Brittle-Ductile Transition Behavior Of Near Lamellar TiAl Intermetallics

TiAl intermetallics has received widely attention in industrial fields due to suchattractive properties as low density, high specific modulus and strength, excellent hightemperature strength stability and etc. TiAl has been thought a future alternative ofhigh-temperature structural material to Ni-based materials and has important applicablevalue. However, its low ductility at room temperature limit it usage. The ductile tobrittle transition of TiAl occurs at high temperature, which can greatly improve itsplasticity and facilitate its plastic processing.The casting and isothermal forging methods were used to obtainTi-47Al-2Nb-2Cr-0.4(W+Mo)(at%) forging billet. The final heat treatment route ofnearly lamellar (NL) microstructure was determined by various tests of the heattreatment at different temperatures. The homogeneous NL TiAl was prepared by heatingat1330℃for2hours with furnace cold to room temperature.Quasi-static tests of NL TiAl under strain rate of1×10-4s-1,1×10-3s-1and5×10-3s-1at different temperatures were carried out on the MTS material test system. Theductile-to-brittle transition temperatures (BDTT) under different strain rates weredetermined. Test results showed that with the increased temperature the flowing stressand elastic modulus were significantly decreased and the plastic deformation wasimproved. The BDTT of NL TiAl was strain rate dependent. The BDTT of NL TiAlunder strain rates of1×10-4s-1was750℃and it increased to1050℃under the strainrate of5×10-3s-1. With the increased strain rate, the BDTT significantly increased.The results of TEM observation showed that both deformation twinning anddislocations were very few below BDTT. The deformation twin was the maindeformation mechanism. Most of the twins were thin and parallel to each other and witha certain angle to the grain boundary. Furthermore, with the increased temperature,numbers of twins and dislocations inside the material gradually increased. Above theBDTT, dislocation density increases significantly and a large number of dislocationswere intensively accumulated in the lamellar and equiaxed grains. Both of the planedislocation cells and the linear dislocation walls were found, which indicated thatdynamic recovery occurred. Above the BDTT a certain number of good slipped <011]super-dislocations were found. The1/2<110] ordinary dislocation and <011] super-dislocation in different directions interacted each other. It showed that with theincreased temperature, the improvement of the mobility of the super-dislocation wasplayed a certain role during brittle-ductile transition process. Part of the grain boundarywas bent due to the thermal active process, which indicated the improvement of thegrain boundary mobility. TLD grain boundaries were found and they continuallytransmitted dislocations to the inner grain to reduce the energy of the grain boundary.