【作者】 王佳；

【导师】 王经涛；

【作者基本信息】 南京理工大学 ， 材料学， 2007， 硕士

【摘要】 块体纳米晶材料的高强度低塑性缺陷已成为限制其工程应用的主要问题，2002年，Y.M.Wang等人在《Nature》上撰文介绍了在液氮温度下通过大变形量轧制+短时退火的方法获得了晶粒尺寸双峰分布的纳米结构纯铜，在保持了纳米铜高强度的同时有效改善了塑性，该结果表明了通过实现晶粒尺寸双峰分布(Bimodal)结构解决这一问题的可能性。但至今尚未有一种方法可以在多相合金中通过定量控制微米晶的比例与分布以获得兼具高强度与大延伸的多相块体纳米合金。针对这一不足，本研究通过合金成分设计+共析相变处理+剧烈塑性变形+退火的工艺组合在铜铝与锌铝共析体系双相合金中成功实现了可控的Bimodal结构，有效地改善了块体纳米晶材料的低塑性，特别是拉伸塑性在强度少量损失的前提下得到了较大提高；同时首次分析了纳米晶基体上微米晶区的分布状态对材料力学性能的影响，并探讨了在上述两种不同合金系中Bimodal结构的不同形成机制。研究结果表明：Cu-10.8wt.％Al合金中先共析相经高压扭转变形(High Pressure Torsion，HPT)细化与退火长大后形成微米晶区，而共析马氏体经变形细化与退火分解后形成超细晶基体，具有该Bimodal结构的试样与完全纳米晶态试样相比，抗拉强度由997MPa降至730MPa，但塑性应变量由0提高到4％；共析相变处理得到呈弥散分布的先共析相原位转变成微米晶，结果表明弥散分布于超细晶基体上的微米晶区使试样抗拉强度上升至1300MPa，塑性应变量为3％；同样具有40％先共析相比例的Zn-41wt.％Al合金在共析相变处理后直接进行室温4道次等径角变形(Equal Channel Angular Pressing，ECAP)后得到Bimodal结构，力学性能测试表明该合金在损失部分抗拉强度的前提下，塑性延伸率显著提高至26％(完全纳米晶态仅为7％)。更多还原

【Abstract】 High strength with loss of ductility has been demonstrated a limitation of engineering utility in bulk nanostructured materials. Y.M.Wang et al reported nano-Cu with bimodal structure obtained by cryo-rolling at liquid nitrogen, which possesses excellent tensile ductility without obvious loss of strength. It suggests micrometer-sized grains with some fractions embedded in nanostructured matrix, namely bimodal structure was possible to be an effective route to eliminate the loss of ductility. But there is no process which can fabricate multi-phase nano-alloys by quantificationally controlling the fraction anddistribution of micro-meter sized grains. Present research succeeds in achieving controllable bimodal structure according to processing routes of composition design + eutectoid-phase-transformation treatment + severe plastic deformation + annealing. Results showed that tensile ductility was greatly improved without much loss of strength. Meanwhile, effect of distribution of micro-meter sized grains on mechanical properties was first discussed. Also analysis about different formation mechanisms of bimodal structure in these two eutectoid systems was included.Experiment results suggested that micrometer-sized grains originated from the refinement during high pressure torsion and then coarsening happened after annealing, while decomposition of eutectoid martensite resulted in formation of nanostructured matrix; compared with its nanostructured counterpart, specimen with this bimodal structure performed a little loss of strength from 997MPa to 730MPa, but much higher in tensile ductility from 0 to 4%; additionally, strength and tensile ductility were improved further to 1300MPa and 3% because of dispersion of micrometer-sized grains resulted from eutectoid transformation treatment. Bimodal structure is also observed in Zn-41wt.%Al with 40% hypo-eutectoid phase in volume fraction after solid solution treatment and 4 passes ECAP. Mechanical test results showed that the tensile uniform ductility increases from 7% to 26% with loss of the strength in this alloy.更多还原