Effect of Modification on Microstructure and Properties of AZ91 Magnesium Alloy

Refinement of α-Mg solid solution grains has a significant influence on the improvement of mechanical properties of cast magnesium alloys. In the article, the effects of three modifiers on microstructure and properties of AZ91 magnesium alloy casted to a sand mould were described. Overheating, hexachloroethane and wax-CaF2-carbon powder were applied. The research procedure comprised microstructure analysis by means of light microscopy, scanning electron microscopy and quantitative analysis with AnalySIS Pro® software and mechanical properties’ investigation. The microstructure of AZ91 alloy in the as-cast condition consists of α-Mg solid solution with precipitates of Mg17Al12, Mg2Si and Al8Mn5 phases. It was reported that all applied modifiers cause refinement of α-Mg solid solution grains and a decrease of the volume fraction of α-Mg+Mg17Al12 compound discontinuous precipitates. The best results were obtained in the case of wax-CaF2-carbon powder.


Introduction
Primary grain size is one of the elementary structural criteria in evaluation of the quality of cast magnesium alloys. Grain refinement results in improvement of mechanical properties of the alloy, e.g., wear resistance and corrosion resistance [1][2][3]. Cooling rate is relatively low in the case of sand casting, which causes the significant growth of solid solution grains and promotes the increase of volume fraction of intermetallic phases. Therefore, refinement of solid solution grains in gravity-casting requires modification.
In the case of Zr-containing magnesium alloys the main factors influencing the refinement of solid solution grains are the volume fraction of strengthening precipitates and the amount of zirconium or applied Mg-Zr master alloy [4], while in case of Mg-Al alloys the most common modification processes are: • overheating of the molten alloy.
In industrial practice, most alloys should not be heated over the liquidus temperature more than necessary to homogenise the chemical composition. High overheating intensifies oxidation of the alloy, absorption of gases and causes the formation of coarse primary grain structure [5]. However, in the case of Mg-Al alloys the described treatment promotes the refinement of solid solution grains. The treatment comprises heating of the molten alloy 150-260 • C over liquidus, temperature arrest for The material was melted in gas crucible furnace under the protective atmosphere with the following composition: 20% argon, 20% CO 2 and 0.2% SF 6. Sand casting was performed at 770 • C. Three types of modification were applied: • overheating of 10 min. at 850 • C with subsequent rapid cooling to pouring temperature, • application of hexachlorethane (commercial name DEGASAL C1/A), 1 kg/100 kg of the alloy at temperature of~740 • C, • application of wax-CaF 2 -carbon powder (commercial name NUCLEANT 5000), 0.3 kg/100 kg of the alloy at temperature of approx. 730 • C.
Then the heat treatment comprising solution treatment of 415 • C/25 h/water cooling and ageing treatment of 170 • C/8 h/air cooling was applied.
An Olympus GX71 light microscope (LM) and a HITACHI S-3400N scanning electron microscope (SEM) with a Thermo Noran energy-dispersive X-ray spectrometer (EDS) equipped with SYSTEM SIX were used for observations of microstructure. EDS analysis was performed with an accelerating voltage Crystals 2020, 10 Conditions of phases detection and evaluation are presented in Figure 1. Conditions of phases detection and evaluation are presented in Figure 1. Mechanical properties were evaluated according to ASTM B 557M standard [15] using an MTS-810 machine. Yield strength at 0.2% offset, elongation and ultimate tensile strength were averaged over three tensile samples.

As-Cast Condition
The microstructure of AZ91 alloy in as-cast condition is composed of α-Mg solid solution with precipitates of Mg17Al12 phase (Figure 2a). Two types of Mg17Al12 precipitates with different morphology were observed: • continuous precipitates located mainly on grain boundaries of α-Mg solid solution; • plate-like precipitates (discontinuous precipitates) formed in discontinuous diffusive transition creating the mixture of α-Mg and discontinuous precipitates of Mg17Al12 phase. The presence of α-Mg+Mg17Al12 discontinuous precipitates is a result of relatively slow solidification of the alloy in the sand mould. Moreover, the presence of Mg2Si and Al8Mn5 phases in the microstructure of investigated alloy was reported [16]. Mechanical properties were evaluated according to ASTM B 557M standard [15] using an MTS-810 machine. Yield strength at 0.2% offset, elongation and ultimate tensile strength were averaged over three tensile samples.

