| ?-TiAl based alloys have excellent plasticities,including excellent strength,good creep resistance and flame resistance at high temperatures,and have become a new lightweight structural material that are currently focusing on practical research.However,the inherent brittleness of?-TiAl based alloys lead to poor plasticity and low ductility at room temperature,it is difficult to form by the conventional processing technology.Therefore,it is very important to study the superplastic forming method for the development of?-TiAl based alloys.The?-TiAl alloys that have achieved superplastic properties are mostly fine-grained near?microstructures or duplex microstructures,and there are few reports on the in-situ synthesis of equiaxed duplex superfine microstructures.The prerequisite for achieving superplasticity is to obtain superfine microstructures.The microstructures of composite materials prepares by the powder sintering technology are fine,no segregation,porosity and other casting defects,easy to add a variety of alloying elements.Therefore,the technology lays a good foundation for the preparation of superfine microstructures.In this paper,Ti-45Al-5Nb?at.%?alloy was prepared by powder metallurgy process.The equiaxed superfine microstructure of?-TiAl based alloy??-TiAl+?2-Ti3Al?was obtained by high-energy ball milling?40h?+vacuum hot-pressing method.The tensile properties of Ti-45Al-5Nb?at.%?alloy at 10001075°C and strain rate of 5×10-51×10-3s-1were studied.The characteristic of stress-strain curves and the superplastic deformation mechanism in the high-temperature process are mainly analyzed,including the mismatch structure,evolution law and interface relationship,etc.The results showed that:?1?The refinement effect of Ti-45Al-5Nb alloy is very significant by high-energy ball milling 40h,and produce partial alloying while achieving nanocrystallization.The equiaxed superfine/nanocrystalline duplex??+?2?alloy is situ-synthesized by vacuum hot-pressing.The grain size of?phase is 500 nm,and the grain size of?2 phase is in100200 nm.?2?The tensile properties of Ti-45Al-5Nb alloy after extrusion are different at the temperature of 10001075°C,strain rate of 5×10-51×10-3s-1.When the peak value of stress is reached at 1000°C and 1×10-33 s-1,the specimen does not enter the steady area that is fractured.The flow stress reached 208 MPa,and the elongation was 14.8%.The peak value of flow stress is only 7.5 MPa at 1075°C and 5×10-55 s-1,the elongation is 251%,shows its superplastic tensile properties.?3?The high temperature deformation process of Ti-45Al-5Nb alloy is the thermally activated process.The strain rate sensitivity index of 1025°C,1050°C and 1075°C is 0.40,0.46 and 0.47.The deformation activation energy?Q?is 387.51 kJ·mol-11 and the constitutive equation is derived as:???=3.75×1011[sinh?0.08??]2.47exp?-387510/RT??4?The tensile fracture of Ti-45Al-5Nb alloy shows different morphology at 1000°C with different strain rates.At the strain rate of 1×10-3s-1,there is a small amount of secondary microcracks on the fracture surface.At the strain rate of 1×10-4s-11 and 5×10-5s-1,the emergence of dimples can be observed at the fracture.With the decrease of the strain rate,the dimples gradually become deeper,and the number increases accordingly.?5?The mechanism of superplastic deformation is dislocation slipping and twinning of?/?phase.The orientation relationship between the?/?phase interface is?001??//?110??.There are a large number of steps consisting of SISF in the?/?twin interface.The width of the twin boundary is 0.963nm,and the angle is 110°.The main softening mechanism is the dynamic recrystallization of?grains. |
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