The effect of Y on the microstructure and mechanical performance of an Mg-Al-Y casting alloy

Abstract

Environmental gains of electric cars can be optimized with the use of lightweight and recyclable magnesium in the vehicle’s structural components. Ductility improvement of low-density Mg-Al alloys will extend their use in automotive body applications. The authors achieved 63% ductility improvement in Mg-6wt%Al with trace Y (1.5 ppm) due to the β-phase refinement and predicted that higher levels would not perform as well. As predicted, 0.3wt% of Y addition investigated in this study led to lower mechanical performance and β-phase refinement than those obtained with trace additions. The tensile ductility and yield strength increased by ~13% and 16%, respectively, and the compression strain to fracture by ~22%. Scanning electron and optical microscopy, X-Rays diffraction, mechanical testing and thermodynamic calculations were used to investigate the effect of 0.3wt% Y on the microstructure of Mg-6wt%Al. The matrix dissolution revealed the close association of the Al₂Y and the β-Mg₁₇Al₁₂ phases.

1. Introduction

Future autonomous cars will rely on computer systems, sensors, and satellite navigation which require extensive electronics that increase the vehicle’s weight (Gao et al., 2014). Due to their high strength-to-weight-ratio, magnesium (Mg) structural alloys are a viable solution for vehicle-weight reduction in future cars. Mg-6wt%Al (Mg-6Al) casting alloys demonstrate the optimum level of strength and ductility for most automotive applications but further increase in their ductility will lead to their extended use in crashworthy car-body components (Friedrich & Mordike, 2006). The Mg₁₇Al₁₂ precipitate, the main second phase, improves strength but reduces ductility (Friedrich & Mordike, 2006; Nave et al., 2000).

Studies have shown that yttrium (Y) additions can improve the ductility of Mg-Al alloys (Su et al., 2010; Tahreen et al., 2016). When Y is in the solid solution of α-Mg then it can improve its mechanical properties via: (1) decrease of the Stacking Fault Energy (SFE) activating the pyramidal <c+a> disclocations (Sandlöbes et al., 2012) or (2) forming long period stacking ordered (LPSO) phases when a transition metal is present (Kawamura & Yamasaki, 2007). However, with Al present, the solubility of rare earths in α-Mg is nil, instead brittle precipitates (Pourbahari et al., 2017; C. Wang et al., 2015; L. Wang et al., 2019) form. Researchers have observed that the modification of β-Mg₁₇Al₁₂ in Mg-Al-Y alloys is responsible for improved mechanical properties (Boby et al., 2013; Cai et al., 2018; Kashefi & Mahmudi, 2012; S.-R. Wang et al., 2009). Previous work (Korgiopoulos & Pekguleryuz, 2020) by the authors has shown that the refinement of β-Mg₁₇Al₁₂ improves the ductility of Mg-6Al cast alloys at trace Y additions. This has been attributed to co-precipitation (at close temperatures) of the Al₄MgY phase with the Mg₁₇A₁₂ phase. Thermodynamics also predicted that the formation temperatures of the two phases deviate at higher Y levels leading to a loss in nucleant effectiveness.

2. Objective

The current work investigates the effect of Y additions (0.3wt%) to see if β-phase refinement and ductility improvement seen in trace levels of Y are also observed at higher Y levels and to elucidate the role of Y on the mechanical properties of Mg-6Al based alloys.

3. Methods

Mg-6Al-0.3Y (in wt%) alloy was synthesized using commercial purity (99.98%) Mg, 99.9% pure Al granules and 99.9% pure Y rods. The total mass of the alloying additions was 617 gr using 95% recovery factor for Al and 60% for Y. The alloying additions were made at 720 °C in a graphite crucible under CO₂/SF₆ protective atmosphere. A graphite crucible was used because it is affordable, and it does not react with magnesium. The cleaning after casting is efficient and leaves no residuals that could possibly contaminate the subsequent castings. The molten alloy was then poured under protective atmosphere into a preheated (400^oC) steel mold to produce a flat plate. The actual composition (in wt%) of the alloy according to inductively coupled plasma atomic-emission spectroscopy is: 6.26%Al, 0.29%Y, 0.02% impurities (Fe, Mn) with Mg as balance. A scanning electron microscope (SEM-Hitachi SU3500) with an energy dispersive X-ray spectroscopy (EDS) detector was used for microstructural investigation. The grain size of α-Mg was measured with the intercept method using a Nikon-Epiphot 200 optical microscope after etching the samples with 4.2gr picric acid, 10 ml acetic acid, 10 ml distilled water and 70 ml ethanol. The ImageJ software (Rasband, 2011) was used to measure the precipitates and the grain size. The crystallographic information was obtained by XRD (Bruker D8 Discovery X-Ray Diffractometer-Cu source) in the bulk sample and after matrix extraction by using 5% acetic acid. The mechanical properties were evaluated by tensile and compression testing (MTS 810) at room temperature with a strain rate of 0.001s^-1. Thermodynamic calculations (FactSage with FTlite database) (Bale et al., 2002) based on the CALPHAD method have been performed in equilibrium and non-equilibrium conditions (Scheil cooling).

