Using a Bridgman vertical vacuum furnace,Ti-45Al-8Nb-（W,B,Y） （at.%） bars,which were prepared from a plasma arc melting （PAM） ingot,were directionally solidified at growth rates of 10,15,and 20 μm/s.Polysynthetic twinned （PST） crystal with an aligned lamellar microstructure was obtained at the growth rate of 15 μm/s because of high Nb addition.The principle of PST crystal growth and the effect of Nb element were discussed.The results of investigations on microstructure and micromechanical properties of the directionally solidified （DS） bars of Ti-45Al-8Nb-（W,B,Y） alloy are briefly summarized.
DING Xianfei LIN Junpin HE Jianping YE Feng CHEN Guoliang
The effect of heat treatment on the microstructure evolution of a high Nb containing TiAl alloy has been studied. The results indicate that β-segregation, β-segregation and S-segregation in the as-cast and as-forged alloys can be effectively eliminated at the temperature above Tα （1350-1400℃） for long holding time （12-24 h） and the full lamellar （FL） microstructure is gained. For the two alloys, the lamellar colony sizes are 120 μm and 2000 μm, respectively after heat treatment at 1400℃ for 12 h. Meanwhile, the sizes are 210 μm and 3000 μm, respectively at 1350℃ for 24 h. To get a fine homogenous microstructure, the primary as-cast alloy is first subjected to preheat treatment for eliminating the segregations. After the preheat treatment, the ailoy is processed by the multi-step canned forging to attain the microstructure with fine grain size.
XU Zhengfang XU Xiangjun LIN Junpin ZHANG Yong WANG Yanli LIN Zhi CHEN Guoliang
The effects of heat treatments on typical microstructures of directionally solidified（DS） Ti-45Al-8Nb-（W,B,Y）（molar fraction,%） alloys prepared by the Bridgeman method were studied.Two typical DS microstructures including full lamellae with cellular growth morphology and massive structure with dendritic growth morphology were examined.The results show that the heat treatment of 1250 ℃ for 24 h ＋ 900 ℃ for 30 min＋air cooling can efficiently eliminate the B2 phase in the DS alloys and change the massive structure of the rapid DS alloy into lamellar microstructure.Columnar lamellar colonies with widths of 150-200 μm and 50-100 μm respectively were observed in intercellular and dendritic arm regions.The heat treatment of 1 400 ℃ for 12 h＋900 ℃ for 30 min＋air cooling could simultaneously remove the B2 phase,massive structure and solidification segregations from the DS alloys,however,it caused severe growth of grains.
