【作者】 朱红梅；

【导师】 罗承萍；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2011， 博士

【摘要】 针对Mg-Zn二元合金铸造性能差、时效析出相粗大,因而时效强化效果不佳的缺点,本研究采用Cu、Zr微合金化以改善Mg-Zn系合金的铸造性能、时效组织和综合力学性能。设计了名义成分为Mg-6Zn-xCu-0.6Zr （x = 0, 0.5, 1.0, 2.0, wt.%）的系列铸造镁合金,并采用金相显微技术、X射线衍射仪技术、扫描电子显微镜技术、透射电子显微镜技术、扫描透射电子显微镜技术、显微硬度计和拉伸试验机等全面研究了合金的时效行为、显微组织及力学性能。重点探讨了微合金化元素Cu、Zr及热处理工艺对合金时效行为、显微组织及力学性能的影响;表征了各种时效析出相的形态和晶体学特征,阐明了其生成机制,并揭示了时效过程中组织的演变进程。铸态Mg-Zn-Zr三元合金添加Cu后,其铸态组织由（α-Mg基体+MgZn2）组成变为由（α-Mg基体+MgZn2+MgZnCu）组成;立方结构的MgZnCu作为共晶相分布于晶界。当Cu添加量大于1 %时对合金铸态组织有一定的细化晶粒的作用。峰时效态时,Cu不仅可提高合金主要强化相细长杆状[0001]_αβ₁′-MgZn₂的析出密度和组织均匀性,还可抑制导致过时效的（0001）_αβ₂′-MgZn₂相的析出,但随着Cu含量的继续增加,[0001]_αβ₁′-MgZn₂的长/径比降低。Cu的添加对合金的力学性能有明显的影响。当Cu含量为0.5%时综合力学性能最佳,峰时效态合金的抗拉强度σ_b、屈服强度σ_0.2和延伸率分别达到266.3 MPa、185.6 MPa和16.7%,特别是塑性指标延伸率增加明显;添加1.0%Cu时的综合力学性能次之;当Cu含量为2%时,因大量MgZnCu颗粒在晶界上呈连续网络状分布,使合金的力学性能明显下降,同时因为大量MgZnCu颗粒的形成而消耗了部分溶质Zn,致使合金的时效硬化效果恶化。同时,微量（0.5%¹.0%）Cu的加入还能延缓合金的过时效进程。Cu的加入使组织均匀化及镁基c/a比的降低是含Cu合金具有较大延伸率的主要原因。Mg-Zn-Zr三元合金主要以沿晶断裂方式为主,Mg-Zn-Cu-Zr四元合金主要以（准）解理断裂和韧窝的混合型方式断裂。铸态合金因组织粗大且不均匀,力学性能较差。经固溶+时效处理后,基体中析出大量弥散、共格的强化相,合金的强度和塑性都显著提高,时效强化效果明显。研究发现,Zr不仅能细化晶粒,而且能促使合金在430°C固溶处理过程中形成四方结构的富Zr相（δ-Zn₂Zr₃）。根据形态和晶体学特征,这些富Zr相分为四类。第1类为细长杆状相,其轴线平行于[0001]_α方向;第2类呈短四棱柱状,以其轴线平行于（0001）_α基面和< 1120 > _α;第3类同样是细杆长状相,其轴线平行于（0001）_α基面和< 1100 > _α;第4类也是细长杆状相,但其轴线与（0001）_α基面斜交,并与[0001]_α方向夹25³5°角。这些δ-Zn₂Zr₃相的形态和晶体学特征具有明显的关联性,其轴线方向都是δ/α间最小或较小晶向错配度的方向。这些固溶处理中形成的δ-Zn₂Zr₃,在后续180°C时效过程中作为前驱体相为β1′-MgZn₂提供有效的异质形核中心。δ-Zn₂Zr₃在β₁′-MgZn₂长大过程中逐渐分解,分解释放出的Zr原子或局部地溶入Mg基体中,或形成新的含Zr化合物,同时为富Zn相的继续长大提供了较好的成份环境。在峰时效之前,合金中先后析出三类杆状的、并且都与（0001）_α基面垂直的[0001] _αβ₁′相。第一、二类都是六方β₁′-MgZn₂,只是与基体的位向关系略有不同;第三类是单斜β1′-Mg₄Zn₇,在靠近峰时效态及后期出现。在过时效合金中还出现另两类析出相,一是盘状的六方β₂′-MgZn₂,盘面平行于（0001）_α基面;二是杆状的棱方平衡相β-MgZn,也与（0001）_α基面垂直。在时效初期还形成了两类GP区,分别平行于{0001}_α和{1120}_α。表征了各析出相的晶体学特征,并讨论了其生成机制。在固溶态合金中的富Zr区（即固溶时析出δ相的区域）可能的析出序列为:δ-Zn2Zr₃→β₁′-MgZn₂→β₂′-MgZn₂→β-MgZn,或者δ-Zn₂Zr₃→β₁′-MgZn₂→β-MgZn。而在富Zn区（即固溶时未析出δ相的区域）可能的析出序列为:过饱和固溶体SSSS→GP区→β₁′-MgZn₂→（β₁′-Mg₄Zn₇和/或β₂′-MgZn₂）→β-MgZn。该研究对了解Mg-Zn-Cu-Zr系合金的时效析出行为和进行Mg-Zn系合金的时效强化设计具有重要的理论和实用意义,并为开发新型低成本、高性能的Mg-Zn-Cu-Zr镁合金提供了理论依据。更多还原

【Abstract】 In view of the poor castability and coarse microstructure which are responsible for the inadequate precipitation strengthening, micro-alloying with Cu and Zr was employed to improve the castability, precipitation microstructure and mechanical properties. A series of cast Mg-Zn base alloys Mg-6Zn-xCu-0.6Zr （x=0, 0.5, 1.0, 2.0, wt.%） were prepared for the study, and optical microscopy （OM）, X-ray diffraction （XRD）, scanning electron microscopy （SEM）, transmission electron microscopy （both conventional and HRTEM）, scanning transmission electron microscopy with energy dispersive spectroscopy （STEM-EDS）, microhardness testing and tensile testing were used to investigate the aging behavior, microstructure and mechanical properties, and the effects of