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On the sequence of clustering and ordering in a meltspun Cu–Ti alloy
Materials Science and Engineering A360 (2003) 220 /227 www.elsevier.com/locate/msea
On the sequence of clustering and ordering in a meltspun Cu Ti alloy /
I.S. Batra *, G.K. Dey, U.D. Kulkarni, S. Banerjee Materials Science Division, Bhabha Atomic Research Centre Trombay, Mumbai 400085, India Received 5 September 2002; received in revised form 20 May 2003
Abstract Although it is well established that dilute Cu /Ti alloys with titanium in the range of 2.5 /5 wt.% decompose by a spinodal mechanism, the sequence of ordering and clustering processes in the early stages has been a matter of controversy. An attempt has been made in this work to resolve some of the issues by carrying out transmission electron microscopy (TEM) investigations on a dilute meltspun Cu /Ti alloy. Decomposition was studied as a function of ageing over a temperature range of 573 /723 K. The results seem to suggest that clustering precedes the formation of long range ordered (LRO) phases in this alloy. Formation of a transitory special point N3M phase was observed for the first time in this work. This disappeared on prolonged ageing giving rise to the metastable Cu4Ti (D1a). # 2003 Elsevier B.V. All rights reserved. Keywords: Spinodal decomposition; Order/disorder transformations; Clustering and ordering; Transmission electron microscopy; Cu /Ti alloys; Phase transformations
1. Introduction Extensively studied [1 /14], age hardenable dilute copper/titanium alloys, with yield strength in hardened condition exceeding 700 MPa, find applications in fabricating high strength springs, electrical contacts, diaphragms, and corrosion and wear resistant materials. The mechanism of their decomposition has, however, been a matter of much debate for about three decades. Whereas Cu /1wt.% Ti has been shown to decompose essentially by nucleation and growth, somewhat more concentrated alloys (with titanium in the range of 2.5 /5 wt.%) exhibit spinodal decomposition [6] */although there are results [13] which contradict the latter finding. It was also found that it is impossible to suppress decomposition during quenching in the concentrated side-band alloys [1] and, consequently, it is difficult to delineate the sequence in which clustering and ordering occur to form the observed metastable Cu4Ti precipitates having D1a structure [6,7]. Despite the above-mentioned difficulty, Laughlin and Cahn [4,5] followed by Datta and Soffa [6] and, more * Corresponding author. E-mail address: [email protected] (I.S. Batra). 0921-5093/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5093(03)00440-4
recently, Eckerlebe et al. [13] and others [8,9,14] attempted to investigate the phenomenon [7,11,15 /21] of concomitant clustering and ordering in these alloys. It was shown by Laughlin and Cahn [5], by a microstructural sequence method that the phase separation reaction in Cu /5.17wt.% Ti alloy is indeed spinodal in nature. It was substantiated by Datta and Soffa [6] in their investigations on Cu /4wt.% Ti alloy using transmission electron microscopy (TEM) and by Biehl and Wagner [8] on Cu /2.7wt.% Ti alloy using atom-probe field ion microscopy. These findings, have, however, been questioned in later investigations by Eckerlebe et al. [13] using small-angle neutron scattering (SANS), by Alvensleben and Wagner [9] using atom-probe field ion microscopy, and by Borchers [14] using TEM technique called the bright field zone axis (BFZA) technique. This is because observations of some features like the sidebands in electron diffraction patterns, the apparently periodic alignment of Cu4Ti precipitates with their major axes along the elastically soft 100 directions, the absence of heterogeneous nucleation at lattice defects and the lack of any measurable incubation period, though expected if the phase-separation is initiated via spinodal decomposition, are, by no means, enough to qualify the reaction [13]. As the spinodal in Cu /Ti involves uphill diffusion of titanium, observation
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of a negative value of coefficient of diffusion of titanium in copper during the reaction could only resolve the issue unambiguously. In their microstructural sequence method, Laughlin and Cahn [5] gave an indirect evidence of this. The controversy, however, continues. Similarly, there has been a debate relating to the sequence of clustering and ordering processes in this system. Laughlin and Cahn [5] concluded that the spinodal reaction occurs first and then the ordering reaction begins in the Ti-enriched regions after a critical concentration is reached in them. The two reactions then occur together. On the other hand, Datta and Soffa [6] concluded that ordering occurs virtually simultaneously with or precedes phase separation. Woychik et al. [10] found faint {1 /1 =2/ 0} short range order (SRO) maxima in [100] TEM diffraction patterns from a supersaturated Cu /15wt.% Ti alloy produced by melt-spinning. Upon ageing the alloy, superlattice reflections due to D1a appeared and the SRO maxima disappeared. According to Laughlin, Alexander and Lee [11], this indicates continuous homogeneous ordering (to the SRO state) prior to spinodal decomposition. These conflicting views, relating to the transformation sequence, emanate essentially because the various scattering effects-like {1 /1 =2/ 0} diffuse scattering, satellites (or side-bands) and D1a reflections-are extremely weak and simultaneously present from the earliest stages observed in the decomposition process. In order to resolve these issues, two different approaches were adopted in this work. In the first approach, an attempt was made to suppress the onset of the spinodal reaction during quenching by a small addition of a third element to Cu /4wt.% Ti. In the second, the alloy was spun into a thin ribbon directly from the liquid state using rapid solidification processing (RSP). Fast cooling rates, in both the solid as well as the liquid state, that accompany melt-spinning, have been known to suppress, in certain systems [21], the early stages of decomposition, which cannot be prevented by a conventional water quench. Both the approaches were found to be successful. The second approach, which obviates the need to add a third element to binary Cu /Ti, forms the subject of this study. The ageing response of this melt-spun alloy was studied using a transmission electron microscope with interesting new results, and interpretations that are relevant to ordering processes in other 1 /1 =2/ 0 ordering alloys as well.
