粉末冶金法Ti<SUB>3</SUB>Al-Nb金属間化合物の金属組織制御による高サイクル疲労強度の改善

Abstract

Ti3Al-based alloys are attractive materials for aircraft and automobile parts because of their higher strength to weight ratio and higher Young’s modulus compared to those of conventional titanium alloys. In the present study, Ti3Al alloys containing large amounts of Nb were produced using a blended elemental (BE) powder metallurgy (P/M). Since high cycle fatigue strength is particularly important for industrial applications, special attention was paid to improve this property by the modification of microstructure.Extra low chlorine titanium powder and Niobium-Aluminum master alloy powder were used as starting powder materials. Blended powders were cold pressed using mechanical pressing or cold isostatic pressing, vacuum sintered at 1573 K for 10.8 ks and finally hot isostatic pressed (HIP’ed) at 1373 K and 200 MPa for 10.8 ks. The compacts produced by this conventional method showed a coarse colony-like microstructure. A refined equiaxed α2 microstructure was obtained by the innovative BE P/M method, in which a water-quenching step is added prior to HIP’ing. Mechanism of the formation of an equiaxed microstructure was analyzed based on the observation of microstructural evolution during the HIP thermal cycle. Remarkable improvement of smooth high cycle fatigue strength was attained by this microstructural modification. For example, high cycle fatigue strength at 107 cycles increased to 440 MPa, from 340 MPa for conventionally processed materials.