Investigation On Solidification Microstructure Characteristics And Evolution Mechanism Of Ultrasonic Treated TiAl-based Alloys

Due to the high specific strength and excellent elevate-temperature properties,Ti Al-based alloys have attracted more and more attention in aerospace and automotive industry,which show broad application prospects as a novel light-weight structure material.Considering the intrinsic characteristics of intermetallic compounds,Ti Al-based alloys always show low ambient ductility,poor high-temperature deformation and difficult processing,so the application in industrial production is greatly restricted.For now,cast metallurgy is the most economic production method for large industrial-scale Ti Al-based alloys.However,for casting Ti Al-based alloys,especially after adding heavy alloying elements,the alloys always exhibit coarse lamellar microstructure with sever element segregation,which will seriously deteriorate performances.Ultrasonic treatment has been widely applied in Al and Mg alloys for high efficiency,economy and environment protection.Previous researches have confirmed that ultrasonic treatment could effectively control solidification,modify microstructure and improve properties.In this study,ultrasonic treatment is applied in the solidification process of Ti Al-based alloys and the effects of ultrasonic treatment on the microstructure evolution,solidification behaviors and mechanical performances are investigated.After ultrasonic treatment,the shrinkage cavity and porosity are almost eliminated,obtaining a high quality casting ingot.Regardless of ultrasonic treatment,Ti Al-based alloys show fully lamellar microstructure at room temperature.However,the lamellar microstructure is gradually refined as the increase of ultrasonic treatment time;after ultrasonic treatment for 60 s,lamellar colony size is decreased to 52 ?m,102 ?m,38 ?m and 56 ?m for Ti44 Al ? Ti48 Al ? Ti44Al6Nb1 Cr and Ti44Al6Nb1Cr2 V alloys,respectively.The microstructure morphology is modified into equiaxed dendrite from developed dendrite after spheroidization and finally forms non-dendritic globular grains.Ultrasonic treatment has no influences on the phase constitution,but the ?2-Ti3 Al and ?-Ti Al phase contents become more close to equilibrium phase content and the B2 phase content is gradually reduced as the increase of ultrasonic treatment time.Besides,ultrasonic treatment is benefit to eliminate element segregation,promotes solute evenly dissolved in the lamellar matrix,but aggravates element segregation in the B2 phases.Ultrasonic treatment accelerates melt heat and mass transfer process,resulting in uniformity temperature and solute fields,which hinders the columnar dendrite growth.As result,the microstructure is transformed from columnar crystal to fine equiaxed grains.For peritectic Ti48 Al alloy,under ultrasonic treatment,the peritectic ? phase could directly precipitate from the melt and the peritectic reaction is reduced.Furthermore,ultrasonic treatment could change the crystalline growth direction leading to randomly distribution of lamellar direction.The results of mechanical performances testing show that microhardness,micro-plastic deformation and compressive properties at room temperature are obviously increased as the increase of ultrasonic treatment time.Especially,the yield strength is improved by 129 %,142 %,148 % and 182 % for Ti44 Al,Ti48Al,Ti44Al6Nb1 Cr and Ti44Al6Nb1Cr2 V alloys,respectively.The strengthening mechanism by ultrasonic treatment is fine-grain strengthening and solution strengthening.Besides,ultrasonic treatment improves the microstructure uniformity,which is also beneficial to improve alloy strength.Ultrasonic treatment enlarges the grain boundary density and facilitates dislocation multiplication,so the plastic deformation is increased by ultrasonic treatment.In this study,solute elements could enlarge the tendency for alloy melt to constitutional supercooling,benefit for higher ultrasonic treatment efficiency.Besides,alloy melt temperature could also affect ultrasonic treatment efficiency: excessively high temperature leads to nucleus remelting and ultrasonic treatment lose efficiency;too low temperature will suppress the cavitation effect and ultrasonic treatment becomes invalid.The optimal melt temperature is nearly crystallizing temperature.In this paper,the grain refinement mechanism by ultrasonic treatment is cavitation-enhanced nucleation: firstly,ultrasonic treatment improves the solute concentration resulting in high constitutional supercooling;secondly,ultrasonic cavitation generates high pressure and improves melt crystallizing temperature,leading to heat supercooling;the enhanced melt supercooling enlarges the nucleation rate and improves the nucleus number.Acoustic streaming accelerates the melt flow and prompts nucleus homogenous redistribution in the whole melt,which diminishs the grain gradient and extends the refining region.Therefore,the microstructure refinement by ultrasonic treatment is the combined results of ultrasonic cavitation and acoustic streaming.