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The microstructures and mechanical properties of Mg–xBi (x = 2, 5, and 8 wt%) were investigated compared with pure Mg. The as-cast Mg–Bi billets consist of dendritic α-Mg grain matrix, divorced eutectic Mg3Bi2 phase and secondary precipitated Mg3Bi2 phase. After homogenization, secondary precipitated Mg3Bi2 particles were all dissolved into the matrix, while most of the divorced eutectic Mg3Bi2 intermetallic compounds retained. All the as-extruded samples exhibit fully dynamic recrystallization (DRX) and the average grain size decreases and the amount of nano-scale Mg3Bi2 precipitates increases with increasing Bi content. Additionally, Bi alloying has little influence on the texture of the as-extruded pure Mg, with all the samples showing typical basal texture. As the results of grain refinement and precipitation hardening, the strengths of the as-extruded samples increase under both tensile and compressive tests. Besides, the yield asymmetry significantly decreases with Bi content. Moreover, all the as-extruded samples represent similar ductility under compression, while the tensile elongation first increases and then decreases with more Bi added. The as-extruded Mg–5Bi alloy demonstrates good combination of strength and ductility. However, Mg–8Bi alloy displays lower tensile ductility than that of Mg–5Bi alloy due to the presence of abundant undissolved coarse Mg3Bi2 particles, which act as cracking sources during tensile test. Furthermore, the strain hardening rate Θ, strain hardening exponent n, and hardening capacity Hc are greatly reduced by Bi addition, mainly owing to the enhanced synergy effects of grain refinement and numerous Mg3Bi2 precipitates.

This study investigates the effect of extrusion temperature on room temperature tensile properties of as-extruded AZ91 plates regarding microstructure and texture development. Forward extrusion was performed at 300, 350 and 400 °C with an extrusion ratio of 25:1. Yield strength rather increases from 212 to 224 MPa as extrusion temperature rises, despite grain growth from 11.9 to 41.2 µm. To understand the underlying mechanisms of this unusual hardening, the effects of microstructural features are discussed with emphasis on grain size, precipitates, stored strain energy and texture. Microscopic analyses and simulation results reveal that grain refinement, continuous precipitates and stored strain have no decisive effect on yield strength. Development of stronger basal texture leads to an increase in yield strength. Texture strengthening makes basal slip more difficult to be activated when loading along the extrusion direction. Based on this, stronger basal texture plays a more dominant role in increasing yield strength.

SiC is well-known as an effective carbon inoculation material for grain refinement of Mg-Al-based alloys. In this study, grain refinement ability of stir-casted Al-SiC composite for Mg-Al-based all...

The effect of Y addition on Fe-removal effect in magnesium melt was investigated using pure magnesium with a high level of Fe impurity. It was confirmed that Fe concentration was significantly reduced even at relatively high melting and holding temperatures and even with very small amounts of Y. It was also found that Fe contents of magnesium alloys are proportional to the corrosion rates. Hence, magnesium alloys containing Y, which has the strongest Fe-removal ability, have the lowest corrosion rates. The corrosion rate of Mg-3Al-0.2Y is only 1.4 mm/y, corresponding to 0.6% of that of the Mg-3Al alloy (214.9 mm/y).

Mg and its alloys are the lightest structural metals available and are extremely attractive for applications as lightweight components, particularly in the automobile, electronic, and aerospace industries. The global market for wrought Mg alloys has steadily expanded over the past decade. And numerous studies have been carried out to meet this increasing demand of high-performance Mg alloys. However, Mg extrusion alloys have had a very limited usage so far. To overcome existing industrial challenges, one desirable approach is the development of low-cost rare earth (RE) free Mg extrusion alloys with superior mechanical properties. This review will introduce the recent research highlights in the extrusion of Mg alloys, specifically focusing on low-cost RE-free Mg alloy. The results from both the literature and our previous study are summarized and critically reviewed. Several aspects of RE-free Mg extrusion alloys are described in detail: (1) novel alloying designs including Mg–Al-, Mg–Zn-, Mg–Ca-, Mg–Sn-, and Mg–Bi-based alloys, (2) advanced extrusion techniques, and (3) extrusion-related severe plastic deformation (SPD) processing. Accordingly, considering the large gap in mechanical properties between the current RE-free Mg alloys and high-performance aluminum alloys, new alloy design, processing route control, and recommendations for future research on RE-free Mg extrusion alloys are also proposed. We hope this review will not only offer insightful information regarding the extrusion of RE-free Mg alloys but also inspire the development of new Mg extrusion technologies.

