温度场作用下Fe-Cr合金中空洞演化与迁移的相场模拟

A phase field study of voids

摘要:

堆芯关键部件材料Fe-Cr基奥氏体钢在高温和辐照环境下长时服役，辐照空位簇聚形成空洞导致辐照肿胀和硬化现象。本文采用相场方法耦合中心温度场和一维温度场研究Fe-Cr合金空位簇聚和空洞演化行为。计算结果表明，中心温度场下，温度梯度促使空位向中心温度高的区域迁移，导致中心高温区域空洞浓度高，优先出现空洞形核，空洞尺寸大，低温区域空位浓度低，空洞尺寸小；一维温度场下，体扩散机制控制下，空洞的迁移速度与温度梯度正相关，与初始孔径无关；表面扩散机制控制下，空洞的迁移速度与温度梯度成正相关，与初始孔径负相关。该研究结果启发局域组织老化引起的导热不均材料或宏观温度梯度下服役部件的组织和性能预测。

Abstract:

The austenite Fe-Cr steels used as the critical in-pile components because of its good high temperature resistance, corrosion resistance and excellent mechanical properties and thermal strength, bear long-period high temperatures and irradiation; the irradiated vacancies aggregate into voids leading to irradiation swelling and hardening, which seriously affects the service safety of the reactor. The phase field method coupling temperature field is employed by solving the phase field equations based the Fourier spectrum method to investigate the voids behavior of austenite Fe-Cr steels upon a central and a one-dimensional temperature field. As the temperature goes down from the center radically in a central temperature field, the vacancies diffuse toward the high-temperature center region driven by the temperature gradient, resulting in the instability of the double voids model, and gradually dissolves to form a new void in the high temperature center region. In the central temperature field, the voids nucleate earlier and grow faster with a sizeable scale in the high temperature than that in the lower temperature region due to the higher vacancy concentration in the central high temperature region. Based on the force-flow relation in the principle of irreversible thermodynamics, the migration behavior of voids upon a one-dimensional temperature gradient is studied by adding advection term to the phase field evolution equation Cahn-Hilliard equation. It has been observed in the experiments that the size of voids in irradiated austenite Fe-Cr steel is nanoscale, and it is generally believed that the nanoscale voids migration is controlled by the bulk diffusion mechanism and surface diffusion mechanism. Therefore, the effects of temperature gradient and initial size of voids on the voids migration upon a one-dimensional temperature gradient in austenite Fe-Cr steel are studied considering both the bulk diffusion mechanism and surface diffusion mechanism respectively. The migration rate governed by the bulk diffusion mechanism positively depends on the temperature gradients but not the initial void"s size. The migration rate governed by the surface diffusion mechanism positively depends on the temperature gradients but is negatively related to the initial void"s size. At the same time, in the process of migration, the shape of the void will also change, and the void will be elongated along the direction of temperature gradient, and the front end is sharper along the direction of temperature gradient, and the back end is wider. The studies inspire the microstructure aging and properties prediction caused by inhomogeneous heat conduct or macroscopic uneven temperature distribution.