【作者】 魏伟；

【导师】 陈光； 王经涛； 陈国良；

【作者基本信息】 南京理工大学 ， 材料学， 2005， 博士

【摘要】 块体超细晶（晶粒尺寸0.1～1μm）金属具有优异的物理、力学性能,是材料领域的研究热点之一。ECAP（Equal Channel Angular Pressing）是一种有效获得块体超细晶金属的强烈塑性变形（Severe Plastic Deformation, SPD）方法,ECAP变形参数及模具设计是制备超细晶材料的关键,而模具外角对材料流变、微观组织与力学性能有重要影响,但至今未得到充分研究。另一方面,电沉积制备的超细孪晶组织使铜的力学性能发生质的飞跃,而在强烈塑性变形制备的超细晶铜中极少出现孪晶,通过强烈塑性变形实现孪晶强化有待研究。此外,超细晶材料的力学行为显著不同于常规材料,建立强烈塑性变形超细晶材料的力学行为模型具有重要意义。 本文详细研究了块体超细晶纯铜（99.98%）的制备、ECAP模具外角、变形道次及室温轧制变形（Cold Rolling, CR）对纯铜微观组织与力学性能的影响、变形孪晶的形成条件等,提出了用于描述超细晶材料力学行为的孪晶复合强化模型。论文主要开展了以下几个方面的工作:（1）在全面分析ECAP变形原理及其主要工艺参数,包括模具角度、变形路径、变形道次、变形速度、变形温度,对比已有等效应变计算公式的基础上,推导了不同条件下ECAP变形力的上限解:网格法的分析表明,ECAP剪切变形的均匀性随模具外角的增大而逐渐减小:有限体积法（FVM）模拟分析了ECAP变形载荷与变形温升,当φ=90°时增大模具外角,变形力相应减小,而变形速度对变形载荷、变形温升的影响很小:自制了ECAP实验装置,并成功制备了12×12×80mm的块体超细晶纯铜。（2）纯铜晶粒尺寸随ECAP变形道次的增加而减小,4道次后平均晶粒尺寸从最初退火态的～100μm细化到～0.2μm,并趋于饱和。（3）ECAP变形道次增加,纯铜拉伸强度增大,延伸率降低,1道次ECAP变形后拉伸强度达到300MPa,是退火态的2.3倍,延伸率则从退火态的54%降低到23%;4道次后,拉伸强度达到420MPa,是退火态的3.3倍,延伸率～30%,并趋于饱和。 （4）对ECAP纯铜施以低应变速率(（?）≈2s^-1)室温轧制变形后,在超细晶基体上获得了变形孪晶,孪晶宽度～0.4μm,而纯铜在同样的轧制变形条件下没有发现变形孪晶,也未见文献报道。通过孪晶形成临界应力σ_c^twin、晶粒直径d和轧制温升的计算,分析了ECAP纯铜在低应变速率(（?）≈2s^-1)轧制变形中变形孪晶的形成条件,结果表明:当d大于1μm时,不会形成孪晶:当d小于0.2μm时,易于形成孪晶,且形成孪晶的倾向对温度不敏感;当d介于0.2～1μm之间时,可以形成孪晶的温度随晶粒尺寸减小而升高。（5）ECAP纯铜在随后的轧制变形过程中,拉伸强度随轧制压下量的增加近乎线性增大。ECAP+CR孪晶/超细晶纯铜在保持延伸率不降低的情况下,屈服强度明显高于ECAP更多还原