As-Cast Condition
The microstructure of AZ91 alloy in as-cast condition is composed of α-Mg solid solution with precipitates of Mg 17 Al 12 phase (Figure 2a). Two types of Mg 17 Al 12 precipitates with different morphology were observed: • continuous precipitates located mainly on grain boundaries of α-Mg solid solution; • plate-like precipitates (discontinuous precipitates) formed in discontinuous diffusive transition creating the mixture of α-Mg and discontinuous precipitates of Mg 17 Al 12 phase.
Average area of solid solution grain cross-section was Ā = 1692 μm 2 (d = 43.6 μm). The volume fraction of continuous precipitates of Mg17Al12 phase was estimated to VV = 4.9%, while in case of α-Mg+ Mg17Al12 discontinuous precipitates it was VV = 7.34%. Mg2Si phase was inhomogenously distributed in the structure of the alloy and its volume fraction was VV = 0.1%.
The applied heat treatment caused the disappearance of continuous precipitates of Mg17Al12 phase and significant increase of volume fraction of α-Mg+Mg17Al12 discontinuous precipitates up to VV = 31.55% (Figure 2b).

Effect of Overheating
The microstructure of AZ91 alloy overheated at 850 °C is shown in Figure 3.  The presence of α-Mg+Mg 17 Al 12 discontinuous precipitates is a result of relatively slow solidification of the alloy in the sand mould. Moreover, the presence of Mg 2 Si and Al 8 Mn 5 phases in the microstructure of investigated alloy was reported [16].
Average area of solid solution grain cross-section wasĀ = 1692 µm 2 (d = 43.6 µm). The volume fraction of continuous precipitates of Mg 17 Al 12 phase was estimated to V V = 4.9%, while in case of α-Mg+ Mg 17 Al 12 discontinuous precipitates it was V V = 7.34%. Mg 2 Si phase was inhomogenously distributed in the structure of the alloy and its volume fraction was V V = 0.1%.
The applied heat treatment caused the disappearance of continuous precipitates of Mg 17 Al 12 phase and significant increase of volume fraction of α-Mg+Mg 17 Al 12 discontinuous precipitates up to V V = 31.55% (Figure 2b).

Effect of Overheating
The microstructure of AZ91 alloy overheated at 850 • C is shown in Figure 3. Average area of solid solution grain cross-section was Ā = 1692 μm 2 (d = 43.6 μm). The volume fraction of continuous precipitates of Mg17Al12 phase was estimated to VV = 4.9%, while in case of α-Mg+ Mg17Al12 discontinuous precipitates it was VV = 7.34%. Mg2Si phase was inhomogenously distributed in the structure of the alloy and its volume fraction was VV = 0.1%.
The applied heat treatment caused the disappearance of continuous precipitates of Mg17Al12 phase and significant increase of volume fraction of α-Mg+Mg17Al12 discontinuous precipitates up to VV = 31.55% (Figure 2b).

Effect of Overheating
The microstructure of AZ91 alloy overheated at 850 °C is shown in Figure 3.  Overheating of the alloy resulted in an approximate 7% decrease of average area of solid solution grain cross-section toĀ = 1585 µm 2 (d = 40.2 µm) in comparison to as-cast condition. No significant changes in volume fraction of continuous precipitates of Mg 17 Al 12 (V V = 5.28%) and Mg 2 Si (V V = 0.08%) phases were observed. However, significant change was observed in the case of α-Mg+Mg 17 Al 12 areas (decrease to V V = 2.15%, approximately 70% in comparison to as-cast condition). Moreover, the presence of particles containing iron, as well as calcium and potassium, was reported (Figure 3b Overheating of the alloy resulted in an approximate 7% decrease of average area of solid solution grain cross-section to Ā = 1585 μm 2 (d = 40.2 μm) in comparison to as-cast condition. No significant changes in volume fraction of continuous precipitates of Mg17Al12 (VV = 5.28%) and Mg2Si (VV = 0.08%) phases were observed. However, significant change was observed in the case of α-Mg+Mg17Al12 areas (decrease to VV = 2.15%, approximately 70% in comparison to as-cast condition). Moreover, the presence of particles containing iron, as well as calcium and potassium, was reported (Figure 3b  Solid solution grain refinement by overheating in the case of investigated alloy results from the occurrence of two different mechanisms of nucleation. First, a significant amount of aluminium oxides acting as effective nucleation sites for α-Mg solid solution was observed. The presence of these oxides results from excessively high overheating temperature, which increased the solubility of gases and in consequence the number of impurities formed [10,17]. The second mechanism relates to the presence of iron in the alloy. At 850 °C, iron diffused from the mild steel crucible to the melt and formed highly dispersive Al-Fe intermetallic phases. These phases acted as nucleation sites for solid solution. At the same time, the decrease of volume fraction of α-Mg+Mg17Al12 areas was an effect of higher supercooling and in consequence higher solidification rate, which significantly decreased the possibility of discontinuous diffusive transition occurrence [6].

