电子束冷床熔炼TC4合金元素挥发机制研究

建立了TC4合金电子束冷床熔炼过程熔体中各元素饱和蒸气压的数学计算模型,利用该模型计算了TC4合金熔体中各元素的饱和蒸气压及其挥发速率,与实验结果基本一致。计算结果表明,在相同温度下,Al元素的饱和蒸气压最大,是其它两种元素的几百倍,挥发损失也最为严重。随着熔体温度的增加,挥发趋势也增大。V元素的挥发很少,可以忽略。此外,电子束冷床熔炼去除杂质效果显著,熔炼出的铸锭成分符合国家标准。     

Ti-15-3合金真空感应凝壳熔炼(ISM)过程研究

利用三元系活度系数计算公式,计算了TiV15Al3Cr3Sn3(Ti-15-3)合金熔体中各组元的活度系数,计算结果表明,各组元的活度系数都小于1。利用活度的计算结果进一步研究了Ti-15-3合金真空熔炼时合金元素的挥发行为,结果表明,合金元素的挥发存在临界真空度,在超过这一真空度后合金元素的挥发损失加剧。建立了间隙元素在Ti-15-3合金中溶解的热力学模型,计算了氧在合金中的溶解度,并分析了影响氧元素在Ti-15-3合金中溶解度的因素。计算结果表明,熔炼室间隙元素分压是决定间隙元素含量的主要因素,并开发了降低间隙元素含量的工艺方法,试验结果与计算结果较符合。为了控制合金熔体温度,利用所开发的温度场计算程序,建立了ISM过程熔体温度与熔炼功率、炉料质量间的关系。利用该程序预测了ISM熔炼过程中凝壳形状及尺寸的变化过程,并对该过程进行了试验研究,计算结果与试验结果一致。     

Ti—6Al—4V合金ISM熔炼过程中Al元素的挥发损失行为

以二元合金的Ｍｉｅｄｅｍａ生成热模型和以Ｋｏｈｌｅｒ建立的三元溶液混合焓模型为基础,计算了Ｔｉ６Ａｌ４Ｖ合金中各组元的活度系数,在此基础上系统研究了该合金的ＩＳＭ熔炼过程中Ａｌ元素的挥发损失行为。研究结果表明：在真空室压力为０１３３Ｐａ（１０－３ｍｍＨｇ）的条件下,当熔体温度低于２１００Ｋ时,Ａｌ元素的挥发受界面挥发反应控制;而当熔体温度高于２１００Ｋ时,则受界面挥发反应和液相边界层中的扩散双重控制。最后计算了不同外压和熔体温度条件下Ａｌ元素的挥发损失量,为实际熔炼过程提供了理论指导。     

含Nb的TiAl合金PAM过程质量变化

对比研究了在相同的真空度条件下纯Ti、纯Al、TiAl二元合金和TiAl-Nb三元合金PAM过程纽扣锭质量变化。结果表明，纯Al在PAM过程中质量随重熔次数的增加而递减。纯Ti在第一次熔炼时质量损失较多，但重熔过程中质量变化趋于稳定。这说明PAM过程中Al的挥发是造成合金质量损失的主要因素。TiAl二元合金和TiAl-Nb系合金在基本相同的熔炼条件下熔炼时，均由于Al过热度的增加而使质量损失增加。Al的质量损失随着熔炼次数的增加而增大，并且随Al含量的增加而增大。Nb的加入方式对纽扣锭的质量变化有影响。以纯Nb方式加入时的损失量高于以中间合金方式加入的损失量。对于TiAl-Nb合金，重熔6次后，其质量变化趋于稳定，也就是合金成分趋于均匀。TiAl-Nb合金重熔6次后质量相对损失只有0．33％，低于其他方法熔炼所引起的Al的损失。     