2. Experimental To prepare the Cu /Ti alloy of the desired composition, small pieces of oxygen free high conductivity (OFHC) copper and iodide titanium, weighed in an appropriate ratio, were melted together in an induction
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furnace in a yttria lined graphite crucible. High purity argon was used as a protective atmosphere in the induction unit. For making the ribbon, approximately 3 g of this alloy was again induction melted in a silica crucible having an orifice of 0.6 mm at the bottom and ejected, with a 1 kg cm 2 back pressure of Argon, onto to a 0.2 m diameter copper wheel rotating at 2000 rpm. The thickness and the width of the resulting ribbon were found to be about 30 mm and 3 mm, respectively. The composition of the ribbon, as analysed at a number of locations using an electron-probe micro-analyzer (EPMA), was found to be 3.72 wt.% Ti, and the balance as Cu. Small pieces of the ribbon were encapsulated in quartz in helium and aged between 573 and 723 K for various periods of time followed by quenching in water. For TEM studies, small pieces of the ribbon were electrothinned by using the window technique. The solution employed for this purpose was a mixture of nitric acid and methanol in a ratio of 1:2 by volume. The temperature of the solution during electro-thinning was maintained at 223 K and the voltage at about 20 V. The samples, after electro-polishing, were immediately loaded in a JEOL 2000 FX microscope and examined at a voltage of 160 kV. Some samples for the TEM studies were also prepared by punching discs from the ribbons and then ion milling them in a Gatan duomill using a gun current of approximately 1 mA and a gun voltage of 4 kV.
3. Results and discussion It is well known [20] that whereas the decomposition of a solid solution in the metastable region of the miscibility gap necessarily takes place by the process of nucleation and growth, unmixing of a solid solution in the unstable region of the miscibility gap can occur by a spinodal reaction. The spinodal reaction is initiated via spontaneous formation and subsequent growth of coherent (also known as homophase) fluctuations. These spontaneous, delocalised long wavelength homophase composition fluctuations can be envisaged as concentration waves, which grow in amplitude with time and lead to separation of an unstable solid solution into well aligned solute-rich and solute-depleted regions extending along elastically soft directions in the crystal. By the same process of spontaneous growth of compositional fluctuations, an unstable disordered state may evolve into an ordered state*/the main difference between these fluctuations and those leading to unmixing being in their wavelengths. Whereas the wavelength of the fluctuations responsible for unmixing is large, the wavelength of the fluctuations, which result in ordering, is of the order of the interplanar spacing of the disordered structure. Of course, phase transformations
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combining an ordering process with an unmixing process are a common occurrence. Many examples [7], including the well-known Fe /Al [21] system, can be cited. Cu /Ti also happens [7,10,11] to be such a system. As pointed out in Section 1 of this paper, the nature of the interplay between the ordering and the unmixing processes in Cu /Ti has remained largely unresolved. Before taking up the discussion of the results of the present effort on Cu /Ti, some points need to be addressed. For example, since in spinodal decomposition (or spinodal ordering), the entire solid solution transforms via ‘continuous’ amplification of the initially small amplitude fluctuations, such decomposition (or ordering) is also called a continuous transformation (or ordering). Although both the spinodal ordering and the ‘‘ordering transformation of the second kind’’ are continuous in nature, de Fontaine [22] prefers to distinguish between these on the basis of thermodynamic considerations. In an ordering transformation of the second kind, the final equilibrium structure evolves and all the three Landau /Lifshitz symmetry criteria are satisfied, while the early stage spinodal structure is often not the equilibrium one and the third Landau /Lifshitz criterion is generally not satisfied. Another term, often employed when ordering precedes unmixing, is the ‘conditional spinodal’. In Cu /Ti literature, occurrence of the conditional spinodal has been talked about [6]. It should also be noted that whereas in a system like stoichiometric Ni4Mo, ordering to a D1a-phase initiates homogeneously [23,24] in the whole of the alloy through the onset of special point {1 /1 =2/ 0} concentration waves, in a system like meltspun Cu /4wt.% Ti, the supersaturation is perhaps not enough for this to occur. It is only when the titanium content in the Ti-rich regions, which form as a result of the spinodal unmixing, goes beyond a critical value that such spontaneous short wave length {1 /1 =2/ 0} concentration waves set in. After this, the ordering and the enrichment occur simultaneously till the observed metastable Cu4Ti (D1a) forms. The results obtained in the present investigation would be discussed in the light of this hypothesis. Thermodynamic modelling of such a situation has earlier been attempted by Kulkarni et al. [19]. Fig. 1(a) shows the BF micrograph obtained from the as melt-spun alloy and Fig. 1(b) the corresponding [001] selected area diffraction (SAD) pattern. Smooth BF contrast, as in Fig. 1(a), was observed in various sample orientations indicating, thereby, absence of concentration modulations in the microstructure. Also, no satellites (sidebands) near the matrix reflections could be discerned. This indicates that copper is supersaturated with titanium in the meltspun ribbon and that no decomposition or precipitation of any kind has occurred. The observed small grain size (B/1 mm) is typical of an RSP alloy.
