【作者】 周涛；

【导师】 陈振华；

【作者基本信息】 湖南大学 ， 材料加工工程， 2009， 博士

【摘要】 镁合金具有密度低、高比强度、高比刚度、导热性好、电磁屏蔽性优异、对振动及冲击能量的吸收高等优点,是非常重要的轻量化结构用绿色工程材料,在汽车、电子、航空航天等科技前沿领域具有广泛的应用前景。其中Mg-Zn合金是一种典型的沉淀强化合金,具有较高的室温和中温强度、良好塑性以及耐腐蚀性能。但该合金的凝固温度区间大,铸造及成型性能差,且晶粒细化困难,因而不能作为铸件或锻件材料,极大地限制了其工业应用。快速凝固(Rapidly solidification,简称RS)技术是制备高性能材料的有效方法之一,可以显著细化合金的组织、减小成分偏析、扩展合金元素在基体中的极限固溶度、形成亚稳相。采用该方法制备的Mg-Zn合金有望解决上述问题。本论文拟采用RS技术和合金化技术相结合,在细化晶粒的同时,通过添加合金化元素,使合金中均匀分布着细小、弥散的、热稳定的沉淀相,从而进一步改善合金的高温性能。本文采用雾化-双辊急冷法,以Mg-6Zn(wt.%)合金为基体,通过Ca、RE(Ce和La)合金化及复合合金化技术,制备出了RS Mg-Zn-Ca、Mg-Zn-Ce和Mg-Zn-Ca-RE合金薄片及片状粉末,系统地研究了Ca、RE合金化以及复合合金化对RS Mg-Zn合金薄片的微观组织、物相种类、热稳定性和等时时效硬化行为的影响。在此基础上,通过热挤压工艺制备了快速凝固/粉末冶金(Rapidly solidification/Powder metallurgy,简称RS/PM)Mg-Zn系镁合金棒材,分为两个系列合金:(1)RS/PM Mg-Zn-Ca系合金,主要包括Mg-6Zn-5Ca和Mg-6Zn-5Ca-RE (3Ce和0.5La)合金;(2)RS/PM Mg-Zn-Ce系合金,主要包括Mg-6Zn-5Ce和Mg-6Zn-5Ce-1.5Ca合金。论文还对合金的微观组织、室温和高温力学性能及抗蠕变性能进行了研究。主要得到以下结论: (1) RS Mg-Zn系合金薄片的组织与性能Ca的加入显著改善了RS Mg-Zn合金薄片的组织和性能。随着Ca含量的增加,合金中低熔点的Mg51Zn20相逐渐被熔点更高的Ca2Mg6Zn3和Mg2Ca相替代,合金的热稳定性逐渐提高。当Ca的含量高于1.5wt.%时,合金的组织显著细化,最小晶粒尺寸达到3～5μm,热稳定的弥散相的体积分数大幅度增加。RS Mg-6Zn-5Ca合金表现出显著的等时时效硬化行为,其硬度最大值达到120.8±4.5Hv。合金的硬化行为主要是由晶粒内部细小、弥散的Ca2Mg6Zn3相的析出引起的,晶界处则形成了尺寸相对较大的Mg2Ca和Ca2Mg6Zn3等热稳定的析出相,这都有助于合金在高温下保持较高的硬度值。Ce有助于细化RS Mg-Zn合金薄片的组织和提高合金的热稳定性。随着Ce含量的增加,合金的组织逐渐细化,最小晶粒尺寸达到4～7μm,同时合金中高熔点的弥散相的体积分数大幅度增加。Ce与Zn、Mg形成了熔点更高(470～500℃)的Mg-Zn-Ce相(T相),使合金中低熔点的Mg51Zn20相显著减少,大幅度提高了合金的热稳定性,合金的等时时效硬化行为却逐渐减弱。但在RT～400℃热处理温度范围,Mg-6Zn-5Ce合金的硬度值明显高于其它成分的Mg-Zn-Ce合金,其硬度最大值为91.5±7Hv,主要强化机制为细晶强化和弥散强化。该合金中高熔点的T相的原子百分比为80.8at.%Mg,12.4at.%Zn和6.8at.%Ce,其中(Zn/Ce)at约为2:1。在RS Mg-6Zn-5Ca合金薄片中添加RE(Ce和La)后,合金中形成了一种固溶有少量(约2～3at.%)Ca的Mg-Zn-RE相(简称为T′相)。T′相的熔点与T相的相近,均明显高于Ca2Mg6Zn3相。与此同时,随着RE含量的增加,Ca2Mg6Zn3相中溶入了少量的RE元素,使Ca2Mg6Zn3相的热稳定性提高了约6～16℃。这些都有助于提高RS Mg-Zn-Ca-RE合金的热稳定性。Mg-6Zn-5Ca-3Ce合金表现出显著的等时时效硬化行为,其最高硬度值达到162.4±5.5Hv,这主要是由晶粒内部细小、弥散的Ca2Mg6Zn3相的析出引起的,而晶界处形成了高熔点的T′相、Mg12Ce和Mg2Ca相,这都有助于合金在高温下保持较高的硬度值。(2) RS/PM Mg-Zn系合金棒材的微观组织、室温和高温力学性能及抗蠕变性能在热挤压过程中,合金的组织显著细化,同时析出相的体积分数大幅度增加,但合金中的主要物相没有发生明显的变化。对于RS/PM Mg-Zn-Ca系合金,Ca和RE(Ce和La)的依次加入可以大幅度提高合金的室温和高温性能。在Mg-6Zn合金中添加5wt.%Ca后,合金的晶界处形成了大量、热稳定的Mg2Ca和Ca2Mg6Zn3相,有效地强化了晶界,使合金在200℃条件下的抗压强度由40.7MPa(Mg-6Zn)大幅度提高到202.3MPa(Mg-6Zn-5Ca) ;而后者的最小蠕变速率是前者的1/134(200℃/50MPa条件下)。进一步加入RE后,除了Mg2Ca相外,合金的晶界处还形成了大量、高熔点的T′相,同时RE还有助于改善Ca2Mg6Zn3相的热稳定性,从而使合金的高温抗压强度(200℃)提高到234.0MPa(Mg-6Zn-5Ca-3Ce合金)和244.6MPa(Mg-6Zn-5Ca-3Ce-0.5La合金)。在175℃/50MPa条件下,后两种合金的最小蠕变速率分别约为Mg-6Zn-5Ca合金的1/2和1/4。对于RS/PM Mg-Zn-Ce系合金,Ce和Ca的依次加入可以大幅度提高合金的室温和高温性能。在Mg-6Zn合金中添加5wt.%Ce后,合金的晶粒内部和晶界处形成了大量细小、弥散的T相,使Mg-6Zn-5Ce合金的高温(200℃)抗压强度高达225.9MPa;而该合金的最小蠕变速率是Mg-6Zn合金的1/1075(200℃/50MPa条件下)。由于T相的热稳定性明显优于Ca2Mg6Zn3相,因此,RS/PM Mg-6Zn-5Ce合金的抗蠕变性能明显优于RS/PM Mg-6Zn-5Ca合金。进一步加入1.5wt.%Ca后,合金的晶界处形成了形状不同的、固溶有微量(约1at.%)Ca的Mg-Zn-Ce相(其中(Zn/Ce)at≈1.5:1),使合金的高温抗压强度(200℃)达到258.2MPa;其最小蠕变速率约为Mg-6Zn-5Ce合金的1/2(200℃/90MPa条件下)。更多还原