the alloying elements Cu and Zr on them were elucidated. The morphology, crystallography, formation mechanism and precipitation sequence of the precipitates were characterized as well.Cu addition to the Mg-Zn-Zr alloys changed the as-cast microstructure of the alloys from being composed of （α-Mg matrix + MgZn₂） to being composed of （α-Mg matrix + MgZn₂ + MgZnCu）, with the cubic-structured MgZnCu distributing at the grain boundaries as an eutectic phase. The Cu addition, when above 1.0%, can to some extent refine the as-cast microstructure, it can also increase the number density and homogeneity of the predominant strengthening precipitate [0001]_α1′-MgZn₂ in the peak-aged condition, and retard the precipitation of the plate-like （0001）_αβ₂′-MgZn₂ formed in the over-aged alloys. The aspect ratio （the length/diameter ratio） of the [0001]_αβ₁′-MgZn₂, however, decreased with further increasing the Cu content, thus slightly deteriorating the mechanical properties.The micro-alloying with Cu also had a striking influence on the mechanical properties of cast Mg-Zn-Cu-Zr alloys. The alloy with 0.5% Cu in its peak aged condition was found to have the optimal composite mechanical property with a UTS of 266.3 MPa, a YS of 185.6 MPa and an elongation of 16.7%, the elongation being appreciably enhanced. Meanwhile, 0.5-1.0% Cu could delay the occurrence of over-aging. The composite mechanical property of the 1.0% Cu alloy was slightly poorer than that of the 0.5% Cu alloy. As the Cu content reached 2.0%, however, the precipitation strengthening effect was greatly deteriorated due to the network-like eutectic phase MgZnCu distributing at the grain boundaries, and to the consumption of Zn by the MgZnCu which would diminish the precipitation of the main strengthening precipitateβ₁′-MgZn₂. The appreciable increase of the elongation value was thought to be due to the homogenization of the microstructure and to the decrease of the c/a ratio of the Mg matrix, both brought about by the Cu addition. The Cu-free Mg-Zn-Zr alloy showed a typical intergranular brittle fracture surface while the Cu-containing alloy exhibited a quasi-cleavage fracture surface mixed with ductile rupture characterized by the dimples and tear ridges.The mechanical properties of the as-cast alloys were poor due to their coarse and inhomogeneous microstructure. A T6 treatment （i.e., solution treatment + aging） led to the precipitation of a large quantity of fine-dispersed and coherent strengthening phases, thus appreciably enhancing the strength and plasticity of the alloys.The microalloying element Zr could not only refine grains, but also promote the precipitation of the tetragonal Zr-richδ-Zn₂Zr₃ in the as-solutionized condition at 430°C. Based on the morphology and crystallograghic features of theδ-Zn₂Zr₃ precipitates, they