SmCo_7纳米晶合金晶粒组织热稳定性的热力学分析与计算机模拟

摘要

The accurate quantitative relationship between the excess volume at the grain boundary and the nanograin size in nanocrystalline alloy is deduced.The fundamental thermodynamic function of nanocrystalline alloy is derived as a function of nanograin size and temperature.By taking the SmCo_7 alloy for example,the thermal stability of the nanocrystalline alloy,as well as its evolution characteristics, is studied based on the calculated excess Gibbs free energy of nanograin boundary.The results show that the nanostructure with grain size below a critical value that corresponds to the maximum excess Gibbs free energy can have higher thermal stability than a coarser nanograin structure.Once the grain size is larger than the critical value,the nanostructure may lose its stability and undergo discontinuous grain growth.By combining the nanothermodynamic model with the cellular automaton algorithm,the quantitative and visual simulations of nanograin growth in nanocrystalline SmCo_7 alloy are performed.The nanograin growth behavior described by the two approaches are consistent with each other,which validates the conclusion of the thermal stability of nanocrystalline alloy,drawn from the present thermodynamic study.%推导出了单相纳米晶合金的晶界过剩体积与晶粒尺寸之间的定量关系,建立了纳米晶合金的晶界热力学性质随温度和晶粒尺寸发生变化的确定性函数.针对SmCo_7纳米晶合金,通过纳米晶界热力学函数计算和分析,研究了单相纳米晶合金的晶粒组织热稳定性.研究表明,当纳米晶合金的晶粒尺寸小于对应于体系中品界自由能最大值的临界晶粒尺寸时,纳米晶组织处于相对稳定的热力学状态：当纳米晶粒尺寸达到和超过临界尺寸时,纳米晶组织将发生热力学失稳,导致不连续的快速晶粒长大.利用纳米晶合金热力学理论与元胞自动机算法相耦合的模型对SmCo7纳米晶合金在升温过程中的晶粒长大行为进行了计算机模拟,模拟结果与纳米晶合金热力学模型的计算预测结果一致,由此证实了关于纳米晶合金晶粒组织热稳定性的研究结论.