*Yang Mingbo 1, 2, Shen Jia 1

Transcription

1 February 2009 Research & Development Modification and refinement mechanism of Mg 2 Si phase in Srcontaining AZ61-0.7Si magnesium alloy *Yang Mingbo 1, 2, Shen Jia 1 ((1. Materials Science and Engineering College, Chongqing Institute of Technology, Chongqing , China; 2. Materials Science and Engineering College, Chongqing University, Chongqing , China) Abstract: The effect of Sr on modifi cation and refi nement of the Mg 2 Si phase in an AZ61-0.7Si magnesium alloy has been investigated and analyzed. The results indicate that Sr can effectively modify and refi ne the Chinesescript shaped Mg 2 Si phase in the AZ61-0.7Si alloy. By adding 0.06wt.%-0.12wt.%Sr to AZ61-0.7Si alloy, the Mg 2 Si phase in the alloy can be changed from the initial coarse Chinese-script shape to fi ne granule and/or irregular polygonal shapes. Accordingly, the Sr-containing AZ61-0.7Si alloy exhibits higher tensile and creep properties than the AZ61-0.7Si alloy without Sr modifi cation. The mechanism on modifi cation and refi nement of the Mg 2 Si phase in Sr-containing AZ61-0.7Si alloy is possibly related to the following two aspects: (1) adding Sr may form the Al 4 Sr phase which can serve as the heterogeneous nucleus for the Mg 2 Si particles and/or (2) adding Sr may lower the onset crystallizing temperature and increase the undercooling level. Key words: AZ61-0.7Si magnesium alloy; Sr; Mg 2 Si phase; modifi cation and refi nement CLC number: TG Document code: A Article ID: (2009) Magnesium alloys are the lightest structural alloys commercially available, and they have great potential for applications in automotive, aerospace and other industries. However, in recent years, improving the elevated temperature properties has become a critical issue for possible applications of magnesium alloys in hot components. It has been shown that the Mg-Al-Si based alloys are potential magnesium alloys for elevated temperatures [1-2], because the Mg 2 Si phase in Mg-Al-Si alloys has high melting point, high hardness, low density, high elastic modulus and low thermal expansion coefficient, and also because the Mg 2 Si phase is very stable and can impede grain boundary sliding at elevated temperatures [3]. However, under the lower solidification rates the Mg-Al-Si based alloys easily form the undesirable, coarse, Chinese-script shaped Mg 2 Si phase, which would damage the mechanical properties of the alloys. Therefore, modification and refinement of the Mg 2 Si phase are believed to be one of *Yang Mingbo Male, born in 1971, Professor, Ph.D, graduated from Dalian University of Technology in 1993 and majored in Foundry. He got his master s and doctor s degree in 1998 and 2001, respectively from Chongqing University. Currently, his research interest is mainly focus on fabrication of light alloy materials and their forming techniques. By now his more than 80 papers have been published and among of them more than 60 papers were cited by SCI and EI. He holds 3 national invention patents. yangmingbo@cqit.edu.cn Received: ; Accepted: the key aspects to improve the mechanical properties of Mg- Al-Si based alloys. At present, the modification and refinement of Mg 2 Si phases in Mg-Al-Si based alloys have received much attention all over the world, and consequently extensive research has been carried out. It was reported that the Chinese-script shaped Mg 2 Si phase in Mg-Al-Si based alloys could be modified and refined by adding Sb [4-6], Ca and/or P [7-9]. However, some researchers also found that Sb was not an effective modifier of the Mg 2 Si phase [7], that Ca resulted in cast defects such as hot-crack [10], and that P produced ignition and the P amount was also difficult to control [4]. Therefore, other microalloying elements for the modification and refinement of the Chinesescript shaped Mg 2 Si phase in Mg-Al-Si alloys need to be considered. Recent results indicated that Sr which has been used in industrial practice especially for the modification of Al- Si alloys [11-12], was an effective modifier and refiner of Mg 2 Si phase in Mg-Al-Si based alloys [13-14]. For example, Srinivasan et al [13] reported that the addition of Sr to a Si-containing AZ91-Mg alloy appeared to refine the microstructure by promoting a smaller grain size, and the coarse Chinese-script shaped Mg 2 Si precipitates also appeared to be smaller and more uniformly distributed. Similar results were obtained by Song et al [14]. In spite of the above studies, the mechanism of modification and refinement of the Mg 2 Si phase in Srcontaining Mg-Al-Si based alloys is not yet clear. The present work is to investigate the effects of Sr microalloying on the 37

