Discontinuous and continuous precipitation in Cu-13 wt.% Sn and Al-20 wt.% Ag alloys

MATERIALS CHEMISTRYAND PHYSICS Materials Chemistry and Physics 53 (1998) 208-216

ELSEVIER

Discontinuous and continuous precipitation in Cu-13 wt.% Sn and A1-20 wt.% Ag alloys D. Hamana *, Z. Boumerzoug, M. Fatmi, S. Chelo'oud University o/" Co~stantine, Reseatz'h Unit in Materials Physics a1~dApplications, A in El Be3' Road, Constantble 25000, Algeria

Received 20 March 1997; received in revised form 28 November 1997; accepted 22 January I998

Abstract

The relation between the continuous and discontinuous precipitations, and the discontinuous precipitation kinetics and mechanisms in CuI3 wt.% Sn and AI-20 wt.% Ag alloys, have been investigated by metallographic observations, differential scanning calorimetry, X-ray diffraction and microhardness measurements. During ageing of the supersaturated solid solution of A1-Ag, after the preprecipitation of the Guinier Preston zones and the continuous precipitation of the plates of the n'ansition phase Y', the cellular precipitation of the equilibrium phase "/occurs and grows from the grain boundaries but lamelIae are not perpendicular to the reaction fi'ont. At high temperature intergranular precipitation occurs without the reaction front or cell. Both types of precipitation occur in the Cu-13 wt.% Sn alloy but very slowly. However, continuous precipitation predominates after a high deformation amount and reduces the amount of chemical driving energy available for both the initiation and propagation of discontinuous precipitation. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Continuous precipitation; Di.,,continuous precipitation; Supersaturated alloys

1. Introduction

In supersaturated alloy, precipitation can take place either continuously or discontinuously. The term continuous precipitation is applied to the formation of precipitate particles that are more or less uniformly distributed throughout the matrix grains. The earliest recognizable stage of continuous precipitation consists of the clustering of solute atoms that will ultimately be concentrated in the new phase. Tile clusters grow into Guinier-Preston (G.P.) zones [ 1 ]. The formation of a solute-depleted matrix phase and precipitates, as a duplex product, behind a boundary advancing into a supersaturated matrix is termed discontinuous precipitation [2]. In most cases, discontinuous precipitation occurs simultaneously with continuous precipitation [ 3 ]. It has long been known that in all alloys there exists a temperature domain close to the sotvus temperature where only continuous precipitation is observed. However, the variation in the relative amounts of the two modes of precipitation outside this domain has been given little attention. For example, Duly et aI. [4] have shown that in Mg-A1 alloys, continuous precipitation is favoured at high and low temperatures, whereas discontinuous precipitation dominates at intermediate temperatures. * Corresponding author. 0254-0584/98/$19.00 c~ i998 Elsevier Science S.A. All rights reserved. PII S0254-0584( 98)00039-X

Cu-Sn and A1-Ag alloys, in which both types of precipitation occur, have already been studied by different authors [5-i1]. The relationship between discontinuous precipitation and continuous precipitation as well as the cell growth kinetics of discontinuous precipitation in a Cu-15 wt.% Sn alloy have been studied mainly by metallographic observations [5]. It has been shown that the decomposition of Cu3Sn ( e phase) from a supersaturated solid solution of tin in copper during ageing was initiated by discontinuous precipitation and followed by continuous precipitation. The cell growth rate decreased with ageing time after linear growth rate of the ceils. This was attributed to the influence of continuous precipitation on the cell growth. The mass transport of tin during the linear cell growth occurred by grain boundary diffusion of tin in a copper-tin solid solution and prior cold work increased the rate of continuous precipitation but had no effect on discontinuous precipitation [ 5 ]. In the Cu-13 wt.% Sn alloy used as the matrix of superconducting N b / C u - S n composites, intensive discontinuous decomposition was discovered after 10% cold deformation followed by prolonged ageing at 280°C [6]. There was almost a complete absence of discontinuous decomposition in the pre-quenched alloy after similar ageing. Moreover, continuous, heterogeneous and homogeneous decomposition was observed [6].

