高塑性Mg-Gd-Zn镁合金轧制过程中的组织、织构演变及力学性能

IMR OpenIR

	高塑性Mg-Gd-Zn镁合金轧制过程中的组织、织构演变及力学性能
	吴迪
学位类型	博士
导师	韩恩厚 ; 陈荣石
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	镁合金热轧冷轧织构塑性 Magnesium Alloy Hot Rolling Cold Rolling Texture Ductility
其他摘要	" 镁合金特别是镁合金板材被誉为“二十一世纪绿色工程结构材料”，在航空航天、汽车、高铁等领域都具有极其重要的应用价值。但是目前的镁合金板材不仅种类少，而且由于具有强烈的基面织构，室温塑性和成形性能差、各向异性强、加工和成形十分困难。利用稀土元素能够弱化镁合金板材轧制织构的特点，本论文设计了Mg-x(x=2,3,5)%Gd-1%Zn镁合金，以此为研究对象，研究了该系列合金的热轧工艺、冷轧工艺和退火工艺，并针对其热轧板材室温及中、高温下的组织、织构、力学性能和成形性能进行了较为系统深入的研究。研究了热轧和退火参数对Mg-x(x=2,3,5)%Gd-1%Zn合金组织和织构演变的影响规律。随轧制温度的升高，合金的轧制性能提高，但轧制温度过高时（≥480℃），基体中细小弥散的第二相会重新固溶消失，组织易发生粗化。轧制态板材的组织以孪晶为主，很少发现再结晶晶粒，织构类型仍属于基面织构，但是强度较低；道次间回炉退火发生的静态再结晶是铸态Mg-x(x=2,3,5)%Gd-1%Zn合金在多道次热轧过程中组织细化、均匀化、以及织构弱化的关键。Mg-3%Gd-1%Zn（GZ31）合金热轧板材在终轧后的静态退火处理中，初期会在孪晶处形成少量再结晶晶粒，随退火温度升高或者退火时间延长，再结晶比例迅速增大，织构显著弱化，并呈现非基面化趋势。发生完全再结晶时，已转变成非基面织构，且织构强度降至最低。进入晶粒长大阶段后，非基面织构的强度会增强。GZ31合金在热轧过程中形成的大量拉伸孪晶在静态退火时会诱发再结晶，且再结晶晶粒取向十分离散，这是GZ31合金热轧板材静态退火后织构显著弱化的主要原因。在最佳轧制和退火工艺条件下制备出Mg-x(x=2,3)%Gd-1%Zn合金板材，研究了其在室温下的组织、织构、力学性能和成形性能，分析了室温高塑性的变形机理，并讨论初了始织构和晶粒尺寸对其室温塑性的影响。最佳工艺条件下制备的GZ21和GZ31合金板材的组织都是由细小均匀的再结晶晶粒组成，基体上分布着大量细小弥散的w相，织构强度极低，且属于非基面织构。板材在室温拉伸时表现出了极佳的室温塑性和成形性能，各方向的断裂伸长率都大于40%，均匀伸长率大于20%，室温Erichsen杯突试验的IE值~8，接近于一些典型的铝合金，可以轻易冷弯至180°，室温的极限冲压比（LDR）可达1.83。室温拉伸时，大量基面滑移的发生，是其具有室温高塑性的主要原因；变形过程中还伴有一些非基面滑移的发生，可以协调一些取向差的晶粒的变形，以及调节c轴方向的应变，对于保证变形的均匀连续性同样十分重要。具有基面织构的GZ31-F锻造样品和具有非基面织构的R-A38板材样品的其它组织特征一样，但都表现出高的室温拉伸塑性，说明GZ31合金热轧板材的室温高塑性主要是由于其极低的织构强度，即弱织构，而不是其非基面的织构类型。晶粒尺寸为10μm~30μm的GZ31合金板材都具有弱织构，室温下也都表现出了优异的拉伸塑性，断裂伸长率大于38%，说明在该晶粒尺寸范围内，织构是决定其室温塑性的关键因素，而晶粒尺寸的影响相对较弱。当晶粒尺寸大于50μm时，板材的均匀伸长率不会有显著变化，而在后均匀变形过程中孪生行为会显著增加，并导致最终的断裂和极低的后均匀伸长率。研究了Mg-3%Gd-1%Zn合金热轧板材在中、高温下的变形行为。结果表明，GZ31合金热轧板材在应变速率1×10-4s-1~1×10-2s-1，温度150℃~250℃的条件下拉伸时，表现出了显著的锯齿流变现象。在锯齿流变的温度区间，出现了屈服强度和抗拉强度的平台，特别是板材的塑性，与室温下的高塑性相比，没有显著提高。发生锯齿流变的机制主要是滑动位错与第二相粒子的剪切作用，动态应变时效（DSA）机制也起到了一定的作用。当拉伸温度≥300℃时，锯齿流变现象基本消失，GZ31合金板材的塑性会随温度升高以及应变速率降低而显著提高，在400℃应变速率1×10-4s-1条件的伸长率可到达~280%，实现了超塑性。研究了Mg-3%Gd-1%Zn合金热轧板材的冷轧性能，冷轧工艺对组织和织构的影响以及冷轧板材的力学性能。单道次轧制的压下量可达到23%，板材的表面和边部均无裂纹产生，且具有较高的表面光洁度。通过引入大量位错和织构的强化以及基面化转变，冷轧板材的屈服强度可提高~110%；而冷轧后辅以适当的热处理工艺，可以使板材在具备室温高塑性的同时也具有较高的强度（屈服强度>200MPa，抗拉屈服>270MPa）。采用每道次小压下量的多道次冷轧工艺，可以获得较高的总压下量，但是由于剪切带的软化效应，该冷轧工艺轧制的板材的强度比预期的略低。" ; " Magnesium alloys are recognized as the “green structural materials” in the 21st century, and have the high potential for improving fuel efficiency of vehicle and reducing CO2 emission because of their high specific strength and stiffness. To achieve more application in industry, it is required to develop rolling technologies for mass production of high performance Mg alloy sheets. Unfortunately, the commercial Mg alloy sheets usually have a pronounced basal texture, which induces poor ductility and formability at room temperature and limits their further forming and industrial applications. The addition of RE metal to Mg alloy has been found to be an effective way of weakening and changing the basal texture of wrought Mg alloys. In this dissertation, Gd, serving