【作者】 陈勇军；

【导师】 翟春泉； 王渠东；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2007， 博士

【摘要】 本文研究了往复挤压(CEC)大变形AZ31、AZ91和AZ31-1Si的组织结构和力学性能。采用金相显微分析(OM)、透射电子显微技术(TEM)和电子背散射技术(EBSD)研究了AZ31、AZ91镁合金在往复挤压过程中挤压道次和温度对晶粒大小、分布形貌和晶界结构等的影响规律,不同性质和含量的第二相对往复挤压镁合金组织结构的影响规律;分析了往复挤压过程中镁合金的织构演变规律;探讨了往复挤压镁合金的晶粒细化机制;考察了往复挤压镁合金的室温力学性能;采用场发射扫描电镜(SEM),原位EBSD拉伸技术分析了往复挤压镁合金的断裂机制和室温拉伸过程中的组织演变;最后分析讨论了往复挤压镁合金的强韧化机制,获得如下结果:往复挤压对镁合金具有强烈的细化能力。在300℃1-25道次往复挤压AZ31镁合金,发现初始道次的细化作用最强,然后随着道次的增加细化作用减小。存在一个临界道次(临界累积应变量,本研究中为7道次)使超过这个临界道次后的晶粒大小不再有明显变化。初始道次往复挤压镁合金试样的中部和边部组织差别较大,随着道次的增加差异性逐渐减小直到基本消失。随着道次的增加,往复挤压镁合金晶粒逐渐细化,组织均匀性增加,小角度晶界有减小的趋势,平均位向差有增加的趋势。AZ31镁合金300℃往复挤压7道次的平均晶粒尺寸为1.77μm,其中细晶粒分布范围为150±50nm,小角度晶界占7%,平均位向差为54.8。往复挤压镁合金组织中细晶粒趋向于聚集在一起形成链形网状结构,随着挤压道次的增加,细晶粒的数量明显增加,原有的链形网状结构被分割和重分使其分布更均匀。随着第二相的增加,细晶粒的聚集程度减小。在225℃-400℃3道次往复挤压AZ31镁合金,发现随着温度的降低,有利于小角度晶界含量和晶粒尺寸减小、平均位向差和晶界密度增加。晶粒大小与Z参数的自然对数满足线性关系,即,ln d = -0.076 ln z+2.571。比较225℃7道次往复挤压AZ31、AZ31-1Si和AZ91镁合金发现,往复挤压对数量少、细小的第二相粒子Mg17Al12具有细化和重新分布的作用,Mg17Al12粒子趋向于网络状分布。Mg17Al12能促进往复挤压镁合金晶粒间位向差的增加、大角度晶界的形成和粗晶粒的细化。往复挤压对大块状Mg2Si也有一定的细化效果而基本没有重新分布的作用。Mg2Si对往复挤压镁合金的晶粒大小、晶粒形貌和晶界结构影响很小。往复挤压镁合金织构组分受挤压道次、温度和第二相的综合影响,挤压道次对织构的影响最大。挤压道次和第二相数量增加,织构强度减小。温度升高,织构强度有增加的趋势。挤压态和往复挤压1道次,大多数晶粒处于硬取向位置,滑移不容易开动。往复挤压3、7道次,大多数晶粒处于滑移的有利位向,变形均匀性增加。研究发现,往复挤压镁合金,是以连续动态回复再结晶(CDRX)和旋转动态再结晶(RDRX)为主,非连续动态再结晶(DDRX)为辅的晶粒复合细化机制。研究了300℃往复挤压AZ31、AZ91和225℃往复挤压AZ31-1Si,发现随着挤压道次的增加,镁合金的延伸率逐渐增加,屈服强度在1道次明显增加,然后随着挤压道次的增加而降低,屈服强度与晶粒大小呈现反Hall-Petch关系。AZ31镁合金7道次延伸率达到35.52%,是挤压态延伸率的2.2倍。屈服强度在1道次比挤压态提高了20MPa,达209.69MPa,7道次降低到140.48MPa。研究了225℃-400℃温度范围3道次往复挤压AZ31、AZ31-1Si和AZ91镁合金,发现随着温度的升高,往复挤压镁合金屈服强度降低,延伸率有增加的趋势。225℃-400℃往复挤压3道次AZ31镁合金的晶粒尺寸与屈服强度满足Hall-Petch关系,即,往复挤压镁合金塑性的改善主要在于断裂方式的转变。研究往复挤压AZ31、AZ31-1Si和AZ91镁合金的室温拉伸断口发现,挤压态镁合金的断裂方式主要为穿晶剪切断裂。往复挤压细晶镁合金断口出现了大量的韧窝,断裂方式主要是沿晶界、基体和第二相界面断裂。细小Mg17Al12相和大块状Mg2Si相是主要的裂纹源。细晶AZ31镁合金室温拉伸过程中,发生了晶粒的旋转和新晶粒的形成。晶粒越小,演变可能性越大。随着拉伸应变的增加,织构强度逐渐降低,大角度晶界、晶粒数量、平均位向差和晶界密度逐渐增加。在拉伸过程中,{0001}晶面平行于拉伸方向的晶粒不容易旋转,其次是{2-1-10}晶面,其余晶面的晶粒,都有调整自己的位向,朝{0001}晶面或者{2-1-10}晶面旋转的趋势。研究了晶粒大小、位错密度、晶界结构、织构、第二相粒子的含量、性质和大小等对往复挤压镁合金强韧性的影响。发现位错密度和小角度晶界增加,细小第二相(≦ 1μm)数量和均匀性增加使往复挤压镁合金屈服强度提高。晶粒细化和高Schmid因子的织构优化使往复挤压镁合金的变形均匀性增加,延伸率明显提高。更多还原