12t级Ti80合金铸锭的成分均匀性及其偏析规律

利用Melt Flow模拟获得优化的熔炼工艺,制备出国内第1个12t级Ti80合金铸锭,并完成2炉稳定性验证。成分检测结果表明,铸锭头、中、尾3点成分极差小于1000μg/g,冒口17点成分极差小于3000μg/g,成分均匀性与现行5t级铸锭基本相当。分析冒口径向成分分布规律发现,Al、Nb和Mo元素呈现边部高、心部低,从边部到心部逐渐降低的趋势,而Zr元素的分布规律则与之相反。利用EDS对不同部位晶内和晶界的微区成分分析发现,Nb、Mo元素呈现晶内高、晶界低,而Al、Zr元素呈现晶内低、晶界高的分布规律。其中,Nb、Mo、Zr元素的宏、微观偏析规律基本一致,即随着凝固的进行,固相中Mo、Nb含量逐渐降低,Zr含量逐渐升高。然而,Al元素的宏、微观偏析规律正好相反,分析认为主要是由于Al元素的饱和蒸气压较高,与其他合金元素相比更容易挥发,在长时间、高真空熔炼和溶质再分配的共同作用下,铸锭中的宏、微观偏析规律出现不一致的现象。     

12吨级Ti80合金铸锭的成分均匀性及其偏析规律研究

随着海洋装备的大型化发展及其各项性能的提升,对高均匀超大规格钛合金铸锭的研发有了更加迫切的需求。本文利用MeltFlow模拟获得优化的熔炼工艺,制备出国内第一个12吨级Ti80合金铸锭,并完成2炉稳定性验证。成分检测结果表明,铸锭头、中、尾3点成分极差小于1000ppm,冒口17点成分极差小于3000ppm,成分均匀性与现行5吨级铸锭基本相当。分析冒口径向成分分布规律发现,Al、Nb和Mo元素呈现边部高、心部低,从边部到心部逐渐降低的趋势,而Zr元素的分布规律则与之相反。利用EDS对不同部位晶内和晶界的微区成分分析发现,Nb、Mo元素呈现晶内高、晶界低,而Al、Zr元素呈现晶内低、晶界高的分布规律。其中,Nb、Mo、Zr元素的宏、微观偏析规律基本一致,即随着凝固的进行,固相中Mo、Nb含量逐渐降低,Zr含量逐渐升高。然而,Al元素的宏、微观偏析规律正好相反,分析认为主要是由于Al元素的饱和蒸气压较高,与其他合金元素相比更容易挥发,在长时间、高真空熔炼和溶质再分配的共同作用下,铸锭中的宏、微观偏析规律出现不一致的现象。     

耐热铝合金Al-Fe-V-Si高温熔炼过程物理-化学反应

以Al-8．5％Fe-1．3％V-1．7％Si合金为例，利用热力学数据从理论上研究了合金在高温熔炼时挥发、氧化和吸气等现象。结果表明，在大气中以1473K熔炼时，挥发损失以铝的挥发损失为主，Si、Fe次之，V最小，铝的蒸汽分压是普通熔炼温度(1073K)时铝的10^4倍；铝熔体的氧化膜不再起保护作用，氧化继续进行。温度对合金熔体的氢含量影响最大，合金元素Fe、Si对氢含量的影响不大。     

Ti6Al7Nb和Ti6Al4V合金的热氧化行为

在600～900℃温度下,0.5～72 h时间范围内空气气氛下对Ti6Al7Nb进行热氧化,根据增重曲线计算其氧化动力学规律,利用XRD、XPS分析表面氧化层的相组成、成分和价态,并以Ti6Al4V合金做为比照.结果表明,Ti6Al7Nb合金较Ti6Al4V合金抗氧化能力更强.同等氧化条件下,Ti6Al7Nb合金的氧化速率常数(k)更小.对短时间(1 h)氧化的样品的表面分析显示:各合金元素均以最高价态或稳定价态存在,其中Al和V被富集,而Nb则贫化;另外,Ti6Al7Nb合金和Ti6Al4V合金氧化层主要由金红石型TiO2(R-TiO2)组成,Al2O3相仅出现在900 ℃Ti6Al4V合金样品中.     