2 CHINA FOUNDRY modification and refinement of the Mg 2 Si phase in AZ61-0.7Si magnesium alloy, and to analyze the corresponding mechanism. 1 Experimental procedure The Sr-containing AZ61-0.7Si alloy was prepared using following materials: commercial AM60 alloy, pure Al, Mg and Zn, Al-30wt.%Si and Al-10wt.%Sr master alloys. The experimental alloy was melted in a crucible resistance furnace, and the melt was protected by a flux addition. At a temperature around 740, the melts were treated respectively with 0.06wt.%, 0.09wt.% and 0.12wt.%Sr using the Al-10wt.%Sr master alloy. After holding at 740 for 60 min, the melts were homogenized by mechanical stirring and then poured into a preheated permanent mould. For comparison, the AZ61-0.7Si alloy without Sr modification was also cast under the same conditions. The actual chemical compositions of the experimental alloys are listed in Table 1. Table 1 Chemical compositions of the experimental alloys (wt.%) Alloys Al Zn Mn Si Sr Mg 1# Bal. 2# Bal. 3# Bal. 4# Bal. The samples for microstructural analysis were etched with an 8% nitric acid distilled water solution, and then were examined with a JOEL JSM-6460LV type scanning electron microscope (SEM) equipped with Oxford energy dispersive spectrometer (EDS). The microstructural phases in the experimental alloys were also analyzed with D/Max-1200X Vol.6 No.1 type X-ray diffraction (XRD) operated at 40 kv and 30 ma. The differential scanning calorimetry (DSC) was carried out using a NETZSCH STA 449C system. Samples weighing around 30 mg were heated in a flowing argon atmosphere from room temperature to 700 for 5 min before being cooled down to 100. The cooling curve was recorded at a controlling rate of 15 /min. In addition, the tensile and creep properties of the experimental alloys were also tested. Their tensile properties at ambient temperature and at 150 were determined from a complete stress-strain curve. The 0.2% yield strength (YS), ultimate tensile strength (UTS) and elongation at breakage were obtained based on the average of three tests. The constant-load tensile creep tests were performed at 150 and 50 MPa for creep extension up to 100 h. The total creep strain and minimum creep rate were measured respectively from each elongation-time curve, and then the results from three tests were averaged out. 2 Results and discussion 2.1 Modification and refinement of Mg 2 Si phase Figure 1 shows the XRD results of 3# and 4# alloys. It is well known that the 1# alloy is composed of a-mg, Mg 17 Al 12 and Mg 2 Si phases. According to Fig.1(a), the 3# alloy is also composed of these three phases, indicating that adding small amounts of strontium (<0.09wt.%) to AZ61-0.7Si alloy does not cause the formation of any other new phases. This result is also consistent to that of Zhao et al [15]. However, it is discovered from Fig.1(b) that, after 0.12wt%Sr was added to AZ61-0.7Si alloy, four phases including a-mg, Mg 17 Al 12, Mg 2 Si and Al 4 Sr with a body centered tetragonal structure of DI3 (a=4.46 nm, c=11.07 nm) [16], are detected in the alloy. Fig.1 XRD results of the experimental alloys: 3# alloy (a); 4# alloy (b) 38 Figure 2 shows the SEM images of the AZ61-0.7Si alloys with and without Sr modification. Figure 3 shows the magnified local images of Fig.2(c) and (d). Combining SEM images with the EDS results of experimental alloys (Table 2), it is found that the Mg 2 si phase in the 1# alloy exhibits coarse Chinese-script shape. However, after adding 0.06wt.%- 0.12wt.%Sr to the AZ61-0.7Si alloy, the Mg 2 Si phase in the alloy changes from the initial coarse Chinese-script shape to granule and/or irregular polygonal shapes (Fig.2(c)-(d) and Fig.3), indicating that adding small amounts of Sr can modify and refine the Chinese-script Mg 2 Si phases in the AZ61-0.7Si alloy. It is well known that the coarse Chinese-script shaped Mg 2 Si phase would introduce a detrimental effect on the mechanical properties of Si-containing magnesium alloys because long cracks can easily form along the interface between Chinese-script Mg 2 Si particles and a-mg matrix [8]. Therefore, it is inferred that the modification and refinement of Mg 2 Si phases in Sr-containing AZ61-0.7Si alloys affirmably

