PRODUCTION OF HIGH FRACTURE TOUGHNESS AND HIGH FATIGUE STRENGTH BULK METALLIC GLASSES AND EXAMINATION OF THE MECHANISM

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Single phase (SF) bulk glassy alloys (BGAs) and nanocrystalline (NC) EGAs of some Zr-, Cu- and Pd-based systems have been synthesized by some fabrication methods. Where, the NC BGA is the glassy alloy which contain nano-scale crystalline particles embedded homogeneously in glassy matrix. Then fatigue strength and fracture toughness in these BGAs were examined.The ratio of fatigue limit (σ_w ; half a full stress amplitude) under stress ratio of 0.1 to tensile strength (σ_B) of the Zr-based NC BGA showed a value of 15% (σ_B=1.7 GPa), and it was approximately three times larger compared with the ratio of the SF BGA of the same composition. The ratio of σ_w to σ_B of the Cu-based BGA exhibited a very large value of 24% (σ_B=2.0 GPa). These σ_w s show the enough values for applications. It is thought that the drastic rise in the σ_w of these NC BGAs is derived from the prevention of slip initiation and propagation by the NC particles.A Zr-based BGA with a high degree of amorphousness exhibited a fracture touness value (Kic) of 53 MPa・m^<1/2>. It was clarified that the Zr-based BGA has a sufficient Kic for practical uses. The Kic s of a Zr-based NC BGA and considerably crystallized Zr-based BGA were small even in the maximum value with 40 and 11 MPa・m^<1/2>, respectively. This is thought to be due to the embrittlement by the crystallization.The Pd-based SF BGA could not have fatigue cracks, and the value of notched fracture toughness exhibit 36 MPa・m^<1/2> and cracks propagated in three or more directions at the same time. An estimated K/c showed a small value of 12 MPa・m^<1/2>.