Microdendritic precipitation in the Ni-Ni3Si and Ni-Ni3Al systems

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Journal of Crystal Growth 58 (1982)522—526 North—I--Iolland Publishing (‘ompau~

MICRODENDRITIC PRECIPITATION IN THE Ni-Ni 3Si AND Ni-Ni3 Al SYSTEMS G. Scott GARDINER, John HUMMEL, All RAHNEMA. Mary A. RUGGIERO and John W. RUTTER Department oJ 1t4etaIIurg~iand Materials Science, (ni tersitr

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Joronto, Toronto, Canada M55 /714

Received 24 February 1982

During a study of Ni — Ni 5Si 2 eutectic specimens, it v.as observed that one of the eutectic phases contained another phase that svas present in the form of very small dendrites. These “microdendrites” were shown to originate from a solid state precipitation reaction. A similar morphology was also observed for alloys in the Ni—Ni~Alsystem. It is postulated that the dendritic precipitate morphologs is most likely to occur in systems in which the precipitating phase and the matrix phase have closely similar crystal structures and similar lattice parameters.

1. Introduction During a study of the nhicrostructure of Ni Ni~Si.,eutectic alloys solidified from undercooled melts, it was observed that one of the eutectic phases contained another finely dispersed phase. When the matrix phase was preferentially removed and the sample viewed using the scanning electron microscope, the finely dispersed phase was found to have a dendritic morphology. The dendrites were a few p.m in length and so the term “microdendrites” was coined to describe them. The purpose of this work was to determine the origin of these microdendrites.

2. Background The observation of a solid state dendritic reaction has been reported tn several copper based alloys [1—31.MehI and Marzke [11 were the first to observe that isothermal heat treatments in the alloy Zn—47.8wt%Cu produced gamma-phase precipitates which had a “starlike” form in the betaphase matrix. They observed that the orientation of the “stars” had a definite relationship with the parent lattice. More recently, Malcolm and Purdy [2] have used the transmission electron microscope to study

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the morphology and crystallographic ortentatton of dendritic precipitates in an alloy composed of 53.5 wt% Cu, 42.4 wt% Zn. and 4.1 wt% Sn. They found that the dendritic precipitates and the matrix phase were crystallographically aligned along their cube axes. Furthermore, precipitates formed at grain houndartes were found to grow wtth a non-planar interface into a neighbouring gamma phase grain that had the same orientation as the beta phase, while growth into the other neighbouring beta phase grain, not so ortented,occurred wtth a planar interface. Malcolm and Purdy concluded that the observed dendritic morphology resulted from a reaction that was diffusion controlled. They reasoned that the morphological instability required for dendritic protrusions resulted from the high mobility of a semicoherent interface. Observations of dendritic precipitation in the alloy Cu—l3wt%Al have been reported by Chadwick [31.As in the ternary CuZnSn system, the reaction involved gamma-phase precipitation in a beta-phase matrix. The gamma and beta cube axes were reported to he parallel. However, in the case of Cu l3wt%Al the dendrites are much larger in scale than those observed in the other copper based systems. Chadwick proposed that the mechanism which resulted in a dendritic morphology was controlled not by diffusion, as proposed by Malcolm and

1982 North- Holland

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Microdendritic precipitation in Ni — Ni

Purdy, but rather by the intrinsic higher mobility of the semicoherent interface formed between gamma and beta. This disagreement has not been resolved. Consequently, a theory which describes the mechanism leading to dendritic morphology in solid state precipitation reactions has not yet been formulated.

3Si and Ni — Ni~A/

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croscopy studies, the samples were given an etching treatment which revealed the microdendrites. The etchant used for all alloys had the following composition: 40 ml nitric acid, 60 ml glacial acetic acid, and 15 drops hydrochloric acid.

