調和組織を有するチタン系材料の疲労き裂発生および伝ぱメカニズム

抄録

Commercially pure (CP) titanium and titanium alloy (Ti-6Al-4V) with a bimodal harmonic structure, which is defined as a coarse-grained structure surrounded by a network structure of fine grains, was fabricated using powder metallurgy to improve both the strength and ductility. Small fatigue crack propagation in bimodal harmonic structured titanium was examined by four-point bending under a stress ratio of 0.1 in the ambient laboratory atmosphere. The crack profiles were observed using optical microscopy and scanning electron microscopy (SEM), and analyzed using an electron backscattered diffraction (EBSD) to examine the mechanism of small fatigue crack propagation on the basis of fractography and crystallography. Fatigue crack paths were not influenced by the bimodal harmonic structure, and the crack growth rates, da/dN, in the harmonic structured titanium were almost the same as those in a material with coarse microstructure for comparable values of stress intensity range, ΔK. In contrast, the harmonic structured titanium had a higher resistance of fatigue crack initiation due to the grain refinement induced by mechanical milling, which resulted in an increase of the fatigue life and fatigue limit. Furthermore, an EBSD analysis was conducted on the facets observed on the fracture surfaces of the harmonic structured titanium to determine the microstructure near the crack initiation site. A transgranular fatigue crack was initiated in the α-facet of coarse-grained structure of the harmonic structured titanium.