Research On Microstructural Evolution And Mechanical Properties Of Ti-44Al-4Nb-2.2Cr Intermetallic Alloy Fabricated By Isothermal Forging

TiAl-based alloys are continually developing as attractive materials for aerospaceapplications due to their desirable properties like low densities, high specific strengthand stiffness. They were developed to compete and replace nickel-based alloys forhigh temperature applications, i.e. aerospace engine parts. However, the poor elevatedworkability and low room temperature ductility of the as-cast and as-HIPedTiAl-based alloys with coarse microstructure, limited its development.In the present work, Ti-44Al-4Nb-2.2Cr alloy ingot was produced by three timesvacuum electricity arc melting and subsequent homogenizing annealing and hotisostatic pressing (HIP) processing. Microstructural observation was carried out forthe heat treatment samples in order to analysis the phase transformation. Compressivedeformation behavior of Ti-44Al-4Nb-2.2Cr alloy was studied on a Gleeble1500Thermal Simulator at950-1200℃and1-0.001s-1.Based on the compressive results, as-cast Ti-44Al-4Nb-2.2Cr alloy ingot wasdeveloped by multi-step isothermal forging and intermediate heat treatment. Theresults indicated that a typical equiaxed coarse microstructure of the as-castTi-44Al-4Nb-2.2Cr alloy ingot can be converted into an fine-grained microstructureby such multi-directional isothermal forging. The aim of the present work was,therefore, as follows:(1) to study the microstructural evolution and mechanicalproperties of the Ti-44Al-4Nb-2.2Cr alloy during the muti-directional isothermalforging,(2) identify the elevated temperature mechanical properties,(3) evaluatesuperplastic properties of the as-forged Ti-44Al-4Nb-2.2Cr alloys with finemicrostructure.As-HIPed Ti-44Al-4Nb-2.2Cr alloy was characterized with large size colonies ofabout50-60μm. The Y2O3was distributed in between the lamellar structures. Theelevated tensile tests result indicated that the elongation of as-HIPed alloy was as highas57%at a strain rate of1×10-3s-1and1200℃. After one-step isothermal forging, themicrostructure was significantly refined. However, there was still residual lamellarstructures were observed. The elongation of one-step isothermal forging alloy was127%at a strain rate of1×10-3s-1and1200℃. After two or three-step isothermalforging, the microstructure of the as-forged alloy was refined to1.92μm. Theelevated tensile test results indicated that the elongation was up to791%at a strainrate of5×10-4s-1and900℃. Obviously, after three-step isothermal forging, theas-forged alloy with fine-grain microstructure exhibted excellent superplasticdeformation performance. Based on the superplastic properties, the strain rate sensitivity factor was0.53at900℃and different strain rates. The deformationactivation was298kJ/mol at850-1000℃and a strain rate of1×10-3s-1. alloysuperplastic deformation mechanism of as-forged alloy was dislocation creepcoordination of grain boundary sliding.