Effect of gadolinium on aged hardening behavior, microstructure and mechanical properties of Mg-Nd-Zn-Zr alloy

LI Jie-hua(李杰华), JIE Wan-qi(介万奇), YANG Guang-yu (杨光昱)

College of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China

Received 12 June 2008; accepted 5 September 2008

Abstract: Mg-3.4Nd-0.1Zn-0.40Zr alloy samples with and without containing gadolinium (0.6%, mass fraction) were prepared by sand casting. The aged hardening behavior, solidification microstructures and mechanical properties of the alloys were investigated by using the analysis methods of OM, XRD, TEM, hardness tests and mechanical property tests. The main research results are as follows. 1) Compared with the alloy without the addition of gadolinium, the alloys with the addition of gadolinium shows the more remarkable age-hardening response. 2) The as-cast microstructure of the alloy with and without containing gadolinium consists of α-Mg grains with Mg₁₂Nd phase on the grain boundary. After solution heat-treatment, Mg₁₂Nd phase of the alloy without containing gadolinium is dissolved in the matrix, however, there is still discontinued Mg₁₂Nd phase at grain boundary of the alloy with containing gadolinium. The more finely dispersed precipitates in Mg matrix are formed in the alloy with containing gadolinium during age-treatment. 3) The room temperature and high temperature mechanical properties of the alloy are satisfactory, with σ_b=280 MPa, σ_0.2=165 MPa at RT and σ_b=215 MPa, σ_0.2=155 MPa at 250 ℃. The high temperature mechanical properties decrease slightly with the increase of temperature.

Key words: Mg-Nd-Zn-Zr alloy; microstructure; mechanical properties; TEM; precipitates

1 Introduction

The low density and high specific strength of magnesium alloys are particularly suitable for automotive, aeronautical and electronic products, in which mass reduction is important[1-2].  However, their poor mechanical properties, especially low creep strength hamper the widespread applications of conventional Mg-based alloys[3-5]. Recently, the development of high temperature creep resistant magnesium alloys is focused on the new alloys, such as Mg-Y, Mg-Sc, Mg-Dy, Mg-Gd and Mg-Sm system[6-12]. Among them, Mg-Gd system is one of the most promising candidates due to the remarkable age-hardening response and very good thermal stability of the main strengthening phase up to 250 ℃. Many investigations have been done on the Mg-Gd based system alloys[13-14]. The high strength of these alloys is mainly attributed to the dispersive β″ (D0₁₉) and/or β′ (bco) precipitates in the magnesium matrix. In addition, some researchers also studied new precipitate phase or structure (LPSO) in Mg-Gd alloys via the use of micro-alloying additions of low-cost element Zn[15]. As a micro-alloying element, Gd can also play important role in improving the mechanical properties of conventional Mg-based alloys, however, up to now, few works have focused on it. In this work, the Mg-3.4Nd- 0.1 Zn-0.40Zr alloy samples with and without containing gadolinium (0.6%, mass fraction) were prepared. The effects of gadolinium on the aged hardening behavior, solidification microstructures and mechanical properties of the alloys were investigated by using the analysis methods of OM, XRD, TEM, hardness tests and mechanical property tests.

2 Experimental

The experimental alloys were prepared with high purity Mg (99.9%), Zn (99.9%), Nd (99.9%), Mg-28Gd (mass fraction, %) and Mg-33Zr (mass fraction, %) master alloys in an electric resistance furnace under the protection of an anti-oxidizing flux. The chemical compositions of the experimental alloys were determined by inductively coupled plasma atomic emission spectrum (ICP-AES) apparatus, as listed in Table 1. DSC traces were made with Q600SDT apparatus in a protective pure argon atmosphere, using pure aluminum as a reference. According to Fig.1, the specimens cut from the alloys ingot were solution treated at 350 ℃ for 3 h and 515 ℃ for 18 h, followed by quenching into hot water at 70-80 ℃, and then subsequently aged at 205 ℃ for 16 h, and air cooling. Hardness was measured using the Vickers-hardness with 50 N load. The microstructures of the specimens prepared by standard techniques and etched in 5% HNO₃ with ethanol were observed by optical microscope (OM). The characterization of phases was carried out in TEM (JEOL-2100F) with an attached Oxford INCA energy dispersive X-ray spectrometer operated at 200 kV. The thin foils for TEM investigation were prepared by thinning small plates cut from the specimens, grounding to 0.03-0.05 mm in thickness, punching the discs of 3 mm in diameter from the plates, milling by MIP-IA precision ion polishing system until a hole was appeared. Tensile test was performed using standard tensile testing machine.

