FRACTURE-TOUGHNESS AND FATIGUE-CRACK PROPAGATION IN A ZR-TI-NI-CU-BE BULK METALLIC-GLASS
by Gilbert-CJ (*R) Ritchie-RO Johnson-WL
APPLIED PHYSICS LETTERS v71 (4) : pp476-478 (1997 Jul 28)
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 (similar to 10 K/s), has permitted novel measurements of important mechanical properties. These include, for example, fatigue-crack growth and fracture toughness behavior, representing the conditions governing the subcritical and critical propagation of cracks in these structures. In the present study, bulk plates of a Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy, machined into 7 mm wide, 38 mm thick compact-tension specimens and fatigue precracked following standard procedures, revealed fracture toughnesses in the fully amorphous structure of K(lc)similar to 55 MPa root m, i.e., comparable with that of a high-strength steel or aluminum ahoy. However, partial and full crystallization, e.g., following thermal exposure at 633 K or more, was found to result in a drastic reduction in fracture toughness to similar to 1 MPa root m, i.e., comparable with silica glass. The fully amorphous alloy was also found to be susceptible to fatigue-crack growth under cyclic loading, with growth-rate properties comparable to that of ductile crystalline metallic alloys, such as high-strength steels or aluminum alloys; no such fatigue was seen in the partially or fully crystallized alloys which behaved like very brittle ceramics. Possible micromechanical mechanisms for such behavior are discussed. (C) 1997 American Institute of Physics.
SMALL ATOM DIFFUSION AND BREAKDOWN OF THE STOKES-EINSTEIN RELATION IN THE SUPERCOOLED LIQUID-STATE OF THE ZR46.7TI8.3CU7.5NI10BE27.5 ALLOY
by Geyer-U (*R) Johnson-WL Schneider-S Qiu-Y Tombrello-TA Macht-MP
APPLIED PHYSICS LETTERS v69 (17) : pp2492-2494 (1996 Oct 21)
Be diffusivity data in the bulk metallic glass forming alloy Zr46.7Ti8.3Cu7.5Ni10Be27.5 are reported for temperatures between 530 and 710 K, extending 85 K into the supercooled liquid state of thr alloy, At the glass transition temperature T-g, a change in temperature dependence of the data is observed, and above T-g the diffusivity increases more quickly with temperature than below, The data in the supercooled liquid can be described by a modified Arrhenius expression based on a diffusion mechanism suggested earlier. The comparison with viscosity data in the supercooled liquid state of Zr46.7Ti8.3Cu7.5Ni10Be27.5 reveals a breakdown of the Stokes-Einstein relation, indicating a cooperative diffusion mechanism in the supercooled liquid state of Zr46.7Ti8.3Cu7.5Ni10Be27.5. (C) 1996 American Institute of Physics.
BULK METALLIC GLASSES - A NEW ENGINEERING MATERIAL
by Johnson-WL (*R)
CURRENT OPINION IN SOLID STATE & MATERIALS SCIENCE v1 (3) : pp383-386 (1996 Jun)
The development of new metallic alloys which form bulk glasses at low cooling rates has led to significant advances in the study of undercooled liquid metals and the glass transition in metallic systems. For the first time it has become possible to carry out measurements of thermophysical properties such as specific heat, viscosity, atomic diffusion, and crystal nucleation rates for liquid alloys over the full temperature range of the undercooled melt. These new materials exhibit a rich variety of phenomena, such as liquid crystal phase separation and nanocrystalline phase formation, and have interesting potential as engineering materials.
NONCONTACT MODULATION CALORIMETRY OF METALLIC LIQUIDS IN LOW-EARTH-ORBIT
by Wunderlich-RK Lee-DS (*R) Johnson-WL Fecht-HJ
PHYSICAL REVIEW B CONDENSED MATTER v55 (1) : pp26-29 (1997 Jan 1)
Noncontact modulation calorimetry using electromagnetic heating and radiative heat loss under ultrahigh-vacuum conditions has been applied to levitated solid, liquid, and metastable liquid samples. This experiment requires a reduced gravity environment over an extended period of time and allows the measurement of several thermophysical properties, such as the enthalpy of fusion and crystallization, specific heat, total hemispherical emissivity, and effective thermal conductivity with high precision as a function df temperature. From the results on eutectic glass forming Zr-based alloys thermodynamic functions are obtained which describe the glass-forming ability of these alloys.