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Chemical composition optimization of gravity cast Mg–yNd–xZn–Zr alloy
Materials Science and Engineering A 496 (2008) 177–188
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Chemical composition optimization of gravity cast Mg–yNd–xZn–Zr alloy Fu Penghuai, Peng Liming ∗ , Jiang Haiyan, Ma Lan, Zhai Chunquan National Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composite, Shanghai Jiaotong University, Shanghai 200030, PR China
a r t i c l e
i n f o
Article history: Received 5 January 2008 Received in revised form 5 May 2008 Accepted 6 May 2008 Keywords: Composition optimization Mg–Nd–Zn–Zr Microstructure Mechanical properties
a b s t r a c t The microstructure and mechanical properties of gravity cast Mg–2.75Nd–xZn–Zr (x = 0, 0.2, 0.5, 1.0, 2.0 wt.%) and Mg–yNd–0.2Zn–Zr (y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25 wt.%) alloys in as-cast, solution-treated and 200 ◦ C peak-aged conditions were investigated at room temperature. The effect of Zn and Nd addition on microstructure and mechanical properties in different thermal conditions were discussed in Mg–2.75Nd–xZn–Zr alloys and Mg–yNd–0.2Zn–Zr alloys, respectively. When Zn addition was 0.2 wt.% and Nd addition was 3.0 wt.%, the alloy revealed the best combination of strength and elongation and was chosen as optimal chemical composition. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The extremely low density, high specific strength and stiffness of Mg alloy make it attractive for engineering applications [1,2]. It has been demonstrated that rare earth metals (RE) are the most effective elements to improve the strength properties of magnesium alloys especially at elevated temperatures [3,4]. More recently, a lot of work has been focused on magnesium alloy containing heavy rare earth elements, such as Mg–Gd–Y–Zr [5–8], Mg–Y–Sm–Zr [9,10], Mg–Dy–Gd–Nd [11] and Mg–Gd–Nd–Zr [12] alloys. Though these alloys show high strength, high content of heavy rare earth elements enhances alloys’ cost, which would probably limit their engineering application. Rational use of neodymium (Nd) could make it possible to develop a high strength and low cost magnesium alloy. Nd is one of the light rare earth elements with maximum solubility in solid Mg of 3.6 wt.% at eutectic temperature 545 ◦ C. Mg–Nd binary alloys have already had significant strengthening effect [13,14]. Addition of 0.5 wt.% Zn to Mg–3 wt.% Nd alloy would further increase its peak-aged hardness [15]. Despite these interesting findings, there is a lack of chemical composition optimization and systematic investigation of Zn containing Mg–Nd alloy, both on microstructure and mechanical properties, which are sensitive to alloy’s chemical composition.
∗ Corresponding author. Fax: +86 21 34202794. E-mail address: [email protected] (P. Liming). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.05.010
Early studies using transmission electron microscopy (TEM) reported that the precipitation process in binary Mg–Nd alloys involves formation of G.P. zones,  ,  , and  phases [16,17]. The ¯ G.P. zones have a form of discs on (1100) ␣ planes of ␣-Mg, and the  phase has a DO19 structure (a = 0.64 nm, c = 0.52 nm), and the  phase has a face-centered cubic structure with a = 0.736 nm. The equilibrium phase  has a body-centered tetragonal structure with a = 1.03 nm and c = 0.593 nm.  phase was also reported to have a hexagonal structure with a = 0.52 nm, c = 1.30 nm [18]. Ternary addition of Zn to binary Mg–Nd alloys results in a different precipitation sequence. The precipitation sequence in Mg–2.8Nd–1.3Zn (wt.%) alloy [19] was proposed to involve the formation of G.P. zones, ␥ and ␥ phases. The meta-stable ␥ phase has a hexagonal structure (a = 0.556 nm, c = 1.563 nm) and precipitates as plates on the basal plane of ␣-Mg. The stable ␥ phase is Mg–Nd–Zn ternary compounds and has an fcc structure (a = 0.7444 nm). In Mg–3Nd–0.5Zn and Mg–3Nd–1.35Zn (wt.%) alloys [15], basal precipitate plates are also observed to gradually replace the prismatic plates in Mg–Nd binary alloy with Zn addition increase. In present study, the effects of Zn and Nd addition on the microstructure and mechanical properties of Mg–2.75Nd–xZn-Zr (x = 0, 0.2, 0.5, 1.0, 2.0 wt.%) and Mg–yNd–0.2Zn–Zr (y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25 wt.%) alloys in as-cast, solutiontreated and 200 ◦ C peak-aged conditions are examined at room temperature, respectively. The optimal chemical composition of gravity cast Mg–yNd–xZn–Zr alloy is determined.
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Table 1 Chemical compositions of Mg–yNd–xZn–0.4Zr alloys (wt.%) Nominal compositions
Nd
Zn
Zr
Mg
Mg–2.75Nd–0Zn–Zr Mg–2.75Nd–0.2Zn–Zr Mg–2.75Nd–0.5Zn–Zr Mg–2.75Nd–1.0Zn–Zr Mg–2.75Nd–2.0Zn–Zr Mg–1.25Nd–0.2Zn–Zr Mg–1.75Nd–0.2Zn–Zr Mg–2.25Nd–0.2Zn–Zr Mg–3.00Nd–0.2Zn–Zr Mg–3.25Nd–0.2Zn–Zr
2.81 2.61 2.79 2.77 2.70 1.35 1.90 2.29 3.02 3.31
0.02 0.19 0.63 1.02 2.04 0.23 0.23 0.22 0.26 0.21
0.42 0.47 0.39 0.32 0.44 0.43 0.48 0.43 0.41 0.46
Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal.
2. Experimental Alloys of nominal composition Mg–2.75Nd–xZn–Zr (x = 0, 0.2, 0.5, 1.0, 2.0 wt.%) (hereafter, all compositions are in weight percents unless stated otherwise) and Mg–yNd–0.2Zn–Zr (y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25) were prepared by high purity Mg, Zn and Mg–25Nd,
Table 2 Average grain size of gravity cast Mg–2.75Nd–xZn–Zr alloys in different thermal conditions (m) Zn(wt.%)
As-cast Solution-treated Grain growtha
0
0.2
0.5
1
2
39.4 48.1 8.7
42.9 48 5.1
42.5 47.7 5.2
39.3 43.45 4.15
47.3 60.85 13.55
a Grain growth = average grain size of solution-treated alloys − average grain size of as-cast alloys.
Mg–30Zr master alloys by melting in an electrical resistance furnace under the protection of mixture gas of SF6 , CO2 and air and cast in permanent mould [20] at pouring temperature 740 ± 5 ◦ C and mould temperature 500 ± 5 ◦ C. The actual chemical composition of the alloy was determined by an inductively coupled plasma analyzer (ICP) and listed in Table 1. Specimens cut from the cast ingots were first solution-treated at 540 ◦ C for 10 h and quenched into hot water at ∼70 ◦ C, then subsequently aged at 200 ◦ C in an
Fig. 1. Optical micrographs of as-cast Mg–2.75Nd–xZn–Zr alloys, in etched condition, etched in a solution of 12 g picric acid + 80 ml acetic acid + 80 ml water + 350 ml ethanol, (a) x = 0, (b) x = 0.2, (c) x = 0.5, (d) x = 1.0, (e) x = 2.0 and in polished condition (f) x = 2.0 wt.%.
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Table 3 Average grain size of gravity cast Mg–yNd–0.2Zn–Zr alloys in different thermal conditions (m) Nd (wt.%)
As-cast Solution-treated Grain growtha
1.25
1.75
2.25
2.75
3
3.25
54.6 58.4 3.8
46.1 49.6 3.5
45.5 48.6 3.1
42.9 48 5.1
40.4 42.7 2.3
39.2 42.1 2.9
a Grain growth = average grain size of solution-treated alloys − average grain size of as-cast alloys.
