Effects Of Cyclic Heat Treatment On Microstructures And Mechanical Properties Of TiAl-based Alloy

Through feasibility analyses, simulated tests and induction heat-treatments, the effects of rapid heating cyclic heat treatments on microstructures and mechanical properties of a TiAI-based alloy (Ti-33Al-3Cr, mass fraction , %) were systematically studied by means of optical microscopy. scanning electron microscopy. transmission electron microscopy. X-ray diffractometry and compression tests, and the following main conclusions are drawn.1)The L16(45) orthogonal cyclic heat treatment tests show that, according to different heat treatment conditions(heating rate vh=25?100 /s. holding temperature 0=1 310?I 340 , holding time t=2? mm, cooling rate vp20?160 /s. cycling number n1 ?), four kinds of microstructures can be obtained, i.e. typical lamellar structure. mixed microstructure of fine lamellae, relatively coarse lamellae and massive phase, fine nearly-lamellar microstructure, and massive microstructure. In the selected ranges of the above parameters, when the heating rate is lower than 25 /s and the holding temperature is lower than 1 310 or the cycling number is smaller than 3, only lamellar coarsening and formation of some new nuclei occur. The hardness (HV5) can be increased by rapid heating cyclic heat treatment, and the error range analysis shows that the effects of heating rate. holding temperature, holding time, cooling rate and cycling number on hardness, from large to small, are as follows: vh; t, 6, v~ and n, among which the effect of heating rate is by far the most evident.2)The optimized single factor test of heating rate shows that, rapidly heated at a rate of 100~? 200 /s ( ~=l 330 , t=2 mm, v~50-?80 C/s, n=:5). the hardness of the TiAl-based alloy can be increased by 30?0 HV5 compared to that of the as-cast alloy and the microstructure can be refined to be less than 50 I~ m.3)The inicrostructure of the thermo-mechanicallly treated TiAI alloy can be further refined by rapid heating cyclic heat treatment. For the two-step microstructure, with increasing cycling number, the lamellar structure is continuously refined. When the cycling number reaches 7, the colony size can be refined from about 50 i~ m to be lessIIthan 20 ii m.4)The microstructure of the hAl-based alloy with a colony size of about 50 I.~ m arida lamellar spacing of about 12X 1W2 j.A m can be obtained. The HV5 hardness can besubstantially increased and the highest hardness reaches 412 HV5. After aging at 1 000 C for 12?8 h, the hardness remains above 350 HV5, and when the aging time is longer than 12 h, the hardness almost keeps unchanged.5)The compression mechanical properties of the hAl-based alloy can also besubstantially enhanced by rapid heating cyclic induction heat treatment. The maximumvalues of yield strength. largest flow stress and compression ratio respectively reach806.6MPa, 1 740 MPa and 19.4%. The optimized mechanical properties are obtained byaging at 1 COOt for 24 h, and the corresponding values are respectively yield strength745.1MPa, largest flow stress 1 740 MPa and compression ratio 19.4%.6)The compression fracture of TiAl-based alloy at room temperature after cyclic induction heat treatment and aging is still typical cleavage and the crack propagations occur transgranularly.7)In the process of rapid heating and short-time holding cyclic heat treatment, the nucleations of fine lamellar microstructure mainly occur along the grain boundaries and also at the phase interfaces. The growth of new lamellar colonies can be described by the ledge mechanism.