EFFECT OF THERMAL EXPOSURE AT 750 ℃ ON ROOM TEMPERATURE TENSILE DUCTILITY OF CAST TiAl ALLOY WITH DIRECTIONAL LAMELLAR MICROSTRUCTURE
ZHU Chunlei, LI Sheng, LI Haizhao, ZHANG Ji()
Beijing Key Laborary of Advanced High Temperature Materials, Central Iron and Steel Research Institute,Beijing 100081
Cite this article:
ZHU Chunlei, LI Sheng, LI Haizhao, ZHANG Ji. EFFECT OF THERMAL EXPOSURE AT 750 ℃ ON ROOM TEMPERATURE TENSILE DUCTILITY OF CAST TiAl ALLOY WITH DIRECTIONAL LAMELLAR MICROSTRUCTURE. Acta Metall Sin, 2014, 50(12): 1478-1484.
The effect of thermal exposure on room temperature tensile ductility of cast TiAl alloy with directional lamellar microstructure was evaluated at 750 ℃ for 48~300 h in atmosphere. By preloading, unloading, dye-penetrating followed by reloading until fracture for exposed samples, initiation and propagation behavior of the microcrack triggered by surface brittle layer was mainly analyzed in order to explain that the directional lamellar structure retains a better ductility at room temperature after thermal exposure. The results show that room temperature tensile ductility is still retained above 2.0% and 1.0% after exposure for 150 and 300 h at 750 ℃, respectively. The embrittlement of the directional lamellar microstructure caused by thermal exposure is much less than that of duplex microstructure and the other lamellar microstructures. At a stress of 430 MPa, the microcrack forms at the Al-depleted brittle layer and propagates into the substrate during subsequent loading. Just as the sharp notch, the microcrack can constrain the plastic deformation, which is the main mechanism of the brittlement for TiAl alloy by thermal exposure. The directional lamellar microstructure with the lamellae interface parallel to the substrate surface is obtained, which is good for restraining the micro-crack propagation into the substrate and retaining higher ductility at room temperature after thermal exposure.
Fig.1 Strength (a) and plastic elongation (b) of TiAl alloy with directional lamellar microstructure after exposure for different times at 750 ℃
Fig.2 Morphology of surface reaction layer of TiAl alloy with directional lamellar microstructure after exposure at 750 ℃ for 200 h
Distance from substrate / μm
O
Ti
Al
V
Cr
1
0
63.88
31.36
3.61
1.15
3
5.23
60.34
30.24
3.27
0.93
5
20.78
46.22
29.73
1.80
1.48
Table 1 EPMA results for Al-depleted layer after exposure at 750 ℃ for 200 h
Fig.3 Morphology of microcrack near fracture surface for TiAl alloy with directional lamellar microstructure exposed at 750 ℃ for 200 h (The arrow indicates the microcrack)
Fig.4 Morphologies of microcrack initiating at Al-depleted layer and running through surface reaction layer when unloaded at 430 MPa (a) and microcrack running through surface reaction layer followed by propagating into the substrate when unloaded at 450 MPa (b) of TiAl alloy
Fig.5 Morphology of microcrack propagating into the substrate unloaded at 500 MPa of TiAl alloy
Fig.6 Fractography of TiAl alloy unloaded at 500 MPa followed by dye-penetrating and reloading until fracture
Fig.7 Morphologies of microcrack deflecting along the lamellae interface (a) and ligament forming in the lamellar zone (b) with the lamellar interface parallel to the surface for TiAl alloy unloaded at 450 MPa
[1]
Lu M, Barrett J R, Kelly T J. In: Hemker K J, Dimiduk D M, Clemens H, Darolia R, Inui H, Larsen J M, Sikka V K, Thomas M, Whittenberger J D eds., Structural Intermetallics 2001, Warrendale PA: TMS, 2001: 225
[2]
Tetsui T. Mater Sci Eng, 2002; A329-331: 582
[3]
Kim Y W. Mater Sci Eng, 1995; A192-193: 519
[4]
Huang S C. In: Darolia R, Lewandowski J J, Liu C T, Martin P L, Miracle D B, Nathal M V eds., Structural Intermetallics 1993, Warrendale PA: TMS, 1993: 299
[5]
Dowling W E, Donlon W T. Scr Metall Mater, 1992; 27: 1663
[6]
Kelly T J, Austin C M, Fink P J, Schaeffer J. Scr Metall Mater, 1994; 30: 1105
[7]
Lee D S, Stucke M A, Dimiduk D M. Mater Sci Eng, 1995; A192-193: 824
[8]
Draper S L, Lerch B A, Locci I E, Shazly M, Prakash V. Intermetallics, 2005; 13: 1014
[9]
Pilone D, Felli F, Brotzu A. Intermetallics, 2013; 40: 131
[10]
Wu X H, Huang A, Hu D, Loretto M H. Intermetallics, 2009; 17: 540
[11]
Pather R, Mitten W A, Holdway P, Ubhi H S, Wisbey A, Brooks J W. Intermetallics, 2003; 11: 1015
[12]
Zhang J, Zhong Z Y. Mater China, 2010; 29(2): 9
(张 继, 仲增墉. 中国材料进展, 2010; 29(2): 9)
[13]
Zhu C L, Zhang X W, Li S, Zhang J. Rare Met Mater Eng, 2014; 43(9): 2124