High punty Ti-(35-58)at.% alloy buttons were prepared by melting and casting in a vacuum arc furnace under Ar almosphere. The as-cast microstructures of near-top surface have been studied as a function of composition using optical microscope, scanning election microscope, and transmission electron microscope. The transition compositions of primary dendrite from β to α phase and from α to r phase were determined to be 49-50%at.%Al and 55-56at.%Al respectively. The two periectic compositions for β +L→α_0 and for α+L → r_0 were determined to be ∼47at.%Al respectively. The dendrite cell size was observed to be minimum at 46-47at.%Al. The mode of solid state transformation of primary β to α phase during cooling continuously varied from the massive transformation to the precipitation of Widmanstatten α plates and of α grains as the composition increased from 35at.%Al to 49at.%Al. Onthe other hand, the mode of transformation of primary α(or interdendritic α) to r phase varied from the precipitation(as a lamellar structure) to the massive transformation, as the Al conent increased form 44at.%Al to 55at.%Al. This variation of transformation modes could be well explained in terms of the variation of the driving force and atomic mobility as a function of composition.
Microstructural characterization and mechanical testing has been also performed to clarify the effects of the Al content and heat treatment temperature on the microstructural evolution and mechanical properties. The heat-treated cast structure as a function of Al content varied from the fully lamellar to near lamellar, near gamma, and single r microstructure as the composition increased from 42at.% to 55at.%Al. On the other hand, the heat-treated cast structure of a Ti-47at.%Al alloy as a function of heat treatment temperature varied from the duplex to near gamma, duplex, near lamellar, and fully lamellar microstructure as the temperature increased from 1000℃ to 1380℃. The tensile ductility was primary affectea by overall grain size and volume fraction of $α_2$ phase. The lamellar-r grain boundaries are stronger obstacies to dislacation motion than r-r grain boundarie, but are weaker obstacles than lamellar grain boundaries in the two-phase TiAl alloy. Hall-Petch slope constant of lamellar grain boundaries evaluated to be ∼2.45amPa√m . The tersile yield strength results calculated in this study suggest that a two-phase Ti-Al alloy having higher lamellar volume fraction and refined overall grain size leads to a higher yield stress.
The growth kinetics of secondary cells in Ti-(40-46)at.%Al and Ti-44at.%Al-0.5at.%Cr intermetalic compounds have been studied using optical microscope, scanning electron microscope, high temperature X-ray diffractometer and transmission electron microscope. The evolution of cell sizes of the same boundaries hsa been moniterd by interupting the aging treatment for each measurement. The sizes of secondary cells increase lineariy with the aging time up to a considerable stage of at temperature below 1050-1100℃. The primary intercelluar spacing and the volume fraction of phase establish a nearly constant value at each temperature of aging. The dependence of undercooling on the intercellular spacing was observed to be -1.16 and -0.61(Ti-44Al), -1.14 and -0.67(Ti-44Al-0.5Cr) respectively, for both the primary and secondary cells. The primary and secondary intercellular spacings as a function of volume fraction of r phase have been showed to be minimum at ∼50%. Whereas the growth rate of secondary cells has been showed to be maximum at ∼50%. The growth kinetics of secondary cells was calculated for Fournelle's model. Then, activation energies have been estimated to be ∼312.7(Ti-44Al), ∼314.4kJ/mole(Ti-44Al-0.5Cr) respectively. However, these values appear to be excessively high for grain boundary diffusion. The activation energy also estimated from the modified Fourmelle's model is based on the assumption that driving force for the secondary cell growth comes mainly from the strain energy within the primary cells. Then, these values have been estimated to be ∼226.2(Ti-44Al), ∼229.7kJ/mole(Ti-44Al-0.5Cr) respectively, consistent with that of grain boundary diffusion. The measured growth rates of secondary cells as a function of volume fraction of r phase were consistant linearly with the calculated values based on the strain energy.
