【作者】 郑海忠；

【导师】 鲁世强；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2009， 博士

【摘要】 Laves相NbCr₂合金因其优异的高温力学性能而具有作为新型高强高温结构材料应用的潜力。针对其室温脆性及较差的高温抗氧化性,本文研究了不同制备方法及工艺过程对NbCr₂合金组织及性能的影响,重点探讨了晶粒细化对合金断裂韧性及高温氧化行为的作用机制;研究了相组成对Cr-Nb合金高温抗氧化性的影响规律;探究了合金元素Al及稀土元素Y对NbCr₂合金断裂韧性和高温氧化行为的影响,并探讨了多元合金化对Cr-20Nb高温抗氧化性的影响及采用涂层技术提高Cr-50Nb合金高温抗氧化性的可行性。研究中发现,采用机械活化热压合成的Cr-33Nb及Cr-25Nb合金晶粒分别细化到250nm和193.3nm,其维氏硬度和断裂韧性要高于相应成分的熔铸态合金,而且单相Laves相NbCr₂的Cr-33Nb热压合金的断裂韧性达到了5.7 MPam^1/2 ;随着球磨时间的延长,有利于NbCr₂晶粒的细化及其热固相反应合成,维氏硬度不断增加,而断裂韧性则从球磨20h的5.7 MPam^1/2减小到球磨100h的3.7 MPam^1/2 ,但远远高于熔铸态合金的1.3 MPam^1/2 ,充分体现了细晶增韧的效果。熔铸态及热压合金950℃～1200℃氧化后均形成了以Cr₂O₃为外层、CrNbO₄为内层的双层结构氧化膜,热压合金氧化速率均接近抛物线规律;虽然熔铸态合金在950℃空气中氧化接近于抛物线规律,但在1200℃空气中氧化类似于直线规律。随着热压温度的升高,合金氧化速率减小;保压时间由15min延长到80min时,合金氧化速率逐渐减小,但保压时间达到120min时,氧化速率又有所提高。随着球磨时间的延长,热压合金1100℃及1200℃的氧化增重并不是一直呈现下降的趋势,在球磨时间为20-35h左右,其抗氧化性最好。提出了晶粒细化对Cr-33Nb合金高温抗氧化性有着双重作用机制,即晶粒的细化促进了体系的扩散及氧化膜内应力的释放,形成了Cr₂O₃外膜;当晶粒过细时,导致氧化膜内生长应力的增加,引起氧化膜的开裂、脱落,从而加剧了氧化。通过系统地研究相组成对Cr-Nb合金在950℃～1200℃空气中氧化行为的影响,揭示了软第二相Cr能显著增加NbCr₂合金950℃的抗氧化性能,但降低了NbCr₂合金1200℃的抗氧化性能;而软第二相Nb不利于合金950℃～1200℃高温抗氧化性的提高,甚至产生了灾难性氧化。通过掺杂Al和Y来提高NbCr₂合金的断裂韧性和高温抗氧化性,结果表明Al和Y分别占据了Laves相NbCr₂合金中Cr原子和Nb原子的晶格位置,从而形成了反位置缺陷。合金元素Al在一定程度上改善了NbCr₂合金的断裂韧性,当Al含量达到12%时,合金断裂韧性达到了6.8 MPam^1/2 ,且提高了NbCr₂合金1100℃的抗氧化性。Y对NbCr₂合金断裂韧性和高温抗氧化性的影响均呈现先增后减的规律;当Y含量为0.1wt.%时,合金的断裂韧性最高,达到6.15 MPam^1/2 ;Y含量在0.07-0.14wt.%时,NbCr₂合金抗氧化性能得到提高。同时研究了Al、Si、Y多元合金化对Cr-20Nb合金高温抗氧化性的影响,结果表明,多元合金化的Cr-20Nb合金抗氧化性要好于加入单一合金元素的及纯Cr-20Nb合金,并随着Si含量的增加,合金的氧化增重越小,抗氧化性越好;SEM结果表明,添加了合金元素后,合金氧化膜与基体的粘附性得到了明显的提高。采用包埋渗Si、Si-Al共渗及Si-Al共渗+sol-gel工艺+热压的复合工艺在Cr-50Nb合金制备涂层,发现采用复合工艺制备的涂层最致密,与基体的结合最紧密,并形成了以Al₂O₃为外层,Si、Al、Cr及Nb扩散层为内层的复合结构;复合涂层大大提高了Cr-50Nb合金的高温抗氧化性,在1100℃经10次共100h氧化后,氧化膜仅脱落0.049mg/cm²,特别是在1200℃,氧化膜脱落量也只有0.13 mg/cm²,而且氧化增重也只有3.38 mg/cm²。更多还原

