氟化钪熔盐-铝镁热还原反应机理及铝镁钪中间合金制备新工艺研究
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摘要
本文在对氟化钪、氧化钪金属热还原的还原反应进行了热力学计算和分析的基础上,提出了采用ScF_3-MF-Mg(Al)热还原法制备Al-Mg-Sc中间合金的新思路;利用热分析技术结合相图资料方法研究了ScF_3-MF-Mg热还原反应机理,采用Freeman-Carrol法和Kissinger法计算了反应级数与反应活化能,并在此基础上进行了还原反应熔盐体系优选;对ScF_3-MF-Mg(Al)热还原法制备Al-Mg-Sc中间合金的新工艺实验条件进行了优化,制备出了含钪量为2%的Al-Mg-Sc中间合金,制备新工艺中钪回收率达80%;利用研制的中间合金制备了Al-5Mg-Sc系列合金,通过对加钪和不加钪的合金组织与性能的对比研究,对Al-5Mg-Sc合金的应用进行了评价。
     其热力学计算结果表明,ScF_3与Mg之间的还原反应在热力学上是可行的,而Sc_2O_3与Mg及ScF_3与Al之间的反应是不可行的。制备Al-Mg-Sc中间合金时应采用ScF_3为原料,Mg为还原剂,在过量Al熔体中进行。还原产生的初生态Sc与Al形成稳定的Al_3Sc金属间化合物,从而使金属热还原的Δ_rG_m~0(T)减少了138KJ/mol,由此增加了反应体系的熵变值,大大提高了金属热还原进行的趋势。在温度为1100K的Al熔体中Mg热还原制备Al-Mg-Sc中间合金,当使用ScF_3为原料且在氟化物熔盐体系中进行时,还原反平衡常数计算值可达6.6×10~9。
     研究了Sc_2O_3-NH_4HF_2固相氟化及ScF_3-Mg热还原机理,利用DTA分析方法研究了ScF_3-Mg热还原反应的热力学及动力学机理。研究结果表明Sc_2O_3与NH_4HF_2分解产生的HF反应生成ScF_3。ScF_3-Mg体系590℃发生ScF_3-Mg(s)固-固还原反应;740℃发生ScF_3-Mg(1)固-液还原反应;ScF_3-LiF-Mg体系在496℃发生ScLiF_4(s)-Mg(s)固-固还原反应;650℃发生(ScF_3-LiF)-Mg(1)液-液还原反应。计算了Mg热还原的动力学参数,其反应活化能E=48.62KJ/mol,反应级数n=1.03。
     研究制定了ScF_3-MX熔盐在Al熔体中Mg热还原制备Al-Mg-Sc中间合金的新工艺,研究结果表明新工艺能制备Sc含量大于1.5wt%的Al-Mg-Sc中间合金,最佳的还原反应温度为1100℃,还原时间40min;二次还原后,Sc收率大于80%。
     微量Sc在Al和Al-Mg合金中,除少量固溶在基体中。主要以两种析出的Al_3Sc金属间化合物的形式存在。其中一种为合金凝固过程中析出的一次Al_3Sc粒子,起非均质晶核的作用,可强烈细化合金的铸态晶粒组织,并由于细晶强化作用可使Al-Mg合金的强度大大提高。由于在Sc含量较低时,Sc与Mg并不形成金属间化合物,在研究的成分范围内Sc在Mg合金中多以固溶体形式存在。
     利用研制的Al-Mg-Sc中间合金制成Al-5Mg-Sc合金,当Sc含量为0.2%时,合金晶粒细化效果不明显;当Sc含量大于0.4%时,铸态合金晶粒得到明显细化,当Sc含量为1.45%时,铸态合金晶粒最细小。表明研制的中间合金是Al-Mg合金的变质剂。
Based on thermodynamics calculation and analysis about the reduction reaction of ScF3, 80203 metallic thermoreduction, the paper put forward to a new preparation method of Al-Mg-Sc master alloys by means of ScF3-Mx-Mg (Al) metallic thermoreduction; It studied the mechanism of ScF3-MF-Mg thermoreduction reaction using DTA and phase diagram analysis and computed reaction progression and reaction activation energy using the Freeman-Carrol method and the Kissinger method, and then on the basis of these results chose the most suitable molten salt system of the thermoreduction reaction. It also optimized the experimental conditions of the new technics preparing Al-Mg-Sc master alloys in which the recovery of Sc is 80% and got the Al-Mg-Sc master alloys with 2% Sc content; And then a sheet of Al-Mg alloys with and without Sc was manufactured by using these Al-Mg-Sc master alloys, whose microstructure and properties were studied contrastively and whose application was valued.
    The results of thermodynamics calculation indicated that ScF3 can be reduced to scandium by magnesium, but Sc2O3 can not be reduced to scandium by magnesium and aluminum .If reduction reaction are proceeded in aluminum bath, the entropy of system will be increased because of the formation of the stable intermetallic compound Al3Sc whose standard Gibbs formation free energy is estimated to be about -138KJ/mol. Owing to the sequent formation of Al3Sc, thermodynamics tendency of reduction will be greatly enhanced and equilibrium constants and conversion ration of reduction will be greatly enlarged. Therefore Al-Mg-Sc master alloy can be gained well by reducing ScF3 using magnesium in aluminum bath. At 1100K ,the empirical equilibrium constants of ScF3-MX-Mg(Al) reduction system is 6.6 109 .
    Principles of scandium oxide solid fluorinating and ScF3-Mg thermoreduction reaction were researched by thermal analysis (DTA). The results indicated that scandium oxide was fluorinated by the reaction with hydrogen fluoride gas coming from NH4HF2 decomposing and ScF3-Mg system starts ScF3(s)-Mg(s) solid-solid phase reduction at 590 C, ScF3(s)-Mg(l) solid-liquid phase reduction at 740 C, ScF3-LiF-Mg system starts ScLiF4(s)-Mg(s) solid-solid phase reduction at 496 C ,(ScF3-LiF)(l)-Mg(l) liquid-liquid phase reduction at 650 C .The kinetics parameters of ScF3-Mg reaction were calculated. The activation energy E=48.62KJ/mol, reaction progression n=1.03.
    The optimum reduction temperature of ScF3-MX-Mg(Al) to manufacture Al-Mg-Sc master alloy is 1100 C,time is 40 min, the concentration of scandium in
    Sc-bearing master is more than 1.99%. All recovery can be up to 91.9% after
    
    
    
    
    fluoridating and reduction secondly.
    Most part of minor scandium in pure Al and Al-Mg alloys exists in form of primary of secondly A13Sc,only a few of then stayed in a (Al) based solid solution .The primary AlsSc particles precipitating from melt of alloy during cooling act as inhomogeneous nuclei and fining grain size effectively. Most part of scandium stayed in magnesium based solid solution in our composition because magnesium and scandium don't act each other to form intermetallic compound when scandium concentration is low.
    Al-5Mg-Sc ingots were manufactured by mixing Al-Mg-Sc master alloy with metals needed. When scandium concentration is about 0.2%, the effect of grain fining of master alloy prepared was not remarkably. When scandium concentration is about 0.4%, the effect of grain fining of master alloy prepared was remarkably. The effect of grain fining of master alloy prepared was most remarkably, when scandium concentration is about 1.45%.
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