真空熔炼TiAl金属间化合物过程中合金元素的挥发行为

真空熔炼的目的是避免间隙元素对ＴｉＡｌ金属间化合物的污染。但在真空下,由于钛铝合金熔体中铝的挥发损失将影响金属间化合物成分的准确度。作者利用计算得到的钛铝合金中钛、铝的活度系数,计算了钛铝合金熔体中钛、铝元素的挥发损失速率。结果表明,２０００Ｋ时,Ｔｉ４８ａｔ％Ａｌ熔体中铝的挥发速率是钛挥发速率的５０倍左右,即铝优先挥发。２０００Ｋ时,Ｔｉ４８ａｔ％Ａｌ在真空下保持３０分钟,铝的自由挥发将使铝含量由４８ａｔ％降至２６ａｔ％,与实验室试验结果基本一致。     

Ti-Al合金熔体与氧化物陶瓷界面反应的热力学分析(英文)

研究CaO、MgO、Al2O3和Y2O3与熔融Ti及Ti合金之间的界面反应热力学。根据氧化物在Ti合金熔体中的溶解反应机制建立热力学模型,通过热力学分析预测各氧化物相对于Ti-Al(0≤x(Al)≤0.5)合金的热力学稳定性。根据氧化物坩埚熔炼实验结果来研究氧化物与Ti-xAl合金熔体的界面反应,对熔炼效果进行定量分析,发现随着合金中Al含量的提高和熔体温度的降低,合金中杂质元素含量成比例的减少,这与氧化物稳定性计算结果一致。     

Evaporation loss of components during induction skull melting of Ti—13Al—29Nb—2·5Mo

Saturated vapour pressures of the Al, Ti and Nb components in Ti—13Al—29Nb—2·5Mo (wt-%) melts were calculated at arbitrary temperatures. The calculated values at 2150 K for Ti, Al and Nb were 2·0, 30·7 and 0·17 Pa, respectively. From the thermodynamic point of view, the principal evaporation loss is of the Al component. Based on the saturated vapour pressure, the mass transport coefficient of the Al component, evaporation loss rates of the Ti, Al and Nb components and the evaporation loss of the Al component were calculated. When the melt temperature is 2100 K, the evaporation control changes from the double control mode to the single interface volatilisation control mode with increasing chamber pressure at a transition pressure of about 4 Pa. The rates of evaporation loss of the components are N_Al≈15N_Ti≈46N^Nb. A critical chamber pressure of about 13·33 Pa exists during induction skull melting of the Ti—13Al—29Nb—2·5Mo alloy. In order to prevent the evaporation loss of Al, the chamber pressure should be kept above 13·33 Pa. The theoretical calculation of the evaporation loss of the Al component is in good agreement with experimental values. IJCMR/432     

Evaporation of multi—components in Ti—25Al—25Nb melt during induction skull melting process

Based on activity calculation model, t he activity coefficients of Ti, Al and Nb components of Ti-25Al-25Nb (mole fraction, %) melt, the vapor pressu res of correspo nding components and the evaporation loss rates were calculated. Utilizing these activity coefficients and the vapor pressures, the relative evaporation coeffic ient is used to judge the evaporation tendency of these components. The evapora tion tendency among the three components were compared and the result shows that the evaporation tendency is that: AlTi>Nb. Evaporation loss rate increases with the increase of melting temperature and decreases with the increase of chamber pressure. There exists an impeding pressure pimpe of Al element evapo ration during induction skull melting process of Ti-25Al-25Nb alloy. The impeding pressu re can be written as pimpe=8.1pe, where perepresents the equilibrium partial pressure. The calculation of evaporation loss of Al element also showed that when chamber pressure exceeds pimpe, the Al volatilization losses could be ignored. In order to prevent the evaporation loss of components, the pressure in the vacuum chamber should not below pimpe.     

