【作者】 程仁菊；

【导师】 潘复生；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2010， 博士

【摘要】 镁及镁合金由于其密度低、比强度高、电磁屏蔽性好、弹性模量高及其它一些优异的性能而被认为是一种很有应用前景的材料。但由于镁合金密排六方的晶体结构(HCP),使得目前镁合金的机械性能和塑性加工能力仍难以满足一些汽车零部件及其它方面应用的需求。晶粒细化对提高铸造镁合金和变形镁合金的机械性能都是行之有效的方法,铸件中微观组织的细化能使溶质元素分布更加均匀,第二相更加细小。晶粒细化还可以提高合金的挤压性能、轧制性能、抗热裂性能和表面光洁度等,并能降低变形镁合金加工件的生产成本。锶及含锶合金作为一种有效的晶粒细化剂已被广泛的应用于铝合金,但关于其在镁合金中的作用的研究还相当缺乏和零散。AZ31和ZK60分别是Mg-Al-Zn和Mg-Zn-Zr合金系中最有代表性,也是目前应用最广泛的商用变形镁合金。而铸态组织的好坏对材料的力学和使用性能有极其重要的影响。因此,系统研究含锶中间合金对AZ31和ZK60镁合金铸态组织的影响,有重要的理论研究价值和实际指导意义。本文采用金相显微镜、扫描电镜及能谱分析仪、X射线衍射分析仪、差热分析仪和Thermo-Calco软件计算等手段,系统研究了锶加入量、锶添加形式、锶中间合金状态、熔体保温温度和保温时间等对AZ31和ZK60合金铸态组织的影响,并探讨分析了锶对AZ31和ZK60合金组织细化的机理.研究结果表明:Al-Sr和Mg-Sr中间合金对AZ31合金,Mg-Sr中间合金对ZK60合金晶粒大小及二次枝晶平均臂间距均有很好的细化作用,在本实验条件下细化效率均随锶含量的增加而提高。中间合金原始状态、中间合金的类型、熔体保温温度和时间等对晶粒大小有重要影响,虽然其对二次枝晶臂间距的影响规律与对晶粒大小的影响相似,但以上各因素对二次枝晶臂间距的影响较小。锶中间合金的类型对于锶细化AZ31合金晶粒的效率存在较大影响。总体而言,在相同条件下以Mg-Sr中间合金形式添加锶较以Al-Sr中间合金形式添加锶可使AZ31合金获得更高的晶粒细化效率。同时,Al-Sr和Mg-Sr中间合金细化AZ31合金晶粒的效率还随熔体保温时间的变化而呈不同的变化规律。在熔体保温温度相同的条件下,Al-Sr中间合金的细化效率随熔体保温时间的延长而逐渐增加,而Mg-Sr中间合金虽然可在很短的保温时间内获得较高的细化效率,但此后随着保温时间的延长细化效率提高不明显。此外,Al-Sr和Mg-Sr中间合金细化AZ31合金晶粒的效率受熔体保温温度的影响也较大。在740℃熔体保温温度下,Mg-Sr中间合金的晶粒细化效果较Al-Sr中间合金好。而在780℃熔体保温温度下,虽然Al-Sr和Mg-Sr中间合金的细化效率均随着保温时间的延长先增加后减小,但Al-Sr中间合金在长的熔体保温时间下似乎能获得更高的细化效率。锶中间合金的原始状态不同,对晶粒大小的细化效率不同。对于Al-Sr中间合金,尽管以商用原始态、热处理态、变形态和重熔快速凝固态Al-Sr中间合金形式添加锶均能有效细化AZ31合金的晶粒,但不同状态中间合金的晶粒细化效果存在一定差异。重熔快速凝固态Al-Sr中间合金的晶粒细化效果好于商用原始态、热处理态和变形态,而商用原始态、热处理态和变形态的晶粒细化效果差别不大。对于Mg-Sr中间合金,尽管以常规铸态、热处理态、变形态和快速凝固态Mg-Sr中间合金形式添加锶均能有效细化AZ31和ZK60合金的晶粒,但不同状态中间合金的晶粒细化效果也存在一定差异。变形态Mg-Sr中间合金的晶粒细化效果最好,其次依次是热处理态和常规铸态,而快速凝固态的细化效果则相对较差。熔体保温温度和时间对含锶中间合金处理AZ31和ZK60合金的晶粒细化效果有重要影响。对于AZ31合金,当以Al-Sr中间合金形式添加锶时,在熔体保温温度相同的条件下,随着熔体保温时间的从20min延长到120min,锶细化AZ31合金晶粒的效率逐渐增加。而在熔体保温时间相同的条件下,随着熔体保温温度从700℃升高到780℃,锶细化AZ31合金晶粒的效率逐渐增加;当以Mg-Sr中间合金形式添加锶时,在700℃、740℃和780℃三个熔体保温温度中,以740℃时锶对AZ31合金的晶粒细化效果最好。对于ZK60合金,当熔体保温温度为740℃时,随着熔体保温时间从20min延长到120min,锶细化ZK60合金晶粒的效率先增加后减小,其中在80min熔体保温时间下可获得最高的晶粒细化效率。AZ31合金中的主要第二相有片状和棒状离异共晶β-Mg17Al12、少量片层状共晶α-Mg+β-Mg17Al12相,花瓣状Al8Mn5相。当锶含量超过0.07%后,还生成了片状Al4Sr相,Al4Sr相分布在晶界。ZK60合金中的主要第二相有片层状或骨骼状MgZn、少量片状离异共晶MgZn2相、少量ZnxZry相。添加锶后合金中没有含锶相形成,锶分布在晶界与MgZn和MgZn2相共存。锶细化AZ31和ZK60合金晶粒的机制在于增加凝固时的过冷度从而增大形核几率和降低结晶再辉温差从而限制α-Mg晶粒的长大,以及提高了合金的生长抑制因子而起到晶粒细化的效果。而Al-Sr和Mg-Sr中间合金细化AZ31合金晶粒的差异则主要与中间合金中的Al4Sr和Mg17Sr2相分解和/或熔解为游离态锶的速率不同有关。类似地,不同状态Al-Sr和Mg-Sr中间合金细化AZ31和/或ZK60合金晶粒的差异也主要与不同状态中间合金中含锶相的尺寸不同从而导致其分解和/或熔解为游离态锶的速率不同有关。此外,熔体保温温度和时间对锶细化AZ31合金晶粒的影响与锶中间合金中的含锶相分解和/或熔解为游离态锶的速率密切相关。更多还原

【Abstract】 Magnesium and magnesium alloys are attractive materials for practical applications due to their low density, high strength to weight ratio, good electromagnetic shielding characteristics and high elastic modulus, as well as other desirable properties. But due to their HCP crystal structure the mechanical properties and processing performances of magnesium alloys are still can not meet the needs of some important parts in vehicles and other application fields. Grain refinement is an effective way to improve the mechanical properties of magnesium alloys. The refined microstructure in as-cast components leads to an uniform distribution of solute elements and secondary phases on a fine scale. It also provides such advantages as superior extrudability and rollability, excellent resistance to hot tearing, good surface finish and considerable cost reduction in the production of wrought magnesium parts. As an effective refining element, strontium and its master alloys have been