Study Of Surface Microcrack Initiation And Fatigue Life For A Thermal-Exposured γ-Tial Based Alloy

The high cycle fatigue behaviour of a high strength y-TiAl based near lamellar alloy Ti-44A1-4Nb-4Zr-0.2Si-1B(at.%) has been studied under a) without exposure, b) with exposure and c) with exposure+surface oxidation conditions to assess the effects of four types of surface manufactured. At the same time, the oxidation behavior of this alloy subjected to an air exposure for up to7000hours at700℃has been investigated.It has been found that the maximum stresses of fatigued specimens are always lower than the yield stress of the alloy. Local plastic deformation is difficult to occur on the maximum-stressed surface. The surface quality with or without prior cracks/defects therefore becomes critical in determining the fatigue strength. Different surface processing methods significantly affects fatigue strength of the alloy, whether before and after long-term thermal exposure at700℃for10000hours. Thermal exposure can not change the influence of the surface-manufacturing quality on fatigue strength which the fatigue strength of the four surface conditions is still in an order of electropolishing> shot peening> electrodischage machining> V-notch. The study has also found that under the condition of σmax<σ0.1in the alloy, microcrack initiation always occurs before yielding. The relatively weak microstructures and phases therefore become the preferential initiation sites and propagation routes. They were observed to be equiaxed y grains, B2+ω grains and α2-γ lamellar interface in soft orientations.The alloy after10000h-long thermal exposure+surface oxidation at700℃demonstrates a "thermal exposure strengthening" phenomenon in S-N fatigue strength, compared to the same types of surface condition before exposure. The thermal exposure strengthening is attributed to the profound stress relaxation occurring when samples stayed in the isothermal environment for10000hours. The sustained high temperature environment induces decrease of the EDM surface tensile stress, thinning the compressively stressed layer, reduction of the stress concentration/defect sites both on the surface and on the subsurface, alleviation of the stress concentration near the V notch, etc. The present results suggest that the beneficial effects of the stress relaxation caused by such a long-term isothermal exposure in air outweigh the harmful effects of oxidation layers formed on the sample surface. It has been found that the degree of the "thermal exposure strengthening" is in a reverse relationship with the quality of the surface. The worse surface produced, the more profound stress relaxation induced, and the higher degree of thermal exposure strengthening obtained.However, the fatigue strength of plane-sided specimens decreased noticeably after the same long-term exposure at700℃, compared to the same surface condition without exposure. This thermal exposure condition involves no stress relaxation since all specimens were machined after exposure. Such a decrease in fatigue strength is attributed to embrittlement induced by phase transformation and microstructural changes. V-notched specimens (kt=3.0) show a significant reduction in fatigue life and become the key factor in determining fatigue behavior for all the three exposure conditions. The study has found that it is difficult to have plastic deformation occurred at the notch root under the condition of σmax<σ0.1, under which no notch strengthening is observed.Both the specimens subjected to air exposure at700℃for7000h and the specimens after exposure+oxidation treatment at700℃for10000h have demonstrated good oxidation resistance. This is attributed to the added elements Nb and Si that are capable of improving oxidation resistance for the alloy. The oxide scales were mainly composed of TiO2and Al2O3. The composite oxidation layer is non-uniform in terms of composition and distribution. The migration rate and diffusion distance of Ti, Al and O elements are considered to be the main factor in controlling the morphology of the oxide layer.The results reveal that surface-machining process is the all-important factor that plays a leading role in fatigue performance. Surface quality is therefore crucial in manufacturing y-TiAl alloy components, which is simply a matter of the component life and the security of human being. Concerning the impacts of high-temperature atmospheric environment on TiAl alloy components, the present study indicates clearly that the positive impacts outweigh the negative impacts in terms of fatigue life.