Directionally solidified （DS） specimens of Nb-Ti-Si based ultrahigh temperature alloy were heat-treated at （1 500 ℃, 50 h） and （1 500 ℃, 50 h） ＋ （1 100 ℃, 50 h）, respectively. The results show that the mic...Directionally solidified （DS） specimens of Nb-Ti-Si based ultrahigh temperature alloy were heat-treated at （1 500 ℃, 50 h） and （1 500 ℃, 50 h） ＋ （1 100 ℃, 50 h）, respectively. The results show that the microstructures become uniform, the long and big primary （Nb,X）sSi3 （X represents Ti and Hf elements） plates in the DS specimens are broken into small ones, and the eutectic cells lose their lamellar morphology and their interfaces become blurry after heat-treatment. Meanwhile, the （Nb,X）sSi3 slices in the eutectic cells of the DS specimens coarsen obviously after heat-treatment. Homogenizing and aging treatments could effectively eliminate elemental microsegregation, and the segregation ratios of all elements in niobium solid solution （Nbss） in different regions tend to 1. After heat-treatment, the microhardness of retained eutectic cells increases evidently, and the maximum value reaches HV1 404.57 for the specimen directionally solidified with a withdrawing rate of 100 μm/s and then heat-treated at （1 500 ℃, 50 h） ＋ （1 100 ℃, 50 h）, which is 72.8 % higher than that under DS condition.展开更多
To investigate the effects of homogenizing and aging treatments on the microstructure of an Nb-Ti-Cr-Si based ultrahigh temperature alloy,coupons were homogenized at 1 200-1 500 °C for 24 h,and then aged at 1 000...To investigate the effects of homogenizing and aging treatments on the microstructure of an Nb-Ti-Cr-Si based ultrahigh temperature alloy,coupons were homogenized at 1 200-1 500 °C for 24 h,and then aged at 1 000 °C for 24 h.The results show that the heat-treated alloy is composed of Nb solid solution（Nbss）,（Nb,X）5Si3 and Cr2Nb phases.With the increase of heat-treatment temperature,previous Nbss dendrites transformed into equiaxed grains,and petal-like Nbss/（Nb,X）5Si3 eutectic colonies gradually changed into small（Nb,X）5Si3 particles distributed in Nbss matrix.A drastic change occurred in the morphology of the Laves phase after homogenizing treatment.Previously coarse Cr2Nb blocks dissolved during homogenizing at temperature above 1 300 °C,and then much finer and crowded Cr2Nb flakes precipitated in the Nbss matrix in cooling.Aging treatment at 1 000 °C for 24 h led to further precipitation of fine particles of Laves phase in Nbss matrix and made the difference in concentrations of Ti,Hf and Al in Nbss,（Nb,X）5Si3 and Cr2Nb phases reduced.展开更多
Integrally directional solidification of an Nb-Ti-Si based ultrahigh temperature alloy was performed in an ultrahigh temperature and high thermal gradient furnace with the use of ceramic crucibles. The microstructural...Integrally directional solidification of an Nb-Ti-Si based ultrahigh temperature alloy was performed in an ultrahigh temperature and high thermal gradient furnace with the use of ceramic crucibles. The microstructural evolution with the withdrawing rate increasing during directional solidification was revealed. The integrally directionally solidified microstructure was composed of couple grown lamellar (Nbss+(Nb,X)5Si3) eutectic colonies and a few hexagonally cross-sectioned (Nb,X)5Si3 columns (X represents Ti and Hf elements). All the directionally solidified microstructure was straightly aligned along the longitudinal axis of the specimens. With increasing of the withdrawing rate, the average diameter of the eutectic cells and inter lamella spacings in the eutectic cell decreased. The near-planar solid/liquid interface appeared when the withdrawing rate was 1μm/s, and the cellular solid/liquid interface formed when the withdrawing rate was 5 μm/s.展开更多
The directionally solidified samples of an ultra-high temperature Nb-Si-Ti-Hf-Cr-Al alloy have been prepared with the use of an electron beam floating zone melting (EBFZM) furnace, and their microstructural characteri...The directionally solidified samples of an ultra-high temperature Nb-Si-Ti-Hf-Cr-Al alloy have been prepared with the use of an electron beam floating zone melting (EBFZM) furnace, and their microstructural characteristics have been analyzed. All the primary dendrites of Nb solid solution (Nbss), eutectic colonies of Nba, plus (Nb, Ti)3 Si/(Nb, Ti)5 Si3 and chains of (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates align along the growth direction of the samples. With increasing of the withdrawing rate, the microstructure is refined, and the amounts of Nbss+ (Nb, Ti)3 Si/(Nb, Ti)5 Si3 eutectic colonies and (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates increase. There appear nodes in the (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates.展开更多
The oxidation behaviors of Nb67-xW15Si18Hfx (x=0, 5, 10) alloys were studied at 1 250℃in air. It is found that the Nb67W15Si18 alloy has the best oxidation resistance among the three alloys; and Hf addition is harmfu...The oxidation behaviors of Nb67-xW15Si18Hfx (x=0, 5, 10) alloys were studied at 1 250℃in air. It is found that the Nb67W15Si18 alloy has the best oxidation resistance among the three alloys; and Hf addition is harmful to the oxidation resistance of the Nb67W15Si18 alloy. The oxides formed on the Nb67W15Si18 alloy are mainly Nb12WO33 and NbO2, and that on the Nb62W15Si18Hf5 and Nb57W15Si18Hf10 alloys is Nb2O5. Effect of Hf on the oxidation behavior of the Nb67-xW15Si18Hfx alloys has been discussed based on microstructures and kinetics of oxidation.展开更多
基金Project(51071124)supported by the National Natural Science Foundation of ChinaProject(CX200605)supported by the Doctorate Foundation of Northwestern Polytechnical University,ChinaProject(20096102110012)supported by a Special Research Fund for Doctoral Disciplines in Colleges and Universities of the Ministry of Education,China
文摘Directionally solidified （DS） specimens of Nb-Ti-Si based ultrahigh temperature alloy were heat-treated at （1 500 ℃, 50 h） and （1 500 ℃, 50 h） ＋ （1 100 ℃, 50 h）, respectively. The results show that the microstructures become uniform, the long and big primary （Nb,X）sSi3 （X represents Ti and Hf elements） plates in the DS specimens are broken into small ones, and the eutectic cells lose their lamellar morphology and their interfaces become blurry after heat-treatment. Meanwhile, the （Nb,X）sSi3 slices in the eutectic cells of the DS specimens coarsen obviously after heat-treatment. Homogenizing and aging treatments could effectively eliminate elemental microsegregation, and the segregation ratios of all elements in niobium solid solution （Nbss） in different regions tend to 1. After heat-treatment, the microhardness of retained eutectic cells increases evidently, and the maximum value reaches HV1 404.57 for the specimen directionally solidified with a withdrawing rate of 100 μm/s and then heat-treated at （1 500 ℃, 50 h） ＋ （1 100 ℃, 50 h）, which is 72.8 % higher than that under DS condition.
