The Effect Of Thermal Stabiliza Tionon Microstructure Of Directionall Ysolidified Ti-53at%al Alloy

In this paper, Ti-53at.%Al alloy as the research object, will be in Bridgmandirectional solidification furnace to 1700°C, then taking the treatment of the thermalstability with the time from 0min to 60min, followed by direct quenching or by pullingsome distances in a certain speed then quenched. And some analysises have been doneabout the formation of mushy zone during the processing of thermal stability,and the thevariation of the interface shape and the microstructure and solute compositiondistribution with the time of thermal stability processing. Further, the directionalsolidificational experiments in different heat stable time have been done to study theeffect on the directional solidificational microstructure and the formation of lamellarstructure. Then we determine the proper time of thermal stability for the directionalsolidification of Ti-53at.%Al alloy, Among the proper thermal stability zone somedirectional solidification experiments in different pull speed have been done to get thetransition and microstructure evolution from the initial to the stable phase.The imposed temperature gradient and the existence of alloy solidification range leadto the formation of the mushy zone during the thermally stable process, and the mushyregion is composed with horizontal and vertical liquid passage. With the increase of thetime of the thermal stability processing, the length of mushy zone become graduallyshorter, the solid phase grains ofαbecome aligned along the direction of temperaturegradient, and the fusion occurred between the grains and the number of grains becomesless with the gradually increasing volume fraction. Horizontal liquid channel disappearfirst, followed by the gradually disappear of the vertical liquid passage. All of this isbecause that the Al-rich droples transfer in the mushy zone.According to theory ofTGZM we calculated the migration of droplets that the time about droplet migrating outthe mushy region is around 20min, which is consistent with the experimentalobservations. The solute distribution in the mushy zone and the front of mushy zone withthermal stability in different time is analysised. And it's found that along the thermalgradient direction, the solid gredient in the mushy region changes with the line of solidphase in the equilibrium phase diagram; When the heat stability treatment is more than30min, the liquid composition gradient in front of the interface is disappeared and itbecame uniform.The directional solidification experiments followed the different time of heat stabilized process show that in the exactly same conditions, the heat stabilized time has a greatinfluence on the microstructure of directional solidification. When the time of the thermalstability processing is too short (0-20min), and the temperature field and solute field hasnot yet reached a stable condition in the mushy zone, leading to the competitional growthbetween the grain orientation in a long distance. it is hard to get the solidificationalmicrostracture parallelled to the direction of heat flow. When the time of the thermalstability processing is longer than 30min, the distance of the initial competitive growth isvery short, leading that the stable grains parallelled to the growth direction growcontinuouly. With the increase of the time of the thermal stability processing, because thegrains gradually integrates within the mushy zone before the start of the directionalsolidification, the layers become a single orientation, and the lamellar orientation in themushy zone can lead crystal lamellar along its orientation during the followingdirectional growth, which lead to a single lamellar orientation within the specimen duringthe directional solidification, showing typical anisotropic. Further after 30min and 60minrespectively thermal stability alloy directional solidification experiments has been donein different pull rates, with analysis of the effect on the microstructure and formation ofphase and lamellar with the changes of the time of thermal stability processing, Samplesobtained have a good directional solidification result, the grains have continuous andstable growth parallel to the temperature gradient direction. However, when the time ofthe heat stable treatment is 60min, due to the less grains and the single lamellarorientation in mushy zone before the start of the directional solidification, it gets coarsegrains,a single layer orientation, anisotropy serious from the directional solidification;While the time of Thermal stable treatment is 30min, We get the typical PST .Owing to the integrated grain orientation and ultimately the growth of PST crystals, tothe Ti-53at% Al, the thermal stability time before the start of the directional solidificationis appropriate about 30min.