Effect of Gd content on microstructure and mechanical properties of Mg-Y-RE-Zr alloys
ZHANG Kui(张 奎), LI Xing-gang(李兴刚), LI Yong-jun(李永军), MA Ming-long(马鸣龙)
State Key Laboratory for Fabrication and Processing of Nonferrous Metals,
Beijing General Research Institute for Nonferrous Metals, Beijing 100088, China
Received 12 June 2008; accepted 5 September 2008
Abstract: Four kinds of Mg-Y-RE-Zr alloys with different Gd contents were prepared, and the effect of Gd content on microstructure and mechanical properties of the alloys was researched. Based on the experimental investigation, the compounds at the grain boundaries are mainly Mg24Y5, Mg41Nd5, and Mg5Gd phases. The average grain size of as-cast alloys is 50-60 μm. After T4 (535 ℃, 24 h) treatment, Mg5Gd phases mostly decompose and dissolve into the matrix, and the disperse spotted phases are mainly Mg24Y5 and Mg41Nd5 phases. After extruding and ageing (250 ℃, 5 h), the grain size is refined and some grains abnormally grow up to about 40 μm. With Gd content increasing, the ultimate tensile strength, yield strength of as-cast alloys and the extruded bars after ageing are improved, but the elongation is decreased.
Key words: Mg-Y-RE-Zr alloy; Gd; microstructure; mechanical properties
1 Introduction
As the lightest constructional alloys of all used metals, magnesium alloys have a strong potential used as structural components for aerospace and military products applications. However, traditional heat resisting magnesium alloys such as Mg-Al-RE, Mg-Al-Si and Mg-Al-Ca alloys which could be used at below 150 ℃couldn’t meet the need of aerospace, and military products which were usually used at higher temperature (more than 200 ℃) because of the rapid degradation of mechanical properties at elevated temperatures[1-8]. The effects of rare earths on the mechanical properties of magnesium alloys at elevated temperatures have been analyzed by many researchers, and their results indicated that the effectiveness of the rare earths was obvious. It was reported that Mg-9Gd-4Y-0.6Zr alloys exhibited higher specific strength at both room and elevated temperatures, and the yield strength was more than 300 MPa when tested at 523 K[9-15]. Considering the important effect of RE elements on magnesium alloys, it is necessary to study the relationship between the content of RE elements, the microstructure and mechanical properties of the alloys. In this paper, by changing Gd content, four kinds of Mg-Y-RE-Zr alloys were prepared. The effects of different content of Gd elements on the structure and mechanical properties of Mg-Y-RE-Zr magnesium alloys were analyzed.
2 Experimental
2.1 Materials
The pure metals of magnesium, yttrium, neodymium, and gadolinium, and Mg-30Zr master alloys were used as raw materials. The Mg-Y-RE-Zr alloys were melted in a mild steel crucible with the protection of home-made flux. When the fused mass temperature reached 750 ℃, the pure metals of yttrium, neodymium, and gadolinium were added into the crucible, and then the temperature was raised to 850 ℃ and kept for 20 min. Finally, the Mg-30Zr master alloys were put into the molten metals and stirred for about 5 min, and then poured into a steel mold with a diameter of 98 mm. The chemical compositions of the designed Mg-Y-RE-Zr alloys are listed in Table 1.
Table 1 Chemical composition of Mg-Y-RE-Zr alloys (mass fraction, %)
2.2 Experiment
Fig.1 shows the DSC curves of as-cast alloys. The decomposing temperature of low-melting eutectic phases formed by Mg and Gd elements was 548 ℃, and the phase transition temperature of alloys 1 and 2 were 548.4 ℃ and 549.4 ℃, respectively (Fig.1). The change of Gd content has little effect on the phase transition temperature. That could set the homogenization parameter of different alloys at 535 ℃ for 24 h. After homogenization, the ingots were hot extruded into rods with diameter of 21 mm. The extrusion ratio was 20:1.
The as-cast samples and extruded rods after ageing at 250 ℃ for 5 h were machined into tensile specimens of 5 mm gauge diameter and 35 mm gauge length. Then the samples were tested on universal material testing machine (the speed of tensile was 2 mm/min). The microstructures of both as-cast and homogenized alloys were observed with an optical microscope(OM) and scanning electron microscope(SEM). The phase analyses were performed with an X-ray diffractometery(XRD) and energy dispersion spectrograph(EDS).
3 Results and discussion
3.1 Microstructure
Fig.2 and Fig.3 how the microstructures of as-cast alloy and T4-treated alloy, respectively. As observed in Fig.2, the microstructure of the ingot is composed of α-Mg and large number of second-phases at the grain boundaries (Fig.2). In solidification process, the second-phases aggregate and grow up along grain boundaries that make great composition segregation of the alloys and also prevent the grains from growing up by anchoring the grain boundaries. The grains of different as-cast alloys are small and the average size is 50-60 μm. With the increasing of Gd content in the alloy, the second-phase along grain boundaries become more and bigger while the grain size has little change.