3 February 2009 Research & Development (a) (b) (a) 1# alloy (b) 2# alloy (c) (d) (c) 3# alloy (d) 4# alloy Fig.2 SEM images of the experimental alloys: (a) (b) Fig.3 Local magnification images of Fig.2(c) (a) and Fig.2(d) (b) Table 2 EDS results of the experimental alloys Positions Elements (at.%) Total Mg Al Si Sr % Fig.2(a)-A Fig.2(b)-A Fig.2(c)-A Fig.3(b)-A Fig.3(b)-B lead to improvement of tensile and creep properties. Table 3 lists the tensile and creep properties of experimental alloys. It can be seen from Table 3 that the tensile and creep properties of Sr-containing AZ61-0.7Si alloys are higher than those of the AZ61-0.7Si alloy without Sr modification. Apparently, the testing results of tensile and creep properties of experimental alloys are consistent with the above microstructural analysis. 2.2 Discussion It is well known that the Mg 2 Si phase in unmodified Mg- Al-Si based alloys is prone to forming in the coarse Chinese 39

4 CHINA FOUNDRY Vol.6 No.1 Table 3 As-cast tensile and creep properties of the experimental alloys Tensile properties Creep properties Alloys Room temperature and 50 MPa for 100 h UTS YS Elong. UTS YS Elong. Total creep strain Minimum creep rate MPa MPa % MPa MPa % % 10-3 %/h 1# # # # script shape under lower solidification rates [1-2]. Under the present experimental conditions, the Mg 2 Si phase in the AZ61-0.7Si alloy demonstrates the typical Chinese script shaped morphology (Fig.2(a)). However, after adding 0.06wt%- 0.12wt%Sr to AZ61-0.7Si alloy, the Mg 2 Si phase in the alloy changes from the initial coarse Chinese script shape to fine granule and/or irregular polygonal shapes (Fig.2(b)- (d) and Fig.3). Previous investigations showed that, when the microalloying method was adopted, the modification and refinement of Mg 2 Si phase in Si-containing magnesium alloys were mainly related to the formation of nucleation sites for Mg 2 Si precipitates. For example, Yuan et al [4] reported that, after adding 0.5wt%Sb to Mg-5Al-1Zn-1Si alloy, the Mg 3 Sb 2 particles which could act as a nucleus for Mg 2 Si phase, would form in the alloy, resulting in morphology change in Mg 2 Si particles from coarse Chinese-script to a small polygonal type. In addition, Kim et al [8] reported that, after adding small amounts of Ca and P to AZ Si alloy, the CaSi 2 and Mg 3 (PO 4 ) 2 particles which could act as a nucleus for Mg 2 Si phases would also form in the alloy. Generally, in the process of nucleus formation, the boundary energy of the heterogeneous nucleus with crystallization phase has an effect on nucleus formation, and it depends on the structure of the two contacting crystalline faces. One criterion of heterogeneous nucleation is that the disregistry of nucleation planes is less than 6% [4]. According to the work by Bramfitt [4], the two-dimensional lattice misfit mathematical model is: 3 i i ( ) 1 d[ uvw] s cosθ d[ uvw ] hkl n s = 100% ( hkl) n 3 i i= 1 d[ uvw] n δ (1) Where the (hkl) s and (hkl) n are the low index planes of the substrate and nucleated solid; the [uvw] s and [uvw] n are the low index directions in the (hkl) s and (hkl) n ; d[uvw] s and d[uvw] n are the atomic spacing along the [uvw] s and the [uvw] n, i is the angle between the [uvw] s and [uvw] n. Figure 4 shows the relationship of some possible crystal faces of Al 4 Sr to Mg 2 Si. At the same time, the results of calculation for some possible crystallographic orientation for Mg 2 Si nucleation on the Al 4 Sr particles are listed in Table 4. Table 4 shows that when the orientation relationship between Al 4 Sr and Mg 2 Si is (100)Al 4 Sr//(100)Mg 2 Si, the disregistry is the lowest (0.69%), which is less than 6%. Therefore, Al 4 Sr can act as the heterogeneous nucleus for the Mg 2 Si particles by this orientation relationship. Obviously, the modification and refinement of Mg 2 Si phases in the 4# alloy is easily explained according to the disregistry mechanism. However, the disregistry mechanism is not suitable for the 2# and 3# alloys because the Al 4 Sr phases are not detected in these two alloys. [011] Al4Sr [011] Al4Sr [01 1 ] Mg2Si [010] Al4Sr [1 1 2 ] Mg2Si [11 0] Mg2Si [010] Al4Sr [11 2 ] Mg2Si [011] Al4Sr [001] Mg2Si [010] Al4Sr [011] Mg2Si [01 1] Al4Sr [1 01 ] Mg2Si [01 1] Al4Sr [001 ] Mg2Si [01 1] Al4Sr [010] Mg2Si (a) (100) Al4Sr //(100) Mg2Si (b) (100) Al4Sr //(110) Mg2Si (c) (100) Al4Sr //(111) Mg2Si Fig.4 Relationships of crystal faces of (100) Al4 Sr to (100) Mg2 Si, (100) Mg2 Si and (100) Mg2 Si, respectively 40

