Investigation On The Fatigue Properties Of Directionally Solidified Superalloy DZ445

DZ445is a directionally solidified alloy and was newly developed for the blades ofgas turbine. In order to examine its working properties under different conditions, thethermal fatigue (TF, contrast to DZ444and IN738alloy), low cycle fatigue (LCF) andhigh cycle fatigue (HCF) behaviors of DZ445alloy were investigated. The mechanismsof fatigue fracture were subsequently discussed, and they can be concluded as follows:The TF cracks of three alloys almost initiate from the V-typed notches. The mainstyle of crack initiation is the formation, gathering and interconnecting of oxidationcavities induced by thermal stress. The three alloys showed different mechanisms of TFcrack propagation due to the distinct chemical compositions and microstructures. TheTF cracks of equiaxial IN738alloy propagate intergranularly, while that of thedirectional solidification (DS) DZ444alloy propagate along interdendritical regions.The paths of TF crack propagation of DZ445(DS) transfer from interdendrite tocrystallographic orientation with increasing temperatures, i.e.(111)<110>, which is theeasy slip system of face centered cubic alloy. The interaction of high-temperatureoxidation and thermal stress is the main fractural mechanism in TF process. Somemicrostructural evolution at950℃accelerates the reduction of the TF property of threealloys.For DZ445alloy, there isn’t transition fatigue life during LCF tests at all totalstrain amplitudes. It is revealed from the strain-fatigue life curves that elastic strainshows much more influence on fatigue life than plastic strain, indicating the fatigue lifeis determined by the strength of the alloy. Under total strain amplitude ranging from0.5%to0.7%, DZ445alloy initially shows continuous cyclic hardening, followed bycyclic stability. Under the total strain amplitude of0.8%and1.0%, the alloy merelyshows cyclic hardening through its whole fatigue life. Cyclic hardening is resulted fromthe interaction between dislocations and γ′phases or precipitated M23C6carbides. Infatigue source zone, fatigue cracks are predominantly generated at PSBs or somedefects on the surface or subsurface of the specimen such as casting shrinkage and machining trails. In the final fracture region, dendrite fracture is the main fracture modeof the alloy. The slip bands on the surface of the LCF specimens promote the generationof fatigue cracks and fractural planes.The HCF fatigue limit of smooth and notched specimens of DZ445alloy at650℃is440MPa,340MPa respectively. The sensitivity coefficient of fatigue notch of DZ445alloy at650℃is equal to0.1471. In fatigue source zone of smooth specimen, fatiguecracks are predominantly generated at casting shrinkage. The crack propagation zone ofsmooth specimen is characterized by considerable slip bands and fatigue striations. Thefatigue cracks of notched specimens are initiated at the root of the notch, then propagatealong fatigue steps existing in crack propagation region. In the final fracture region,dendrite fracture is the main fracture mode for both specimens. There are abundant slipbands on the surface of both specimens which incline at an angle to stress axis. Themovement mechanism of dislocation in both specimens is γ′shearing by dislocation.During HCF process, M23C6carbides are also precipitated like the LCF.