Investigation On The High Temperature Deformation Of TiAl Based Alloy And The Microstructure Evolution

TiAl based alloys have excellent high temperature strength, oxidation resistance and creep resistance, which are considered as most promising structurual materials. However, their low ductility at room temperature seriously restricts their applications.In this paper, the hot deformation behaviors of as-cast TiAl based alloy and PM-TiAl composite were investigated using isothermal compression tests. The flow stress of alloys at high temperatures was corrected by the friction and temperature, the material parameters were calculated and theconstitutive equations and processing maps of both materials were constructed to based on the corrected flow stress, then the hot deformation mechanisms were analysised according to the processing maps. It will be helpful to obtain good thermomechanical parameters of TiAl-based alloys. Also, the elimination of the β (B2) phase segregation in as cast Ti-43Al-4Nb-1.4W-0.6B-0.1Y alloy were systematically investigated to improve the room temperature plasticity and fracture toughness.With an increasing B content in Ti-43Al-4Nb-1.4W-B alloy, the refining effect becomes obvious. Room temperature plasticity and fracture toughness. The most efficient refinement effect could be got when the B content was0.6%. As-cast TiAl alloy is sensitive to deformation temperature and strain rate, its flow stress decreases with increasing temperature and decreasing strain rate. The relationship among the strain rates, flow stress and temperature could be expressed by thehyperbolic sine function, at the strain of0.4, which could be expressed as: ε=e31.9[sinh(0.0057σ)]2.8exp(-419.21/RT)The deformation behavior of as-cast TiAl alloy could be described by a quintic polynomial which could predict the flow stress accurately. The hot deformation mechanisms of as-cast TiAl alloy mainly include the kinking, bending, and recrystallization of the lamellar colonies. The specimen were succeed deformated at the condition simulating industrial production based the processing map. The flow stress of PM-TiAl composite reinforced with Nb particle was corrected by by the friction and temperature, the flow stress decreases with increasing temperature and decreasing strain rate. The relationship among the strain rates, flow stress and temperature could be expressed by thehyperbolic sine function, at the strain of0.7, which could be expressed as: ε=e34.71[sinh(0.0087σ)2.47exp(-455.46/RT) The deformation behavior of PM-TiAl composite could be described by a quintic polynomial which could predict the flow stress accurately.The processing map at the strain of0.7was constructed based on corrected flow stress, At the domain located at low strain rate and high temperature which was suitable for hot demormation, Power dissipation efficiency η reaches peak value of60～75%. The deformation resistance was decreased and the plastic deformation ability increased as a result of addition of Nb particles.Heat treatment was carried out for Ti-43Al-4Nb-1.4W-0.6B-0.1Y alloy,the β (B2) phase content was decreased but could not be eliminated. It is better for the elimination of β (B2) phase when the alloy was deformed before heat treatment for the broken and high concentrating stresses.For specimen which was deformed at1200℃,0.1s-1the content of0(B2) phase was only2.3%when it was heated at1250℃for2h, and the duplex microstructure was acquired. When the deformed specimen was heated at1300℃,though the content of β (B2) phasephase decreased, the grain size increased and fully lamellar microstructure acquired.