The as meltspun alloy was aged at 573 K for 360 000 s to detect if any SRO, of the 1 /1 =2/ 0 kind, seen by Woychik et al. [10] in as meltspun Cu /15wt.% Ti, develops as a result of ageing at this low a temperature. Instead, as seen from Fig. 2(a), (b), spinodal decomposition appears to have taken place. The modulated microstructure in Fig. 2(a) and the presence of satellites near the matrix spots are the indicators. The absence of either Cu4Ti superlattice reflections or {1 /1 =2/ 0} SRO maxima in the [001] SAD pattern in Fig. 2(b) indicates that, even though the decomposition of the supersaturated solution into solute-depleted and solute-enriched regions has taken place, the enrichment level is still not enough to trigger short range or long range ordering (LRO) reactions. Also, no signature of any SRO or LRO reactions prior to the occurrence of the spinodal is evident. These observations indicate that occurrence of 1 /1 =2/ 0 SRO or LRO in the supersaturated solution may not be a prerequisite to the onset of the spinodal reaction in the Cu /4wt.% Ti alloy. [001] SAD pattern (Fig. 3(a)) from the melt /spun alloy aged at 673 K for 9000 s shows some important features. Apart from 1/5{420} and equivalent spots belonging to two variants of Ni4Mo (D1a), {1 /1 =2/ 0} and equivalent, and {/1 =2/ /1 =2/ 0} and equivalent, spots of the N3M phase [25 /28] (refer to the key in Fig. 3(b)) can also be seen. Further ageing at 673 K leads to disappearance of N3M and emergence of sharp D1a superlattice reflections. These can be seen clearly in Fig. 4. N3M, therefore, appears to be only a transitory phase and a precursor to the formation of the metastable D1a. Most of the ordered phases that appear in the early stages of decomposition of a supersaturated solid solution belong to a class of structures known as special point structures. The list of special point structures is given by Khachaturyan [27]. The N3M phase (space group P42nm) does not appear in this list, although it comprises superimposition of two perpendicular {1 /1 =2/ 0} concentration waves. A competing phase [25,27], D022, which has been observed in several ordering systems has the same A3B stoichiometry and is generated by a superimposition of a {1 /1 =2/ 0} and a {1 0 0} concentration waves. Although the existence of the N3M phase has been proposed on the basis of energy considerations as a candidate for the SRO state in Ni / Mo alloys, this is the first work to report its occurrence in the LRO form. The structure of the N3M phase is quite intriguing in the sense that it can be visualised as a space filling arrangement of unimolecular clusters of Ni4Mo (D1a) and Ni2Mo (Pt2Mo-type) */both of which are found to occur in Ni-base ordering systems. The LRO N3M phase would be expected to give rise to superlattice reflections at {1 /1 =2/ 0} and equivalent, and {/1 =2/ /1 =2/ 0} and equivalent positions in the SAD patterns
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Fig. 1. (a) BF micrograph from as meltspun Cu /3.72wt.% Ti alloy; (b) the corresponding [001] SAD pattern. Note the absence of concentration modulations in (a) and the absence of satellites and superlattice reflections in (b).
(Fig. 3(b)). However, the latter set of reflections are conspicuous by their absence in the SRO state in Ni / Mo alloys and also in the 1 /1 =2/ 0 state of SRO observed by Woychik et al. [10] in Cu /15wt.% Ti. This has been discussed in greater detail in [28]. Another important observation in Fig. 3(a) is the absence of curved streaks connecting the {1 /1 =2/ 0} maxima with D1a reflections. Such streaks of diffuse intensity characterise continuous transformation from the 1 /1 =2/ 0 state of SRO to LRO D1a phase in Ni /Mo. The N3M and the D1a phases, therefore, appear to coexist as discrete and disjoint
entities in this alloy unlike in the Ni /Mo case. However, microdomains of neither phase could be imaged in the dark field owing to the low intensity of superlattice reflections. Presence of N3M and D1a in the sample indicates that regions sufficiently enriched in titanium have formed here due to the spinodal reaction. In contrast, at 573 K, even after a prolonged ageing of 360 000 s, this could not be achieved. A careful observation of the fundamental reflections in Fig. 3(a) reveals their splitting in 100 directions. This would imply a longer wavelength of spinodal modulations vis-
Fig. 2. (a) BF micrograph from meltspun Cu /3.72wt.% Ti alloy aged at 573 K for 360 000 s showing concentration modulations in [100] directions; (b) a quadrant of the [001] SAD pattern showing presence of satellites near (200) and (220) reflections. Absence of any SRO maxima or superlattice reflections suggests that clustering precedes ordering at this ageing temperature.
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Fig. 3. (a) A quadrant of [001] SAD pattern from meltspun Cu /3.72 wt.% Ti alloy aged 673 K for 9000 s showing superlattice reflections from N3M and two variants of D1a superlattice structures; (b) the key to (a) showing a 000-200-220-020 unit square of [001] reciprocal lattice section of fcc. Superlattice positions of two variants of D1a (solid and open circles) and a single variant of N3M */both with their c /axes parallel to [001]. Solid squares show {1 /1 =2/ 0} and equivalent and open squares show {/1 =2/ /1 =2/ 0} and equivalent superlattice reflections of N3M; (c) the corresponding BF micrograph showing concentration modulations in two perpendicular 100 directions.
a`-vis Fig. 2(b). An increase in wavelength of the modulations results in reduced spacing between the matrix spots and the satellites. This is clearly seen in Fig. 3(c) where the modulations appear to have coarsened. Aging at 723 K for 1800 s results in the SAD pattern and the microstructure shown in Fig. 5(a) and (d), respectively. In the [001] SAD pattern in Fig. 5(a), faint superlattice spots due only to the two variants of Cu4Ti (D1a) can be seen. {1 /1 =2/ 0} maxima or the N3M superlattice reflections were not seen. An ageing treatment for the same period at 673 K, it may be recalled, shows N3M as a precursor to the formation of D1a. Salt and pepper contrast arising from the early stages of
spinodal modulations can be seen in the BF micrograph in Fig. 5(d). A tendency for the formation of the triaxially modulated microstructure and, also, Cu4Ti (D1a) is quite apparent here. Further ageing at this temperature results in coarsening of the modulated structure with significant increase in the quantity of Cu4Ti (D1a), and a concomitant increase in the intensity of D1a superlattice spots (Fig. 5(b), (c)). A well defined structure consisting of regularly spaced modulations aligned in 100 directions can be seen in Fig. 5(e) corresponding to 9000 s of ageing. On prolonged ageing, the triaxially modulated microstructure coarsens significantly and D1a domains of ellipsoidal morphology can be seen within these modulations (Fig. 5(f)).
