Structural relaxation kinetics defines embrittlement in metallic glasses

Abstract

Structural relaxation during isothermal annealing, quantified by enthalpy recovery of Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ towards its metastable equilibrium and correlation to embrittlement, quantified through fracture toughness, K_Q, is studied. Enthalpy relaxation over time obeys the Kohlrausch-William-Watts (KWW) stretch exponent with β = 0.74 and τ = 11,000 s. Such β and τ are used to fit experimental K_Q(t) with KWW, resulting in R² = 0.79. This finding combined with a controlled characterization of the glasses'K_Q versus temperature, fictive temperature, and their combination, revealed that embrittlement in metallic glasses is predominantly controlled by structural rearrangements, whereas volume changes from thermal expansion have negligible influence.