J. Cent. South Univ. Technol. (2007)03-0305-05

DOI: 10.1007/s11771-007-0060-x               

Effects of cryogenic treatment on mechanical properties of

extruded Mg-Gd-Y-Zr(Mn) alloys

XIONG Chuang-xian(熊创贤)^{1, 2}, ZHANG Xin-ming(张新明)¹, DENG Yun-lai(邓运来)¹,

XIAO Yang(肖 阳)¹, DENG Zhen-zhen(邓桢桢)¹ , CHEN Bu-xiang(陈部湘)¹

（1.   School of Materials Science and Engineering, Central South University, Changsha 410083, China;

2. Departments of Civil Engineering, Hunan City College, Yiyang 413000, China）

Abstract: The influence of cryogenic treatment on the mechanical properties of the extruded Mg-Gd-Y-Zr(Mn) alloys was investigated by the tensile tests, scanning electron microscopy(SEM), transmission electron microscopy(TEM), and energy dispersive X-ray spectroscopy (EDS). The results show that the mechanical properties of both alloys are improved greatly during the in situ tensile test by soaking the samples in liquid nitrogen for 10 min. The ultimate tensile strength, yield tensile strength and elongation of cryogenic treated magnesium alloy added with zirconium or manganese are largely elevated. And remarkable microstructure change is observed in both alloys by cryogenic treatment. There are a large number of twins，rod-like, tree-like and chrysanthemum-like precipitated phases in the microstructures and the fracture surfaces exhibit the characteristics of ductile rupture when they are observed at room temperature.

Key words: magnesium alloy; cryogenic treatment; mechanical property; microstructure

1 Introduction

Due to high specific strength, specific stiffness, damping and electromagnetism shield properties, magnesium alloys are widely used as structural materials in the field of automobile, aerial and spacing engineering. Recent literatures show that it is possible to improve the strength of Mg alloys substantially by addition of heavy rare earth elements^[1-4], and the addition of Gd and Y might improve properties of Mg alloy, especially high temperature strength^[5-6]. As a high temperature magnesium alloy, Mg-Gd-Y-Zr alloy has been the focus of the research, for instance, the investigation of microstructure^[7-8], and the high temperature creep behavior^[9-10]. The phase structure and phase transformation behavior have been reported^{[11, 12]}. And scientists have paid attention to its use in automobile filed for several years^[13]. The investigation of its low temperature behavior is very important to expand the use of this alloy. However, reports on the low temperature properties of the alloys have not been paid enough attention. In this study, the tensile properties of the alloys sunk into liquid nitrogen were tested in situ and their mechanical performance and fracture behaviors were investigated.

2 Experimental

Two as-cast alloys were prepared from 99.95% Mg (mass fraction), 99.96% Gd (mass fraction), and 99.94% Y (mass fraction) by melting in a resistance furnace. Both rare earth metals and Zr were added into melts in the form of Mg-Gd, Mg-Y and Mg-Zr master alloys prepared before. Mn was added in the form of MnCl₂. The nominal compositions (mass fraction) of two alloys were Mg-9.5Gd-4.6Y-0.65Zr(Mn). The ingots of the alloys were homogenized at 500 ℃ for 12 h and then hot extruded into rods of about 15 mm in diameter with extrusion ratio of 16. The rods were machined into the standard tensile specimens, which were aged at 520 ℃ for 24 h processed before. The tensile tests of these specimens were carried out in situ after being sunk into liquor nitrogen for 10 min on CSS-41000 electronic stretcher at rate of 2 mm/min. The microstructure and fracture surface were investigated with Sirion200 scanning electron microscope at room temperature.

3 Experimental results

3.1 Mechanical properties at low temperature

Table 1 displays the low temperature and room temperature tensile test results of the investigated alloys.

The following main features of the low temperature properties should be noted. The tensile strengths of the cryogenic treated alloys increase significantly compared with those without treatment and the same phenomenon is true to the elongation. At low temperature the alloy with Zr shows the mechanical properties: σ _b = 510 MPa, σ_0.2 = 460 MPa and δ= 8.5%, and the alloy with Mn shows σ _b=505 MPa, σ_0.2=450 MPa and δ=9.0%. These properties are rather high for magnesium alloys. So both alloys with Zr and with Mn exhibit excellent mechanical properties at low temperature.

Table 1 Tensile properties of alloys at room temperature and low temperature

3.2 Fracture characteristics of alloys

Fig.1 shows the SEM micrographs of the ruptured surfaces of the tensile specimens with and without cryogenic treatment. The characteristics of the rupture differ substantially. The fracture surface of the sample without cryogenic treatment shows some kind of cleavage or quasi-cleavage that is the characteristic of brittle rapture. While no such kind of cleavage or quasi-cleavage is found in the cryogenic tensile fracture surface, and it shows a kind of ductile rapture. The conversion of fracture mechanism is attributed to the changes of the microstructures of materials under both conditions. The fracture surface of the specimen cryogenic treated shows more boundaries than the room temperature one, which is probably due to certain kind of grain refinement resulting from cryogenic treatment. The phenomena of promoting of the strengths and improving of the ductility show that their properties at low temperature are excellent potentially. This experimental study has confirmed the fact that both alloys are good metallic materials for low temperature application.