Fig. 2. XRD patterns of as-cast Mg–2.75Nd–xZn–Zr alloys, x = 0, 0.2, 0.5, 1.0, 2.0 wt.%.
oil-bath. Vickers hardness testing was taken using 5 kg load and holding time of 30 s. Tensile test samples were cut into rectangular tensile specimens with dimensions of 10 mm width, 2 mm thickness and 25 mm gauge length by an electric-sparking wire-cutting machine. Tensile testing was carried out on a Zwick/Roell-20 kN material test machine at a cross head speed of 1 mm/min at room temperature. Three tensile samples were tested for each condition that they followed ISO 6892:1998 standard.
Fig. 3. Optical micrographs of as-cast Mg–yNd–0.2Zn–Zr alloys etched in a 4 vol.% nital, (a) y = 1.25, (b) y = 1.75, (c) y = 2.25, (d) y = 2.75, (e) y = 3.0 and (f) y = 3.25 wt.%.
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Fig. 4. Mechanical properties of as-cast Mg–2.75Nd–xZn–Zr (a) and Mg–yNd–0.2Zn–Zr (b) alloys at room temperature, x = 0, 0.2, 0.5, 1.0, 2.0 and y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25 wt.%.
Fig. 5. Optical micrographs of a longitudinal section of the typical fracture surface in as-cast Mg–2.75Nd–xZn–Zr alloys in polished condition, (a) x = 0, (b) x = 2.0 wt.%. The secondary cracks are mainly locate along the grain boundaries and associated with eutectic compounds.
Fig. 6. Typical fractography of tensile specimens of as-cast Mg–2.75Nd–xZn–Zr alloys. (a and b) x = 0; (c and d) x = 2.0 wt.%, in which (b) and (d) are back scattered images, in which eutectic compounds are brighter than the matrix. Obviously, more fractured eutectic compounds can be observed on the fracture surface of 2.0 wt.% Zn containing alloy.
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Fig. 7. Optical micrographs (a) y = 1.75, (b) y = 3.0 wt.%.
of
a
longitudinal
section
of
the
typical
Specimens were etched in a 4 vol.% nital or in a solution of 12 g picric acid + 80 ml acetic acid + 80 ml water + 350 ml ethanol for microstructure observation. Microstructure was examined in an optical microscope (OM), a scanning electron microscope (SEM) and a JEOL-2010 TEM operating at 200 kV. The average grain size was measured by linear intercept method in an OM. Phase analyses were carried out with X-ray diffractometer (XRD). Fracture surface was investigated in a SEM. 3. Results 3.1. As-cast alloy Fig. 1 shows the optical micrographs of as-cast Mg–2.75Nd–xZn–Zr alloys (x = 0, 0.2, 0.5, 1.0, 2.0). They all
fracture
surface
in
as-cast
Mg–yNd–0.2Zn–Zr
181
alloys
in
polished
condition,
consist of dendrites of ␣-Mg matrix separated by eutectic compounds. In low Zn containing alloys, some particles are visible within grains (Fig. 1a–c) and their amount gradually reduces with Zn addition. The volume fraction of eutectic compounds increases with Zn addition and a network located along the grain boundaries gradually forms (Fig. 1e). In Mg–2.75Nd–2.0Zn–Zr alloy, besides the Mg12 Nd phase, new eutectic compounds forms and it is clearly visible in as-polished condition (Fig. 1f). By XRD analysis (Fig. 2) it was confirmed that the new eutectic compounds is probably Mg–Nd–Zn ternary compound, which was once reported as ␥ phase in Mg–2.8Nd–1.3Zn alloy [19]. The average grain size of as-cast Mg–2.75Nd–xZn–Zr alloys first slightly increases from 0 to 0.2Zn, and then decreases from 0.2 to 1.0Zn, while increases again more obviously at 2.0Zn (Table 2).
Fig. 8. Typical fractography of tensile specimens of as-cast Mg–yNd–0.2Zn–Zr alloys. (a and b) y = 1.25; (c and d) y = 3.0 wt.%, in which (b) and (d) are back scattered images.
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Fig. 9. Optical micrographs of solution-treated Mg–2.75Nd–xZn–Zr alloys etched in a solution of 12 g picric acid + 80 ml acetic acid + 80 ml water + 350 ml ethanol, (a) x = 0; (b) x = 0.2; (c) x = 0.5; (d) x = 1.0; (e) x = 2.0 wt.%.
Fig. 10. XRD patterns of solution-treated Mg–2.75Nd–xZn–Zr alloys, x = 0, 0.2, 0.5, 1.0, 2.0 wt.%.
Fig. 11. TEM bright filed image (a) and selected area diffraction patterns of ␥ phase with fcc structure in solution-treated Mg–2.75Nd–2.0Zn–Zr (wt.%) alloy: (b) along [1 1 0]␥ and (c) [0 1 0]␥ .
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Fig. 12. SEM micrographs of solution-treated Mg–2.75Nd–xZn–Zr alloys etched in a 4 vol.% nital, (a) x = 0; (b) x = 0.2; (c) x = 0.5; (d) x = 1.0; (e) x = 2.0 wt.%.
Fig. 3 shows the optical micrographs of as-cast Mg–yNd–0.2Zn–Zr alloys (y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25). The volume fraction of eutectic compounds increases and the average grain size decreases (Table 3) with Nd addition. By XRD analysis (not shown here), the eutectic compounds are all Mg12 Nd phase. Fig. 4 shows the mechanical properties of as-cast Mg–2.75Nd–xZn–Zr and Mg–yNd–0.2Zn–Zr alloys at room temperature. When Zn addition increases from 0 to 2.0, the yield strength (YS) of as-cast Mg–2.75Nd–xZn– Zr alloys slightly reduces at beginning (at 0.2Zn), then gradually increases from 0.2 to 1.0Zn and slightly drops at 2.0Zn. Elongation and ultimate tensile strength (UTS) first increase (at 0.2Zn) and then gradually decrease. 2.0Zn containing alloy has the lowest elongation and UTS. When Nd addition increases from 1.25 to 3.25, the YS of as-cast Mg–yNd–0.2Zn–Zr alloys gradually increases and elongation gradually decreases, while UTS slightly increases from 1.25 to 3.0Nd and then drops slightly at 3.25Nd. Fig. 5 shows the typical optical micrographs of ruptured samples perpendicular to the fracture surface of as-cast Mg–2.75Nd–xZn–Zr
alloys, which was deformed at room temperature by tensile test. Secondary cracks near the fracture surface are observed. As-cast Mg–2.75Nd–xZn–Zr alloys show nearly the same secondary crack morphologies with different Zn addition. The secondary cracks are mainly observed inside or along the eutectic compounds (Fig. 5). The typical fracture surface images of as-cast Mg–2.75Nd– xZn–Zr alloys are shown in Fig. 6. The fracture surfaces are mainly composed of ruptured eutectic compounds and their amount increases with Zn addition. The fracture surface of Zn-free alloy consists of some cleavage planes (Fig. 6a), which reveals part of trans-granular fracture. While Zn-containing alloy fractures totally inter-granularly (Fig. 6c) and nearly no cleavage planes can been found. Fig. 7 shows the typical optical micrographs of ruptured samples perpendicular to the fracture surface of as-cast Mg–yNd–0.2Zn–Zr alloys. When Nd addition is less than 2.25, secondary trans-granular cracks are often observed (Fig. 7a). With Nd addition increases, inter-granular cracks gradually become dominant (Fig. 7b), which mainly initiate from eutectic compounds. The typical fracture surface images of as-cast Mg–yNd–0.2Zn–Zr alloys are shown in Fig. 8.
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Fig. 13. Optical micrographs of solution-treated Mg–yNd–0.2Zn–Zr alloys etched in a solution of 12 g picric acid + 80 ml acetic acid + 80 ml water + 350 ml ethanol, (a) y = 1.25, (b) y = 1.75, (c) y = 2.25, (d) y = 2.75, (e) y = 3.0 and (f) y = 3.25 wt.%.