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Fig. 4. [001] SAD pattern from meltspun Cu /3.72wt.% Ti alloy aged at 673 K for 61 200 s. Note that N3M reflections in Fig. 3(a) have disappeared on longer ageing, while D1a reflections have intensified.
Some important points relevant to clustering and ordering processes in this dilute Cu /Ti alloy have emerged on the basis of the observations in the foregoing. The severity of the quench experienced by the liquid alloy during RSP and, presumably, the high cooling rates experienced by the thin film after solidification appear to have suppressed occurrence of the clustering spinodal as well as 1 /1 =2/ 0 or D1a type of ordering. Apart from irradiation, RSP is known to be an effective means of producing a disordered solid solution in certain alloys [21,29]. A significant finding in the low temperature (573 K) ageing sequence was that neither {1 /1 =2/ 0} nor D1a type ordering developed prior to the occurrence of spinodal decomposition. Ordering, therefore, does not appear to be a prerequisite for phase separation, at least at this ageing temperature for this composition. It is likely that some highly localised SRO may have evolved prior to clustering. It would be difficult to detect the diffuse intensity, if any, arising from such a state of order in an electron diffraction pattern principally for two reasons. The degree of SRO would be rather low owing to the dilute nature of the alloy. Secondly, the insignificant difference between atomic scattering factors of Cu and Ti would make the diffuse intensities as well as superlattice reflections rather weak relative to intensities of fundamental spots. This is in clear contrast to the Ni / Mo ordering [23 /26], wherein formation of localised atomic clusters in the evolutionary stages gives rise to various patterns of diffuse intensity. At a somewhat higher ageing temperature of 673 K, both the clustering as well as the ordering processes seem to have occurred in the initial stages. An important
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finding in this context was the occurrence of the LRO N3M structure, which was observed for the first time in this work [28]. This structure differs from the 1 /1 =2/ 0 state of order observed in earlier studies on Cu /Ti and, also in Ni /Mo system in the sense that it shows {/1 =2/ /1 =2/ 0} and equivalent reflections in addition to {1 /1 =2/ 0} and equivalent reflections. The N3M phase appears to be a precursor to the formation of the metastable Cu4Ti (D1a). The decomposition kinetics appears to be quite rapid at 723 K. Both the concentration modulations and the D1a ordered phase are seen to coexist right from the initial stages. Modulations coarsen quite rapidly resulting in a triaxially modulated microstructure, which is known [6,7] to be characteristic of this alloy system. A curious observation in some [001] SAD patterns encountered in this study was the occurrence of four closely spaced sharp reflections around {1 1 0} positions. In the ageing sequence at 723 K, these appear to be quite strong (Fig. 6(a)) and weaken progressively on ageing (Fig. 6(b), (c)). The spacing between these four reflections, which form a square, appears to change with ageing time. Such reflections are often encountered [30] in long period superlattice derivatives of fcc having long period along 110. Such clusters of four reflections having different spacings have also been observed by Menon and Martin [31] in another 1 /1 =2/ 0 ordering system, namely, Ni /Mo/Al. This aspect of the decomposition process needs to be examined further.
4. Conclusions The decomposition of the supersaturated fcc phase in meltspun Cu /3.72wt.% Ti alloy was investigated in the temperature range of 573/723 K. It was not possible to clearly demarcate the various steps in the ageing sequence at every temperature that was studied owing to the fact that kinetics of the processes appears to be strongly temperature dependent. However, on the basis of available results, some comments can be made about the apparent ageing sequence in this alloy. Spinodal clustering seems to be the first step of the decomposition process. This has come out clearly during ageing at 573 K. Ordering seems to follow clustering in the titanium enriched regions. However, a critical level of enrichment appears to be necessary for the onset of formation of LRO N3M and Cu4Ti (D1a structure) phases. It is likely that short range atomic correlations might have evolved during the clustering process. However, no diffuse intensity could be observed in the SAD patterns, possibly, because of the dilute nature of the alloy and the nearly equal atomic scattering factors of copper and titanium. N3M appears to be only a transitory phase, which disappears on prolonged ageing giving way to the metastable D1a.
226 I.S. Batra et al. / Materials Science and Engineering A360 (2003) 220 /227 Fig. 5. Intensification of D1a spots and change in the modulated structure on ageing at 723 K for various periods of time. (a) and (d) for 1800 s; (b) and (e) for 9000 s; and (c) and (f) for 72 000 s. Development of the triaxially modulated structure is quite evident in (e) and (f).
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Fig. 6. [001] SAD patterns showing four closely spaced sharp reflections forming a square around {1 1 0} positions. Intensity as well as the spacing between these spots appears to reduce with ageing at 723 K (a) 1800 s; (b) and (c) 9000 s.
Acknowledgements Authors are thankful to Dr P.K. De, Head, Materials Science Division for his keen interest in this work. Thanks are also due to Mrs P. Agashe for help in photographic work and to Mrs A.B. Menon for carefully typing the manuscript.
