Phase Field Simulation Of The Nucleation And Growth Of The Lamellar Microstructure In TiAl Alloys

Titanium aluminide(TiAl)based alloys have attracted a lot of attention in recent years as candidates for a variety of structural applications such as parts in aero or car engines.In particular,the two-phase alloys containing TiAl(γ)and Ti3Al(α2)having a fully lamellar microstructure exhibit excellent balanced properties,including low density,good oxidation resistance and high strength at elevated temperatures.But the lamellae formation mechanisms,especially the dynamic nucleation and growth process have not been well understood due to the complexity of the process and the difficulties in experimental characterization.The microstructure of TiAl alloys,such as lamellar thickness distribution,interface types,and the mechanical properties depend strongly on the nucleation and growth,therefore,the understanding of the nucleation and growth processes is crucial for the mechanical properties improvement through microstructure control.The nucleation and microstructure evolution during α/α’2→α2+γ transormation under different conditions and their dependence on elastic strain energies,interfacial energies,chemical driving forces and external applied stress have been simulated systematically using phase field method,the stress and interaction energy distributions of different lamellar configurations have been calculated.The following conclusions can be drawn:A single y disk can induces shear stress along opposite directions in the matrix,with two maxima of tensile and compressive stress in the matrix.The elastic interaction energy are determined by the configuration of the pre-existing variant,and the relative orientations and distance of the two interacting variants.The most negative elastic interaction energy exists between twin related variants due to their opposite shear tranformation strain.With the increases of elastic strain energy,the γ phase nucleation manner changes from independent to correlated and then to collective nucleation.The twin frequency in the lamellar formed increases with the increase of elastic strain energy.Correlated or collective nucleation induced by elastic strain energy minimization is the main reason for the high twin frequency.Increasing of twin boundary energy makes the embryo of the new neighboring twin variant less stable so that reduce the twin probability as the precipitates grow,therefore,the frequency of twin related variants decreases in the final microstructure.The depletion zone exists in front of the growing interface of the nucleus under lower temperature when the diffusion rate is low,but the effect on the twin frequency change is small.Twin frequency decreases with the increase of chemical driving force.Nucleation manner changes from collective to correlated and independent nucleation with the increase of chemical driving force.The nucleation manners obtained from simulations are in accordance with the results of theoretical calculation.Compression perpendicular or tension parallel to the basal plane is found to promote twin variant selection and reduce the overall nucleation rate,while isostatic pressing,tension perpendicular,compression and shear parallel to the basal plane does the opposite.Experimentally,elastic strain energy and interfacial energy may be altered by adjusting the lattice mismatch between coexisting phases by adding different alloying elements,while the chemical driving force depends on temperature and can be controlled by choosing an appropriate heat treatment temperature,therefore the properties of TiAl alloys can be improved by adjusting lamellar thickness and twin frequency through alloying element selection and heat treatment route design.