Effect of Hexachlorethane Refiner
The application of hexachloroethane caused the decrease of the average area of α-Mg solid solution grain cross-section to Ā = 1513 μm 2 (d = 38.7 μm). That value was slightly (~5%) lower than after the overheating process (Figure 5a). Solid solution grain refinement by overheating in the case of investigated alloy results from the occurrence of two different mechanisms of nucleation. First, a significant amount of aluminium oxides acting as effective nucleation sites for α-Mg solid solution was observed. The presence of these oxides results from excessively high overheating temperature, which increased the solubility of gases and in consequence the number of impurities formed [10,17]. The second mechanism relates to the presence of iron in the alloy. At 850 • C, iron diffused from the mild steel crucible to the melt and formed highly dispersive Al-Fe intermetallic phases. These phases acted as nucleation sites for solid solution. At the same time, the decrease of volume fraction of α-Mg+Mg 17 Al 12 areas was an effect of higher supercooling and in consequence higher solidification rate, which significantly decreased the possibility of discontinuous diffusive transition occurrence [6].

Effect of Hexachlorethane Refiner
The application of hexachloroethane caused the decrease of the average area of α-Mg solid solution grain cross-section toĀ = 1513 µm 2 (d = 38.7 µm). That value was slightly (~5%) lower than after the overheating process (Figure 5a). No significant change in volume fraction of continuous precipitates of Mg 17 Al 12 (V V = 5.56%) and Mg 2 Si (V V = 0.1%) phases were observed in comparison to as-cast alloy. However, as in the case of overheating, significant decrease of the volume fraction of α-Mg+Mg 17 Al 12 areas to V V = 1.97% (approximately 75% in comparison to as-cast condition) was reported. Apart from precipitates typical for this alloy, the particles of the Al 8 Mn 5 phase containing carbon were observed ( Figure 6). No continuous precipitates of the Mg 17 Al 12 phase were observed in the alloy after age-hardening treatment as in case of non-modified alloy. However, a significant increase of volume fraction of α-Mg+Mg 17 Al 12 discontinuous precipitates to V V = 36.45% (approximately 15% in comparison to non-modified alloy after heat treatment) was reported. No significant change in volume fraction of continuous precipitates of Mg17Al12 (VV = 5.56%) and Mg2Si (VV = 0.1%) phases were observed in comparison to as-cast alloy. However, as in the case of overheating, significant decrease of the volume fraction of α-Mg+Mg17Al12 areas to VV = 1.97% (approximately 75% in comparison to as-cast condition) was reported. Apart from precipitates typical for this alloy, the particles of the Al8Mn5 phase containing carbon were observed ( Figure 6). No continuous precipitates of the Mg17Al12 phase were observed in the alloy after age-hardening treatment as in case of non-modified alloy. However, a significant increase of volume fraction of α-Mg+ Mg17Al12 discontinuous precipitates to VV = 36.45% (approximately 15% in comparison to non-modified alloy after heat treatment) was reported.  Because the presence of carbon was observed mainly within Al8Mn5 precipitates (Figure 6), more complex mechanism of α-Mg solid solution grain refinement is very probable. In the first stage, Al4C3 particle is formed. On its surface, the Al8Mn5 phase forms and then the α-Mg solid solution grain nucleates on it [18].  No significant change in volume fraction of continuous precipitates of Mg17Al12 (VV = 5.56%) and Mg2Si (VV = 0.1%) phases were observed in comparison to as-cast alloy. However, as in the case of overheating, significant decrease of the volume fraction of α-Mg+Mg17Al12 areas to VV = 1.97% (approximately 75% in comparison to as-cast condition) was reported. Apart from precipitates typical for this alloy, the particles of the Al8Mn5 phase containing carbon were observed ( Figure 6). No continuous precipitates of the Mg17Al12 phase were observed in the alloy after age-hardening treatment as in case of non-modified alloy. However, a significant increase of volume fraction of α-Mg+ Mg17Al12 discontinuous precipitates to VV = 36.45% (approximately 15% in comparison to non-modified alloy after heat treatment) was reported. The presence of particles containing carbon may result from the formation of numerous Al4C3 phase particles in the reaction of aluminium with the C2Cl6 phase during the mixing of the melt. Al4C3 particles act as sites for heterogenic nucleation of α-Mg solid solution. Effective refinement is associated with the same type of crystalline structure. Both Al4C3 phase and α-Mg solid solution have hexagonal unit cell with similar lattice parameters (a = 0.333 nm, c = 0.49900 nm in Al4C3 and a = 0.32030 nm, c = 0.52002 nm in α-Mg). Because the presence of carbon was observed mainly within Al8Mn5 precipitates (Figure 6), more complex mechanism of α-Mg solid solution grain refinement is very probable. In the first stage, Al4C3 particle is formed. On its surface, the Al8Mn5 phase forms and then the α-Mg solid solution grain nucleates on it [18]. The presence of particles containing carbon may result from the formation of numerous Al 4 C 3 phase particles in the reaction of aluminium with the C 2 Cl 6 phase during the mixing of the melt. Al 4 C 3 particles act as sites for heterogenic nucleation of α-Mg solid solution. Effective refinement is associated with the same type of crystalline structure. Both Al 4 C 3 phase and α-Mg solid solution have hexagonal unit cell with similar lattice parameters (a = 0.333 nm, c = 0.49900 nm in Al 4 C 3 and a = 0.32030 nm, c = 0.52002 nm in α-Mg). Because the presence of carbon was observed mainly within Al 8 Mn 5 precipitates (Figure 6), more complex mechanism of α-Mg solid solution grain refinement is very probable. In the first stage, Al 4 C 3 particle is formed. On its surface, the Al 8 Mn 5 phase forms and then the α-Mg solid solution grain nucleates on it [18].
Carbon hexachloride was effective both in grain refinement and in degassing of the melt. Nevertheless, its application is limited due to emission of toxic gases, like hydrocarbon chlorates, which cause severe pollution of the environment [19].