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On the sequence of clustering and ordering in a meltspun Cu Ti alloy /
I.S. Batra *, G.K. Dey, U.D. Kulkarni, S. Banerjee Materials Science Division, Bhabha Atomic Research Centre Trombay, Mumbai 400085, India Received 5 September 2002; received in revised form 20 May 2003
Abstract Although it is well established that dilute Cu /Ti alloys with titanium in the range of 2.5 /5 wt.% decompose by a spinodal mechanism, the sequence of ordering and clustering processes in the early stages has been a matter of controversy. An attempt has been made in this work to resolve some of the issues by carrying out transmission electron microscopy (TEM) investigations on a dilute meltspun Cu /Ti alloy. Decomposition was studied as a function of ageing over a temperature range of 573 /723 K. The results seem to suggest that clustering precedes the formation of long range ordered (LRO) phases in this alloy. Formation of a transitory special point N3M phase was observed for the first time in this work. This disappeared on prolonged ageing giving rise to the metastable Cu4Ti (D1a). # 2003 Elsevier B.V. All rights reserved. Keywords: Spinodal decomposition; Order/disorder transformations; Clustering and ordering; Transmission electron microscopy; Cu /Ti alloys; Phase transformations
1. Introduction Extensively studied [1 /14], age hardenable dilute copper/titanium alloys, with yield strength in hardened condition exceeding 700 MPa, find applications in fabricating high strength springs, electrical contacts, diaphragms, and corrosion and wear resistant materials. The mechanism of their decomposition has, however, been a matter of much debate for about three decades. Whereas Cu /1wt.% Ti has been shown to decompose essentially by nucleation and growth, somewhat more concentrated alloys (with titanium in the range of 2.5 /5 wt.%) exhibit spinodal decomposition [6] */although there are results [13] which contradict the latter finding. It was also found that it is impossible to suppress decomposition during quenching in the concentrated side-band alloys [1] and, consequently, it is difficult to delineate the sequence in which clustering and ordering occur to form the observed metastable Cu4Ti precipitates having D1a structure [6,7]. Despite the above-mentioned difficulty, Laughlin and Cahn [4,5] followed by Datta and Soffa [6] and, more * Corresponding author. E-mail address: [email protected] (I.S. Batra). 0921-5093/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5093(03)00440-4
recently, Eckerlebe et al. [13] and others [8,9,14] attempted to investigate the phenomenon [7,11,15 /21] of concomitant clustering and ordering in these alloys. It was shown by Laughlin and Cahn [5], by a microstructural sequence method that the phase separation reaction in Cu /5.17wt.% Ti alloy is indeed spinodal in nature. It was substantiated by Datta and Soffa [6] in their investigations on Cu /4wt.% Ti alloy using transmission electron microscopy (TEM) and by Biehl and Wagner [8] on Cu /2.7wt.% Ti alloy using atom-probe field ion microscopy. These findings, have, however, been questioned in later investigations by Eckerlebe et al. [13] using small-angle neutron scattering (SANS), by Alvensleben and Wagner [9] using atom-probe field ion microscopy, and by Borchers [14] using TEM technique called the bright field zone axis (BFZA) technique. This is because observations of some features like the sidebands in electron diffraction patterns, the apparently periodic alignment of Cu4Ti precipitates with their major axes along the elastically soft 100 directions, the absence of heterogeneous nucleation at lattice defects and the lack of any measurable incubation period, though expected if the phase-separation is initiated via spinodal decomposition, are, by no means, enough to qualify the reaction [13]. As the spinodal in Cu /Ti involves uphill diffusion of titanium, observation
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of a negative value of coefficient of diffusion of titanium in copper during the reaction could only resolve the issue unambiguously. In their microstructural sequence method, Laughlin and Cahn [5] gave an indirect evidence of this. The controversy, however, continues. Similarly, there has been a debate relating to the sequence of clustering and ordering processes in this system. Laughlin and Cahn [5] concluded that the spinodal reaction occurs first and then the ordering reaction begins in the Ti-enriched regions after a critical concentration is reached in them. The two reactions then occur together. On the other hand, Datta and Soffa [6] concluded that ordering occurs virtually simultaneously with or precedes phase separation. Woychik et al. [10] found faint {1 /1 =2/ 0} short range order (SRO) maxima in [100] TEM diffraction patterns from a supersaturated Cu /15wt.% Ti alloy produced by melt-spinning. Upon ageing the alloy, superlattice reflections due to D1a appeared and the SRO maxima disappeared. According to Laughlin, Alexander and Lee [11], this indicates continuous homogeneous ordering (to the SRO state) prior to spinodal decomposition. These conflicting views, relating to the transformation sequence, emanate essentially because the various scattering effects-like {1 /1 =2/ 0} diffuse scattering, satellites (or side-bands) and D1a reflections-are extremely weak and simultaneously present from the earliest stages observed in the decomposition process. In order to resolve these issues, two different approaches were adopted in this work. In the first approach, an attempt was made to suppress the onset of the spinodal reaction during quenching by a small addition of a third element to Cu /4wt.% Ti. In the second, the alloy was spun into a thin ribbon directly from the liquid state using rapid solidification processing (RSP). Fast cooling rates, in both the solid as well as the liquid state, that accompany melt-spinning, have been known to suppress, in certain systems [21], the early stages of decomposition, which cannot be prevented by a conventional water quench. Both the approaches were found to be successful. The second approach, which obviates the need to add a third element to binary Cu /Ti, forms the subject of this study. The ageing response of this melt-spun alloy was studied using a transmission electron microscope with interesting new results, and interpretations that are relevant to ordering processes in other 1 /1 =2/ 0 ordering alloys as well.
