Discontinuous Precipitation and Dissolution in Cu-4 . 6 at % In Alloy under the Effect of Plastic Deformation and the Temperature

The study of the discontinuous precipitation reaction and the lamellar precipitate dissolution in the alloy Cu-In system provoked a considerable benefit and has been the subject of many theoretical and experimental investigations. The aim of this work is to make the evidence on the one hand the effect of the plastic deformation on the mechanism of the discontinuous precipitation reaction such as nucleation, growth and lamellar coarsening and in other hand the effect of temperature on the characteristics and front behavior movement of the opposite reaction (discontinuous dissolution). Different techniques of analysis have been used in this respect such as the optical microscopy, the differential thermal analysis and the microhardness Vickers. The obtained results confirm various works achieved in this field.


Introduction 1.Discontinuous Precipitation
The precipitation phenomena take a considerable place in metal solutions, because it modifies the alloy properties, sometimes in favourable way leading to the rise of hardness and load breaking.The final state of the precipitation process is of several phases.
Generally the reaction of precipitation consists of the decomposition of a supersaturated solid solution α 0 (mother phase) in a mixture of two phases of different compositions [1], according to the following reaction: α 0 → α + δ where α is the girl phase, depleted in alloy element and has the same structure as α 0 the mother phase and δ the precipitated phase rich in alloy element and can be:  A mixed crystal with the same structure, the case of discontinuous precipitation in the alloy system Au-Ni [2]. A mixed crystal with a different structure, the case of the alloy system Pb-Sn [3]. An intermetallic phase, the case of the alloy system Cu-Zn [4]. A liquid phase, the case of the alloy system Pb-Bi [5].
Discontinuous precipitation in the alloy Cu-In system has been the subject of many research tasks [6][7][8] and the supersaturated solid solutions of this alloy system is decomposed in continuous and discontinuous modes, appearing at high and low temperature respectively.The favourable sites supporting the appearance of the cellular precipitate are the grain boundaries of large orientation [9,10].The average size of the initial grains has also a considerable influence on the precipitate morphology.The mechanisms of discontinuous precipitation nucleation suggested by Fournelle and Clark [11], then by You and Turnbull [9] are most plausible in this alloy system, i.e. it is a transformation related to the grain boundary dynamics.The growth model in this alloy system is that proposed by Frebel and Schenk [12] and of which the speed growth of the precipitated lamellas depends primarily on the speed of ageing annealing and the solute content of this alloy system, Figure 1 [13].
Two types of cellular reactions were observed by Spenger and Mack [14] in this alloy system, one fine and the other coarse, in both cases the lamellas are uniformly distributed.Predel and Gust [15] has shown also the same observation Figure 2, where the formation of the coarse lamella proceeds mainly between two fine lamellas and in both cases the process is controlled by the diffusion on the grain boundaries.The fine lamellas are uniform-and the Temperature ly distributed; in contrast the distribution of thick lamellas is disordered, so there is a competition between the primary reaction of precipitation and the coalescence of the lamellas.
In old work one could not observe the coarse lamellas, because probably they appear only after long annealing time.This sam issolution of the lamellate structure β + α (biphasic carried out according e phenomenon was observed in many alloys such as (Al-Ag [16], Al-Cu [17], Au-Fe [18], Cu-Ag [19,20], Fe-Zn [21], Fe-Ni-Ti [22], Ni-Sn [23], Pb-Na [24], Cu-Ga [25], Cu-In [26] and Zn-Al [27].The types of precipitate in this alloy system have the shapes of hem [28], quadratic and fissure [29].Experimentally, it was shown that a predeformation with the ageing annealing affects considerably the mechanism and the kinetics of precipitation [30].D. B. Williams [31] affirmed that under the influence of the deformation, the speed of continuous precipitation increases consequently, the degree of supersaturation in solute atom decreases, which implies a reduction in the driving force of the cellular reaction.

