Effect Of Ti On High-temperature Compression Properties Of ?-TiAl Based Alloy

The high temperature properties of ?-TiAl based alloys are excellent.It has high specific strength,good creep resistance and flame resistance.However,due to inherent brittleness,the plasticity of ?-TiAl based alloys is poor and make it difficult to be manufactured by conventional processing techniques,limits its wide application.Therefore,it is of great research significance to improve the plasticity of ?-TiAl based alloy through some new research ideas and methods.In this paper,Ti powders with mass percentages of 6%,8%and 10%were added into Ti-48Al-2Cr-2Nb prealloy powder.The phase coexisting ?-TiAl based alloy with the structure of ?-TiAl and lamellar ?-TiAl/?2-Ti3Al phase clusters coated with ?-Ti phase as shell was prepared by powder metallurgy vacuum hot pressing sintering.The "hard phase"was coordinated by softer ?-Ti phase,when ?-TiAl based alloy were compressed at high-temperature,the ?-Ti phase with good plasticity deform first and then the structure deform coordinates,so as to improve the plasticity and high-temperature compression performance of the material.This paper mainly studies the high-temperature compression properties of 6%Ti,8%Ti,and 10%Ti alloys at 800?1000? and 1×10-3?5×10-5 s-1 strain rate.Focus on analysis of the effect of adding Ti on the high-temperature compressive properties of ?-TiAl based alloys and the high-temperature deformation mechanism and microstructure evolution of materials Research indicates:(1)The microstructure of ?-Ti,?-TiAl and ?2-Ti3Al was prepared by powder metallurgy.In 6%Ti,8%Ti and 10%Ti alloy,the microstructure of Ti-48Al-2Cr-2Nb alloy with ?-Ti phase as shell coating(equiaxed ?-TiAl phase and lamellar ?-TiAl/?2-Ti3Al phase clusters)was formed.With the increase of Ti content,the thickness of the coating layer,the integrity of the coating and the ratio of the coating gradually increased,and the thickness of the formed coating layer was between 2.6 and 4.26 ?m.(2)With the increase of Ti content,the high temperature compressive strength of the alloy decreased first and then increased,the maximum compressive strength of 10%Ti alloy was obtained under the same conditions.The compressive strength of the Ti-48Al-2Cr-2Nb alloy(0%Ti content)is 119.43%of that with 10%Ti content,which is the maximum value under the same conditions.It can be concluded that the improvement of the coating ratio,coating integrity and coating layer thickness is beneficial to the improvement of the compressive strength of the material.(3)The high-temperature deformation process of the coated alloy is a thermal activation process.During the high-temperature compression process,the deformation activation energy Q value of the 10%Ti alloy is 415.18 kJ·mol-1,and the strain rate sensitivity factor m value is 0.2877,less than 0.3.Under this condition,no superplastic deformation occurs,and the constitutive equation of high-temperature thermal deformation of 10%Ti alloy is:?=e31.623[sinh(0.0045?)]3.4755exp(-415180/RT)(4)After high temperature compression,microcracks and holes were found in the alloy structure.Studies results shown that the high-temperature microcracks of the alloy structure belong to micropore-gathering cracks.The micropores are mainly formed at the junction of the two-phase grain group,the phase boundary,the two sheets and the junction of the sheets in different growth directions.The micropores are aggregated to form holes,which generate microcracks after expansion and polymerization.At the initial stage of deformation,the micropores are conducive to of grain boundary sliding.As he deformation continues to increase,the connection of the micropores is likely to cause deformation instability and destruction.(5)The coating layer of the 10% Ti alloy is thick enough and the coating is complete enough.The soft Ti will first move and coordinate the deformation of the structure during high temperature compression,which plays a role in protecting the Ti-48Al-2Cr-2Nb prealloy.Therefore,the micropore volume of the 10%Ti alloy after compression is the least and the microstructure is the most complete.A certain thickness and integrity of Ti layer can improve the plasticity and hot working properties of Ti-48Al-2Cr-2Nb alloy.(6)The high-temperature compression deformation mechanism of the alloy is mainly dislocation slip and twinning in the ? layer.The high-temperature deformation softening mechanism is mainly based on the dynamic recrystallization of equiaxed ?-phase grains and lamellar structures.After plastic deformation,the lamellar crystals ? and ?2 still maintain the intact interface structure,which is combined with a 108° large-angle crystal plane,and satisfies the Burgers orientation relationship(001)?//(1010)?2.