Zr含量对Nb-Ti-Si基超高温合金组织及抗氧化性能的影响

EFFECT OF Zr ADDITION ON MICROSTRUCTURE AND OXIDATION RESISTANCE OF Nb-Ti-Si BASE ULTRAHIGH-TEMPERATURE ALLOYS

采用真空非自耗电弧熔炼法制备了成分为Nb-22Ti-15Si-5Cr-3Hf-3Al-xZr（x=0，0．5，1，2，4和8，原子分数，％）的合金，研究了Zr含量对合金电弧熔炼态组织及高温抗氧化性能的影响．结果表明，6种不同Zr含量的合金均由Nb固溶体和γ-（Nb，X）5Si3（X为Ti，Hf,Cr和Zr）组成，添加Zr并未改变合金的相组成，但随着Zr含量增加，合金中初生γ-（Nb，X）5Si3的尺寸增大，含量提高．对不同Zr含量合金在1250℃分别氧化1—50h发现，随着Zr含量的增加，合金氧化膜黏附性及致密性均得到显著改善，Zr含量较高的合金（F4和8）氧化50h后，氧化膜出现明显的分层现象：最外层为致密的单相TiO2层，中间层主要由ZrO2，TiNb2O7和TiO2组成，而内层主要由Si的氧化物组成．随着合金中Zr含量增加，氧化膜厚度及单位表面积的氧化增重均显著降低，抗氧化性能得到明显改善．

Nb-Ti-Si base in situ composites which consist of Nb solid solution （Nbss） and silicides （α- NbsSi3, β-NbsSi3, γ-NbsSi3 and/or Nb3Si） phases, have shown great potential as altemative materials to Ni-based su- peralloys due to their high melting points （beyond 1700 ℃ ）, good formability, low density （6.6-7.2 g/cm^3） and high strength. However, a major hindrance to the applications of these alloys at elevated temperatures is their poor oxidation resistance. Alloying is an effective method to improve the integrated properties of the alloys, especially for the oxidation resistance. Up to now, many beneficial elements such as Ti, Al, Cr and Sn have been employed to ameliorate their oxidation resistance. Nevertheless, there is no systematic and comprehensive investigation on the effect of Zr contents on the microstructure and oxidation behavior of the alloys based on Nb-Ti-Si system. The aim of this work is to clarify the effects of Zr contents on phase selection, microstructure and high temperature oxidation resistance of Nb-Ti-Si based alloys in detail. The constituent phases, microstrueture and composition of the alloys under as-cast state and after oxidation were investigated by OM, XRD, SEM and EDS. Thus, six Nb-Ti-Si base ultrahigh-temperature alloys with compositions of Nb-22Ti-15Si-5Cr-3Hf-3Al-xZr （x=0, 0.5, 1, 2, 4, 8, atomic fraction, %） were prepared by vacuum non-consumable arc-melting. The results show that the alloys with different Zr contents are mainly composed of Nbss and γ-（Nb, X）5Si3 （X represents Ti, Hf, Cr and Zr）. However, the addition of Zr has an obvious affect on the microstructure of Nb-Ti-Si base alloys. Both the sizes and amounts of primary γ- （Nb, X）sSi3 increase with increase in Zr contents. Alloys with different Zr contents were oxidized at 1250 ℃ for 1- 50 h, respectively. It is found that both adhesion and compactness of the scales are improved effectively by increase in Zr contents. The scales of alloys with higher Zr contents （x=4 and 8） after oxidation for 50 h show an obvious layered structure： the outmost layer is only composed of TiO2, the middle layer mainly consists of ZrO2, TiNb207 and TiO2, and the inner layer is mainly comprised of Si-rich oxides. The mass gain per unit area and the thickness of the scale after oxidation decrease with increase in Zr contents in the alloys, indicating that the addition of Zr can improve the oxidation resistance of the alloys significantly.