【作者】 刘莹；

【导师】 陈立佳； 吴伟；

【作者基本信息】 沈阳工业大学 ， 材料学， 2009， 硕士

【摘要】 等通道转角挤压(ECAP)技术作为超细晶金属和合金的制造技术已经引起人们的普遍关注。等通道转角挤压技术能够在不改变材料横截面积的前提下,通过纯剪切的剧烈塑性变形有效地细化材料的组织,改善材料的力学性能。本文主要研究了经过常规热挤压以及不同道次和路径等通道转角挤压的Al-4%Mg-0.3%Ce合金的显微组织、拉伸性能和疲劳性能,并利用扫描电子显微镜对拉伸和疲劳断口形貌进行了观察,确定了等通道转角挤压Al-4%Mg-0.3%Ce合金的拉伸和疲劳断裂机理。显微组织观察结果表明,经一道次等通道转角挤压后,合金的平均晶粒尺寸约为1μm,而经二道次等通道转角挤压后,合金的平均晶粒尺寸约为0.5μm。室温拉伸实验结果表明,等通道转角挤压Al-4%Mg-0.3%Ce合金表现出更高的室温屈服强度,经过不同道次和路径等通道转角挤压的Al-4%Mg-0.3%Ce合金的拉伸断裂方式为切断型,而常规热挤压Al-4%Mg-0.3%Ce合金的拉伸断裂方式则为正断型。低周疲劳实验结果表明,经过常规热挤压以及不同道次和路径等通道转角挤压的Al-4%Mg-0.3%Ce合金的弹性应变幅、塑性应变幅与断裂时的反向循环周次之间的关系表现为单斜率线性行为,并分别服从Basquin和Coffin-Manson公式。此外,经过常规热挤压以及不同道次和路径等通道转角挤压的Al-4%Mg-0.3%Ce合金在较高的外加总应变幅下疲劳变形时,其循环滞后回线上出现了锯齿状起伏的现象,说明合金在疲劳变形期间发生了动态应变时效。断口形貌观察表明,在拉伸加载条件下,等通道转角挤压Al-4%Mg-0.3%Ce合金表现为韧性断裂,而在疲劳加载条件下,等通道转角挤压Al-4%Mg-0.3%Ce合金的疲劳裂纹均以穿晶方式萌生于试样表面,并以穿晶方式扩展。更多还原

【Abstract】 The equal-channel angular pressing(ECAP) has emerged as a widely-known procedure for the fabrication of ultrafine-grained metals and alloys.The equal channel angle pressing can refine the microstructures of materials and thus improve the mechanical properties of materials by introducing severe shearing plastic deformation without changing the cross section.In this investigation,the ECAP processes with different routes and passes have been conducted for the Al-4%Mg-0.3%Ce alloy.The microstructures as well as tensile and the low cyclic fatigue properties of the ECAPed Al-4%Mg-0.3%Ce alloy have been studied.In addition,the morphologies of the tensile and fatigue fracture surfaces for the ECAPed Al-4%Mg-0.3%Ce alloy are observed and analyzed using scanning electron microscope,and the corresponding tensile and fatigue failure mechanisms are determined.The microstructural observations of the Al-4%Mg-0.3%Ce alloy reveal that the average grain size is about 1μm after one pass of ECAP,and about 0.5μm after two passes of ECAP.The results of tensile experiments show that the ECAPed Al-4%Mg-0.3%Ce alloys give the higher yield strength.Under the tension loading condition,the shear fracture occurs for the Al-4%Mg-0.3%Ce alloys ECAPed with different passes and routes,while the normal tension break takes place for the conventional hot extruded Al-4%Mg-0.3%Ce alloy.The results of low-cyclic fatigue tests reveal that for the conventional hot-extruded and ECAPed Al-4%Mg-0.3%Ce alloys,the relation between elastic and plastic strain amplitudes with reversal cycles to failure shows a monotonic linear behavior,and can be well described by the Basquin and Coffin-Manson equations,respectively.In addition,at the higher total strain amplitudes,a serrated flow can be observed on theσ-εhysteresis loop for the conventional hot-extruded and ECAPed Al-4%Mg-0.3%Ce alloys.It means that the so-called dynamic strain aging takes place during fatigue deformation.The fractographic results reveal that under tensile loading condition,the Al-4%Mg-0.3 %Ce alloys ECAPed with different passes and routes exhibit the typical ductile fracture, while under low-cycle fatigue loading condition,the cracks initiate transgranularly at the surface of fatigue samples and propagate in a transgranular mode.更多还原