Ni基超合金単結晶の疲労強度に及ぼす結晶方位と応力波形の影響

Abstract

The stress-controlled fatigue tests were performed at 700°C. In this study, two kinds of fatigue testing procedures were adopted. One is the tension-compression triangle-waveform test (R=−1). The other is the tensile or compressive stress hold testing procedures. Cracks initiated at casting pores under tensile stress conditions. Only in compressive stress hold test, crack was initiated by stage I cracking (crystallographic cracking on the {111} slip plane). In [011] and [\bar111], the transition from mode I cracking (perpendicular to the principal stress axis) to mode II-type crystallographic cracking on the {111} slip plane was occurred easily because there were not enough non-co-planar slip systems in order to relax the stress concentration near the crack-tip.The [001] crystals showed stable mode I crack growth because the stress concentration at crack-tip was relaxed by homogeneous multiple slip. The creep-fatigue damage mechanisms were found to be divided into two groups. In the case of the [001]-compressive and [011]-tensile stress hold waveforms, deformation occurred by mechanical-twin shear through γ and γ′. On the other hand, in the case of the [001]-tensile and [011]-compressive stress hold waveforms, each γ′ particle was sheared by superlattice partial with the fault bounded Shockley partial because the direction of shear stress on the (111) plane was opposite to that of microtwin formation. Fatigue lifetimes were decreased by the formation of deformation twinning.