【作者】 张磊；

【导师】 刘纯国； 王慧远；

【作者基本信息】 吉林大学 ， 材料加工工程， 2015， 博士

【摘要】 镁合金具有低密度、高比强度、易于回收等诸多优点，在航空、航天、汽车和通讯等领域有着巨大的应用前景。然而，由于镁合金塑性变形能力较差，且在成形过程中易形成织构，导致镁合金变形件制备难度大，各向异性严重等问题，极大的限制了变形镁合金的工业应用。针对上述问题需要对镁合金成形过程及其变形机制进行深入系统的研究。为此，本文研究了新型Mg–6Al–3Sn（AT63）镁合金在单轴变形和平面压缩两种应变状态下的组织演变规律和力学变形行为，着重探讨了两种应变状态下AT63镁合金的孪生和滑移机制，在此基础上提出了镁合金新型衬板轧制方法。主要结论如下：（1）研究了挤压双织构AT63镁合金板材单轴压缩下的孪生行为，阐明了双织构镁合金变形过程中拉伸和压缩孪生共存的孪生特点，揭示了双织构镁合金通过取向互补降低拉伸和压缩孪生对应变方向（ED、TD、ND）依赖性的作用机制，为减小镁合金各向异性的织构设计提供借鉴。（2）基于轧制AT63镁合金板材单轴拉伸下的变形行为研究，发现了板材沿RD和TD拉伸变形中织构演变是由位错滑移主导，并建立了单轴拉伸下织构演变与滑移机制的关系；其中，(0002)织构演变由基面a滑移主导，而(1010)织构演变由非基面a和非基面c a滑移共同主导，和织构随拉伸变形逐渐转变为Schmid因子最低的硬取向。（3）研究了轧制AT63镁合金板材平面压缩孪生行为，揭示了平面压缩下镁合金的孪生模式取决于压缩应变方向，而孪晶生长速度和孪晶形貌受扩展应变方向影响；提出了平面压缩下孪生行为的最小孪生应变准则，即束缚方向孪生应变小的孪生系优先产生，为平面应变下镁合金孪生机制分析提供了依据。（4）提出镁合金衬板轧制新方法，基于单衬板轧制法制备AT63镁合金板材组织及性能研究表明，通过在轧制过程中添加硬质合金衬板能够显著减少轧制试样表面裂纹，提高单道次压下量，获得了平均晶粒尺寸4m、抗拉强度σb为330MPa的细晶高强AT63镁合金轧板。（5）波浪衬板轧制法制备AT63镁合金板材组织及性能研究表明，通过添加波浪形表面衬板能够有效改善轧制过程中剪切应力对称性，从而弱化轧板基面织构，提高板材加工硬化能力（n=0.295）和断裂延伸率（δf=22.5%）；波浪衬板轧制法为弱化镁合金轧板基面织构提供了新思路。（6）双衬板轧制法制备AT63镁合金板材组织及性能研究表明，通过上下表面同时添加硬质合金衬板，能够实现350oC下压下量85%以上单道次轧制成形，并获得综合性能良好的AT63镁合金轧板（σb=340MPa，δf=18.2%）；大压下量（85%+5%）双衬板轧制同样能够用于制备轧制成型能力差的AZ91镁合金，并获得综合性能良好的AZ91镁合金轧板（σb=370MPa，δf=27%）；该双衬板轧制法在提高轧制效率以及难轧合金板材轧制成型方面具有较大的应用潜力。总之，本文研究了单轴变形下挤压和轧制AT63镁合金的变形行为，揭示了双织构镁合金孪生机制，建立了单轴应变下织构演变与滑移机制的关系；研究了平面压缩下轧制AT63镁合金的变形行为，揭示了平面压缩下孪生行为的最小孪生应变准则，为平面应变下镁合金的孪生机制分析提供了依据；提出了新型衬板轧制法，为弱化镁合金轧板基面织构以及实现镁合金大压下量轧制成型提供了新思路。更多还原