4. Observations and results 3. Experimental procedure Alloys for the present experiments were produced using high purity raw materials. In the Ni—Ni5Si2 system four alloys were studied (please refer to the phase diagram in fig. 1): — Ni—7.5wt%Si, an alloy in the solid solubility range of Si in Ni at the eutectic temperature; — Ni — 1 l.Swt%Si, the eutectic composition; — Ni—12.lwt%Si, the composition of Ni1Si [4]; — and, Ni—16.lwt%Si, the composition of Ni5Si2

[4,51. In the Ni—Ni3A1 system, an alloy of composition Ni—8wt%A1 was studied. The alloys were melted, solidified, and cooled in a vacuum induction furnace under a partial pressure of argon. The resulting specimens were sectioned and examined. Some samples were encapsulated under vacuum in vycor tubes and subsequently given selected heat treatments. Optical and scanning electron microscopy were used to examine the specimens. For scanning electron mi-

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Microdendrites were first observed in furnace cooled eutectic samples (11.5 wt% Si, as shown in fig. 1). They were found to occur exclusively in one of the eutectic phases. Fig. 2 is a scanning electron micrograph of a eutectic sample showing the morphology of the many microdendrites present. As shown in the figure, the microdendrites possess a main stalk and well defined secondary branches and have a maximum length of about 2 p.m. Heat treatment of the eutectic sample for 2h at 1050°C followed by a water quench was effective in removing any microstructural traces of microdendrites. However, a heat treatment for l~ h at 985°C, followed by furnace cooling, produced a microstructure containing microdendrites. Similar microdendrites were also observed in samples of composition Ni—7.5wt%Si and Ni— l2.lwt%Si; however, none were observed in the sample containing l6.lwt%Si. In the Ni—7.5wt%Si sample, as solidified, mi-

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Fig. I. Portion of the Ni—Ni5Si2 phase diagram pertinent to this study 141.

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Fig. 2. Mierodendrites in Ni—1l.Swt%Si alloy furnace cooled following solidification.

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G.S. Gardiner

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.t!iirndendritic precipitation in Ni

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Fig. ~. Microdendrites in alpha-phase dendrites of a Ni-7.5s~t%Si alloy furnace cooled following solidification.

crodendrites occurred within, but near the edge of, primary alpha-phase dendrites (fig. 3). This sample also contained some eutectic formed as a result of nonequilibrium solidification conditions. Solution treatment, followed by furnace cooling, removed the coring, as well as the nonequilibrium eutectic. and resulted in a morphology composed of microdendrites in an alpha-phase matrix (fig. 4). In the Ni—12.lwt%Si sample, the microdendrites were uniformly dispersed within the alpha phase produced by eutectic solidification. Traces

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Fig. 5. Ni—I 2. lwt%Si sample heat treated for 12 h at 1025°C.

of the peritectoid reaction expected from the phase diagram (fig. 1) were found only when the sample was heat treated for 12 h at 1025°C.The resulting morphology is shown in fig. 5. An incomplete perttectoid reaction has resulted in a three-phase structure: alpha (black), beta (grey), and gamma (white). Microdendrites were also observed in the Ni-Ni 3Al alloy. In the as-cast alloy, microdendrites were found around the cored primary alpha-phase dendrites. Heat treatment at 1250°Cfor 48 h followed by furnace cooling resulted in a more homo-

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from 1075 C.

from 1250°C.

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Microdendritic precipitation in Ni —Ni

geneous structure composed of finely dispersed dendritic precipitate in the alpha-phase matrix. The shape of the dendrites was observed to be mainly cruciform and the dendrite arms appear to have a preferred orientation in the matrix (fig. 6).