【Abstract】 Magnesium alloys have great potentials to be used as structural materials in the fields of electronic, automobile and airspace industries mainly because of their low densities, high specific strength, good dimension stability, electromagnetism shield and etc. As one of the precipitation strengthening alloys, the Mg-Zn alloy exhibits moderate strength, good plasticity and corrosion resistance. However, its wide crystallization temperature range leads to the poor casting property and the difficulty in grain refinement. Thus, the Mg-Zn alloy can not be used as industrial castings or forgings, and its applications is remarkably limited. Rapidly solidification (RS) is an effective method to prepare high performance materials, characteristics of grain refinement, microstructural homogeneity, solid solubility extension and the formation of non-equilibrium phases. The problems in the Mg-Zn alloy as mentioned above are desirable to be resolved by rapid solidification processing. In the present work, the combination of rapid solidification processing and alloying has been adopted to develop the Mg-Zn based alloys with uniform fine-scale dispersions of thermal stable intermetallic precipitates in order to further improve the high-temperature performance of the alloys.In the present dissertation, the atomization-twin rolls quenching technology has been utilized to successfully prepare the RS Mg-Zn based alloys in the form of flakes, including the RS Mg-Zn, Mg-Zn-Ca, Mg-Zn-Ce and Mg-Zn-Ca-RE (Ce and La) alloys. The effects of the sole Ca and RE additions and the combined additions on the microstructures, phase compositions, thermal stability and isochronal age-hardening behaviors of the RS Mg-Zn alloy have been systematically investigated. On the basis of the study mentioned above, the rapidly solidification/powder metallurgy (RS/PM) Mg-Zn based alloys in the form of rods have been prepared by hot extrusion. These alloys can be divided in two series: (1) RS/PM Mg-Zn-Ca system alloys, mainly including Mg-6Zn-5Ca and Mg-6Zn-5Ca-RE (3Ce and 0.5La) alloys; (2) RS/PM Mg-Zn-Ce system alloys, mainly including Mg-6Zn-5Ce and Mg-6Zn-5Ce-1.5Ca alloys. The microstructures, mechanical properties at room and elevated temperatures and creep resistance have been investigated. The main results are listed as follows: (1) Microstructures and properties of the RS Mg-Zn based alloy flakesThe Ca addition improves the microstructures and properties of the RS Mg-Zn alloy remarkably. With the increase of Ca, the Mg51Zn20 phase with a low melting point in the alloys are gradually replaced by the Ca2Mg6Zn3 and Mg2Ca phases, which have relatively high melting points. Therefore, the thermal stability of the alloys increases gradually with the increment of Ca. With the Ca content higher than 1.5wt.