结晶器导热铜合金表面梯度纳米层的形成机制及高温耐磨性能研究

Continuous casting mold is the final container of molten steel. The mold, usually made by copper alloys of high thermal conductivity, suffers heat vibration, thermal stress and mechanical abrasion during working, and therefore wear resistance and high strength are required at high temperature. The most common way for improving surface property of the alloy, at present, is to combine a high strength surface layer. Nevertheless, the combination between the copper alloy and the layer, i.e. the interface between them, is usually poor, which impedes the wear resistance to be largely improved and decrease the life time of the alloy. Based on previous work of the applicant during the past 10 years, the project first employs high-energy ball milling, spark plasma sintering and aging to fabricate Cu-Ag/Cr alloys, and then uses sliding wear to form surface layers of gradient nanograined microstructure, and makes use of second phase particles due to thermal treatment to stabilize the nanostructure, and final obtains a target alloy which has good high temperature wear resistance. Through using in-situ experimental techniques and molecular dynamics simulation, it is expected to get deep insight into the mechanisms of formation of the surface nanostructure, pinning of boundaries of nanograins by second phase particles, thermal stability and wear of the nanostructure. Based on the inherent relationship between the microstructure and the wear performance, it can provide new ideas and theoretical guidance and benefit development of surface treatment for copper alloys.

连铸结晶器是钢液凝固的最终容器，拉坯过程中受高温热负荷、热应力、机械摩擦力的影响，要求结晶器导热铜合金材料具备优良的耐高温磨损和抗高温形变能力。当前，导热铜合金表面处理最常用的方法是表面涂层，但存在涂层与基体结合不佳的难题，耐磨性能提升有限，限制了结晶器导热材料的使用寿命。结合申请人研究基础，本项目提出利用高能球磨、等离子烧结和时效处理制备Cu-Ag/Cr合金，采用表面滑动摩擦磨损技术制备连续过渡的梯度纳米结构表面层，并利用热处理对第二相稳定梯度纳米结构进行有效控制，最终获得具有优良表面特征的高温耐磨导热铜合金材料。利用分子动力学模拟等手段，深入揭示导热铜合金表面梯度纳米层的形成机制和演化规律，第二相对纳米晶的钉扎机制，纳米晶热稳定机制和梯度表面的磨损机制，阐明表面微观结构与耐磨性能和热稳定性的内在关系，可为高温耐磨铜合金表面处理技术的开发提供新的思路和科学依据，具有重要的理论和应用价值。

本项目采用球磨和等离子烧结辅以一定的时效热处理可控制备了高强纳米晶Cu-Cr/Ag合金，继而采用滑动摩擦磨损表面处理进一步强化了合金表面，最终获得了一种应用于连铸结晶器，室温和升温条件下均有优良耐磨性能的铜合金材料。本项目阐明了纳米晶长大动力学的核心影响因素是晶界迁移动力学，晶界类型、多晶拓扑结构和温度影响晶界迁移动力学，进而对晶粒长大产生影响，该发现可以很好地解释实验发现的长大指数多变且常大于2的现象，解决了这一长期困扰学界的问题；明晰了第二相纳米颗粒对晶界迁移的钉扎机制，探索了晶界类型、颗粒大小形状和取向对钉扎作用的影响规律。以上两方面对合理设计和制备高强纳米晶铜合金提供了理论依据，利用该依据成功提高了铜合金的热稳定性。同时，本项目阐明了耐磨性能提高的关键在于生成表面纳米层状梯度组织而不在于提高表面层的强度。基于实验表征和模拟探索，明晰了层状组织的形变机制，包括位错滑移、孪生和界面滑移。在小形变量情况下，位错滑移和孪生造成晶粒旋转生成典型的剪切取向/织构，在大形变量阶段，主导机制转变为界面滑移，使得剪切取向/织构得以稳定的同时极大地抑制了应力集中和裂纹萌生，从而使材料表现出极佳的耐磨性能。本项目的研究成果不仅极大地推动了高强耐磨铜合金的发展，亦可推广应用于其他异构纳米晶材料。