5. Discussion Two phase diagrams for Ni--Ni5Si2 are found in the literature [4,5]. In this study, proper heat treatment of the Ni—12.lwt%Si alloy demonstrated that a peritectoid reaction could be produced; this confirmed that the phase diagram given in the ASM Handbook [4] is the correct one. The role that this reaction plays in the formation of microdendrites in the unhomogenized samples was mitially not evident. In the Ni—7.5wt%Si, Ni— 1l.5wt%Si, and Ni—12.lwt%Si alloys, solidification resulted in a morphology containing the alpha and gamma phases. The microdendrites found in the alpha phase could have resulted from either precipitation or a peritectoid reaction. The former was shown to be responsible by the results of the homogenization and heat treatment of the Ni— 7.Swt%Si sample. Since, under equilibrium conditions, a peritectoid reaction does not occur at this composition, heat treatment demonstrated conclusively that microdendrites result from a solid state precipitation reaction in which beta phase precipitates in the alpha phase. In the Ni—16.l wt%Si alloy, the absence of microdendrites can be explained by the fact that little or no alpha phase was formed during solidification, The presence of microdendrites in the Ni—Ni3 Al sample can only be attributed to a solid state precipitation reaction in which Ni3 Al precipitates in the nickel-rich solid solution phase. This is predicted directly by the phase diagram. The mechanism responsible for the dendritic morphology of the precipitates is not known to the authors. In the copper based alloys, the dendritic morphology of gamma precipitates in beta has been attributed to the high mobility of semicoherent interphase boundaries [3]. The existence of such boundaries in the nickel based alloys studied could not be determined. However, there are some striking similarities between the nickel based alloys

3Si and Ni—Ni 1At

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studied and the copper based alloys in which dendritic precipitation occurs. Both Ni3Si and Ni3 Al have the same crystal structure as the nickel-rich solid solution phase. Ni3Si and Ni3AI have an ordered face centred cubic crystal structure [6]; the matrix phase has a face centred cubic crystal structure. This is analogous to the situation in the copper based alloys discussed where the gamma phase and the beta phase have lattices that are nearly identical [1,3]. In the Cu—Zn. Cu—Zn— Sn, and Cu—Al systems the beta phase has a body centred cubic crystal structure and the gamma phase has an ordered body centred cubic crystal structure [6]. This similarity in lattices between the precipitate and matrix phase indicates a common mechanism for dendritic precipitation in the nickel based systems studied and copper based alloys in which dendritic precipitates have been observed. This argument may be used to predict dendritic morphology of precipitates in other systems. For example, in nickel based superalloys some precipitates have a face centred cubic crystal structure and thus may be expected to form with a dendritic morphology. In these systems, the morphology of the precipitates may not have been observed due to the metallographic techniques used. Other studies have produced precipitates with features similar to those observed in the present work. For example, in the study of gamma-prime precipitates in the Ni—Al—Ta ternary, Merz et al. [7] produced scanning electron micrographs which show distinctly dendritic features. However, no comment was made by the authors on the morphology of the precipitates.

6. Conclusions Microdendrites in the Ni—Ni5Si2 and Ni—Ni3AI systems result from a solid state reaction in which the beta phase, Ni3Si or Ni3A1, precipitates into the alpha solid solution phase. The mechanism which results in a dendritic morphology is cxpected to be similar to that in copper based alloys which also exhibit precipitates with a dendritic morphology. A dendritic morphology is expected for precipitates with an ordered face centred cubic

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Microdendritic precipitation in Ni —Ni ~Si and Ni—Ni ~Al

crystal structure in nickel based superalloys. Generally. dendritic precipitation is expected in any system in which the lattice of the precipitate and that of the matrix are nearly identical.

[21J.A.

Malcolm and G.R. Purdy, Trans. AIME 239 (1967)

1391.

131

GA. Chadwick. Metallography of Phase Transformations (Crane, Russak and Company. 1972). [4] ASM Metals Handbook. Vol 8, 8th ed (1973) [5] M. Hansen. Constitution of Binary Alloys (McGraw-Hill. 1958).

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[II R.F.

l~]G D. Merz. T.Z. 14(1979) 663.

123.

Mehl and O.T. Marzke, Trans. TMS-AIME 93 (1931)

Pearson. Handbook of Lattice spacings and Structures of Metals (Pergamon. 1967). Kattamis and A.f. Giansei, .J. Mater. Sci.