The fracture surfaces of low Nd containing alloys are mainly composed of cleavage planes (Fig. 8a and b), which indicate that trans-granular fracture is dominant. When Nd addition increases, the fractured eutectic compounds on the fracture surface increase (Fig. 8c and d). The fracture pattern of the alloys gradually changes to inter-granular fracture. In as-cast alloys, the volume fraction of eutectic compounds has significant influence on the fracture behavior. The more eutectic compounds, the easier micro cracks initiate, the shorter elongation. It seems that keeping the volume fraction of eutectic compounds low is important to achieve better tensile properties. Trace amount of Zn addition (0.2Zn) can improve both UTS and elongation, while more Zn addition leads to the decreased mechanical properties. Increase Nd addition improves the alloys’ YS and UTS, but reduces the elongation. 3.2. Solution-treated alloy Fig. 9 shows the optical micrographs of solution-treated Mg–2.75Nd–xZn–Zr alloys. When Zn addition is less than 1.0, the
eutectic compounds along the grain boundaries are nearly all dissolved into the matrix during solution treatment (Fig. 9a–c), which is also indicated by the XRD patterns in Fig. 10. When Zn addition is 1.0, there are residual eutectic compounds at the triple points of grain boundaries (Fig. 9d) and they are Mg12 Nd compounds indicated by XRD (Fig. 10). When 2.0Zn is added, coarse network-shaped compounds (Figs. 9e and 11a) are located mainly at the triple points of grain boundaries after solution treatment. They are Mg–Nd–Zn ternary compounds, ␥ phase, with fcc structure (a = 0.7444 nm) [19], both confirmed by XRD (Fig. 10) and TEM selected area electron diffraction (SAED) patterns (Fig. 11b–c). Besides the different residual particles along the grain boundaries, precipitates formed at grain interiors during solution treatment (Figs. 9 and 12) are also different. These precipitates gradually change their shape from globular-like to long-rod-like when Zn addition increases from 0 to 2.0. As previously reported [21], these precipitates are Zr-containing particles, in which the globular-like particles in Mg–Nd–Zr alloy mainly contain Zr and some of them are ZrH2 phase, while long-rod-like particles in Mg-Nd–Zn–Zr alloy mainly contain Zn and Zr. Up to now, most of the particles are still
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Fig. 14. Mechanical properties of solution-treated Mg–2.75Nd–xZn–Zr (a) and Mg–yNd–0.2Zn–Zr (b) alloys at room temperature, x = 0, 0.2, 0.5, 1.0, 2.0 and y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25 wt.%.
Fig. 15. Typical fractography of tensile specimens of solution-treated Mg–2.75Nd–xZn–Zr alloys. (a and b) x = 0; (c and d) x = 1.0; (e and f) x = 2.0 wt.%, in which (b), (d) and (f) are back scattered images.
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Fig. 16. Typical fractography of tensile specimens of solution-treated Mg–yNd–0.2Zn–Zr alloys. (a) y = 1.25; (b) y = 1.75; (c) (d) y = 3.0, in which (d) is back scattered image.
unidentified and need further work. Different from as-cast alloys, the average grain size of solution-treated Mg–2.75Nd–xZn–Zr alloys first decreases from 0 to 1.0Zn, and then obviously increases at 2.0Zn (Table 2). Compared with as-cast alloys, except 2.0Zn containing alloy, during solution treatment, higher Zn containing alloys seem to have higher resistance to grain growth: the average grain size of Mg–2.75Nd–Zr alloy grows by 9 m, while that of Mg–2.75Nd–1.0Zn–Zr alloy only grows by 4 m. However, the grains in 2.0Zn containing alloy grow the fastest. Fig. 13 shows the optical micrographs of solution-treated Mg–yNd–0.2Zn–Zr alloys. The eutectic compounds are completely dissolved into the matrix except 3.0 and 3.25Nd containing alloys. The residual eutectic compounds in 3.0Nd containing alloy are finer
than those in 3.25Nd containing alloy (Fig. 13e and f). The average grain size after solution treatment also decreases with Nd addition (Table 3), similar to the variation in as-cast alloys. Higher Nd containing alloy seems to have higher grain growth resistance during solution treatment (Table 3). Fig. 14 shows the mechanical properties of solution-treated Mg–2.75Nd–xZn–Zr and Mg–yNd–0.2Zn–Zr alloys at room temperature. When Zn addition increases from 0 to 2.0, the YS of solution-treated Mg–2.75Nd–xZn–Zr alloys slightly decreases at beginning (at 0.2Zn), which is similar to as-cast alloys (Fig. 4a), and then gradually increases from 0.2 to 1.0Zn. When Zn addition is 2.0, due to the pre-exist cracks formed during quenching, the alloy shows the lowest YS. The UTS gradually increases from 0
Fig. 17. Hardness evolution as a function of aging time during isothermal aging at 200 ◦ C for (a) Mg–2.75Nd–xZn–Zr and (b) Mg–yNd–0.2Zn–Zr alloys.
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Fig. 18. HV (HVpeak-aged − HVsolution-treatd ) change with Zn and Nd addition in Mg–2.75Nd–xZn–Zr alloys and Mg–yNd–0.2Zn–Zr alloys, respectively.
187
surfaces. More importantly, the fracture surfaces of no-quenchingcrack area in 2.0Zn containing alloy is similar to those of 1.0Zn containing alloy. It consists of cleavage planes and ␥ phases. Similar fracture surface morphology variation is observed in Nd addition series alloys. When Nd addition is less than 3.0, the fracture surfaces all consist of cleavage planes (Fig. 16a and b). Only when Nd addition is 3.0 or more, the residual eutectic compounds can be observed (Fig. 16c and d). In solution-treated alloys, both residual particles along the grain boundaries and precipitates at grain interiors vary with Zn addition. The ␥ phase in 2.0Zn containing alloy grows coarser during solution treatment and distributes along the grain boundaries, which probably leads to crack formation during quenching. Therefore, in order to improve the mechanical properties, the formation of ␥ phase should be avoided. Compared with as-cast alloys, both groups of alloys are improved in their ductility and UTS, rather than YS, which is mainly due to the dissolution of eutectic compounds. However, the coarse residual eutectic compounds also have negative influence on mechanical properties, such as in solution-treated Mg–3.25Nd–0.2Zn–Zr alloy. 3.3. 200 ◦ C peak-aged alloy
to 1.0Zn, then significantly reduces when Zn addition is 2.0 (also due to the quenching cracks). Elongation first increases (at 0.2Zn) and then gradually decreases, which is also similar to as-cast alloys (Fig. 4a). When Nd addition increases from 1.25 to 3.25, the YS of Mg–yNd–0.2Zn–Zr alloys increases and elongation decreases gradually. When Nd addition is 3.25, there is an obvious decrease of elongation, which may be related to the coarse residual eutectic compounds (Fig. 13f). The UTS gradually increases first and then decreases with the peak at 3.0Nd. Figs. 15 and 16 show the typical fracture surface images of solution-treated Mg–2.75Nd–xZn–Zr and Mg–yNd–0.2Zn–Zr alloys, respectively. When Zn addition is low (less than 1.0Zn) in Mg–2.75Nd–xZn–Zr alloys, the fracture surfaces mainly consist of cleavage planes and no residual eutectic compounds are observed (Fig. 15a and b). The alloys mainly fracture trans-granularly. When more Zn is added, such as 1.0Zn, the residual eutectic compounds are often observed on the fracture surface (Fig. 15c and d), which suggests that the micro-cracks probably first initiate around the residual eutectic compounds, then propagate trans-granularly, remaining lots of cleavage planes. When much more Zn (2.0Zn) is added, the crack forms even during quenching after solution treatment, which leads to totally inter-granular fracture (Fig. 15e and f). The ␥ phases are easily found on the “smooth” grain boundary
Fig. 17 shows the hardness curves of solution-treated Mg–2.75Nd–xZn–Zr and Mg–yNd–0.2Zn–Zr alloys isothermally aged at 200 ◦ C. When Zn addition increases from 0 to 1.0 in Mg–2.75Nd–xZn–Zr alloys, the peak hardness of all the alloys are nearly the same (Fig. 17a), about 73HV. The peak is delayed by Zn addition: Mg–Nd–Zr alloy takes about 8 h to reach peak hardness, while Zn containing alloys need 16 h. The overage hardness decreases much slower in Zn containing alloys than that in Mg–Nd–Zr alloy. It is obviously that HV (HVpeak-aged − HVsolution-treatd ) gradually decreases with Zn addition (Fig. 18). The decreased hardening effect at beginning may be due to the transformation from the prismatic precipitates in Mg–Nd binary alloy to the basal precipitates in Mg–Nd–Zn ternary alloy [15,19], as the prismatic precipitates have a better strengthening effect than the basal ones [22]. When Zn addition is 2.0, the alloy shows nearly no aging hardening effect (Fig. 17a) and HV is only about 6HV (Fig. 18). It is probably due to the growth of ␥ phase during solution treatment, which consumes so much Nd and Zn element that there are no more atoms to precipitate when subjected to aging. When Nd addition increases from 1.25 to 3.25 in Mg–yNd–0.2Zn–Zr alloys, the peak hardness increases while the time to reach peak hardness remains about 16 h (Fig. 17b). HV
Fig. 19. Mechanical properties of 200 ◦ C peak-aged Mg–2.75Nd–xZn–Zr (a) and Mg–yNd–0.2Zn–Zr (b) alloys at room temperature, x = 0, 0.2, 0.5, 1.0, 2.0 and y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25 wt.%.