Discontinuous Dissolution
The d structure) formed after annealing is to the reaction: This can take place at any temperature higher from 20 to 50˚C at the solvus temperature (critical temperature of solubility) corresponding to the alloy composition.Dissolution is easy and rapid if this critical temperature is raised, and supported by diffusion of the addition element in the matrix.The solid solution results after disappearance of the precipitate, has however a uniform composition only if the dissolution heat treatment were sufficiently prolonged (of about an hour) at a temperature higher than the solvus temperature, to ensure by diffusion a perfect homogenization (a complete dissolution).
The dissolution process is perhaps explained by Figures 3(a) and 3(b) represented on the one hand the discontinuous precipitation diagram on the grain boundary with α 0 is the mother phase (single-phase structure), the β precipitate in the lamellate form and the new phase α, and in other hand by the dissolution of precipitate with grain boundary displacement (GB) according to the reaction front (RF) [32].
According to a study made on several alloy systems and among the Al-38 at %Ag alloy based on the microhardness measure, the dissolved phase leads to the alloy hardening, it increases the lattice tension due to the precipitate rich in solute by the introduction of this last into the solid solution.In the same way, the fragmentation of the lamellate of the β phase causes the same effect.The mechanical properties of the alloy during the discon-  1.
sed and on the same aged samples, see Table 2.
ermal analysis and the microhardness Vickers.

matic of discontinuous precipitation (a) and discontinuous dissolu
tion treatment depend on the alloy proper-ples, where two samples are deformed by ti S. P. Gupta [33] noticed that the dissolution of the lamellate precipitates in the same allo s type.I. G. Solorzano and W. Gust [34] have shown that an adequate heat treatment makes it possible to develop two different mechanisms of dissolution, one at 480°C leading to a discontinuous dissolution where the atoms diffusion is carried out through the reaction front and the other at 580˚C leading after 60 seconds to a continues dissolution or the atoms diffusion is carried out in volume, the latter is accomplished by a recrystallization due to the presence of dislocations in the precipitated lamella, which supports the formation of new grains.The increase in the dislocations rate is due to the thermal cycle.Schapiro and Kirkaldy [35] have observed the existence of twins inside the precipitated lamella which supports the process of recrystallization.S. P. Gupta and B Prasard [36] found that the applied technique of the thermal cycle to a Cu-In alloy causes a refinement of the grain.

Experimental Methods
The significant part of the equilibriu alloy Cu-In system is illustrated in responds to the small percentage of Indium.The maximum solubility of the α phase is 18.1 at% In at 574˚C, the δ precipitated phase (Cu 9 In 4 ) from the supersaturated phase with a composition of 29 to 30.6 at % In at all the temperatures below 613˚C [37].
The alloy used for this investigation is the Cu-4.6 at% In alloy obtained by vacuum i ert atmosphere (Argon) from Copper (5N5Cu) and Indium (5N5In) very pure.The samples investigated in the experimental study are homogenised for 27 days at T = 600˚C, quenched in water.For the effect of deformation on the discontinuous precipitation, we have used 3 sam-The ageing treatments were carried out at 400˚C to cause only the discontinuous precipitation.For dissolution, three different temperatures of 460˚C, 500˚C and 600˚C are u The structural evolution is of microhardness Vickers (HV 0,1 ).The structural morphology is followed by various techniques such as optical microscopy, the differenttial th

Discontinuous Precipitation
The temperature of ageing of 400˚C reveals only the discontinuous precipitation (lamellate) and not of other morphology precipitates whatever the deformation rate, the precipitate are of the double hems type, Figures 5(b), 5(d) and 5(f) with the reaction front has a corrugated pace.The mechanism of folding noticed by the authors [9,11] was observed, i.e. in the early times of nucleation, the grain would be folded locally to absorb the precipitates, Figure 6, which confirms a precipitation with the grain boundary dynamics.
However precipitation is much slower on a grain boundary of small size than on a grain boundary of large size, an average critical size of grains boundaries is deserved to be in evidence for such cases.
The deformation does not have any effect on the precipitates growth, but however we have observed two cellular reactions, one of fine lamellas and the other coalesced, Figure 6, which implies a competition between the primary reaction of precipitation and the coalescence of the lamellas.The formation of the coalesced lamellas is carried out mainly between two very close fine lamellas; this enlargement probably starts when the totality or less most of the structure is precipitated.For the coalesced lamellas it is always about the same phase as that of discontinuous precipitation.The lamellas preserve

