Microscopic dynamics perspective on the relationship between Poisson's ratio and ductility of metallic glasses.
Microscopic dynamics perspective on the relationship between Poisson's ratio and ductility of metallic glasses.
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DOI：
10.1063/1.4862822
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发表时间：
2013-11
期刊：
影响因子：
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通讯作者：
KL Ngai;LM Wang;RP Liu;WH Wang
中科院分区：
文献类型：
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作者：
KL Ngai;LM Wang;RP Liu;WH Wang
在金属玻璃中，塑性或延性与泊松比和弹性体积模量与剪切模量的比值K/G之间建立了明确的相关性。这两种宏观力学性能之间的这种相关性是有趣的，并且从微观水平上的动力学解释是具有挑战性的。最近的一项实验研究发现，金属玻璃的延展性与二次β松弛有关。韧脆转变的应变速率和温度依赖关系与二次β-松弛时间τβ的倒数相似。β-弛豫强度越大，τβ越短，金属玻璃的延展性越好。研究结果表明β-松弛与延性有关，并有助于延性。另一方面，K/G或ν泊松与有效的Debye-Waller因子（即非遍过性参数），f0有关，表征了由其他单元组成的笼内结构单元的动力学特性，表现为频率相关磁化率中显示的几乎恒定的损失。我们将f0与结构α-松弛函数的Kohlrausch拉伸指数相关函数中的非指数参数n联系起来，其中φ (t)=exp[-(t/τα)(1-n)]。这种联系源于这样一个事实，即f0和n都是由粒子间势决定的，1/f0或（1 - f0）和n都随着势的非调和性而增加。利用耦合模型的一个经过充分检验的结果表明，τβ完全由τα和n决定。从(i) K/G或ν泊松与1/f0或（1 - f0）的关系，（ii） 1/f0或（1 - f0）与n的关系，(iii) τα和n与τβ的关系中，我们得到了K/G或ν泊松与τβ的期望关系。将这一关系与延性与τβ的关系结合起来，我们最终得到了延性与泊松比ν泊松比或K/G之间基于微观动力特性的经验相关性的解释。
In metallic glasses a clear correlation had been established between plasticity or ductility with the Poisson's ratio νPoisson and alternatively the ratio of the elastic bulk modulus to the shear modulus, K/G. Such a correlation between these two macroscopic mechanical properties is intriguing and is challenging to explain from the dynamics on a microscopic level. A recent experimental study has found a connection of ductility to the secondary β-relaxation in metallic glasses. The strain rate and temperature dependencies of the ductile-brittle transition are similar to the reciprocal of the secondary β-relaxation time, τβ. Moreover, metallic glass is more ductile if the relaxation strength of the β-relaxation is larger and τβ is shorter. The findings indicate the β-relaxation is related to and instrumental for ductility. On the other hand, K/G or νPoisson is related to the effective Debye-Waller factor (i.e., the non-ergodicity parameter), f0, characterizing the dynamics of a structural unit inside a cage formed by other units, and manifested as the nearly constant loss shown in the frequency dependent susceptibility. We make the connection of f0 to the non-exponentiality parameter n in the Kohlrausch stretched exponential correlation function of the structural α-relaxation function, ϕ(t)=exp[-(t/τα)(1-n)]. This connection follows from the fact that both f0 and n are determined by the inter-particle potential, and 1/f0 or (1 - f0) and n both increase with anharmonicity of the potential. A well tested result from the Coupling Model is used to show that τβ is completely determined by τα and n. From the string of relations, (i) K/G or νPoisson with 1/f0 or (1 - f0), (ii) 1/f0 or (1 - f0) with n, and (iii) τα and n with τβ, we arrive at the desired relation between K/G or νPoisson and τβ. On combining this relation with that between ductility and τβ, we have finally an explanation of the empirical correlation between ductility and the Poisson's ratio νPoisson or K/G based on microscopic dynamical properties.