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Tensile properties and fracturecharacteristics of spray cast alloyIC6 (Ni-7.8Al-14Mo-0.05B)

*MI Guo-fa1, WANG Hong-wei2, TIAN Shi-fan3, LI Zhou3, ZENG Song-yan2

(1. Henan Polytechnic Institute, Jiaozuo 454000, China; 2. Harbin Institute of Technology, Harbin 150001, China; 3. Institute of Aeronautical

Materials, Beijing 100095, China)

Abstract: Tensile testing results of spray cast Ni3Al-based superalloy indicated that the yield strength and tensile

strength increase with the increasing of temperature and reached maximum at around 760 , then decrease withthe increasing of temperature. After high temperature isostatic pressing (HIP), yield strength decreased and ductilityand tensile strength increased. Stereographic projection showed that no matter at room temperature, mediumtemperature or high temperature, cracks extend along (111).

Keywords: spray casting; Ni3Al-based superalloy; tensile properties; fracture characteristics

CLC number: TG146 Document Code: A Article ID: 1672-6421(2007)01-010-03

Male, born in 1966, professor, Ph.D. Research interests: metalsolidification and new materials.E-mail: peter@hpu.edu.cn

Received date: 2006-07-10; Accepted date: 2006-11-15

*MI Guo-fa

studied by S-570 SEM. TEM specimens were cut axially fromrupture specimen and the thickness was about 200 µm. Theywere ground to a thickness of 50 µm, and then followed byjet polishing using a 15% chloric acid and 85% methanolsolution.

2 Results and discussion

2.1 Tensile property of spray cast alloy

Figure 1 shows tensile property of the spray cast alloy. It canbe seen that with the increase of experimental temperature theyield strength and tensile strength increase and they reachedmaximum at about 760 , then decrease with further increaseof the temperature. This result demonstrated that the spray castalloy has “R” characteristic. The “R” characteristic is atemperature reverse effectiveness and it is common to Ni

3Al

alloy. Figure 1 indicated that although the ductility of the alloydecreases with the increase of temperature, it still possesseshigh ductility at 900 . After high temperature isostatic pressing

( ) SC; ( ) SC+HIP (1 150 , 150 MPa, 2 h )

Superalloy castings have coarse grain and severe ingredientsegregation that lead to mechanical properties decrease.

Ni3Al-based superalloy of Ni-Al-Mo system is a casting

material with high potential. By directional solidification andsingle crystal growth, the stress rupture and creep propertiescan be improved [1, 2]. However, excellent mechanical propertiesdepend on the crystallographic direction. Although the coolingrate of spray casting is lower than that in powder productionprocess, the spray casting process can get rapidly solidifiedmicrostructure that possesses fine grains and little segregation,which will enhance mechanical properties. Also, the atomizationprocess can avoid severe contamination and reduce particleboundaries that exist in powder metallurgy process. The tensilestrength and ductility of spray cast materials may be higher thanthat of the powder metallurgy materials [3-5].

In this paper, through the mechanical testing of spray castNi

3Al-based superalloy of Ni-Al-Mo system the tensile and

fracture behaviors were studied by means of SEM (ScanningElectron Microscope) and TEM (Transmission ElectronMicroscope).

1 Experimental procedure

Mechanical properties were studied by Instron tensile machine.The experimental temperatures were 20 , 650 , 760 , 800and 870 . The displacement rate was 8.33 10-3mm/s.Fractography and dynamic tensile tested specimens were

Tensile properties of spray cast superalloytemperature

Fig. 1

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(HIP), the “R” characteristic retains, the yield strengthdecreases, and the tensile strength and ductility increases. Thedecrease of yield strength is due to the decreasing ofdeformation resistance. When the alloy was HIP treated itsdislocation features was adjusted. The increasing of tensilestrength is due to the incorporation of microporosities and theminimization of crack initiation.

a) Formation and primary development of cracks; b), c) Further development of cracks; d) Breakage

