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The effects of different pouring temperatures on the reaction behaviour of the ceramic shell with the molten Mg-9%Al-1%Zn-0.4%Mn (all are in wt-%) (AZ91) alloy during investment casting were studie...

The tensile and bending behaviours of the as-cast AM50 alloy were studied after adding different amounts of Sb/Cu (x = 0, 0.5, 1, 3 and 5 wt-%). The optimum levels of the Ultimate Tensile Strength ...

In this study, the effects of Ni, Cr additions and T6 heat treatment on the microstructure and tensile properties of Ti-refined Al-12%Zn-2.5%Mg-2.5%Cu alloy were investigated. Microstructural and tensile properties evaluation was conducted by optical microscopy and scanning electron microscopy (SEM) coupled with energy-dispersive spectrometry, x-ray diffractometry and standard tensile testing. The microstructural observations showed that the base alloy consists of lamellar eutectic structure in dendritic regions (α(Al) + η(MgZn2)) and some interdendritic phases (T(Al2Mg3Zn3)/S(Al2CuMg)). Addition of Ni and Cr elements (from 0 to 5 wt.%) introduced two new intermetallic compounds in the microstructure as Al3Ni and Al7Cr, respectively. With the enhancement of Ni and Cr contents from 3 to 5 wt.%, the size and volume fraction of Al3Ni and Al7Cr particles increased from 2.1 μm and 8% (for Ni) and 32 μm and 3.2% (for Cr) to 5 μm and 18% and 45 μm and 12%, respectively. Tensile testing revealed that the highest ultimate tensile strength (UTS) and elongation (El.%) values are achieved by adding 3 wt. % Ni and 0.5 wt.% Cr to the alloy, which altered the quantities from 215 MPa and 5.5% for the base alloy to 295 MPa and 7.1% (for 3 wt.% Ni) and 230 MPa and 5.8% (for 0.5 wt.% Cr). These interesting results were concluded from size and morphology of new intermetallic particles, in which Ni additions had more effective influences on tensile properties of the alloy. In addition, T6 heat treatment had the adequate influence on the increment of the UTS and elongation to fail values by breaking eutectic structure and dissolution of η-phase and homogeneous distribution of new intermetallics. SEM images of several fractured surfaces showed an overall brittle failure mechanism with cracking mainly through the interdendritic region or within the intermetallics. Al3Ni phases caused an improvement in the fracture mode to more ductile, whereas Al7Cr intermetallics turned the mode to more brittle form.

Effects of different Cu concentrations on the microstructure, mechanical, and corrosion properties of the extruded Mg-2%Zn alloy were studied by the use of x-ray diffraction, optical microscopy, scanning electron microscopy, energy-dispersive spectroscopy, tensile testing, polarization, and electrochemical impedance spectroscopy (EIS) measurements. The hot-extruded Mg-2%Zn comprised an equiaxed grain structure with the average size of about 13 μm. By adding 5 wt.% Cu, the average grain size decreased to 4 μm, due to the grain refinement behavior of Cu addition. Microstructural observations confirmed the existence of the α-Mg matrix, Mg(Zn,Cu), and Mg(Zn,Cu)2 intermetallics. The results obtained from mechanical testing revealed that Cu addition along with applying hot extrusion increased the hardness significantly from 80 HBN, for Cu-free alloy, to 108 HBN for the extruded Mg-2%Zn-5%Cu alloy. By the addition of 0.5 wt.% Cu, the values of ultimate tensile strength (UTS), elongation, and ultimate compressive strength (UCS) changed from about 233 MPa, 16.8%, and 294 MPa to 260 MPa, 21.5%, and 335 MPa, respectively, as optimum levels. However, more Cu addition (i.e., 5 wt.%) decreased UTS, elongation, and UCS values to 242 MPa, 15.9%, and 272 MPa, respectively, by introducing a high volume fraction of the second phase. On the other hand, tensile yield strength and compressive yield strength increased gradually at higher Cu additions from 128 to 105 MPa in the case of the Cu-free extruded alloy to 165 and 156 MPa for the alloy containing 5 wt.% Cu, respectively. The polarization and EIS results indicated that the extrusion process eliminates the protective film from the Mg-2%Zn alloy surface. The Mg-2%Zn alloy exhibited the best anti-corrosion property among the studied alloys, as further Cu addition increased the intermetallic volume fractions and enhanced the corrosion rate, due to the galvanic couple effect.

