Discontinuous Precipitation in Al-8% Mass.Mg Alloy under the Effect of Temperature

doi:10.4236/msa.2011.29172

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Materials Sciences and Applicatio ns, 2011, 2, 1279-1284

doi:10.4236/msa.2011.29172 Published Online September 2011 (http://www.SciRP.org/journal/msa)

1279

Discontinuous Precipitation in Al-8% Mass.Mg

Alloy under the Effect of Temperature

Said Bensaada, Mohamed Tewfik Bouziane, Ferhat Mohammedi

Laboratoire Larhyss, Université Mohamed Khider BP Biskra, Biskra, Algeria.

Email: [email protected]

Received February 4th, 2011; revised April 20th, 2011; accepted June 23rd, 2011.

ABSTRACT

The precipitation in the alloy Al-Mg system has been the subject of many theoritical and experimental investigations

that have contributed to the understanding of the different mechanisms which control them. The purpose of this work is

to clarify the effect of temperature on the mechanisms of these during ageing of Al-8% mass.Mg alloy. The techniques

of analysis used in this respect are the optical microscopy, the X-ray diffraction and the micro hardness Vickers.

Keywords: Alloy Al-Mg, Precipitation, Temperature

1. Introduction

The transformations of the most significant phases are

those which appear in the supersaturated solid solutions

representing only one phase α0 in equilibrium and of

which the steady state corresponds to two phases. One α

depleted in alloy components and of the same structure

of the initial phase α0 and other phase precipitate β rich in

alloy components and of different structure. This trans-

formation is indicated by the term of precipitation and

corresponds to the reaction [1]:







The processes of precipitation which are gouverned by

diffusion phenomenon are generally classified in two

principal categories:

—The continues precipitation (or nucleation) is homo-

geneous and completely appears randomely in the whole

alloy. One of the principal characteristics of this reaction

is the continues variation of the lattice parameter of the

initial phase α0 and of the concentration of the solute

content.

—And the discontinuous precipitation where nuclea-

tion is heterogeneous leading to the presence of two dis-

tinct regions. One is transformed into alternate lamellar

form (





) behind a mobile grain boundary and the

other not. They takes place initially and preferably on

heterogeneities of the matrix phase α0 which they can be

dislocations, surfaces of impurities or grain boundaries.

Its characteristic during ageing is the discontinuous

variation of the lattice parameter of the matrix phase [2].

Precipitation in the Alloy Al-Mg System

Precipitation in this alloys system is characterized by

intermediate states exibits to the metastable phases ac-

cording to this scheme [3-10].

0G.P Zones





 







The stage of pre-precipitation in this system of alloy

leads to the formation of G.P zones (small domain rich

on solute content). Different technique were used for the

description of G.P zones in this alloy system, such as the

diffraction of X-rays with high angles used by Dauger

and all [11] in alloy Al-12% mass.Mg. The measure-

ments of the resistivity carried out in alloy Al-10%

mass.Mg and which show an increase in the resistivity

during ageing [12-14] and an electron microscopy is used

on the same alloy [15].

Some authors [16,17] assume that the formation of G.P

zones at room temperature is only for magnesium rates

beyond 10% in atoms, whereas others estimate that this

critical rate is about 5% in magnesium atoms [18, 19].

An increase in the temperature of ageing around 100˚C

[20] led to the dissolution of G.P zones and the parti-

cles





. Thus allowing also the formation of intermedi-

ate phase





, metastable, semi-coherent and with a com-

position of Al3 Mg2 of hexagonal structure. The precipi-

tates





are formed by nucleation and growth on the

structural defects of the matrix. Their growth leads to a

total loss of coherence by arrangement of the dislocations

to the







interface and consequently to the formation

of the stable phase β of the same composition than the

Discontinuous Precipitation in Al-8% Mass.Mg Alloy under the Effect of Temperature

1280

phase



 but of cubic with centered faces structure [20].

However, the transformation mechanisms of





 phase



 and of



 on β in equilibrium phase are not yet

clear. It is known that plastic deformation introduces

defects into the matrix after quenching leading to the

acceleration of the precipitation process. However in this

alloy system the ageing at high temperatures can start in

addition to the process of precipitation, the process of

recrystallization which can involve a slow down of the

precipitation process gouverned by diffusion. This means

that the recrystallisation led to the formation of a new

matrix containing much less dislocations and of fine

grains than those of the annealed alloy. But the nucleus

formation of a new phase within a solid solution super-

saturated and deformed after quenching, can oppose to

the movement of grain boundaries and consequently,

slow the recrystallisation and leads of an interaction be-

tween the precipitation and the recrystallization.

Indeed for the creation of the homogenoues structure

of the supersaturated solid solutions generally consist

with a an elevation of the load breaking by hardeness.

The increase in hardness in the alloy Al-Mg system per-

haps obtained by an increase in the magnesium content

[21-25]. However in alloys hardened by ageing, the me-

chanical properties are related on the size and the disper-

sion of the particles and their interaction with mobile

dislocations. In the same way the mechanism of harden-

ing is controlled by the shearing of these particles by

mobile dislocations. This would explain the reduction in

ductility accompanying the hardening observed [26,27]

du to the stackings of dislocations, caused by the stress

fields surrounding the particles.

