AMR.264-265.66.pdf

Microstructural Evolution of AZ91 Magnesium Alloy during Deformation and Heat Treatment

Jing-Yuan Lia, Jian-Xin Xieb

University of Science & Technology Beijing, China

[email protected], [email protected]

Key words: microstructure; AZ91 alloy; heat treatment; deformation.

Abstract. Microstructural evolutions of AZ91 magnesium alloy, which applied to homogenizing

annealing treatment, hot extrusion and ageing treatment respectively, are investigated in this paper.

Results exhibit that the massive -Mg17Al12 phase appearing in the initial structure dissolves to

eliminate in -Mg matrix mostly due to the homogenizing annealing treatment; dynamic

recrystallization and consequent grain refinement occur during extrusion; banded structure, in

which the -Mg17Al12 phase precipitates parallel to the extrusion direction in -Mg matrix, are

observed in the ageing treated specimen. Furthermore, the effects of temperature, keeping time of

homogenizing annealing treatment and ageing treatment, and the extrusion processing parameter on

the microstructural evolution of this alloy are also discussed according to the experimental results in

details. In addition, the various mechanisms of the morphology and quantity of -Mg and

-Mg17Al12 phase are analyzed corresponded to the various states. As for the banded structure

appearing in the ageing treated specimen, it can be attributed to the banded segregation which

remained in the extruded one and resulted from the casting dendrite and subsequent extrusion.

Introduction

Magnesium alloy is gradually becoming important since it has low density, high specific strength,

high dimensional stability, is easy to machine, and is easy recyclable. As the lightest material for

structural applications, Mg alloy is a potential candidate for replacing steel and aluminum alloy,

especially in some automotive area. Thus, research and applications on Mg alloys have surged in

recent years [1].

Su-Hai Hsiang et al [2] investigated the mechanical properties of the hot extruded magnesium

alloy AZ31 and AZ61 in the optimal processing conditions. N.Ogawa [3] suggests that the

deformation temperature should be avoided higher than 400, because severe oxidation occurs on

the billet surface in a furnace for Mg alloys. Mechanical properties of model magnesium AZ91 cast

alloy in initial and application next solution heat treatment (T4) state were tested by Cizek [4]. The

extrusion properties of AZ91 alloys were investigated at 335, 370 and 415 [5].

However, it is still largely unknown about the microstructural evolution of AZ91 Mg alloy

during the whole forming process, which includes previous homogenization, extrusion and

subsequent ageing treatment. Especially, since the morphology of -Mg17Al12 phase exercise a

great influence on the ductility in Mg-Al alloys, the present study aims to explore the morphologies

of -Mg17Al12 obtained in the above processes.

Experimental Procedure

The AZ91 alloy used in the present study, the composition of which is 8.4%Al-0.88%Zn-0.34%Mn

mainly, was supplied by General Research Institute for Nonferrous Metals, China. The alloy is

provided in the form of 93mm diameter bars, and then the bars are cut to the extrusion specimen

with 30mm in diameter and 45mm in length.

This study is performed in three test processes, which are homogenizing annealing treatment,

extrusion and aging treatment of the specimen respectively. The homogenizing annealing treatment,

which is subsequent with air cooling, is performed in tubular resistance furnace at 350, 380, 420

and 450 for 5, 10, 15, 24hours. The extrusion of specimen without lubricant is performed at

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temperature 320, 350, 380 and 400, and extrusion ratio of 10 and 40 after applied to

homogenization with the optimized parameter, which is 380 for 15h. The exit speed of extrusion

is 1.6m/min. Subsequently, the ageing treatment is performed at 200 for 2, 5, 10, 15 hours.

The microstructures of the specimens applied to various treatment or deformation are observed

by OM and SEM. Prior to observe, the specimens were etched with acetic glycol (20ml acetic acid,

1ml HNO3, 60ml ethylene glycol and 20ml water) and the average grain size was measured by the

linear intercept method.