A simple miniaturized disc bend test(MDBT) apparatus has been constructed and tested. The results on speciments of homogeneous materials were reproducible and in good agreement with those from uniaxial tensile tests. The dependence of yield stress on the volume fraction of r phase obeys the typical rule of mixture. The interlamellar spacing, ordered domain size, and thickness of grain boundary r-layer in the lamellar structure decreased with an increase in the cooling rate. Hall-Petch relations in the dependence of the yield stress on the grain size were determined to be 6.31(1℃/min), 4.83910℃/min), and 4.72MPa√m(30℃/min) respectively, with an increase in cooling rate. The yield stress as a function of the ordered domain size clearly reveals the following relationship : $o_y=60.4+0.16d^-1/2$, consistent with the results obtained by the easy mode deformation of PST crystals. These results imply that slips of polycrystalines occur parallel to the lamellarboundaries in the lamellar grains favorably oriented for easy mode deformation.
The effects of B addition on the as-cast and heat-treated structures of Ti-(35-55)Al-2Cr-(2-4)Nb alloys have been studied by microstructural observation. X-ray diffractometer. The addition of B results in a refinement of microstructures of the as-cast structures due to the presence of primary titanium borides. This has an effect to increase the volume fraction of interdendritic r phase in the as-cast structure due to its Ti depletion effect in overall composition. This also gives rise to formation of primary α dendrite instead of that of primary β dendrite at larger B content. The boride morphology change from lacy type at a low B concentration to needle or plate type at ∼2at.%B. Finally, blocky type borides appear at B content larger than ∼ 3at.%. The lamellar grain size of as-cast struvture greatly varies with both the Al and the B contents and it tends to have discreate changes at near 46-47at.%Al and 1.0at.%B respectively. B addition encourages the precipitation of Widmanstatten plates rather than massive β→$\alpha_m$ transfomation and results in the basketweave Widmanstatten structure. B addition also enhances the formationkinetics of r plates(as a lamellar structure) in as-cast structure.
The influence of elemental B addition on the heat-treated cast structure of Ti-47Al-2Cr-(2-4)Nb alloy has investigated using X-ray diffractometer, optical microscopy, scanning and transmission election microscopy, and tensile testing. The phase sequence β→β+α→α→α+r→α+β+r→β+r. The addition of (0-2at.%)B does not change the phase sequence. It however lends to stabilize α phase by shifting the (α+β+r) three-phase region towards a higher (Cr+Nb) content. The B addition does not significantly alter the equilibrium composition within (α+r) two-phase field. Nonetheless it remarkedly accelerates the lamellar formation kinetics and enhances the thickening rate of r plates the lamellar formation kinetics and enhances the thickening rate of r plates. The misifit between α and r plates rather increase from 1.236 to 1.460%. The B addition also enhances the α/r interfacial energy. The accelerration of lamellar formation kinetics is thus belived to primarily due to th enhancement of chemical diffusivity of the alloy with the B addition. The B addition induces a significant refinem ofen heat-treated cast structure. This primarily due to the B effect to promote the appearance of interdendric r region. Futher refinement aries from then boride;s role to refine the dendrite and rain size and from the intrinsic B effect to enhance the chemical diffusivity and the r thickening rate.
The addition of small amount of B enhances both the strength and tensile ductility of near gamma structure. The strengthening arises from grain size refinement and from the boride dispersion. The calculation of fracture strain suggests that an enhancement of ductility for B addition(up to ∼0.2at.%) mostly due to its grain refinement effect. Hall-Petch slope constant in the dependence of the yield stress on the grain size for near gamma structure has been determined to be ∼0.5MPa√m. The yield stress as a function of inverse square root of interparticle spacing of boride in a Ti-47Al-2Cr-4Nb alloy clearly reveals Hall-Petch relations, whereas it shows abnomal behavior in a 0.4at.%B-containing alloy. The lamellar delamination of lamellar grains have been suppressed in the duplex and near lamellar structures of the alloys containing above ∼0.4at.%B. This implies that cohesiveness of r/r and $r/\alpha_2$ lamellar interfaces was strengthened by B addition.
The yield stress rose with the decreasw in the Al content and the tensile ductillity showed a maximum at ∼47at.%Al Ti-(43-51)Al-2Cr-4Nb-(0.4, 1.0B) alloys. Apparent Hall-Petch slope constant in the dependence of the yield stress on the grain size for fully lamellar structure has been determined to be 2.61MPa√m.
