構造用チタン合金の疲労特性に及ぼす金属組織の影響

抄録

Ti-6Al-4V alloy is widely used as aero engine parts such as low pressure compressor disks and blades. For these applications, fatigue properties are very important. It is well known that fatigue properties of titanium alloys are severely affected by their microstructures. In this study, Ti-6Al-4V samples with bi-modal microstructure (combination of equiaxed α phase and lamellar α/β phase) were produced through upset forging, and the effects of microstructure on high cycle fatigue properties were investigated. 100 mm diameter and 200 mm height Ti-6Al-4V cylinder shape samples were isothermally forged to 50 mm height pan-cake shape samples at two different temperature of 1173 K or 1073 K using NIMS 1500 t forging simulator. To obtain bi-modal microstructure, forged samples were heat treated at 1223 K, α/β phase region, followed by aging at 1023 K. High cycle fatigue specimens were machined along the hoop direction of heat treated samples. Microstructural observation by optical microscopy and scanning electron microscopy, and crystal orientation analysis by electron backscattered diffraction (EBSD) were conducted and compared with the results of high cycle fatigue tests. Material forged at lower temperature of 1073 K exhibited longer fatigue life compared to that of material forged at higher temperature of 1173 K. Materials forged at 1173 K has strong texture (high directivity of crystal orientation) of α phase compared to materials forged at 1073 K, and it seems to affect their fatigue life.