Effect of yttrium, calcium and zirconium on ignition-proof principle and mechanical properties of magnesium alloys

JOURNAL OF RARE EARTHS, Vol. 30, No. 1, Jan. 2012, P. 74

Effect of yttrium, calcium and zirconium on ignition-proof principle and mechanical properties of magnesium alloys FAN Jianfeng (῞ᓎ䫟)1, 2, 3, CHEN Zhiyuan (䰜ᖫ䖰)1, 2, 3, YANG Weidong (ᴼӳϰ)1, 2, 3, FANG Shuang (ᮍ ⠑)1, 2, 3, XU Bingshe (䆌ᑊ⼒)1, 2, 3 (1. Key laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China; 2. Shanxi Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China; 3 College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China) Received 9 June 2011; revised 30 June 2011

Abstract: Good ignition-proof principle and mechanical properties were realized in Mg-Y-Ca-Zr alloy system. By adding Y and Ca elements, the ignition point of Mg-3.5Y-0.8Ca alloy was improved to over 1173 K, and the alloy could be melted in air without any protections. The effect of Zr addition on the microstructures and mechanical properties of Mg-3.5Y-0.8Ca alloys were investigated, and Mg-3.5%Y0.8%Ca-0.4%Zr alloy had good comprehensive properties with tensile strength of 190 MPa and elongation of 11%. Auger electron spectroscopy (AES) and X-ray diffraction (XRD) analysis revealed that the oxide film formed on the surface of Mg-3.5Y-0.8Ca alloy was mainly composed of Y2O3. Thermogravimetric measurements in dry air indicated that the oxidation dynamics curves measured at 773, 873 and 973 K followed the cubic law. Moreover, the semiconductor characteristic of Y2O3 film and its effect on ignition-proof properties of Magnesium alloys were discussed from the viewpoint of electrochemistry. Keywords: magnesium alloys; ignition proof principle; mechanical properties; rare earths

Due to the excellent physical and mechanical properties, such as low density, good damping capacity and ease of manufacturing, magnesium alloys have great potential to be used in the aerospace and automotive industries. However, because of their high affinity to oxygen, magnesium alloys are particularly prone to surface degradation during high temperature manufacturing stages such as melting, welding or heat treatment[1]. Commonly, magnesium alloys are melted under the protection of the fluxes or gases (CO2, SO2, and SF6) to prevent the melts from serious oxidation and even burning[2,3]. But the above methods have inherent disadvantages such as environmental pollution, required complicated equipment, increased cost, etc. Since the 1950s, much interest has been focused on the investigation and development of ignition-proof magnesium alloys[4–6]. Beryllium and calcium were proved to be effective elements for improving the oxidation resistance of magnesium alloys. You et al.[7] investigated the increase of the oxidation resistance of Mg-Ca alloys for the formation of MgO/CaO protective film at elevated temperatures. Zeng et al.[8] found that the addition of Be improved the oxidation resistance during alloying process. However, ignition-proof magnesium alloys with enough Be and Ca additions have not been extensively applied in industry due to their poor mechanical properties and the high toxicity of Beryllium. For these reasons, it is

necessary to find new ignition-proof methods. Rare earths are often used as addition elements to improve alloys’ properties, especially Y has been used in many alloys to improve oxidation resistance[9–12]. Moreover, the oxidation mechanism of magnesium alloys is relatively complex and some physical phenomena are still not very clear. Therefore, it is necessary and also of interest to understand the oxidation behavior of magnesium alloys with addition of rare earths. In the present research, the effect of Y and Ca additions on the oxidation behavior of magnesium alloys at high temperatures was studied so as to provide a new idea for the preparation of ignition-proof magnesium alloys. On the other hand, as a structural material, mechanical properties are very important. So the effect of Zr on the microstructure and mechanical properties of Mg-Y-Ca alloy were also investigated.

