Research On Fabrication, Toughening, Oxidation Behavior And Mechanism Of Laves Phase NbCr₂ Based Alloys

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.