The microstructure and oxidation behavior of directionally solidified(DS) Nb-15 Si-24 Ti-4 Cr-2 Al-2 Hf alloys with separate vanadium,tantalum,tungsten and zirconium additions were investigated by X-ray diffraction(XR...The microstructure and oxidation behavior of directionally solidified(DS) Nb-15 Si-24 Ti-4 Cr-2 Al-2 Hf alloys with separate vanadium,tantalum,tungsten and zirconium additions were investigated by X-ray diffraction(XRD),electron probe microanalyzer(EPMA) equipped with wave-dispersive spectroscopy(WDS),scanning electron microscopy(SEM) and transmission electron microscopy(TEM) equipped with an energy-dispersive spectroscopy(EDS).Results show that the five alloys are all composed of primary(Nb,Ti) solid solution((Nb,Ti)ss)phase and eutectic(Nb,Ti)ss/(Nb,Ti)_(5) S_(3) structure.After oxidation at 1250℃ for 100 h in air,the surfaces of the five alloys are covered by the oxides of Nb_(2)O_(5),TiNb_(2) O_(7),Ti_(2) Nb10O29,TiO_(2) and amorphous SiO_(2).It is found that the alloying elements of V and W are detrimental for oxidation resistance and the addition of Ta has no obvious effect.Zr addition obviously benefits the oxidation resistance at high temperature,by decreasing the weight gain from 242.75 to184.83 mg·cm^(-2).The oxidation mechanism of Nb-Sibased alloys and the effects of different alloying elements on the oxidation resistance of Nb-Si-based alloys were discussed.展开更多
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 micros...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.展开更多
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 12...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.展开更多
An oxidation-resistant composite coating,including Mo-Si-B layer and Al_(2)O_(3)diffusion barrier,was prepared on Nb-Si-based alloy by a two-step method.The Al_(2)O_(3)film was fabricated by firstly deposition of Al o...An oxidation-resistant composite coating,including Mo-Si-B layer and Al_(2)O_(3)diffusion barrier,was prepared on Nb-Si-based alloy by a two-step method.The Al_(2)O_(3)film was fabricated by firstly deposition of Al on the alloy through pack cementation,and then by in situ oxidation of the aluminide coating.Mo-Si-B coating was then prepared on the Al_(2)O_(3)film by spark plasma sintering.The Mo-Si-B coating is mainly composed of MoSi_(2)and a small amount of MoB and SiO_(2)particles.After long-term exposure at 1250℃,the growth of the interdiffusion zone(IDZ)was suppressed effectively at the interface with the Al_(2)O_(3)diffusion barrier.The results indicate that the diffusion barrier can significantly inhibit the outward diffusion of Nb,Ti and Cr alloying elements as well as the sluggish inward consumption of Si element.The Mo-Si-B coating with Al_(2)O_(3)diffusion barrier exhibited a good oxidation resistance with a mass gain of about 0.97 mg/cm^(2)after 500 h of oxidation.展开更多
基金financially supported by the National Nature Science Foundation of China(Nos.51471013 and51571004)。
文摘The microstructure and oxidation behavior of directionally solidified(DS) Nb-15 Si-24 Ti-4 Cr-2 Al-2 Hf alloys with separate vanadium,tantalum,tungsten and zirconium additions were investigated by X-ray diffraction(XRD),electron probe microanalyzer(EPMA) equipped with wave-dispersive spectroscopy(WDS),scanning electron microscopy(SEM) and transmission electron microscopy(TEM) equipped with an energy-dispersive spectroscopy(EDS).Results show that the five alloys are all composed of primary(Nb,Ti) solid solution((Nb,Ti)ss)phase and eutectic(Nb,Ti)ss/(Nb,Ti)_(5) S_(3) structure.After oxidation at 1250℃ for 100 h in air,the surfaces of the five alloys are covered by the oxides of Nb_(2)O_(5),TiNb_(2) O_(7),Ti_(2) Nb10O29,TiO_(2) and amorphous SiO_(2).It is found that the alloying elements of V and W are detrimental for oxidation resistance and the addition of Ta has no obvious effect.Zr addition obviously benefits the oxidation resistance at high temperature,by decreasing the weight gain from 242.75 to184.83 mg·cm^(-2).The oxidation mechanism of Nb-Sibased alloys and the effects of different alloying elements on the oxidation resistance of Nb-Si-based alloys were discussed.
基金financially supported by the National Natural Science Foundation of China (No.51101005)
文摘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.
基金financially supported by the National Natural Science Foundation of China(No.51101005)
文摘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.
基金financially supported by the National Natural Science Foundation of China (Nos. 51971177 and 51971181)the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2020JM-106)
文摘An oxidation-resistant composite coating,including Mo-Si-B layer and Al_(2)O_(3)diffusion barrier,was prepared on Nb-Si-based alloy by a two-step method.The Al_(2)O_(3)film was fabricated by firstly deposition of Al on the alloy through pack cementation,and then by in situ oxidation of the aluminide coating.Mo-Si-B coating was then prepared on the Al_(2)O_(3)film by spark plasma sintering.The Mo-Si-B coating is mainly composed of MoSi_(2)and a small amount of MoB and SiO_(2)particles.After long-term exposure at 1250℃,the growth of the interdiffusion zone(IDZ)was suppressed effectively at the interface with the Al_(2)O_(3)diffusion barrier.The results indicate that the diffusion barrier can significantly inhibit the outward diffusion of Nb,Ti and Cr alloying elements as well as the sluggish inward consumption of Si element.The Mo-Si-B coating with Al_(2)O_(3)diffusion barrier exhibited a good oxidation resistance with a mass gain of about 0.97 mg/cm^(2)after 500 h of oxidation.