1 Experimental All the alloys, i.e., Mg-Y and Mg-Y-Ca were prepared by commercially pure Mg (99.9 wt.%), pure Y (99.3 wt.%) and pure Ca (99.5 wt.%) in a steel crucible under a cover gas mixture of CO2 and SF6. The chemical compositions of the resulting alloys and the oxidation films were analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-AES) and Auger electron spectroscopy (AES). Speci-

Foundation item: Project supported by the National Natural Science Foundation of China (50901048, 51174143), the fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP201003), Program for Changjiang Scholar and Innovative Research Team in University (IRT0972), Program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi and Natural Science Foundation of Shanxi (2010021022-5) Corresponding author: FAN Jianfeng (E-mail: [email protected]; Tel.: +86-358-6014852) DOI: 10.1016/S1002-0721(10)60642-4

FAN Jianfeng et al., Effect of yttrium, calcium and zirconium on ignition-proof principle and mechanical properties of …

mens about 20 mm×20 mm×5 mm were cut for ignition point testing in air immediately after mechanical polishing and degreasing in acetone. The crystallographic features were examined mainly by X-ray diffraction with a Cu K source. The analyses of microstructure morphologies were accomplished by using a scanning electron microscope which was equipped with an energy dispersive spectrometer (SEM-EDS). The oxidation kinetics experiment was conducted in air under isothermal conditions at temperatures of 673, 773 and 873 K using the Universal V2.4F thermogravimetric analyzer (TGA), which is capable of accommodating a specimen with the maximum mass of 2 g and has a measurement accuracy of 1 g. To minimize the amount of the oxide formed during room temperature exposure, the samples were installed in the thermogravimetric analyzer apparatus immediately after polishing and degreasing in ethanol.

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tion film is mainly composed of Y and O elements. If the depth where the oxygen concentration decreases by about 5% is considered as the film/matrix interface, the film thickness is about 5.2 m, which is also consistent with the XRD results. 2.3 Thermogravimetric analysis A high chemical affinity of magnesium with oxygen

2 Results and discussion 2.1 Ignition points testing The results of the ignition-point tests in air are shown in Table 1. When the contents of Y are less than 10 wt.% the ignition point increases only slightly with increasing Y content. But when the Y content is higher than 10 wt.% the alloys do not burn even at high temperatures up to 1173 K. Moreover, alloys with higher Y contents are uneconomic because of their high cost, so Ca is added to reduce the Y content. Under the same conditions, when 0.8 wt.% Ca is added to Mg-Y alloys, the critical concentration of Y needed for “no ignition” is decreased significantly to 3.5 wt.%. So, the ignition of Mg alloys at elevated temperatures can be inhibited by the combination addition of Y and Ca elements. Some Mg-Y-Ca alloys can melt safely at 1173 K in air without any protection. Mechanism of influence of Y and Ca is relative to the selective oxidation and the third element effect, which has been discussed in detail in another paper[13].

Fig. 1 SEM morphology of oxidation surface of Mg-3.5Y-0.8Ca alloy oxidized at 1173 K for 0.5 h

Fig. 2 X-ray diffraction pattern of oxide film formed on Mg-3.5Y0.8Ca alloys oxidized for 0.5 h at 1173 K in air

2.2 Microstructure and phase composition SEM morphologies (Fig. 1) of cross section of the oxidation films formed on the surfaces of Mg-Y-Ca alloys reveal that the film is compact and its thickness is about 6 m. Moreover, in order to determine the phase composition of the oxidation films, XRD analysis of the surface of the Mg-Y-Ca alloys was carried out, and the results are shown in Fig. 2. The protective film of Mg-Y-Ca alloy is composed of Y2O3, CaO and MgO. The preceding conclusion is consistent with the AES concentration-depth profiles of the elements in the oxidation film of the Mg-Y-Ca alloys which was exposed to air for 30 min at 1173 K, as shown in Fig. 3. It can be seen that the oxida-

Fig. 3 AES surface analysis of oxidation films formed at 1173 K for 30 min on Mg-3.5Y-0.8Ca alloy

