【作者】 黄小勇；

【导师】 蒋斌；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2013， 硕士

【摘要】 稀土元素的添加可以有效地细化铸态Mg-Al的晶粒组织，但以往的稀土元素多以混合稀土的形式添加，单一的稀土元素在镁合金当中所起的作用没有明确定论。因此，本文选取稀土元素La，添加到Mg-Al合金当中原位生成金属间化合物，研究了Al-La化合物对Mg-Al合金晶粒细化的影响，并对合金化合物的热力学形成机制进行了初步探索。制备了挤压态Mg-Al-La中间合金，将其添加进AZ31合金中，研究其对AZ31的显微组织的影响。主要研究结果如下：单一La元素的添加可以细化纯镁的铸态晶粒，这主要是由于在凝固过程中La成分过冷所起的作用。复合添加Al、La两种元素到Mg-Al合金当中的细化效果与元素含量的多少有很大关系。当Al、La元素含量相对较低时，如在Mg-xAl-0.8La合金中，随着合金元素含量的增多，晶粒细化效果显著；Mg-xAl-1.5La合金中，最终细化效果不明显；在Mg-xAl-3.5La合金中，随着合金元素含量的增多，反而出现晶粒的粗化。晶粒的大小是由合金元素的成分过冷与合金化合物的含量和尺寸影响共同决定的。在本文的所有Mg-Al-La合金系列当中，随着Al含量的增加，合金中的第二相由Mg17La2，转变为Al2La、Al2.12La0.88、Al11La3、Mg17Al12等，Al-La化合物形态从细小的颗粒状，小棒状，小针状变为骨骼状，网络状以及粗大针状。热力学计算认为，Al-La化合物的生成热小于Mg-La和Mg-Al，故Mg-Al-La合金中通常会优先生成Al-La化合物，通过铜模淬火试验，证实了Al-La化合物确实存在于合金的凝固初期中，证实了热力学计算结果。通过热挤压变形，Mg-Al-La中间合金中的第二相种类没有改变，而形貌由铸态时的粗大且呈网状分布转变为细小且弥散地分布。适量的挤压态Mg-Al-La中间合金可以较好地细化铸态AZ31的晶粒，随着中间合金的添加量的逐渐增多，AZ31的平均晶粒尺寸呈现出先增大后减小最后又增大的趋势，当中间合金的添加量为20%，即(Al+La)元素的绝对量为1.10%时，晶粒细化效果最显著。更多还原

【Abstract】 The microstructure of Mg-Al alloys can be refined effectively by the addition ofRare Earth elements. However, the effect of every single Rare Earth element on theMg-Al alloys has not been revealed as they are always introduced as Misch-metal.Therefore, this article has been researched on generating the metallic compoundsthrough in situ formation by adding single La element into Mg-Al alloys, and the effectof Al-La compounds on the grain refinement was inverstigated, as well as thethermodynamic formation mechanism.Mg-Al-La master was prepared by melting andhot extrusion, then the extruded master alloys wasadded into the AZ31alloy toinvestigate the grain refinement effect.The study concluded as follows:As-cast microstructure of pure magnesium can be refined by single La addition, therefinement mechanism is related to constituent supercooling of La element during thesolidification process.The effect of grain refinement on the Mg-Al alloy is deeplyinfluenced by the contents of Al and La.When Al and La content is relatively low, suchas in Mg-xAl-0.8La alloys, grain refinement effect is obvious; while in Mg-xAl-1.5Laalloys, grain refinement effect is little weakened;when come to Mg-xAl-3.5La alloys,onthe contrary,the grain coarsening was very obvious.In Mg-Al-La alloys of this study, with the increase of Al content, the type ofsecond phase transformed from Mg17La2to Al2La, Al2.12La0.88, Al11La3and Mg17Al12,and their morphology evolved from small granular, small bar, small needle into bones,reticular and thick needle shape.Through thermodynamic calculation it can be conjectured that the formationenthalpy of Al-La compounds is much smaller than that of Mg-La and Mg-Al, whichsuggestedthat the formation of Al-La compounds is preferred in the Mg-Al-Laalloys.With copper quenching experiment, it was confirmed that the Al-La compoundsindeed generated in initial solidification of Mg-Al-La alloys.Through hot extrusion, the second phase of Mg-Al-La master alloy was obviouslyrefined, and the type of second phase was not changed. Moderate amount addition ofextruded master alloy can refine AZ31alloy obviously.The initial small mount ofalloying element leaded to slightly grain coarsening, and then the grain size decreased with further increase of alloying elements, the most significant refining effect wasobtained with the1.10%(Al+La) addition.更多还原