Fig. 2 SEM in-situ observation of the spray cast alloy

2.3 Morphology of fracture

Figure 3 to Fig. 5 show the fracture morphology of spray castIC6 alloy. Tensile fractures at different temperatures have a rangeof shear zone. The room temperature tensile fracture consistedof ductile nests with different size (Fig. 3). With the increase oftemperature, little planar zones occurred and different slip beltsexist in these little planar zones. Furthermore, the fine ductilenests can also be found in high temperature tensile fracture. Whenthe temperature is greater than 650 , secondary crack can beseen obviously (Fig. 4). With the increasing of temperature thenumber of secondary cracks increases significantly (Fig. 5). Thedifferent glide bands can be found on the facet, which coincideswith the reduction of toughness when temperature increases.

2.2 SEM observation on fracture behavior

Figure 2 shows the results of room temperature tensile deformationof spray cast IC6 alloy at different stages. It can be seen that withthe increase of deformation stress cracks were formed at grainboundaries and then quickly propagate. Ahead of the cracks, therewere a lot of dislocation lines. Cracks can propagate along grainboundaries and also propagate traversing grains.

2.4 Crystallographic characteristics of tensile fracture

Figure 6(a) shows the morphology of cracks near fracture zoneafter room temperature tensile fracture. It can be found that crackpropagates along vertical directions. From Fig. 6(b), it can be

seen that the crystal planes consisting cracks correspond to (110)and (111), and the crack propagation direction is (110). Fig. 6(c)is the computer simulation of the three dimension projection. Atroom temperature, the tensile crack turns to initiate along (111)plane octahedron slip. Also, at the temperature of 650 and870 , respectively, the crystal plane consisting crackscorresponds to (111) and the tensile crack also occur along (111).

d

50 µm

b

10 µm

c

10 µm

a

10 µm

a) Low magnification b) High magnification

Fractured surface of tensile tested bar atroom temperature

Fig. 3

a

50 µm

b

20 µm

a) Low magnification b) High magnification

Fractured surface of tensile tested bar at650

Fig. 4

b

10 µm

a

100 µm

a) Low magnification b) High magnification

Fractured surface of tensile tested bar at870

Fig. 5

a b

10 µm100 µm

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3 Conclusions

1) Yield strength and tensile strength increase with theincreasing of temperature and they reached maximum at around760 . After that, they decrease with the increasing oftemperature. After HIP treated, yield strength decreases, whileductility and tensile strength increase.

2) No matter at room temperature, medium temperature or hightemperature, crack turns to initiate along (111) plane.

(a) Bright field (BF)

(c) Computer simulated stereographic projection

Fig. 6 Crack induced by tensile deformation at 650

ZHAO Xi-hong, HAN Ya-fang, TAN Yong-ning, et al. Thedirectionally solidified Ni

3Al-based alloy IC6. Journal of Materials

Engineering, 1997(9): 13-14, 30. (in Chinese)

[1]

References

XIAO Cheng-bo, HAN Ya-fang, ZHANG Xing-an. Study ondirectionally solidified Ni3Al based high temperature structuralmaterial IC6A alloy. Journal of Materials Engineering, 2001(6):3-6. (in Chinese)MI Guo-fa, LI Zhou, TIAN Shi-fan, et al. Spray Formed Nickel-Base Superalloys. Acta Metallurgica Sinica (English letters),1996, 9(6): 523-529.MI Guo-fa, LI Zhou, TIAN Shi-fan, et al. Microstructure andmechanical properties of particulate reinforced matrix composite.Chinese Journal of Aeronautics, 1997, 10(1): 127-130. (inChinese)MI Guo-fa, TIAN Shi-fan, ZENG Song-yan, et al. The status andprospect of the metal spray atomization and depositiontechnique. Materials science & Engineering, 1996, 14(4): 8-13.(in Chinese)

[2]

[3]

[4]

[5]

a

c

(b) Electron diffraction pattern (EDP) with g = [112]

b