【Abstract】 Bulk ultrafine-grained metals (grain size 0.1~1μm) with unique physical and mechanical properties is one of most interesting topic in the field of materials research. Equal channel angular pressing (ECAP) is an effective route to obtaining bulk ultrafine-grained metals through severe plastic deformation (SPD). Deformation parameters of ECAP and die design are the key factors for ultrafine-grained metals. Although the outer arc angle of the ECAP die has important effects on materials flow, microstructure and mechanical properties, it has not been well investigated to this day. On the other hand, mechanical properties of electronic deposited copper with ultrafine twins have been greatly enhanced. The present work tries to find some possibilities to strengthen ultrafine-grained metals by deformation twins via changing SPD routes. Furthermore, mechanical behavior of ultrafine-grained materials is distinctly different to that of coarse-grained materials. To model mechanical behavior of ultrafine-grained materials produced by SPD is very important to deeply understand strengthening theories and the relationship of microstructure and mechanical properties.Investigations on fabrication of bulk ultrafine-grained copper (99.98%), the effects of outer arc angle of ECAP die, the number of passes of ECAP and cold rolling on microstructure and mechanical properties as well as the factors for the formation of deformation twins were conducted in detailed. A composite model of twin strengthening was proposed to describe the mechanical behavior of ultrafine-grained materials. The main research work for the dissertation focused on eight aspects of ultrafine-grained materials. (1) Based on the detailed analysis of ECAP principle and mainly technical parameters including die angles, pressing routes, the number of ECAP passes, pressing speed and temperature, the equivalent strain and the upper-bound solution of ECAP is compared and deduced respectively. The influence of the outer arc angle on materials flow in the ECAP process is investigated by etched-grids, which shows that the homogeneity of shear deformation decreases with increasing the outer arc angles in the ECAP process. Finite volume method is conducted to simulate the ECAP load and deformation temperature increment in the ECAP process, which shows that increasing outer arc angle will lead to depress ECAP load as φ =90 ° . And the pressing speed has little effect on ECAP load anddeformation temperature increment. Bulk ultafine-grained copper with the size of 12 X 12 X 80mm is successfully obtained by self-designed ECAP instrument. (2) Grain size of pure copper can be refined as increasing the number of ECAP passes. The average grain size is effectively refined from the initial -lOOum to ~0.2um after four passes, and then be saturated. (3) Tensile strength of copper increases abruptly with increasing the number of ECAP passes. After one ECAP pass, tensile strength of copper arrives at 300MPa, which is ~2 times than that of as-annealed state, however, the ductility (elongation to failure) is reduced from the initial 54% to 23%. After four ECAP passes, tensile strength reaches 420MPa, which is ~3 times than that of as-annealed state, the ductility is -30% and then be saturated. (4) When ultrafine-grained copper produced by ECAP is rolled at room temperature and low strain rate (e ? 2 s’1), deformation twins (the width ~0.4um) are found inside the matrix of ultrafine grains. However, no reports on deformation twins have been found in pure copper at the same rolled conditions so far. Meanwhile the factors for the formation of deformation twins are discussed. Based on the critical stress for twins formation a’"’", grain size d and deformation temperature increment in the cold rolling process, it shows that (a) deformation twins can’t come into being as of is larger than lum; (b) The formation of deformation twins is easy and independent on temperature as d is smaller than 0.2uxn; and (c) The temperature for the formation of deformation twins rises when d is reduced among of 0.2um to lum. (5) Tensile strength of ECAP-ed copper after cold rolling almost linearly increases with increasing the rolling reduction. Retaining the same ductility, yield strength of twins/ultrafine-grained copper is much larger than that of copper after ECAP. Yield strength of copper after two ECAP passes is 348MPa, however, yield strength of twins/ultrafine-grained copper is 422MPa after two ECAP passes and 83% CR deformation. (6) The saturation of strain-hardening and strain-softening occurs in copper subjected to ECAP and ECAP+CR deformation, namely, the flow stress increases with increasing strain, and then be saturated, which can be contributed to the saturation of grain size, the width of deformation twins and dislocation density after SPD. (7) The experimental saturated yield strength of copper after ECAP is 378MPa, which is consistent with reported saturation for pure copper (about 380~390MPa). The experimental saturated yield strength of copper after ECAP+CR deformation is 422MPa. Both of them are larger than that of Voce’s extrapolated saturation for copper ~305MPa. The Voce model is validated reasonable to describe the relationship between yield strength and strain over a wide range of strain by experimental results from ECAP, ECAP+CR deformation. (8) Considering the composite strengthening effects of grain refinement and twin, a composite更多还原