Microstructure and mechanical properties of hot-rolled Mg-Zn-Y-Zr magnesium alloy

YAN Hong(闫 宏), CHEN Rong-shi(陈荣石), HAN En-hou(韩恩厚)

Environmental Corrosion Center, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China,

Received 28 July 2006; accepted 15 September 2006

Abstract: Fine-grained magnesium alloys strengthened by quasicrystalline particles were easily developed by thermomechanical process for Mg-Zn-Y-Zr alloys. The microstructure evolution of Mg-Zn-Y-Zr alloys hot rolled with different reductions at different temperatures was studied. Tensile tests and fracture observation were carried out to study the mechanical properties of this alloy. The thin magnesium sheets hot rolled at 380 ℃ exhibit better combination of high strength and ductility than that hot rolled at lower temperature. The results show that the grains become equiaxed and uniform as compared with those of the extruded materials because of recrystallization and repeated heating between rolling passes. It is also found that with the increasing rolling temperature and strain the I-phase particles become much smaller and are homogeneously distributed in the matrix, which enhances both strength and ductility.

Key words: Mg-Zn-Y-Zr alloy; hot rolling; quasicrystalline particles; recrystallization; mechanical properties

1 Introduction

Nowadays, magnesium alloys, as the lightest structural material, have attracted more and more attentions in aerospace, automobile and 3C industries because of its high specific strength and stiffness and good damping capacity.

Currently, effective methods of improving mechanical properties of magnesium alloys are taken, such as alloying and hot deformation. Mg-Zn-Y-Zr alloy has better mechanical properties than conventional ZK60 due to addition of Y, which can form three new kinds of ternary equilibrium phases, i.e. W-phase (Mg₃Zn₃Y₂, cubic structure), I-phase (Mg₃Zn₆Y, icosahedral quasicrystal structure, quasi-periodically ordered), and Z-phase (Mg₁₂ZnY)[1], and increase the eutectic temperature of this alloy significantly[2]. I-phase has an important strengthening effect on Mg-Zn-Y-Zr alloys because of high hardness, thermal stability, high corrosion resistance, low friction coefficient, low interfacial energy, etc. It is stable against coarsening at high temperature due to the low interfacial energy of the quasicrystals[3], supplying high bonding properties at the I-phase/matrix interface. Most researches focus on the structure of I-phase and the effect of ratio of Zn to Y on the properties of this alloy[4-7]. Recently, some researchers bring their attention to the area of improving the mechanical properties through thermomechanical process. It has been reported that Mg-Zn-Y alloys containing thermally stable I-phase exhibit significantly high yield strength and ductility at ambient temperature, depending on the volume fraction of the I-phase[8]. Also, enhanced mechanical properties have been achieved due to small quasicrystalline particles of the I-phase in the extruded Mg-Zn-Y alloys[9]. In this paper, an conventional thermomechanical process, i.e. hot-rolling which is much effective in industry were utilized and the correlated microstructure evolution was investigated during hot rolling process. It aims to provide proper instructions to hot rolling process of producing Mg-Zn-Y-Zr alloy thin sheets.

2 Experimental

The magnesium alloy used in the present study has a chemical composition of Mg-5.8%Zn-1.0%Y-0.48%Zr (mass fraction). The ingots were homogenized at 380 ℃ for 6 h and extruded into plates with thickness of 13.8 mm with a reduction of 10?1. These plates were repeatedly rolled and the rolls were heated to 100 ℃ prior to rolling. Cross rolling was taken and the first rolling direction was parallel to the extrusion direction. The blocks were heated at 320, 350 and 380 ℃ for 1.8×10^-3 s and then rolled with an average reduction of 13.8%, 15.3% and 17.3% per one pass, respectively. The heating and rolling were repeated for 12, 11, 10 passes and finally, the thickness of sheets reached 2.2 mm with total reduction of 84%. Uniaxial tensile tests were carried out on dog-bone specimens cut from the hot-rolled sheets and as-annealed sheets (specimen gauge length is 25 mm) under a constant cross head speed with an initial strain rate of 10^-3 s^-1 at room temperature. The microstructures of hot-rolled sheets with total strain of 0.48, 0.68, 0.84 and their tensile fracture surfaces were observed using a scanning electron microscope (SEM).

3 Results and discussion

3.1 Microstructure

Fig.1(a) shows a SEM image of the as-cast alloy, and eutectic pockets can be clearly seen. This alloy            consists of two phases, i.e. a-Mg and I-phase. Fig.1 (b) shows the SEM images of the alloy extruded at 380 ℃. It is seen that the eutectic pockets have been destroyed to small particles (less than 1 μm) which were uniformly distributed in the matrix. Some big particles (about 5 μm in size) were also found to be along the extrusion direction. After extrusion, the grain was significantly refined probably because of occurrence of dynamic recrystallization during extrusion.

The SEM images of alloys at different temperatures to total strains of 0.48, 0.68 and 0.84 are shown in Fig.2. From this figure, effects of strain and rolling temperature on the microstructures of this alloy can be studied.