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Ageing of Al-rich Ag-A1 alloys, quenched from the f.c.c. solid solution has been sufficiently studied [7-1 I]. In addition to the G.P. zones, a metastable h.c.p. "y' phase is observed. It appears at ageing times longer that those required for the initiation of zone formation, and grows in the form of Widmanst~.tten plates. The interfaciaI structure and growth kinetics of T' and T (Ag2AI) precipitate plates in AI-Ag alloys have been studied by conventional and in-situ transmission electron microscopy (TEM) techniques because these precipitates represent one of the simplest diffusional transformations involving a distinct change in crystaI structure, i.e. from f.c.c, to h.c.p. [ 8 ]. G.P. zones formation in A1Ag alloys has been observed by previous investigators [9,10]. Legoues et aI. [9] have shown that, because of very low interfaciaI energies, homogeneous nucleation of G.P. zones is so easy that it begins almost as soon as the solvus is crossed during quenching from the solution annealing treatment, well before the spinodal region is reached. The influence of the prior formation of a continuous precipitate upon the growth kinetics of the cellular reaction has been evaluated in an AI-17.9 wt.% Ag alloy [ i 1]. The continuous precipitate, in the form of intragranular pIates of the T' transition phase, was shown to have reduced the upper bound of the driving force for the cellular reaction from the silver content of the untransformed alloy to that corresponding to the coherent solvus. When this reduction ( > 9 8 % ) is taken into account, the growth of cells is found to be controlled by cell boundary rather than by volume diffusion.

Changing the mode of heat treatment from the usual quenching and ageing to that of isothermal transformation reduces both the rate of ceil growth and the proportion of cellular structure formed by about an order of magnitude and increases the interlamellar spacing by 50%-100%. These effects appear to result from a further decrease in the driving force. This decrease is attributed to a higher rate of introduction of misfit dislocations into the broad faces of the T' plates constituting the continuous precipitate, and thus to smaller values of the coherent solvus [ l 1]. Few detailed investigations have been reported on the effect of prior continuous precipitation on the discontinuous precipitation. In most of the previous investigations, the effect of the continuous precipitation has simply been treated as having a role to reduce the driving force for the discontinuous precipitation, since a prior decomposition into the metastable phase reduces the matrix composition to an equilibrium composition with the metastable phase (G.P. zones or second phase) [ 12,13 ]. Thus a detailed study of various microstructure evolutions during ageing of undeformed and deformed AI-20 wt.% Ag and Cu-13 wt.% Sn supersaturated solid solutions by different experimental techniques is still interesting. Consequently, the aim of this work is to investigate the relation between the continuous and discontinuous pre-

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D. Hamana et aL / Materials Chemistry a,M Physics 53 (1998) 208-216

cipitations, and the kinetics and the mechanisms of the discontinuous precipitation by metallographic observations, differential scanning calorimetry, X-ray diffraction and microhardness measurements.

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Fig+ 4. Microstructure of Cu-13 wt.% Sn alloy h o m o g e n i z e d for 3 h at 600°C, quenched, deformed ( 4 0 % ) then aged at 280°C for (a) and (b) 1700 h.

D. H~m~ana et al. /Moterials Chemistry and Physica 53 (1998) 208-216

99.999% purity) and Sn (of 99.99% purity) in a vacuum of 1.3 mPa (I0 -'~ Ton'). Homogenization annealing is alternated with a 20% reduction to homogenize the alloys sufficiently, as determined by electron microprobe analysis. Additionally, some polycustalline samples were cold-rolled before the homogenization annealing. Samples for calorimetric measurements were in a disk shape of 3 mm diameter and 1.5 mm thichness. The calorimetric measurements were performed with a METLER TA 400 DSC. A protective atmosphere of pure argon was used. For standard metallographic investigations, the specimens were etched in a solution of 5 ml HF, 23 ml HNO> 9 ml HC1 and 87 ml H20 (for AI-Ag) and diluted HNO~ (for Cu-Sn) at room temperature. The Vickers microhardness was determined on the At-20 wt.% Ag alloy under a load of 200 g. Prior cold working is achieved by rolling at room temperature.

211

endothermic peak due to the dissolution of a few precipitate particles appears (Fig. 1(b) and (c)). This result is confirmed by the metallographic study. Indeed discontinuous precipitation is retarded but a grain boundary deformation owing to a localized displacement, is observed (Fig. 2). This migration of the reaction front which is locally impeded by precipitate particles may be the result of a cellular precipitation. One can also note the absence of continuous precipitation. After quenching and slight prior cold working (15%) discontinuous precipitation takes place concomitantly with the normal (continuous) precipitation (Fig. 3). During discontinuous decomposition, the alloying element leaves the supersaturated solid solution oeo and arrives in the cell by diffusion either along the boundary towards plates of precipitate ephase or perpendicular to the boundary into the denuded solid

3. Results and discussion

3.1. Case of Cu-13 wt.% Sn alloy Precipitation in this type of alloy is very sIow and is slightly stimulated by a prior cold work, As shown in Fig. 1, DSC scans from 20°C to 440°C with a heating rate of 2°C rainof the homogenized (3 h at 600°C) and quenched sample (Fig. 1 (a)) and homogenized, quenched then aged sample at 280°C for 1120h (Fig. l ( b ) ) and 3210h (Fig. i ( c ) ) confirm the slowness of the precipitation reaction. No precipitation (exothermic) peak is observed during heating of quenched or aged samples. After a long ageing time only an

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Fig. 6. Debye-Scherrerhack plane films of Cu-13 wt.% Sn alloy homogenized for 3 h at 600°C, quenched, deformed (a) 15~ (b) 60% then aged for 1500 h at 280°C. CuKa radiation ( 10 mA, 30 kV).