Effect of Wax-CaF 2 -Carbon Refiner
In the first stage of the process (addition of 0.2 kg of modifier per 100 kg of the alloy), it was reported that the applied modification had no significant influence on the size of solid solution grains and did not cause the increase of mechanical properties of the casting, which was caused by improper amount of modifier being added, inappropriate technology of its introduction to the melt, and insufficient temperature of the modification process (720 • C). Therefore, in the second stage the amount of modifier was increased from 0.2 kg to 0.4 kg per 100 kg of the alloy. The technology of introduction of modifier to the melt was changed as well. The technology utilised in the first stage comprised the introduction of the modifier on the surface of molten metal with subsequent intensive mixing, which resulted in the modifier burnout. Hence, in the second stage the modifier powder was wrapped in aluminium foil and introduced to the bottom of the crucible. Simultaneously the temperature of the process was raised to 740 • C to increase the reactivity of the modifier. The change of the modification process parameters and increased reactivity of the modifier resulted in solid solution grain refinement. The average area of solid solution grain cross-section decreased toĀ = 1457 µm 2 (d = 37.5 µm) and was reported to be the lowest among all investigated smelts (Figure 7a).

Effect of Wax-CaF2-Carbon Refiner
In the first stage of the process (addition of 0.2 kg of modifier per 100 kg of the alloy), it was reported that the applied modification had no significant influence on the size of solid solution grains and did not cause the increase of mechanical properties of the casting, which was caused by improper amount of modifier being added, inappropriate technology of its introduction to the melt, and insufficient temperature of the modification process (720 °C). Therefore, in the second stage the amount of modifier was increased from 0.2 kg to 0.4 kg per 100 kg of the alloy. The technology of introduction of modifier to the melt was changed as well. The technology utilised in the first stage comprised the introduction of the modifier on the surface of molten metal with subsequent intensive mixing, which resulted in the modifier burnout. Hence, in the second stage the modifier powder was wrapped in aluminium foil and introduced to the bottom of the crucible. Simultaneously the temperature of the process was raised to 740°C to increase the reactivity of the modifier. The change of the modification process parameters and increased reactivity of the modifier resulted in solid solution grain refinement. The average area of solid solution grain cross-section decreased to Ā = 1457 μm 2 (d = 37.5 μm) and was reported to be the lowest among all investigated smelts (Figure 7a). After modification with wax-CaF2-carbon (as in the case of modification with carbon hexachloroethane) the volume fraction of continuous precipitates of Mg17Al12 phase (VV = 5.13%) and Mg2Si phase (VV = 0.09%) remained unchanged in comparison to as-cast alloy, whereas the volume fraction of α-Mg+Mg17Al12 areas was decreased by approximately 80% to VV = 1.54% (the most significant change among all applied modifiers). Precipitates containing carbon were observed in the microstructure of the alloy, in the vicinity of