Key Engineering Materials Vols. 297-300

Abstract: New Ti-base alloys were designed to replace conventional expensive elements with low cost elements of iron and silicon. The prepared Ti-4wt%Fe-(0~4)wt%Si alloys were creep tested at 600oC in air, tensile tested at room temperature and 400oC in air, and exposed to hot air for oxidation tests between 700 and 1000oC. When compared to the Ti-6Al-4V alloy, Ti-4wt%Fe-(0.5~2)wt%Si alloys displayed poorer creep resistance, but exhibited good tensile properties and superior oxidation resistance.

Abstract: The Small Punch Creep Tests (SP-C tests) are simulated by a Finite Element Method (FEM). The objective of the present study is to establish a foundation for the SP-C test method by investigating the deformation and stress state of the SP-C test specimen. The emphasis is placed on the dependence relation of the creep strain and the stress on the measurable quantities, such as applied loads and the central deflections. Simulation works are conducted for two different materials, one is the tungsten-alloyed 9% Cr ferritic steel and the other is 12Cr1MoV steel. The numerical results for the central deflection versus time curves are quantitatively similar to the experimental results obtained on tungsten-alloyed 9% Cr ferritic steels. From the numerical results, the relationship between the central deflection and the equivalent creep strain is approximately independent of load, temperature, and material properties. The magnitude of the equivalent stress in the central region of the SP-C specimen shows no significant change with respect to time at the secondary creep stage, an approximate equation is proposed for the assessment of the equivalent stress in the SP-C specimen. As a farther result, the high temperature creep properties of the 12Cr1MoV steel and tungsten-alloyed 9% Cr ferritic steel are appraised by numerical simulation. The results are in good agreement with the results from the standard test method. The results indicate that the small punch test technique is an effective method for the evaluation of the high-temperature creep properties of materials.

Abstract: Polymers are vital materials in better performance of specific strength. However their application can be restricted by the lower glass transition temperature, Tg. Some polymers have been developed as engineering plastics for the high temperature applications. We examined the high temperature strength of polymers at constant applied stress. The creep rupture and viscoelastic behavior were scrutinized for PC (polycarbonate) and PMMA (polymethyl methacrylate), which were quite different in the molecular structures. The former contains benzene rings and the latter is a single polymer. Tg is 423 K for PC and 378 K for PMMA. The large difference in the creep behavior was observed near Tg. The creep life strongly depends upon the applied stress just below Tg. The creep life is a function of the applied stress as follows. n life t − µ s . The stress exponent, n depends upon the temperature. Mechanical models were applied to evaluate the viscoelastic properties of the polymers at high temperatures. The viscosity rapidly decreased near Tg , regardless of the smaller decrease in the elastic constant. The results would be due to the difference in the molecular structures. The benzene ring could contribute to the higher resistance against the creep deformation through the higher viscosity.

Abstract: The so-called structure parameters are firstly introduced to indicate their influence on nonlinear viscoelastic behavior and the corresponding nonlinear evolution equation is then derived based on the linear one proposed by Biot by means of thermodynamics of irreversible process. As a worked example, the above nonlinear evolution equation is applied to a viscelastic rod of the ideal viscoelastic solid to predict its uniaxial creep behavior. Under the strain-induced anisotropic effect due to the configurationally structure changes, the originally isotropic rod gradually becomes transversely isotropic one during the creep deformation process. The final results show that (i) when the applied longitudinal stress is below a certain value, the creep strain will get towards an asymptotic value at a decreasing strain rate, and that (ii) when over this value, it will initially increase at a decreasing strain rate and then at an accelerative rate after overcoming a critical point.

Abstract: Most heat resisting materials in structural components are used under multi-axial stress conditions and under such conditions ductile materials often exhibit brittle manner and low creep ductility at elevated temperature. Creep crack initiation and growth properties are also affected by multi-axial stress and it is important to evaluate these effects when laboratory data are applied to structural components. Creep crack growth tests using circumferential notched round bar specimens are a simple method to investigate multi-axial stress effects without using complicated test facilities. Creep crack growth tests have been performed using a 12CrWCoB turbine rotor steel. In order to investigate the effects of multi-axial stress on creep crack growth properties, the tests were conducted for various notch depths at 650°C. The circumferential notched round bar specimen showed brittle crack growth behaviour under multi-axial stress conditions. Creep crack growth rate was characterized in terms of the C* parameter. A 12CrWCoB turbine rotor steel has been tested using circumferential notched round bar specimens with different multi-axiality. Circumferential notched round bar specimens show increased brittle creep crack growth behaviour due to the multi-axial stress condition. Creep crack growth properties could be predicted by allowing for the decrease of creep ductility under multi-axial conditions.

