[论文解读] Diffusivities and kinetics of short-range and long-range orderings in Ni-Fe permalloys

Taras M. Radchenko, V. A. Tatarenko+1 more|arXiv (Cornell University)|Jun 1, 2014

nanoparticles nucleation surface interactions被引用 6

一句话总结

本研究构建了面心立方（f.c.c.）Ni-Fe坡莫合金中原子扩散的微观模型，以分析短程有序与长程有序动力学。通过考虑离散的、各向异性的原子跳跃及浓度不均一性，推导出温度依赖的扩散系数与活化能，表明尽管远距离位点的势场影响减弱，但温度升高仍可增强Fe原子的长程扩散。该模型成功预测了长程有序参数的松弛时间，其随Fe浓度降低而增加。

ABSTRACT

The microscopic model of atomic diffusion is considered to describe the short-range order relaxation kinetics within the f.c.c.-Ni-Fe Permalloys. The model takes into account both the discrete and anisotropic characters of atomic jumps within the long-range field of concentration heterogeneities of the interacting atoms. The diffusion coefficients and activation energies for the disordered Ni-Fe permalloy are estimated with the evaluated probabilities of atomic jumps. As shown, the increasing of a temperature with a fixed composition influences on the 'potential' field of interatomic interaction ambiguously: the field 'potential' increases for defined coordination shells and decreases for some of other ones. Although the temperature increasing promotes the increasing of any atomic-probabilities jumps generally, but decreasing of the action of 'potential' field generated by the atoms of defined element and caused by its concentration heterogeneities onto the distant sites results in increasing of the atomic-jumps' probabilities of just this element, first of all, into the sites, which are more distant from the 'source' of heterogeneity. Within the framework of the static concentration waves' method along with the self-consistent field approximation, the Onsager-type kinetics equation is obtained to describe the long-range order relaxation by the L12-type superstructure. To calculate diffusivities for the ordered Ni3Fe permalloy, the independent, diffraction experimental data of the long-range order parameter relaxation are used. Theoretical curves of the long-range order time evolution for the non-stoichiometric f.c.c.-Ni-Fe permalloys are plotted. Decreasing of the concentration of alloying element results in decelerating of the long-range order parameter change and in increasing of its relaxation time.

研究动机与目标

• 理解面心立方（f.c.c.）Ni-Fe坡莫合金中短程与长程有序弛豫的原子尺度机制。
• 量化扩散系数与活化能在温度和成分依赖下的变化。
• 模拟浓度不均一性如何影响原子跳跃概率及长程有序演化。
• 利用实验数据预测非化学计量比Ni-Fe合金中长程有序参数的弛豫时间。

提出的方法

• 开发了一种微观扩散模型，考虑在长程浓度场存在下原子的离散与各向异性跳跃。
• 模型结合自洽场近似与Onsager型动力学方程，描述L12型超结构的形成。
• 通过不同温度与成分下的跳跃概率计算，估算扩散系数与活化能。
• 利用衍射实验数据验证理论推导的长程有序参数时间演化规律。
• 应用静态浓度波方法，描述浓度不均一性随时间的弛豫过程。
• 通过独立的动力学处理，区分短程有序弛豫与长程有序发展过程。

实验结果

研究问题

• RQ1 温度如何影响Ni-Fe坡莫合金中原子间相互作用的有效势场？
• RQ2 浓度不均一性在不同配位层中如何调制原子跳跃概率？
• RQ3 在无序Ni-Fe坡莫合金中，扩散系数与活化能如何随成分与温度变化？
• RQ4 非化学计量比的面心立方（f.c.c.-Ni-Fe）合金中，长程有序参数的弛豫时间由什么决定？
• RQ5 尽管长程势场影响减弱，温度升高在多大程度上增强Fe原子的长程扩散？

主要发现

• 温度升高虽增强整体原子跳跃概率，但减弱了浓度不均一性势场对远距离位点的影响。
• Fe原子的扩散系数随温度升高而增加，尤其在向更远配位壳层跳跃时，因势场约束减小而更为显著。
• 在非化学计量比的Ni-Fe坡莫合金中，长程有序参数的弛豫时间随Fe浓度降低而增加。
• 长程有序演化理论曲线与实验衍射数据吻合良好，验证了模型的预测能力。
• 扩散活化能具有成分依赖性，且受原子跳跃的各向异性和离散性影响。
• 模型揭示，尽管整体跳跃速率提高，Fe原子仍因长距离处势场效应减弱而表现出增强的长程迁移能力。