Nb对锆合金LOCA淬火相变及力学性能影响的分子动力学研究

Molecular dynamics study of the effect of Nb on mechanical properties of Zirconium LOCA after quenching

摘要:

失水事故(LOCA,Loss of coolant accident)下锆合金中原β-Zr层的力学性能对核安全具有重大意义,Nb作为锆合金中重要的合金元素对其有重要影响。本文运用分子动力学方法研究了Zr-xNb(x=0, 0.5, 1, 2.5; wt.%)合金在淬火过程中的相变行为和淬火后的拉伸行为,分析了Nb对其力学性能的作用机制。结果表明Nb淬火相变模拟过程产生了类似原β-Zr组织的层片状多晶,这种层片状多晶主要由fcc和hcp结构原子组成；多晶的bcc→fcc相变路径遵从Brain位相关系,bcc→hcp相变路径遵从P-S位相关系；在降温过程中,Nb的添加会降低bcc与hcp相的自由能之差,从而使得合金β→α+β相变温度降低；Zr、Zr-0.5Nb和Zr-1Nb合金模型中Nb促进了晶界处bcc相的生成,使得变形集中于晶界,导致晶界易发生断裂；而Zr-2.5Nb中,晶粒内部的Nb含量较高,易形成bcc相,从而协调变形,提高其塑性；此外Zr-2.5Nb中团簇呈弥散分布,这使得其整体强度得到提高。

Abstract:

The mechanical properties of the prior-β Zr layer in zirconium alloys in loss of coolant accident (LOCA) are of great significance to nuclear safety. Nb as an important alloying element in zirconium alloys, has an important influence on its mechanical properties. In this paper, the phase transformation behavior of Zr-xNb (x=0, 0.5, 1, 2.5; wt.%) alloy during LOCA quenching and tensile behavior after quenching were investigated by molecular dynamics method, and the Nb"s effect on mechanical properties of Zr-xNb was analyzed. The results show that the simulated phase transformation process of quenching leads to lamellar polycrystals similar to the structure of prior-β Zr , which are mainly composed of fcc and hcp structural atoms. The bcc→fcc phase transformation path follows the Brain phase relationship, while the bcc→hcp phase transformation path follows the P-S phase relationship. During the cooling process, the addition of Nb reduces the difference between the free energy of the bcc and hcp phases, thus making the temperature of β to α + β phase transition lower. In the Zr, Zr-0.5Nb and Zr-1Nb alloy models, Nb promotes the generation of bcc phase at the grain boundaries, which makes the deformation concentrated at the grain boundaries thus leads to the fracture of the grain boundaries. In Zr-2.5Nb, the content of Nb in the grain is also higher for the formation of bcc phase, which makes the deformation homogeneous and improves its plasticity. In addition, the clusters of Zr-2.5Nb alloy models are diffusely distributed, which makes the tensile strength of Zr-2.5Nb improved.