After solution heat-treatment at 535 ℃ for 24 h, the distribution of the second phases at grain boundaries becomes dispersed spot state from island structure of as- cast state, especially at the triple grain junctions (Fig.3). The discontinuous particles which have high thermal stability can improve the creep resistance of the alloy by nailing the grain boundaries and preventing the migration of the grains and dislocations. With the increasing of Gd content, the particles of undecomposed second-phases become more and bigger (Fig.3).
SEM images and micro-area chemical composition analysis results of alloy 2 are shown in Fig.4 and Table 2. The second-phase along grain boundaries mainly are proeutectic α-Mg and rare earth compounds which are Mg-RE phases such as Mg24Y5, Mg41Nd5, and Mg5Gd by XRD analysis (Fig.4(a) and Fig.5(a)). After solution heat-treatment, the content of Y and Gd elements in matrix obviously increases but Nd element has little change. Mg5Gd phases mostly decompose and dissolve into matrix, and the particles of undecomposed second phases mainly are Mg24Y5 and Mg41Nd5 phases (Fig.4(b)).
After being extruded and ageing at 250 ℃ for 5 h, the distribution of undecomposed interphases become streamline form and the average grain size is refined to about 15 μm. In the area where there is little interphases, some grains abnormally grow up to about 40 μm. With the increasing of Gd content, the grains become more fine and homogeneous (Fig.5).
Fig.1 DSC curves of as-cast alloys: (a) Alloy1; (b) Alloy 2
Fig.2 Microstructures of as-cast alloys: (a) Alloy 1; (b) Alloy 2; (c) Alloy 3; (d) Alloy 4
Fig.3 Microstructures of as-cast alloys by T4 (535 ℃, 24 h) treatment: (a) Alloy 1; (b) Alloy 2; (c) Alloy 3; (d) Alloy 4
Table 2 Micro-area chemical composition analysis results of alloys 2 in Fig.4
3.2 Mechanical properties
Fig.6 shows the tensile properties of the alloys at different states. With the increasing of Gd content, for as-cast alloys, the ultimate tensile strength and yield strength are improved but the elongation drastically decreases. When the Gd content changes from 5.16%
Fig.4 SEM images of alloy 2: (a) As-cast state; (b) T4 (535 ℃, 24 h) state
Fig.5 Longitudinal microstructures of extruded bars after ageing (250 ℃, 5 h): (a) Alloy 1; (b) Alloy 2; (c) Alloy 3; (d) Alloy 4
Fig.6 Tensile properties of alloys: (a) As-cast state; (b) Extruded bars after ageing
(alloy 1) to 10.60% (alloy 4), both the ultimate tensile strength and the yield strength just increase by about 15% but the elongation decreases by 90% (Fig.6(a)). This indicates the increment of casting defects such as inclusions and segregations because the increasing of Gd content has much more disadvantages to the elongation than the advantages of increasing second phases to the strength.
After being hot extruded and ageing, the casting defects such as gas cavity, shrinkage porosity and heat cracking basically disappear. The second-phases are well- distributed in matrix and the structure of the ingots is refined. The mechanical properties of the alloy become homogeneous. The ultimate tensile strength, yield strength and elongation all greatly increase. Compared to those of alloy 1, the yield strength of alloys 2, 3 and 4 increases by 30.5%, 41.2% and 54.9% respectively, the elongation decreases by 42.5%, 71.4% and 85.7%. The tensile properties at 250 ℃ of alloy 2 after being extruded and ageing was tested. Its ultimate tensile strength, yield strength and elongation are 260 MPa, 230 MPa and 23.0% respectively. Synthetically, alloy 2 has more applicable possibility than other designed alloys in the experiment.
4 Conclusions
1) The compounds at grain boundaries mainly are Mg24Y5, Mg41Nd5, and Mg5Gd phases. The average grain size of as-cast alloys is 50-60 μm. With the increasing of Gd content, the size of compounds at grain boundaries become bigger.
2) After T4 (535 ℃, 24 h) treatment, the distribution of compounds becomes dispersed spot state; Mg5Gd phases mostly decomposes and dissolves into the matrix, the particles of undecomposed second-phases mainly are Mg24Y5 and Mg41Nd5.
3) After being extruded and ageing, the average grain size is refined to about 15 μm on an average and some grains abnormally grow up to about 40 μm. With the increasing of Gd content, the grain size becomes more fine and homogeneous.
4) With the increasing of Gd content, the strength of the alloys at different state largely increases, while the elongation decreases greatly. In designed alloys, alloy 2 exhibits preferably general performance.
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(Edited by CHEN Ai-hua)
Foundation item: Project(2007CB613705) supported by the National Basic Research Program of China; Project(2006BAE04B01) supported by the National Science and Technique Support Program during the 11th Five-Year Period of China.
Corresponding author: LI Yong-jun; Tel: +86-10-82241161-221; 13601076520; E-mail: liyongjun8158@163.com
