高强高韧超细/纳米晶时效铝合金的微观结构演变及其强韧化机制

Increasing strength significantly and maintaining toughness is an everlasting scientific question confronted with metallic structural materials. In our previous works, ultrafine grained age-hardenable Al alloys with both high-strength and high-ductility have been successfully fabricated by severe plastic deformation (SPD) processes. However the integrated toughening and strengthening mechanisms as well as the quantitative relationship between the structural parameters and properties in these alloys are not clear. In this project, ultrafine and nano-grained Al-Mg-Si-(Cu) aluminum alloys with different Mg and Si contents will be fabricated by severe plastic deformation (SPD). Aging behaviors of pre-aging before SPD, dynamic aging during SPD and post-aging after SPD will be systematically investigated.aging behaviors of pre-aging before SPD, dynamic aging during SPD and post-aging after SPD will be systematically investigated. Precipitation sequence and kinetics, microstructures & nanostructures including the structural parameters such as dislocation density, misorientation angle, and the volume of precipitates will be analyzed and measured applying necessary characterization technologies, e.g., high-resolution transmission electron microscopy (HRTEM), electron backscatter diffraction (EBSD), aberration-corrected scanning transmission electron microscopy (STEM), atom probe tomography (APT) and in-situ TEM etc. Special emphasis is laid on the interactions of precipitates, dislocations, grain boundaries and solutes, as well as the effects of the ageing precipitation on the accumulation of dislocations, strain hardening, strength and ductility. Combined with microstructural analysis, theoretic calculation, nanoindentation and tensile tests including jump tests and relaxation experiments, the contributions of the strengthening and toughening mechanisms such as ageing precipitation, grain refinement, dislocation and solid solution will be quantitatively analyzed. The integrated toughening and strengthening mechanisms, mechanical behaviors and ageing precipitation mechanisms in the ultrafine and nano-grained aluminum alloys will be finally revealed. The achievements may help to effectively control the microstructures and properties of the bulk nanostructured materials, and promote the development of new forming technologies for age-hardenable aluminum alloys with both high-ductility and high-strength.

大幅提高强度并兼顾其韧性，是金属结构材料面临的永恒科学问题。前期工作已制备出最高强度518MPa、均匀伸长率大于10%的几种高强高韧兼备的超细晶6000系时效铝合金，但结构与性能之间的量化关系尚不清楚。本项目拟在现有基础上，用HRTEM、EBSD、Aberration-corrected STEM、APT和in-situ TEM等表征手段，研究超细晶和纳米晶Al-Mg-Si-(Cu)铝合金时效相种类、结构参数、析出序列和析出动力学等的演变规律，阐明时效析出相与位错、晶界、溶质等晶体缺陷的交互作用规律，探讨时效析出相对提高塑韧性的机制，结合理论计算和性能测试，建立晶粒、晶界、位错、时效析出相等微观结构参数与强度和加工硬化速率等强韧性指标之间的定量关系。研究结果将揭示超细晶和纳米晶铝合金的时效析出规律、复合强化和韧化机制等关键科学问题，为纳米晶材料的工程化应用提供理论基础。

用ECAP和HPT分别制备出了多种不同成分和不同工艺的超强高韧和良好耐磨性的6000系铝合金超细晶/纳米晶材料，用DSC、HRTEM、EBSD、Aberration-corrected STEM和APT 等表征手段，系统研究了时效相种类、结构参数、析出序列和析出动力学，探讨了时效析出相与位错和溶质等晶体缺陷的交互作用，结合理论计算和性能测试，阐明了时效析出相等微观结构参数对提高塑韧性的机制,揭示了超细晶和纳米晶铝合金的时效析出相演变规律、复合强化和韧化机制等关键科学问题。项目研究成果对全面认识超细晶/纳米晶铝合金的强韧化机制和摩擦学行为，有效控制组织和性能，推动我国高性能铝合金原创性成形技术的自主创新和应用具有重要意义。本项目得到的基本结论如下：.(1) 分别获得了几种平均晶粒尺寸为超细晶和纳米晶的超强高韧和良好耐磨性的6000系铝合金，最小平均晶粒尺寸低于50 nm，最高抗拉强度和屈服强度分别超过600 MPa和550 MPa，且均匀延伸率均超过10%，为已知6000系铝合金中综合性能最佳，完全能满足应用的需要。.(2) 大变形铝合金过程中时效析出相演变规律为：在高密度位错和强应变的交互作用下，经历了“时效相细化球化回溶过饱和固溶体再析出时效相”的循环往复周而复始的动态过程。.(3) 铝合金存在纳米时效相，高密度时效析出相的钉扎作用同时增加了变形抗力和位错储存能力，是合金兼具高强高韧和良好耐磨性的主要原因。.(4) 在材料内部引入纳米时效析出相，获得“纳米尺度时效析出相 + 高密度位错 + 纳米晶/超细晶”的复合结构，是获得高强高韧和和良好耐磨性纳米晶/超细晶材料的重要途径。.(5) 各种强化机制中，纳米时效析出相强化为最主要的强化机制；APT等分析证实了纳米时效相与位错之间存在明显的交互作用；获得了用DSC分析法预测6000系铝合金时效析出程度及析出相种类的方法。