Effect of different amounts of Sr and Ti additions with T6 heat treatment on microstructure and mechanical properties of the AZ91 alloy was investigated. The microstructure of the AZ91 alloy is consisted of the α-Mg phase surrounded by the semicontinuous network of β-Mg17Al12 intermetallics. The average grain size and volume fraction of β phase decreased up to 1 wt.% Sr and 0.1 wt.% Ti additions. Exceeding Ti additions led to increase in both grain size and volume fraction. It was found that the grain refinement mechanism of AZ91 alloys with Sr additions was grain restriction factor. Also, 0.1 wt.% Ti addition could refine the grain structure of the AZ91 alloy by the formation of Al3Ti particles that act as nucleation sites for the α-Mg phase. Furthermore, adding 1 wt.% Sr and 0.1 wt.% Ti enhanced the hardness values for both cast and aged AZ91 alloy. Moreover, ultimate tensile strength and tensile elongation values reached their maximum amounts with the 0.5 wt.% Sr and 0.1 wt.% Ti additions and more Sr and Ti additions deteriorated the mechanical properties of cast and aged AZ91 alloys. The fracture surface characterizations had good relations with the tensile properties. The fracture modes were cleavage and quasi-cleavage in the cast and aged AZ91 alloys. Furthermore, the optimum levels of Sr and Ti additions altered the fracture modes to more quasi-cleavage fractures.

Effects of adding different amounts of SiCp on the microstructure and mechanical properties of the as-cast and as-extruded Mg–7% Zn–1.5% Cu (ZC71) alloys were studied. The as-cast ZC71 alloy consisted of α-Mg phase that encircled with the MgZnCu and Mg(Zn,Cu)2 intermetallics. Hot extrusion has led to a grain-refined structure with distributed intermetallics along the extrusion direction. Adding SiCp decreased the grain size values for the as-extruded composites. The Vickers hardness values increased with SiCp addition for both conditions. The ultimate tensile strength and tensile elongation (El%) values reached the optimum level with 5 wt% SiCp addition. More SiCp additions led to more agglomerations and decrement in strength and elongation. The yield tensile strength also increased with SiC additions. Adding 5 wt% SiCp changed the brittle fracture to the more quasi-cleavage. Hot extrusion altered the fracture mode to more ductile for all composites.

The effects of different Cu concentrations and heat treatment on the microstructure, mechanical and corrosion properties of the as-cast and hot-extruded Mg-2%Zn alloys were studied. The results showed that T6 heat treatment led to the dissolution of intermetallics and breaking the continuous networks of intermetallics. Also, the grain size of the base alloy decreased with further Cu additions after solution and aging heat treatments. Microstructural characterization indicated that Mg(Zn,Cu) and Mg(Zn,Cu)2 intermetallics were formed which their volume fractions increased with exceeding Cu additions. Moreover, the addition of 3 wt.% Cu and T6 heat treatment increased the Brinell hardness of both as-cast and hot-extruded specimens reached to 94 HBN and 116 HBN, respectively. The optimum amounts of the ultimate tensile strength and elongation values obtained from 0.1 wt.% Cu addition for both conditions after T6 treatment which were 253 MPa, 11.5% and 263 MPa, 14.7%, respectively. However, more Cu addition (> 0.5 wt.%) deteriorated the tensile properties of the alloy due to the high volume fraction of intermetallics. The yield tensile strength increased continuously with different Cu additions for the cast and extruded alloys after solution and aging heat treatments. After T6 heat treatment, extruded alloys showed better anti-corrosion properties than those of the cast alloys, while the extruded Mg-2%Zn-0.1%Cu alloy exhibited the best anti-corrosion property. Further Cu addition increased the volume fraction of intermetallics and enhanced the corrosion rate which was due to the galvanic couple effect.

The microstructure, tensile properties and corrosion behavior of the Mg-8 wt.% Mg2Si-x%Ca alloy have been studied by the use of optical microscopy, scanning electron microscopy equipped with energy-dispersive spectroscopy, x-ray diffraction, standard tensile testing, polarization test and electrochemical impedance spectroscopy (EIS) measurements. Microstructural studies indicated that Ca modifies both primary and eutectic Mg2Si phase. It was found that the average size of primary Mg2Si particles is about 60 μm, which is dropped by about 82% in the alloy containing 0.05 wt.% Ca. By the addition of different Ca contents, Ca-rich intermetallics (i.e., CaSi2 and CaMgSi) were formed. The modification mechanism of adding Ca during solidification was found to be due to the strong effect of CaMgSi phase as a heterogonous nucleation site, apart from CaSi2 which was reported before, for Mg2Si intermetallics. Tensile testing results ascertained that Ca addition enhances both ultimate tensile strength (UTS) and elongation values. The optimum amount of Ca was found to be 0.1 wt.%, which improved UTS and elongation values from about 130 MPa and 2% to 165 MPa and 5.5%, whereas more Ca addition (i.e., 3 wt.%) reduced the tensile properties of the alloy to about 105 MPa and 1.8%, which can be due to the formation of CaMgSi intermetallics with deteriorating needle-like morphology. Polarization and EIS tests also showed that the Mg-3%Si-0.5%Ca alloy pronounces as the best anti-corrosion alloy. Nevertheless, further added Ca (up to 3 wt.%) deteriorated the corrosion resistance due to predominance of worse galvanic coupling effect stemmed from the presence of stronger CaMgSi cathode in comparison with Mg2Si. With higher Ca additions, an adverse effect was seen on corrosion resistance of the Mg-3%Si alloy, as a result of forming a weak film on the alloy specimen surface.