【Abstract】 Magnesium (Mg) alloys have enormous potential for applications in the aerospace,automotive and automobile industries, due to their outstanding properties such as low density,high specific strength and easy recovery. However, the developments of wrought Mg alloysare really hindered for their limited room temperature formability and strong anisotropy.Solving these problems needs a better understanding of the deformation mechanisms of Mgalloy. In this paper, we investigated the microstructure evolution and mechanical behavior ofnew Mg–6Al–3Sn (AT63) Mg alloys, during uniaxial deformations and plane straincompressions (PSC). The focus was on the twinning and dislocation slip mechanisms. Basedon the plastic theory of Mg alloy under various strain modes, we proposed an innovativerolling approach, i.e., hard-plate rolling. The main conclusions can be summarized asfollows:(1) Uniaxial compressions were carried out on an extruded AT63Mg alloy plate with dualbasal texture. Due to the dual basal texture, this plate exhibited a coexistence ofextension and contraction twinning. It was found that orthogonal texture componentsresulted in both extension and contraction twins during the compressions along theextrusion direction (ED), transverse direction (TD) and normal direction (ND). Thisresult is helpful for the texture design to reduce the mechanical anisotropy of Mg alloy.(2) Uniaxial tensions were carried out on a rolled AT63Mg alloy plate. It was found that,the texture evolution during the tensions along rolling direction (RD) and TD resultsmainly from dislocation slip. The texture variations in (0002)poles were related tobasal slip, while those in (1010)poles were attributed to both non-basal a andnon-basal c a slips. The final textures of the two tensile samples exhibit thelowest SF values for all slip systems. This conclusion shed light on the relationship ofbetween the dislocation slip and texture evolution during uniaxial strain.(3) PSCs were applied on a rolled AT63Mg alloy plate. The twinning mode depended mianly on the compressive strain direction, meanwhile the amount and morphology oftwins varied significantly with the expensive strain direction. It was shown that thetwinning behaviors during PSCs were conducted by the twinning strain, i.e., the twinvariants with small twinning strain along constraint direction will be activatedpreferentially. This criterion is helpful for the twinning analysis during PSCs.(4) Single hard-plate rolling (SHPR) was tests were conducted on AT63Mg alloy. Owingto the addition of one hard steel plate, the cracks of the SHPRed samples wassignificantly decreased. Thus, large thickness reduction can be induced in one rollingpass, and the resulting AT63Mg alloy sheet exhibited small average grain size of4mand high tensile strength of330MPa.(5) Wave-shaped die rolling (WDR) were conducted on AT63Mg alloy. The addition ofwave-shaped die was found to modify the symmetric shear stresses during rolling,resulting in a significant weakening in the basal texture of WDRed sheets. Due to theweakened basal texture, the WDRed sheets presented strong strain hardening capacity(n=0.295) and high elongation to failure (δf=22.5%).(6) Double hard-plate rolling (DHPR) tests were conducted on AT63Mg alloy. Due to theaddition of two hard steel plates, large thickness reduction above85%can be inducedat350oC by just one pass. The DHPRed AT63Mg alloy sheets presented concurrentlyhigh tensile strength and high elongation (σb=371MPa, δf=27%). Moreover, DHPRwith large thickness reduction (85%+5%) was also suitable for AZ91Mg alloy whichhas very limited formability. The DHPRed AZ91sheet exhibited excellentstrength/ductility combination (σb=371MPa, δf=27%). DHPR is applicable to otheralloys having poor formability, and paves a way for their industrial applications.In summary, this paper investigated the deformation behaviors of extruded and rolledAT63alloys during uniaxial deformations, which presented the twinning mechanism indually basal-textured alloy and revealed the relationship between dislocation slip and textureevolution. The deformation behaviors of rolled AT63alloy during PSC were alsoinvestigated, which shed light on the twinning criterion during PSC. Additionally, innovativerolling approaches were proposed in this paper, and were found to be effective forweakening the rolling basal texture and improving the rolling efficiency of Mg alloys.更多还原