Table 1 Analyzed chemical composition of experimental alloys

Fig.1 DSC curves of experimental alloy A2

3 Results

3.1 Aging hardening behaviour

From the hardness evolutions as a function of ageing time during isothermal ageing at 478 K and 523 K, shown in Fig.2, it can be seen that the hardness of the specimen aged at 478 K without containing gadolinium increases gradually with the increase of aging time, then starts to increase sharply and the peak hardness (HV105) is obtained at about 80 h. The hardness increases more quickly, the peak hardness (HV95) becomes lower, and the time to get the peak hardness becomes shorter with the age temperature increasing when it is aged at 523 K. The more remarkable aged-hardening response is shown in the alloy with the addition of gadolinium. The trend is the same as that of the alloy without containing gadolinium. However, the hardness increases more slowly, the peak hardness (HV120) becomes higher, and the time to get the peak hardness becomes longer at the same aged temperature.

Fig.2 Hardness evolutions as function of ageing time during isothermal ageing: (a) Mg-Nd-Zn-Zr alloy; (b) Mg-Nd-Gd-Zn- Zr alloy

3.2 Microstructures

Fig.3 shows the optical microstructures of the as-cast experimental alloys. Fig.4 shows the X-ray diffraction patterns of the as-cast experimental alloys. It can be seen that the microstructures of the alloys consist of α-Mg grains with Mg₁₂Nd phase on the grain boundary. No other binary or ternary phases are formed. This is further confirmed by the TEM images of as-cast sample shown in Fig.5, where some spicule and thread-shaped phase distribute in the matrix and at α-Mg grains boundaries.

Fig.3 Microstructures of as-cast experimental alloys: (a) Alloy Al; (b) Alloy A2

Fig.4 X-ray diffraction patterns of as-cast experimental alloys: (a) Alloy A1; (b) Alloy A2

Fig.5 TEM image and selected area electron diffraction (SAED) of as-cast sample in matrix: (a) Alloy A1; (b) Alloy A2

The optical microstructures of the alloys after solution treatment are shown in Fig.6. It can be seen that Mg₁₂Nd phase of alloy A1 is dissolved in the matrix, however, there is still discontinued Mg₁₂Nd phase at grain boundary in alloy A2. The discontinued phase is observed with TEM corresponding energy dispersive X-ray spectra and selected area electron diffraction (SAED) as shown in Fig.7. Moreover, the spicule and thread-shaped distributed in the matrix exist all the same, which can be seen more clearly from Fig.8.

Fig.9 shows the optical microstructures of experimental alloys after ageing. More and finer dispersed precipitates in Mg matrix as well as at α-Mg grain boundaries are formed, which can be seen more clearly from TEM images in Fig.10.

3.3 Mechanical properties

The room temperature and high temperature mechanical properties of experimental alloys without and with the addition of 0.6% gadolinium after ageing- treatment are listed in Tables 2 and 3, respectively. It can be seen that the room temperature mechanical property of experimental alloys is satisfactory, with σ_b=275 MPa, σ_0.2=158 MPa for alloy A1 and σ_b=280 MPa, σ_0.2=165 MPa for alloy A2, respectively. With the temperature increasing, the ultimate strength and yield strength of alloy A1 decrease sharply.While the high temperature mechanical properties of alloy A2 decrease slightly with the increase of temperature, and the UTS and YS are still kept at 215 and 155 MPa at 250 ℃, respectively.

Fig.6 Optical microstructures of experimental alloys after solution-treatment: (a) Alloy A1; (b) Alloy A2

Fig.7 TEM image with energy dispersive X-ray spectra and selected area electron diffraction of discontinued phase at grain boundary in experimental alloy A2 after solution- treatment at 515 ℃ for 18 h

Fig.8 TEM image of experimental alloy A2 after solution- treatment at 515 ℃ for 18 h: (a) On grain boundary; (b) In α-Mg matrix

Fig.9 Optical microstructures of experimental alloys after ageing-treatment at 250 ℃ for16 h: (a) Alloy A1; (b) Alloy A2

Fig.10 TEM image and selected area electron diffraction of experimental alloy A2 after ageing-treatment at 205 ℃ for 16 h in matrix

Table 2 Room temperature and high temperature mechanical properties of experimental alloy A1

Table 3 Room temperature and high temperature mechanical properties of experimental alloy A2

The fractured surfaces of the experimental alloy A2 after ageing-treatment at RT and 250 ℃ are shown in Fig.11. Some micro-cracks and cleavage-type facets are observed. Those cracks show the typical quasi-cleavage fracture mode. The more cleavage surface and ductile dimples are seen on the fracture surface at 250 ℃.