widely used in the industrial application of aluminum alloys while the investigation of their effects on magnesium alloys is very lack and scattered. As the most representative Mg-Al-Zn and Mg-Zn-Zr system alloys respectively, the AZ31 and ZK60 magnesium alloys are the most widely commercial applicated wrought magnesium alloys at present. Consequently, it is theoretically important and practically valuable to systematically investigate the influence of Sr-containing master alloys on the as-cast microstructures of AZ31 and ZK60 alloys.In the present work the effect of Sr amount, Sr addition type, Sr-containing master alloy state, melt holding temperature and holding time on the as-cast microstructures of AZ31 and ZK60 alloys and the refining mechanisms have been systematically investigated by means of Optical Microscopy(OM), Scanning Electron Microscopy(SEM), Energy Dispersive Spectrometry(EDS), X-ray Diffraction(XRD), Differential Scanning Calorimetry (DSC) and Thermo-Calco software etc.The research results show that the grain sizes and secondary dendrite arm spacings (SDAS) of the AZ31 alloys treated by Al-Sr and Mg-Sr master alloys and the ZK60 alloys treated by Mg-Sr master alloys are been greatly refined and the refinement efficiency increased with the increasing of the Sr content. The master alloy states and types, the melt holding temperatures and times are having great influence on the grain sizes, although the influence rules of those factors on the grain sizes and SDAS are similar but the influences on the SDAS are not great. The types of the Sr-containing master alloys have obvious influences on the grain refinement efficiency of Sr to the AZ31 alloys. In general, under the same condition the Mg-Sr master alloy has relatively higher grain refinement efficiency in the AZ31 alloys than the Al-Sr master alloy. Meanwhile, along with the change of the melt holding time the grain refinement efficiency of Al-Sr and Mg-Sr master alloys to AZ31 alloys have different change rules. For a given melt holding temperature, with the melt holding time increased the grain refinement efficiency of Al-Sr master alloys increases. As for the Mg-Sr master alloy, although it can obtain high refinement efficiency in a very short melt holding time, after that time the increase of the grain refinement efficiency is not very obvious. In addition, the melt holding temperature also has an obvious effect on the the grain refinement efficiency of the Al-Sr and Mg-Sr master alloys to the AZ31 alloy. Under the melt holding temperature of 740℃, the Mg-Sr master alloy has relatively higher grain refinement efficiency than the Al-Sr master alloy. However, under the melt holding temperature of 780℃, although with the melt holding time increased both the grain refinement efficiency of the Al-Sr and Mg-Sr master alloys first increases and then decreases, for a long melt holding time the Al-Sr master alloy seems to can obtain higher grain refinement efficiency than the Mg-Sr master alloy.The different original states of the master alloys have different efficiency on the grain refinement to the AZ31 and ZK60 alloys. For Al-Sr master alloys, although the commercial original, the heat-treated, the rolled and the remelting rapid cooled master alloys can effectively refine the grains of the AZ31 alloys, the master alloys with different states exhibit different grain refinement efficiency. The remelting