%, the alloys are characteristics of the notably refined microstructures and the remarkably higher volume fractions of dispersions. The minimal grain size of the alloys is about 3～5μm. For the RS Mg-6Zn-5Ca alloy, the isochronal age-hardening behavior of the alloy is distinct and the maximum hardness is 120.8±4.5Hv, mainly due to the precipitation of fine and dispersed Ca2Mg6Zn3 within the grains. In addition, the Mg2Ca and Ca2Mg6Zn3 phases with relatively larger sizes are observed at the grain boundary. All of them are beneficial for the alloy to maintain a relatively high hardness at high temperatures.The Ce addition is beneficial for the refinement of the microstructures and remarkable improvement of the thermal stability of the alloy. With the increase of Ce, the microstructures are gradually refined and the volume fractions of dispersions are increased remarkably. The minimal grain size of the alloys is about 4～7μm. The stable intermetallic compound i.e. the Mg-Zn-Ce ternary phase (T phase) with a high melting point(470～500℃) is formed in the RS Mg-Zn-Ce alloys at the expense of the Mg51Zn20 phase and thus the thermal stability of the alloys is enhanced. However, the isochronal age-hardening behavior of the alloys gradually decreases with the increase of Ce. The hardness of the RS Mg-6Zn-5Ce alloy is much higher than the other Mg-Zn-Ce alloys at the range of RT to 400℃and the highest hardness of the alloy is 91.5±7Hv, mainly caused by fine microstructure and the precipitation of dispersed precipitates. In the RS Mg-6Zn-5Ce alloy, the atomic percentage of the T phase is 80.8at.%Mg,12.4at.%Zn and 6.8at.%Ce,and the (Zn/Ce)at is about 2:1.With the addition of RE (Ce and La) in the RS Mg-6Zn-5Ca alloy, a Mg-Zn-RE phase with a few Ca (about 2～3at.%) is formed in the alloy, which is shortened as the T′phase. The melting point of the T′phase is close to that of the T phase, but much higher than that of the Ca2Mg6Zn3 phase. Moreover, with the increase of RE in the Mg-6Zn-5Ca alloy, a little of RE is detected in the Ca2Mg6Zn3 phase and leads to the improvement of the thermal stability of the Ca2Mg6Zn3 phase. Both the′T phase a nd the Ca2Mg6Zn3 phase containing RE are beneficial to the improvement of the thermal stability of the RS Mg-Zn-Ca-RE alloys. The Mg-6Zn-5Ca-3Ce alloy exhibits an obvious isochronal age-hardening behavior and the highest hardness of the alloy is 162.4±5.5Hv, mainly derived from fine and dispersed Ca2Mg6Zn3 within the grains. Moreover, the T′phase, Mg12Ce and Mg2Ca are detected at the grain boundaries, which contributes to the enhancement of the hardness. (2) Microstructures, strength at room temperature (RT) and