2. Experimental Methods

Alloy Al-8%mass.Mg was prepared for the investigation

by vacum induction melting of the elements (Al and Mg)

of high purity. The ingots were then homogenized at

440˚C for 44 hours followed by an ice quenching. The

deformation of the samples used for ageing at 160˚C,

220˚C and 270˚C are obtained by colled rolling. The

ageing conditions are simultaneously obtained from lit-

erature and from the equilibrium diagram of the alloy

Al-Mg system. In this respect the vacuum annealing was

performed. The optical microscopy, the diffraction of

X-rays and the microhardness are the principal methods

of analysis. The metallographic observation were per-

formed after etching in the solution of Keller (07 ml of

HF, 09 ml of hydrochloric acid, 20 ml of nitric acid and

85 ml of distilled water).

3. Results and Discussion

In order to make in evidence the effect of the tempera-

ture on the mechanisms of precipitation in alloy Al-8%

mass.Mg, we have chosen three temperatures of ageing

(160˚C, 220˚C and 270˚C) favorable to this type of

transformation.

3.1. Ageing at 160˚C

The interesting feature of this alloy is that the plastic

deformation induced twins and dislocations which can

play an essential role to stimulate the process of precipi-

tation. However ageing at the temperature of 160˚C is

only favorable to the continuous precipitation and with

the intermediate phase





is characterized by an inter-

esting aspect as shown in Figure 1(a). This structure is

called Widmannsttaten, it is probably the direct cones-

quence of the germination of the phase



. The particles

of this phase are developed in the form of needles (Fig-

ure 1(b)) and when the nucleation and the growth of the

particles





are performed, i.e. the totality of the phase





is formed and total coherence is lost, thus we have

the transformation of





on β in equilibrium phase. The

(a)

(b)

Figure 1. Structural evolution of Al-8% mass.Mg alloy,

homogeneised at 440˚C during 44 h, quenched in ice, de-

formed of 35%, aged at 160˚C during 4 h (a) and during 18

h (b).

Discontinuous Precipitation in Al-8% Mass.Mg Alloy under the Effect of Temperature

1281

evolution of microhardness HV during ageing is repre-

sented in Figure 5(a). We can notice that a maximum

hardening is obtained after an annealing of 90 hours then

decreases during the extension of ageing time.

3.2. Ageing at 220˚C

During ageing at 220˚C, the presence of the lamellar pre-

cipitate is confirmed by optical microscopy as shown in

Figure 2(a) and (b). This means that this temperature is

only favorable to discontinuous precipitation developing

on the grain boundaries. The morphology of the precipi-

tate varies from a region to another. In the same way the

mechanism S is more dominating Figure 3(a) and (b)

The microhardness starting increases from the value ob-

tained at the quenched condition until a maximum corre-

sponding to a duration of 63 hours then to decrease Fig-

ure 5(b). One can deduce that the maximum hardening is

obtained when the totality of the intermediate phase





is formed. The transformation of



into equilibrium phase

Figure 3. Structural evolution of Al-8% mass.Mg alloy,

homogeneised at 440˚C during 44 h, quenched in ice, aged

at 270˚C during 22 h and with S-mechanism.

β leads to the decrease of hardness. In the same way the

diffraction of X-rays confirms the precipitation in this

alloy Figure 4.

(a)

3.3. Ageing at 270˚C

With the temperature of ageing at 270˚C, only the conti-

noues precipitation appears. For high temperatures dis-

continuous precipitation disappears, because the volume

diffusion prevents its growth and its nucleation becomes

difficult (slows down the movement of the reaction

front). The increase in the temperature accelerates rela-

tively the diffusion process. Best hardening is obtained

after 54 hours of duration Figure 5(c) and more we pro-

long ageing, we lose hardness more.

4. Conclusions

The whole results presented in this work reflects in par-

ticular the effect of the temperature on the mode of pre-

cipitation in alloy Al-8% mass.Mg. A predeformation

with ageing at 160˚C reveals the structure of Wid-

mannsttaten, with a growth in needles form and leading

(b)

Figure 2. Structural evolution of Al-8% mass.Mg alloy,

homogeneised at 440˚C during 44 h, quenched in ice, aged

at 220˚C during 22 h.

Discontinuous Precipitation in Al-8% Mass.Mg Alloy under the Effect of Temperature

1282

(a)

(b)

Figure 4. Xrays difraction of Al-8% mass.Mg alloy, homogeneised at 440˚C during 44 h, quenched in ice (a), aged at 160˚C

during 22 h (b).

Discontinuous Precipitation in Al-8% Mass.Mg Alloy under the Effect of Temperature1283

(a)

(b)

(c)

Figure 5. Microhardness (HV) evolution structural of

Al-8% mass.Mg alloy, homogeneised at 440˚C during 44 h,

quenched in ice, aged at 160˚C (a), at 220˚C (b) and at

270˚C (c).

to a continuous precipitation. Continues precipitation is

favoured at high and low temperature, while discontinu-

ous precipitation dominates at intermediate temperatures.

High temperatures accelerate relatively the diffusion

process and the mechanism S is more dominating in dis-

continuous precipitation. Hardening was observed at all

temperatures of ageing, however the extension of ageing

time of bearing led to a hardness fall.

5. Acknowledgements

The authors gratefully acknowledge the financial support

by the M.E.S.R.S Ministry for the Higher Education and

the Requirend scientist of Algeria under research labo-

ratory LARHYSS, University of Biskra.

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