Characteristics of the structural and johari-goldstein relaxations in pd-based metallic glass-forming liquids

The dynamics of Pd-based metallic glass-forming liquids (Pd ₄₀Ni₁₀Cu₃₀P₂₀, Pd _42.5Ni_7.5Cu₃₀P₂₀, Pd ₄₀Ni₄₀P₂₀, and Pd₃₀Ni ₅₀P₂₀) was studied by mechanical spectroscopy and modulated differential scanning calorimetry (MDSC). We found that the change in composition has a significant effect on the α relaxation dynamics; the largest difference corresponds to an increase of the glass transition temperature T_g of ∼15 K, for materials in which 30% Ni was substituted by 30% Cu (i.e., from Pd₄₀Ni₄₀P₂₀ to Pd₄₀Ni₁₀Cu₃₀P₂₀). We also found that all Pd-based metallic glasses have very similar fragilities, 59 < m < 67, and Kohlrausch stretched exponents, 0.59 < β_KWW < 0.60. It is interesting that the values of m and β_KWW correlate well with the general relation proposed by Böhmer et al. for nonmetallic glass formers (Böhmer, R.; et al. J. Chem. Phys. 1993, 99, 4201-4209), which for the observed β_KWW values predicts 58 < m < 61. From a linear deconvolution of the α and β relaxations, we find that the substitution of the Ni with Cu induced a large change in the time constant of the Johari-Goldstein relaxation, τ_β. The activation energy, U_β, of the β relaxation was largely independent of chemical composition. In all cases, 25 < U_β/RT < 28, a range in agreement with results for other glass formers (Kudlik, A.; et al. Europhys. Lett. 1997, 40, 649-654 and Ngai, K. L.; et al. Phys. Rev. E 2004, 69, 031501). From the heat capacity and mechanical loss, estimates were obtained for the number of dynamically correlated units, N_c; we find significantly larger values for these metallic glass-forming liquids than N _c for other glass-forming materials.