Creep Behavior Of High Nb-TiAl Alloys Prepared By Cold Crucible Directional Solidification

TiAl-based alloys exhibit the superiority of lightweight for application in aero engine blade,due to the low density,high specific strength,excellent oxidation and creep resistance.The high Nb-TiAl alloy can improve the high temperature properties and ensure room temperature performance,but the effect of temperature and applied stress on creep behavior lacks systematic research.The cold crucible directional solidification?CCDS?technique can prepare the no contamination TiAl-based ingots with the industry size and offer the defect free directional solidification?DS?microstructure,and one of the primary purposes for technique is to improve the creep performance of TiAl alloy,but there is a lack of research in this area.In this paper,the creep behavior of high Nb-TiAl alloy is systematically investigated by combining conventional casting with CDDS,and the brittle-ductile transition mechanisms during creep is revealed.Based on the creep research results,the composition gradient TiAl alloys were prepared to optimize the composition of high-Nb TiAl alloy using CCDS technique,and a high-Nb TiAl alloy with both room temperature and high temperature properties was obtained.The relation among microstructure,room temperature properties and high temperature creep properties was established.There is brittle-ductile transition temperature?BDTT?during creep tests in TiAl alloy,and the creep strain increases exponentially and the creep life decreases largely at this temperature range.The slip system in?₂ lamellae was activated during creep above BDTT,and the strain rate of lamellae increases dramatically,which improves the local stress concentration at colony boundary,accelerates the necking of specimen and leads to significant reduction in creep properties.The thermal activation is so active that dislocations can overcome obstacle under a small effective stress during creep above BDTT,and the deformation depends on dislocation slip assisted by the thermal activation during creep below BDTT.Cavities and cracks usually nucleate at interface of?B2+??phases for high Nb-TiAl alloys,which reduce creep properties dramatically.The CCDS technique can improve the high temperature creep properties,the B2/?lamellae are observed,which exhibit a better creep resistance.The high Nb-TiAl alloys with a high creep properties should have the following characteristics.Firstly,the TiAl alloys should have a high Nb content to ensure a high BDTT.Secondly,prevention the formation of?B2+??phases in alloy matrix.Thirdly,the appropriate high Al content can limit the?B2+??phases and contribute to room temperature performance.Fourthly,DS technique is used to eliminate transverse grain boundaries.In order to optimize the microstructure and mechanical properties of TiAl-based alloys,a novel gradient alloy method has been invented using the CCDS technique.The molten pool can be thoroughly mixed under electromagnetic force,and the composition gradient can be controlled by the height of molten pool.This method can be used to study the effect of solute content on the microstructure and mechanical properties in a CCDS ingot,which overcomes the problem of large difference of component by preparation of multiple ingots.The optimized alloy composition is acquired by studying the microstructure and mechanical properties of Ti-48Al/Ti-44Al and Ti-47Al-10Nb/Ti-47Al gradient alloys,and the Ti-47Al-6Nb-0.1C alloy with both room temperature and high temperature properties were obtained.The room temperature tensile properties of CCDS Ti-47Al-6Nb-0.1C strongly depend on the lamellar spacing and the content of?B2+??phases,while the DS structure has little effect on room temperature ultimate strength.The creep properties of CCDS Ti-47Al-6Nb-0.1C strongly depend on the DS structure and the content of?B2+??phases,and the minimum strain rate strongly depends on lamellar spacing.Optimum preparation parameters of CCDS Ti-47Al-6Nb-0.1C are 0.5mm/min and 45kW.At the optimum preparation parameters,this alloy exhibits the good room-temperature tensile properties with ultimate strength 582MPa and elongation 1.62%,and the minimum strain rate is7.55×10^-10s^-1 during creep at 760°C/250MPa.When the CCDS Ti-47Al-6Nb-0.1C creeps above BDTT,the creep is controlled by dislocation glide at the low stress region,both the well-DS and bad-DS Ti-47Al-6Nb-0.1C exhibit the high strain rate,the necking of specimen and creep fracture occurs at a shorter time.The creep properties of the well-DS Ti-47Al-6Nb-0.1C improve to a certain degree compared to the bad-DS Ti-47Al-6Nb-0.1C.When the CCDS Ti-47Al-6Nb-0.1C creeps below BDTT at the middle and low stress region,the interface sliding mechanism is the dominant in this regime.The well-DS Ti-47Al-6Nb-0.1C exhibits a very low minimum strain rate compared to the bad-DS Ti-47Al-6Nb-0.1C.There is local high angle grain boundary at colony boundaries in the CCDS alloys.The lamellae on both sides of vertical grain boundary usually have different lamellar directions,and the lamellar colonies with different lamellar directions usually exhibit different deformation capacities.The plastic incompatibility of lamellar colonies leads to the local stress concentration at the steps of colony boundary,which promotes the formation of cavities and cracks at colony boundary and accelerates the creep failure.