Superplasticity of Directionally Solidified NiAl-15Cr Alloy at High Temperature

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JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2006, 13(4): 44-48

Superplasticity of Directionally Solidified NiAl-15Cr Alloy at High Temperature ZHANG Guang-ye'"

,

ZHANG Hua'

,

ZHANG Hou-an'

,

W U Xian-ming'

,

G U O Jian-ting'

(1. School of Electromechanism Engineering, Hunan University of Science and Technology, Xiangtan 411201, Hunan, 2. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, Liaoning, China) China; Abstract: Ni-25A1-15Cr (atomic percent, %) alloy was directionally solidified (DS) under argon atmosphere in an A12 O3-SO2 ceramic mold by standard Bridgman method. The microstructure of the as-fabricated alloy was studied using optical microscope, X-ray diffractometer, and scanning electron microscope. The alloy consisting of dendritic P NiAl phase, interdendritic 7/7' phase, and transient layer 7' phase, has been investigated, This alloy exhibits superplastic deformation behavior at 1 273- 1 373 K over an initial strain rate range of 8.35 X lo-' - 1.67 X lo-' s-l. The maximum elongation of 280% with strain rate sensitivity index m=O. 22 was obtained at the temperature of 1 323 K and an initial strain rate of 8. 35X s-'. Transmission electron microscopy (TEM) observations indicate that the superplastic deformation stems from the balance between high resistance (by dislocation sliding) and strain softening (by dynamic recovery and recrystallization). Key words: intermetallics; directional solidification; superplasticity ; microstructure

NiAl-based intermetallic alloys are potential materials for high temperature structural applications. However, both limited fracture toughness at room temperature and lack of creep strength at high temperature impede their practical use. While separate approach, either the increase of the high temperature strength or the low temperature toughness, has been investigated, only directional solidification shows promise to simultaneously improve both of these performances['-31. Ni-20A1-30Fe, which can be toughened by ductile phase, has been extensively studied because of its promising ductility at room temperatureC4-". The extruded Ni-20A1-30Fe alloy investigated by Guha S et alCS1exhibits an elongation of 20% at room temperature, and has an elongation more than 70% at 827 %.Unfortunately, the higher temperature tension had not been done. Recently, it is encouraging that Ni-20A1-30Fe has been found to exhibit superplasticity exceeding 470 %[", HOW-

ever, the high temperature strength of Ni-20A1-30Fe is rather poor. T h u s , a Ni-20A1-27Fe-3Nb alloy, in which the iron has been partially replaced by the element niobium, was selected and directionally solidified to improve the high temperature strength by solid solutioning or precipitation strengthening. There has been considerable interest devoted to superplastic deformation of various interrnetallics, as reviewed by Nieh T G et al"']. So far, the intermetallic alloys that possess superplasticity are basically equiaxial crystalline grain, most of which are fine crystalline grain, and some are large crystalline grain. T h e superplasticity of non-equaxial crystalline intermetallic alloys has not been reported except single crystal NiAl that exhibited a large tensile elongation of In the present investigation, the super170%["'. plastic behavior of DS NiAl-15Cr alloy was developed, and indeed it can possess superplastic elongation exceeding 280 %.

Foundation Item: Item Sponsored by National Natural Science Foundation of China (59895152) and National Advanced Materials Committee of China (863-715-005-0030) Biography:ZHANG Guang-ye(l974-), Male, Doctor, Lecturer! E-mail:zhangguangye7411@163. corns Revised DatelJanuary 4. 2005

Superplasticity of Directionally Solidified NiAl-15Cr Alloy a t High Temperature

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1 Experimental The Ni-25Al-15Cr alloy used in present study was prepared by induction melting and directionally solidifying in vacuum at a pulling rate of 5 mm/min, using a modified Bridgman method with the temperature gradient through the solid and liquid interfaces at about 80 K/cm (hereinafter the alloy will be denoted as DS NiAl-15Cr). Tensile specimens with a gauge section of 2 mm X 2. 5 mm X 10 mm were electro-discharge machined,from the ingots parallel to the growth direction. T h e tensile tests were conducted on a SHIMADZU AG-2500KNE test machine equipped with a furnace in air over initial strain rate range from 8.35X s-' to 1. 67 X lo-' s-' and at temperature from 1 273 K to 1 373 K. Specimens for microstructural observation were etched by FeC13( 5 g > HCI ( 15 mL CH, COOH ( 60 mL 1. Microstructures and fracture surfaces were examined by scanning electron microscope (JSM-6301F) and transmission electron microscopy (Philips T E M 420).

