Improved Tensile Ductility by Severe Plastic Deformation for Nano-Structured Metallic Glass

Abstract

The effect of severe plastic deformation by high-pressure torsion (HPT) on the structure and plastic tensile properties of two Zr-based bulk metallic glasses, Zr_55.7Ni₁₀Al₇Cu₁₉Co_8.3 and Zr₆₄Ni₁₀Al₇Cu₁₉, was investigated. The compositions were chosen because, in TEM investigation, Zr_55.7Ni₁₀Al₇Cu₁₉Co_8.3 exhibited nanoscale inhomogeneity, while Zr₆₄Ni₁₀Al₇Cu₁₉ appeared homogeneous on that length scale. The nanoscale inhomogeneity was expected to result in an increased plastic strain limit, as compared to the homogeneous material, which may be further increased by severe mechanical work. The as-cast materials exhibited 0.1% tensile plasticity for Zr₆₄Ni₁₀Al₇Cu₁₉ and Zr_55.7Ni₁₀Al₇Cu₁₉Co_8.3. Following two rotations of HPT treatment, the tensile plastic strain was increased to 0.5% and 0.9%, respectively. Further testing was performed by X-ray diffraction and by differential scanning calorimetry. Following two rotations of HPT treatment, the initially fully amorphous Zr_55.7Ni₁₀Al₇Cu₁₉Co_8.3 exhibited significantly increased free volume and a small volume fraction of nanocrystallites. A further increase in HPT rotation number did not result in an increase in plastic ductility of both alloys. Possible reasons for the different mechanical behavior of nanoscale heterogeneous Zr_55.7Ni₁₀Al₇Cu₁₉Co_8.3 and homogeneous Zr₆₄Ni₁₀Al₇Cu₁₉ are presented.

Keywords: high-pressure torsion; metallic glass; nano-heterogeneous structure; tensile plasticity.