This paper investigates the evolution of microstructures and precipitations of an ultra-low iron alloy 625 subjected to long term aging treatment by scanning electron microscope (SEM) and X-ray diffraction(XRD). Use ASTM G28A acid Fe3(SO4)2 erosion to represent intergranular corrosion weightlessness and corrosive morphology. The result shows that alloy at 750°C by aging treatment for 40h, precipitated γ'' phase in the grain boundary. In high density area of γ'' phase, occurs γ'' phase to δ phase degeneration transformation by aging treatment for 200h and the needle-like δ phase becomes more with time prolonged. And γ'' phase degenerated to δ phase completely until treated for 1000h. The sample which has aging treatment tends to have intergranular corrosion and mainly because alloy element spreading leads to dilution area and grain boundary precipitated phase, plus interlaced δ phase's dissolving, which makes sample grain particle fall off and this results in apparent weightlessness. The weightlessness rate(WLR) is related with precipitated volume score. With aging sensitization time change, can be described by Johnson-Mehl-Avrami equation, i.e.: = 44.32 1 − exp (− . ) + 44.62 1 − exp (− . ) + 1.267 ( / ).

The microstructures evolution of precipitations for an ultra-low iron Alloy 625 subjected to long term aging treatment at 750°C was investigated using scanning electron microscope (SEM) and X-ray diffraction (XRD). The intergranular corrosion behaviors of Alloy 625 were evaluated by using ASTM G28A. The result shows that the precipitated phase γ′′-Ni3Nb was mainly precipitated at the grain boundaries and twin boundaries. The number and volume fraction of γ′′ increased with the prolonging of aging time. The transformation of γ′′ to δ-Ni3Nb occurred after aging periods of 200 h. The corrosion resistance of Alloy 625 was significantly reduced during aging treatment. The decrease in intergranular corrosion resistance of Alloy 625 was attributed to the dissolution of precipitated phase and chromium depleted zone. The mass loss rate of Alloy 625 after aging treatment is related to the volume of precipitated phase and can be simulated by Johnson-Mehl-Avrami equation.

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Abstract.A beta phase containing titanium aluminum compound was prepared. Isothermal Fatigue(IF) were subjected at 650 ℃ at three strain rates, such as 6.67×10-3 s-1, 6.67×10-4 s-1, 6.67×10-5 s-1 to determine the effect of strain rate on cyclic stress-strain response (CSSR) of TiAl alloy during IF tests. The curves of cyclic stress-strain response were also discussed. The results show that strain rates have an apparent effect on CSSR of TiAl alloy during IF tests. IntroductionGamma titanium aluminum compounds are currently receiving much attention as candidates of high temperature structural application in aerospace and automobile industries because of weight saving in combination with excellent mechanical properties such as large stiffness, high strength and excellent creep and oxidation resistance at elevated temperature [1][2][3].In the last decades, considerable efforts on TiAl alloys, such as alloying [4,5], heat treatment [4], thermo-mechanical treatment [6,7], have been devoted to improving their low room temperature ductility and poor fracture toughness which will be a serious handicap for their actual applications. Recently, βphase containing TiAl alloys have been developed widely, due to good hot deformability. For example, Kim et al. [6], put forward a new concept of beta-TiAl alloys, which existed within a broad composition range of Ti-(40-45)Al-(2-8)Nb-(1-8)(Cr, Mn, V, Mo)-(0-0.5)(B, C) (in at.%).Its cyclic deformation behaviors of as casted β phase containing Ti-44Al-2Cr-2Nb-0.15B alloy during isothermal fatigue (IF) tests under different strain rate were explored in the present work thus its deformation behaviors, fatigue endurance and resistance to oxidation at elevated temperature can be acquired for conducting its applications for aerospace and automobile industries as important high temperature structural.