Results and Discussion

Eutectic Structure Observed in the Initial Material. Based on the Mg-Al binary equilibrium

phase diagram [6], the maximum solid solubility of Al in Mg is W(Al)=12.7% at the eutectic

temperature, 437, while it shows a minimum solid solubility, about 2%, at room temperature.

Therefore, the microstructure of AZ91 alloy with composites of Al about 9%, should be a mixture

structure of -Mg solid solution matrix and -Mg17Al12 precipitate. However, the eutectic reaction

occurs always at the end of solidification since the speed of actual solidification is impossible to be

slow enough to keep the equilibrium solidification. The actual solidification can be described using

the dashed line in the non-equilibrium phase diagram, as shown in figure 1.

Fig.1 Parts of Mg-Al Binary Non-equilibrium Phase Diagram[6]

The as-received microstructure with an average grain size of 100m is shown in figure 2, in

which the massive divorced eutectic appears at the boundary of -Mg grains. The second

-Mg17Al12 precipitate is identified near the eutectic. The divorced eutectic structure forms in the

interdendritic and grain boundary region, surrounded by eutectic -Mg which is enriched in

aluminum after solidification of the primary -Mg dendrites. Then the second -Mg17Al12 phase

precipitates on the -Mg matrix and near the primary eutectic -Mg17Al12 phase during the

subsequent cooling, since these regions have higher aluminum contents than the centers of the

dendrites, where the aluminum concentrations may be as low as 2 wt% Al. Zn, which is another

important alloy element besides Al in AZ91 alloy, is mainly dissolved in -Mg17Al12 phase but not

in -Mg matrix.

Fig.2 (a) optical and (b) scan electronic micrographs of as-received AZ91 alloy

L

Al W(Al)%

Tem

per

atu

re/

A

E C

C

L+

++++

12.7 9.0

Mg

(a)

Advanced Materials Research Vols. 264-265 67

Solution of -Mg17Al12 during Homogenizing Annealing Treatment. As is well known,

Mg17Al12 is a very brittle phase in Mg-Al alloys, and its morphology, size, quantity and distribution

exercise a great influence on the plasticity of Mg-Al alloys. The casting structure, in which reticular

-Mg17Al12 phase exists at the -Mg grain boundary, leads to fracture and limited ductibility during

deformation. In contrast, fine, homogenous and equiaxial -Mg17Al12 particular improve the

formability significantly. In order to obtain such a perfect morphology of microstructure, several

homogenization tests are carried out on AZ91 alloy employed in this investigation. Since the solid

solution point of AZ91 is about 360 and the solidus point is 437, the experimental temperature

is determined at 350, 380 and 420, and keeping for 5h, 10h, 15h and 24h.

Fig.3 Microstructures of AZ91 alloy homogenized at (a) 350 for 5h, (b) 380 for15h;

(c) 420 for 24 h respectively

The optical microstructures of homogenization treated specimen are shown in figure 3, in which

the lighter background corresponds to the -Mg matrix and the black regions or lines represent the

remained -Mg17Al12 phase. It is obvious that the -Mg17Al12 phase decreases as the temperature

elevating and the keeping time extending. As can be seen in figure 3a, no significant change is

detected in the experimental material after treated at 350 for 5h, while comparing with the initial

state described in section 3.1. As the keeping time extending at this treatment temperature, the

morphology gradually transforms from dendritic to equiaxial. In addition, more -Mg17Al12

dissolved into -Mg matrix and the remained -Mg17Al12 is investigated being at the boundary of

grain. But till kept for up to 24h, the reticular structure can still be seen in AZ91 alloy. When the

thermal temperature is elevated to 380, -Mg17Al12 phase dissolves more quickly than treated at

350. Kept for 15h at this temperature, the massive -Mg17Al12 has eliminated and changed to

discontinuous linear existing at boundary of -Mg grains. The aluminum has mostly solid dissolved

in -Mg matrix, and the alloy becomes to a supersaturated solid solution at room temperature. At

this state, the grains still retain comparably small, with average diameter of 130m. However, when

the keeping time is extended to 24h, the gains grow up to 200m. When treated at 420 for 5h,

most -Mg17Al12 phase has dissolved additionally the coarsening of structure can not be avoided.

