Zr 基バルク金属ガラスのガラス遷移温度以下の温度におけるクリープ変形挙動

Abstract

Bulk Metallic Glasses (BMGs) have high strength, high fatigue limit and high fracture toughness. However, the creep property at temperatures enough less than the glass transition temperature (Tg) has not been examined yet, although it is needed for practical uses. Therefore, in this report, the creep test under a constant load in the atmosphere at comparatively low temperature less than the Tg {573 K(T/Tg=0.84) and 473 K (T/Tg=0.69)} was carried out using the Zr55Cu30Al10Ni5 at% BMG. As the result, typical creep curves just like as ones in the crystalline alloys were shown, that is, there were transient, steady state and accelerated creeps, and creep strain rate increased with increasing the test temperature and load. The creep deformation occurred uniformly along the parallel part. Just before the break, a necking occurred and several shear bands came at the region due to induced high stress, and finally fractured along those shear bands in the direction of about 50 degrees for the loading axis like as the tensile fracture in the BMG. The specimen surface was oxidized and got into hard ZrO2. In the ZrO2 layer, many cracks were observed. However, in the inner region eliminating the oxide layer, no cracks, no shear bands and on the fracture surface typical vein patterns were observed. Therefore, it was presumed that the uniform creep deformation along the parallel part occurred through a viscous flow under the equilibrium state maintained glass structure.