Diffusion induced grain boundary migration and recrystallization in boron-doped Ni3Al intermetallics

Scripta METALLURGICA et MATERIALIA

Vol.

27, pp. 1235-1239, 1992 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

DIFFUSION INDUCED GRAIN BOUNDARY MIGRATION AND RECRYSTALLIZATION IN BORON-DOPED Ni3Al INTERMETALLICS C.Y. Ma, W. Gust and R.A. Fournelle* Max-Planck-Institut fiir Metallforschung and Institut ffir Metallkunde, Seestr. 75, D-7000 Stuttgart Germany and *Department of Mechanical and Industrial Engineering,Marquette University, Milwaukee, WI 53233, USA (Received July 13, 1992) (Revised September 8, 1992)

Introduction In the past two decades it has been found that in various alloy systems diffusion of a solute into or out of polycrystalline specimens can induce a migration of grain boundaries resulting in alloyed (solute enriched) or dealloyed (solute depleted) zones being left behind [1]. This phenomenon, which was first observed b y den Broeder in W-Cr diffusion couples [2], is called diffusion induced grain boundary migration (DIGM). Within the same time period it was also observed that new grains could nucleate and grow in a metal when an alloy element was diffused into it. This phenomenon, which was first reported by Li and Hillert [3] for the diffusion of Zn into Fe, is called diffusion induced recrystallization (DIR) and involves the growth of the new recrystallized grains by DIGM. Except for reports of DIGM in ternary MoNi-X alloys [4-6] and Ni-based superalloys [7] all studies of DIGM and DIR were performed on binary alloys [1]. No studies have yet been reported on whether or not DIGM and DIR can occur in intermetallic compounds, although it is known that DIGM can occur in various oxide ceramic systems. Therefore, one of the objectives of the present work was to determine if DIGM and DIR could occur in intermetallic compounds by evaluating what happens when Cu and Zn are diffused into B-doped polycrystalline Ni3A1. This alloy was selected because of its commercial interest and because certain conclusions about the nature of the driving force for DIGM and DIR could be drawn from the results.

Experimental Ni (99.97%), A1 (99.99%) and Ni-B master alloy weighing a total of 1 kg were melted in a vacuum induction furnace. After casting, the ingot was chemically analyzed by inductive-coupled plasma-optical emission spectroscopy (ICP-OES) and then cut into plates 4 mm thick. These plates were rolled and annealed at 1323 K for 2 h several times until a thickness of I mm was attained. This sheet was given a final anneal at 1323 K for 10 h in order to produce a grain size between 50 and 150 ~m. All recrystallized specimens were then mechanically ground and polished through 1 p.m diamond on both surfaces. After polishing, two diffusion treatments were employed to produce DIGM and DIR. One involved zincification; i.e., specimens were annealed at temperatures between 623 and 1073 K for 4 days in evacuated quartz glass capsules with a zinc source consisting of brass filings (Cu-30 wt.% Zn). The other involved a diffusion couple arrangement; i.e., specimens were electroplated with a thickness of 30 ~m Cu and then heat treated in evacuated quartz gIass capsules at temperatures between 773 and 1073 K for a period of 4 days. After these treatments, the microstructures of specimen cross sections were observed by light microscopy (LM) and scanning electron microscopy (SEM). The chemical compositions of the alloyed zones resulting from DIGM and DIR-were also examined using energy dispersive X-ray spectral analysis (EDX). Results As can be seen in Table 1 the aluminum and boron contents were very close to the desired values. After annealing the specimens electroplated with Cu, significant amounts of DIGM were observed to have occurred for those specimens annealed in the temperature range from 873 to 1073 K for 4 days. DIR occurred only for those specimens annealed at 1073 K, as shown in Table 2. Compared to the coated specimens the zincified specimens did not exhibit either DIGM or DIR for temperatures ranging from 623 to 1073 K. Figures l(a) and Co) show the DIGM and DIR developed on cross sections of the Cu coated specimens 1235 0956-716X/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press Ltd.

1236

DIGM

Vol.