were classified as four types. The thin, long rod-likeδ-Zn₂Zr₃ of type 1 was with its axis perpendicular to the basal plane （0001）_α; type 2, while being nearly tetrahedral in shape, was lying in the basal plane （0001）_αand with its axis parallel to < 1120 >α; type 3, also being thin, long rod-like, was lying in the basal plane （0001） _αand with its axis parallel to < 1100 >α; and type 4, while being also thin, long rod-like, was inclining to [0001]_αby 25-35°. A correlation between the morphology and crystallography existed for each of the former three types ofδ-Zn₂Zr₃, i.e., their axes adopted a crystal direction in which the directional misfit between theδ-Zn₂Zr₃ andα-Mg matrix was minimal. Theseδ-Zn₂Zr₃ particles precipitated during solution treatment at 430°C could serve as a heterogeneous nucleation site for the precipitation ofβ₁′-MgZn₂ during the subsequent aging treatment at 180°C. Theδ-Zn₂Zr₃ particles gradually decomposed with the growth of the successorβ₁′-MgZn₂, with the released Zr atoms diffusing into the matrix or taking part in forming new Zr-containing compounds.Before peak-aging, three types of [0001]_αβ₁′precipitates were formed, which were all rod-like shapes with their axis perpendicular to the basal plane （0001）_α. Types 1 and 2 were all the hexagonalβ₁′-MgZn₂, only with slightly different orientation relationships with theα-Mg matrix; and type 3 was the monoclinicβ₁′-Mg₄Zn₇ occurring close to the peak-aged stage. Two other types of precipitates were formed in the over-aged alloys. They were the plate-like hexagonalβ₂′-MgZn₂, with its plate face parallel to the basal plane, and the rod-like rhombohedral equilibrium phaseβ-MgZn, also with its axis perpendicular to the basal plane （0001）_α. In the early stages of aging, two types of GP zones, parallel to {0001}_αand {1120}_αrespectively, were formed. The crystallographic features of the above five types of aged precipitates were characterized, and their formation mechanisms were discussed on the basis of the crystallographic features.The possible precipitation sequence in the Zr-rich region （i.e., the region withδ-Zn₂Zr₃ precipitated in the solution treated condition） was proposed as follows:δ-Zn₂Zr₃→β₁′-MgZn₂→β₂′-MgZn₂→β-MgZn, orδ-Zn₂Zr₃→β₁′-MgZn₂→β-MgZn. And, the possible precipitation sequence in the Zn-rich region （i.e., the region free ofδ-Zn₂Zr₃ in the solution treated condition） was proposed as follows: Super saturated solid solution （SSSS）→GP zones→β₁′-MgZn₂→（β₁′-Mg₄Zn₇ and/orβ₂′-MgZn₂）→β-MgZn.The study is significant in clarifying the aging behavior of the Cu-modified Mg-Zn-Cu-Zr alloys, and helpful to design a precipitation-strengthening procedure for the Mg-Zn series alloy. It may also provide a theoretical basis for developing new Mg-Zn-Cu-Zr alloys with low cost and high performance.更多还原