| By means of measuring creep properties under different conditions and microstructure observation after 1380?/2h/FC solution treatment and 900?/100 h aging treatment,the influence of heat treatment on the microstructures and creep behaviors of forged TiAl-Nb alloy are investigated.Results show that the microstructure of as-forged Ti Al-Nb alloy mainly consists of the lamellar ?/?2 phases.A grain consists of the lamellar ?/?2 cluster with same orientation,the grain boundaries consisted of broken ? phase with bigger volume fraction are located between the lamellar ?/?2 clusters with different orientations.After the as-forged alloy is aging treated,the grain size with fine and uniform is obtained,but the coarsening of the lamellar ?/?2 phases occurs,and the broken ? phase with bigger volume fraction still exists in the boundary regions.Wherein,the grain boundaries with broken structure are still the weaker link of creep strength during creep of alloy.After the as-forged alloy is solution treated at 1380?/2h/FC,the original broken structure at the grain boundary turns into the serrated configuration,and volume fraction of boundary and the space of the lamellar ?/?2 phase decrease.Compared with the forged alloy,the creep life of aging alloy at 700?/300 MPa increases from 335 h to 485 h.The creep property of the solution stated alloy in the range of 800?-920? is obviously improved,the creep life of the solution stated alloy at 800?/200 MPa enhances from 143 h to 436 h.The deformation mechanism of the alloy in the temperature near 700? is dislocation slipping in ? and ?2 phases.Wherein,the dislocations slipping to the interfaces of lamellar ?/?2 phases are hindered and the plied up in the region near the interfaces,which may cause the stress concentration.When the value of the stress concentration exceeds the yield strength of the grain boundary,the initiation of the cracks occurs and propagated along the lamellar ?/?2 interfaces perpendicular to stress axis until the occurrence of transgranular fracture.This is thought to be the damage and fracture mechanism of the alloy during creep near 700?.The deformation mechanism of alloy during creep between 800?-920? is dislocations slipping in ? and ?2 phases.The dislocations of slipping to the interfaces are hindered to form the ones rows or tangled to cause the stress oncentration,which can promote dislocations shearing through the interface and slipping in another lamellar phases,so that the lamellar structure in the alloy is thansformed into the bending configuration.Compared with ?2-Ti3 Al phase,the ? phase of TiAl-Nb alloy possesses a weaker strength,the boundaries with single ? phase is the weaker link of strength during creep between 800?-920?.In the late stage of creep,the crack is easily initiated and propagated along the grain boundaries at about 45° relative to the stress axis and parallel to the lamellar phases,which is thought to be the fracture mechanism of the alloy during creep between 800?-920?. |
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