两种Ti—Al金属间化合物基合金ISM熔炼过程中Al元素挥发控 …

对水冷铜坩埚真空感应熔炼过程中合金元素的挥发动力学进行了深入的分析,并在此基础上建立了ＩＳＭ熔炼过程中Ａｌ元素挥发控制方式的判断模型。对Ｔｉ－４８Ａｌ－２Ｃｒ－２Ｎｂ（ａｔ％）和Ｔｉ－２４Ａｌ－１１Ｎｂ（ａｔ％）合金中Ａｌ元素挥发控制方式的判断结果表明：随着熔体温度的升高和外压的增大,Ａｌ元素的挥发由低温和高压时的界面挥发控制向高温和低压时的界面挥发和液相中扩散同时起作用的双重控制方式转变,只是对     

Ti—24Al—11Nb合金感应凝壳熔炼（ISM）过程中Al元素的挥发损失行为

计算了Ｔｉ－２４Ａｌ－１１Ｎｂ中各组元的活度系数,在此基础上系统地研究了该合金在冷坩蜗感应凝壳熔炼过程中Ａｌ的发挥发损失行为。计算结果表明外压为０．１３３Ｐａ时,在１９００－２２００Ｋ的范围内,Ａｌ的挥发由界由挥发反应和液相界面层中的扩散同时控制作用。     

The solidification path related columnar-to-equiaxed transition in Ti–Al alloys

Columnar-to-Equiaxed Transition (CET) of binary Ti–Al alloys and multi-component Ti–48Al–2Cr–2Nb alloys is studied using Bridgman solidification technique. The effect of aluminum concentration and growth rate on CET is determined. It is found in Ti–46Al and Ti–50Al alloy ingots equiaxed grains develop ahead of the moving solid–liquid interface with a growth rate of 500 μm/s; microstructures in Ti–49Al alloy stay columnar dendrites with the same growth rate. CET in Ti–Al alloys are not only influenced by growth rate, but also by the solidification path that is related to alloying composition. CET in Ti–Al alloys is predicted using the dendritic growth model based on the criterion of growth at marginal stability. According to the calculated results and directionally solidified microstructures, values of the nucleation undercooling for α and β phases are given. The growth rates to avoid CET in Ti–48Al–2Cr–2Nb alloy are suggested.     

Oxidation behavior of Ti–46Al–8Nb alloy with boron and carbon addition under thermal cycling conditions

Oxidation behavior of Ti–46Al–8Nb (in at.%) alloy with boron and carbon addition under thermal cycling conditions was investigated. Oxidation of Ti–46Al–8Nb, Ti–46Al–8Nb–1B and Ti–46Al–8Nb–1B-0.25C (in at.%) alloys was carried out at 1073 K in laboratory air for 42 cycles (1 cycle, 24 h), 1008 h in total. The mass loss rates of Ti–46Al–8Nb and Ti–46Al–8Nb–1B measured during the oxidation were similar. The best oxidation resistance was found for Ti–46Al–8Nb–1B-0.25C alloy with the smallest mass change. XRD and SEM-EDS investigations showed that in all cases, the oxide scales compositions were substantially similar. The scale consisted of an outer layer built of Al₂O₃ with the presence of some amounts of TiO₂, an intermediate layer of the scale consisting of TiO₂, an inner layer composed of oxides and nitrides. Additionally, niobium rich particles at the scale/substrate interface were present. The oxidation mechanism of Ti–46Al–8Nb was studied via two-stage isothermal oxidation (24 h in ¹⁶O₂ followed by 24 h in ¹⁸O₂) at 1073 K combined with secondary neutral mass spectroscopy (SNMS). These results indicate that the oxidation mechanism depends on a mixed diffusion process, consisting of outward transport of cations and simultaneous oxygen inward transport.