5 February 2009 Research & Development Table 4 Calculated values of planar mismatch d between Al 4 Sr and Mg 2 Si Matching interface (100) Al4 Sr//(100) Mg2 Si (100) Al4 Sr//(110) Mg2 Si (100) Al4 Sr//(111) Mg2 Si (hkl) Al4 Sr [011] [010] [0 1] [011] [010] [0 1] [011] [010] [0 1] (hkl) Mg2 Si [001] [011] [010] [1 0] [1 ] [00 ] [0 ] [ ] [ 0 ] i ( ) d (%) Figure 5 shows the DSC cooling curves of the experimental alloys. It is observed from Fig.5 that, after adding 0.06wt%- 0.12wt%Sr to the AZ61-0.7Si alloy, the onset crystallizing temperature of the alloy, T l, decreases. According to the classic solidification theory, the relationship between critical nucleus radius and the degree of undercooling is given as follows [17] : * 2σ 2σ Tm r = = (2) G L T r Where r* is the critical nucleus radius, DG r is the variation of volume free energy, v is the interfacial energy of unit surface area, T m is the equilibrium crystallizing temperature, L m is the crystallizing latent heat and DT is the degree m of undercooling, which can be expressed as: DT=T m -T l. According to Equation 2, the critical nucleus radius decreases with the decrease of T l, then the nucleation energy of crystal nucleus reduces and the probability of nucleation increases, which would result in grain and precipitate refinement. Obviously, the undercooling degree mechanism can explain the modification and refinement of the Mg 2 Si phase in the 2# and 3# alloys besides the 4# alloy. Actually, this situation is similar to the modification and refinement of Chinese script shaped Mg 2 Si phases under a fast cooling condition such as in die casting which has higher undercooling degree [4, 7]. Fig.5 DSC cooling curves of the experimental alloys 3 Conclusion (1) Sr can effectively modify and refine the Chinese script shaped Mg 2 Si phase in AZ61-0.7Si magnesium alloy. After adding 0.06wt%-0.12wt%Sr to the AZ61-0.7Si alloy, the Mg 2 Si phase in the alloy changes from the initial coarse Chinese script shape to fine granule and/or irregular polygonal shapes. Accordingly, the Sr-containing AZ61-0.7Si alloy exhibits higher tensile and creep properties than the AZ61-0.7Si alloy without Sr modification. (2) The mechanism for the modification and refinement of Mg 2 Si phases in Sr-containing AZ61-0.7Si alloy is possibly related to the following two aspects: (1) adding Sr may form the Al 4 Sr phases which can act as the heterogeneous nucleus for the Mg 2 Si particles and/or (2) adding Sr may decrease the onset crystallizing temperature and increase the undercooling degree. 41

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