THERMEC 2006

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Authors: Janny Lindemann, Anja Kutzsche, Michael Oehring, Fritz Appel
Abstract: The effect of shot peening and roller burnishing on the fatigue performance of the γ(TiAl) alloy Ti-45Al-9Nb-0.2C was investigated over a wide range of processing intensities. At optimized conditions shot peening and roller burnishing can markedly improve the fatigue strength at ambient temperatures. For temperatures above 650 °C, the residual compressive stresses induced by shot peening and roller burnishing quickly relax. This indicates that, at elevated temperatures, surface roughness and dislocation strengthening become more important for the fatigue performance of mechanically surface-treated components. Roller burnishing leads to much lower surface roughness than shot peening, resulting in more effective improvement of high temperature fatigue performance. However, surface strengthening by shot peening can also be beneficial for the fatigue performance at elevated temperatures, when the surface roughness is reduced by subsequent polishing.
1553
Authors: M. Roth, Horst Biermann
Abstract: The efficiency of aircraft and industrial gas turbines and combustion engines depends on the maximum operation temperature and, therefore, on the properties of the commercial high temperature materials. In the temperature range 500°C to 750°C γ-titanium aluminides especially alloys of the third generation represent an attractive alternative to the established nickel-base superalloys which have the double density. Due to superimposed cyclic thermal and cyclic mechanical loadings during start-up and shut-down operations structural components in gas turbines and combustion engines may not only be exposed to isothermal but also to thermo-mechanical fatigue (TMF). In this study the cyclic deformation and fatigue behaviour under thermo-mechanical load of the γ-TiAl alloy TNB-V5 with near-gamma microstructure is evaluated. To set a fatigue-life relation strain-controlled thermo-mechanical fatigue tests were carried out with two different strain ranges, different temperature-strain cycles and different temperature ranges from 400°C to 800°C. Additional low-cycle fatigue (LCF) tests were performed at 400°C, 600°C and 800°C for comparison. Cyclic deformation curves, stress-strain hysteresis loops and fatigue lives of the tests are presented. The shortest fatigue lives are always observed in out-of phase (OP) tests, the longest in in-phase (IP) tests. Clockwise-diamond (CD) and counter-clockwise diamond (CCD) testing yield similar fatigue lives intermediate between those of OP and IP tests. For a general life prediction the double-logarithmic plot of the damage parameter by Smith, Watson and Topper vs. fatigue life is well suitable.
1559
Authors: Yoshihisa Harada, David C. Dunand
Abstract: The microstructure of ternary Al3(Sc1-yREy) intermetallic compounds (where RE is one of the rare-earth elements La, Ce, Nd, Sm, Eu, Yb or Lu), was investigated as a function of RE concentration for 0
1565
Authors: Wen Fang Cui, Chun Ming Liu, V. Bauer, Hans Jürgen Christ
Abstract: Isothermal low cycle fatigue (LCF) behaviours of a third generation titanium aluminide based γ-TiAl alloy with duplex microstructure were investigated under the various test conditions, including temperature (550°C-750°C), total strain amplitude (0.3%-0.6%) and environment (air and vacuum), in order to clarify the fatigue life, deformation characters and fracture process of the alloy during LCF. The plastic strain accumulation has a great contribution to LCF damage. With increasing total strain range, LCF life decreases distinctly. Under the small total strain amplitude (≤0.4%), the increase of test temperature enforces microstructure resistance to LCF fracture. However, the increase of test temperature together with large total strain amplitude (>0.5%) accelerates the microstructural degradation, which behaves the dissolution of α2 lamellae and recrystallization of γ phase, resulting in great LCF damage. Moreover, environment brittlement during high temperature exposure to air influences the initiation process of fatigue cracks. The fracture mechanisms at various test conditions were analyzed.
1571
Authors: Han Cheol Choe, S.J. Park, S.W. Eun, Yeong Mu Ko
Abstract: In order to investigate the intergranular and pitting corrosion behavior of Fe-25Al-6Cr intermetallic compounds containing Mo, Nb and B, 11 kinds of Fe-25Al were prepared by arc melting in Ar gas, using high purity aluminum and iron. The fabricated materials were heat treated for homogenious structure and stabilization of the iron aluminides. The electrochemical methods were used for interganular and pitting corrosion. Effects of Mo, Cr, Nb and B on the repassivation current density(Ir) and the active current density(Ia) of Fe-25Al-6Cr were recorded in the following order: MoCr>Nb>B, from the highest to the lowest respectively.
1583
Authors: Won Yong Kim
Abstract: Microstructures and mechanical properties of Ni3Al based intermetallic alloys produced by vacuum arc melting and vacuum induction melting were investigated in terms of phase analysis using scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), X-ray diffractometer and tensile test machine. The duplex microstructural feature consisting of γ’ matrix phase and small intermetallic dispersoids was observed to be distributed over the whole microstructure in the Zr and/or Mo-added samples. From the SEM-EDS analysis of the alloys, it is clearly confirmed that the Mo is solved both into γ’ matrix phase and intermetallic phase while Zr has a role to form an intermetallic Ni5Zr phase for the entire alloys investigated. The ultimate tensile strength of the present alloy was superior to iron-based and Ni-based die materials especially in the high temperature region. The mechanical results obtained will be discussed in correlation with microstructural observations, phase analyses.
1589
Authors: Yoon Uk Heo, Seung Ho Mun, Hu Chul Lee
Abstract: The mechanism of grain boundary embrittlement and the improvement of the tensile ductility afforded by alloy addition or heat treatment was investigated in an Fe-Mn-Ni alloy. The precipitation of θ-MnNi intermetallic particles was observed at the prior austenite or interlath boundaries during the aging treatment and this was believed to be responsible for the grain boundary embrittlement of these alloys. After prolonged aging or aging at higher temperatures above 520°C, these metastable intermetallic particles were transformed into the thermodynamically stable austenite phase, thereby leading to the recovery of the grain boundary strength. The addition of Mo caused the grain boundary precipitate to be changed to austenite and resulted in a significant improvement in the tensile ductility after aging.
1595
Authors: Takehiko Watanabe, K. Oohara
Abstract: This study was carried out to newly develop the fluxes and filler metals for brazing magnesium alloy AZ31B more easily at lower temperatures. Furthermore, surface preparation was developed to improve the brazeability of magnesium alloy. The main results obtained are as follows. We could successfully develop the fluxes that consisted of chlorides containing Ca ion and Li ion, which made the faying surface of the magnesium alloy active at around 450°C. In addition, we succeeded in developing the filler metals with the melting temperatures lower than 490°C which were Mg-Sn-In system containing a small amount of Al to lower the melting temperature. Surface preparation for magnesium alloy by immersion in aqueous solution containing halogen ion improved remarkably the brazeability of the magnesium alloy. Using the surface preparation together, the fluxes and filler metals could achieve the brazed joints with a high strength equivalent to that of the base metal.
1603

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