rapid cooled Al-Sr master alloy has higher refinement efficiency than the commercial original, the heat-treated and the rolled, whose difference in the grain refinement efficiency is not obvious. For Mg-Sr master alloys, although the as-cast, the heat-treated, the rolled and the rapid cooled master alloys can effectively refine the grains of the AZ31 and ZK60 alloys, the master alloys with different states exhibit different grain refinement efficiency. The rolled Mg-Sr master alloy has the best grain refinement efficiency and followed respectively by the heat-treated and the as-cast. Oppositely, the grain refinement efficiency of the rapid cooled Mg-Sr master alloys are relatively poor.The melt holding temperature and time have great influence on the refinement efficiency. For the AZ31 alloy, under the conditions of adding Sr in the form of Al-Sr master alloy and the same melt holding temperature, an increase in the melt holding time from 20min to 120min causes the grain refinement efficiency of Sr in the AZ31 alloy to gradually increase. However, for a given melt holding time, an increase in the melt holding temperature from 700℃to 780℃causes the grain refinement efficiency of Sr in the AZ31 alloy to gradually increase. In addition, under the conditions of adding Sr in the form of Mg-Sr master alloy, the grain refinement efficiency of Sr in the AZ31 alloy treated at 740℃is relatively higher than that at 700℃and 780℃. For the ZK60 alloy, under the melt holding temperature of 740℃, an increase in the melt holding time from 20min to 120min causes the grain refinement efficiency of Sr in the ZK60 alloy to first increase and then decrease, and the melt holding time of 80min can obtain the best grain refinement efficiency.The secondary phases in the AZ31 alloy are mainly composed of sheet- or stick- like divorced eutecticβ-Mg17Al12, few lamellar eutecticα-Mg+β-Mg17Al12 phases and flower-like Al8Mn5 phases. While the Sr content is higher than 0.07(wt.)%, lamellar Al4Sr phase formed and Al4Sr phases were at the grain boundary. The secondary phases in the ZK60 alloy are mainly composed of lamellar- or bone- like MgZn phases, divorced eutectic MgZn2 phases and few ZnxMny phases. Being different from the AZ31 alloy, adding a small amount of Sr to the ZK60 alloy dose not cause the formation of any Sr-containing phases in the alloy.The mechanisms about the grain refinement of the AZ31 and ZK60 alloys with the addition of Sr are mainly related to the nucleation increase resulted from the supper-cooling temperature increase during solidification and that the grain growth of theα-Mg grains are restrained due to the decrease of the recalescence temperature variation. As for the difference in the grain refinement efficiency for the Al-Sr and Mg-Sr master alloys in the AZ31 alloy, the reason is possibly related to the different efficiency which the Al4Sr and Mg17Sr2 phases dissolution and/or melt in order to obtain the free Sr. Similarly, the above mentioned-mechanism is also suitable for the difference in the grain refinement efficiency for the Al-Sr and Mg-Sr master alloys with different states in the AZ31 and ZK60 alloys. In addition, the effects of the melt holding temperature and time on the grain refinement efficiency of Sr in the AZ31 alloy is possibly related to the efficiency which the Al4Sr and Mg17Sr2 phases dissolution and/or melt in order to obtain the free Sr.更多还原