Abstract: Three types of Ti-Al-Nb ternary alloys are obtained by arc-melting and heat treatment, which are γ-TiAl single phase alloy, γ-TiAl + α2-Ti3Al duplex phase alloy, and γ-TiAl +α2-Ti3Al +Nb2Al multiple phase alloy. The phase stability is studied using optical microscope, X-ray diffractometer and electron probe microanalyzer. The oxidation behavior of three Ti-Al-Nb ternary alloys with different microstructures was investigated at 1273K using interrupted oxidation test in air. The compression test was carried out at 298K-1373K. The oxidation resistance of Ti-Al-Nb ternary alloy at high temperature was found to be better than that of the binary Ti-Al alloy. Among the three Ti-Al-Nb ternary alloys, the two-phase alloy with γ+α2 has the best oxidation resistance and mechanical properties. The existence of α2 could enhance the oxidation resistance of the alloy at high temperature. On the contrary, the presence of Nb-enriched phase such as Nb2Al would decrease the oxidation resistance at elevated temperature due to the formation of Nb2O5, which would accelerate the exfoliation of oxide.

Abstract: To understand the effects of nitrogen on high temperature, creep-rupture tests have been conducted at 973 and 1073K for 18Cr-9Ni austenitic stainless steels with 0.14 and 0.08wt% nitrogen contents. It is observed that creep-rupture life of 18Cr-9Ni-0.14N steel is longer than that of 18Cr-9Ni-0.08N steel. To verify the difference in creep-rupture life between two alloys, scanning electron microscope and transmission electron microscopy are used to observe the microstructure. From the observations, it is known that the Cr-rich carbides are precipitated mainly at the grain boundary. Comparing the ratio of the linear density of the precipitate particles, the higher nitrogen content is, the less carbide is precipitated. Nitrogen might retard the formation of carbides at the grain boundary and reduce the density of cavity sites which are one of the main grain boundary damages.

Abstract: The application of nondestructive evaluation to creep-fatigue damage was examined in this paper. Generally, as the hold time of static load increases, the degradation of material becomes more rapid and the creep-fatigue life decreases. Therefore, in the evaluation of creep-fatigue strength and life of high-pressure vessel such as main steam pipe at high temperature is very important in power plants. In this study, the creep-fatigue behavior of P92 steel was evaluated nondestructively by the backscattered ultrasound using the creep-fatigue specimens. The results obtained by Rayleigh surface wave of backscattered ultrasound were compared and analyzed with the experimental parameters. Also, the relation between the SDA (slope of degraded area) and creep-fatigue life was examined. From the result of nondestructive test, we suggest that SDA would be used as the new parameter for the evaluation of creep-fatigue damage. As the degradation increased, the SDA decreased and also the creep-fatigue life decreased.

Abstract: To investigate the degradation of mechanical properties induced mainly by neutron irradiation and operating temperatures, tensile tests were conducted from room temperature to 300°C using irradiated and unirradiated Zr-2.5Nb pressure tube materials. The irradiated longitudinal and transverse specimens collected from the coolant inlet, middle and outlet parts of the tube which had been operated in the CANDU reactor and showed different operating temperatures and irradiation fluence. The different tensile behavior was characterized by the tensile loading direction in the unirradiated tube. The transverse specimen showed higher strength and lower elongation than those of the longitudinal one. The increased strength hardening and the decreased elongation embrittlement of the irradiated material were compared to those of the unirradiated one. While the tensile strength of the inlet was higher than that of the outlet, the elongation of the inlet was lower than that of outlet. Considering the operation condition, it was proposed that the operating temperature could be a more effective parameter than the irradiation fluence for long-time life. Through TEM observation, it was also found that while the a-type dislocation density was increased, the c-type dislocation was not changed in the irradiated material. The fact that the higher dislocation density was sequentially distributed over the inlet to the outlet parts was consistent with the distribution of the tensile strength.

Abstract: Current research efforts in the development of high temperature defect assessment procedure are summarized. Creep exemption criteria are proposed for the assessment of defective structures at high temperature in consideration of the effects of loadings, operating temperature and service time. Time-dependent failure assessment diagram (TDFAD) is developed that covers major failure mechanisms of defective high temperature structures. Challenges due to the welding effect are discussed. TDFAD for weldments is derived for various combinations of materials. In order to develop a unified assessment method to cope with material and loading complexity, a new failure assessment diagram based on continuum damage concept is proposed to reflect the damage effect on ductile creep failure and brittle creep fracture.