Effects Of Temperature On Microstructure Of TiAl And Ti₂AlNb Alloys Investigated By Phase Field Method

In order to meet the requirements of high propulsion ratio and low fuel consumption in the aerospace industry,TiAl alloys and Ti₂AlNb alloys have attracted much attention due to their excellent comprehensive properties,such as low density,oxidation resistance and considerable strength at high temperatures.It is a new generation of lightweight structural materials that are expected to replace some Nickel-based superalloys parts at 600?¹000?.However,due to the poor ambient temperature toughness of two alloys,it is difficult to obtain the ideal microstructure in the deformation and casting process.Therefore,it is important to understand the effects of temperature on microstructure of TiAl and Ti₂AlNb alloys.In this paper,the phase field method is used to simulate the phase transition of the solid phase transformation process of TiAl alloy and Ti₂AlNb alloy.And a comparative analysis with the experimental results reveals its principle,predicts and regulates the microstructure.It also provides new ideas for the phase change simulation of other intermetallic compounds.In the paper,the construction of the free energy of intermetallic compounds is studied.The gradient descent method is used to calculate the necessary parameters in the sub-lattice model—the atomic occupancy ratio.It avoids the complicated process of the first-principles and mathematical analysis methods,and can meet the accuracy requirements of subsequent calculations.Subsequently,when the calculated sub-lattice model is coupled into the phase field model,the potential well function of a quadratic term is be introduced to overcome the problem of the change of the order parameter balance value in other phase-field models.The mathematical error caused by the positive and negative Fourier transform is also reduced,and the whole calculation system is more stable,and the calculation result is more real and effective.Next,the three-dimensional phase-field model of?₂-?phase transition is established,which incorporates various thermodynamic parameters,kinetic parameters and crystallographic knowledge.This not only verifies the accuracy of the sub-lattice model,but also reproduces the formation and evolution process of lamellar structure in TiAl alloys.The results show that the elastic energy is the main reason for the formation of TiAl alloys lamellae.The larger the elastic energy,the flatter the shape of lamellar structure,and the elastic energy has different effects on the shape of the six variants of the?phase.In the temperature range of 1300⁹00 K,the thickness and grain number of TiAl alloys first increase and then decrease,and a large number of twins appear in TiAl alloys lamellae.Also,the thickness of the lamellar structure decreases with increasing cooling rate,which is consistent with the experiment.Finally,the sub-lattice model is used to describe the free energy of intermetallic compounds in Ti₂AlNb alloys.Then the phase-field equation of?₂-O phase transition process is constructed.The evolution of microstructure morphology at different temperatures is studied and the reliability of the sub-lattice model is verified.It is found that the three variants of Ti₂AlNb alloy exhibit a square or rectangular shape under the action of elastic energy.With the increase of temperature,800 K,900 K,1000 K,the number of crystal grains increases gradually,and the growth rate increases.And the grain size is much smaler.