Al8Mn5 phase (Figure 8a,b), as in the case of modification with carbon hexachloride. Moreover, the presence of precipitates containing carbon, calcium and fluorine was reported (Figure 8a,c). In this case no continuous precipitates of Mg17Al12 phase were observed after heat treatment as well (Figure 7b). However, the volume fraction of α-Mg+Mg17Al12 discontinuous precipitates was the largest, VV = 42.39%. After modification with wax-CaF 2 -carbon (as in the case of modification with carbon hexachloroethane) the volume fraction of continuous precipitates of Mg 17 Al 12 phase (V V = 5.13%) and Mg 2 Si phase (V V = 0.09%) remained unchanged in comparison to as-cast alloy, whereas the volume fraction of α-Mg+Mg 17 Al 12 areas was decreased by approximately 80% to V V = 1.54% (the most significant change among all applied modifiers). Precipitates containing carbon were observed in the microstructure of the alloy, in the vicinity of Al 8 Mn 5 phase (Figure 8a,b), as in the case of modification with carbon hexachloride. Moreover, the presence of precipitates containing carbon, calcium and fluorine was reported (Figure 8a,c). In this case no continuous precipitates of Mg 17 Al 12 phase were observed after heat treatment as well (Figure 7b). However, the volume fraction of α-Mg+Mg 17 Al 12 discontinuous precipitates was the largest, V V = 42.39%.
In the case of this modifier, the mechanism of grain refinement is complex. First mechanism comprises the formation of Al 4 C 3 phase particles, which act as nucleation sites for the α-Mg solid solution, while the second is a result of the presence of calcium, which forms Al 2 Ca phase with aluminium. Al 2 Ca phase promotes nucleation, increases supercooling, and limits the grain growth [20]. In the case of this modifier, the mechanism of grain refinement is complex. First mechanism comprises the formation of Al4C3 phase particles, which act as nucleation sites for the α-Mg solid solution, while the second is a result of the presence of calcium, which forms Al2Ca phase with aluminium. Al2Ca phase promotes nucleation, increases supercooling, and limits the grain growth [20].

Influence of Modification on Mechanical Properties
Basing on the conducted research it was reported that the type of modifier does not have a significant influence on the mechanical properties of AZ91 alloy in as-cast condition (Figure 9a). A slight (~5%) decrease of ultimate tensile strength was observed and was attributed to the decrease of the volume fraction of α-Mg+Mg17Al12 discontinuous precipitates areas. The strengthening effect resulting from the grain refinement was relatively insignificant in this case. On the other hand, the effect of grain refinement was noticeable after heat treatment (Figure 9b). Grain boundaries act as nucleation sites for discontinuous Mg17Al12 precipitates, and the more grain boundaries, the higher volume fraction of the Mg17Al12 phase. The highest volume fraction of α-Mg+Mg17Al12 discontinuous precipitates (VV = 42.39%) was reported after modification with wax-CaF2-carbon powder, while the lowest was in non-modified alloy (VV = 31.55%). This results in the highest ultimate tensile strength of the alloy after wax-CaF2-carbon powder modification (UTS = 269 MPa), whereas the lowest ultimate tensile strength was reported in the case of non-modified alloy (UTS = 252 MPa).

