【作者】 房大然；

【导师】 李家俊； 张哲峰；

【作者基本信息】 天津大学 ， 材料学， 2007， 博士

【摘要】 等通道转角挤压(Equal Channel Angular Pressing–ECAP)技术是制备超细晶金属材料的重要方法之一。本论文以铸态Al-0.63%Cu、Al-3.9%Cu和Al-2.77%Mg合金为实验材料,观察了ECAP过程合金的组织变化,系统研究了ECAP后合金的拉伸、疲劳、冲击性能以及其变形与断裂机制。上述合金经四次ECAP后,晶粒都细化到了亚微米级别。而且,铸态Al-3.9%Cu合金中的沿晶界分布的粗大θ相被挤碎,成为了弥散分布的颗粒。因此,合金拉伸强度增加,但延伸率降低。多次挤压后,合金的静力韧度增大。Al-2.77%Mg合金ECAP四次之后在523K退火处理,得到了具有双态晶粒度的组织,提高了材料的综合力学性能。合金的拉伸断裂显示出不同的特征。对于Al-0.63%Cu合金,随着挤压道次增多,拉伸断裂从颈缩方式转变为剪切方式。对于Al-3.9%Cu合金,铸态时表现为正断,而ECAP之后则以剪切方式断裂。铸态Al-2.77%Mg合金拉伸时发生颈缩断裂,经不同道次挤压后则显示为具有不同剪切断裂角的剪切断裂。论文中对合金的拉伸断裂机制进行了探讨。在应变疲劳实验中,Al-0.63%Cu合金表现出明显的循环软化行为。多次ECAP处理使合金滞回环的形状系数变小,同时,使合金的包申格效应增强。疲劳后,在ECAP后合金的XZ面上出现了与循环应力轴成45°夹角的剪切带,而在XY面上,剪切带垂直于循环应力轴。研究表明,剪切带的方向和ECAP模具的剪切平面之间没有对应关系。另外,疲劳裂纹也可以在剪切带区域外出现。在应力疲劳实验中,Al-2.77%Mg合金的疲劳寿命随着ECAP道次的增多而明显提高。ECAP一次的合金表面有沿着剪切带出现的疲劳裂纹,也有横穿过剪切带的之字形疲劳裂纹。在ECAP四道次的Al-2.77%Mg合金疲劳断口上可以看到明显的疲劳裂纹萌生区、扩展区和最后瞬断区。ECAP处理增强了铸态Al-0.63%Cu和Al-2.77%Mg合金的冲击性能。由于Al-3.9%Cu合金中含有较多的第二相,在ECAP后其冲击性能没有提高。研究表明,合金的冲击性能和静力韧度具有一定的相关性。更多还原

【Abstract】 Equal channel angular pressing (ECAP) technique is one of the important techniques for producing ultrafine-grained (UFG) materials. In this paper, cast alloys Al-0.63%Cu, Al-3.9%Cu and Al-2.77%Mg were subjected to ECAP for the experimental materials. After ECAP, the microstructures of the alloys were observed, and tension, fatigue and impact tests were performed, to investigate the corresponding mechanical properties as well as deformation and fracture mechanisms. The following conclusions can be drawn:The grains are refined to submicron-meter scale after four ECAP passes. In addition, the precipitation phaseθalong grain boundaries in cast alloy Al-3.9%Cu can be broken into disperse particles. As a result, the tensile strength is improved, while elongation decreases. It is noted that the static toughness of the alloys increases after multi-passes. For Al-2.77%Mg alloy subjected to four ECAP passes and subsequent annealing at 523K, comprehensive mechanical properties are enhanced due to the bimodal distribution of grain size.The tensile failure modes of the alloys display different features. For Al-0.63% Cu alloy, necking degree decreases gradually, and the shear feature becomes more obvious with increasing the number of ECAP passes. For Al-3.9%Cu alloy, the cast sample displays fracture normal to the tensile axis, while the ECAPed samples fail in shear mode. For Al-2.77%Mg alloy, the cast sample exhibits obvious necking before failure, while the samples subjected to different number of ECAP passes display shear fracture with different shear fracture angles. Based on the results above, the tensile failure mechanisms of the ECAPed alloy are discussed.The ECAPed Al-0.63%Cu alloy displays obvious cyclic softening during strain-controlled fatigue tests. Meanwhile, It is found that multipass ECAP can decrease the shape parameter of hysteresis loops, indicative of a large Bauschinger effect. The shear bands orient at about 45°to the cyclic loading axis on XZ-plane, while make an angle of around 90°with respect to the cyclic loading axis on XY-plane. The current research proves that there is no one-to-one relationship between the shear bands induced by fatigue and the shear plane in the last ECAP pass. In addition, it is noted that fatigue cracks can also occur in the area away from shear bands. Stress-controlled fatigue life of Al-2.77%Mg alloy increases with increasing the number of ECAP passes. Meanwhile, It is found that fatigue cracks can propagate along shear bands, or across the shear bands on the surface of the alloy ECAPed for one pass. The fatigue fractograph consists of several different zones, such as fatigue crack initiation, propagation and final fracture.Impact properties of Al-0.63%Cu and Al-2.77%Mg alloys are enhanced after ECAP. While for Al-3.9%Cu alloy, its impact toughness is not improved as a result of more second-phase in the alloy. It is shown that the impact properties have a close relation with the static toughness of the alloys.更多还原