212

D+ Hamana et al./ Materials C]lemistr3' and Physics 53 (1998) 208-216

solution. Dark-etching discontinuous precipitation cells that have a lamellar structure nucleate at the grain boundaries and advance into the grain interiors (Fig. 3). In rnany systems the decomposition involves the migration of an incoherent interface (reaction front) between the supersaturated solid solution and the cellular structure. The decomposition occurs predominantly by an interracial diffusion process from the solid solution to the corresponding phases in the cellular structure. The incoherent interface (reaction front) is usually formed by the migration of a high-angle grain boundary that was present in the supersaturated solid solution. The individual lamellae are arbitrarily oriented in the cells of the discontinuous reaction and continuous precipitation occurs on slip planes in the grain interior (Fig. 3). A higher prior cold work (40%) leads to more important continuous precipitation in the grain interiors identical to a Widmanstrttten structure (Fig. 4). After the appearance of continuous precipitates, the celt advance almost stops. Finally, continuous precipitation of the e-phase tends to occur simultaneously with discontinuous precipitation and intragranular • precipitates may pin the reaction front. All these instabilities may affect the shape of the boundary, its migration speed and the concentration profile in the oe lamellae. From the X-ray diffraction pattern it has been established that both the precipitates of discontinuous and continuous modes were the equilibrium phase • ( C u 3 S n ) . dQldT

A higher amount of prior plastic deformation (60%) leads to the development of intensive and more homogeneous continuous decomposition during I000 h ageing at 280°C ( F i g . 5) and a fine polycrystalline structure in comparison to a slight deformation (15%) confirmed by the observation of continuous diffraction rings on the Debye-Scherrer back plane film (Fig. 6). Moreover, one can note the absence of discontinuous precipitation.

3.2. Case of Al-20 wt. % Ag alloy DSC scans recorded at 2°C min-~ for two samples of this alloy, which have been homogenized 16 h at 550°C and quenched (Fig. 7(a)) then aged for 1 year at room temperature (Fig. 7(b) ) showed the following: 1. an endothermic effect in the temperature range (150185°C) due certainly to the dissolution of G.P. zones formed just after quenching and before the start of the heating; 2. an exothermic peak between (220°C and 300°C) due to the precipitation of precipitate phases (y' and y). The second endothermic peak due to their dissolution appears at higher temperature and is not detected at T < 440°C. The quantity of dissolved G.P zones is more important after 1 year of ageing at room temperature. Moreover, the endothermic peak due to their dissolution vanishes after 1 h (Fig. 8(a)) and 20h (Fig. 8(b)) ageing at 350°C of a quenched and deformed sample (15%). However, an exothermic peak due to the phase precipitation is less impoctan~

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D. H amana el: al. / Materials Chemist 0' a/~d Physics 53 (1998) 208-216

213

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Fig. 9. Microstructure of A1-20 wt.% Ag alloy homogenized for 16 h at 550°C, quenched and aged for (a)-(d) t3 h at 280°C.

after 20 h of ageing when a big part of the precipitate particles is already formed (Fig. 8). The precipitate formed by discontinuous precipitation need not be of the same phase as that which may simultaneously be forming by continuous precipitation. Fig. 9 of the quenched and aged sample shows the metastable 3,' transition phase formed continuously competing with the equilibrium Y phase, which formed discontinuously. Those parts of the grains that have not been reached by the cellular reaction are occupied by plates of the transition phase 3,'. The continuous precipitate, consisting almost entirely of intragranular plates of the h.c.p, transition phase ",/, forms very rapidly. The cells consist of alternate lamellae of Ag-depleted & and of the equilibrium precipitate, h.c.p. 7 which are not perpendicular to the reaction front. There is, however, another mechanism through which "7 forms. This is simply the in-situ transformation of the continuous precipitate from 7' + Y. These two phases differ only slightly in lattice parameter and in composition. The transformation from T' to Y is accomplished by

the introduction of a sufficient number of misfit dislocations of appropriate Burgers' vector into the broad faces of 7' plates [I1]. As Watanabe and Koda [I4] have recognized, the ",/' --+ ",/transformation progressively reduces the driving force for the cellular reaction, and consequently this reaction is usually terminated at an early stage in its development. A prior cold work (30%) leads to the development of intensive discontinuous reaction during ageing for 15 rain at 280°C (Fig. 10). The formation of cells with lamellae is observed inside grains as well as on the boundaries, at structure defects, and the intensity of decomposition inside grains being no less than on the boundaries. The localized precipitation results from the promotion by lattice imperfections of accelerated nucleation and growth of precipitate cells. The microstructural effect of localized precipitation is to produce groupings of particles at preferred locations, such as dislocation networks, slip planes and incoherent twin boundaries. It is also clear from the micrographs that the cell's morphology is quite different than that observed in Fig. 9 and the

