Ichitsubo T, Matsubara E, Chen H S, Saida J, Yamamoto T, Nishiyama N
Department of Materials and Science Engineering, Kyoto University, Kyoto 606-8501, Japan.
J Chem Phys. 2006 Oct 21;125(15):154502. doi: 10.1063/1.2346672.
It has been reported that the structural stability is significantly deteriorated under radio-frequency-ultrasonic perturbation at relatively low temperatures, e.g., near/below the glass transition temperature T(g), even for thermally stable metallic glasses. Here, we consider an underlying mechanism of the ultrasound-induced instability, i.e., crystallization, of a glass structure to grasp the nature of the glass-to-liquid transition of metallic glasses. Mechanical spectroscopy analysis indicates that the instability is caused by atomic motions resonant with the dynamic ultrasonic-strain field, i.e., atomic jumps associated with the beta relaxation that is usually observed for low frequencies of the order of 1 Hz at temperatures far below T(g). Such atomic motions at temperatures lower than the so-called kinetic freezing temperature T(g) originate from relatively weakly bonded (and/or low-density) regions in a nanoscale inhomogeneous microstructure of glass, which can be straightforwardly inferred from a partially crystallized microstructure obtained by annealing of a Pd-based metallic glass just below T(g) under ultrasonic perturbation. According to this nanoscale inhomogeneity concept, we can reasonably understand an intriguing characteristic feature of less-stable metallic glasses (fabricated only by rapid melt quenching) that the crystallization precedes the glass transition upon standard heating but the glass transition is observable at extremely high rates. Namely, in such less-stable metallic glasses, atomic motions are considerably active at some local regions even below the kinetic freezing temperature. Thus, the glass-to-crystal transition of less-stable metallic glasses is, in part, explained with the present nanoscale inhomogeneity concept.
据报道，即使对于热稳定的金属玻璃，在相对较低的温度下，例如接近/低于玻璃化转变温度T(g)时，其结构稳定性在射频-超声扰动下会显著恶化。在此，我们考虑玻璃结构超声诱导不稳定性（即结晶）的潜在机制，以理解金属玻璃从玻璃态到液态转变的本质。机械光谱分析表明，这种不稳定性是由与动态超声应变场共振的原子运动引起的，即与β弛豫相关的原子跳跃，β弛豫通常在远低于T(g)的温度下，频率约为1 Hz的低频时观察到。在低于所谓的动力学冻结温度T(g)时，这种原子运动源于玻璃纳米级不均匀微观结构中相对弱键合（和/或低密度）的区域，这可以从在超声扰动下，刚好低于T(g)对钯基金属玻璃进行退火得到的部分结晶微观结构中直接推断出来。根据这种纳米级不均匀性概念，我们可以合理地理解较不稳定金属玻璃（仅通过快速熔体淬火制备）的一个有趣特征，即在标准加热时结晶先于玻璃转变，但在极高加热速率下可以观察到玻璃转变。也就是说，在这种较不稳定的金属玻璃中，即使在动力学冻结温度以下，原子运动在某些局部区域也相当活跃。因此，较不稳定金属玻璃从玻璃态到晶态的转变，部分可以用当前的纳米级不均匀性概念来解释。