Influence of Modification on Mechanical Properties
Basing on the conducted research it was reported that the type of modifier does not have a significant influence on the mechanical properties of AZ91 alloy in as-cast condition (Figure 9a). In the case of this modifier, the mechanism of grain refinement is complex. First mechanism comprises the formation of Al4C3 phase particles, which act as nucleation sites for the α-Mg solid solution, while the second is a result of the presence of calcium, which forms Al2Ca phase with aluminium. Al2Ca phase promotes nucleation, increases supercooling, and limits the grain growth [20].

Influence of Modification on Mechanical Properties
Basing on the conducted research it was reported that the type of modifier does not have a significant influence on the mechanical properties of AZ91 alloy in as-cast condition (Figure 9a). A slight (~5%) decrease of ultimate tensile strength was observed and was attributed to the decrease of the volume fraction of α-Mg+Mg17Al12 discontinuous precipitates areas. The strengthening effect resulting from the grain refinement was relatively insignificant in this case. On the other hand, the effect of grain refinement was noticeable after heat treatment (Figure 9b). Grain boundaries act as nucleation sites for discontinuous Mg17Al12 precipitates, and the more grain boundaries, the higher volume fraction of the Mg17Al12 phase. The highest volume fraction of α-Mg+Mg17Al12 discontinuous precipitates (VV = 42.39%) was reported after modification with wax-CaF2-carbon powder, while the lowest was in non-modified alloy (VV = 31.55%). This results in the highest ultimate tensile strength of the alloy after wax-CaF2-carbon powder modification (UTS = 269 MPa), whereas the lowest ultimate tensile strength was reported in the case of non-modified alloy (UTS = 252 MPa). A slight (~5%) decrease of ultimate tensile strength was observed and was attributed to the decrease of the volume fraction of α-Mg+Mg 17 Al 12 discontinuous precipitates areas. The strengthening effect resulting from the grain refinement was relatively insignificant in this case. On the other hand, the effect of grain refinement was noticeable after heat treatment (Figure 9b). Grain boundaries act as nucleation sites for discontinuous Mg 17 Al 12 precipitates, and the more grain boundaries, the higher volume fraction of the Mg 17 Al 12 phase. The highest volume fraction of α-Mg+Mg 17 Al 12 discontinuous precipitates (V V = 42.39%) was reported after modification with wax-CaF 2 -carbon powder, while the lowest was in non-modified alloy (V V = 31.55%). This results in the highest ultimate tensile strength of the alloy after wax-CaF 2 -carbon powder modification (UTS = 269 MPa), whereas the lowest ultimate tensile strength was reported in the case of non-modified alloy (UTS = 252 MPa).

Conclusions
In the article the effect of three methods of modification on microstructure and properties of gravity-casted and heat-treated AZ91 alloy were presented. The most important outcomes are the following: (1) Microstructure of AZ91 alloy in as-cast condition consists of α-Mg solid solution with continuous precipitates of Mg 17 Al 12 phase, discontinuous precipitates of α-Mg+Mg 17 Al 12 compound, and the particles of Mg 2 Si and Al 8 Mn 5 phases. (2) Overheating of the alloy caused the decrease of the average area of α-Mg solid solution grain cross-section and the increase of the volume fraction of α-Mg+Mg 17 Al 12 areas. Numerous aluminium oxides and iron diffused from the crucible acted as the nucleation sites. Their presence was effective in grain refinement but had a negative effect on the quality and mechanical properties of the castings. (3) Carbon hexachloride turned out to be highly effective both in grain refinement and degassing of the melt. Al 4 C 3 phase and Al 8 Mn 5 phase nucleating on the surface of Al 4 C 3 acted as nucleation sites. Unfortunately, the application of carbon hexachloride is limited due to the emission of toxic gases, which cause severe environment pollution. (4) Wax-CaF 2 -carbon was the most effective modifier. The mechanism of grain refinement was complex. In the first stage, as in the case of carbon hexachloride, the precipitates of Al 4 C 3 phase were formed, while the second stage comprised the formation of Al 2 Ca phase, which promotes nucleation and increases the supercooling. (5) The type of applied modifier does not influence the mechanical properties of AZ91 alloy in the as-cast condition. A slight decrease of ultimate tensile strength resulted from the decrease of the volume fraction of α-Mg+Mg 17 Al 12 discontinuous precipitates' areas. After age-hardening treatment, the increase of mechanical properties with the grain refinement was observed and was attributed to the nucleation of discontinuous precipitates of Mg 17 Al 12 phase on the grain boundaries.