State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083
Cite this article:
Liang YANG,Shubo GAO,Yanli WANG,Teng YE,Lin SONG,Junpin LIN. EFFECT OF Si ADDITION ON THE MICROSTRUCTURE AND ROOM TEMPERATURE TENSILE PROPERTIES OF HIGH Nb-TiAl ALLOY. Acta Metall Sin, 2015, 51(7): 859-865.
High Nb-TiAl alloys, which being regarded as a new generation TiAl alloy, had attracted more and more attention for their higher operating temperature and better oxidation resistance than conventional TiAl alloys. It was found that silicide particles in high Nb-TiAl alloys were Nb5Si3 rather than Ti5Si3 precipitated in TiAl alloys. In this work, the effect of Nb5Si3 phase on the microstructure and room-temperature tensile properties of high Nb-TiAl alloy was studied. The experimental results showed that the precipitation temperature of silicide was between 1000~1200 ℃. Precipitates located in the colony boundary, b(B2) segregation and between g/a2 lamella. The tensile properties of as-cast alloy with Si addition increased. Because the formation of Nb5Si3 precipitates resulted in the reduction of Nb content, which was one of b(B2) phase stable elements. Therefore, the volume fraction of b(B2) phase obviously decreased due to Si addition. However, after heat treatments, the tensile properties of Si containing high Nb-TiAl alloy gradually reduced with the increasing of heat treatment temperature. Silicide particles which precipitated along lamella leaded to generation and propagation of cracks. Moreover, silicide particles further precipitated due to tensile stress which increased the rate of crack propagation. Si addition leaded to g phase area expanded.g single-phase region formed between 1280~1300 ℃. Silicide precipitated in colony boundary resulted in bulkg+b(B2) phases, which weaken the grain boundaries. While silicide precipitated in lamella leaded to formation of secondary g lath which split the initial lamella microstructure.
Table 1 Processes of heat treatments for as-cast Ti-45Al-8Nb-2Mn-0.5Si (US) alloy
Fig.1 SEM-BSE images (a, b) and EBSD images (c, d) of Ti-45Al-8Nb-2Mn (UM) alloy (a, c) and US alloy (b, d) (Inset in Fig.1b shows the high magnified image of rectangular area)
Alloy
g
a2
b(B2)
e
UM
94.6
0.827
4.54
-
US
97.7
0.026
0.56
1.665
Table 2 Phase compositions of UM and US alloys
Fig.2 SEM-BSE images of sample HT1 (a), HT2 (b), HT3 (c) and HT4 (d) in US alloy (Inset in Fig.2b shows the high magnified image of rectangular area)
Fig.3 TEM images and SAED patterns (insets) of e2 phase (a) and e3 phase (b) in sample HT3, and HRTEM image of the e3/g interface (c) (The corresponding FFT image is shown in the lower right, the inverse FFT image of interface dislocation is shown in the upper right in Fig.3c)
Fig.4 Room temperature tensile properties of samples
Fig.5 The quasi-phase diagram of 8Nb-TiAl[25]
Fig.6 DSC curves of UM (a) and US (b) alloys
Fig.7 Secondary electron image (a) and EBSD image (b) of tensile fracture interface of sample HT2
Fig.8 SEM-BSE images of e1 precipitation in grain boundary (a), and e2 and e3 precipitations in lamella in sample HT4 (Inset in Fig.8a shows the high magnified image of rectangular area)
Fig.9 TEM image of cross lamella (rectangular area in Fig.8b) in sample HT4
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