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Results and Discussion

The microstructures at transverse and longitudinal section of DS NiAl-15Cr alloy are shown in Fig. 1 ( a ) and (b>, respectively. The results show that this alloy exhibits dendritic structures. SEM and EDS observations show that the dendritic arms with darker contrast have a composition of Ni-36. 9A1-6. 9Cr (atomic percent, %) , which are surrounded by interdendritic region with lighter contrast having a composition

of Ni-12. 9Al-23.2Cr (atomic percent, %> and the transient layer with gray contrast having a composition of Ni-21. 8A1-12. OCr ( atomic percent, %). Further observation suggests that the microstructure of interdendritic region consists of dispersive fine ordered y' precipitations in a disordered y matrix, as shown in Fig. 1 (d). T E M selected area electron diffraction ( SAED > patterns from interdendritic regions show y' superlattice [Fig. 1 (el]. This kind of structure of relatively brittle dendritic arms surrounded by relatively ductile interdendritic region, which is analogous to the cladding during the extruding process, may be beneficial for obtaining good plasticity. T h e a-Cr precipitation in p phase has previously been reported in Ni-Al-Cr p sing'le phase alloys and multiphase /?based alloys, but four-phase equilibrium of a , p, y , and y' can exist only at invariant temperature for Ni-Al-Cr ternary system according to phase rule thermodynamic restrictions. Since chromium content in p phase is 6.9 % in atomic percent, exceeding the room-temperature solubility limit of chromium content in p phase which has been estimated to be at the level of approximately 2% in atomic percent, it is, therefore, considered that a-Cr phase precipitation from p phase of the alloy is not an equilibrium one, but that local equilibrium is reached in p phase. The DS NiAl-15Cr alloy indeed displays superplasticity in the temperature range of 1 273-1 373 K and at an initial strain rate ranging from 8. 35 X to 1. 67X lo-' s - ' . The macrograph of the fractured specimens

(a) SEM micrograph of transverse section; (b) SEM micrograph of longitudinal section; longitudinal section; ( d ) TEM centered dark field image from interdendritic region;

Fig. 1

Microstructures of DS NiAI-1SCr alloy

(c) High magnified SEM micrograph of ( e ) SAD of interdendritic region

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tested in tension at 1 323 K and strain rates ranging from 8.35 X to 1. 67 X lo-' s-' is shown in Fig. 2. Note that there is no obvious necking near the failure section. The plastic flow curves of the alloy are shown in Fig. 3, in which tensile tests are performed at an initial strain rate of 1. 67 X l o p 3 s-' Below 1 273 K , the true stress-strain curves reach rapidly a peak stress followed by stress decreasing gradually with increasing strain, where strain softening behavior occurs. At high temperature, such as 1 323 K and 1 373 K , the true stress-strain curves exhibit extensive steady flow behavior, indicating continuous plastic deformation up to large tensile strains to

.

original 1.67 x

s-l

8.35 x

s-'

1.67 x

s-'

8.35 x 10"

s-1

Fig. 2 Some fractured specimens after superplastic deformation at 1 323 K 350

250

150

50

f-

0 0

0.2

0.4

0.6

1.67 x

'.a

i!

0.8

1.0

1.2

1 323 K

s-'

b

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fracture. This situation suggests that strain hardening and strain softening is nearly balancing, which contributes to the obtaining of rather large elongation. Comparing with the flow behavior at 1 173 K , the effect of strain hardening is sufficiently large to postpone the occurrence of necking. Uan J Y et a1c123 believe that it is the strain softening resistance that governs the ductility. T h e relationship of the tensile elongation with temperature and strain rate for the alloy is shown in Table 1. A maximum elongation of 280% was obtained a t the temperature of 1 323 K and an initial strain rate of 8.35 X s-'. An interesting appearance is the relationship between temperature and initial strain rate for obtaining the largest elongation at each testing temperature. T h e fastest strain rate (1.67 X lo-* s-' ) corresponds to the highest temperature ( 1 373 K), and the slowest strain rate (8.35 X l o p 3 s-' corresponds to the lowest temperature ( 1 273 K ) . Under the investigating conditions, it seems that the optimum condition of obtaining superplasticity is obtained at 1 323 K and an intermediate strain rate. The strain rate sensitivity index ( m ), which is traditionally applied to evaluate the capability of the superplastic deformation of the polycrystalline alloys, can be obtained from the equation of u=Kim (a is true flow peak stress; iis strain rate; and K is constant incorporating structure and temperature dependence). It is generally believed that the superplastic deformation in the polycrystalline alloys occurs when the strain rate sensitivity index is close to or more than 0. 3. Using the Backofen's method of strain rate sudden ~ a r i a t i o n " ~ ' ,a value of m was obtained in this experiment. With increasing temperature, the m value increases. At 1 232 K , the m value reaches its maximum value (m=O. 2 2 ) , and at 1 373 K , it slightly decreases. And the maximum elongation of 280% with m value of 0.22 is obtained at the strain rate of 8.35 X l o p 3 s-' and at 1 323 K , which is consistent with that law. Comparing with the general suTable 1 Dependence of elongation on temperature and strain rate Temperature/

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

True strain

Fig. 3 Tensile true s t r e t r u e strain curves of NiAI-15Cr alloy at various temperatures and strain rates

Elongation/% 1. 67 X lop3 s-l 8 . 3 5 X l o r 3 s-'

