【作者】 周振平；

【导师】 李荣德；

【作者基本信息】 沈阳工业大学 ， 材料加工工程， 2008， 博士

【摘要】 Al-Fe合金是一种新型的轻质耐热合金。普通熔铸Al-Fe合金中存在粗大针状和针片状Al₃Fe化合物,割裂基体,极大地降低了合金强度,限制了合金应用。细化组织是提高合金性能最直接最有效的手段,但到目前为止,Al-Fe合金的许多凝固学问题还不甚了解,组织细化有很大困难。为了搞清Al-Fe合金的某些凝固学问题,本文对液态结构转变、组织遗传性、熔体过热处理及合金元素对Al₃Fe化合物的作用机理作了研究。Al-5Fe合金加热到1200℃以上,DTA曲线上出现一个显著的吸热峰,这是Al₃Fe化合物熔解所导致;从1300℃冷却到1200℃时,DTA曲线上出现一个显著的吸热峰,这是Al₃Fe化合物析出所导致。研究认为,高熔点Al₃Fe化合物是Al-Fe合金的遗传因子。将合金液过热到1300℃进行激冷可以很大程度上抑制Al₃Fe化合物在液相线温度以上的析出,显著细化最终凝固组织,其原因是Al₃Fe化合物充分熔解,消除了合金的遗传性。经1300℃的熔体过热处理,Al-5Fe合金的抗拉强度有了较大幅度的提高,从未处理时的107MPa提高到145MPa,增长幅度达35.5%。Al-5Fe合金的断裂机制也由熔体处理前的解理断裂变为熔体处理后的部分韧窝断裂加部分解理断裂。研究发现,在950℃,Fe在Al液中不是直接溶解,而是先与Al化合形成Al₃Fe化合物,而后Al₃Fe化合物再向Al液中溶解。通过这种方式Fe得以不断地溶解到Al液中。熔炼完毕后,Al-5Fe合金液中含有Al₃Fe化合物。1200℃以下的熔体过热处理对Al-5Fe合金中的初生Al₃Fe化合物有较好的细化作用,但细化效果随激冷工艺而有所不同。高低温熔体混合处理效果最佳;加铝铁混合粉末激冷,Fe粉溶解完毕后产生细小的Al₃Fe化合物,可以作为结晶核心,因而效果较好;加铝锭激冷可以保留高温熔体的优良结构,而且不会带入粗大的Al₃Fe化合物,因此效果也较好;加Al-5Fe合金中间合金激冷会带入粗大的Al₃Fe化合物,对合金的结晶起到负面作用,因而细化效果最差。1200℃以下的熔体过热处理不能使初生Al₃Fe化合物完全转变为针点状和颗粒状,是因为高熔点的Al₃Fe化合物未能充分熔解。Mg、Cr、Co合金化元素对初生Al₃Fe化合物的生长有重要影响。Mg主要分布在基体和初生Al₃Fe化合物边缘。Mg在初生Al₃Fe化合物边缘的富集抑制了Al₃Fe化合物的择优生长,使其长成块状。Mg富集还能导致成分过冷,如果成分过冷足够大,初生Al₃Fe化合物在成分过冷区内会出现内生形核和内生生长。加入Mg后,部分块状初生Al₃Fe化合物类似沉积岩,表面有波纹,其原因是初生Al₃Fe化合物以层状堆垛方式生长。Cr和Co主要固溶在初生Al₃Fe化合物内,通过原子占位抑制初生Al₃Fe化合物的择优生长,使其长成块状。Mg和Mn的复合使初生Al₃Fe化合物长成梅花状、块状或穗状。Mn主要分布在初生Al₃Fe化合物内,有利于初生Al₃Fe化合物长成梅花状和块状;Mg主要分布在基体中,在初生Al₃Fe化合物边缘有轻微富集,有利于初生Al₃Fe化合物长成块状和穗状。由于Mg的抑制作用,穗状初生Al₃Fe化合物中存在缩颈和微裂纹。Cr和Mg的复合有利于初生Al₃Fe化合物长成块状。Cr主要分布在初生Al₃Fe化合物内。当Cr含量较低时,由于Mg在初生Al₃Fe化合物边缘富集,阻碍了Cr固溶于初生Al₃Fe化合物内部,削弱了Cr的细化作用:当Cr含量较高时,Mg的阻碍作用不太明显。在某些特定条件下,针状或针片状初生Al₃Fe化合物可以长成花朵状或星状。花朵状或星状初生Al₃Fe化合物实际上是侧向具有十个分枝的棒柱状Al₃Fe相的横截面。形成花朵状或星状Al₃Fe相的外因是过冷度。只有当冷却速度或过冷度足够大时,才能产生花朵状或星状Al₃Fe相。形成花朵状或星状Al₃Fe相的内因是（100）和（201）晶面的复合孪生生长。更多还原

