Discontinuous Precipitation and Dissolution under the Influence of the Plastic Strain and Temperature in Al-15 at.%Zn and Al-30 at.%Zn Alloys ()
1. Introduction
1.1. Discontinuous Precipitation
The first appearance of the DP was in 1930 [1]. Since then, more than 700 publications on DP have been reported in the literature. However, it is surprising that the precise conditions in which it occurs or is preferred to continuous precipitation (CP) have not yet been clearly identified [2]. Discontinuous precipitation involves the decomposition of a supersaturated solid solution into two phases α0 lamellar structure downstream of a moving grain boundary, one α-depleted alloy element and having the same structure as the phase and α0 the other rich in β phase precipitated alloying element, it is symbolized by the following equation:
α0 → α + β and Figure 1 [3]. This reaction is governed by the grain boundary diffusion at wide angle, which is always effective to allow formation cellular.
The interest which attaches to the study of discontinueous precipitation in alloys of the Al-Zn comes from the fact that it has a detrimental effect on mechanical properties, physical and chemical properties of many engineering alloys. Unlike the continuous precipitation or the matrix is depleted in solute continuously over time, in the case of discontinuous precipitation, both the existence of a discontinuous change of direction and a sudden change concentration of solute on either side of the front migration reaction [4].
In a study by S. Abdou [5] on alloys of this system, it was found that the nucleation occurs not only at grain boundaries at all angles, but also in structural defects. Generally, studies on discontinuous precipitation in this alloy system are made in isothermal conditions in preference to high supersaturations and low temperatures (62˚C to 150˚C), although the grain boundary mobility is low in alloys of this system [6].
The plastic deformation introduced additional structural defects in the matrix such as dislocations, which increase the favourable sites for cellular precipitation, because the dislocations are aligned to form additional grain boundaries, which promotes this type of decomposition [7,8]. However, in some alloys, the restrain delays or completely eliminates the discontinuous precipitation [9,10]. The previous work does not cover a wide range of
Figure 1. Schematic of discontinuous precipitation [3].
strain rates and found in the study of the effect of cold plastic deformation during ageing at 470˚C 2mass% CuBe, that a deformation rate of 0% - 20% discontinueous precipitation retards by cons rate of 20% - 50% accelerates and 50% - 70% slow it down and over 70% of total abolition. However, a localized deformation such as a fingerprint or a scratch hardness of the surface can initiate discontinuous precipitation in these areas [11-14].
Discontinuous precipitation in alloys of Al-Zn system is characterized by complex metastable phases formed and they are is sensitive to quench rate, the homogenization temperature and amount of impurity [5]. These types of alloys does not remain homogeneous after quenching, because the matrix contains α0 concentration fluctuations, which are known by the Guinier-Preston zones (GP) or stage of pre-precipitation could affect the growth of cellular precipitates.
However, before the formation of the equilibrium phase β-rich element Zn, appear first metastable phases named GP form of segregation or clusters that retain the structure of the solid solution α0 and are fully consistent and second phase appears in R. The sequence of formation of precipitate in the solid solutions of Al-Zn system is as follows [5,15,16].
Supersaturated solid solution (α0)
↓
GP zones
↓
Phase R
↓
Equilibrium Phase (β)
The process of formation of metastable phases is dc [17,18], affecting the kinetics of discontinuous precipitation, either by reducing the driving force and/or by provoking a physical obstruction to the movement of reaction front [19]. This phenomenon has been observed in alloys of the Al-Zn [20,21]. In the alloy Ti-Fe-Co this is due mainly to the reaction of spinodal decomposition [22,23].
The formation of metastable phases is accompanied by an increase in elastic strain due to the coherence and distortions resulting network and when they reach the value of the shear strength of the matrix, the consistency is broken and dislocations of accommodation form, which gives the lowering of hardness. In reality these metastable phases of transition that may occur are difficult to distinguish from the stable phase by metallography.
However, the mechanism of Tu and Turnbull [24] is unlikely in this alloy system, an alternative mechanism suggested by Fournelle and Clark [25] is the most plausible. According to Peterman et al. [26], the cells appear only on one side of the grain boundary, then one seam is developed, however, the grain boundary deforms and moves in two opposite directions from its initial position, which gives a double seam (S mechanism). Bauman et al. [27] showed that low temperatures generally lead to the ageing mechanism S, against high ageing temperatures promote the unique morphology of the seam.
1.2. Dissolution of Cellular Precipitate
The reverse process of discontinuous precipitation is the dissolution of the precipitate in the matrix cell α0, or the reaction front formed during the discontinuous precipitation migrates in opposite directions in the two-phase structure [28,29] and according to the reaction
α + β → α'
Studies done by Pawlowski and Truszkowski [30] showed that the reaction of dissolution of precipitates in the cellular system Al-Zn alloy is controlled by volume diffusion process and can be done in two modes continuous and discontinuous. M. Sulonen [31] showed that the prestrain leads to faster dissolution of the latter to continued dominance by the dissolution reaction cons is an intermittently in samples not deformed. The dissolved phase leads to hardening of the alloy, it increases the voltage due to the precipitate rich in solute, by introducing it into the matrix [30]. Cons by continuous dissolution can have a positive effect on the mechanical properties of alloys in general, because it can cause the grain refining [31]. It was established that only a long homogenization annealing of several hours may lead to a complete dissolution of the particles of the second phase β formed during the discontinuous precipitation.
2. Experimental Methods
The alloys in question were prepared by melting under inert atmosphere (Argon) from aluminum (3NAL) and Zinc (Zn 4N5) very pure. The ingots obtained were subjected to a first homogenization annealing for 2 days at T = 350˚C, followed by a second homogenization annealing for 21 days at T = 400˚C and quenched in a methanol-salt solution Ice-T = –10˚C. For the demonstration of the effect of temperature we chose two aging temperatures of 75˚C and 160˚C and the effect of cold plastic deformation several samples at different strain rates were used. So that samples do not undergo oxidation during annealing processes, a set of vacuum annealing was designed. The used reagent is a solution consisting of 15 ml of hydrofluoric acid, nitric acid 15 ml and 30 ml of glycerin with the attack time is 10 to 30 seconds. The analytical techniques used in this regard are: optical microscopy, X-ray diffraction and Vickers hardness (HV).
3. Results and Discussion
3.1. Ageing at 75˚C
The structural evolution during ageing at 75˚C and a holding time of 22 hours of the alloy Al-15 at.% Zn for the samples showed that the deformation rate is between 45% and 55% discontinuous precipitation located mainly along the grain boundary (Figure 2(a)), by cons in the samples to strain rates below 45% and above 55% or it develops very slowly and in interaction with recrystallization, or it is totally absent (Figures 2(b) and (c)). However it is certain that if recrystallization occurs at lower ageing temperature, it substantially eliminates defects promote the precipitation and the effect of the deformation is more pronounced at this temperature.
The appearance of precipitate in the samples at strain rates between 45% and 55% is probably a weak interacttion between the discontinuous precipitation and recrystallization. So the plastic deformation does not support the discontinuous precipitation especially at low temperature in the alloy Al-15 at.% Zn. It is not a major factor contributing to the stimulation of the precipitate in this type of alloy. The best contribution to hardening structural proved to be one of the intermediate phase, given the size of its particles and their degree of dispersion in the matrix, the alloy softens and its properties fall more with the precipitate of equilibrium (Figure 3).
Figure 4 shows the early development of the mechanism S (double seam), which is the most dominant in this type of alloy and at this temperature. Similarly the spectra of X-ray diffraction confirm the emergence of a second phase during annealing at 75˚C (Figure 5).