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Softening of phonon spectra in metallic glasses(金属玻璃中声子谱的软化)
发布时间:2016-11-11

Softening of phonon spectra in metallic glasses(金属玻璃中声子谱的软化)

摘要:采用密度泛函理论和实施小位移法计算,可研究MgZnCa系列非晶合金的振动谱,采用从头算分子动力学模拟方法可得到这些合金的原子结构。基于德拜模型计算结果来看,振动的热力学性能是温度的函数,尤其是低温比热可近似为温度的3次函数。本研究计算了Mg的振动对比热的贡献,研究了Mg声子谱的软化,其中最大允许振动频率被降低,而高度集体扩散过程却被促进。结果在合金所报道的临界铸造厚度与Mg声子的软化之间获得了统计相关性。采用类似方法对两个明显不同的ZrTiCuAl非晶合金及他们大、小不同的关键铸件厚度分别进行了计算,发现ZrTiCuAl的计算结果与MgZnCa的一致。

Abstract: The vibrational spectra of a series of MgZnCa amorphous alloys were computed using density functional theory and implementing the small displacement method. The atomic structures of the alloys were obtained by ab initio molecular dynamics simulations. The vibrational thermodynamic properties were calculated as a function of temperature and, in particular, the specific heat at low temperature was approximated by temperature cubed based on the Debye model. We computed the contribution of Mg vibrations to the specific heat and investigated the softening of Mg phonon spectra, where the maximum allowed vibrational frequency is lowered and highly collective diffusion processes are promoted. The statistical correlation between the reported critical casting thickness of the alloys and softening of Mg phonons was obtained. Similar calculations were performed for two distinctively different amorphous ZrTiCuAl alloys with large and small reported critical casting thickness, respectively. The findings were consistent with those of the MgZnCa alloys.

Editorial Summary Lower vibrational frequencies of atoms in metallic glasses(金属玻璃中原子的较低振动频率)

金属玻璃是一类具有高硬度、高回弹性、耐腐蚀性和耐久性的结构材料。金属玻璃中原子按一定频谱振动,对确定热性能(如热容量)起重要作用,还可影响合金的稳定性。基于量子分子动力学模拟对金属玻璃的振动谱进行计算很有挑战性,原因是这样的计算既复杂,又密集。澳大利亚的新南威尔士大学Michael Ferry及其同事对此作了模拟,并对几种不同的金属玻璃作了振动谱计算。他们发现,在某些合金中,原子会作较低频率的振动。他们找到了向低频振动谱的转变对热容量、熵和自由能的影响规律,并讨论了这些方面对金属玻璃的动力学性能和稳定性的影响。

Metallic glasses are a class of structural materials exhibiting high hardness, high resilience, corrosion resistance and durability. The spectrum of frequencies at which atoms vibrate in metallic glasses play an important role in determining thermal properties such as heat capacity and can affect alloys stability. The calculation of vibrational spectrum for metallic glasses based on quantum molecular dynamic simulations is challenging due to the complexity and resource intensive nature of such calculations. Michael Ferry and colleagues at the University of New South Wales in Australia performed these simulations and computed vibrational spectra for a number of different metallic glasses. They found that in certain alloys atoms tend to vibrate at lower frequencies. They obtained the effect of such shifts to lower frequencies on heat capacity, entropy and free energy and discussed their implications on dynamical properties and observed stability of metallic glasses.

 
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