【Abstract】 Al-Fe alloy is a new type of lightweight heat resistant alloy. Thick needle-like and needle plate-like Al₃Fe compound in cast Al-Fe alloy are found in the matrix and greatly reduce the strength of the alloy, wich restricts the applications of the alloy. The refinement of the microstructure is the most effective means of improving the alloy properties. However, many solidification problems of Al-Fe alloy are not well understood. Therefore, the refinement of the microstructure is difficult. In order to understand certain solidification problems, the liquid transformation, the heredity of microstructure, melt overheating treatment and the effect of alloying elements on the growth of Al₃Fe compound have been investigated in this thesis.A remarkable endothermic peak occurs on DTA curve when Al-5Fe alloy is heated above 1200℃. The reason is because of the fusion of Al₃Fe compound. A remarkable exothermicl peak occurs on DTA curve when Al-5Fe alloy is cooled from 1300℃to 1200℃, which is due to the precipitation of Al₃Fe compound. The result indicates that the dystectic Al₃Fe compound are the genetic genes. Overheating the liquid Al-5Fe alloy to 1300℃and chilling subsequently can greatly retard the precipitation of Al₃Fe compound above the liquidus temperature and markedly refine Al₃Fe compound, which is because the Al₃Fe compound is melted thoroughly and their heredity is eliminated. The melt overheating treatment at 1300℃can enhance the tensile strength of Al-5Fe alloy from 107MPa before treatment to 145MPa after treatment, increased by 35.5%. the fracture mechanism of Al-5Fe alloy changes from cleavage fracture before treatment to part dimple fracture and part cleavage fracture.The experimental results show that at 950℃pure Fe element does not directly dissolve in liquid Al, but combines with Al and forms Al₃Fe compound, then Al₃Fe compound dissolve in liquid Al. In this way, Fe continuously dissolves in liquid Al. Al₃Fe compound are present in liquid Al-5Fe alloy at normal smelting temperature. Overheating treatment below 1200℃has a good effect on the morphology of primary Al₃Fe compound. However, the refinement effects vary from chilling processes. The refinement effect of high temperature and low temperature melt treatment is the best; the dissolution of Fe in liquid Al produces fine Al₃Fe cpmpounds which serve as crystalline nuclei, so the refinement of chilling in Al&Fe powders mixture is better; chilling in Al ingots fixes the superior structure of high temperature melt and does not introduce the thick Al₃Fe cpmpounds, hence, the refinemet is also better; chilling in Al-5Fe master alloy introduces the thick Al₃Fe cpmpounds and aggravates the crystalline, therefore, the refinement is worst. The morphology of primary Al₃Fe compound can not change into needle spot-like and grain-like after melt overheating treatment below 1200℃because dystectic Al₃Fe compound are not entirely melted.The alloying elements of Mg, Cr and Co have an important effect on the growth of primary Al₃Fe compound. The majority of Mg distributes in the matrix and around primary Al₃Fe compound. The enrichment of Mg around primary Al₃Fe compound restrains the preferential growth of Al₃Fe and causes the formation of grain-like primary Al₃Fe compound. The enrichment of Mg also gives rise to the constitutional undercooling. If the undercooling is enough, the endogeny nucleation and endogeny growth of primary Al₃Fe compound occur in the constituent undercooling zone. After addition of Mg, a part of grain-like Al₃Fe compound with ripples on their surfaces look like stratified rocks. The reason may be that the Al₃Fe compound grows in the mode of lamellar stacking. Cr and Co distribute mainly inside Al₃Fe compound. They suppress the preferential growth of Al₃Fe compound by means of atom occupation space. Therefore, the block-like Al₃Fe compound forms.The combination of Mg and Mn produces the blossom-like, block-like or fringe-like Al₃Fe compound. Mn is advantageous to the formation of blossom-like and block-like Al₃Fe compound, but Mg to the formation of fringe-like Al₃Fe compound. Necking and tiny cracks are present in fringe-like Al₃Fe compound due to the suppression of Mg.The combination of Cr and Mg is advantageous to the formation of block-like Al₃Fe compound. Cr mainly exists inside Al₃Fe compound. When the content of Cr is low, the dissolution of Cr in Al₃Fe compound is hindered by the affluence of Mg around Al₃Fe compound, which impairs the refinement of Cr. However, the hindrance of Mg is not evident when the amount of Cr is high. Needle-like or needle plate-like Al₃Fe compound may be the flower-like or star-like at some specific conditions. The nature of flower-like or star-like Al₃Fe compound is the transverse profiles of rod Al₃Fe compound whose sides have ten branches. The external reason of the formation of flower-like or star-like Al₃Fe compound is the undercooling. Only when the cooling rate or undercooling is enough high does flower-like or star-like Al₃Fe compound occur. The another reason, i.e., the intrinsic reason, is the multiple twinning of （100） and （201） of Al₃Fe compound.更多还原