4. Results and discussion

Mg-6Al-0.3Y alloy consists of partially divorced β-Mg₁₇Al₁₂, interdendritic Y-enriched precipitates and the Al enriched a-Mg phase as determined by EDS (Fig. 1). Y is detected in Mg-Al-Y and Mg-Al-Mn-Fe-Y precipitates. Both precipitates are closely associated with Mg₁₇Al₁₂ suggesting that they act as nucleation sites. The Y-enriched precipitates range from fine to coarse and present in three different morphologies, namely, spherical, square and plate-like, with the expectation that only the finer precipitates can act as refiners for the β phase. Table 1 shows the composition of the β-phase and Al₂Y as per the EDS analysis. No Y was detected in the a-Mg solid solution. There is close association (Figs 1 and 2) of the Al₂Y precipitates with the Mg₁₇Al₁₂. Similar association has been observed before by the authors (Korgiopoulos & Pekguleryuz, 2020) in Mg-6%Al based alloys for lower Y additions. XRD detects (Fig. 3a) Mg-Al solid solution and β-Mg₁₇Al₁₂ peaks in the bulk alloy. After the matrix extraction (Fig. 3b), the matrix disappears and the peaks for Al₂Y and Mg₁₇Al₁₂ become stronger. The peak intensity of spectra b is lower due to the low amount and small size (average size ~10-50 microns) of the extracted precipitates.

Figure 1. SEM/BSE as-cast microstructure of Mg-6Al-0.3Y and EDS maps. The red square shows the close associations of Y enriched precipitates with Mg₁₇Al₁₂.

Table 1. EDS on the precipitates after matrix extraction

Figure 2. Mg-Al-0.3Y alloy after matrix extraction. The red circles show the Al₂Y precipitates associated with the Mg₁₇Al₁₂ phase.

Figure 3. XRD results (a) Bulk alloy, (b) after matrix extraction.

The Mg₁₇Al₁₂ (Table 2) in Mg-6Al-0.3Y is 25% finer than that in the binary Mg-6Al, but 41% coarser than the alloy with trace Y amount. Additionally, high Y addition forms even coarser (~87μm²) Mg-Al-Mn-Fe-(Y) and Mg-Al-Y precipitates. The grain size of the cast Mg-6Al-0.3Y is also higher than the alloy with trace Y level and it is attributed to the coarsening of the Al₂Y phase (Chang et al., 2013; Pan et al., 2008; Zou et al., 2005).

Table 2. Precipitates size and α-Mg grain size as measured with Image J

* Only precipitates in Mg-6Al-0.3Y alloy contain Y.

Mg-6Al-0.3Y has the highest tensile yield strength (YS) of the three alloys (Table 3). Compared to the Mg-Al with trace Y, the tensile ductility (% El), ultimate tensile strength (UTS), and the compressive strength are lower (Tables 3 and 4). According to thermodynamic simulations conducted by the authors (Table 5), the solubility of Y in Mg-Al-Y is practically nil. Instead, Y forms precipitates (ordered intermetallics) with Al such as Al₄MgY, Al₃Y and Al₂Y. In Mg-Al-0.3Y, Al₂Y forms earlier (at higher temperature) than the β-phase (Table 5) and has time to coarsen losing its effectiveness as a nucleant of the β-phase and embrittling the alloy.

Table 3. Tensile properties of as cast samples at room temperature

Table 4. Compression properties of as cast samples at room temperature

Table 5. Equilibrium and non-equilibrium (Scheil) thermodynamic calculations

5. Conclusions

The addition of 0.3wt% Y improves the tensile properties and the strain to fracture in compression of the binary cast alloys Mg-6wt%Al. The improvement in ductility and UTS is not as significant as lower (trace level) Y additions. Al₂Y precipitates (determined via XRD and SEM/EDS) are in close association with the Mg₁₇Al₁₂ phase indicating their role as nucleants but the early formation of the Al₂Y in the liquid in Mg-Al-0.3Y leads to its coarsening, resulting in some loss in mechanical properties compared to the alloy with the trace level of Y.