Fig.11 Typical fracture surface images of experimental alloy A2 after age-treatment at 205 ℃ for 16 h: (a) Room temperature; (b) High temperature (250 ℃)

4 Conclusions

1) In the specimen without containing gadolinium aged at 478 K, the hardness increases gradually with the increase of aging time, then starts to increase sharply and the peak hardness (HV105) is obtained at about 80 h. The hardness increases more quickly, the peak hardness (HV95) becomes lower, and the time to get the peak hardness becomes shorter with the increase of ageing temperature. The addition of gadolinium shows the more remarkable age-hardening response. However, the hardness increases more slowly, the peak hardness (HV120) becomes higher, and the time to get the peak hardness becomes longer at the same ageing temperature.

2) The as-cast microstructure of experimental alloys consists of α-Mg grains with Mg₁₂Nd phase on the grain boundary. Compared with the alloy A1 without the addition of gadolinium, the mechanical properties of the alloy A2 at room temperature and high temperature are much higher. Moreover, the high temperature mechanical properties decrease slightly with the increase of temperature.

References

[1] SMOLA B, STUL?KOV? I, VON BUCH F, MORDIKE B L. Structural aspects of high performance Mg alloys design [J]. Materials Science and Engineering, 2002, A324(1/2):113-117.

[2] GROBNER J, SCHMID-FETZER R. Selection of promising quaternary candidates from Mg-Mn-(Sc, Gd, Y, Zr) for development of creep resistant magnesium alloys [J]. Journal of Alloys and Compounds, 2001, 320(2): 296-301.

[3] von BUCH F, LIETZAU J, MORDIKE B L, PISCH A, SCHMID-FETZER R. Development of Mg-Sc-Mn alloys [J]. Materials Science and Engineering, 1999, A263(1): 1-7.

[4] MORDIKE B L. Development of highly creep resistant magnesium alloys [J]. Journal of Materials Processing Technology, 2001, 117: 391-394.

[5] MORDIKE B L. Creep-resistant magnesium alloys [J]. Materials Science and Engineering, 2002, A324: 103-112.

[6] SUZUKI M, SATO H, MARUYAMA K, OIKAWA H. Creep deformation behavior and dislocation substructure of Mg-Y binary alloys [J]. Materials Science and Engineering, 2001, A319-321: 751-755.

[7] ROKHLIN L L, DOBATKINA T V, TARYTINA I E, TIMOFEEV V N, BALAKHCHI E E. Peculiarities of the phase relations in Mg-rich alloys of the Mg-Nd-Y system [J]. Journal of Alloys and Compounds, 2004, 367(1/2): 17-19.

[8] BAE D H, KIM S H, KIM D H, KIN W T. Deformation behavior of Mg-Zn-Y alloys reinforced by icosahedral quasicrystalline particles [J]. Acta Materialia, 2002, 50(9): 2343-2356.

[9] NEUBERT V, STUL?KOV? I, SMOLA B, MORDIKE B L, VLANCH M, BAKKAR A, PELCOV? J. Thermal stability and corrosion behavior of Mg-Y-Nd and Mg-Tb-Nd alloys [J]. Materials Science and Engineering, 2007, A462(1/2): 329-333.

[10] SMOLA B, STUL?KOV? I. Equilibrium and transient phases in Mg-Y-Nd ternary alloys [J]. Journal of Alloys and Compounds, 2004, 381(1/2): L1-L2.

[11] NIE J F, MUDDLE B C. Characterisation of strengthening precipitate phases in a Mg-Y-Nd alloy[J]. Acta Mater, 2000, 48(8): 1691-1703.

[12] STULIKOVA I, SMOLOA B, MORDIKE B L. New high temperature creep resistant Mg-Y-Nd-Sc-Mn alloy[J]. Phisica Status Solid (A), 2002, 190(2): 5-7.

[13] ROKHLIN L L, NIKITINA N I. Recovery after ageing of Mg-Y and Mg-Gd alloys[J]. Journal of Alloys and Compounds, 1998, 297(2): 166-170.

[14] HONMA T, OHKUBO T, HONO K, KAMADO S. Chemistry of nanoscale precipitates in Mg-2.1Gd-0.6Y-0.2Zr (at.%) alloy investigated by the atom probe technique[J]. Materials Science and Engineering, 2005, A395(1/2): 301-306.

[15] HONMA T, OHKUBO T, KAMADO S, HONA K. Effect of Zn additions on the age-hardening of Mg-2.0Gd-1.2Y-0.2Zr alloys [J]. Acta Materialia, 2007, 55(12): 4137-4150.

(Edited by CHEN Can-hua)

Foundation item: Project(2006CB605202) supported by the National Basic Research Program of China; Project(CX200705) supported by the Doctorate Foundation of Northwestern Polytechnical University, China

Corresponding author: LI Jie-hua; Tel: +86-29-88495414; E-mail: lijiehua13032980772@hotmail.com