Spark Plasma Sintering Densification,microstructures And Properties Of Ti-46.5Al-2.15Cr-1.90Nb Alloy

TiAl alloy has high specific strength,high melting point,high modulus and good corrosion resistance,but its application is limited due to its low plasticity at room temperature and difficulty in processing.The rapid solidification technology can prepare Ti-46.5Al-2.15Cr-1.90 Nb alloy powders with refined grain size and homogeneous microstructure.In this study,the samples are prepared by spark plasma sintering.Through the study of microstructure and mechanical properties of alloys with different sintering parameters,the densification mechanism is analyzed.The constisutive equation and processing map of the alloy are established based on thermal deformation experiments of the alloys fabricated using optimum technological parameters.Furthermore,the microstructure evolution of deformed samples is also investigated to determine the suitable hot deformation process window.The microstructure of gas atomized powders is metastable ?2 phase and the cooling rate of powders is higher than 104K/s.With the increasing of cooling rate,the microstructure of powders transforms from dendric crystal to cellular crystal.The eutectoid temperature and ? phase transformation temperature are determined as about 1250 oC and 1310 oC respectively via DSC analysis.Ti-46.5Al-2.15Cr-1.90 Nb alloys have been prepared by spark plasma sintering of metallic powders.With the increasing of sintering temperature,the microstructure evolves from initial non-equilibrium microstructure to near ?,duplex and fully lamellar.Under sintering pressure 50 MPa,when the sintering temperature is above 1150 ?,the density of alloy reaches maximum value.The optimal sintering parameters of the sample with best comprehensive mechanical properties are as follows: the sintering temperature 1150 ?,sintering pressure 50 MPa,sintering time 7 min and the atomized powders as raw material,and its maximum compressive stress at room temperature is 1895.6MPa,the value of plastic deformation is 32.59%.The main factors affecting the densification are temperature and pressure.When the sintering temperature is relatively lower(<1100 ?),the densification of alloy is mainly caused by the plastic deformation under sintering pressure,because deformation stress reduces with rising the temperature.When the sintering temperature becomes higher(>1100?),the density is improved by both the plastic deformation under pressure and the atom diffusion at high temperature.Meanwhile,the atom diffusion can also help to further fill the boundaries between the powders.According to high temperature compression experiments,Ti-46.5Al-2.15Cr-1.90 Nb alloy exhibits strain rate and temperature sensitive feature,i.e.the flow stress decreases with the increasing of deformation temperature and decreasing of deformation rate.Moreover,the hot deformation activation energy is calculated to be Q=414.514 k J/mol based on the stress-strain curve,and the high-temperature constitutive equation is also obtained.Ti-46.5Al-2.15Cr-1.90 Nb alloy processing map is further achieved by considering the instability criterion,and the suitable hot working parameters are as following: the strain rate is not higher than 0.01/s,and the temperature is higher than 1150?.In addition,when the deformation temperature is about 1200?,the microstructure of deformed sample is uniform with fine grains.As to comprehensive analysis of the dynamic recrystallization GOS and ln Z values,the results show that: with the decreasing of ln Z,namely increasing the deformation temperature and reducing the deformation rate,the recrystallization rate of increases initially and then decreases.When the deformation rate is 0.01/s,the dynamic recrystallization at lower temperature(1100?)is insufficient,and thus the microstructure is heterogeneous.When the deformation temperature increases to 1250?,duplex microstructure with fine grains can be obtained.For the deformation rate of 0.001/s,duplex microstructure with fine grains can be obtained through dynamic recrystallization at relatively low temperature(1100?).However,when the deformation temperature increases to 1250?,the recrystallization rate of reduces,and the abnormal growing up of grains can be observed.Therefore,the hot deformation is more appropriate to be performed in temperature range of 1200?~1250? and at deformation rate of 0.01/s.