Microstructure evolution of casting Mg alloy AM60B subjected to

compression deformation

CAO Han-xue(曹韩学)^{1, 2}, LONG Si-yuan(龙思远)^{1, 2}, YOU Guo-qiang(游国强)^{1, 2}, LIAO Hui-min(廖慧敏)¹

1. College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China;

2. National Engineering Research Center for Magnesium Alloys, Chongqing University,

Chongqing 400030, China

Received 12 June 2008; accepted 5 September 2008

Abstract: In order to research the microstructure evolution of casting Mg alloy AM60B after compression, the isothermally compressive deformation of different compression ratios followed by metallographic observation was performed. The influence of grain boundaries and second phases on the deformation and recrystallization behavior of the alloy was investigated with optical microscopy, followed by transmission electron microscopy (TEM) to gain an insight into the interplay between the dislocations and microstructure features. The investigation results show that the deformation structure featured by refined grains forms first at as-cast grain boundary when the compression ratio is low, and then spreads throughout the whole cross-section of the casting when the deformation ratio approaches 70%. TEM observation indicates that, dislocations preferentially distribute in the region next to the grain boundaries and second phases, which leads first to the recrystallization occurring there and bounds the recrystallization process in later deformation. Therefore, the grain boundaries and second phases are beneficial to keeping the recrystallized microstructure with fine grains, and may contribute to the formation of an inhomogeneous grain size distribution on the cross-section of the    alloy.

Key words: magnesium alloy; microstructure; compression deformation

1 Introduction

The utilization of magnesium for engineering components in the automotive industry has signi?cantly increased in the past few years. Despite magnesium has cost disadvantage compared with steel, it offers a high mass reduction potential when being used as a structural material because of its low density and acceptable strength[1]. Due to its excellent die filling properties and competitive processing cost, Mg alloys for structural applications are mainly formed by gravity casting techniques. However, poor processing quality hinders the use of Mg in mechanical performance critical applications. Hot deformation (300-400 ℃) proves itself a desirable shape forming technique for high performance Mg parts[2].

Generally, there are three main slip systems in magnesium alloy: basal, prismatic and pyramidal. According to the von Mises criterion, the activity of ?ve independent slip systems is required for plastic deformation of polycrystals. However, magnesium does not possess ?ve independent crystallographically equivalent slip systems. So twinning is necessary to retain the compatibility of deformation in magnesium polycrystals[3]. In the meantime, magnesium alloy might undergo the dynamic recrystallization (DRX) phenomena during hot working processes[4].

Because of their well-known diverse orientation, the shearing stresses on different slip systems in the grains of a solidified Mg alloy vary widely under certain external loads. As the shearing stress reaches its critical peak value of its slip system, dislocations start to move, which, in turn, leads first to preferential plastic deformation within the alloy[5]. On the other hand, the as-cast grain boundaries and secondary phases impose influence on the dislocation movement and the resultant recrystallization behavior, which consequently leads to the development of an inhomogeneous deformation microstructure. However, how the deformation process progresses in a grain and how the as-cast grain boundaries and second phases affect the recrystallizationbehavior remain unclear to date.

Therefore, the influence of grain boundaries and second phases on deformation, and microstructure evolution of AM60B Mg alloy were investigated by experimental deformation followed by numerical simulation, optical microscopy and transmission electron microscopy.

2 Experimental

To systematically observe the plastic deformation features of the as-cast microstructure of AM60, a series of experiments were designed to axially compress round bar specimens of the same geometry with the parameters in Table 1. To do so, AM60B Mg alloy was first cast into round bar of 16 mm in diameter by permanent mould gravity casting, and then sectioned to obtain round specimens for compression with a diameter-to-height ratio of 1.

Table 1 Compression parameters

In this method, specimens and pressing equipment were heated together to selected temperature. The specimens were then compressed at a steady speed of 50 mm/min, so that it took about 10 s to produce a sample.

Subjected to the designed deformation ratio, microstructure observation by optical microscopy and TEM on the zones of deferent deformation characteristics was performed after compression. In the meantime, numerical simulations of the compression process with the deformation parameters same as those used in experiments were performed to visualize the internal stress and strain distribution of specimens.

3 Experimental results

Fig.1 shows the appearance of the specimens subjected to axial compression at a temperature of 400 ℃, a compressing speed of 50 mm/min, but varied com-pression ratios of 30%, 45%, 60% and 70%. Fig.2 shows the micrographs of the central deformation zones in the specimens.