【Abstract】 Laves phase NbCr₂ has potential application as high temperature structural materials because of its excellent mechanical properties at elevated temperature. However, its wide application is limited by the ambient brittleness and high temperature oxidation. The main objective of this work focused on the investigation of grain size effect on fracture toughness and high temperature oxidation behaviors of Laves phase NbCr₂ based compounds which were fabricated by ingot metallurgy and powder metallurgy, respectively. In addition, the influence of phase constitution on the oxidation behavior of the hot pressed Cr-Nb alloys, the effect of Al and Y on fracture toughness and high temperature oxidation behaviors of the Laves phase NbCr₂ based alloys, the effect of mutilalloying on the oxidation of Cr-20Nb alloys and the effect of coating technique on the Cr-50Nb alloys were investigated.The results indicate that the average grain sizes of Cr-33Nb and Cr-25Nb fabricated by mechanical alloying and hot pressing are 250 nm and 193.3 nm, respectively. The microhardness and fracture toughness of hot pressed compacts are higher than those of cast alloys. Especially, the fracture toughness of hot pressed Cr-33Nb alloys with single phase NbCr₂ is up to 5.7 MPa m . With increasing the powder milling time from 0 h to 100 h, the content of Laves phase NbCr₂ and the microhardness of the hot pressed compacts increase, the grain size decreases gradually. The fracture toughness values measured using the indentation method decrease from 5.7 MPam^1/2 to 3.7 MPam^1/2, which are two to four times higher than that of cast materials (1.3 MPam^1/2), indicating that nanostructured NbCr₂ is much tougher than the conventional coarse-grained.It is found that both cast alloys and hot pressed alloys form a complex oxidation film containing an outer layer of Cr₂O₃ plus an inner layer of NbCrO₄, instead of a single and continuous Cr₂O₃ film. The oxidation kinetic curves of the hot pressed alloys at 950℃～1200℃and the cast alloys at 950℃are approximately parabola. However, the oxidation kinetic curves of the cast alloys at 1200℃are approximately linear. The oxidation resistance of the hot pressed compacts is found to increase with increasing sintering temperature and sintering time from 15 min to 80 min; however, as sintering time is prolonged to 120 min, the oxidation resistance declines gradually. The oxidation resistance of hot pressed alloys at 1100℃and 1200℃was found to increase rapidly with increasing mechanical alloying time from 20 h to 35 h, followed by a decrease slowly. The differences observed above are attributed to the fine grain size which can increase the diffusion of Cr atom to form Cr₂O₃ on the oxide film and increase the relaxation of the oxide scale stress and the adhesion of the oxide layer on the matrix. However, when the grain sizes were more refined, the oxidation resistance is reduced, which is attributed to the greater grain boundary areas those are the short-circuit diffusion paths for alloys and the locations of higher growth stresses leading to spallation.The influence of phase constitution on the oxidation behavior of the hot pressed Cr-Nb alloys were investigated at 950℃～1200℃in air. The results show that Cr phase improves the oxidation resistance of Laves phase NbCr₂ based compounds at 950℃. However, Nb phase deteriorates the oxidation resistance; moreover, Nb phase results in a catastrophic failure during oxidation.The influences of Al and Y on the fracture toughness and oxidation resistance of NbCr₂ alloys were studied. The results show that Al mainly occupies the Cr site and Y mainly occupies the Nb site in the NbCr₂ Laves phase. The fracture toughness of the NbCr₂ alloys increased slightly with the increase of Al addition. At a higher Al addition level （i. e. 12 at. %）, the NbCr₂ alloy has a fracture toughtness of 6.8 MPam^1/2 . The addition of Al can also increases the oxidation resistance of NbCr₂ at 1100℃. The optimum content of Y addition can increase the fracture toughness and high temperature oxidation-resistance of NbCr₂ alloys. The fracture toughness reaches its maximum 6.15 MPam^1/2 when the Y content is 0.1wt.%. The hot pressed alloy with 0.07-0.14wt.% Y has the best oxidation resistance. The influence of Al, Si and Y multialloying on the oxidation resistance of Cr-20Nb alloys is also investigated. The results show that the oxidation resistance of Cr-20Nb alloys alloyed by multielements is higher than that of unalloyed Cr-20Nb and Cr-20Nb alloyed by single element. Moreover, the oxidation resistance of Cr-20Nb alloys increases with increasing the Si content. SEM analysis shows that the additions improve the adhesion of oxide film on the matrix.To protect Cr-50Nb alloys from high-temperature oxidation, the silicide diffusion coating and Si-Al coating were produced by pack cementation process and the multilayer coating was fabricated by Si-Al co-deposition pack cementation process, followed by sol-gel process and hot pressing. The results indicate that the multilayer coating has a two layers structure: the outer layer is a compact Al₂O₃ layer; the inner layer is an adherent diffusion layer consisting of Si, Al, Cr and Nb. The multilayer coating improves the oxidation resistance of Cr-50Nb alloys significantly. After 10 cycles and 100 h exposure at 1100℃and 1200℃, the spallation amounts of Cr-50Nb oxide scales are 0.049 mg/cm² and 0.13 mg/cm², respectively. Moreover, the weight gain of Cr-50Nb coated by the multilayer coating was 3.38 mg/cm².更多还原