-SiC) was fabricated by hot press method with nano-SiC powder which a particle size is about 30nm, and alumina, yttria and silica were used for sintering additives. Process condition of NITE-SiC was given temperature of 1800oC, pressure of 20Mpa and holding time of 1Hr. NITE-SiC was optimization for densification and good mechanical properties by preceding researches. As a result, weight loss was occurred heat treatment temperature of all. Surface of heat treated specimens was occurred reacting of oxidation or erosion. All Point was occurred residual stress, but residual Stress was show Relaxant properties by effect of heat treatment.

Abstract: Microstructure can have a significant effect on the resistance to intergranular stress corrosion cracking. Certain grain boundaries are susceptible to corrosion while others have high resistance and may form crack bridging ligaments as the crack deviates around them. To investigate the mechanics of crack bridging, 3D computational model has been previously developed. An extension to the model, to include stress corrosion crack growth kinetics is presented in this paper. An analysis of the effects of resistant grain boundary fraction demonstrates that the bridging ligaments can significantly retard short crack propagation rates. Increasing the fraction of resistant boundaries is shown to improve microstructure resistance by reducing the crack propagation rate.

Abstract: Effect of ceramic coating (TiN, TiCN, TiAlN or Al2O3) on the wear resistance and thermal oxidation behavior of Inconel 617 alloy (Ni-22Cr-12.5Co-9Mo-1Al) have been examined. TiAlN and Al2O3 coatings showed lower wear loss than bare Inconel 617 and the enhanced thermal oxidation resistance at 1000oC by suppressing the inhomogeneous formation of Cr2O3 at the surface region through the reaction between oxygen and grain boundary-diffused Cr, one of the main component of Inconel 617 alloy, while TiN and TiCN coatings showed poor temperature stability at the temperature region above 600 oC.

Abstract: The long-term corrosion strength properties for the carbon steels under pressurized water atmosphere have been investigated, in the conjunction with the detailed analysis of their microstructures. The corrosion test for carbon steels was carried out at the temperature of 200°C under a water pressure of 10 MPa. The corrosion test samples were maintained up to 50 weeks in the tube shaped reactor. The mechanical strength and the microstucture of carbon steels suffered from the long term corrosion test were evaluated by SEM, XRD and tensile test. The weight loss of carbon steel by the corrosion test was also examined. The tensile strength of carbon steels decreased with the increase of corrosion time under a pressurized water atmosphere, accompanying the creation of severe corrosion damages like stress corrosion crack.

Abstract: The interaction between two or more independent causes of materials degradation results in a duplex mode of materials degradation. The materials degradation rate whether expressed in loss of material or loss of mechanical strength is often faster than the sum of each form of materials degradation acting separately. For this reason, duplex modes of materials degradation are of great practical importance. Synergisms between unrelated forms of materials degradation are not immediately obvious and have in some instances been only recently recognized. In this paper a chemically aggressive environment over a geo-synthetic polymeric membrane is investigated. The membrane free energy depends upon the mono-axial strain and the degree of reaction with an external agent. We suppose chemical potential assigned and the minimization problem is formulated. Practically the minimum problem is non-convex, and coexistence of equilibrium phases is possible. The proposed general model is able to describe the complex phenomenon when polymeric geo-synthetic membrane are undergoing to heterogeneous and complex action of the package system built in dump of urban waste. Finally a FEM simulation is compared with minimization problem theoretical results.

Abstract: Thermal barrier coating (TBC) of a gas turbine blade suffers from high temperature oxidation. It is known that thermally growth oxidation (TGO) grows at the interface between ZrO2- 8%Y2O3 and CoNiCrAlY, and the TGO degrades the adhesive strength. The purpose of this study is to clarify the TGO growth process. Thermal aging tests of ZrO2-8%Y2O3 / CoNiCrAlY TBC systems under various temperature conditions were carried out. TGO growth process was observed by an electron probe micro analyzer (EPMA). Both TGO of Al and TGO of other elements were examined, and the thickness of the TGOs were examined. Results are summarized as follows. (1) The delamination occurred at a ZrO2-8%Y2O3 layer beside the interface. (2) The growth rate of complicated TGO which consisted of Co, Ni and Cr increased with an increase of temperature. However, the complicated TGO wasn’t related with the delamination life. (3) The delamination could occur if the thickness of Al TGO increased more than about 3 μm.

Abstract: The low cycle fatigue tests of SA508 Gr.1a low alloy steel were carried out to investigate the fatigue crack growth mechanisms in high temperature water. The fatigue life in 310oC deoxygenated water was shorter than that in air. Furthermore, the reduction in the fatigue life in 310oC deoxygenated water was enhanced with a decreasing strain rate, from 0.4 to 0.008 %/s. The ductile striations with the streamed down features, which may indicate the occurrence of the metal dissolution, were mainly observed at the strain rate of 0.008 %/s. And the flat facets and the brittle cracks, which may be evidences for the HIC, were primarily observed in the strain rate range from 0.04 to 0.4 %/s. From the analysis of microstructure, it is thought that the HIC contribute dominantly to the reduction in the fatigue life in the strain rate range from 0.04 to 0.4 %/s and the metal dissolution is mainly responsible for the reduction in the fatigue life at the strain rate of 0.008 %/s.

Abstract: In this research, corrosion fatigue tests using 490MPa TMCP steel were performed in synthetic seawater condition to investigate the corrosion fatigue crack propagation characteristics of TMCP steel plate which is often used for ships and offshore structures from its weldability and weight reduction purpose. In addition, relationships between boundary correlation factor F(α,β) and α(= 2a/W) of the fracture mechanics formula for the test specimen which was exerted by the pin load were calculated for the evaluation of da/dN-.K. To get the boundary correlation factor F(α,β), the calculation of J as a path-independent line integral was carried out. The solutions of J integral showed similar value in spite of the different path. Crack propagation tests for the same steel in air condition was carried out also for comparison. Finally, it was clear from the crack propagation test results that the propagation rate of TMCP steel plate in synthetic seawater condition is faster than that of in air condition at least twice.