【作者】 顾春飞；

【导师】 张新平；

【作者基本信息】 南京理工大学 ， 材料学， 2012， 硕士

【摘要】 镁合金作为最轻质的结构材料,已经广泛应用于通讯、运输及航天航空等工业领域。这些构件常处于变幅载荷、非对称循环应力或应变服役条件,因此除了要有较高比强度、比刚度,还要求其抗循环应变能力强。构件的棘轮行为是其循环塑性变形积累的表征,对棘轮行为的研究可以有效地防止构件由于过大的塑性变形而导致尺寸失效甚至破坏。本文通过单向热轧获得AZ31镁合金板材,研究不同压下率、不同取样角度对镁合金微观组织、力学性能、棘轮行为规律的影响。并建立了基于Voronoi图的多晶体模型,数值模拟其棘轮行为规律及微观组织变化。结果表明：1)轧制过后AZ31镁合金出现孪晶,压下率越大,孪晶越多,晶粒尺寸越小；随着压下率的增大,强度增高,延伸率降低,各向异性明显；取样角度为900时的强度较高,0°时延伸率较差。2)轧制AZ31镁合金的棘轮行为规律与挤压态类似,存在循环硬化特性,增大平均应力和幅值应力都会使得棘轮应变增大,平均应力决定了棘轮应变的水平,幅值应力决定变形量的范围,并且峰值应力是棘轮行为开动的动力。3)压下率越大,轧制镁合金硬化能力越强；45°取样时,轧制镁合金的抗棘轮应变性能最好。棘轮应变由压下率和取样角度共同决定,当压下率为45%时,取样角度为0°,试样过早断裂。4)多晶体模型中平均相对晶粒尺寸与晶粒数目成负指数关系。棘轮行为数值模拟的结果与实际试验吻合较好。多晶体的应力应变分布与外应力的大小及内部晶粒取向有关。更多还原

【Abstract】 As the lightweight structural material, magnesium alloys have been widely used in communication, transportation and aviation industry. These components are often subjected to variable amplitude loading, non-symmetrical cyclic stress or strain conditions. So in addition to a higher specific strength and specific stiffness, also requires a smaller cycle strain. Ratcheting is the characterization of the accumulation of plastic strain in the cycle fatigue. Research on ratcheting can effectively prevent excessive size failure or damage which due to the plastic deformation.In this paper, the unidirectional hot rolling was applied to prepare AZ31 magnesium alloy sheets. The microstructures, static mechanical properties and ratchetings of the sheets were obtained with different reduction rates and angles. A polycrystal model which based on Voronoi figure with [0,1] fixed boundarys was built to simulate the variation of ratcheting behaviors and microstructures.The results shown:1) After rolling, twins were appeared in AZ31 magnesium alloy sheets. There were more twins and the grain size was smaller with the increase of reduction rate. Besides, the strength increased, and the elongation decreased. The anisotropy was obvious. The strength was higher when the angle was 90°while the elongation was smaller when the angle was 0°.2) Similar to the as-extruded AZ31 magnesium alloy, cyclic hardening occurred in the rolled sheets. The ratcheting strain of AZ31 magnesium alloy increased with mean stress and stress amplitude.The mean stress determined the level of the ratcheting strain, the stress amplitude determined the range of the deformation and the peak stress was the motive power of ratcheting.3) The cyclic hardening ability of sheets were better with the increase of reduction rate. The ratcheting strain was smaller when the angle was 45°. The ratcheting strain was determined by reduction rate and angle. The samples premature broke when the reduction rate was 45% and the angle was 0°.4) In the polycrystal model, a negative exponent relationship existed between the average relative grain size and the grain number. The results of the numerical simulation were similar to the practical ratcheting test. The stress and strain distribution of the polycrystal model was determined by the stress outside and the grain orientation inside.更多还原