As Fig.2 shows, a compressive deformation of α-Mg matrix grain geometry is observed in the specimen subjected to a small compression ratio of 30%. The grain refinement resulting from the recrystallization of the heavily plastic deformation occurs mainly in eutectic enriched grain boundaries (as shown in Fig.2(a)). Along with the increase of deformation ratio, the fine grain zone spreads into central region of the as-cast α-Mg grain accompanied by the further compression of the as-cast grain geometry (as shown in Fig.2(b)), and the whole cross-section of the as-cast microstructure gets refined when the compression ratio approaches 60% (as shown in Fig.2(c)). When the deformation ratio reaches 70%, a typical deformation microstructure uniformly consisting of equiaxial fine α-Mg grains is obtained (as shown in Fig.2(d)) [6-7].

Fig.3 shows the results of the numerical simulation of a round bar subjected to axial compression at a temperature of 400 ℃, a compressing speed of 50 mm/min, but varied compression ratios of 30%, 45%, 60% and 70%. As the deformation maps indicate, the grids elongate in the direction perpendicular to compressive stress. As shown in Fig.4, the strain in central zone grids increases from 0 to 1.3 with the increase in deformation ratio, in consistence with the observed microstructure evolution subjected to deformation.

TEM observation indicates that dislocations preferentially distribute in the region next to the grain boundaries and second phases.

As shown in Fig.5, dislocations in different planes cross slip in α-Mg matrix and pile up in the region next to grain boundary to form dislocation networks. As a result of the orientation preference of their generation and high mobility, the density of the dislocations in central region and in the unfavorable slipping directions remains low, leading to an inhomogeneous distribution in the grains[8].

Fig.6 shows dislocations piled up in the matrix next to the Mg₁₇Al₁₂ phases. The observation indicates that when the moving dislocations confront the second phases, they are blocked and piled up[9-11], consistent with the  observation by other institutions[12-13].

Fig.1 Appearance of specimens subjected to different compression ratios: (a) As-cast; (b) 30%; (c) 45%; (d) 60%; (e) 70%

Fig.2 Typical microstructures of central zone of specimens subjected to compression at different ratios: (a) 30%; (b) 45%; (c) 60%;  (d) 70%

Fig.3 Simulated strain distribution in specimens subjected to different compression ratios: (a) Before compression; (b) 30%; (c) 45%; (d) 60%; (e) 70%

As Fig.6 shows, there are a great variation of the dislocation density and obvious existence of sub-grain structure, a mid-product of the dynamic recrystallization process during plastic deformation, around the second phase. This observation naturally reflects the preferential slipping of dislocation along some favorable lattice planes, and gives an explanation to the formation of the refined equiaxial fine zone along the as-cast grain boundary, where the as-cast secondary phases prefer to form.

4 Discussion

It is well-known that, in the process of dislocation movement in as-cast alloys under compression plastic deformation, the frontal dislocations by different activated lattice planes will be blocked on obstacles, such as fixed dislocations, impurity, second phase mainly consisting of eutectics in cast alloys, and grain boundaries, resulting in piling up and tangling each other there. The preferential activation of the dislocation move-movement and dislocation reactions in the pile-ups or tangles in front of obstacles lead to the formation of deformation sub-structures, accompanied by the appearance of lower dislocation density zones among the dislocation pile-ups. Furthermore, as indicated by the microscopy observation, the extent of recrystallization in a plastic deformation is associated with deformation ratio and the recrystallizing process is bounded by the grain boundary and the second phases preferentially present there[14].

Fig.4 Effect of compression ratio on deformation strain at central zone

Fig.5 Dislocation pile-ups on grain boundary

Fig.6 Dislocations blocked on secondary phases

In a progressive plastic deformation, more dislocations form, move and are blocked by the boundaries of sub-structure (or named sub-grain), second phases and as-cast grain boundaries. Under the drive of the unfavorably high energy state of the concentrated dislocations, the deformation sub-structure transforms gradually into sub-grain by polygonizing, and some of the sub-grain will grow gradually, leading to the formation of refined grain zone in the region around the as-cast grain boundary via a dynamic recrystallization process. When the plastic deformation progresses to a certain extent, the as-cast microstructure is swallowed by the recrystallization matrix, leading to a full replacement of as-cast microstructure of coarse grains bounded by second phases on grain boundaries with the fine equiaxial grains, as observed above and presented in the work of other institutions[15-16].

