| As a substitute for nickel-base superalloys, Nb containing TiAl alloys are candidate materials for new generation aero-engine application due to their low density, excellent high-temperature (HT) mechanical properties and good oxidation resistance. However, the poor room-temperature (RT) ductility and processibility limit their industrial applications. The directional solidification (DS) technique is developed to control the lamellar microstructure orientation in TiAl alloys for improving RT and HT mechanical properties, such as RT ductility and HT creep resistance, which has become the research focus in this field.The Bridgman DS experiments and the related thermodynamic simulation were performed in this research. In view of inadequate researches on controlling the lamellar microstructure orientation with seeding technique, this research presents a new way to control the orientation through directly modifying the solidification procedure based on the complete peritectic transformation (PT) method. The sample characteristics before starting DS and the influence on the subsequent directional growth were analyzed. Crystal growth, control of phase transformation process and extent of peritectic transformation (EPT), etc. were also discussed systematically. The influence of sample history, alloy composition and the DS conditions (temperature gradient G and withdrawal rate V) on crystal orientation of parimary phase, peritectic transition and EPT were obtained, which provides scientific and theoretical basis for controlling the lamellar microstructures of multiple P-solidifying TiAl-Nb alloys by the method of adjusting the solidification path. The mechanical properties of the directionally solidified alloys were also evaluated. Several conclusions and innovations can be drawn as follows.(1) Length of the mushy zone decreases but the β dendrite spacing in directional growth increases with the increase of thermal stabilization (TS) time. The P dendrite spacing is more homogeneous and its growth direction is more inclined parallel to the axial direction with the increase of TS time. Al solute concentration in the mushy zone is always lower than that in original as-cast alloys. TS treatment results in the redistribute of solute Al thus changes the phase constituent in the mushy zone. The mushy zone with the columnar β and a grains is easily produced after TS treatment on the alloys with microstructures of the directional dendrite segregation morphology before DS starting. An appropriate TS is necessary to produce the L+β+α region in the mushy zone, which is of great benefit to control DS microstructure of TiAl peritectic alloys.(2) The dendrite morphology changes from equiaxial to columnar, but the directional dendrite segregation almost keeps the continuity across the initial growth interface after single directional solidification (SDS) and double directional solidification (DDS) processes, respectively. Crystal orientations in the regions neighboring the initial growth interface have preferential modifications along the growth direction. Compared with the SDS, the length of the initial mushy zone significantly decreases, but the β dendrite is well-orientated in the mushy zone in the DDS. Crystal orientation can be well-controlled in DDS by modifying β growth morphology as well as the orientation in initial mushy zone of the two DS processes. The directional dendrite parallel to the growth direction and the preferred β orientation are considered as the main reasons for β seed crystallization.(3) The results by Thermo-Calc showed that the composition of the complete PT is around 46.5% Al for TiAl binary alloy: in the TiAl-Nb ternary alloy, the composition gradually shifts to high Nb content with the increase of solute Al; the complete PT still occurs at about 49% Al if the content of Nb is around 8%, and EPT distribution on both sides of complete PT band is symmetrical; the maximum of the temperature gap (ATp) of the PT occurs if the chemical compositions of Nb and Al range between 5-8% and 47-49%, respectively. Compared with the simulation results, it is predicted that the optimized EPT band (i.e. composition interval of complete PT) in the experimental observations shifts toward to low Al content with a offset of 1.5-2.0%. Meanwhile, the experimental composition band broadens with the increase of Nb content.(4) The values of λ1, λ2, R and d decrease as the V increases for a given CAl value. The values of λ1, λ2. and R increase with the increase of CAl value for a constant V, while the value of d increases and then decreases with the increase of CAl value for a given V. The average index for λ1 dependence on v is 0.29, which is in accordance with the previous experimental observations, and that for λ2 dependence on v is close to the previous experimental results. The growth rate exponents of R and d are lower than the values got in other alloy systems. The value of λ1 for a given x or v value in a constant G measured in present work was compared with the calculated values of λ2 from the Hunt, the Kurz-Fisher, the Trivedi, the Bouchard-Kirkaldy and the Lu-Hunt models. Besides the values of λ1 calculated with Lu-Hunt model which are lower than the present results, it is shown that the experimental results are mostly in a good agreement with the calculated values with the others models. The experimental values of λ2 are closer to those calculated with Trivedi-Somboonsuk model and the values of R obtained by experiment are slightly higher than the line of R calculated with Kurz-Fisher model. The average value of λ2/R for the experimental TiAl-Nb alloys is 1.44, which is lower than theoretical value.(5) Some microstructure characteristics including the grain size, lamellar orientation, morphorlogy, distribution and degree of microsegregation and so on vary dramatically for the different solidification modes, such as single β solidification(B), hypo-peritectic solidification(BA), hyper-peritectic solidification(BBA) and single a solidification(A). B mode alloys have a chemically homogeneous microstructure free of sharp textures and peritectic solidification leads to significant chemical inhomogeneity, which is virtually not removable. The transformation Lâ†'α occurs in the interdendritic regions with the BA mode while both Lâ†'α and Lâ†'γ coexist with BAA mode. With the increase of Nb from 2% to 8%, the typical colony size reduces by 1/3, and the size of primary dendrite also shows a trend of decline. While the volume fractions of both β-segregation and the interdendritic Al segregation gradually increase. The peritectic solidification alloys including the BA and BAA modes are of great interest because they have a potential for producing coarse columnar grains. With a further investigation to assess the influence of these two modes on lamellar structure, it was found that the alloys with a higher Al content in the BA mode could be considered as the most preferred alloy composition to form the well-aligned α2+γ lamellar structure. Because of the microsegregation, the colonies boundary changes from the core of primary β dendritics with a BA mode to the interdendritic regions with a BAA mode. For the alloys with a lower Al content in the BA mode, both the changes of lamellar spacing and the slight tilt of lamellar structures lead to the "bending" phenomenon, which would disappear with the increase of Al content in the alloy.(6) For the compression test, the values of both yield strength and compressive strength in longitudinal direction of samples are higher than that in transversal direction, while the maximum compression strain varies barely between two directions. The DS samples with the peritectic solidification mode exhibit tsuperior room temperature and elevated tensile properties and fracture toughness than any other alloy solidification modes. Peritectic solidification alloys, especially that with the hypo-peritectic compositions are the most preferred DS materials using to control lamellar orientation and to improve the mechanical properties. |
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