Table 1 Ignition points of magnesium alloys with different compositions Composition/wt.% Mg Mg1.1Y Mg2.2Y Mg3.8Y Mg5.8Y Mg10.6Y Ignition points/ºC 615 643

641

660

665

Mg1.6Y 1Ca Mg2.5Y 0.6Ca Mg3Y 0.9Ca Mg3.5Y 0.3Ca Mg3.5Y0.5 Ca Mg3.5Y0.8Ca

No ignition 955

1015

1085

1090

1120

No ignition

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makes it difficult to prepare samples surfaces without any oxidation during conventional polishing in air. Hence, the experiments reported here are conducted with an initial oxide film having an estimated thickness of 2.5 nm[1]. Fig. 4 shows the oxidation dynamics curves of Mg3.5Y0.8Ca alloy at different temperatures which reflect the relationship of the mass change in unit surface area and the exposed time. The oxidation dynamics curves of Mg3.5Y0.8Ca alloy follow the cubic-line law, which can be fitted as (Fig. 4): 773 K: x3=0.01004t+0.00466 (1) 873 K: x3=0.09915t+0.43126 (2) 3 973 K: x =1.31198t–5.10143 (3) In the above equation, t and x are time and mass gain respectively. From the oxidation dynamics curves and the fitting equation, it can be seen that the mass gain rates are becoming increasingly smaller with the thickening of the oxidation film, which indicates that the protective oxidation films have been formed on the surface of Mg3.5Y0.8Ca alloy and the controlling factor of the oxidation film growth is the diffusion of metal and oxygen though the oxidation films. The above principles of oxidation dynamics can be unified as the following equation: xn=kt+c (4) where n is the power function constant, k is the growth rate constant of the oxidation film, and c is the integral constant. When n=2, the oxidation dynamic curve following parabolic principle is a standard protective oxidation course, and such oxidation was controlled by the diffusion of metal and oxygen ions, which was discussed in detail by Wagner[14]. Commonly, the actual oxidation courses always deviate from the parabolic principle. When n<2, the increase of diffusion retarding with the thickening of the oxidation film is not absolutely of direct ratio, and the stress of the film, crystal imperfection and grain boundary etc may be all causes why the diffusion deviates the parabolic law. If n2, the diffusion retarding is larger than the retarding resulted from the thickening of the film, which is likely to be caused by the doping of oxides, diffusion velocity of ions and the formation of the compact barrier layer. Y2O3 has a bixbyite-type crystal structure, which is a modified fluorite-type cubic structure (CaF2) with one fourth of oxygen vacancy sites[15]. The oxygen vacancies sites are regularly arranged in the c-type structure and do not effec-

tively contribute to oxygen diffusion, so the mobility of oxygen in Y2O3 is much less than that in the fluorite structure. Ishihara et al.[16] thought that the fluorite structure has a high oxygen conductivity, but the oxygen conductivity is decreased by an introduction of large amounts of oxygen vacancies more than 5% in the fluorite structure, because the introduction of oxygen vacancies leads to the formation of cation-oxygen vacancy clusters, and oxygen vacancies are ordered in the anion sublattice. Moreover, the doping of ions with a lower valence than Y3+ into Y2O3 contributes to the introduction of oxygen vacancy sites. On the other hand, According to Tallan et al.[17], electrical conductivity of Y2O3 decreased with a slope of 1/6 at temperatures lower than 1400 ºC and oxygen partial pressure higher than 10–6 Pa: v(pO2)1/6 (5) Eq. (5) indicates that pure Y2O3 exhibits p-type semiconductivity. According to Hauffe valence rule[18], doping p-type semiconductor by lower valence ions weakens oxidation of metal matrix; on the other hand, doping by higher valence ions enhances oxidation of metal matrix. During the oxidation course of Mg-Y-Ca alloy, Mg2+ and Ca2+ will be inevitable introduced into Y2O3 film, so the n values of the oxidation dynamics curves of Mg-Y-Ca alloy are higher than 2, and the Y2O3 film formed on Mg-Y-Ca alloy is a protective film. 2.4 Effect of Zr addition on microstructure and mechanical properties of Mg-3.5Y-0.8Ca alloy Fig. 5 illustrates the optical microstructure of as-cast alloy samples. It can be seen that Mg-3.5Y-0.8Ca alloy without Zr addition has a very coarse grain size of 50–100 m and a wide grain boundary size of about 5 m. The grain morphology of Mg-3.5Y-0.8Ca alloy is irregular with some dendrite crystals. With 0.1 wt.% Zr addition, the microstructure of Mg-3.5Y-0.8Ca-0.1Zr alloy is hardly changed. When 0.4 wt.% Zr is added, the microstructure is apparently changed. As seen from Fig. 5(c), the dendrite grain structure has disappeared and transformed into a uniform polygonal-shape equiaxed grains with an average grain size of 50 m while the grain boundary are also refined. When Zr content increases continuously (Fig. 5(d)), the grains are more granular and the grain boundaries are more refined, but the grain sizes grow slightly.

