Microstructure and mechanical properties of Mg-6Al-4RE-0.4Mn alloy
WANG Jian-li(王建利)1, 2, PENG Qiu-ming(彭秋明)1, 2, WU Yao-ming(吴耀明) 1, WANG Li-min(王立民)1
1. Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China;
2. Graduate School, Chinese Academy of Sciences, Beijing 100049, China
Received 28 July 2006; accepted 15 September 2006
Abstract: Mg-6Al-4RE-0.4Mn (mass fraction, %, RE=Ce rich mischmetal) alloy was prepared by steel mould casting technique. Solid solution and artificial aging (T6) were carried out. The microstructure and mechanical properties of both as-cast and T6 treated alloys were investigated at room temperature and 423 K. It is found that the intermetallic compounds of the as-cast and T6 treated alloys were mainly composed of Al11RE3, Mg17Al12 and Al6REMn6. The Vickers hardness (Hv) of the as-cast alloy is 63 and it increases to 70 after T6 heat treatment. T6 heat treatment plays a slight role in the tensile properties at room temperature, but the yield strength and elongation of T6 state at 423 K are distinctly improved. The yield strength and elongation of the as-cast alloy at 423 K are 71 MPa and 18%, respectively, and the values increase after T6 heat treatment, which is attributed to Al11RE3, Mg17Al12 and Al6REMn6 precipitates that impede the dislocation movement and gliding of grain boundaries effectively.
Key words: Mg-Al-RE alloy; casting; heat treatment; microstructure; mechanical property
1 Introduction
Recently, the application of casting magnesium alloys in automotive has being increasing expanded[1-3]. Die casting is one of the most effective fabricating methods to produce magnesium components. AZ91D and AM60B are widely used for manufacturing powertrain components such as the transmission case and engine block, since these alloys exhibit superior die castability and a good balance of strength and ductility[1]. However, the application temperature of these alloys is limited to about 393 K, above which the mechanical properties degrade sharply because of the poor thermal stability of Mg17Al12 precipitate[3-4]. There are several ways to improve the mechanical properties of Mg-Al system alloys. Firstly, the mechanical properties could be remarkably improved by adding other alloying elements such as Ca, Sr and RE. to suppress the formation of Mg17Al12[5-6]. Rare earth elements(RE) are considered due to their special properties and a few investigations have been done[7-10]. When added 2% misch metal to Mg-4Al matrix (AE42), the mechanical properties increase greatly which is ascribed to the formation of highly thermal stable Al11RE3 precipitates and the completely suppression of Mg17Al12[11]. Secondly, the mechanical properties could also be enhanced by heat treatment. WANG et al[12] reported that the ductility of rheo-diecast AZ91D was improved without losing mechanical strength by Tx heat treatment. While Refs.[13-14] have revealed that the mechanical properties were improved through dispersion strengthening mechanism by heat treatment process. However, the investigation about effect of heat treatment on die cast Mg-Al-RE alloys is limited[15-17]. Considering the high potential of Mg-Al-RE alloys, the microstructure and mechanical properties of as-cast and T6 heat-treated Mg-6Al-4RE-0.4Mn alloy were investigated in the present study.
2 Experimental
Mg-6Al-4RE-0.4Mn alloy was prepared in electric resistance furnace in graphite crucible using flux as protective cover. The purity of magnesium and aluminum used in the present work was higher than 99.9%. Manganese was added as Mg-1.6%Mn master alloy, while RE was added in the form of Mg-20%Ce rich mischmetal master alloy (51.46%Ce, 27.03%La, 15.64%Nd and 5.88%Pr). The alloy was melt at 1 023 K and homogenized at 993 K for 0.5 h and then cast into iron mould which was preheated to about 473 K. The size of ingot was 70 mm×50 mm×12 mm. The experimental alloy ingot was solid solution treated at 683 K for 35 h and then quenched into about 293 K water. Subsequently aging treatment was carried out at 483 K (T6 heat treatment). The microstructural characterization was conducted using optical microscopy (OM), scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDX) and transmission electron microscope (TEM). The specimens for OM and SEM were prepared by the standard technique of grinding and polishing, followed by etching. The as-cast sample was etched using a solution of 4% nitric acid in ethyl alcohol, while the heat-treated sample was etched with 0.5% HF water solution, TEM foils were prepared by twin-jet polishing in an electrolyte of 5%(volume fraction) perchloric acid in alcohol. The phase constitutions of the studied alloy were analyzed by X-ray diffraction (XRD). Vickers hardness test was carried out under a load of 0.245 N with a dwell time of 15 s. The final results were given as an average value of more than 10 measurements. Tensile tests were performed using universal testing machine at room temperature (RT) and 423 K with an initial strain rate of 7.5×10-4 s-1. For tensile tests at 423 K, the samples took about 10 min to balance the temperature. The values of elongation were given by manual measurement after tensile test.
3 Results and discussion
3.1 Aging hardening behavior
The as-cast samples after solution treated at 683 K for 35 h have been aged at 483 K for 0-40 h. The variation of Vickers hardness with aging time is shown in Fig.1. It can be seen that at first aging for longer time results in an increase in Vickers hardness and reaches peak hardness at about 30 h. And then the hardness decreases with the increasing aging time. The increase of Vickers hardness during the first stage is due to the increasing precipitation of Mg17Al12. After reaching the peak hardness, the Mg17Al12 precipitate becomes coarsening and results in the decrease of the Vickers hardness. It should be pointed out that the Mg17Al12 precipitate is based on XRD analysis, EDX and TEM observation to be presented later.