3.3 Microstructures of investigated alloys

Figs.2(a), (b) show the room temperature treated SEM microstructures of the low temperature treated Mg-Gd9.5-Y4.6-0.65Zr alloy, and Figs.2(c), (d), (e), (f) show the longitudinal section SEM microstructures. It is obvious that some new phases form, some of which are branch-like, some are chrysanthemum-like and others are rod-like. These new phases are not observed in the room temperature treated specimen. Fig.3 shows the SEM image of the new phase and EDS analysis result of the new phase. It seems that the chemical constituents of the new phases are some compounds of Mg and rare earth elements, which may be the result of cryogenic treatment. And after a detailed comparison of all figures in Fig.2, it can be seen that there are certain new boundaries, which

Fig.1 Microstructure morphologies of ruptured surfaces and near ruptured surfaces of Mg-Gd-Y-Zr and Mg-Gd-Y-Mn extruded alloys at low temperature (-196 ℃) in T5 peak aging state

(a), (b) Mg-Gd-Y-Mn alloy; (c), (d) Mg-Gd-Y-Zr alloy

Fig.2 SEM images and phase morphologies of longitudinal sections of specimens cryogenic treated and ones at RT

(a) Grain-interface morphology at RT; (b) Block-like particles at RT; (c) Branch-like precipitations at LT; (d) Chrysanthemum-like precipitations at LT; (e) Some precipitations at LT; (f) Rod-like precipitations at LT

Fig.3 SEM image(a) and EDS analysis (b) of new phase on longitudinal sections of specimens cryogenic treated

further confirms that the grains are refined after cryogenic treatment. It is evident that the changes of microstructures and phase structures of the materials cryogenic treated are the reason of excellent mechanical properties at low temperature.

Fig.4 shows the TEM images of the specimens cryogenic treated. They show the changes of dislocation configurations and the reciprocity between the second phase particles and dislocations.

4 Discussion

In a common sense, the tensile strength increases along with the decrease of the temperature, but the

Fig.4 TEM images of longitudinal sections of specimens cryogenic treated

elongation decreases. However, this is not a general rule. For example, Al-Li alloys exhibit higher ductileness and fracture toughness at low temperature than those at room temperature. The mechanical properties of Al-Li alloys at low temperature have been chiefly investigated at present. The extremely high values of their fracture toughnesses shown at low temperature mostly throw light on something as follows:

1) The eutectic phases formed on crystal boundaries of Al-Li alloys at room temperature due to the segregation of Li, Na, K and H elements will solidify at low temperature and increase the binding force between crystal interfaces, so improve the cryogenic toughness of the alloy^[13].

2) The change of dislocation constituents during cryogenic deformation could lead to increase of hardening exponent and elevate the ductility of the materials^[14-15].

3) The increase of the cryogenic ductility of the alloy can be related to the reduction of distance between dislocation glide bands and the increase of space of the band, elastic modulus and yield strength^[16].

Mg-9Gd-4Y-0.65Zr alloy in this study exhibits the phenomena of higher strength, ductibility and fracture toughness at low temperature than those at room temperature. As everyone knows, magnesium alloy has close-packed hexagonal crystalline structure. Multilevel twins and simultaneously pile-up dislocation, second phase precipitate particles etc. would be formed on crystal boundary and in crystalline grain while the alloy is extruded at room temperature. Interaction of these microstructures (dislocations, twins and precipitates, etc) and the change of dislocation constituents during cryogenic deformation could lead to refined grains and increased hardening exponent, and elevate the ductility of the material. The increase of the cryogenic ductility of the alloy can be related to the reduction of distance between dislocation glide bands and the increase of space of the band, elastic modulus and yield strength^[17].

5 Conclusions

1) The ultimate tensile strength, yield tensile strength and elongation of the cryogenic treated magnesium alloy added with zirconium are 510 MPa, 460 MPa and 8.5%, respectively, which means an increment of approximately 38%, 57% and 280% as compared with the same alloy without cryogenic treatment.

2) The ultimate tensile strength, yield tensile strength and elongation of the alloy added with manganese are 505 MPa, 445 MPa and 9.0%, respectively, which means an increment of approximately 40%, 56% and 250% as compared with the same alloy without cryogenic treatment.

3) Remarkable microstructure change is observed in cryogenic treated alloys. There are a large number of twins, rod-like, tree-like and chrysanthemum-like precipitated phases in the microstructures and the fracture surfaces exhibit the characteristics of ductile rupture when they are observed at room temperature.

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(Edited by YANG Bing)

Foundation item: Project (51412020304QT7106) supported by the National Defense Pre-investigation Foundation of China; Project(2003AA741043) supported by the National High-Tech Research and Development Program of China; Project(5133001E) supported by the State Key Fundamental Research and Development Program of China

Received date: 2006-07-24; Accepted date: 2006-08-29

Corresponding author: XIONG Chuang-xian, Doctoral candidate; Tel: +86-737-4628095; E-mail: xiongchuangxian@163.com