Characteristicsof the Structural and Johari–GoldsteinRelaxations in Pd-Based Metallic Glass-Forming Liquids.

Thedynamics of Pd-based metallic glass-forming liquids (Pd40Ni10Cu30P20, Pd42.5Ni7.5Cu30P20, Pd40Ni40P20, and Pd30Ni50P20)was studied by mechanical spectroscopy and modulated differentialscanning calorimetry (MDSC). We found that the change in compositionhas a significant effect on the α relaxation dynamics; the largestdifference corresponds to an increase of the glass transition temperature Tgof ∼15 K, for materials in which 30%Ni was substituted by 30% Cu (i.e., from Pd40Ni40P20to Pd40Ni10Cu30P20). We also found that all Pd-based metallic glasses havevery similar fragilities, 59 < m< 67, andKohlrausch stretched exponents, 0.59 < βKWW<0.60. It is interesting that the values of mandβKWWcorrelate well with the general relation proposedby Böhmer et al. for nonmetallic glass formers (Böhmer,R.; et al. J. Chem. Phys.1993, 99, 4201–4209), which for the observed βKWWvalues predicts 58 < m< 61. Froma linear deconvolution of the α and β relaxations, wefind that the substitution of the Ni with Cu induced a large changein the time constant of the Johari–Goldstein relaxation, τβ. The activation energy, Uβ, of the β relaxation was largely independent of chemical composition.In all cases, 25 < Uβ/RT< 28, a range in agreement with results for otherglass formers (Kudlik, A.; et al. Europhys. Lett.1997, 40, 649–654 and Ngai,K. L.; et al. Phys. Rev. E2004, 69, 031501). From the heat capacity and mechanical loss,estimates were obtained for the number of dynamically correlated units, Nc; we find significantly larger values for thesemetallic glass-forming liquids than Ncfor other glass-forming materials. [ABSTRACT FROM AUTHOR]