Study On The Solidified Microstructure Of TiAl Alloys With High ? Stabilizing Elements

Intermetallic ?-TiAl based aloys have attracted more attention,because of its low density,high specific strength,good resistance against oxidation and creep resistance.In recent years,General Electric have equipped the last stage of the low-pressure turbine of the so-called GEnX jet engine with cast TiAI blades.It heralds that lightweight TiAl alloys have bright future in aerospace high temperature structural parts.In the past decades,peritectic solidified cast ?-TiAl as 4822 alloys have been applied in turbine and automobile engines,while the alloys' strength and applied temperature is relatively low.In order to meet the needs of new aircraft engine structure parts,next generation of ?-TiAl based alloys must be developed.By stabilizing the ?-phase through alloying with Nb,Mo,W,Ta or other refractory elements,an improvement in properties can be achieved,particularly at strength and service temperature.But it is easier to form brittle phase ?0 or ? in final microstructure,which is not conducive to the mechanical properties of the new ?-TiAl based alloys.Therefore,it is very important to study the solidification microstructures and the phase transformation of this type of alloys with high ?-stable elements,which is of great significance for the development and application of high strength TiAl alloy parts.In the present paper,various compositions of TNM alloys(reinforced by high Nb and Mo elements)were designed to clarify the high ? stabilizing elements strengthening mechanism on the solidified microstructure.The main contents are as follows:(1)the effect of alloy composition on solidification microstructure;(2)the effect of cooling rate on solidification microstructure and final microstructure after heat treatments;(3)?-phase evolution mechanism under heat treatment and thermal exposure.Based on the above results,we hope to establish the relationship of alloy composition,heat treatment process and microstructure,so as to provide theoretical basis for the development of much casting y-TiAl alloys with higher applied temperature and strength.The main findings are summarized as below:the microstructure of ?-TiAl based alloys consist of ?2/? lamellar grains,equiaxed y grains,and a certain ?/B2 phase at grain boundary.? phase solidified alloys can be achieved when the Al content is much lower.The results show that Al content was 45.5 at%,a peritectic solidification pathway L+???+L will occur,corresponding solidification microstructure is coarse-grained columnar microstructure with significant component segregation.The method of addition of B element is able to produce cast microstructures with significantly refined colony sizes.With the increase of B content,boride morphology changed from needle-like to lamellar.B element can effectively refine the lamellar grains and reduced the degree of segregation.The cooling rate has a great influence on the solidification microstructure.When the cooling rate is slow,it will be present coarse lamellar grains and higher volume content residual? phase with large size,because of slight super-cooling and a small amount of nucleation of ?phase.When the cooling rate is high,it is just the opposite.The result indicated that the content of ? phase can be adjusted by the subsequent heat treatment,under the heat exposure condition,the corresponding microstrcture is relatively stable,the grain size and the content change of phase is much smaller.