27, No.

annealed at 973 and 1073 K for 4 days, respectively. Figure 1(c) shows the absence of these reactions in a zincified specimen. The Cu contents along profiles through DIGM and DIR zones for the Cu coated specimens obtained by EDX measurements are shown in Figures 2 and 3. The Cu concentration in the alloyed zones produced by DIGM ranged from 10 to 11 wt.%, while that near the migrating boundary was lower at about 5.0 wt.%. The average Cu content in the DIR zones was about the same as that of the DIGM zones; however, a p e a k value of 11.6wt.% Cu was observed near the migrating DIR boundary. In addition, it was noted that A1 content in both the DIGM and DIR zones remained the same as that of the untransformed matrix (24.8 + 0.5 at.%) indicating that the Cu atoms had displaced Ni atoms in these zones and implying that they had taken up Ni lattice sites in the crystal structure. As can been seen in Fig. 4, DIGM was found at depths up to 30 , m below the Cu coated surface. This is ossible because, as can been seen in Fig 5, Cu enetrated along grain boundaries to depths greater than 3 ~ ~m from the Cu source. " P TABLE 1 Chemical analysis obtained by ICP-OES.

A1 ( at.% )

B ( at.% )

Ni (at.%)

Nominal

24.0

0.24

Balance

Analyzed

23.9

0.22

Balance

TABLE 2 Conditions for which DIGM and DIR have been observed in Ni3A1 + B (Cu). Conditions 773 K 873 K 973 K 1073 K

96 h 96 h 96 h 96 h

DIGM

DIR

No Yes Yes Yes

No No No Yes

FIG. 1 Light micrographs showing DIGM and DIR in the Ni~AI(Cu) system after annealing for 4 days at 973 K (a) and 1073 K (b). No evidence of DIGM and DIR is observed under the same conditions in the NiaAI(Zn ) system (c).

9

Vol.

27, No.

9

DIGM

1237

Discussion

The results have clearly shown that both DIGM and DIR occur in B-doped Ni3A1 when Cu is diffused into it and that they do not when Zl~ is diffused into it. The question is why. In the twenty years since DIGM was first described by den Broeder a number of theories have been developed to explain how and why it occurs. Presently two of these theories, the coherency strain theory [8] and the dislocati'on climb theory [9] are the most widely accepted by workers in the field. The coherency strain theory assumes that the driving force for boundary migration during DIGM is the coherency strain developed ahead of the migrating boundary as the result of dlffusion of the solute species into it from the grain boundary. The dislocation climb theory assumes that the boundary migrates as the result of the climb

Cu

Ni3Al/'104111y j

s.oy.

/o.o

FIG. 2 Cross sectional view (a) and schematic diagram (b) of DIGM in the Ni3AI(Cu) system after annealing at 973 K for 4 days indicating points of EDX measurements of Cu content.

Cu

(,b)

Ni3A1

FIG, 3 Cross sectional view (a) and schematic diagram (b) showing DIR after annealing at 1073 K for 4 days. The results of EDX measurements of Cu content are indicated.

1238

"

DIGM

Vol.

27,

No.

of grain boundary dislocations due to the net flow of vacancies along the grain boundaries during solute diffusion. Only the first of these theories lends itself to the evaluation otthe results in this study and, as such, only this theory will be applied to the discussion of the present results. It is well known that the solubilities of both Zd ( 8 at.% ) and Cu ( 24 at.% ) [10] in Ni3A1 are great enough for volume diffusion of these elements to occur ahead of a boundary mi~ating by DIGM if the temperatures are high enough. Thus, it is conceivable that coherency strains could develop as the result of the diffusion of either Cu orZn. The question is whether or not the energies associated with these strains are great enough to cause the boundaries to migrate. One way by which one can qualitatively evaluate the effects of coherency strains is to compare the lattice parameters and atomic sizes of the elements involved. For example, the greater the atomic size mismafch between solute and solvent atoms, the greater the strain. With respect to Cu diffusion into Ni3AI it is known that the lattice parameter of Cu is about the same as that of Ni3AF [11] and that the Goldschmidt atomic radii for Cu and Ni (0.128 and 0.125 nm, respectively) are quite close to each other [12]. Thus, one would expect very little strain to develop ahead of a migrating boundary in Ni3AI as the result of Cu diffusion into it. Zn, on the other hand, has a Goldschmidt atomic radius of 0.137 nm [12], which is considerably greater than that of Ni. Thus, one would expect a high coherency strain for the diffusion of Zn into Ni3AI. Considering the above, one should expect DIGM to occur

FIG. 4 DIGM occurring at a position ( A and B ) 30 ~m away from the Cu source after annealing at 973 K for 4 days.