Fig. 4 Curves of mass gain vs. time for Mg-3.5Y-0.8C alloy in dry air under isothermal conditions (a) 773 K; (b) 873 K; (c) 973 K

FAN Jianfeng et al., Effect of yttrium, calcium and zirconium on ignition-proof principle and mechanical properties of …

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Fig. 5 Optical microstructure of Mg-3.5%Y-0.8%Ca-xZr alloys (a) 0%Zr; (b) 0.1%Zr; (c) 0.4%Zr; (d) 0.5%Zr

Fig. 6 shows the tensile mechanical properties of the as-cast Mg-3.5%Y-0.8%Ca-xZr alloys at room temperature. It is seen that the yield strength, ultimate tensile strength and elongation of the alloy increase significantly with the increase of Zr contents. Mg-3.5%Y-0.8%Ca-0.4%Zr alloy has good comprehensive properties with tensile strength of 190 MPa and elongation of 11%. The elongation is equivalent with common wrought magnesium alloys (Table 2).

Zr addition has no effect on the ignition-proof property of Mg-3.5Y-0.8Ca-xZr, which can also be melted at 1173 K directly in air without any protections, and the AES concentration-depth analysis result of the oxidation film formed on Mg-3.5Y-0.8Ca-0.4Zr alloy at 1173 K for 30 min is shown in Fig. 7. Compared with Fig. 3, the oxidation film of Mg-3.5Y-0.8Ca-0.4Zr alloy is also mainly composed of Y and O elements, and Zr content is very little.

Fig. 6 Tensile properties of Mg-(3%–3.5%)Y-(0.5%–0.8%)Ca-xZr alloys Table 2 Mechanical properties of new alloys and some common Mg alloys Materials Mg-3Y-0.5Ca

Tensile strength/MPa 120

Elongation/%

States

1.5

Cast

Mg-3Y-0.5Ca-0.1Zr

140

4

Cast

Mg-3Y-0.5Ca-0.4Zr

183

11

Cast

AZ31

262

14.4

Extrusion

AZ61

295

12

Cast

ZK60

340

14.1

Extrusion

Fig. 7 AES surface analysis of oxidation films formed at 1173 K for 30 min on Mg-3.5Y-0.8Ca-0.4Zr alloy

3 Conclusions (1) The XRD, SEM, and AES analyses suggested that the protective films formed on the surface of Mg-Y-Ca alloys were mainly composed of Y2O3. (2) Thermogravimetric measurements in dry air revealed

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that the oxidation dynamics curves measured at 773, 873 and 973 K followed the cubic law. (3) With addition of Zr element, good comprehensive properties were acquired in Mg-3.5Y-0.8Ca-0.4Zr alloy, the tensile strength and elongation of the as cast alloy were 183 MPa and 11%.

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