2. Experimental To prepare the Cu /Ti alloy of the desired composition, small pieces of oxygen free high conductivity (OFHC) copper and iodide titanium, weighed in an appropriate ratio, were melted together in an induction
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furnace in a yttria lined graphite crucible. High purity argon was used as a protective atmosphere in the induction unit. For making the ribbon, approximately 3 g of this alloy was again induction melted in a silica crucible having an orifice of 0.6 mm at the bottom and ejected, with a 1 kg cm 2 back pressure of Argon, onto to a 0.2 m diameter copper wheel rotating at 2000 rpm. The thickness and the width of the resulting ribbon were found to be about 30 mm and 3 mm, respectively. The composition of the ribbon, as analysed at a number of locations using an electron-probe micro-analyzer (EPMA), was found to be 3.72 wt.% Ti, and the balance as Cu. Small pieces of the ribbon were encapsulated in quartz in helium and aged between 573 and 723 K for various periods of time followed by quenching in water. For TEM studies, small pieces of the ribbon were electrothinned by using the window technique. The solution employed for this purpose was a mixture of nitric acid and methanol in a ratio of 1:2 by volume. The temperature of the solution during electro-thinning was maintained at 223 K and the voltage at about 20 V. The samples, after electro-polishing, were immediately loaded in a JEOL 2000 FX microscope and examined at a voltage of 160 kV. Some samples for the TEM studies were also prepared by punching discs from the ribbons and then ion milling them in a Gatan duomill using a gun current of approximately 1 mA and a gun voltage of 4 kV.
3. Results and discussion It is well known [20] that whereas the decomposition of a solid solution in the metastable region of the miscibility gap necessarily takes place by the process of nucleation and growth, unmixing of a solid solution in the unstable region of the miscibility gap can occur by a spinodal reaction. The spinodal reaction is initiated via spontaneous formation and subsequent growth of coherent (also known as homophase) fluctuations. These spontaneous, delocalised long wavelength homophase composition fluctuations can be envisaged as concentration waves, which grow in amplitude with time and lead to separation of an unstable solid solution into well aligned solute-rich and solute-depleted regions extending along elastically soft directions in the crystal. By the same process of spontaneous growth of compositional fluctuations, an unstable disordered state may evolve into an ordered state*/the main difference between these fluctuations and those leading to unmixing being in their wavelengths. Whereas the wavelength of the fluctuations responsible for unmixing is large, the wavelength of the fluctuations, which result in ordering, is of the order of the interplanar spacing of the disordered structure. Of course, phase transformations
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combining an ordering process with an unmixing process are a common occurrence. Many examples [7], including the well-known Fe /Al [21] system, can be cited. Cu /Ti also happens [7,10,11] to be such a system. As pointed out in Section 1 of this paper, the nature of the interplay between the ordering and the unmixing processes in Cu /Ti has remained largely unresolved. Before taking up the discussion of the results of the present effort on Cu /Ti, some points need to be addressed. For example, since in spinodal decomposition (or spinodal ordering), the entire solid solution transforms via ‘continuous’ amplification of the initially small amplitude fluctuations, such decomposition (or ordering) is also called a continuous transformation (or ordering). Although both the spinodal ordering and the ‘‘ordering transformation of the second kind’’ are continuous in nature, de Fontaine [22] prefers to distinguish between these on the basis of thermodynamic considerations. In an ordering transformation of the second kind, the final equilibrium structure evolves and all the three Landau /Lifshitz symmetry criteria are satisfied, while the early stage spinodal structure is often not the equilibrium one and the third Landau /Lifshitz criterion is generally not satisfied. Another term, often employed when ordering precedes unmixing, is the ‘conditional spinodal’. In Cu /Ti literature, occurrence of the conditional spinodal has been talked about [6]. It should also be noted that whereas in a system like stoichiometric Ni4Mo, ordering to a D1a-phase initiates homogeneously [23,24] in the whole of the alloy through the onset of special point {1 /1 =2/ 0} concentration waves, in a system like meltspun Cu /4wt.% Ti, the supersaturation is perhaps not enough for this to occur. It is only when the titanium content in the Ti-rich regions, which form as a result of the spinodal unmixing, goes beyond a critical value that such spontaneous short wave length {1 /1 =2/ 0} concentration waves set in. After this, the ordering and the enrichment occur simultaneously till the observed metastable Cu4Ti (D1a) forms. The results obtained in the present investigation would be discussed in the light of this hypothesis. Thermodynamic modelling of such a situation has earlier been attempted by Kulkarni et al. [19]. Fig. 1(a) shows the BF micrograph obtained from the as melt-spun alloy and Fig. 1(b) the corresponding [001] selected area diffraction (SAD) pattern. Smooth BF contrast, as in Fig. 1(a), was observed in various sample orientations indicating, thereby, absence of concentration modulations in the microstructure. Also, no satellites (sidebands) near the matrix reflections could be discerned. This indicates that copper is supersaturated with titanium in the meltspun ribbon and that no decomposition or precipitation of any kind has occurred. The observed small grain size (B/1 mm) is typical of an RSP alloy.
