【作者】 石凤健；

【导师】 王雷刚；

【作者基本信息】 江苏大学 ， 材料学， 2012， 博士

【摘要】 剧烈塑性变形技术是制备块体超细晶材料、充分挖掘材料潜力的有效途径,其中等径角挤压技术(Equal channel angular pressing, ECAP)和反复镦压(Cyclic channel die compression, CCDC)技术具有工业化应用前景。本文以纯铜和CuCrZr合金为研究对象,对纯铜的反复镦压和CuCrZr合金的等径角挤压进行了数值模拟和实验研究。所开展的探索性工作及取得的成果如下：1)通过光学显微镜、扫描电镜观察了纯铜反复镦压后的组织变化,通过硬度实验、拉伸实验以及电导率测试研究了纯铜的性能变化。实验结果表明：反复镦压与其他剧烈塑性变形技术一样能使纯铜的晶粒明显细化,纯铜的硬度和抗拉强度明显提高,但断后伸长率和电导率下降。其断裂性质仍为韧性断裂,电导率仍较高,表明反复镦压具有制备高强度、高导电纯铜的潜力。高宽比对组织细化过程有影响,但对纯铜多道次变形后的抗拉强度影响不是很明显。B路线镦压更容易得到较为等轴的组织。剧烈塑性变形导致纯铜的热稳定性下降,在230～270℃发生再结晶。2)采用数值模拟方法研究了纯铜反复镦压过程,分析了试样高宽比、工艺路线以及摩擦对反复镦压变形分布及载荷的影响。模拟结果表明：由于摩擦的影响,镦压后等效应变分布不均匀,中心区域等效应变较高,摩擦系数越大,应变分布越不均匀；高宽比越大,道次镦压变形程度越大,但所需载荷也越大。工艺路线对变形载荷以及平均等效应变影响较小,但对镦压后的应变分布影响较为明显。3)通过数值模拟方法研究了CuCrZr合金的等径角挤压过程,分析了模具结构参数和挤压条件对变形分布和载荷的影响。模拟结果表明：通道夹角越小,平均等效应变、挤压载荷以及试样温升都明显增加。通道夹角处内外侧均采用圆角可有效降低挤压过程中的最大载荷,提高应变分布的均匀性。摩擦系数越大,最大挤压载荷明显增加,试样下表面区域的等效应变增加。Bc路线挤压4道次后等效应变分布更均匀。4)通过光学显微镜、扫描电镜研究了固溶态CuCrZr合金等径角挤压后的组织变化,通过硬度实验和电导率测试研究了不同处理状态合金的性能。实验结果表明：ECAP 10道次后CuCrZr合金的组织细化至亚微米级,晶粒较等轴、均匀。ECAP使合金的硬度大幅提高,合金的电导率下降。未经ECAP变形的固溶态CuCrZr合金500℃时效2h可获得较好的电导率和硬度的配合,电导率为80.2%IACS,硬度为158 HV。ECAP6道次CuCrZr合金375℃时效8h后,硬度达到了202 HV; 400℃时效1h后,硬度即达到了峰值200 HV。与固溶后直接时效相比,峰值硬度明显提高,达到硬度峰值的时间和温度都降低。ECAP 6道次CuCrZr合金在400℃时效1h可获得较好的电导率和硬度的配合,电导率为81.1% ACS,硬度为200HV。说明时效前的ECAP处理明显加速了合金的时效动力学过程,与常规冷塑性变形手段相比,能使材料获得更好的综合性能。ECAP 10道次CuCrZr合金的软化温度约为530℃,但在550℃时,合金的硬度仍高达161HV,说明ECAP后合金的抗软化能力并没有降低。因为ECAP细化了合金组织,形成了大量的畸变区,累积了更高的储存能,促进了时效时第二相的析出,使得第二相更为弥散、细小,钉扎了位错和晶界的运动,使合金在较高温度下仍保持了良好的综合性能。本文的研究工作表明,反复镦压和等径角挤压技术与其它剧烈塑性变形技术一样能极大地细化材料组织、提高材料的性能。将剧烈塑性变形与合金化相结合,通过后续的组织调控,为制备高强度、高导电、高耐热铜合金提供了一种新的途径。更多还原

