Localized atomic segregation in the spalled area of a Zr ₅₀ Cu ₄₀ Al ₁₀ bulk metallic glasses induced by laser-shock experiment

Abstract

Abstract Laser-shock experiments were performed on a ternary <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mstyle displaystyle="false"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">Z</mml:mi> <mml:mi mathvariant="normal">r</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>50</mml:mn> </mml:mrow> </mml:msub> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> <mml:mi mathvariant="normal">u</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>40</mml:mn> </mml:mrow> </mml:msub> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">A</mml:mi> <mml:mi mathvariant="normal">l</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:mstyle> </mml:math> bulk metallic glass. A spalling process was studied through post-mortem analyses conducted on a recovered sample and spall. Scanning electron microscopy magnification of fracture surfaces revealed the presence of a peculiar feature known as cup-cone. Cups are found on sample fracture surface while cones are observed on spall. Two distinct regions can be observed on cups and cones: a smooth viscous-like region in the center and a flat one with large vein-pattern in the periphery. Energy dispersive spectroscopy measurements conducted on these features emphasized atomic distribution discrepancies both on the sample and spall. We propose a mechanism for the initiation and the growth of these features but also a process for atomic segregation during spallation. Cup and cones would originate from cracks arising from shear bands formation (softened paths). These shear bands result from a quadrupolar-shaped atomic disorder engendered around an initiation site by shock wave propagation. This disorder turns into a shear band when tensile front reaches spallation plane. During the separation process, temperature gain induced by shock waves and shear bands generation decreases material viscosity leading to higher atomic mobility. Once in a liquid-like form, atomic clusters migrate and segregate due to inertial effects originating from particle velocity variation (interaction of release waves). As a result, a high rate of copper is found in sample cups and high zirconium concentration is found on spall cones.