Solidified Microstructure Evolution and Mechanical Properties of TiAl Alloys Reinforced by In-situ Ti2AlC

Author:Fang Hong Ze

Supervisor:chen rui run

Database:Doctor

Degree Year:2018

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Pages:170

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The TiAl-based alloys are the materials of aero-engine blade with potential advantages,due to the advantages of low desity,high specific strength and high elevated temperature properties.These advantages can improve the thrust-to-weight ratio of the airspace engine and reduce energy consumption.Based on the previous investigations,the technology of TiAl-based composites reinforced by in-situ Ti2AlC can effectively improve the disadvantages of single intermetallics.The directionally solidified technolony can acquire the fully lamellar structure growing in a single direction,which can significantly improve the creep resistance properties,the fatigue resistance properties and the elevated temperature properties.However,the correlational studies need further research and improve,which are the forming process of in-situ Ti2AlC,the coupling interaction of Nb and Ta acting as the solid solution elements and the TiAl alloys reinforced by in-situ Ti2AlC with directionally solidified microstructure.This paper is systematically studied that the foming process of carbides during casting,the coupling interaction of Nb and Ta,the influence mechanism of TiB with different morphologies and forming conditions of the TiAl alloys reinforced by in-situ Ti2AlC with directionally solidified microstructure.The correlation influencing mechanisms are revealled.The TiAl alloys reinforced by in-situ Ti2AlC with directionally solidified microstructure are successfully prepared.To investigate the forming process of in-situ Ti2AlC during the casting,the experimental results can be explained that the length-diameter ratio of Ti2AlC decreases with increasing carbon content and there are the residualγphases around the Ti2AlC particles in the solidification microstructure.The formation mechanism of carbide in TiAl alloys is deduced as:during heating,aluminum is melted firstly at 660oC and the liquid aluminum reacts with Ti to form TiAl3 near 900oC;TiC is formed above 900oC;Ti2AlC will be formed from the reaction of TiAl3,TiC and Ti at approximately 1200oC;The formed Ti2AlC particles decompose into TiC and liquid TiAl in the melting when all the metal particles are melted above the 1625±10℃temperature.The TiC(melting point of 3200℃)particles can act as the nucleation particles and form the Ti2AlC particles again.During cooling,TiC and liquid TiAl react as Ti2AlC at 1625oC±10oC.Hence,during the solidification process,the TiC acts as heterogeneous nucleation particles for the Ti2AlC and the Ti2AlC acts as heterogeneous nucleation particles for the matrix.The microstructure are significantly refined with increasing carbon content.By regulating the Al content,the residual TiC phases can completely transform the Ti2AlC phases.The TiAl binary alloy can effectively improve the room and elevated temperature properties.Based on the improving TiAl alloys reinforced by in-situ Ti2AlC phases,the Ti42Al2.6C and Ti46Al2.6C alloys have been chose as the matrix for the contrast experiment.To add the solid solution element of Nb into the two alloys,the experiment study the effect of Nb on microstructure evolution and mechanical properties during solidification process.Experimental results show that the B2 phases exist between the lamellar colonies when the Nb content is more than 4 at.%and the B2content increases with increasing Nb content in the Ti42Al2.6C-Nb alloys.There areγphases exists between the lamellar colonies and there are no B2 phase in matrix in the Ti46Al2.6C-Nb alloys.Because theγphase has the high solid solubility for Nb element,theγphase content is higher,and the more Nb solid solutes into theγphase.The B2phase will be decreased because the Nb element segregates.The addition of Nb will improve the room and elevated temperature mechanical properties of the two alloys.However,the more content of B2 phase exist and form the more source of cracks,which will limit the improving the mechanical properties of alloys.To eliminate the B2 phase,the Ta element with low diffusion rate has been added into the Ti42Al6Nb2.6C and Ti46Al8Nb2.6C alloys,which two alloys act as the matrix for the contrast experiment.The Ta can limite the Nb segregation and decrease the B2phase content during solidification process.Experimental results show that the addition of Ta will eliminate the B2 phase between lamellar colonies in the Ti42Al6Nb2.6C-Ta alloys.When the Ta content is more than 0.6 at.%,there are B2 phases in the lamellar colonies in the Ti46Al8Nb2.6C-Ta alloys.Because the Ta element with low diffusion rate will restrain the diffusion of Nb and make Nb solid solute into matrix.The addition of Ta increases the lamellar colony size about 2.2 times in the Ti42Al6Nb2.6C-Ta alloys and decreases the lamellar colony size about 1.4 times in the Ti46Al8Nb2.6C-Ta alloys,which is concerned with the forming and disappearing B2 phases.The forming B2phase will cause the constitutional supercooling at the solidification front,which will refine the microstructure.The nanoscale Ti1.4Al phase forms at lamellar colony boundaries and inγphase,because the transformation needs a change of stacking sequence and composition by low diffusion rate of Ti with pinning effect of Ta in the Ti42Al6Nb2.6C-Ta alloys.Around the nanoscale Ti1.4Al phase,there are some dislocations.Because the lattice parameters of the Ti1.4Al precipitate and theγ-TiAl phase are similar,moiréfringes appear.The B2 content decreases and the solid solution of Nb and Ta play an important role to further improve the room and elevated tempreature mechanical properties.The addition of Ta can increase the lamellar colony size.In order to adjust and control the microstructure and mechanical properties,the B element has been added,because the refining effect of B and precipitation strengthening of in-situ TiB.Experimental results show that the boride content increase with increasing B content.The Nb and Ta are the strongly stable element for TiB and they solid solute into TiB.Hence the pinning effect of Ta for Nb decreases with addition of B.There are some B2phases between lamellar colonies in the Ti42Al6Nb2.6C0.8Ta-B alloys and the B2content decreases with increasing TiB content.The B2 phase existing in lamellar colony will be disappeared in the Ti46Al8Nb2.6C0.8Ta-B alloys.During solidification process,the F borides increase remarkably in the Ti42Al6Nb2.6C0.8Ta-B alloys and the C borides increase remarkably in the Ti46Al8Nb2.6C0.8Ta-B alloys with increasing boron content.The addition of B can improve the room and elevated temperature mechanical properites,especially inthe Ti42Al6Nb2.6C0.8Ta2.0B alloy the tensile strength is664MPa and the tensile strain is 6.89%at 850℃.The mainly mechanism is the precipitation strengthening of in-situ TiB and the grain boundary strengthing is limited.Based on the above research,the Ti46Al4Nb1.0C0.8Ta(nominal composition)alloy has been designed.To control the drawing velocity and heating power,the TiAl alloys reinforced by in-situ Ti2AlC phases have been successfully prepared.When the drawing velocity is 0.2 mm/min and the heating power is 39kW,the solid-liquid interface is relatively flat and the angle between the columnar crystal and temperature gradient direction is smaller in the steady state region.And the continuous growth of the columnar crystal is better.The primary phases areβphase andαphase at the solid-liquid interface front.The reason for forming the primary phases ofαphase is that the forming Ti2AlC phase will consume a part of Ti,which the more Al content segregates between the dendritic crystal to form theαphase.There are the incompletely transforming lamellar microstructure in the fuzzy region.The Ti2AlC particles mainly exist at the bottom of the directionally solidified specimen and the skin layer of the steady state region.And the site of forming Ti2AlC particles has the distance from sample edge.Under this experimental parameters,the initial stability area of the directional solidification has the minimum distance and the stability of the directional solidification microstructure is best.