The as meltspun alloy was aged at 573 K for 360 000 s to detect if any SRO, of the 1 /1 =2/ 0 kind, seen by Woychik et al. [10] in as meltspun Cu /15wt.% Ti, develops as a result of ageing at this low a temperature. Instead, as seen from Fig. 2(a), (b), spinodal decomposition appears to have taken place. The modulated microstructure in Fig. 2(a) and the presence of satellites near the matrix spots are the indicators. The absence of either Cu4Ti superlattice reflections or {1 /1 =2/ 0} SRO maxima in the [001] SAD pattern in Fig. 2(b) indicates that, even though the decomposition of the supersaturated solution into solute-depleted and solute-enriched regions has taken place, the enrichment level is still not enough to trigger short range or long range ordering (LRO) reactions. Also, no signature of any SRO or LRO reactions prior to the occurrence of the spinodal is evident. These observations indicate that occurrence of 1 /1 =2/ 0 SRO or LRO in the supersaturated solution may not be a prerequisite to the onset of the spinodal reaction in the Cu /4wt.% Ti alloy. [001] SAD pattern (Fig. 3(a)) from the melt /spun alloy aged at 673 K for 9000 s shows some important features. Apart from 1/5{420} and equivalent spots belonging to two variants of Ni4Mo (D1a), {1 /1 =2/ 0} and equivalent, and {/1 =2/ /1 =2/ 0} and equivalent, spots of the N3M phase [25 /28] (refer to the key in Fig. 3(b)) can also be seen. Further ageing at 673 K leads to disappearance of N3M and emergence of sharp D1a superlattice reflections. These can be seen clearly in Fig. 4. N3M, therefore, appears to be only a transitory phase and a precursor to the formation of the metastable D1a. Most of the ordered phases that appear in the early stages of decomposition of a supersaturated solid solution belong to a class of structures known as special point structures. The list of special point structures is given by Khachaturyan [27]. The N3M phase (space group P42nm) does not appear in this list, although it comprises superimposition of two perpendicular {1 /1 =2/ 0} concentration waves. A competing phase [25,27], D022, which has been observed in several ordering systems has the same A3B stoichiometry and is generated by a superimposition of a {1 /1 =2/ 0} and a {1 0 0} concentration waves. Although the existence of the N3M phase has been proposed on the basis of energy considerations as a candidate for the SRO state in Ni / Mo alloys, this is the first work to report its occurrence in the LRO form. The structure of the N3M phase is quite intriguing in the sense that it can be visualised as a space filling arrangement of unimolecular clusters of Ni4Mo (D1a) and Ni2Mo (Pt2Mo-type) */both of which are found to occur in Ni-base ordering systems. The LRO N3M phase would be expected to give rise to superlattice reflections at {1 /1 =2/ 0} and equivalent, and {/1 =2/ /1 =2/ 0} and equivalent positions in the SAD patterns
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Fig. 1. (a) BF micrograph from as meltspun Cu /3.72wt.% Ti alloy; (b) the corresponding [001] SAD pattern. Note the absence of concentration modulations in (a) and the absence of satellites and superlattice reflections in (b).
(Fig. 3(b)). However, the latter set of reflections are conspicuous by their absence in the SRO state in Ni / Mo alloys and also in the 1 /1 =2/ 0 state of SRO observed by Woychik et al. [10] in Cu /15wt.% Ti. This has been discussed in greater detail in [28]. Another important observation in Fig. 3(a) is the absence of curved streaks connecting the {1 /1 =2/ 0} maxima with D1a reflections. Such streaks of diffuse intensity characterise continuous transformation from the 1 /1 =2/ 0 state of SRO to LRO D1a phase in Ni /Mo. The N3M and the D1a phases, therefore, appear to coexist as discrete and disjoint
entities in this alloy unlike in the Ni /Mo case. However, microdomains of neither phase could be imaged in the dark field owing to the low intensity of superlattice reflections. Presence of N3M and D1a in the sample indicates that regions sufficiently enriched in titanium have formed here due to the spinodal reaction. In contrast, at 573 K, even after a prolonged ageing of 360 000 s, this could not be achieved. A careful observation of the fundamental reflections in Fig. 3(a) reveals their splitting in 100 directions. This would imply a longer wavelength of spinodal modulations vis-
Fig. 2. (a) BF micrograph from meltspun Cu /3.72wt.% Ti alloy aged at 573 K for 360 000 s showing concentration modulations in [100] directions; (b) a quadrant of the [001] SAD pattern showing presence of satellites near (200) and (220) reflections. Absence of any SRO maxima or superlattice reflections suggests that clustering precedes ordering at this ageing temperature.
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Fig. 3. (a) A quadrant of [001] SAD pattern from meltspun Cu /3.72 wt.% Ti alloy aged 673 K for 9000 s showing superlattice reflections from N3M and two variants of D1a superlattice structures; (b) the key to (a) showing a 000-200-220-020 unit square of [001] reciprocal lattice section of fcc. Superlattice positions of two variants of D1a (solid and open circles) and a single variant of N3M */both with their c /axes parallel to [001]. Solid squares show {1 /1 =2/ 0} and equivalent and open squares show {/1 =2/ /1 =2/ 0} and equivalent superlattice reflections of N3M; (c) the corresponding BF micrograph showing concentration modulations in two perpendicular 100 directions.
a`-vis Fig. 2(b). An increase in wavelength of the modulations results in reduced spacing between the matrix spots and the satellites. This is clearly seen in Fig. 3(c) where the modulations appear to have coarsened. Aging at 723 K for 1800 s results in the SAD pattern and the microstructure shown in Fig. 5(a) and (d), respectively. In the [001] SAD pattern in Fig. 5(a), faint superlattice spots due only to the two variants of Cu4Ti (D1a) can be seen. {1 /1 =2/ 0} maxima or the N3M superlattice reflections were not seen. An ageing treatment for the same period at 673 K, it may be recalled, shows N3M as a precursor to the formation of D1a. Salt and pepper contrast arising from the early stages of
spinodal modulations can be seen in the BF micrograph in Fig. 5(d). A tendency for the formation of the triaxially modulated microstructure and, also, Cu4Ti (D1a) is quite apparent here. Further ageing at this temperature results in coarsening of the modulated structure with significant increase in the quantity of Cu4Ti (D1a), and a concomitant increase in the intensity of D1a superlattice spots (Fig. 5(b), (c)). A well defined structure consisting of regularly spaced modulations aligned in 100 directions can be seen in Fig. 5(e) corresponding to 9000 s of ageing. On prolonged ageing, the triaxially modulated microstructure coarsens significantly and D1a domains of ellipsoidal morphology can be seen within these modulations (Fig. 5(f)).