According to the results stated above, homogenization keeping at 380 for 15h exhibits the

optimized microstructure morphology, which are the least -Mg17Al12 phase and relatively fine

structure and high mechanical properties. This treatment processing is employed in further

extrusion test described in next section.

Grain Refinement and Incomplete Recrystallization during Extrusion. As is well known, the

activation of various slip systems of Mg greatly depends on the deformation temperature. Although

Mg alloys exhibits limited formability at room temperature, slip systems (1011) [1120] and

(1010)[1120] are also activated when the temperature is beyond 300. After the optimized

homogenizing treatment (380 for 15h), extrusion are carried out at 380 and various extrusion

ratio. The result is shown in figure 4. Figure 4(a) illustrates that, even after a 10 extrusion ratio,

significant microstructural changes occur: grains get significant refinement, grain boundary become

wary and clear, recrystallized grains are visible along some grain boundary areas, non-crystallized

grains are elongated to a clubbed morphology. In addition, it can also be seen that the

non-recrystallized grains appear mostly in the center rather than at the edge of the specimen because

the deformation in the center is not sufficient for complete recrystallization. On the other hand, an

equiaxed microstructure with an average grain size of 10m is present after extrusion at 380, ratio

of 40, as shown in figure 4(b).

200m

(a)

200m

(c)

200m

(b)

68 Advances in Materials and Processing Technologies II

Fig.4 Microstructure of AZ91 Mg alloy extruded at 380

with extrusion ratio of (a) 10 and (b) 40

Figure 5 is the microstructures of AZ91 alloy extruded at various temperatures with the same

extrusion ratio of 40. Very fine recrystal, which is 1m or so, are observed at the boundary of the

elongated initial grains and still some non-recrystallized grains are remained when the working

temperature is 320 and 350, as shown in figure 5(a) and (b). When the extrusion temperature is

elevated to 400, an equiaxed, homogeneous and completely recrystallized microstructure is

obtained. Consequently, the growth of new grain occurs and the average size of the grain is 10m,

as shown in figure 5(c).

Fig.5 Extruded microstructure of AZ91 alloy at (a) 320; (b) 350; (c) 400

with extrusion ratio of 40

Many researchers has reported that the severe shearing deformation, such as equal channel

angular extrusion(ECAE), high pressure torsion(HPT), can lead to grain refinement in metal alloys.

The results obtained in this investigation suggest that significant grain refinement can also be

achieved in Mg alloys using very traditional processing, such as extrusion. That should be attributed

to the low stacking fault energy (SFE) of magnesium, which has a value of 50-78mJ/m-2

, compared

with titanium (>300mJ/m-2

), aluminum (200mJ/m-2

). The SFE is a material property on a very small

scale and it modifies the ability of a dislocation in a crystal to glide onto an intersecting slip plane.

When the SFE is low, the mobility of dislocations in a material decreases. It means that the dynamic

recrystallization is easy to occur during the hot working, even at relatively low temperature and

small deformation, for instance, temperature of 320 and extrusion ratio of 10 respectively in this

investigation. Nevertheless, the recrystallization performs incompletely and a mixed crystal

structure forms under the low temperature and small extrusion ratio conditions. With elevating the

working temperature or increasing the extrusion ratio, the fraction of recrystallized grains increased

progressively. An equiaxed microstructure is present when the temperature and extrusion ratio are

beyond 380 and 40 respectively. Consequently, the growth of the new grains occurs at elevated

temperature.

Banded Precipitation during Ageing Treatment. After extrusion at 380 and extrusion ratio

of 40 accompanied with air cooling, the specimens of AZ91 alloy are ageing treated at 200 for

various times. Figure 6 shows the microstructural evolution of the specimens after ageing treatment.

