The discontinuous precipitation and dissolution in the alloy Al-Zn system has been the subject of many theoretical and experimental investigations that have contributed to the understanding of the different mechanisms which control them. However, many questions remain unanswered because of the complexity of the constituted phases which are affected by the speed of the quenched, deformation, the temperature of homogenization and ageing effect. The purpose of this work is to clarify the effect of temperature and deformation on the mechanisms of these two reactions during ageing of Al-15 at.% Zn and Al-30 at.% Zn alloy. The techniques of analysis used in this respect are the optical microscopy, the X-ray diffraction and the hardness Vickers.
The first appearance of the DP was in 1930 [
α0 → α + β and
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 [
In a study by S. Abdou [
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
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 [
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 [
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 [
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
Studies done by Pawlowski and Truszkowski [
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).
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 (
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 (