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Fig. 4. [001] SAD pattern from meltspun Cu /3.72wt.% Ti alloy aged at 673 K for 61 200 s. Note that N3M reflections in Fig. 3(a) have disappeared on longer ageing, while D1a reflections have intensified.
Some important points relevant to clustering and ordering processes in this dilute Cu /Ti alloy have emerged on the basis of the observations in the foregoing. The severity of the quench experienced by the liquid alloy during RSP and, presumably, the high cooling rates experienced by the thin film after solidification appear to have suppressed occurrence of the clustering spinodal as well as 1 /1 =2/ 0 or D1a type of ordering. Apart from irradiation, RSP is known to be an effective means of producing a disordered solid solution in certain alloys [21,29]. A significant finding in the low temperature (573 K) ageing sequence was that neither {1 /1 =2/ 0} nor D1a type ordering developed prior to the occurrence of spinodal decomposition. Ordering, therefore, does not appear to be a prerequisite for phase separation, at least at this ageing temperature for this composition. It is likely that some highly localised SRO may have evolved prior to clustering. It would be difficult to detect the diffuse intensity, if any, arising from such a state of order in an electron diffraction pattern principally for two reasons. The degree of SRO would be rather low owing to the dilute nature of the alloy. Secondly, the insignificant difference between atomic scattering factors of Cu and Ti would make the diffuse intensities as well as superlattice reflections rather weak relative to intensities of fundamental spots. This is in clear contrast to the Ni / Mo ordering [23 /26], wherein formation of localised atomic clusters in the evolutionary stages gives rise to various patterns of diffuse intensity. At a somewhat higher ageing temperature of 673 K, both the clustering as well as the ordering processes seem to have occurred in the initial stages. An important
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finding in this context was the occurrence of the LRO N3M structure, which was observed for the first time in this work [28]. This structure differs from the 1 /1 =2/ 0 state of order observed in earlier studies on Cu /Ti and, also in Ni /Mo system in the sense that it shows {/1 =2/ /1 =2/ 0} and equivalent reflections in addition to {1 /1 =2/ 0} and equivalent reflections. The N3M phase appears to be a precursor to the formation of the metastable Cu4Ti (D1a). The decomposition kinetics appears to be quite rapid at 723 K. Both the concentration modulations and the D1a ordered phase are seen to coexist right from the initial stages. Modulations coarsen quite rapidly resulting in a triaxially modulated microstructure, which is known [6,7] to be characteristic of this alloy system. A curious observation in some [001] SAD patterns encountered in this study was the occurrence of four closely spaced sharp reflections around {1 1 0} positions. In the ageing sequence at 723 K, these appear to be quite strong (Fig. 6(a)) and weaken progressively on ageing (Fig. 6(b), (c)). The spacing between these four reflections, which form a square, appears to change with ageing time. Such reflections are often encountered [30] in long period superlattice derivatives of fcc having long period along 110. Such clusters of four reflections having different spacings have also been observed by Menon and Martin [31] in another 1 /1 =2/ 0 ordering system, namely, Ni /Mo/Al. This aspect of the decomposition process needs to be examined further.
4. Conclusions The decomposition of the supersaturated fcc phase in meltspun Cu /3.72wt.% Ti alloy was investigated in the temperature range of 573/723 K. It was not possible to clearly demarcate the various steps in the ageing sequence at every temperature that was studied owing to the fact that kinetics of the processes appears to be strongly temperature dependent. However, on the basis of available results, some comments can be made about the apparent ageing sequence in this alloy. Spinodal clustering seems to be the first step of the decomposition process. This has come out clearly during ageing at 573 K. Ordering seems to follow clustering in the titanium enriched regions. However, a critical level of enrichment appears to be necessary for the onset of formation of LRO N3M and Cu4Ti (D1a structure) phases. It is likely that short range atomic correlations might have evolved during the clustering process. However, no diffuse intensity could be observed in the SAD patterns, possibly, because of the dilute nature of the alloy and the nearly equal atomic scattering factors of copper and titanium. N3M appears to be only a transitory phase, which disappears on prolonged ageing giving way to the metastable D1a.
226 I.S. Batra et al. / Materials Science and Engineering A360 (2003) 220 /227 Fig. 5. Intensification of D1a spots and change in the modulated structure on ageing at 723 K for various periods of time. (a) and (d) for 1800 s; (b) and (e) for 9000 s; and (c) and (f) for 72 000 s. Development of the triaxially modulated structure is quite evident in (e) and (f).
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Fig. 6. [001] SAD patterns showing four closely spaced sharp reflections forming a square around {1 1 0} positions. Intensity as well as the spacing between these spots appears to reduce with ageing at 723 K (a) 1800 s; (b) and (c) 9000 s.
Acknowledgements Authors are thankful to Dr P.K. De, Head, Materials Science Division for his keen interest in this work. Thanks are also due to Mrs P. Agashe for help in photographic work and to Mrs A.B. Menon for carefully typing the manuscript.
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