Structural relaxation and rejuvenation in a metallic glass induced by shot-peening

Abstract

The metallic glass Pd₄₀Cu₃₀Ni₁₀P₂₀ in as-cast or pre-annealed states was shot-peened successively at room temperature or at 77 K. The structural state of the glass was characterized by the relaxation spectrum measured in a differential scanning calorimeter. Mechanically induced relaxation of the as-cast glass and mechanically induced rejuvenation of pre-annealed samples are both more evident at 77 K than at 298 K, enabling deductions about the underlying mechanisms. The relaxation spectrum of the glass as a function of temperature displays two broad maxima, which occurring at the higher temperature is attributed to the part of the free-volume distribution associated with flow defects. In samples shot-peened at 77 K, the stored energy after deformation can be as high as 20% of the cold work. Shot-peening simultaneously generates flow defects within shear bands and destroys them in the matrix between bands: whose effect dominates is principally dependent on the initial state of relaxation of the glass. Shot-peening of partially crystallized samples appears capable of breaking up and dispersing crystallites without inducing any further significant crystallization.

Keywords:

• differential scanning calorimetry
• metallic glass
• plastic deformation
• structural relaxation

Acknowledgements

The authors are grateful to N. Nishiyama (Institute for Materials Research, Tohoku University, Japan) for providing samples. A. Concustell acknowledges support from MICINN (Spain) for postdoctoral contract EX-2007-0391, and the other authors acknowledge the support from European Commission under MCRTN contract “Ductile Bulk Metallic Glass Composites”.