Discontinuous Dissolution
their morphology anisotropy during th owth of the lamellas is observed in the deformed than in the not deformed samples, it is a growth in width leading to narrow hems.The differential thermal analysis, Figure 7 which the curve is characterized by a variation of energy during the rise in the temperature and the exo-The various temperatures used during dissolution of precipitates lamellate have developed in two different mechanisms of dissolution.The temperatures of 600˚C and 500˚C led on the one hand to a dissolution of continues type where the atoms diffusion is carried out in volume (Figures 9(b) and 9(d)) and on the other hand supported the formation of twins after dissolution of precipitates.
On the other hand the temperature of 460˚C has shown a discontinuous dissolution of discontinuous type, Figure 9(f), with opposite migration of the reaction front (RF) and where the atoms diffusion is carried out mainly through the grain boundary.However the effect of the temperature on the speed of dissolution is observed, because in comparison with the last two temperatures (600˚C and 500˚C), the process of dissolution in the case of the temperature of 460˚C is relatively slow.The dissolution of precipitates in the three cases has lead to a fall of the microhardness, Figure 10.

Conclusions
The results obtained by different technique of analysis used in this respect are coherent between them and confirm several work devote to the study of the reaction of precipitation and discontinuous dissolution in the alloy Cu-In system.However the temperature of 400˚C supports only discontinuous precipitation and not other precipitates morphology was observed.The lamellas are of double hems type with a corrugated pace.The presence of the fine lamellas and coalescences confirms the existence of the competition between precipitation and the coalescence of the lamellas.The grain boundary is characterized by a crumpling to absorb the precipitate during their movement.The effect of the deformation rate is observed on the precipitation speed during the initial stage; on the other hand no effect was observed on the coalescence of the precipitate.Precipitation is carried out mainly in the coarse grains zones, because the initial average size of th   the temperature of 460˚C, slow down probably by the spheroidisation of the lamellate precipitate.

Figure 1 .
Figure 1.Growth speed of precipitate during annealing time in the Cu-In alloy system [13].

Figure 4 .
Figure 4. Part of equilibrium diagram of Cu-In alloy system [37].

Figure 6 .
Figure 6.Structural evolution of Cu-4.6 at% In alloy, after homogenization during 27 days at 600˚C, quenched in water, deformed of 20% and annealing at 400˚C during 67 h.

Figure 8 .
Figure 8. Hardness evolution HV 0,1 during annealing time Cu-4.6 at% In alloy, after homogenization during 27 days at 600˚C, quenched in water (a), deformed of 20% (c) and of 70% (e) and annealing at 400˚C during.nisms of precipitation.The appearance of the new intermetallic phase Cu 9 In 4 led to the hardening of alloy.The effect of the temperature on the mechanism of the dissolution of the lamellate precipitate has shown on the one hand that dissolution is carried out into two different e grains has an influence on the mecha-

Figure 9 .
Figure 9.Structural evolution of Cu-4.6 at% In alloy, after ho nealing at 400˚C during 97 h (a), 67 h (c) and 53 h (e) and dis und at 460˚C during 8 h (f).mo sol genization during 27 days at 600˚C, quenched in water, anution at 600˚C during 12 mn (b), at 500˚C during 25 mn (d)

Figure 10 .
Figure 10.Hardness evolution HV 0,1 during dissolution time of Cu-4.6 at% In alloy, after homogenization during 27 days at 600˚C, quenched in water and annealing at 600˚C (a), at 500˚C (b), and at 460˚C (c).