密排六方纳米材料的变形机制及缺陷演化与力学性能的关系

Nanocrystalline (nc) materials exhibit unique structural characteristics and superior mechanical properties which are fundamentally different from those of conventional coarse-grained materials. Deformation mechanism is one of the most important research areas for nc materials. In recent years,great achievement has been gained on understanding the deformation mechanism of nc materials with face-centered cubic (FCC) structure, which includes: emission of partial dislocations from grain boundaries to form twins, grain rotation, grain boundary sliding, etc., while the deformation mechanism for nc materials with hexgonal close-packed (HCP) structure is still unclear. For nc materials with HCP structure,which have limited glide systems, it's difficult for the glide planes in the same family in neighboring grains to coalesce via grain rotation, as which occurs in FCC nc materials. In addition, deformation twinning has been observed in HCP nc materials recently, which breaks down the traditional knowledge that twinning is prohibited in nc materials with HCP structure. Nevertheness, the twinning mechanism is still unclear. Therefore，in this project, an HCP Ti with anverage grain size less than 100 nm, will be processed by severe plastic deformation techniques including high-pressure torsion and equal channel angular pressing. The evolution of crystalline defects such as dislocation, twin, grain boundary, the mutual interactions among them,and the contribution on accommodating the deformation made by each of them will be investigated by high-resolution transmitted electron microscope to deduce the deformation mechanism for HCP nc materials. The mechanical properties such as strength and ductility of the material will be measured to find out the structure-property relationship. The results will provide significant scientific guidance for designing HCP nc materials with high strength and high ductility simultenously.

纳米材料具有独特的结构和优异的力学性能，其变形机制是国际纳米材料研究领域的重要前沿方向。目前对FCC纳米材料变形机制（从晶界发射不全位错产生孪晶、晶粒旋转、晶界滑动等）认识已较全面，而对HCP纳米材料变形机制的认识仍有限。由于HCP纳米材料中滑移系少，相邻晶粒的同一族滑移面很难像FCC一样通过晶粒旋转而合并。此外，近期在HCP纳米材料中也观察到变形孪晶，打破了对该类材料中孪晶被抑制的传统认识，但孪生机理仍有待研究。据此，本研究拟采用高压扭转和等径角挤压技术，对晶粒尺寸<100nm的HCP钛进行剧烈塑性变形，通过高分辨透射电镜表征，研究变形引起的位错、孪晶、晶界等缺陷的产生和发展，缺陷之间相互作用，及对协调变形所做贡献，揭示HCP纳米材料变形机制；表征材料强度、塑性等力学性能演化，结合缺陷演化特点，探究结构与力学性能关系。研究结果有望为发展同时具有高强度与高塑性的HCP纳米材料提供理论参考。

纳米材料具有独特的结构和优异的力学性能，其变形机制是国际纳米材料研究领域的重要前沿方向之一。研究表明，当晶粒尺寸从粗晶的微米级减小到超细晶、纳米晶的亚微米、纳米量级时，晶界协调变形，包括从晶界发射不全位错、晶粒转动、晶界滑动等，对变形起主导作用。值得指出的是，上述晶界协调变形机制主要是从具有面心立方（FCC）晶体结构的纳米材料的研究中揭示，而对非FCC结构（如HCP结构等）纳米材料变形机制的认识非常有限。具有HCP晶体结构的金属材料，如Mg、α-Ti、α-Co、α-Zr、α-Hf等金属及其合金，是一类重要的结构材料，在经济与国防建设中具有重要的战略地位。系统研究该结构纳米材料变形机制不仅能完善人们对于纳米材料变形的认识，更能为发展具有优良综合力学性能的HCP结构材料提供理论参考。为此，本项目以HCP结构Ti、Ti-5at.%Al合金为对象，分别通过粉末冶金法和高压扭转剧烈塑性变形法制备了超细/纳米晶Ti及Ti-Al合金，研究了不同晶粒尺寸下材料的主导变形机制，分析了结构演变与力学性能的关系。研究表明：(1) 经放电等离子烧结法制备的块体纳米钛显示出高达5-6倍于熔铸钛的超高硬度。烧结温度、保温时间、烧结压强等工艺参数对硬度值有直接影响。细晶强化、少量TiO2颗粒产生的弥散强化是硬度提高的主要原因。（2）<a>型位错增殖和孪生对粗晶Ti的变形起主导作用，而当晶粒尺寸减小到纳米级时，<c>及<a+c>型位错增殖对变形起主导作用。（3）与Ti的变形不同，Ti-5at.%Al合金的变形过程中发生了部分HCP到FCC结构的转变，相变主要通过不全位错在基面滑移产生。同时，FCC结构的生成及FCC孪生促进了材料的晶粒细化过程。项目研究成果不仅能为本项目组进一步研究HCP结构材料变形机制、纳米材料变形机制提供理论指导，也能为其他科技同行的研究工作提供参考。