3.2 Microstructures
Fig.2 shows the XRD patterns of the as-cast and T6 (peak aged) state alloys. It indicates that the intermetallic compounds of the both states samples are mainly composed of Al11RE3 (RE=Ce, La, Nd, Pr) and Mg17Al12 with a small amount of Al6REMn6. The optical micrographs of as-cast state and T6 state alloys are shown in Fig.3. Some plate-like precipitates with lath-like and polygonal particles are distributed in the as-cast alloy matrix (Fig.3 (a)). This similar morphology can also be observed by following SEM analysis in Fig.4(a). The results of dot EDX analysis (Figs.4(b), (c) and (d)) suggest that the chemical formulas plate-like, lath-like and polygonal precipitates are Al11RE3, Mg17Al12 and Al6REMn6 phases, separately. Because of the similar XRD patterns between Al6REMn6 and Al10RE2Mn7, Al-RE-Mn compound has been considered to be Al10RE2Mn7 in Ref.[16]. Fig.5 shows the TEM image of as-cast Mg-6Al-4RE-0.4Mn alloy. The plate-like, lath-like and polygonal precipitates can be seen in Fig.5(a). The lath-like precipitate is identified as Mg17Al12 (body centered cubic structure, a=1.054 38 nm)[18] by selected area electron diffraction.
Fig.1 Vickers hardness as function of aging time at 483 K for Mg-6Al-4RE-0.4Mn alloy
Fig.2 XRD patterns of Mg-6Al-4RE-0.4Mn alloy at as-cast and T6 treated states
Fig.3 Optical micrographs of Mg-6Al-4RE-0.4Mn alloy: (a) As-cast state; (b) T6 state
Fig.4 SEM image and EDX analysis of as-cast Mg-6Al-4RE-0.4Mn alloy
Fig.5 TEM image of as-cast Mg-6Al-4RE-0.4Mn alloy (a) and selected area electron diffraction pattern of [012] zone axis of Mg17Al12 (b)
After T6 heat treatment, the Mg17Al12 phase changes into spherical particle and is distributed homogenously in the matrix as shown in Fig.3(b). In the meantime, there is no variation in size or shape of the Al11RE3 precipitates and they are mainly distributed along or across grain boundaries. It is noted that Al11RE3 intermetallic compound exhibits higher thermal stability compared with Mg17Al12 phase. Furthermore, the morphology and distribution of Al6REMn6 precipitates are similar to those of as-cast state. The main phases of the commonly used Mg-Al system alloys such as AM60 and AZ91 are α-Mg matrix and Mg17Al12 compound. Adding RE to Mg-6Al-0.4Mn alloy results in the formations of Al11RE3 and Al6REMn6 precipitates. This is because that the difference in electronegativity between Al and RE is larger than that between Al and Mg[19], Al preferentially reacts with RE to form Al-RE compounds. Similar results are also reported by WEI et al in Refs.[15, 20].
3.3 Mechanical properties
Tensile properties including ultimate tensile strength (UTS), yield strength (YS) and elongation (ε) of the as-cast and T6 heat-treated alloys at RT and 423 K are listed in Table 1. The Vickers hardness of as-cast alloy is 63, and it increases to about 70 after T6 heat treatment, which is about 1.1 times enhanced. It can be seen in Table 1 that the effect of T6 heat treatment on the UTS at RT is obscure, but the YS and ε of T6 state alloy at 423 K are increased clearly. The YS and ε of the as-cast alloy at 423 K are 71 MPa and 18%, and 80 MPa and 20% after T6 heat treatment, they are increased by 13% and 11%, respectively. This is associated with Al11RE3 particles which are distributed along or across the grain boundaries. The high melting point (above 1 473 K) of Al11RE3 precipitates makes them more thermal stable in the interesting temperature range and diffusion in these phases will be slow compared with Mg17Al12 phase[16]. This stability is also confirmed as no changes in the morphology which is observed after heat treatment in Fig.2(b). Thus, the tensile properties at 423 K can be improved since the dislocation movement and grain boundary sliding at elevated temperatures are effectively prohibited by the Al11RE3 precipitates. On the other hand, the mechanical properties can also be enhanced by dispersion strengthening[1, 14]. Mg17Al12 phase is dissol- ved during solid solution treatment. However, it will recrystallize during aging and finely distribute in the matrix as seen in Fig.3(b). The homogeneous dispersion of Mg17Al12 precipitates could act as barriers to dislocations and cause a strengthening effect on the mechanical properties.
Table 1 Tensile properties of Mg-6Al-4RE-0.4Mn alloy at as- cast and T6 treated states
4 Conclusions
1) The intermetallic compounds of Mg-6Al-4RE- 0.4Mn alloy at as-cast and T6 states are mainly composed of Al11RE3 and Mg17Al12 with a small amount of Al6REMn6.
2) The yield strength and elongation of T6 treated Mg-6Al-4RE-0.4Mn alloy at 423 K are greatly improved. The YS and elongation of as-cast alloy at 423 K are 71 MPa and 18%. While after T6 heat treatment, they increase by 13% and 11%, respectively.
3) The high thermal stability of Al11RE3 particles and fine dispersion of Mg17Al12 precipitates account for the improvement of tensile properties at elevated temperatures for T6 state alloy.
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(Edited by YANG Hua)
Foundation item: Project supported by Chinese Academy of Science for Distinguished Talents Program; Project(KGCX2-SW-216) supported by the Science Program of the Promotes Northeast of CAS; Project(05GG54) supported by Science Technology Program of Changchun, China
Corresponding author: WANG Li-min; Tel: +86-431-85262447; E-mail: lmwang@ciac.jl.cn