【Abstract】 The purpose of this paper is to study the microstructure and mechanical properties of AZ31, AZ91 and AZ31-1Si alloys after cyclic extrusion compression (CEC). The effects of CEC pass and CEC temperature on the grain size, grain boundaries structure and texture of AZ31 and AZ91 Mg alloys were investigated by optical microscopy (OM), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). The effect of the second phase with different volume and properties on CEC microstructure and texture was studied. The grain refining mechanism for Mg alloys during CEC was discussed. The mechanical properties at room temperature of as-extruded and CECed AZ31, AZ31-1Si and AZ91 were measured. The fracture mechanism of as-extruded and CEC Mg alloys were investigated by SEM and in-situ EBSD tension. At last the strength and ductility of Mg alloys after CEC were discussed. The main results can be summarized as follows,The microstructure of AZ31 alloy after CEC passes of 1-25 at 300℃was studied. The results show that magnesium alloys can be refined effectively by CEC. The most effective CEC pass is the first pass and then decrease. There will be a critical CEC pass to obtain the final grain size. There is no difference in microstructure of middle area between cross section and longitudinal section. However, significant difference in microstructure between central and peripherial areas of longitudinal section can be obtained at initial strains, which decreases and almost disappearance with increasing strains.The grain refines continuously and low angle grain boundaries (LAGBs) tend to decrease while the average misorientation tends to increase with the increase of CEC pass. The grain size of 25μm, LAGBs of 28.7% and average misorientation of 34.6 are obtained in as-extruded AZ31 alloy and the mean grain size of 1.77μm with fine grains of 150±50nm, LAGBs of 7% and average misorientation of 54.8 can be obtained in AZ31 alloy after CEC 7 passes and 300℃. The fine grains in CEC microstructure tend to form network structure. The original network structure is subdivided to more even network due to more fine grains formed with the increase of accumulated strains. The gather level of fine grains decreases with increasing second phase.The microstructure of AZ31 alloy after CEC 3 passes at temperature of 225℃-400℃was investigated. The results show that it is good for the decrease of grain size and LAGBs and the increase of average misorientation and grain boundary line length/ area as CEC temperature increase. The relationship between grain size and Z parameter can be described as ln d = -0.076 ln z+2.571The microstructure of AZ31, AZ31-1Si and AZ91 alloy after CEC 7 passes at 225℃was compared. The results show that the Mg17Al12 phase can be refined and redistributed by CEC and tends to present network distribution in microstructure. Fine Mg17Al12 phase can help the refinement of coarse grains, the increase of misorientation and the formation of high angle grain boundaries (HAGBs) but little affect on fine grains with the size less than Mg17Al12. The efficiency of Mg17Al12 to contribute grain refinement is better at lower CEC temperature. The coarse mass Mg2Si phase can also be refined but can not be redistributed by CEC. The grain size and grain boundary structure are little affected by Mg2Si.The texture components of Mg alloys are affected by CEC pass, CEC temperature and the second phase. Among them, CEC pass is the most important factor. The texture intensity decreases as CEC pass increases. The texture intensity tends to increase as CEC temperature increases. The texture intensity decreases as the second phase increases. Most grains are difficult to slip at as-extruded and CEC AZ31 1 pass while most grains with available misorientation to slip after CEC 3 and 7 passes.The grain refining mechanism of Mg alloys during CEC can be described as a compound grain refining mechanism, which combined both the Continuous Dynamic Recovery and Recrystallization (CDRR) and Rotation dynamic recrystallization (RDRX) assistanted by Discontinuous Dynamic Recrystallization (DDRX).The mechanical properties of AZ31 and AZ91 alloys after CEC at 300℃and AZ31-1Si alloy after CEC at 225℃were studied. The results show that the elongation of Mg alloys increases as CEC pass increases. The yield strength increases obviously at CEC 1 pass. However, which decreases sharply and presents inverse Hall-Petch relationship with increasing strains. The elongation of AZ31 after CEC 7 passes at 300℃reaches 35.52%, which is 2.2 times of as-extruded AZ31 alloy. The yield strength of AZ31 after CEC 1 pass increases 20MPa, up to 209.69MPa and next decreases to 140.48MPa after CEC 7 passes. The mechanical properties of AZ31, AZ31-1Si and AZ91 alloy after CEC 3 passes at 225℃-400℃were investigated. The results show that the yield strength of Mg alloys after CEC decreases and the elongation tends to increase as CEC temperature increases. When CEC temperature increases from 225℃to 400℃, the yield strength of AZ31 alloy continuously decreases from 166.61 MPa to 103.89 MPa. The elongation is always above 30%. The grain size a1nd yield strength is consistent with Hall-Petch relationship, that is,σs = 48.88 + 280.48d?2The improvement of ductility for Mg alloys after CEC depends on the change of fracture mode. AZ31 alloy with coarse grains is intracrystalline and shear fracture. The fine grained AZ31 alloy fractures along grain boundaries and the boundaries between matrix and the second phase. The Mg17Al12 and Mg2Si phases are the main crack source during deformation.Grain rotation and the formation of new fine grains appear during tensile deformation of fine grained AZ31 Mg alloy. HAGBs, grain number and the average misorientation increases but the texture intensity decreases during tension. The grains having {0001}or {2-1-10} plane being parallel to tensile direction tend to be stable, while grains having other planes are unstable and tend to rotate to {0001} and {2-1-10} during tension.The effects of dislocation intensity, grain boundary structure, texture, grain size and the second phase et, al on the strength and ductility of Mg alloys after CEC were studied. The results show that the yield strength improves as the increase of dislocation intensity, LAGBs and the fine second phase (≦ 1μm). The improvement of ductility depends on grain refinement and texture optimization with high value of Schmid factor.更多还原