【作者】 周鹏；

【导师】 辛仁龙；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2020， 硕士

【摘要】 本论文旨在探索复合Schmid因子(CSF)在评测镁合金力学性能各向异性和孪生启动方面的应用。为此,论文以商业AZ31热轧板和挤压棒为初始材料,通过热处理获得一系列不同晶粒尺寸的样品。然后利用万能试验机测得单轴拉伸和压缩等力学性能数据。结合扫描电镜(SEM)、电子背散射衍射(EBSD)及Schmid因子(SF)分析,研究单轴变形条件下的位错滑移和孪生机制。利用CSF系统研究了镁合金室温变形屈服强度与滑移和孪生启动之间的关系,并重点考察了镁合金在基面滑移主导变形过程的孪生启动及变体选择机理。主要结论如下:(1)沿RD拉伸的板材比沿ED拉伸的棒材具有更大的平均CSF值,即板材对宏观应力与局部应力的总体协调能力比棒材更好,从而板材会更容易发生屈服;压缩时,板材的平均CSF值大于棒材,即沿RD压缩的板材具有更好的总体应变协调能力,所以板材的压缩屈服强度低于棒材。上述结果表明CSF可以很好地解释板材和棒材由于织构差异引起屈服强度不同的现象。多晶镁合金CSF值越大,屈服强度越低。(2)通过设置合理的CRSS比值,可以获得归一化的CSF值,并以此判断{10-12}孪晶、基面滑移和柱面滑移之间的协调模式。对比研究不同初始织构样品的力学性能,发现CSF可以基本反映镁合金的各向异性和拉压不对称性。(3)沿45°方向拉伸和压缩时孪生的启动趋势相似,但宏观应力与局部应力对变形过程孪晶变体启动的影响不同。计算孪晶的SF和SF rank分布,发现镁合金在基面滑移主导变形过程激活孪晶的SF值不高且分散,表现出明显的non-Schmid行为。相对而言,T-45°样品中孪晶的平均SF要高于C-45°样品。对滑移-孪生交互作用分析,发现{10-12}孪晶主要由相邻晶粒中基面滑移的刺激而形核,变体选择主要取决于局部应变协调效应。此外,压缩变形中的滑移-孪生传递效率要优于拉伸变形。(4)CSF能有效地评估镁合金滑移主导变形下的孪生启动趋势和屈服行为。计算表明45°方向拉伸和压缩变形启动的孪晶全部具有CSF rank 1或2。对比SF分析,揭示了结合宏观应力和局部应力的CSF能够更好地解释滑移诱导孪晶的变体选择规律。更多还原

【Abstract】 This article is committed to exploring the application of composite Schmid factor(CSF)in evaluating the anisotropy of mechanical properties and twinning activation of magnesium alloys.To this end,the commercial AZ31 hot-rolled plate and extruded rod were used as raw materials and a series of specimens with different grain sizes were obtained by heat treatment.Then the mechanical properties including uniaxial tension and compression were measured by a universal testing machine.Combined with scanning electron microscopy(SEM),electron back-scattered diffraction(EBSD)and Schmid factor(SF)analysis,the dislocation slip and twinning mechanism under various deformation conditions were studied.The relationship between the yield strength of magnesium alloys at room temperature and the activation of slip and twinning was systematacially studied using CSF,and the mechanism of twinning activation and variant selection in magnesium alloys dominated by slip deformation wasfocally investigated.The major conclusions are summarized below:(1)For tension,the plate deformed along RD has a larger average CSF value than the rod deformed along ED,that is,the overall accommodation ability of the macro stress and local stress of the plate is better than that of the rod,so that the plate will be more likely to yield;For compression,the average CSF value of the plate is greater than that of the rod,that is,the plate compressed along RD has better overall strain accommodation ability,so the compressive yield strength of the plate is lower than that of the rod.The above results show that CSF can well explain the phenomenon of different yield strength caused by the difference in texture of the plate and rod.The greater the CSF value of the polycrystalline magnesium alloys,the lower the yield strength.(2)By setting a reasonable CRSS ratio,the normalized CSF value can be obtained,and the accommodation mode between extension twinning,basal slip and prismatic slip can be judged by this.By comparing the mechanical properties of specimens with different initial textures,it was found that CSF can basically reflect the anisotropy and the tension-compression yield asymmetry of magnesium alloys.(3)The trend of twinning activation is similar when tension and compression along the 45°from c-axis of the plate,but the effects of macro stress and local stress on the twinning activation and variant selection are different.Calculating the distribution of SF and SF rank of the twins,it was found that the SF of the twins activatedin magnesium alloys dominated by basal slip is scattered and not high,and showed distinct non-Schmid behavior.The average SF of twins in T-45° specimens is relatively higher than that in C-45° specimens.Analysis of the slip-twin interaction revealed that {10-12}twins were mainly nucleated by the stimulation of basal slip in adjacent grains,and the variant selection primarily depends on the local strain accommodation.In addition,the efficiency of slip-twin transfer in compression is better than that in tension.(4)CSF can effectively evaluate the trend of twinning activation and yield behavior of magnesium alloys dominated by slip deformation.The calculation indicates that all twins activated in tension and compression along the 45° from c-axis of the plate have CSF rank 1 or 2.Compared with SF analysis,it was revealed that CSF combining macro stress and local stress can better explain the variant selection of slip-induced twins.更多还原