2 I4

D. Hamana et al./Materials Chemist O' and Physics 53 (1998) 208-216

Fig. 10. Microstructure of AI-20 wt.% Ag alloy homogenized 16 h at 550°C, quenched, deformed (30%) and aged for (a) and (b) 15 rain at 280°C. intensity of continuous decomposition has relatively decreased. The presence of discontinuous precipitation is confirmed by X-ray diffraction; two diffraction rings of the same peaks (331) and (420) are observed and are due to supersaturated and depleted c~ solid solution. The relative displacement of these rings is due to the continuous precipitation (Fig. 1 1 ). Thus the most important effect of high plastic deformation on the development of discontinuous precipitation of a new

Fig. 1l. Debye-Scherrer back plane films of AI-20 wt.% Ag alloy homogenized for 16 h at 550°C, quenched, deformed (30%) and aged for 13 h at 280°C. CuKceradiation ( 10 rnA, 30 kV).

phase consists in the appearance of large numbers of cells of discontinuous decomposition at and inside grains with a different morphology. Typical microstructures of aged specimen at higher temperature (350°C) are shown in Fig, 12. Contrary to most alloys that exhibit discontinuous precipitation, the aspect of precipitate particles at grain boundaries is quite different. One can note the absence of cells and reaction front (Fig. 12(a) and (b)) but continuous precipitation in the grain interior is always identical to a Widmanstrttten structure (Fig. 12(c) ). The initial position of the reaction front is marked by a regular arrangement of parallel and coarser lamellae ( Fig. 12 (b) ). As the temperature increases, bulk diffusion becomes faster, which tends to favour volume precipitation. Moreover, discontinuous precipitation is limited in two respects, since both its initiation and propagation tend to be inhibited at high temperature. As shown by Hillert, volume diffusion in front of a moving grain boundary during the growth of a discontinuous precipitation colony tends to equal the solute content on both sides of the boundary, thereby reducing the chemical driving force for the propagation of discontinuous precipitation [ 15,16]. In the A1-Ag system, optical metallography is therefore a straightforward method to study the precipitation morphology. Moreover, it was observed that maximum Vickers hardness corresponds to the formation of an important quantity of ,/'-precipitate particles at 280°C, i.e. when the volume fraction of continuous precipitation was higher (Fig. t 3 (a)). The decline in hardness was thus probably associated with the , / ' ~ 31transformation and a progressive coarsening of the structure. Both subsequent continuous and discontinuous

215

D. Hamana el aL / Materials Chemistr3' and Physics 53 (1998) 208-216

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coarsenmgs were indeed observed during ageing at 350°C and the rnicrohardness values were less important (Fig. 13(b)). 8Note that as expected when discontinuous precipitation leads to the formation of lamellae, the coarsening did not lead to a globulization but only to an increase in the lamellae spacing. It is thus always possible to distinguish between continuous and discontinuous precipitation.

4. Conclusions

In the Cu- 13 wt.% Sn alloy the decomposition of the supersaturated solid solution was very slow and was stimulated by prior cold working. Discontinuous precipitation tended to be accelerated after low deformation, but both continuous and discontinuous precipitations occurred. The disappearance of

discontinuous precipitation after high deformation was

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D. Hamana et al. / MateriaLs Chemistry asTd Physics 53 (1998 ) 208-216

caused by both volume diffusion phenomena owing to the introduction of a high defects density, that impeded the propagation of a reaction front and by the impossibility for discontinuous precipitation to nucleate. In the A1-20 wt.% Ag alloy the metastabte T' transition phase formed continuously competing with the equilibrium T phase, which formed discontinuously. The continuous precipitate, consisting almost entirely of intragranular plates of the h.c.p, transition phase T', formed very rapidly, identical to a Widmanst~itten structure. The cells consisted of alternate lamellae of Ag-depleted c~and of the equilibrium precipitate, h.c.p. 3,. As far as the high temperature behaviour is concerned, it is likely that continuous precipitation will always be dominant near the sotvus temperature: volume diffusion always acts to decrease the driving energy for discontinuous precipitation, and at the same time the high temperature nucleation mechanism for discontinuous precipitation is quite different (absence of reaction front and cell). At low temperature things are less clear: if the reaction front and the cells are present, the lamellae are not perpendicular to the reaction front which led to think about a new nucleation and growth mechanism in this type of alloy. As a conclusion, it can be said that such an investigation, which is extremely important for a deeper understanding of

the phenomenon of discontinuous decomposition and the effect of prior continuous precipitation on it, is still important and deserves undivided attention.

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