Blocked by the second phases, the crystal growth of recrystallized microstructure is not observable in the region around the original as-cast grain boundaries, while the grains become coarser in the central area of the original as-cast grains due to the lower dislocation density and higher grain growth potential without the second phases. Consequently, an inhomogeneous grain size distribution forms as shown in Fig.2(d)[17].

5 Conclusions

1) A fine-grain recrystallized deformation zone forms at the as-cast grain boundary when the deformation ratio is low, and a microstructure consisting of refined grains on the whole cross-section can be achieved when the deformation ratio reaches 70%.

2) The grain boundaries and second phases play an important role in recrystallization. Recrystallization occurs first on grain boundaries and then expands into α-Mg matrix with the increase of deformation ratio. However, the grain size distributes inhomogeneously in the centre of the original as-cast grains.

3) The second phase is beneficial to keeping the recrystallized microstructure with fine grain, and may contribute to the formation of an inhomogeneous grain size distribution on the cross-section of the alloy.

References

[1] CERRI E, LEO P, DE MARCO P P. Hot compression behavior of the AZ91 magnesium alloy produced by high pressure die casting [J]. Journal of Materials Processing Technology, 2007, 189: 97-106.

[2] MORDIKE B L, EBERT T. Magnesium properties—applications—potential [J]. Material Science & Engineering, 2001, A302: 37-45.

[3] M?THIS K, CHMEL?K F, JANE?EK M, HADZIMA B, TROJANOV? Z, LUK?? P. Investigating deformation processes in AM60 magnesium alloy using the acoustic emission technique [J]. Acta Materialia, 2006, 54: 5361-5366.

[4] FATEMI-VARZANEH S M, ZAREI-HANZAKI A, BELADI H. Dynamic recrystallization in AZ31 magnesium alloy [J]. Materials Science and Engineering, 2007, A456: 52-57.

[5] ZHENG Xiu-lin. The mechanical behavior of engineering material [M]. Beijing: Beijing University of Technology Press, 2004: 51-52. (in Chinese)

[6] LI Feng, SANG Yu-bo, ZHAO Li-wei. Influence of hot deformation on microstructure of AZ31 alloy [J]. Hot Working Technology, 2006, 35: 8-9. (in Chinese)

[7] HELIS L, OKAYASU K, FUKUTOMI H. Microstructure evolution and texture development during high-temperature uniaxial compression of magnesium alloy AZ31 [J]. Materials Science and Engineering A, 2006, 430: 98-103.

[8] BOHLEN J, YI S B, SWIOSTEK J, LETZIG D, BROKMEIER H G, KAINER K U. Microstructure and texture development during hydrostatic extrusion of magnesium alloy AZ31 [J]. Scripta Materialia, 2005, 53: 259-264.

[9] YI S B, ZAEFFERER S, BROKMEIER H G. Mechanical behavior and microstructural evolution of magnesium alloy AZ31 in tension at different temperatures [J]. Materials Science and Engineering A, 2006, 424: 275-281.

[10] NIE J F. Effects of precipitate shape and orientation on dispersion strengthening in magnesium alloys [J]. Scripta Materialia, 2003, 48: 1009-1015.

[11] SUN H Q, SHI Y N, ZHANG M X, LU K. Plastic strain-induced grain refinement in the nanometer scale in a Mg alloy [J]. Acta Materialia, 2007, 55: 975-982.

[12] CUI Zhong-qi. Metallography and heat treatment [M]. Beijing: China Machine Press, 1998: 174-176. (in Chinese)

[13] WANG R M, ELIEZER A, GUTMAN E. An investigation on the microstructure of an AM50 magnesium alloy [J]. Materials Science and Engineering A, 2003, 355: 201-207.

[14] SPIGARELLI S, EL MEHTEDI M, CABIBBO M, EVANGELISTA E, KANEKO J, J?GER A, GARTNEROV V. Analysis of high-temperature deformation and microstructure of an AZ31 magnesium alloy [J]. Materials Science and Engineering A, 2007, 462: 197-201.

[15] CHEN Zhen-hua. Deformed magnesium alloy [M]. Beijing: Chemical Industry Press, 2004: 68. (in Chinese)

[16] ISHIKAWA K, WATANABE H, MUKAI T. High strain rate deformation behavior of an AZ91 magnesium alloy at elevated temperatures [J]. Materials Letters, 2005, 59: 1511-1515.

[17] NAVE M D, BARNETT M R. Microstructures and textures of pure magnesium deformed in plane-strain compression [J]. Scripta Materialia, 2004, 51: 881-885.

(Edited by YANG Bing)