Nb-24Ti-18Si-2Al-2Hf-4Cr and Nb-24Ti-18Si-2Al-2Hf-8Cr alloys were prepared by arc melting in a water-cooled crucible under argon atmosphere.Microstructural characteristics and oxidation resistance of the alloys at 1250 ℃ were investigated.The results show that,when the Cr content is 4 at%,the microstructures consist of （Nb,Ti）ss and Nb5Si3;as Cr content increases to 8 at%,C14 Laves phase Cr2Nb is formed.The isothermal oxidation tests show that the oxidation kinetics of the two alloys follow similar features.The weight gains of the two alloys after oxidation at 1250 ℃ for 100 h are 235.61 and 198.50 mg.cm-2,respectively.During oxidation,SiO2,TiO2,Nb2O5 and CrNbO4 are formed at first.Then,Ti2Nb10O29 is formed after oxidation for 20 min and begins to change into TiNb2O7 as the oxidation proceeds.SiO2 is formed as solid state at first but later evolves into glassy state to improve the cohesion of the scale.After oxidation for 100 h,oxidation products consist of SiO2,TiNb2O7,Nb2O5 and CrNbO4.
An Nb-14Si-22Ti-4Cr-2AI-2Hf-0.15Y（at.%） alloy was prepared by directional solidification （DS） with liquid metal cooling, and the withdrawal rates selected were 1.2, 6, and 18 mm-min1, respectively. The Influence of withdrawal rate and heat treatment on the microstructural evolution, fracture toughness and tensile strength at room temperature were investigated. Results show that the directionally solidified microstructure is composed of primary （Nb, X）ss dendrites and （Nb, X）ss/a-（Nb, X）5Si3 eutectic cells aligning with the growth direction. The formation of bulk Nb3Si is suppressed. With an increase in withdrawal rate, the dendrite arm spacing of （Nb, X）ss decreases, and the （Nb, X）ss/a-（Nb, X）5Si3 eutectic cells become finer and distribute homogeneously. Directional solidification can significantly improve the room temperature fracture toughness, especially the alloy with a withdrawal rate of 6 mm.min-1; its average value reaches 14.1 MPa.m^0.5, about 34% higher than that of the alloy without directional solidification. The withdrawal rate has obvious effect on tensile strength, and the tensile strength is improved from 200 MPa to 429 MPa as the withdrawal rate increases from 1.2 mm.min-1 to 1.8 mm-min-1. After heat treatment, the primary （Nb, X）ss branches become coarser; both the room temperature fracture toughness and tensile strength of the alloys solidified at 1.2 and 6 mm.min 1 are somewhat lower than the corresponding values of the alloy without heat treatment, while they are higher than the corresponding values of the alloy without heat treatment when solidified at 18 mm-min4.
Wang BinJia LinaYuan SainanMa LiminSu LinfenZhang Hu
Nb-22Ti-14Si-2Hf-2Al-xCr （x=2, 6, 10, 14, 17 at%） alloys are prepared by arc-melting under argon atmosphere. Microstructural characteristics, mechanical properties and oxidation resistance of the arc-melted alloys are investigated. At 2 at% Cr content, the microstructure is composed of Nbss, Nb3Si and a small quantity of Nb5Si3, when the Cr contents increase, Nb3Si disappears. For the high Cr content （x ≥ 10 at%） alloys, besides the Nbss and Nb5Si3, Cr2Nb is also detected. With the increase of Cr content, the volume fractions of Cr2Nb and Nb5Si3 increase, while that of Nbss increases firstly and then begins to degrade when the Cr content is higher than 6 at%. For the alloy with 2 at% Cr, the room temperature fracture toughness is about 14.5 MPa·m1/2, but badly decreases to about 8.5 MPa·m1/2, when the Cr contents increase. Vickers hardness of Nbss tends to increase linearly from about 400 to 500, while that of silicides is not sensitive to Cr contents, about 950. The appearance of Cr2Nb phase significantly improves the high temperature oxidation resistance of the alloys with high Cr contents. The isothermal oxidation tests show that the oxidation kinetics of the alloys with various Cr contents follows parabolic oxidation kinetics.
In this work, the Nb-14Si 24Ti-10Cr-2Al-2Hf-0.1Y alloy （at.%） was processed by the liquid-metal-cooled directional solidification （DS） at 1750 ℃ with withdrawal rates of 1.2, 6, 18 mm/min and post heat treatment （HT） at 1450 ℃ for 10 h. The microstructures of the direction- ally solidified and heat treated samples were investigated. The results show that the microstructure of directionally solidified alloy mainly consists of petaloid Nbss ＋ Nb5Si3 eutectics and Ti-rich Nbss ＋ Nb5Si3 ＋ Cr2Nb eutectics. With the increase of withdrawal rate, the primary NbsSi3 is eliminated, Nbss ＋ Nb5Si3 eutectic cells turn round and connected with the microstructure refine- ment and Nbss ＋ Nb5Si3 ＋ CrzNb eutectics turn to a river-like morphology. After heat treatment, Nbss ＋ Nb5Si3 ＋ Cr2Nb eutectics disappeared and petaloid Nbss ＋ Nb5Si3 eutectics turn to a spe- cific fiber-mesh structure gradually, which is promoted by higher withdrawal rates. Furthermore, both the volume fraction of Cr2Nb and the content of Cr in Nbss of Nbss ＋ Nb5Si3 eutectics change regularly with the increase of withdrawal rate and heat treatment at 1450 ℃ for 10 h.
Ding FeiJia LinaYuan SainanSu LinfenWeng JunfeiZhang Hu
Nb-16Si-24Ti-10Cr-2A1-2Hf alloy was directionally solidified with withdrawal rates of 1.2, 6, 18, 36 and 50 mrn/min and then heat treated at 1400, 1450 and 1500℃with withdrawal rate of 50 mm/min for 10 h. The effects of withdrawal rate and heat treatment temperature on the microstructure were studied. The microstructure of directionally solidified alloy was composed of the primary NbsSi3, Nbss/NbsSi3 eutectic cells and Cr2Nb, which distribute paralleled to the growth direction. The microstructure becomes more refined with the increasing withdrawal rate, accompany with the evolution of eutectic cells morphology. After heat treatment, Nbss phase connects and forms a continuous matrix, and the Cr2Nb phase becomes smaller and distributes more dispersedly. After heat treatment at 1450 ℃ for 10 h, the alloy achieves balance between the optimization of microstructure and alleviation of solute segregation.
In this work, the near-eutectic Nb-24Ti-15Si-4Cr-2Al-2Hf（at%） alloy was directionally solidified at 1900 ℃ with withdrawal rates of 6, 18, 36, 50 mm·min^-1 and then heat-treated at 1450 ℃ for 12 h. The microstructure evolution was investigated. The results show that the microstructure of the directionally solidified（DS） alloy is composed of Nbss＋Nb5Si3 eutectics within the whole withdrawal rate range, while the variation of rates makes a great difference on the solidification routes,the morphology and size of Nb_（ss）＋Nb_5Si_3 eutectic cells.With the increase in withdrawal rates, the petaloid Nbss＋Nb5Si3 eutectic cells transform into granular morphology. After the heat treatment, a mesh structure Nbssis formed gradually which isolates the Nb5Si3, and the phase boundaries become smoother in order to reduce the interfacial energy. Moreover, two kinds of Nb5Si3 exist in the heat-treated（HT） samples identified by crystal form and element composition, which are supposed as α-Nb5Si3 and γ-Nb5Si3, respectively. This study exhibits significant merits in guiding the optimization of Nb-Si-based alloys＇ mechanical properties.