基金Project (51071124) supported by the National Natural Science Foundation of ChinaProject (20096102110012) supported by the Ministry of Education, China Project (07-TP-2008) supported by the State Key Laboratory of Solidification Processing in Northwestern Polytechnical University,China
文摘To investigate the effects of homogenizing and aging treatments on the microstructure of an Nb-Ti-Cr-Si based ultrahigh temperature alloy,coupons were homogenized at 1 200-1 500 °C for 24 h,and then aged at 1 000 °C for 24 h.The results show that the heat-treated alloy is composed of Nb solid solution（Nbss）,（Nb,X）5Si3 and Cr2Nb phases.With the increase of heat-treatment temperature,previous Nbss dendrites transformed into equiaxed grains,and petal-like Nbss/（Nb,X）5Si3 eutectic colonies gradually changed into small（Nb,X）5Si3 particles distributed in Nbss matrix.A drastic change occurred in the morphology of the Laves phase after homogenizing treatment.Previously coarse Cr2Nb blocks dissolved during homogenizing at temperature above 1 300 °C,and then much finer and crowded Cr2Nb flakes precipitated in the Nbss matrix in cooling.Aging treatment at 1 000 °C for 24 h led to further precipitation of fine particles of Laves phase in Nbss matrix and made the difference in concentrations of Ti,Hf and Al in Nbss,（Nb,X）5Si3 and Cr2Nb phases reduced.
基金Supported by the National Natural Science Foundation of China (No 500671081) the Doctorate Foundation of Northwestern Polytechnical University (No CX200605)
文摘Integrally directional solidification of an Nb-Ti-Si based ultrahigh temperature alloy was performed in an ultrahigh temperature and high thermal gradient furnace with the use of ceramic crucibles. The microstructural evolution with the withdrawing rate increasing during directional solidification was revealed. The integrally directionally solidified microstructure was composed of couple grown lamellar (Nbss+(Nb,X)5Si3) eutectic colonies and a few hexagonally cross-sectioned (Nb,X)5Si3 columns (X represents Ti and Hf elements). All the directionally solidified microstructure was straightly aligned along the longitudinal axis of the specimens. With increasing of the withdrawing rate, the average diameter of the eutectic cells and inter lamella spacings in the eutectic cell decreased. The near-planar solid/liquid interface appeared when the withdrawing rate was 1μm/s, and the cellular solid/liquid interface formed when the withdrawing rate was 5 μm/s.
基金supported by the National Natural Science Foundation of China(No.50271056)National High Technical Research and Development Programme of China(No.2003AA305810)the Special Research Fund for Doctoral Disciplines in Colleges and Universities of M.0.E,China(No.20020699025).
文摘The directionally solidified samples of an ultra-high temperature Nb-Si-Ti-Hf-Cr-Al alloy have been prepared with the use of an electron beam floating zone melting (EBFZM) furnace, and their microstructural characteristics have been analyzed. All the primary dendrites of Nb solid solution (Nbss), eutectic colonies of Nba, plus (Nb, Ti)3 Si/(Nb, Ti)5 Si3 and chains of (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates align along the growth direction of the samples. With increasing of the withdrawing rate, the microstructure is refined, and the amounts of Nbss+ (Nb, Ti)3 Si/(Nb, Ti)5 Si3 eutectic colonies and (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates increase. There appear nodes in the (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates.
基金Project(50671002) supported by the National Natural Science Foundation of China
文摘The oxidation behaviors of Nb67-xW15Si18Hfx (x=0, 5, 10) alloys were studied at 1 250℃in air. It is found that the Nb67W15Si18 alloy has the best oxidation resistance among the three alloys; and Hf addition is harmful to the oxidation resistance of the Nb67W15Si18 alloy. The oxides formed on the Nb67W15Si18 alloy are mainly Nb12WO33 and NbO2, and that on the Nb62W15Si18Hf5 and Nb57W15Si18Hf10 alloys is Nb2O5. Effect of Hf on the oxidation behavior of the Nb67-xW15Si18Hfx alloys has been discussed based on microstructures and kinetics of oxidation.
