Microstructure and mechanical properties of

rolled Mg-12Gd-3Y-0.4Zr alloy sheets

WANG Rong(王 荣)¹, DONG Jie(董 杰)², FAN Li-kun(范立坤)¹,

ZHANG Ping(张 平)¹, DING Wen-jiang(丁文江)^{1, 2}

1. National Engineering Research Centre of Light Alloys Net Forming, Shanghai Jiao Tong University,

Shanghai 200240, China;

2. The State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University,

Shanghai 200240, China

Received 12 June 2008; accepted 5 September 2008

Abstract: The extruded Mg-12Gd-3Y-0.4Zr alloy sheets were rolled from 30 mm to 2.3 mm at 723 K by electric heated rollers, and then different heat treatments were performed to improve their properties. The microstructures and tensile properties of the alloy sheets were investigated, including as-rolled, annealed and T5 treated. The experimental results show that the grains are effectively refined by the rolling process, and the strength of the rolled alloy is greatly enhanced. The annealed alloy exhibits lower strength and higher elongation than the rolled one, while the aged alloy shows higher strength and lower elongation. After being aged at 498 K for 17 h, the alloys get the highest strength, namely, the ultimate tensile strength is 456.8 MPa, yield strength is 348.9 MPa, and elongation is 3.8%.

Keywords: Mg-Gd-Y alloy; hot rolling; precipitation; heat treatment; mechanical properties

1 Introduction

Because of their low density and high specific strength, magnesium alloys have been applied in various fields, e.g. automotive, aerospace and military industries, and played attractive efforts. However, magnesium alloys containing high rare earth metals have remarkable precipitation hardening effects and heat-stability, so these alloys have not only the merits of commercial magnesium alloys, but also high strength and good creep resistance[1-3]. Mg-RE system alloys have been widely studied in recent years, especially the Mg-Gd-Y series alloys, which are regarded as potential high strength magnesium alloys. The microstructure evolution and the strengthening mechanism of as-cast Mg-Gd-Y alloy have been investigated recently[4-7]. Besides, Mg-Gd-Y alloys have been extruded and then T5 treated to promote their mechanical properties, which achieved remarkable results[8-9]. CHEN et al[10] reported that rolling process can strengthen Mg-9Gd-4Y-0.65Mn alloy, which was rolled at 773 K with the reduction of 10% per pass. And Mg-Zn based system alloys have also been rolled to improve the alloy properties[11-12].

To enhance the mechanical property of Mg-12Gd- 3Y-0.4Zr alloy sheet through deformation and heat- treatment, the hot rolling process of Mg-12Gd-3Y- 0.4Zr alloy was investigated in this work, then the microstructure and mechanical properties of the alloys at both rolled and heat-treated conditions were studied.

2 Experimental

The alloy used in this study was Mg-12Gd-3Y-0.4Zr (GW123K, nominal composition of 12.1%Gd, 3.46%Y, 0.38%Zr, (mass fraction) balance Mg). The alloy was hot extruded to a rectangular bar with a cross-section of 130 mm×30 mm at 673 K. The extruded bar was pre-heated at 723 K for 0.5 h, and then hot rolled to the thickness of 2.3 mm in hot rolling mills. The temperature of the rollers was controlled to 473 K by electric heaters inside. The total thickness reduction was about 92%, and the reduction per pass was about 30%. The rolled plates needed to be re-heated to 723 K between passes to keep their deformability. The rolling direction was parallel to the extrusion direction of the as-received sheets.

The alloy was investigated in as-rolled, annealed and T5 heat-treated condition. For annealing treatment, the specimens were annealed both at 723 K and 773 K for 2 h, and for T5 heat-treatment, the specimens were held at 489 K for 17 h. The microstructures of the specimen were examined with optical microscope and scanning electronic microscope. For mechanical testing, specimens were machined with load axis parallel to rolling direction, and the room temperature tensile test was performed on Zwick/Roell material test machine.

3 Results and discussion

3.1 Microstructures

GW123K alloy sheets can be rolled with the reduction of 30% per pass at 723 K from 30 mm to 2.3 mm without any edge crack (Fig.1).

Fig.1 Macrograph of GW123K rolled sheet

The optical microstructures of the as-extruded and as-rolled GW123K alloy are shown in Fig.2. It can be seen that the grain size is refined after rolling. The average grain sizes of alloys in Fig.2(a) and (b) are 20 μm and 11 μm, respectively. It’s obvious that dynamic recrystallization occurs during the rolling process, meanwhile there are a lot of precipitates along the grain boundaries or inside the grains (Fig.2(b)). Fig.3 shows the SEM image of the rolled sample, from which it can be seen the precipitates are mainly stone-shaped compound and cuboid-shaped compound. The size of stone-shaped compound is about 1-3 μm, while that of the cuboid-shaped one is less than 1 μm. The results of EDX analysis indicate that the precipitates are MgGdY metallic compounds, as shown in Table 1. It can be inferred that the composition of cuboid-shaped compound is Mg₅(GdY) and the stone-shaped compound is Gd+Y supersaturated solid solution. The cuboid-shaped

Fig.2 Microstructure of GW123K alloy: (a) As-extruded;    (b) As-rolled

Fig.3 SEM image of as-rolled sample

Table 1 EDX result of as-rolled Mg-Gd-Y alloy

compound is similar to the results that HE et al[13] and WANG et al[4] reported in the as-cast Mg-10Gd-xY-Zr alloy, but the report of stoned- shaped compound hasn’t been found.

