【作者】 熊姝涛；

【导师】 潘复生； 蒋斌；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2011， 硕士

【摘要】 铸态镁合金的晶粒粗大,影响了其性能的提高和后续的应用。因此,本文通过添加合金元素,原位生成金属间化合物,并利用它们与镁基体间的晶体学关系来异质形核细化镁合金的晶粒,优化铸态镁合金的性能。本文采用金相分析（OM）、扫描电镜（SEM）、能谱分析（EDS）及X射线衍射分析（XRD）等试验手段,研究了Al/Ca比、Al/Ce比对镁合金铸态组织的影响,初步探索了化合物的形成机制,取得以下研究结果:Mg-Al-0.5Ca合金中Al含量在0.5 2 wt%范围内时,随着Al含量的增大,晶粒明显细化,由420μm减小至287μm,Ca含量较高时,Al₂Ca尺寸呈增大的趋势,细化效果减弱;在Mg-Al-1Ce合金中Al含量在0.2 2.5 wt%范围内变化时,随着Al含量的增大,合金的晶粒尺寸先减小再增大最后又减小,Ce含量较高时,Al₄Ce尺寸亦增大,细化效果减弱。热力学计算认为:Al-Ca的生成热小于Mg-Ca和Mg-Al,故向Mg-Ca合金中加入Al元素会优先形成Al₂Ca金属间化合物;Al-Ce的生成热小于Mg-Ce和Mg-Al,在Mg-Al-Ce熔体中,元素Al优先与Ce元素形成金属间化合物Al₄Ce。通过液态淬火试验,Al₂Ca、Al₄Ce化合物存在于液相中,证实了热力学计算结果。在Mg-Al-Ca合金中,随着Al含量的增加,第二相种类由Al₂Ca和Mg2Ca转变为Al₂Ca和Mg₁₇Al₁₂,其形状由颗粒状和杆状变成网状。Mg-Al-Ce合金中,随着Al含量的增加,第二相由Mg12Ce、Mg17Ce2和Al₄Ce转变为Mg₁₇Al₁₂和Al₄Ce,其形貌有颗粒状和杆状变成骨骼形网络状。晶体学计算结果认为:Al₂Ca和α-Mg基体间存在Mg Al₂Ca{1 010}/{220}2[1120]_Mg /[111]Al Ca和Mg Al₂Ca{1011}/{311}2[1120]_Mg /[112]Al Ca两组位向关系,匹配度均小于10%,Al₂Ca颗粒可作为镁合金的有效晶粒细化剂;Al₄Ce和α-Mg基体间两组位向关系4{1010}M g /{112}_Al4Ce 4[1120]_Mg /[111]_Al4Ce和4{1010}M g /{112}_Al4Ce 4[1120]_Mg /[110]_Al4Ce证明了Al₄Ce的异质形核核心的作用。因此,在凝固过程中Al₂Ca和Al₄Ce可作为镁基体的异质形核质点,细化铸态镁合金的晶粒。更多还原

【Abstract】 The coarse grains in as-cast magnesium alloys have great effect on the improvement of their performance and applications. Therefore, this article has been researched on generating the metallic compounds through in situ formation by adding alloy elements to refine grains through heterogeneous nucleation which depends on the crystallographic relationship between the compounds and magnesium matrix, thus optimizing their performance.In this paper, the investigation on the effects of the value of Al/Ca or Al/Ce on the as-cast microstructure of magnesium alloys has been carried out by means of optical microscope, electrical resistivity measurements, X-ray diffraction technique, etc. Thermodynamic and crystallographic calculation are also been used to explore the formation mechanism of the compounds preliminarily. Following research results were obtained.Grains refined obviously when the Al content changed in 0.5 2wt% in Mg-Al-0.5Ca alloys, with the increase of Al content, the grain size decreased from 420μm to 287μm. When Ca content was higher, the Al₂Ca dimension showed an increasing trend and weak refinement. With the adding 0.2 2.5 wt% Al to Mg-Al-1Ce alloy, the grain size decreased at first, but with the Al content increasing the grain size increased in a short range and decreased finally. When Ce content was higher, the Al₄Ce size showed an increasing trend and weak refinement.From thermodynamic calculation it is clear that: the formation enthalpy of Al-Ca is smaller than that of Mg-Ca or Mg-Al, which can promote the formation of Al-Ca metallic compounds; Al-Ce metallic compounds have smaller the formation enthalpy than Mg-Ce and Mg-Al, in Mg-Al-Ce system, and Al and Ce would react firstly and generate metallic compound Al₄Ce. It was confirmed that Al₂Ca and Al₄Ce presented in the liquid by liquid quenching experiments.In Mg-Al-Ca alloys, with the increase of Al content, the type of second phase transfromed from Al₂Ca and Mg2Ca to Al₂Ca and Mg₁₇Al₁₂, and the morphology evolved from granular and nod to net-like. In Mg-Al-Ce alloys, with the Al content increased, the second phases changed from Mg₁₂Ce, Mg₁₇Ce)2 and Al₄Ce to Mg₁₇Al₁₂ and Al₄Ce, and the morphology evolved from granular and nod to net-like bone-shape. Crystallographic calculation results showed that: there are two ORs between Al₂Ca andα-Mg matrix which misfits are less than 10%, Mg Al₂Ca{1 010}/{220}2[1120]_Mg /[111]Al Ca and Mg Al₂Ca{1011}/{311}2[1120]_Mg /[112]Al Ca , i.e. Al₂Ca could play as the effective nucleant for Mg alloys. The two ORs 4{1010}M g /{112}_Al4Ce 4[1120]_Mg /[111]_Al4Ce and 4{1010}M g /{112}_Al4Ce 4[1120]_Mg /[110]_Al4Ce between Al₄Ce andα-Mg matrix which is got by crystallographic calculation also proved the role of heterogeneous nucleant of Al₄Ce. Therefore, Al₂Ca and Al₄Ce could play the role of heterogeneous nucleant for Mg matrix and refine grains of the as-cast Mg alloys.更多还原