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Mg–yNd–0.2Zn–Zr (y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25 wt.%) alloys in as-cast, solution-treated and 200 ◦ C peak-aged conditions are studied at room temperature and the following conclusion can be drawn:
Fig. 20. Typical fractography of Mg–2.75Nd–2.0Zn–Zr (wt.%) alloy.
tensile
specimens
of
200 ◦ C
peak-aged
gradually increases with Nd addition (Fig. 18), which suggests that the Nd addition increases the strengthening effect of the precipitates. Fig. 19 shows the mechanical properties of 200 ◦ C peak-aged Mg–2.75Nd–xZn–Zr and Mg–yNd–0.2Zn–Zr alloys at room temperature. The YS of 200 ◦ C peak-aged Mg–2.75Nd–xZn–Zr alloys slightly increases from 0 to 1.0Zn, and drops at 2.0Zn due to the pre-exist cracks formed during quenching. The UTS increases from 0 to 0.5Zn, and quickly decreases after that. Elongation first increases from 0 to 0.2Zn, and then gradually decreases. When Nd addition increases from 1.25 to 3.25, the YS of 200 ◦ C peak-aged Mg–yNd–0.2Zn–Zr alloys linearly increases (Fig. 19b). The UTS first increases and then decreases, with the peak at 3.0Nd. Elongation gradually decreases from 1.25 to 3.0Nd, and then quickly drops at 3.25Nd. Fig. 20 shows the fracture surface image of 200 ◦ C peakaged Mg–2.75Nd–2Zn–Zr alloys, which mainly consist of cleavage planes. Similar fracture surfaces can be observed in other 200 ◦ C peak-aged Mg–yNd–xZn–Zr alloys, which are similar to those in solution-treated condition (Figs. 15 and 16). The 200 ◦ C peak-aged alloys fracture mainly trans-granularly. In 200 ◦ C peak-aged alloys, the aging hardening effect decreases with Zn addition. However, trace amount of Zn addition (0.2Zn) both increase the strength (YS and UTS) and elongation. Increased Nd addition leads to the increased strength and aging hardening effect, but decreased elongation. Based on mechanical properties of Mg–yNd–xZn–Zr alloys in as-cast, solution-treated and 200 ◦ C peak-aged conditions at room temperature, the best combination of strength and elongation is achieved in Mg–3.0Nd–0.2Zn–Zr alloy. Therefore, the optimal chemical compositions are x = 0.2, y = 3.0 for gravity cast Mg–yNd–xZn–Zr alloys. The alloys have good combination of strength and elongation when x = 0.2 – 0.5, y = 2.75 – 3.0. 4. Conclusion The microstructure and mechanical properties of gravity cast Mg–2.75Nd–xZn–Zr (x = 0, 0.2, 0.5, 1.0, 2.0 wt.%) and
(1) When x = 0.2, y = 3.0 wt.%, the Mg–yNd–xZn–Zr alloy has the best combination of strength and elongation in all thermal conditions and this is considered as optimal chemical composition. (2) The eutectic compounds at grain boundaries will initiate microcracks, facilitate the alloys’ fracture during tensile test and should be avoided by solution treatment. (3) When Zn addition is higher than 1.0 wt.%, Mg–Nd–Zn ternary ␥ phase forms. It coarsens during solution treatment and reduces the alloys’ mechanical properties significantly. (4) The shape of grain interior precipitates formed during solution treatment gradually changes from globular-like to long-rod-like with the addition of Zn. These precipitates are all Zr-containing particles with different chemical composition and crystal structure. (5) Aging hardening effect of Mg–yNd–xZn–Zr alloys decreases with Zn addition from 0 to 2.0 wt.%, but increases with Nd addition from 1.25 to 3.25 wt.%. (6) Addition of trace amount of Zn (0.2 wt.%) improves the alloys’ elongation and ultimate tensile strength (UTS) in all thermal conditions, however further increase in the Zn content leads to the reduced elongation. (7) Increase in the Nd content improves the alloys’ yield strength (YS) and UTS, but reduces the ductility in all thermal conditions. Nd addition should be limited to 3.0 wt.%. References [1] B.L. Mordike, T. Ebert, Mater. Sci. Eng. A 302 (2001) 37–45. [2] I.M. Baghni, Y.-S. Wu, J.-Q. Li, et al., Trans. Nonferrous Met. Soc. China 13 (2003) 1253–1259. [3] L.L. Rokhlin, T.V. Dobatkina, N.I. Nikitina, Mater. Sci. Forum 419 (2003) 291– 296. [4] T. Mohri, M. Mabuchi, N. Satio, M. Nakamura, Mater. Sci. Eng. A 257 (1998) 287–294. [5] S.M. He, X.Q. Zeng, L.M. Peng, X. Gao, J.F. Nie, W.J. Ding, J. Alloy Compd. 427 (2007) 316–323. [6] S.M. He, X.Q. Zeng, L.M. Peng, X. Gao, J.F. Nie, W.J. Ding, J. Alloy Compd. 421 (2006) 309–313. [7] X. Gao, S.M. He, X.Q. Zeng, L.M. Peng, W.J. Ding, J.F. Nie, Mater. Sci. Eng. A 431 (2006) 322–327. [8] Q.M. Peng, J.L. Wang, Y.M. Wu, L.M. Wang, Mater. Sci. Eng. A 433 (2006) 133– 138. [9] D. Li, Q. Wang, W. Ding, Mater. Sci. Eng. A 448 (2007) 165–170. [10] D. Li, Q. Wang, W. Ding, Mater. Sci. Eng. A 428 (2006) 295–300. [11] D. Li, J. Dong, X. Zeng, C. Lu, W. Ding, J. Alloy Compd. 439 (2006) 254–257. [12] K.Y. Zheng, J. Dong, X.Q. Zeng, W.J. Ding, Mater. Sci. Eng. A 454–455 (2007) 314–321. [13] L.L. Rokhlin, Magnesium Alloys Containing Rare Earth Metals, Taylor and Francis, London, 2003. [14] Z.-P. Luo, S.-Q. Zhang, L.-Q. Lu, G. Wei, J. Rare Earths 12 (1994) 296–298. [15] R. Wilson, C.J. Bettles, B.C. Muddle, J.F. Nie, Mater. Sci. Forum 419–422 (2003) 267–272. [16] M.M. Avedesian, H. Baker, Magnesium and Magnesium Alloys, ASM, USA, 1999. [17] M. Hisa, J.C. Barry, G.L. Dunlop, Philos. Mag. 82 (2002) 497–510. [18] T.J. Pike, B. Noble, J. Less-Common Metals 30 (1973) 63–74. [19] P.A. Nuttall, T.J. Pike, B. Noble, Metallography 13 (1980) 3–20. [20] H. Gao, G. Wu, W. Ding, J. Mater. Sci. 39 (2004) 6449–6456. [21] F. Penghuai, P. Liming, J. haiyan, Z. Chunquan, X. Gao, J.F. Nie, Mater. Sci. Forum 546–549 (2007) 97–100. [22] J.F. Nie, Scripta Mater. 48 (2003) 1009–1015.