After annealing treatment, the grain size grows up to above 20 μm, but the amount and the shape of stone- shaped compound haven’t changed heavily (Fig.4(a)). The microstructure of the alloy at T5 condition is shown in Fig.4(b). Except the growth of the grain, no significant difference can be observed by OM compared with as-rolled condition. And the result of SEM shows that heat-treatment has no significant effect on MgGdY metallic compounds.

Fig.4 Microstructures of heat-treatment alloys: (a) 723 K, 2 h; (b) 498 K, 17 h

3.2 Mechanical properties

The hardness(HV) of the alloy increases from 110 in as-rolled condition to 129.8 in T5 condition, which can be seen from the precipitation hardening curves (Fig.5). The peak hardness and corresponding aging time

Fig.5 Precipitation hardening curves of GW123K alloy aged at different temperatures

decrease with the increasing of aging temperature, and GW123K alloy exhibits the highest peak hardness(HV) of 129.8 when aged at 498 K.

Table 2 shows the room temperature tensile mechanical properties of GW123K alloy in various conditions. It can be seen that both the yield strength and tensile strength increase over 100 MPa after rolling, but the elongation has a slight decrease. Grain refinement strengthening combined with the work hardening effect as well as precipitation strengthening contribute to the increase of strength[14]. The decrease of elongation may due to the large MgGdY compounds, which may become crack sources during the tensile test.

Table 2 Room temperature tensile properties of GW123K alloy

Compared with as-rolled condition, the as-annealed alloy has a large decrease in strength and an increase in elongation. The overall mechanical properties of the specimen at 723 K for 2 h are better than those at 773 K, and this may be due to the grain coarsening. The alloy in T5 condition has the highest strength, showing ultimate tensile strength of 456.8 MPa, yield strength of 348.9 MPa, while the ductility decreasing isn’t severe, i.e. the elongation decreases from 4.4% to 3.8%. Precipitation strengthening is the main factor for the great increase of the strength[15], so the artificial aging fully brings out the strengthening effect of the rare earth metals, and the main precipitation at peak-aged condition should the β′ phase[7].

3.3 Fractography

The SEM micrographs of the fracture surfaces of GW123K alloy are shown in Fig.6. As can be seen from Fig.6(a), the as-rolled specimen shows quasi-cleavage fracture morphology with tear ridges, and the close-up of the fracture surface shows that the fracture surface is composed of a lot of small dimples with particles inside (Fig.6(b)), indicating that crack initiates from the broken MgGdY particles[16]. Cleavage surfaces and tear ridges can be seen on the fracture surface of the as-annealed specimen (Fig.6(c)), so the annealed alloy shows mixed fracture mechanism. Fig.6(d) shows lots of cleavage steps and cleavages on the fracture surface, so the T5 treated specimen exhibits typical cleavage fracture mechanism.

Fig.6 Fractographs of tensile specimen (SEM): (a) As-rolled; (b) A close-up of (a); (c) 723 K, 2 h; (d) 498 K, 17 h

4 Conclusions

1) GW123K alloy sheets can be consecutively rolled with the reduction of 30% per pass at 723 K. The rolling processing has an effect on grain refinement and the precipitation of second phases.

2) The annealed and T5 heat-treatments have no significant effect on the MgGdY metallic compounds, but the artificial aging can further bring out the strengthening effect of the rare earth metals.

3) Rolling processing can greatly enhance the strength of GW123K alloy. The annealed alloy exhibits lower strength and higher elongation than the rolled one, while the aged alloy shows higher strength and lower elongation. GW123K alloy gets the highest strength in T5 condition, namely, the ultimate tensile is 456.8 MPa, yield strength is 348.9 MPa, and elongation is 3.8%.

 4) The rolled alloy shows up quasi-cleavage fracture surface, while the annealed one shows mixed fracture mechanism, and the aged one exhibits typical cleavage fracture mechanism.

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(Edited by ZHAO Jun)

Foundation item: Project(2007CB613703) supported by the Major State Basic Research Program of China; Project(06DJ14005) supported by the Shanghai Council of Science and Technology, China

Corresponding author: DONG Jie; Tel: +86-21-54742630, Email: jiedong@sjtu.edu.cn