多尺度异质界面复相结构金属的微区力学性能研究

Having both high strength and good ductility is the goal of advanced metallic materials. In most cases, the strength is in inverted relationship with toughness, which greatly restricts the use of high-strength metals in industry and has become one of the bottlenecks in the development of metallic materials. Multi-scale and multiple-phase structure is a new idea for developing advanced metallic materials. Materials with both high strength and good ductility might be achieved. However, because of the heterogeneous microstructures, it is difficult to establish a quantitative link between the local microstructure and the macroscopic mechanical properties. This project will introduce micro-zone mechanical properties as bridge between heterogeneous microstructures and macroscopic mechanical properties. Typical micro-units will be picked up from Cu-Nb microcomposite wires with fcc/bcc interfaces and Mg-Al composite with hcp/fcc interfaces. Cs-corrected TEM, 3D-AP etc. will be used to characterize the microstructures of each typical micro-units. SEM with in-situ specimen holders and nanoindentation will be employed to study the mechanical properties of local micro-units. Infinite element method and mathematical modeling will be utilized to establish the quantitative link among local microstructures, local micro-zone mechanical properties and macroscopic mechanical properties. The aim of this project is to reveal the strengthening mechanism of this kind of materials, to design and optimize the microstructure of materials, and to ultimately improve both the strength and toughness of metallic materials.

高强高韧是金属材料的追求目标，而大多数情况下，强度跟韧性呈倒置关系，制约了高强度金属的工业应用，成为金属材料发展的瓶颈问题。发展多尺度复相结构是开发先进金属结构材料的新思想，可以得到良好的强度与韧性的匹配，但由于其微观组织的不均匀性，导致微观组织与宏观力学性能间很难直接建立定量联系，使得开发此类金属材料缺乏具体的理论指导。本项目在微观组织与宏观性能间引入微区力学性能这一桥梁，以fcc/bcc界面的Cu-Nb复合线和hcp/fcc界面的Mg-Al复合材料为研究对象，从中选取典型微区单元，利用球差校正透射显微镜、三维原子探针等对其微观组织进行表征，利用搭载原位台的扫描电镜及纳米压痕仪对微区单元的力学性能做原位动态的研究，利用有限元及数学建模的方法建立微观组织-微区单元力学性能-宏观力学性能的定量关系，从本质上揭示该类材料的强化机制，指导材料微观组织结构的设计与优化，从而最终提高金属材料强韧性。

本项目选用具有fcc/bcc异质界面的铜铌复合线材为研究对象，利用FIB，纳米压痕，高分辨透射电镜等，系统研究了材料典型微观组织结构及对应微区单元的力学性能，包括（不同尺度铜基体织构演变分析；（反复轧制退火后铜铌复合线材的力学性能及微观组织研究；不同温度退火处理下铜铌复合线材微观组织演变；不同温度退火处理对铜铌复合线材微区力学性能的影响和分子动力学模拟位错与不同孪晶界面的交互作用。得到的主要结论有：1. 反复轧制退火过程中，铜铌复合线材的抗拉强度和弹性模量表现为先增加，后减少，再增加的趋势。处于纳米级别的基体Cu和增强体Nb对材料性能的变化起主导作用。2. 经过不同温度退火，Cu-Nb复合线材的显微硬度随着退火温度的升高，表现出单调下降的趋势。退火温度小于300℃时，硬度值的降低主要是由于大尺度的Cu基体发生了回复和再结晶。随着退火温度的升高，Cu基体发生完全再结晶随后进一步的粗化和Nb相的再结晶导致硬度值继续降低。3. 在相同的加载力和加载时间条件下，CuNb复合区的位移总是比Cu1和Cu2区域少。Cu1和Cu2区域的硬度值随着退火温度的升高呈逐渐降低的趋势，CuNb纳米复合区的硬度值随退火温度的升高也呈现逐渐下降的趋势，与维氏显微硬度值的变化趋势相同。4. 层错能对位错与孪晶交互作用有很大影响。当位错为混合型位错时，位错可能在共格孪晶界面上分解。螺型位错会交滑移到非共格孪晶界面上；混合型位错只能在界面上特殊的点通过界面。在低层错能合金中面内滑移是重要的变形方式，退火孪晶会与位错交互作用起到强化材料的作用。整个过程位错在共格孪晶界面发生穿越与反射。.