Cu

FIG. 5 Depth of Cu solute diffusion along the GBs in NisA1 (a) and schematic diagram Co). Chemical analysis results of Cu are indicated.

9

Vol.

27, No.

9

DIGM

1239

more readily for the case of Zn diffusion than for Cu diffusion, but the opposite is true. Thus, the fact that DIGM occurs for Cu diffusion into Ni3AI but not for Zn diffusion seems to contradict the prediction of the coherency strain theory. This leads one to consider whether some other driving forces, such as free energy of mixing or interface free energy [13-15], are causing grain boundaries to migrate. Could it be that the dislocation climb mechanism [9] or some other unknown atomistic mechanism involving individual atomic jumps across the migrating boundary is active in this system ? This should be the subject of further studies. While the above observation t)y itself does not disprove the coherency strain theory, it does indicate that other factors may also play a role in whether or not intermetallics like Ni3A1 exhibit DIGM or not. One of these factors may be related to the nature of the sublattice which the solute atom species occupy. It is known that Cu atoms substitute for Ni atoms in Ni3AI, while Zn atoms substitute for A1 atoms [10]. Perhaps it is just coincidence that DIGM occurred for the diffusion of Cu atoms which occupy Ni sites but not for the diffusion of Zn atoms which substitute for A1 atoms. In this context it is interesting to note that both DIGM and DIR are observed for both Cu and Zn diffusion into polycrystalline Ni [15,16]. In addition, DIGM has been also reported in the system of Al(Zn) by Tashiro and Purdy [17,18]. These facts may simply emphasize the differences in behavior between a binary alloy and an interrnetallic compound. Acknowledgements The authors are grateful to Mr. C. S. Liu from Chung Shan Institute of Science and Technology in Lung-Tan, Taiwan, for ingot melting. We also thank Miss S. K~ihnemann for SEM examination and useful discussions. References

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

A.H. King, Int. Mat. Rev. 32, 173 (1987). F.J.A. den Broeder, Acta metall. 20, 319 (1972). C. Li and M. Hillert, Acta metall. 29, 1949 (1981). Y.J. Baik and D.N. Yoon, Acta metall. 33, 1911 (1985). W.H. Rhee, Y.D. Song and D.N. Yoon, Acta metall. 35, 57 (1987). Y.J. Baik and D.N. Yoon, Acta metall. 35, 2265 (1987). B. Radhakrishnan and R.G. Thompson, Scripta metall, mater. 24,537 (1990). M. Hillert, Scripta metall. 17, 237 (1983). R.W. Balluffi and J.W. Cahn, Acta metall. 29, 493 (1981). S. Ochiai, Y, Oya and T. Suzuki, Acta metall. 32, 289 (1984). J.Mishima, S. Ochiai and T. Suzuki, Acta metall. 33, 1161 (1985). M. Hansen, Constitution of Binary Alloys, p. 1266, 1267, 1269, McGraw-Hill, New York (1958). R.A. Fournelle, Mater. Sci. Engng. A 138, 133 (1991). T.K. Chaki, Phil. Mag. A 62, 465 (1990). D. Liu, W.A. Miller and K.T. Aust, Acta metall. 37, 3367 (1989). R.A. Fournelle, B. Giakupian, W. Gust and B. Predel, J. Physique 49, C5-593 (1988). K. Tashiro and G.R. Purdy, Scripta metall. 17, 455 (1983). G.R. Purdy and K. Tashiro, in "Interface Migration and Control of Microstructure '%p.33, Am. Soc. Met., Metals Park, Ohio (1986).