【Abstract】 Severe plastic deformation (SPD) is an effecitive method for fabricating bulk ultrafine-grained materials and sufficiently excavating the materials’latent capacity. Equal channel angular pressing (ECAP) and cyclic channel die compression (CCDC) have a potential for industrial application. In this paper, numerical simulations and experiments were conducted on CCDC of pure copper and ECAP of CuCrZr alloy. The developed exploratory work and achievements are listed as follows:1) The microstruture evolution of pure copper was examined by optical microscopy and scaning electron microscopy after CCDC. The properties change were investigated by microhardness test, tensile test and electrical conductivity measurement. The results show that grains can be obviously refined by CCDC same as other SPD technologies. The microhardness and tensile strength increase obviously, but the elongation to failure and electrical conductivity decrease. The fracture feature is still ductile and the electrical conductivity is high relatively. These illustrate that CCDC technolgy has the potential to fabricate pure copper with high strength and high electrical conductivity. The height-width ratio has effect on the grain refinement process, but has little effect on tensil strength. The equiaxed grains can be obtained easily by route B than route A. The thermal stability of pure copper decreases after subjecting to severe plastic deformation, static recrystallization occurs at the temperature range of 230 to 270℃.2) The cyclic channel die compression process of pure copper was investigated by numerical simulation. The effect of height-width ratio of the sample, processing routes and friction on effective strain distribution and deformation load were analysed. The simulation results show that the effective strain distribution is inhomogeneous after CCDC due to the effect of friction between tools and sample. The higher effective strain appears in the center of sample. The strain inhomogeneity increases with increasing the friction coefficient. The deformation degree of per pass increases with increasing the height-width ratio of the sample, but the deformation load is also enhaced. The effect of the processing route is samll on the deformation load and average effective strain, but its effect is obvious on the effective strain distribution.3) The equal channel angular pressing process of CuCrZr alloy was investigated by numerical simulation. The effect of die geometry and extrusion condition on effective strain distribution and deformation load were analysed. The simulation results show that the effective strain, deformation load and temperature rise increase obviously with the decrease of die angle. The appropriate outer and inner corner angle can effectively decrease maximum deformation load and increase the strain homogeneity. With increasing the friction coefficient, the maximum extrusion load is enhanced, while the effective strain at the bottom of the sample also is increased. The distribution of effective strain is more homogeneous after four passes ECAP by route Bc.4) The microstruture evolution of solid solution CuCrZr alloy was examined by optical microscopy and scaning electron microscopy after ECAP. The properties of CuCrZr alloy after different treatments were investigated by measuring the hardness and electrical conductivity. The results show that the grain size of the alloy can be refined to submicrometer level after 10 passes ECAP. The ultrafine grain is equaixed and uniform. The hardness increases drastically after ECAP, while the electrical conductivity decreases appreciably. The well combination of hardness and electrical conductivity can be obtained after aging 2 hours at 500℃for the unECAPed alloy. The electrical conductivity is 80.2%IACS and the Vickers hardness is 158 HV. The peak Vickers hardness respectively reaches 202 HV aging 8 hours at 375℃and 200 HV aging 1 hour at 400℃for the alloy after 6 passes ECAP. The ECAP treatment before aging enhances the peak hardness and reduces the time and temperature of peak hardness compared with direct aging after solid solution. The well combination of hardness and electrical conductivity can be obtained after aging 1 hour at 400℃for the 6 passes ECAPed alloy. The electrical conductivity is 81.1%IACS and the Vickers hardness is 200 HV. The results illustrate that the aging kinetics process can be accelerated after ECAP before aging, better comprehensive properties can be obtained compared with conventional cold plastic deformation technologies. The softening temperature is about 530℃, however, the Vickers hardness even remains 161HV at 550℃for the 10 passes ECAPed sample, which indicates that the softening resisitance of CuCrZr alloy do not decrease after ECAP. It can be attributed to the grain refinement and the precipitation of precipitates is accelerated due to abundant lattice distortion and high stored energy, which makes the precipitates are more dispered and fine. The precipitates pin the movement of grain boundaries and dislocation. The excellent comprehensive properties are still attained at elevated temperature.The investigation results show that CCDC and ECAP technology similarily can greatly refine the grain and enhance properties of materials same as other SPD technologies. Combining severe plastic deformation and alloying, a new approach is provided for fabricating high strength, high elecrical conductivity and high heat resistance copper alloy by subsequent microstructure control.更多还原