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Chemical composition optimization of gravity cast Mg–yNd–xZn–Zr alloy Fu Penghuai, Peng Liming ∗ , Jiang Haiyan, Ma Lan, Zhai Chunquan National Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composite, Shanghai Jiaotong University, Shanghai 200030, PR China
a r t i c l e
i n f o
Article history: Received 5 January 2008 Received in revised form 5 May 2008 Accepted 6 May 2008 Keywords: Composition optimization Mg–Nd–Zn–Zr Microstructure Mechanical properties
a b s t r a c t The microstructure and mechanical properties of gravity cast Mg–2.75Nd–xZn–Zr (x = 0, 0.2, 0.5, 1.0, 2.0 wt.%) and Mg–yNd–0.2Zn–Zr (y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25 wt.%) alloys in as-cast, solution-treated and 200 ◦ C peak-aged conditions were investigated at room temperature. The effect of Zn and Nd addition on microstructure and mechanical properties in different thermal conditions were discussed in Mg–2.75Nd–xZn–Zr alloys and Mg–yNd–0.2Zn–Zr alloys, respectively. When Zn addition was 0.2 wt.% and Nd addition was 3.0 wt.%, the alloy revealed the best combination of strength and elongation and was chosen as optimal chemical composition. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The extremely low density, high specific strength and stiffness of Mg alloy make it attractive for engineering applications [1,2]. It has been demonstrated that rare earth metals (RE) are the most effective elements to improve the strength properties of magnesium alloys especially at elevated temperatures [3,4]. More recently, a lot of work has been focused on magnesium alloy containing heavy rare earth elements, such as Mg–Gd–Y–Zr [5–8], Mg–Y–Sm–Zr [9,10], Mg–Dy–Gd–Nd [11] and Mg–Gd–Nd–Zr [12] alloys. Though these alloys show high strength, high content of heavy rare earth elements enhances alloys’ cost, which would probably limit their engineering application. Rational use of neodymium (Nd) could make it possible to develop a high strength and low cost magnesium alloy. Nd is one of the light rare earth elements with maximum solubility in solid Mg of 3.6 wt.% at eutectic temperature 545 ◦ C. Mg–Nd binary alloys have already had significant strengthening effect [13,14]. Addition of 0.5 wt.% Zn to Mg–3 wt.% Nd alloy would further increase its peak-aged hardness [15]. Despite these interesting findings, there is a lack of chemical composition optimization and systematic investigation of Zn containing Mg–Nd alloy, both on microstructure and mechanical properties, which are sensitive to alloy’s chemical composition.
∗ Corresponding author. Fax: +86 21 34202794. E-mail address: [email protected] (P. Liming). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.05.010
Early studies using transmission electron microscopy (TEM) reported that the precipitation process in binary Mg–Nd alloys involves formation of G.P. zones,  ,  , and  phases [16,17]. The ¯ G.P. zones have a form of discs on (1100) ␣ planes of ␣-Mg, and the  phase has a DO19 structure (a = 0.64 nm, c = 0.52 nm), and the  phase has a face-centered cubic structure with a = 0.736 nm. The equilibrium phase  has a body-centered tetragonal structure with a = 1.03 nm and c = 0.593 nm.  phase was also reported to have a hexagonal structure with a = 0.52 nm, c = 1.30 nm [18]. Ternary addition of Zn to binary Mg–Nd alloys results in a different precipitation sequence. The precipitation sequence in Mg–2.8Nd–1.3Zn (wt.%) alloy [19] was proposed to involve the formation of G.P. zones, ␥ and ␥ phases. The meta-stable ␥ phase has a hexagonal structure (a = 0.556 nm, c = 1.563 nm) and precipitates as plates on the basal plane of ␣-Mg. The stable ␥ phase is Mg–Nd–Zn ternary compounds and has an fcc structure (a = 0.7444 nm). In Mg–3Nd–0.5Zn and Mg–3Nd–1.35Zn (wt.%) alloys [15], basal precipitate plates are also observed to gradually replace the prismatic plates in Mg–Nd binary alloy with Zn addition increase. In present study, the effects of Zn and Nd addition on the microstructure and mechanical properties of Mg–2.75Nd–xZn-Zr (x = 0, 0.2, 0.5, 1.0, 2.0 wt.%) and Mg–yNd–0.2Zn–Zr (y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25 wt.%) alloys in as-cast, solutiontreated and 200 ◦ C peak-aged conditions are examined at room temperature, respectively. The optimal chemical composition of gravity cast Mg–yNd–xZn–Zr alloy is determined.
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Table 1 Chemical compositions of Mg–yNd–xZn–0.4Zr alloys (wt.%) Nominal compositions
Nd
Zn
Zr
Mg
Mg–2.75Nd–0Zn–Zr Mg–2.75Nd–0.2Zn–Zr Mg–2.75Nd–0.5Zn–Zr Mg–2.75Nd–1.0Zn–Zr Mg–2.75Nd–2.0Zn–Zr Mg–1.25Nd–0.2Zn–Zr Mg–1.75Nd–0.2Zn–Zr Mg–2.25Nd–0.2Zn–Zr Mg–3.00Nd–0.2Zn–Zr Mg–3.25Nd–0.2Zn–Zr
2.81 2.61 2.79 2.77 2.70 1.35 1.90 2.29 3.02 3.31
0.02 0.19 0.63 1.02 2.04 0.23 0.23 0.22 0.26 0.21
0.42 0.47 0.39 0.32 0.44 0.43 0.48 0.43 0.41 0.46
Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal.
2. Experimental Alloys of nominal composition Mg–2.75Nd–xZn–Zr (x = 0, 0.2, 0.5, 1.0, 2.0 wt.%) (hereafter, all compositions are in weight percents unless stated otherwise) and Mg–yNd–0.2Zn–Zr (y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25) were prepared by high purity Mg, Zn and Mg–25Nd,
Table 2 Average grain size of gravity cast Mg–2.75Nd–xZn–Zr alloys in different thermal conditions (m) Zn(wt.%)
As-cast Solution-treated Grain growtha
0
0.2
0.5
1
2
39.4 48.1 8.7
42.9 48 5.1
42.5 47.7 5.2
39.3 43.45 4.15
47.3 60.85 13.55
a Grain growth = average grain size of solution-treated alloys − average grain size of as-cast alloys.
Mg–30Zr master alloys by melting in an electrical resistance furnace under the protection of mixture gas of SF6 , CO2 and air and cast in permanent mould [20] at pouring temperature 740 ± 5 ◦ C and mould temperature 500 ± 5 ◦ C. The actual chemical composition of the alloy was determined by an inductively coupled plasma analyzer (ICP) and listed in Table 1. Specimens cut from the cast ingots were first solution-treated at 540 ◦ C for 10 h and quenched into hot water at ∼70 ◦ C, then subsequently aged at 200 ◦ C in an
Fig. 1. Optical micrographs of as-cast Mg–2.75Nd–xZn–Zr alloys, in etched condition, etched in a solution of 12 g picric acid + 80 ml acetic acid + 80 ml water + 350 ml ethanol, (a) x = 0, (b) x = 0.2, (c) x = 0.5, (d) x = 1.0, (e) x = 2.0 and in polished condition (f) x = 2.0 wt.%.
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Table 3 Average grain size of gravity cast Mg–yNd–0.2Zn–Zr alloys in different thermal conditions (m) Nd (wt.%)
As-cast Solution-treated Grain growtha
1.25
1.75
2.25
2.75
3
3.25
54.6 58.4 3.8
46.1 49.6 3.5
45.5 48.6 3.1
42.9 48 5.1
40.4 42.7 2.3
39.2 42.1 2.9
a Grain growth = average grain size of solution-treated alloys − average grain size of as-cast alloys.
Fig. 2. XRD patterns of as-cast Mg–2.75Nd–xZn–Zr alloys, x = 0, 0.2, 0.5, 1.0, 2.0 wt.%.
oil-bath. Vickers hardness testing was taken using 5 kg load and holding time of 30 s. Tensile test samples were cut into rectangular tensile specimens with dimensions of 10 mm width, 2 mm thickness and 25 mm gauge length by an electric-sparking wire-cutting machine. Tensile testing was carried out on a Zwick/Roell-20 kN material test machine at a cross head speed of 1 mm/min at room temperature. Three tensile samples were tested for each condition that they followed ISO 6892:1998 standard.