1.67X

lo-'

1273

91

52. 2

1323

134

280

153

1373

151

159

232

s-'

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Superplasticity of Directionally Solidified NiAl-15Cr Alloy at High Temperature

47

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perplastic deformation of polycrystalline , the value of m is smaller; the similar phenomena is present in single-crystalline NiAl approximately at m=O. 17. The high temperature deformation behavior can be described according to the following classical equation, ;=Aa”exp(-Q/RT) ( n is stress exponent; Q is activation energy; R is universal gas constant; T is absolute temperature; A is constant). At given temperature, initial strain rate, and true flow peak stress, Q value is calculated to be approximately 514 kJ/mol, which is dramatically larger than that of lattice diffusion in NiAl (220-300 kJ/mol) and that of creep in NiAl (250-300 k J / m ~ l ) [ ’ ~ ’This . value corresponds to those for creep deformation of a NiA1-28Cr-6M0”~~ alloy. In general, two types of superplasticity have been reported: “structural superplasticity” observed in polycrystalline materials having fine equiaxial , and “internal stress superplasticity” observed in metal matrix composites which are subjected to a cyclic temperature changec17’. It is seemed that the superplastic deformation mechanism of the present alloy could not belong to either of these two superplastic deformation mechanisms. T h e T E M observations of the sample tested at 1 323 K and an initial strain rate of 8. 35 X l o p 3 s-l and interrupted

Fig. 4

a t strain of 10% and 60% , which corresponds to the steady state flow of the stress-strain curves, are shown in Fig. 4. At the strain of 0. 1, distinctive subgrains and new small grains free of dislocations [Fig. 4 ( a ) ] can be observed, which indicated that dynamic recovery and recrystallization occur. In addition, the chromium particles block the movement of dislocations, as shown in Fig. 4 ( b ) . As the strain increased to 60% [Fig. 4 ( c ) and ( d ) ] , the density of dislocations in the grains decreased and that at the grain boundaries increased. It is noted that, in the present study, dynamic recrystallization occurred at the early stage of deformation, while, in superplastic binary NiAl, the process begins at a relatively large strainC’*’. T h e difference may come from the effect of chromium particles in NiAl dendritic arms. A s shown in Fig. 4 ( b ) , the chromium particles block the dislocations movement; therefore, the trend of recovery was reduced and the trend of recrystallization was enhanced. There were large dislocation densities in the P and y / y ‘ phases, with higher density of dislocations in the y / y ’ phase than that in the P phase. A very large density of dislocation networks and subboundaries, which intersect with two slipping directions near the phase boundary of the dendritic arm and the interdendritic

( a ) , (b) E = O . 1; ( c ) , (d) E = O . 6 TEM image of alloy tested at 1 323 K and 8.35 X

s-’

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region, are also present in both phases. The interaction of a high density of dislocation may contribute to the strain hardening, which can provide resistance t o strain softening and then retard necking. A t the same time, dynamic recrystallization occurs in both ,8 and y / y ' phases, which contributed t o the strain uftening. Guo J T et alc191investigated the superplasticity of NiAl intermetallic alloy macroalloyed with iron, in which the recrystallization occurs only in ,8 phase. It is the balance between the strain hardening and the strain softening that is beneficial to the superplasticity of the present alloy. This is also coniirmed from the observation of Fig. 3, in which a steady state exists in the stress-strain curves. An anomalously large elongation almost like superplascic deformation (about 170%) of Stoichiometric NiAl single crystal was reported by Tagasugi T et a1 over a very limited temperature range of 0.35 -0.40 T, (where T, is melting temperature)c111,in which the glide motion and the climb motion of < 100 > dislocation were evenly dominant and thereby contributed t o the large tensile elongation. T h u s , that is nearly analogous to our results.

3

Conclusions

(1) The alloy is composed of p N i A l dendritic arms, y interdendritic region, and transient phase y' as well as fine a-C within the p N i A l dendritic arms. ( 2 ) Superplasticity has been observed in the I X NiAl-15Cr alloy a t the temperatures from 1 273 K to 1 373 K and initial strain rates ranging from 8. 35 X l o p 4 s-' to 1. 67 X lo-' s - I . A maximum elongation of 280% with the strain rate sensitivity index m = 0. 22 is obtained at 1 323 K and an initial strain rate of 8.35X 10-3 s-'. ( 3 ) T h e superplastic deformation mechanism of DS NiAl-15Cr is due t o the balance between high resistance (by dislocation sliding) and strain softening (by dynamic recrystallizaton). References : [l]

[2]

Noebe R D, Bowman R R , Nathal M V. Physics and Mechanical Properties of the B2 Compound NiAl [J]. International Materials Reviews. 1993, 38: 193-201. Johnson D R , Chen X-F, Oliver B F , et al. Processing and M e chanical Properties of In-Situ Composites From the NiAl-Cr and the NiAl-Cr(Mo) Eutectic System [J]. Intermetallics. 1995,

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