as a major alloying element, was added in Mg-Zn alloy, and the rolling process, annealing treatment, microstructures, textures, mechanical properties and formability of the Mg-x(x=2,3,5)%Gd-1%Zn alloys at room termperature and high temperature were investigated. The main work and achievements are summarized as follows. The microstructure and texture evolution of the as-cast Mg-x(x=2,3,5)%Gd-1%Zn alloys during hot-rolling process and annealing treatment was investigated. As the rolling temperature increased, the workability of the Mg-x(x=2,3,5)%Gd-1%Zn alloys was enhanced. However, if the rolling temperature was higher than 480℃, the fine precipitates in the matrix could disappear and the microstructure quickly turned to be coarse. The microstructure of the rolled sheets was mainly composed of a great deal of twins, rather than the DRXed grains. The texture of the rolled sheets belonged to the basal texture, yet the intensity was relative low. The static recrystallization (SRX) during the in-pass annealing treatment played a key role in refining the microstructure and weakening the texture of the Mg-x(x=2,3,5)%Gd-1%Zn alloy sheets during multi-pass hot-rolling process. As for the final annealing treatment, at first, a few of SRXed grains formed in the twins, and then the proportion of SRXed grains sharply increased and the texture turned to be weak and non-basal, as the annealing temperature or annealing time increased. For the GZ31 sheet with fully recrystallized microstructure, the texture went to be a typical non-basal one, and its intensity bottomed out. Nevertheless, the intensity of the non-basal texture could rebound up, as the SRXed grains grew up. A large amount of tensile twins, appearing during hot-rolling process, were the favorable sites for SRX during the annealing treatment, and the recrystallized grains oriented randomly, which is mainly responsible for the weak texture of GZ31 sheet. Mg-x(x=2,3)%Gd-1%Zn alloy sheets were manufactured by the optimal hot-rolling and annealing technics, and the microstructure, texture, mechanical property and formability of the rolled sheets were investigated. In addition, an attempt was made to analyze the deformation mechanism to its high ductility by EBSD and TEM, and the effect of grain size and initial texture on the room-temperature ductility of the GZ31 sheet was also presented. The microstructure of the GZ21 and GZ31 alloy sheets consisted of fine recrystallized grains with a large amount of homogeneously distributed tiny particles in the matrix. These sheets exhibited a quite weak and non-basal texture, which resulted in the basal slips flourished during the tensile deformation at room temperature. Therefore, the GZ21 and GZ31 alloy sheets presented an excellent ductility with the elongation-to-failure larger than 40% in all tensile direction. Moreover, the non-basal slip also played a role in keeping a compatible and