纳米晶铁碳合金应变强化能力优化与强韧化机理研究

Nanocrystalline metals exhibit high strength but usually low ductility, because the nanoscale grains seriously deteriorate strain hardening capability. Therefore, to enhance strain hardening capability is the key point to obtain concurrent high strength and high ductility. In this project, severe plastic deformation at different temperatures (including the temperature lower than room tempearture) and subsequent annealing treatment will be carried out, in order to obtain homogeneous finely dispersed nanosized carbide particles in nanocrystalline ferrite matrix, and to obtain a grain boundary state in combined with an intragranular dislocation configuration contributing to ameliorate strain hardening capability; this project will reveal the optimal combination of various microstructure characteristics in nanostructured Fe-C alloy to enhance strain hardening capability, and the related mechanisms, e.g., how dislocations nucleate and accumulate; the evolution of ferrite matrix and carbide phase induced by strain and temperature, as well as the connection between each other will be investigated, to reveal the control of optimized microstructure evolution for strengthening and toughening; it will also be investigated whether special deformation mechanisms (e.g. grain boundary mechanisms) operate in the nanostructured Fe-based body-centered cubic materials, and how they influence the deformation behaviors and mechanical properties. The implementation of this project will provide reference and basis to develop the design criteria for the processing of technically feasible high-performance nanostructured carbon steels.

纳米晶金属材料具有很高的强度但塑性一般很低，因为细小的晶粒严重的降低了其应变强化能力。因此提高纳米晶材料应变强化能力是同时获得高强度、高塑性，实现强韧化的关键。本项目采用不同温度剧烈塑性变形（包括室温以下低温）结合退火处理，在纳米晶铁素体基体内获得理想均匀分布的纳米碳化物颗粒，获得有利于应变强化能力的晶界状态和晶内位错组态，对应变强化能力进行优化；并且优化出提高纳米晶铁碳合金应变强化能力各组织特征的最佳配合状态，揭示相关的提高应变强化能力的机理，如位错形核、堆积的位置和机制；揭示在形变和温度作用下，铁素体基体与碳化物相的演化及相互作用机理，归纳出获得强韧化最佳组织状态的演变规律；验证体心立方铁基纳米晶材料内是否存在特殊变形机制（如晶界机制），及其对变形行为和力学性能的影响。本项目将为制定高性能纳米结构碳钢的设计标准和实现工程应用提供依据。

利用大塑性变形的高压扭转技术在铁碳合金中获得了铁素体晶粒~70nm的纳米晶结构，渗碳体在变形过程中被破碎分散、溶解的碳原子偏聚于位错和晶界内。纳米结构合金的拉伸强度高达~1700MPa，并且保持了一定的均匀延伸率。晶内渗碳体颗粒及晶界附近碳化物颗粒阻碍回复及诱发几何必须位错的堆积，使应变强化能力得以提高。. 采用温变形加冷轧退火技术，由于冷变形增加位错密度和储存能，在退火过程中增加渗碳体颗粒的形核点，获得了更加弥散细小的碳化物分布。该组织状态有力的提高了应变强化能力，使材料的强韧性获得了提升。. 采用变路径中温累积变形，由于促进多滑移系开动，且使渗碳体颗粒多向流动，致使铁素体晶粒更加细化和等轴化，并伴随碳化物颗粒均匀弥散分布。变路径累积变形在铁素体+珠光体钢中获得双峰晶粒尺寸分布，粗细晶配比、渗碳体弥散程度得以优化，使应变强化能力改善、强韧性获得了提高。. 对中碳钢预热处理获得伪共析组织，之后连续中温变形，获得了铁素体多峰晶粒尺寸分布的层状结构，该结构在拉伸过程中诱发应变梯度和多向应力状态，产生几何必须位错，促进位错的有效堆积，使应变强化能力得以显著提升。该材料强度与超细晶碳钢相当，约1200MPa，并且均匀延伸率 > 6%，展示了较强的均匀变形能力。. 本项目揭示了在纳米晶/超细晶铁碳合金中优化碳化物颗粒分布、铁素体晶粒尺寸分布和形态对提高强韧性的作用，归纳了相应的微观组织演化规律，阐明了提高应变强化能力的机理；相应方法和思路对多种成分铁基纳米晶/超细晶材料的强韧化设计具有借鉴意义。