Effect Of Tial Based Alloy Microstructure Control On Mechanical Properties

The thermal Stability of lamellar microstructure of TiAl alloy was studied. Thethermal stability of as-cast lamellar microstructure of Ti-47Al and Ti-48Al alloysaged at ll50℃ is inferior to that of Ti-46Al alloy, because the dendritic segregationof the Ti-47Al and Ti-48Al alloys is more severely than of the Ti-46Al alloy,. Thethermal stability of coarse lamellar microstructure of the Ti-48Al alloy produced byheat treatment at the αa phase field followed by furnace cooling is superior to that offine lamellar microstwture produced by air cooling.The mechanisms of contnuous coarsening and discontnuous coarsening of the finelamellar microsructure of Ti-48Al alloy aged at ll50℃ were investigated. Thecontinuous lamellar coarsening can be occurred through migration of the interfacialfaults and dissolution of the γ/γ lamellar interfaces. The thermal stability of the brue-twin, pseudo-tWin and 120°-rotational ordered γ/γ lamellar interfoces was compared.The l20°-rotational ordered interface has the worst thermal stability and the true-twininterface has the best one. Because the energy of the l20°-rotational ordered interfaceis very high and the stacking sequence of { l l l } planes is maintained across it, thel20°-rotational ordered interface can migrate through neighboring atoms on theinterface exchanging their Iattice sites durng aging. The discontinuous coarseningcells nucleate on primary lamellar grain boundaries and grow into a grain Without anyorietation relationship with the DC nuclei. Theγ/α₂ interfaces in the DC cellsgenerally parallel to the lamellar interfaces in another primary lamellar grain.Two types of Spheroidization mechanisms of α₂ lamellae were found. The α₂lamellae in the primary lamellar microstructure can be rapidly spheroidized throughthe Rayleigh dissolution mechanism during the growth of the DC cells. Then,incoherent globular α₂ particles form in the DC cells. The coherent α₂ plates in theDC cells can be transformed into the irregular faceted α₂ particles by the sizecoarsening and the shape coarsening.The boride morphology and growth mechanism in Ti-48Al-(0.2～0.8)at%B alloyswere studied. In the Ti-48Al-0.2B and Ti-48Al-0.5B alloys, the borides are all flakysecondary TiB₂, phase which grow coupled with the matrix by the L→β+TiB₂, andL+β→α+TiB₂, eutectic reactions. However, there are a few hexagonal blocky borideswith{l0l0} and (000l) habit planes besides the flaky borides in the Ti-48Al-0.8Balloy. The blocky borides are primary TiB₂, phase growing unconstrained in the melt.Many of ultra-fine plates (thickness of 5～l0nm) of B2 phase growing coupled withthe flacky TiB₂, were found in the center of flaky TiB₂, in the Ti-48Al-0.8B alloywithout containing β stabilizers. There is {l0l0}₍(TiB₂) //{00l}_B2, [000l]₍(TiB₂)//<0l0>_B2oritation reationship between the B2 phase and the flacky TiB₂, phase.The borides at grain boundaries in the Ti-48Al-0.8B alloy restrain the discontinuouscoarsening effectively during aging at ll50℃. However, the new γ grains nucleateivand grow at incoherent TiB,/matfix illterfaces and grain boundaries, as a result, thelamellar microstructore trdrisforms illto fine near ganuna microsttucture when aged atl l50oC for 24h. The as-cast microsmictures are significanly refined when the boronaddition is uP to 0.8at%B due to primary blopky TiB, directly nucleate in the melt.During heat treatmeflt at the a phase field, borides can imPede the a grain groedeffectively. Only addition of 0.2at%B can significanly refine fulIy lamellarmicrostffictures.The mechanism of y-a phase transformation was studied. The cr plates precipitateon the { l l l } planes in the y grain interiors. Concurrently, the irregUlar a pAnclesprecipitate on the y grain boundaries probably due to the disconiinuous precipitation.Addition of 0.8at%B into the Ti-48Al alloy degrades the nucleation rate of a platCs.HREM analysis shows...