Fig. 3. Optical micrographs of as-cast Mg–yNd–0.2Zn–Zr alloys etched in a 4 vol.% nital, (a) y = 1.25, (b) y = 1.75, (c) y = 2.25, (d) y = 2.75, (e) y = 3.0 and (f) y = 3.25 wt.%.
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Fig. 4. Mechanical properties of as-cast Mg–2.75Nd–xZn–Zr (a) and Mg–yNd–0.2Zn–Zr (b) alloys at room temperature, x = 0, 0.2, 0.5, 1.0, 2.0 and y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25 wt.%.
Fig. 5. Optical micrographs of a longitudinal section of the typical fracture surface in as-cast Mg–2.75Nd–xZn–Zr alloys in polished condition, (a) x = 0, (b) x = 2.0 wt.%. The secondary cracks are mainly locate along the grain boundaries and associated with eutectic compounds.
Fig. 6. Typical fractography of tensile specimens of as-cast Mg–2.75Nd–xZn–Zr alloys. (a and b) x = 0; (c and d) x = 2.0 wt.%, in which (b) and (d) are back scattered images, in which eutectic compounds are brighter than the matrix. Obviously, more fractured eutectic compounds can be observed on the fracture surface of 2.0 wt.% Zn containing alloy.
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Fig. 7. Optical micrographs (a) y = 1.75, (b) y = 3.0 wt.%.
of
a
longitudinal
section
of
the
typical
Specimens were etched in a 4 vol.% nital or in a solution of 12 g picric acid + 80 ml acetic acid + 80 ml water + 350 ml ethanol for microstructure observation. Microstructure was examined in an optical microscope (OM), a scanning electron microscope (SEM) and a JEOL-2010 TEM operating at 200 kV. The average grain size was measured by linear intercept method in an OM. Phase analyses were carried out with X-ray diffractometer (XRD). Fracture surface was investigated in a SEM. 3. Results 3.1. As-cast alloy Fig. 1 shows the optical micrographs of as-cast Mg–2.75Nd–xZn–Zr alloys (x = 0, 0.2, 0.5, 1.0, 2.0). They all
fracture
surface
in
as-cast
Mg–yNd–0.2Zn–Zr
181
alloys
in
polished
condition,
consist of dendrites of ␣-Mg matrix separated by eutectic compounds. In low Zn containing alloys, some particles are visible within grains (Fig. 1a–c) and their amount gradually reduces with Zn addition. The volume fraction of eutectic compounds increases with Zn addition and a network located along the grain boundaries gradually forms (Fig. 1e). In Mg–2.75Nd–2.0Zn–Zr alloy, besides the Mg12 Nd phase, new eutectic compounds forms and it is clearly visible in as-polished condition (Fig. 1f). By XRD analysis (Fig. 2) it was confirmed that the new eutectic compounds is probably Mg–Nd–Zn ternary compound, which was once reported as ␥ phase in Mg–2.8Nd–1.3Zn alloy [19]. The average grain size of as-cast Mg–2.75Nd–xZn–Zr alloys first slightly increases from 0 to 0.2Zn, and then decreases from 0.2 to 1.0Zn, while increases again more obviously at 2.0Zn (Table 2).
Fig. 8. Typical fractography of tensile specimens of as-cast Mg–yNd–0.2Zn–Zr alloys. (a and b) y = 1.25; (c and d) y = 3.0 wt.%, in which (b) and (d) are back scattered images.
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Fig. 9. Optical micrographs of solution-treated Mg–2.75Nd–xZn–Zr alloys etched in a solution of 12 g picric acid + 80 ml acetic acid + 80 ml water + 350 ml ethanol, (a) x = 0; (b) x = 0.2; (c) x = 0.5; (d) x = 1.0; (e) x = 2.0 wt.%.
Fig. 10. XRD patterns of solution-treated Mg–2.75Nd–xZn–Zr alloys, x = 0, 0.2, 0.5, 1.0, 2.0 wt.%.
Fig. 11. TEM bright filed image (a) and selected area diffraction patterns of ␥ phase with fcc structure in solution-treated Mg–2.75Nd–2.0Zn–Zr (wt.%) alloy: (b) along [1 1 0]␥ and (c) [0 1 0]␥ .
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Fig. 12. SEM micrographs of solution-treated Mg–2.75Nd–xZn–Zr alloys etched in a 4 vol.% nital, (a) x = 0; (b) x = 0.2; (c) x = 0.5; (d) x = 1.0; (e) x = 2.0 wt.%.
Fig. 3 shows the optical micrographs of as-cast Mg–yNd–0.2Zn–Zr alloys (y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25). The volume fraction of eutectic compounds increases and the average grain size decreases (Table 3) with Nd addition. By XRD analysis (not shown here), the eutectic compounds are all Mg12 Nd phase. Fig. 4 shows the mechanical properties of as-cast Mg–2.75Nd–xZn–Zr and Mg–yNd–0.2Zn–Zr alloys at room temperature. When Zn addition increases from 0 to 2.0, the yield strength (YS) of as-cast Mg–2.75Nd–xZn– Zr alloys slightly reduces at beginning (at 0.2Zn), then gradually increases from 0.2 to 1.0Zn and slightly drops at 2.0Zn. Elongation and ultimate tensile strength (UTS) first increase (at 0.2Zn) and then gradually decrease. 2.0Zn containing alloy has the lowest elongation and UTS. When Nd addition increases from 1.25 to 3.25, the YS of as-cast Mg–yNd–0.2Zn–Zr alloys gradually increases and elongation gradually decreases, while UTS slightly increases from 1.25 to 3.0Nd and then drops slightly at 3.25Nd. Fig. 5 shows the typical optical micrographs of ruptured samples perpendicular to the fracture surface of as-cast Mg–2.75Nd–xZn–Zr
alloys, which was deformed at room temperature by tensile test. Secondary cracks near the fracture surface are observed. As-cast Mg–2.75Nd–xZn–Zr alloys show nearly the same secondary crack morphologies with different Zn addition. The secondary cracks are mainly observed inside or along the eutectic compounds (Fig. 5). The typical fracture surface images of as-cast Mg–2.75Nd– xZn–Zr alloys are shown in Fig. 6. The fracture surfaces are mainly composed of ruptured eutectic compounds and their amount increases with Zn addition. The fracture surface of Zn-free alloy consists of some cleavage planes (Fig. 6a), which reveals part of trans-granular fracture. While Zn-containing alloy fractures totally inter-granularly (Fig. 6c) and nearly no cleavage planes can been found. Fig. 7 shows the typical optical micrographs of ruptured samples perpendicular to the fracture surface of as-cast Mg–yNd–0.2Zn–Zr alloys. When Nd addition is less than 2.25, secondary trans-granular cracks are often observed (Fig. 7a). With Nd addition increases, inter-granular cracks gradually become dominant (Fig. 7b), which mainly initiate from eutectic compounds. The typical fracture surface images of as-cast Mg–yNd–0.2Zn–Zr alloys are shown in Fig. 8.
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Fig. 13. Optical micrographs of solution-treated Mg–yNd–0.2Zn–Zr alloys etched in a solution of 12 g picric acid + 80 ml acetic acid + 80 ml water + 350 ml ethanol, (a) y = 1.25, (b) y = 1.75, (c) y = 2.25, (d) y = 2.75, (e) y = 3.0 and (f) y = 3.25 wt.%.