The effects of calcium addition at microalloying levels and hot extrusion process on the modification of the microstructure and enhancement of the mechanical properties of the Mg–3Si metal matrix composite (MMC) were studied. Electron backscatter diffraction (EBSD), X-ray diffraction (XRD), cooling curve thermal analysis, simulation based on the finite element method (FEM), thermodynamics predictions, tensile testing, and optical microscopy were used for characterization. The Ca addition modified the primary and eutectic Mg2Si phase, resulted in grain refinement of the matrix, led to better tensile properties in the as-cast condition, and gradually changed the isothermal eutectic reaction to a non-isothermal route. Moreover, hot extrusion led to the extreme grain refinement induced by dynamic recrystallization (DRX) and the break up of the eutectic structure into fine and well-distributed particles. For the Ca-containing composites, the modified primary Mg2Si particles enhanced the particle stimulated nucleation (PSN) mechanism, leading to the formation of a higher volume fraction of DRX grains with more random orientations (crystallographic texture). Accordingly, remarkable improvements in tensile properties (such as tensile toughness) were observed via extrusion of the modified in situ composites.

Effects of different concentrations of Cu on the structure, mechanical and corrosion properties of Mg-2%Zn alloy were studied by the use of x-ray diffraction, optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, standard tensile testing, polarization and electrochemical impedance spectroscopy (EIS) measurements. The average grain size of the alloy decreased from above 1000 μm to about 200 μm with 5 wt.% Cu addition in as-cast condition. Microstructural studies revealed that Mg-2Zn-xCu alloys matrix typically consists of primary α-Mg and MgZnCu and Mg(Zn,Cu)2 intermetallics which are mainly found at the grain boundaries. The results obtained from mechanical testing ascertained that Cu addition increased the hardness values significantly. Although the addition of 0.5 wt.% Cu improved the ultimate tensile strength and elongation values, more Cu addition (i.e., 5 wt.%) weakened the tensile properties of the alloy by introducing semi-continuous network of brittle intermetallic phases. Based on polarization test results, it can be concluded that Cu eliminates a protective film on Mg-2%Zn alloy surface. Among Mg-2%Zn-x%Cu alloys, the one containing 0.1 wt.% Cu exhibited the best anti-corrosion property. However, further Cu addition increased the volume fraction of intermetallics culminating in corrosion rate enhancement due to the galvanic couple effect. EIS and microstructural analysis also confirmed the polarization results.

The synergetic effects of strontium (Sr) and rare earth elements (RE) on the modification of the microstructure and enhancement of the mechanical properties of as-cast Mg–9Al–1Zn alloy were studied. Based on the microstructural analysis, phase identification, and tensile tests, it was revealed that the combined RE and Sr addition results in superior mechanical properties compared to those of solo additions of RE or Sr. This was related to both morphological enhancement of the β-Mg17Al12 intermetallics notably by RE towards the disappearance of the intergranular network and modification of the Al11RE3 particles by Sr. The enhancement of tensile properties was achieved by increasing the RE/Sr ratio in the Sr-RE containing alloys and the best combination of mechanical properties was observed for the Mg–9Al–1Zn-0.9RE-0.1Sr alloy. Owing to the dissolution of the β phase and solid solution strengthening effect of Al, the tensile properties of the Mg–9Al–1Zn-0.9RE-0.1Sr alloy were enhanced by homogenization treatment. Moreover, the hot extrusion process remarkably augmented the tensile properties by combined effects of grain refinement and fracturing and distributing the intermetallic particles.

Effect of 0.05, 0.1, 0.5, 1 and 3 (wt. %) Ca addition and extrusion process on the microstructure and tensile properties of Mg-Mg2Si metal matrix composite has been studied by the use of optical microscopy (OM) and standard tensile testing. The average size of primary Mg2Si particles decreased from 34μm to about 10μm with the addition of 0.05 (wt.%) Ca and extrusion process and also the size of eutectic Mg2Si decreased from 20μm to about 2μm. The morphology of Mg2Si particles altered from octahedron to fine polygonal and more round shape and eutectic phases altered to well distuributed fragmented particles on the microstructure that was enhance the mechanical properties in comparison with as-cast specimens. Tensile test showed that UTS value increases with the addition of Ca. The maximum UTS value was achieved with 0.1 (wt.%) Ca addition.