At 320 ℃ and the strain of 0.48, the most obvious change of the microstructure is that the big I-phase particles along the extrusion direction were destroyed to small ones. When the strain increases to 0.68, only a few twins can be observed and the grain shape becomes irregular. At the strain of 0.80, the grains seem to be equiaxed due to recrystallization and no trace of extrusion can be seen in the microstructure. At 350 ℃, and the strain of 0.48, the microstructure looks similar to that rolled at 320 ℃. When the strain reaches 0.68, the volume fraction of twins increases significantly with contrast to that rolled at 320 ℃ and some recrystallized small grains along grain boundaries and twinned regions can be found (see arrows in Fig.2(e)). Also it should be noted that the twin bands are most at 35?-45? with respect to the extrusion direction. At the strain of 0.80, twins disappear and the grains become equiaxed and smaller than that hot- rolled at 320 ℃. At 380 ℃, and the strain of 0.48, there exists a small fraction of twins and a lot of recrystallized small grains along grain boundaries. When the strain reaches 0.68, the recrystallized grains replace twins. At the strain of 0.80, the grains become more equiaxed and the small particles are distributed almost homogeneously across the matrix.

At the strain of 0.48, no twins are observed, which suggests that deformation takes place mainly by dislocation slip. When the strain is up to 0.68, appearance of a large amount of twins both at 320 ℃ and 380 ℃ indicates that both twining and slip act as deformation modes to some extent. But when the temperature is as high as 380 ℃, no obvious twins occur. This is consistent with the fact that twinning becomes less important for deformation at high temperatures[10]. At the strain of 0.80, the twins disappear and the grains become equiaxed because of recrystallization in repeated heating between rolling passes. As a result, with the increasing rolling temperature and strain, the particles become much smaller and more homogeneously distributed in the matrix. However, significant effect in grain refinement was not observed as was expected. This might be ascribed to the result of repeated heating associated with the multiple rolling passes[11].

3.2 Room temperature strength

The results of the tensile tests of the alloy rolled at different temperatures and followed by annealing are shown in Table 1. As shown in the table, both tensile strength (TS) and yield strength (YS) of the hot-rolled alloy increase with the increase of hot rolling temperature, while elongation of the rolled material change with a reverse way. It is attributed to the smaller grains and more homogeneous distribution of destroyed particles, as shown in Fig.2, which have a strong strengthening effect due to the pinning of dislocations . It must be pointed out that the reduction per pass increases with increasing of rolling temperature. In contrast to the as-rolled alloy, both tensile strength and yield strength of the alloy annealed at 350 ℃ for about 30 min decreased, which should be ascribed to the relie of strain hardening during annealing. The extent of reduction of YS decreases with the increase of temperature and is much more than that of TS. It is related to the fact that strain hardening is much intense at low rolling temperature. The elongation of the as-rolled sheets decreases with the increase of rolling temperature, while that of as-annealed sheets increases.

Fig.1 SEM micrographs of Mg-Zn-Y-Zr alloy: (a) as-cast; (b) extruded at 380 ℃

Fig.2 SEM images of Mg-Zn-Y-Zr alloy hot rolled at different temperatures: (a)-(c) 320 ℃; (d)-(f) 350 ℃; (g)-(i) 380 ℃, with strains of 0.48, 0.68 and 0.8, respectively

Fig.3 Tensile fracture surface of Mg-Zn-Y-Zr at various states: (a) hot rolled at 320 ℃; (b) hot rolled at 320 ℃ and then annealed at 350 ℃ for 30 min; (c) hot rolled at 380 ℃; (d) hot rolled at 380 ℃ and then annealed at 350 ℃ for 30 min

Table 1 Tensile properties of hot-rolled and annealed Mg-Zn-Y-Zr magnesium alloy

3.3 Fracture analysis

Fig.3 shows the SEM images of the tensile fracture surfaces of hot-rolled alloy. As shown in Fig.3(a), the failure surface consists of many small deep dimples and a few cracked particles. These brittle particles were broken during the tensile tests and prone to become the crack source[12]. While in Figs.3(c)-(d), a lot of cracked particles (see arrows in Fig. 6(c)), which directly result in the low ductility, are observed. Compared with that of the as-rolled alloy, the failure surface of the as-annealed alloy is composed of a great deal of much small deep dimples and almost no cracked particles. As discussed above, the particles are smaller at high rolling temperature, which means the quantity of particles is big. As a result, stress concentration became intense at the site of particles due to the pinning effect of small particles to dislocation. These sites become the source of fracture. The absence of cracked particles in Fig.3(b)-(d) is because of the relieving of stress concentration at sites of particles during annealing, which also explains the elongation increase after annealing. Thereby it can be concluded that the ductility decreases with the increase of cracked particles.

4 Conclusions

1) The microstructural evolution and mechanical properties of a Mg-Zn-Y-Zr alloy rolled at different temperatures and strain are studied. It was found that with the rolling temperature and strain increasing, the I-phase particles become much smaller, which are homogeneously distributed in the matrix.

2) The grains become equiaxed and uniform as compared with those of the extruded materials because of recrystallization and repeaed heating between rolling passes. After annealing, the TS decreases a little while the ductility almost double.

3) The Mg-Zn-Y-Zr magnesium alloy hot rolled at 380 ℃ exhibits good combination of strength and ductility, because dynamic recystallizaion is prone to happen and coarse I-phase particles are easily broken to small ones which can improve the mechanical properties.

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(Edited by CHEN Ai-hua)

Foundation item: Projects(20373072, 20473091) supported by the National Natural Science Foundation of China

Corresponding author: YAN Hong; Tel: +86-24-23893116; E-mail: hyan@imr.ac.cn