The fracture surfaces of low Nd containing alloys are mainly composed of cleavage planes (Fig. 8a and b), which indicate that trans-granular fracture is dominant. When Nd addition increases, the fractured eutectic compounds on the fracture surface increase (Fig. 8c and d). The fracture pattern of the alloys gradually changes to inter-granular fracture. In as-cast alloys, the volume fraction of eutectic compounds has significant influence on the fracture behavior. The more eutectic compounds, the easier micro cracks initiate, the shorter elongation. It seems that keeping the volume fraction of eutectic compounds low is important to achieve better tensile properties. Trace amount of Zn addition (0.2Zn) can improve both UTS and elongation, while more Zn addition leads to the decreased mechanical properties. Increase Nd addition improves the alloys’ YS and UTS, but reduces the elongation. 3.2. Solution-treated alloy Fig. 9 shows the optical micrographs of solution-treated Mg–2.75Nd–xZn–Zr alloys. When Zn addition is less than 1.0, the
eutectic compounds along the grain boundaries are nearly all dissolved into the matrix during solution treatment (Fig. 9a–c), which is also indicated by the XRD patterns in Fig. 10. When Zn addition is 1.0, there are residual eutectic compounds at the triple points of grain boundaries (Fig. 9d) and they are Mg12 Nd compounds indicated by XRD (Fig. 10). When 2.0Zn is added, coarse network-shaped compounds (Figs. 9e and 11a) are located mainly at the triple points of grain boundaries after solution treatment. They are Mg–Nd–Zn ternary compounds, ␥ phase, with fcc structure (a = 0.7444 nm) [19], both confirmed by XRD (Fig. 10) and TEM selected area electron diffraction (SAED) patterns (Fig. 11b–c). Besides the different residual particles along the grain boundaries, precipitates formed at grain interiors during solution treatment (Figs. 9 and 12) are also different. These precipitates gradually change their shape from globular-like to long-rod-like when Zn addition increases from 0 to 2.0. As previously reported [21], these precipitates are Zr-containing particles, in which the globular-like particles in Mg–Nd–Zr alloy mainly contain Zr and some of them are ZrH2 phase, while long-rod-like particles in Mg-Nd–Zn–Zr alloy mainly contain Zn and Zr. Up to now, most of the particles are still
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Fig. 14. Mechanical properties of solution-treated Mg–2.75Nd–xZn–Zr (a) and Mg–yNd–0.2Zn–Zr (b) alloys at room temperature, x = 0, 0.2, 0.5, 1.0, 2.0 and y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25 wt.%.
Fig. 15. Typical fractography of tensile specimens of solution-treated Mg–2.75Nd–xZn–Zr alloys. (a and b) x = 0; (c and d) x = 1.0; (e and f) x = 2.0 wt.%, in which (b), (d) and (f) are back scattered images.
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Fig. 16. Typical fractography of tensile specimens of solution-treated Mg–yNd–0.2Zn–Zr alloys. (a) y = 1.25; (b) y = 1.75; (c) (d) y = 3.0, in which (d) is back scattered image.
unidentified and need further work. Different from as-cast alloys, the average grain size of solution-treated Mg–2.75Nd–xZn–Zr alloys first decreases from 0 to 1.0Zn, and then obviously increases at 2.0Zn (Table 2). Compared with as-cast alloys, except 2.0Zn containing alloy, during solution treatment, higher Zn containing alloys seem to have higher resistance to grain growth: the average grain size of Mg–2.75Nd–Zr alloy grows by 9 m, while that of Mg–2.75Nd–1.0Zn–Zr alloy only grows by 4 m. However, the grains in 2.0Zn containing alloy grow the fastest. Fig. 13 shows the optical micrographs of solution-treated Mg–yNd–0.2Zn–Zr alloys. The eutectic compounds are completely dissolved into the matrix except 3.0 and 3.25Nd containing alloys. The residual eutectic compounds in 3.0Nd containing alloy are finer
than those in 3.25Nd containing alloy (Fig. 13e and f). The average grain size after solution treatment also decreases with Nd addition (Table 3), similar to the variation in as-cast alloys. Higher Nd containing alloy seems to have higher grain growth resistance during solution treatment (Table 3). Fig. 14 shows the mechanical properties of solution-treated Mg–2.75Nd–xZn–Zr and Mg–yNd–0.2Zn–Zr alloys at room temperature. When Zn addition increases from 0 to 2.0, the YS of solution-treated Mg–2.75Nd–xZn–Zr alloys slightly decreases at beginning (at 0.2Zn), which is similar to as-cast alloys (Fig. 4a), and then gradually increases from 0.2 to 1.0Zn. When Zn addition is 2.0, due to the pre-exist cracks formed during quenching, the alloy shows the lowest YS. The UTS gradually increases from 0
Fig. 17. Hardness evolution as a function of aging time during isothermal aging at 200 ◦ C for (a) Mg–2.75Nd–xZn–Zr and (b) Mg–yNd–0.2Zn–Zr alloys.
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Fig. 18. HV (HVpeak-aged − HVsolution-treatd ) change with Zn and Nd addition in Mg–2.75Nd–xZn–Zr alloys and Mg–yNd–0.2Zn–Zr alloys, respectively.
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surfaces. More importantly, the fracture surfaces of no-quenchingcrack area in 2.0Zn containing alloy is similar to those of 1.0Zn containing alloy. It consists of cleavage planes and ␥ phases. Similar fracture surface morphology variation is observed in Nd addition series alloys. When Nd addition is less than 3.0, the fracture surfaces all consist of cleavage planes (Fig. 16a and b). Only when Nd addition is 3.0 or more, the residual eutectic compounds can be observed (Fig. 16c and d). In solution-treated alloys, both residual particles along the grain boundaries and precipitates at grain interiors vary with Zn addition. The ␥ phase in 2.0Zn containing alloy grows coarser during solution treatment and distributes along the grain boundaries, which probably leads to crack formation during quenching. Therefore, in order to improve the mechanical properties, the formation of ␥ phase should be avoided. Compared with as-cast alloys, both groups of alloys are improved in their ductility and UTS, rather than YS, which is mainly due to the dissolution of eutectic compounds. However, the coarse residual eutectic compounds also have negative influence on mechanical properties, such as in solution-treated Mg–3.25Nd–0.2Zn–Zr alloy. 3.3. 200 ◦ C peak-aged alloy
to 1.0Zn, then significantly reduces when Zn addition is 2.0 (also due to the quenching cracks). Elongation first increases (at 0.2Zn) and then gradually decreases, which is also similar to as-cast alloys (Fig. 4a). When Nd addition increases from 1.25 to 3.25, the YS of Mg–yNd–0.2Zn–Zr alloys increases and elongation decreases gradually. When Nd addition is 3.25, there is an obvious decrease of elongation, which may be related to the coarse residual eutectic compounds (Fig. 13f). The UTS gradually increases first and then decreases with the peak at 3.0Nd. Figs. 15 and 16 show the typical fracture surface images of solution-treated Mg–2.75Nd–xZn–Zr and Mg–yNd–0.2Zn–Zr alloys, respectively. When Zn addition is low (less than 1.0Zn) in Mg–2.75Nd–xZn–Zr alloys, the fracture surfaces mainly consist of cleavage planes and no residual eutectic compounds are observed (Fig. 15a and b). The alloys mainly fracture trans-granularly. When more Zn is added, such as 1.0Zn, the residual eutectic compounds are often observed on the fracture surface (Fig. 15c and d), which suggests that the micro-cracks probably first initiate around the residual eutectic compounds, then propagate trans-granularly, remaining lots of cleavage planes. When much more Zn (2.0Zn) is added, the crack forms even during quenching after solution treatment, which leads to totally inter-granular fracture (Fig. 15e and f). The ␥ phases are easily found on the “smooth” grain boundary
Fig. 17 shows the hardness curves of solution-treated Mg–2.75Nd–xZn–Zr and Mg–yNd–0.2Zn–Zr alloys isothermally aged at 200 ◦ C. When Zn addition increases from 0 to 1.0 in Mg–2.75Nd–xZn–Zr alloys, the peak hardness of all the alloys are nearly the same (Fig. 17a), about 73HV. The peak is delayed by Zn addition: Mg–Nd–Zr alloy takes about 8 h to reach peak hardness, while Zn containing alloys need 16 h. The overage hardness decreases much slower in Zn containing alloys than that in Mg–Nd–Zr alloy. It is obviously that HV (HVpeak-aged − HVsolution-treatd ) gradually decreases with Zn addition (Fig. 18). The decreased hardening effect at beginning may be due to the transformation from the prismatic precipitates in Mg–Nd binary alloy to the basal precipitates in Mg–Nd–Zn ternary alloy [15,19], as the prismatic precipitates have a better strengthening effect than the basal ones [22]. When Zn addition is 2.0, the alloy shows nearly no aging hardening effect (Fig. 17a) and HV is only about 6HV (Fig. 18). It is probably due to the growth of ␥ phase during solution treatment, which consumes so much Nd and Zn element that there are no more atoms to precipitate when subjected to aging. When Nd addition increases from 1.25 to 3.25 in Mg–yNd–0.2Zn–Zr alloys, the peak hardness increases while the time to reach peak hardness remains about 16 h (Fig. 17b). HV
Fig. 19. Mechanical properties of 200 ◦ C peak-aged Mg–2.75Nd–xZn–Zr (a) and Mg–yNd–0.2Zn–Zr (b) alloys at room temperature, x = 0, 0.2, 0.5, 1.0, 2.0 and y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25 wt.%.
