Micromechanisms of Fracture and Fatigue-Crack Growth in Bulk Metallic Glass Alloys
Final rept. 1 May-31 Dec 97
CALIFORNIA UNIV BERKELEY DEPT OF MATERIALS SCIENCE AND MINERAL ENGINEERING
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The recent development of metallic alloy systems which can be processed with an amorphous structure over large dimensions, specifically to form metallic glasses at low cooling rates tilde 10 Ks, has permitted novel measurements of important mechanical properties. These include, fatigue crack growth and fracture toughness behavior, representing the conditions governing the subcritical and critical propagation of cracks. In the present study, bulk plates of a Zr41.2 Ti13.8 Cu12.5 Ni10 Be22.5 alloy, machined into 7 mm thick, 33 mm wide compact tension specimens and fatigue precracked following standard procedures, revealed fracture toughness in the fully amorphous structure of KIc tilde 55 MPasquare root m, i.e., comparable with that of a high strength steel or aluminum alloy. However, annealing to induce partial and full crystallization was found to result in a drastic reduction in fracture toughness to 1 MPasquare root m, i.e., comparable with silica glass. Under cyclic loading, whereas crack propagation behavior of the bulk amorphous metal was similar to that observed in traditional steel and aluminum alloys, the stress-life S-N properties of were markedly different. As in more traditional crystalline metallic alloys, the crack propagation mechanism in the metallic glass was associated with alternating blunting and resharpening of the crack tip as evidenced by striations on fatigue fracture surfaces. Alternatively, during S-N tests flaws apparently initiated quite easily due to the lack of microstructural barriers which would normally provide local crack arrest points, thereby giving rise to poor S-N properties.
- Ceramics, Refractories and Glass
- Metallurgy and Metallography