high temperatures and creep resistance of the RS/PM Mg-Zn based alloy rodsThe microstructures of the alloys are sharply refined and the volume fractions of dispersions are increased remarkably in the hot-extrusion state, but the kinds of the mostly phases are not changed. For the RS/PM Mg-Zn-Ca system, the mechanical properties of the alloys at RT and elevated temperatures can be improved by the sequential additions of 5wt.%Ca and RE (Ce and La) in the RS/PM Mg-Zn alloy. With the addition of 5wt.%Ca in the Mg-Zn alloy, some relatively stable Ca2Mg6Zn3 and high-melting Mg2Ca phases which are found at the grain boundaries lead to the effective strengthening of the grain boundaries. Thus, the compressive strength of the alloy at 200℃is remarkably enhanced from 40.7MPa (Mg-6Zn) to 202.3MPa (Mg-6Zn-5Ca) with Ca additions. The minimum creep rate of the latter one is about 134 folds lower than that of the former (200℃/50MPa). With the further addition of RE in the Mg-Zn-Ca alloy, there are some′T phases with a higher melting point at the grain boundaries besides Mg2Ca phases. Moreover, the RE addition is beneficial to the improvement of the thermal stability of the Ca2Mg6Zn3 phase. Therefore, the high-temperature strength (200℃) of the alloy is further enhanced up to 234.0MPa (Mg-6Zn-5Ca-3Ce alloy) and 244.6MPa (Mg-6Zn-5Ca-3Ce-0.5La alloy), respectively. The minimum creep rate of the latter two alloys is about 2 and 4 folds lower than that of the former (175℃/50MPa).For the RS/PM Mg-Zn-Ce system alloys, the mechanical properties of the alloys at RT and elevated temperatures can be increased by the sequential addition of Ce and Ca in the RS/PM Mg-Zn alloy. With the addition of 5wt.%Ce in the Mg-Zn alloy, a lot of fine and dispersed T phases are formed within the grains and at the grain boundaries. So the compressive strength of RS/PM Mg-6Zn-5Ce alloy at 200℃is enhanced up to 225.9MPa. The minimum creep rate of the alloy is about 1075 folds lower than that of Mg-6Zn alloy (200℃/50MPa). The creep resistance of the RS/PM Mg-6Zn-5Ce alloy is much higher than that of the RS/PM Mg-6Zn-5Ca alloy due to the higher thermal stability of the T phase than the Ca2Mg6Zn3 phase. With the further addition of Ca in the Mg-6Zn-5Ce alloy, a lot of the Mg-Zn-Ce phases in the different shapes with the (Zn/Ce)at about 1.5:1 and containing a few of Ca (about 1at.%) are observed at the grain boundaries. So it can be inferred that the enhanced high-temperature strength and creep resistance of the alloy may be resulted from the dissovement of the Ca in the Mg-Zn-Ce phase. The high-temperature (200℃) strength of the alloy is up to 258.2MPa and the minimum creep rate of the alloy is about 1/2 of that of the Mg-6Zn-5Ce alloy (200℃/90MPa).更多还原