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Mg–yNd–0.2Zn–Zr (y = 1.25, 1.75, 2.25, 2.75, 3.0, 3.25 wt.%) alloys in as-cast, solution-treated and 200 ◦ C peak-aged conditions are studied at room temperature and the following conclusion can be drawn:
Fig. 20. Typical fractography of Mg–2.75Nd–2.0Zn–Zr (wt.%) alloy.
tensile
specimens
of
200 ◦ C
peak-aged
gradually increases with Nd addition (Fig. 18), which suggests that the Nd addition increases the strengthening effect of the precipitates. Fig. 19 shows the mechanical properties of 200 ◦ C peak-aged Mg–2.75Nd–xZn–Zr and Mg–yNd–0.2Zn–Zr alloys at room temperature. The YS of 200 ◦ C peak-aged Mg–2.75Nd–xZn–Zr alloys slightly increases from 0 to 1.0Zn, and drops at 2.0Zn due to the pre-exist cracks formed during quenching. The UTS increases from 0 to 0.5Zn, and quickly decreases after that. Elongation first increases from 0 to 0.2Zn, and then gradually decreases. When Nd addition increases from 1.25 to 3.25, the YS of 200 ◦ C peak-aged Mg–yNd–0.2Zn–Zr alloys linearly increases (Fig. 19b). The UTS first increases and then decreases, with the peak at 3.0Nd. Elongation gradually decreases from 1.25 to 3.0Nd, and then quickly drops at 3.25Nd. Fig. 20 shows the fracture surface image of 200 ◦ C peakaged Mg–2.75Nd–2Zn–Zr alloys, which mainly consist of cleavage planes. Similar fracture surfaces can be observed in other 200 ◦ C peak-aged Mg–yNd–xZn–Zr alloys, which are similar to those in solution-treated condition (Figs. 15 and 16). The 200 ◦ C peak-aged alloys fracture mainly trans-granularly. In 200 ◦ C peak-aged alloys, the aging hardening effect decreases with Zn addition. However, trace amount of Zn addition (0.2Zn) both increase the strength (YS and UTS) and elongation. Increased Nd addition leads to the increased strength and aging hardening effect, but decreased elongation. Based on mechanical properties of Mg–yNd–xZn–Zr alloys in as-cast, solution-treated and 200 ◦ C peak-aged conditions at room temperature, the best combination of strength and elongation is achieved in Mg–3.0Nd–0.2Zn–Zr alloy. Therefore, the optimal chemical compositions are x = 0.2, y = 3.0 for gravity cast Mg–yNd–xZn–Zr alloys. The alloys have good combination of strength and elongation when x = 0.2 – 0.5, y = 2.75 – 3.0. 4. Conclusion The microstructure and mechanical properties of gravity cast Mg–2.75Nd–xZn–Zr (x = 0, 0.2, 0.5, 1.0, 2.0 wt.%) and
(1) When x = 0.2, y = 3.0 wt.%, the Mg–yNd–xZn–Zr alloy has the best combination of strength and elongation in all thermal conditions and this is considered as optimal chemical composition. (2) The eutectic compounds at grain boundaries will initiate microcracks, facilitate the alloys’ fracture during tensile test and should be avoided by solution treatment. (3) When Zn addition is higher than 1.0 wt.%, Mg–Nd–Zn ternary ␥ phase forms. It coarsens during solution treatment and reduces the alloys’ mechanical properties significantly. (4) The shape of grain interior precipitates formed during solution treatment gradually changes from globular-like to long-rod-like with the addition of Zn. These precipitates are all Zr-containing particles with different chemical composition and crystal structure. (5) Aging hardening effect of Mg–yNd–xZn–Zr alloys decreases with Zn addition from 0 to 2.0 wt.%, but increases with Nd addition from 1.25 to 3.25 wt.%. (6) Addition of trace amount of Zn (0.2 wt.%) improves the alloys’ elongation and ultimate tensile strength (UTS) in all thermal conditions, however further increase in the Zn content leads to the reduced elongation. (7) Increase in the Nd content improves the alloys’ yield strength (YS) and UTS, but reduces the ductility in all thermal conditions. Nd addition should be limited to 3.0 wt.%. References [1] B.L. Mordike, T. Ebert, Mater. Sci. Eng. A 302 (2001) 37–45. [2] I.M. Baghni, Y.-S. Wu, J.-Q. Li, et al., Trans. Nonferrous Met. Soc. China 13 (2003) 1253–1259. [3] L.L. Rokhlin, T.V. Dobatkina, N.I. Nikitina, Mater. Sci. Forum 419 (2003) 291– 296. [4] T. Mohri, M. Mabuchi, N. Satio, M. Nakamura, Mater. Sci. Eng. A 257 (1998) 287–294. [5] S.M. He, X.Q. Zeng, L.M. Peng, X. Gao, J.F. Nie, W.J. Ding, J. Alloy Compd. 427 (2007) 316–323. [6] S.M. He, X.Q. Zeng, L.M. Peng, X. Gao, J.F. Nie, W.J. Ding, J. Alloy Compd. 421 (2006) 309–313. [7] X. Gao, S.M. He, X.Q. Zeng, L.M. Peng, W.J. Ding, J.F. Nie, Mater. Sci. Eng. A 431 (2006) 322–327. [8] Q.M. Peng, J.L. Wang, Y.M. Wu, L.M. Wang, Mater. Sci. Eng. A 433 (2006) 133– 138. [9] D. Li, Q. Wang, W. Ding, Mater. Sci. Eng. A 448 (2007) 165–170. [10] D. Li, Q. Wang, W. Ding, Mater. Sci. Eng. A 428 (2006) 295–300. [11] D. Li, J. Dong, X. Zeng, C. Lu, W. Ding, J. Alloy Compd. 439 (2006) 254–257. [12] K.Y. Zheng, J. Dong, X.Q. Zeng, W.J. Ding, Mater. Sci. Eng. A 454–455 (2007) 314–321. [13] L.L. Rokhlin, Magnesium Alloys Containing Rare Earth Metals, Taylor and Francis, London, 2003. [14] Z.-P. Luo, S.-Q. Zhang, L.-Q. Lu, G. Wei, J. Rare Earths 12 (1994) 296–298. [15] R. Wilson, C.J. Bettles, B.C. Muddle, J.F. Nie, Mater. Sci. Forum 419–422 (2003) 267–272. [16] M.M. Avedesian, H. Baker, Magnesium and Magnesium Alloys, ASM, USA, 1999. [17] M. Hisa, J.C. Barry, G.L. Dunlop, Philos. Mag. 82 (2002) 497–510. [18] T.J. Pike, B. Noble, J. Less-Common Metals 30 (1973) 63–74. [19] P.A. Nuttall, T.J. Pike, B. Noble, Metallography 13 (1980) 3–20. [20] H. Gao, G. Wu, W. Ding, J. Mater. Sci. 39 (2004) 6449–6456. [21] F. Penghuai, P. Liming, J. haiyan, Z. Chunquan, X. Gao, J.F. Nie, Mater. Sci. Forum 546–549 (2007) 97–100. [22] J.F. Nie, Scripta Mater. 48 (2003) 1009–1015.