级联碰撞对层状珠光体力学行为的影响

Effects of Collision Cascades on the Mechanical Properties of Pearlites

铁素体合金钢是目前在核能工程界应用最为广泛的一种金属结构材料,而以渗碳体和铁素体基体构成的层状珠光体是铁素体合金钢中常见的金相结构.深入理解辐照效应对层状珠光体力学性能的影响对高辐照条件下铁素体钢的材料设计与寿命评估有着重要的理论参考意义.基于以上考虑,论文采用分子动力学(MD)模拟,研究了连续低能铁原子级联碰撞对渗碳体/铁素体两相界面的破坏情况,探讨了经历不同程度级联碰撞的两相结构在单向拉伸以及压缩荷载下的初始屈服情况.通过对原子模拟结果的深入分析,得到了以下主要结论:(a)辐照会破坏渗碳体/铁素体两相界面的失配位错结构,引起渗碳体的分解,并促进碳原子向铁素体的扩散;(b)在单轴拉伸荷载作用下,级联碰撞会使初始屈服机制由{112}<111>位错滑移系的开动转变为缺陷团簇附近位错环的形核与长大;(c)在单轴压缩荷载作用下,级联碰撞会使初始塑性变形机制由{110}<111>滑移系的开动转变为{112}<111>滑移系的开动;(d)在单轴拉伸和压缩情况下,级联碰撞(及辐照效应)都会提高珠光体的初始屈服应力.论文的研究结果为铁素体合金钢的辐照硬化和辐照脆化行为提供了新的微观层面解释,对于辐照条件下铁素体合金钢的材料组织优化设计有一定的参考意义.

Lamellar pearlite consisting of alternating layers of ferrite and cementite is a kind of common metallographic structure in ferritic-alloy steels,the most widely-used structural materials in nuclear energy engineering.Understanding the irradiation effects on the mechanical behaviors of lamellar pearlites is of importance for designing and life-prediction of such materials under high-irradiation conditions.By implementing molecular dynamics simulations,the microstructure variations of cementite/ferrite interfaces in lamellar pearlites caused by successive low-energy collision cascades are investigated.Besides,the initial yielding behaviors of lamellar pearlites exposed to different doses of irradiation are discussed for both uniaxial tensile and compressive loading conditions.The main conclusions can be summarized as follows:(a)The irradiation will destroy the structure of misfit dislocation arrays on the interface,and then promote the diffusion of carbon atoms from the interface to the ferritic phase.(b)Under uniaxial tensile loading,collision cascades change the initiation of yielding from the activation of{112}<111>slip system to the nucleation and expansion of dislocation loop in the vicinity of interstitial clusters.(c)Under uniaxial compressive loading,collision cascades change the initiation of yielding from the activation of{110}<111>slip system to the activation of{112}<111>slip system.(d)For both tensile and compressive loading types,the dislocation nucleation stress(or yield stress)can be increased under the effect of irradiation.The reasons for the variation of yielding mechanisms and the increase of yield stress are discussed in terms of crystallography.The above results provide two new nanoscale explanations for irradiation hardening and embrittlement.On the one hand,the carbon atoms diffused into the ferrite layer could increase the hindrance to dislocation motions through solid solution strengthening.On the other hand,the destruction of interface dislocation arrays due to collision cascades has shielding effects on the nucleation and gliding of dislocations.The present work may shed light on the optimal design for ferritic-alloy steels working under high-irradiation environments.