homogeneous deformation, especially in those grains with orientations unfavorable to basal slip, and contributed to the high room-temperature ductility. The GZ31 sheet also exhibited excellent room-temperature formability. The Erichsen value was ~8, the limited drawing ratio could reach 1.83, and it could also be bent easily to 180° without any cracks. The forged GZ31-F samples and the R-A38 sheets both presented excellent ductility at room temperature, with the elongation-to-failure larger than 40%, which meant that the excellent tensile ductility of the Mg-3%Gd-1%Zn alloy sheets should be mainly attributed to its low intensity of (0002) pole figure, instead of the non-basal texture type. The GZ31 alloy sheets with a grain size of 10~30μm all exhibited a large elongation-to-failure (>38%); comparing with the weak texture, the grain size had a negligible effect on their room-temperature ductility. As grain size increased further (>50μm), the twining activity increased substantially during the post-uniform deformation, leading to its premature failure and low post-uniform elongation. The tensile deformation behavior of the hot-rolled Mg-3%Gd-1%Zn alloy sheet at elevated temperature was systematically characterized. Serrated flow was observed when the GZ31 alloy sheets were tensile tested at temperatures ranging from 150℃ to 250℃ and at strain rates of 1×10-4s-1 to 1×10-2s-1. The GZ31 alloy sheet exhibited plateaus in the temperature dependence of the yield and ultimate tensile strengths in the serrated flow regime. A loss in ductility was also found. The serrated flow was mainly controlled by the shearing of precipitates by gliding dislocations. In addition, DSA might also play a role in the present PLC phenomenon. When the testing temperature was higher than 300℃, the PLC effect almost disappeared and the ductility of the GZ31 sheet increased sharply. The optimum superplastic condition was obtained at 400ºC and 1×10-4s-1 with the largest elongation-to-failure of ~280%. The cold rollability of the Mg-3%Gd-1%Zn alloy sheet and the effect of the cold rolling and annealing treatment on its microstructure, texture and mechanical properties were investigated. The 0.2% proof stress of the GZ31 hot-rolled sheet could be increased by ~110% through the single-pass cold rolling with the thickness reduction of 23%, due to the high intensity of dislocation and basal texture. Compared with the GZ31 hot-rolled sheet, the cold rolled sheet annealed at 350℃ for 30min might get an enhancement in strength without a great loss of ductility. Although it experienced a larger total thickness reduction, the sheet processed by multi-pass cold rolling did not show a higher strength as expected, due to the softening effect of shear bands."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64470
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	吴迪. 高塑性Mg-Gd-Zn镁合金轧制过程中的组织、织构演变及力学性能[D]. 北京. 中国科学院金属研究所,2012.