Figure 6(a) illustrate that block -Mg17Al12 phase precipitates mainly at grain boundary after treated

for 2h. In addition, the structure appears non-homogeneous and some precipitation bands also can

be seen. As the ageing treatment extending, it becomes obvious that the block -Mg17Al12 phase

precipitates paralleled with each other so as to form a banded structure, and the quantity of

-Mg17Al12 phase increases. Intense banded -Mg17Al12 precipitate is observed after treated at

200 for 15h as shown in figure 6(d).

(b)

20m 20m

(a)

(b) (c)

20m 20m 20m

(a)

Advanced Materials Research Vols. 264-265 69

Fig.6 Microstructures of AZ91 alloy after ageing treated at 200

for (a) 2h; (b) 5h; (c)10h; (d)15h

The banded morphology observed here should be attributed to the banded distributions of

aluminum in the extruded materials. The development of structure and precipitation can be deduced

as follows. As stated in section 3.1, severe aluminum segregation, in which that aluminum is rich at

interdendritic while it is poor in the center of dendrite, occurred during solidification of the AZ91

alloy. The divorced eutectic -Mg17Al12 phase dissolves into the -Mg matrix in the homogenizing

treatment and a supersaturated solid solution forms during the followed cooling. However,

aluminum and other alloy elements can hardly to homogenize wholly in the matrix because the

diffusion of alloy elements is quite difficult in Mg matrix. It means that the dendrite distribution of

aluminum is remained in the specimen for the subsequent extrusion, although this segregation is not

obvious prior to extrusion as shown in figure 3. And then the interdendritic regions enriched in

aluminum were elongated parallel to the deformation direction during the extrusion. In other words,

a banded segregation of aluminum forms. The regions rich in aluminum become the nuclei of the

second -Mg17Al12 precipitation during the subsequent ageing treatment. Moreover, the growth of

-phase is along the bands enriched aluminum as the keeping time the ageing treatment extending.

Thus, a structure of intense banded -Mg17Al12 phase saturated in the -Mg matrix forms.

Conclusions

Reticular divorced eutectic -Mg17Al12 is observed at the -Mg grain boundary in the as-received

AZ91 alloys. Homogenizing treatment may improve the microstructure and morphology of this

alloy. A structure with average grain size of 100m and -Mg17Al12 particular scattered at the -Mg

boundary can be obtained after the optimized thermal treatment at 380 for 15h. Extrusion, in

which recrystallization and consequent refinement occur, is carried out followed the optimized

homogenization. The results illustrate that a mixed grain structure composed of recrystals and

remained initial grains is obtained when the extrusion temperature and ratio are lower than 380

and 40 respectively. Homogenous, equiaxed and fine grains with size of 10m can be obtained at

380, 400 with a ratio of 40. Banded -Mg17Al12 precipitates saturated by -Mg matrix forms in

subsequent ageing treatment. It can be deduced that the solidification segregation regions of

aluminum are elongated during extrusion, and then these regions become the nuclei center and

growth direction of -Mg17Al12 precipitates. To control the working temperature and keeping time

for AZ91 alloy so as to obtain a fine homogenous structure should be performed in further study.

a b

c d

70 Advances in Materials and Processing Technologies II

Acknowledgments

This work was supported in part by the National 973 Major Project of China, The Key

Fundamental Problem of Processing and Preparation for High Performance Magnesium Alloy,

under Grant No. 2007CB613703.

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[3] N.Ogawa, M. Shiomi, K. Osakada, Forming limit of magnesium alloy at elevated temperatures for precision forging, Int. J. of Mach. Tool Manu., Vol.42 (2002), p. 607-614

[4] L. Cizek, M. Greger, L. Pawlica: Study of selected properties of magnesium alloy AZ91 after heat treatment and forming, J. of Mater. Process. Tech Vol.157-158 (2004), p. 466-471

[5] N.V. Ravi Kumar, J.J. Blandin, C. desrayaud, F. Montheillet, M. Suery, Grain refinement in AZ91 magnesium alloy during thermomechanical processing, Mat. Sci. Eng. A Vol.359 (2003),

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Advanced Materials Research Vols. 264-265 71

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