Study On The Formation And Evolution Mechanism Of Modualted Structure In High Nb-Containing TiAl Alloy

As a promising structural material for technological applications at elevated temperature,TiAl alloys have attracted the extensive attention considering their low density,high yield and creep strength,and good oxidation resistance.Nevertheless,the brittleness and poor deformability at low temperature have significantly limited their practical applications.Great efforts have been made to improve the mechanical properties of TiAl alloys by tailoring the compositions and optimizing microstructures.The new generation TiAl alloys often contain high contents of Nb elements,and it has been recognized that high Nb-containing TiAl alloys possess higher yield strength,better creep property and oxidation resistance at high temperature than the conventional TiAl alloys.Recently,a novel in-situ modulated microstructure with tweed-like contrast has been broadly discovered in the high Nb-containing TiAl alloys with the base-line composition of Ti-(40-44)Al-(5-10)Nb(at.%).This novel microstructure is supposed to contribute to the advancement of room temperature ductility and high temperature strength.Therefore,fundamental understanding of the formation and evolution mechanism of modulated structure is the critical issue for further developing high performance high Nb-containing ?-TiAl alloys.This work mainly focuses on the formation and temporal evolution mechanism of the modulated structure inside high Nb-containing ?-TiAl alloys.The main findings are summarized as follows:The formation temperature of modulated microstructure has been firstly investigated through the experimental observations.The nano-scale modulated structure inside ?2 lamellae forms in the Ti-45Al-8.5Nb alloy at temperatures below 675?,but disappears at temperatures above 700?.The formation of the modulated structure can be prevented by air-quenching at high temperatures higher than 700?.TEM results combined with the simulated diffraction patterns indicate the modulated structure is comprised of ?2+O phases.The orthorhombic phase as a constituent of the modulated structure has the 01 structure with random occupancies of Ti and Nb atoms on the Wyckoff sites 8g and 4c2.There are six crystallographic equivalent O phase variants occurring in ?2 parent phase.The orientation relationships between the O and ?2 phase can be described accurately as(001)o//(0001)?2 and[110]o//[1120]?2.A phenomenological theory of martensitic transformation applies to the ?2 to O phase transformation,and the habit planes for the variants lying at?350}o//{1340}a2,which agrees well with those determined by TEM.Although the ternary Al-Ti-Nb thermodynamic phase diagram in the literature cannot predict the existence of the O phase within Ti-45Al-8.5Nb alloy,first-principles calculations of the free energies indicate that the Nb-rich O phase is thermodynamically stable in the Nb-lean ?2 phase in the alloy at 600?.It implies that the formation composition of O phase in ternary Al-Ti-Nb thermodynamic phase diagram needs to be further modified.HAADF images and EDS microanalyses clearly show an enrichment of Nb in O phase in comparison with the ?2 parent phase.The partitioning coefficient for Nb between the O and ?2 phase is about 2 at 600?.Based on the TEM evidences,the formation of the orthorhombic phase can be thought as a result of a phase separation reaction in the ?2 parent phase,which is dominated by a diffusion mechanism.The transformation from the initial ?2 to the O phase takes place through a mechanism of nucleation and growth.The diffusion coefficient of Nb inside ?2 phase derived from the growth kinetics of the O phase is about(1.3±0.2)×10-22 m2s-1.TEM observations also demonstrated that the amount of the O phase in the ?2 parent phase increases with an increase of annealing time at 600?.The morphology of the O phase variants changes from the thin plate-like shape with a low volume fraction at an initial annealing stage to the rectangle/square shape with a high volume fraction after a prolonged annealing.At the same time,the retransformed ?2-? emerges at the intersections of the O phase for minimization of their elastic interaction energy.The retransformed a2-II rotates around the[0001]axis by?3.4° to the ?2-? parent phase,leading to the formation of two orthogonal habit planes for the single O variant.Significant precipitation hardening effect of the O phase for the y/(a2+0)lamellar microstructure has been revealed by nano-indentation and micro-hardness measurements.The hardening effect of the O phase can be attributed mainly to significant refinement of the ?2 lamellae associated with elastic strain energy from the phase transformation.The compression tests of high Nb-containing y-TiAl alloys at room temperature show the great enhancement of compression strength caused by the modulated structure.TEM observations of deformation microstructure illustrate the significant effect caused by modulated structure on the mechanical behavior of y phase.Larger amounts of deformation twins occur in y phase that is sandwiched by modulated lamellae,while high density of dislocations exist inside y phase without modulated lamellae.In this work,the comprehensive investigations have been done to probe the formation and temporal evolution mechanism of the modulated structure,and the influence on mechanical properties caused by modulated structure has also been discussed.The research results offer the theoretical and practical knowledge of the micro structural optimization and mechanical properties enhancement of high Nb-containing TiAl alloys.