The influence of Cr and Si additions on the α→γ continuous cooling transformation kinetics of Ti45Al (all in at.%) has been studied using real time resistivity-temperature-time measurement system, optical microscope, scanning electron microscope (back scattered electron image mode), X-ray diffractometer, and transmission electron microscope. In Ti45Al2Cr alloy, the lamellar reaction predominates the α→γ transformation mode at low cooling rates (below 100℃/s). At high cooling rates (above 100℃/s), the α→γ phase transformation is suppressed by $α→α_2$ ordering reaction. Cr addition lowers the temperature of lamellar formation, but little affects the range of cooling rate over which the lamellar reaction occurs. In Ti45Al1Si alloy, Ti5Si3 phase starts to form just before the start of $α→γ_L$ lamellar reaction. The lamellar formation temperaure is distinctively higher than that of Cr containing Ti452Cr alloy before the $α→α_2$ ordering reaction suppresses the lamellar reaction. The lamellar reaction persists to occur up to much faster cooling rates (180℃/s). In Ti45Al2Cr1Si alloy, the $α→γ_L$ lamellar reaction kinetics is very similar to that in Ti45Al1Si alloy. The only difference is the fact that a small amount of massive reaction occurs. This is because Cr and Si significantly shift the γ phase field to lower Al content. High temperature β phase precipitates at α grain boundary on heating Ti45Al2Cr1Si alloy above 1390℃ and can survive during cooling at sufficiently fast cooling rates on α grain boundary, which can refine the α grain size.
The $α→α_2$ ordering start temperature in Ti45Al2Cr alloy has been measured to be nearly constant of about 1030℃ regardless of cooling rates, which is close to that expected from the extension of α/$α_2$ phase boundary. The antiphase domain size of Cr containing alloy was distinctively larger than in Si containing alloy. The ordering temperature was however not possible to measure in Si containing alloys, although the same ordering reaction also occurs in these alloys. The $α→α_2$ ordering reaction occurred up to extremely high cooling rate of 2860℃ in Ti45Al2Cr1Si alloy.
Si addition shifts the nose of CCT curve toward high temperature and shorter time as compared with the Cr addition. That is, the addition of Si accelerates the $α→γ_L$ lamellar transformation kinetics as compared with Cr. This is because $Ti_5Si_3$ precipitated in Si contained alloy acts as a strong nucleation site for γ lamellar plates. The lamellar spacing of the Si containing alloy is much thicker than that of the Cr containing alloy. This is because the lamellar nucleation rate is significantly low in Si containing alloy because of little number of nucleation sites. These two results indicate the lamellar growth rate is much fasten in Si containing alloys. This suggests that the addition of Si has an effect of increase the interdiffusion coefficient in α phase. In Ti45Al2Cr1Si alloy, the lamellar spacing of the alloy is distinctively finer than in Ti45Al1Si alloy despite of the fact that the start temperature of the lamellar reaction is very similar in both alloys. This strongly suggests Cr addition to Ti45Al1Si alloy has an effect to enhance the lamellar nucleation rate and that is probably because the α/γ interfacial energy in Ti45Al1Si alloy reduces as a result of Cr addition.
The influence of small amount of Si addition on the phase equilibria of Ti(40-51)Al2Cr (all in at.%) alloys at temperatures between 1200℃ and 1330℃ has been studied using optical microscope, scanning electron microscope (back scattered electron image mode), X-ray diffractometer, and differential scanning calorimetry. $Ti_5Si_3$ phase was present at all temperatures and compositions investigated. The addition of Si has an effect to stabilize the β and γ phases in the presence of Cr. Thus the phase transformation sequence of Ti45Al2Cr alloy during cooling from 1330℃ alters from α→α+γ to α→α+γ→α+β+γ→β+γ as 1at.%Si is added. In addition, Si addition raises $T_α$ temperature and extends the γ phase field toward lower Al composition, thereby enlarging the γ phase field in both the composition and temperature scale. This is consistent with the observation that Si atom substitutes Al atom in γ phase. This substitution leads to an increase of c/a ratio from 1.0116 to 1.0133 despite the fact that the atomic size of Si is smaller than that of Al. A pseudo-binary phase diagram of Ti(40-51)Al2Cr1Si alloys has been experimentally determined at temperatures lower than 1330℃ from the present investigation.
$Ti_5Si_3$ phase forms in two different formation mechanisms. Coarse $Ti_5Si_3$ particles precipitate at the interface of dissolving high temperature β phase during cooling. Fine Ti5Si3 particles precipitate at the $α_2$/γ lamellar boundary during aging.
The influences of alloying elements and Al composition on the mechanical properties have been studied using tensile testing, optical microscope, and scanning electron microscope in back scattered electron image mode. In Ti45Al2Cr(0-2)Si alloys heat treated at 1200℃, the elongation tends to be maximum at 1at.%Si. In Ti45Al2Cr(0-1)1Si alloy heat treated at 1400℃ for fully lamellar structure, Si enhances its ductility because the high temperature β phase has an effect to refine the lamellar grain size. In Ti45A2Cr1Si(0-2.5)B alloys, the addition of B results in the refinement of microstructure due to the presence of primary titanium borides. The boride morphology changes from lacy type at 0.4at.%B to needle or plate type at 1.5at.%B. The elongation is low 0.4at.%B addition because $Ti_5Si_3$ particles forms at lacy type boride. In the case of Si containing alloy, optimum mechanical properties was observed at Al composition of 45at.%Al because of the shift of γ phase field toward Al lean side.