The precipitation behavior of Mg-8Al-2Sn-1Zn (ATZ821) alloy was investigated. Sn addition to the AZ81 alloy greatly influenced the initial precipitation behavior in the following two aspects: (i) the segregation of Sn solutes at the interface of Mg17Al12 via Sn partitioning induced during the growth of Mg17Al12 and (ii) high interfacial energy at the edge-tip of growing Mg17Al12. According to the hardness profile and detailed microstructural analyses of ATZ821 cast samples artificially aged at 453 K for different times, it was confirmed that the initial precipitation of Mg17Al12 was delayed due to Sn partitioning at the interface, and all Mg2Sn without exception was nucleated at the edge-tip of pre-existing Mg17Al12 precipitates on the basal planes. In addition, a selected area diffraction pattern analysis confirmed that α-Mg, Mg17Al12, and Mg2Sn had the following orientation relationship: ( 0001 ) α // ( 110 ) M g 17 A l 12 , [ 1 2 ¯ 10 ] α // [ 1 ¯ 11 ] M g 17 A l 12 and ( 0001 ) α // ( 022 ) M g 2 S n , [ 1 2 ¯ 10 ] α // [ 1 1 ¯ 1 ] M g 2 S n .

This study investigated the corrosion protection by electrodeposited copper layer on AZ31 Mg alloy with and without additional silver layer. The single copper layer electrodeposited on AZ31 from copper pyrophosphate bath exhibited nodular structure and crevices between the nodules which resulted in the rapid initiation of pitting corrosion. Employing an additional electrodeposited silver layer onto the copper deposited AZ31 alloy decreased the number of imperfections and changed structure of electrodeposited copper from nodular to fine particle type resulting in improved corrosion resistance. This study further revealed that triple-layer coating of Cu/Ag/Cu exhibited the most efficient corrosion protection of AZ31 Mg alloy.

A new Mg alloy with the composition of Mg-8Al-0.3Zn-0.1Mn-0.3Ca-0.2Y was successfully fabricated by a low-temperature, low-speed indirect extrusion process. The extruded alloy showed excellent mechanical properties, i.e., a tensile yield strength of 379.3 MPa, ultimate tensile strength of 421.7 MPa, elongation of 11.3%, and yield asymmetry of 0.95, as well as a very high chip ignition temperature of 820 °C. The outstanding mechanical properties are attributed to the microstructural characteristics of the extruded alloy, that is, (i) grain boundary strengthening by fine recrystallized grains, (ii) precipitation hardening by abundant fine Mg17Al12 precipitates, and (iii) strain hardening by the deformed non-recrystallized region with a strong basal texture that is unfavorable for basal slip during tension. Electron backscatter diffraction results showed that no twinning occurred in the recrystallized grains, which had a very small size of approximately 800 nm, whereas {10–12} twins were formed in the non-recrystallized grains even at low compressive stress, which consequently increased the yield asymmetry.

This study demonstrates that the precipitation behavior of β-Mg 17 Al 12 phase during aging and the resultant variation in hardness and mechanical properties of cast Mg-Al-Zn alloy are strongly dependent on initial grain size. Grain size reduction accelerates discontinuous precipitation at the early stage of aging treatment by increasing the area fraction of grain boundaries that can act as nucleation sites for discontinuous precipitates (DP), but it does not influence DP growth rate. Grain refinement also prematurely terminates continuous precipitation because the formation of a large number of DP reduces the amount of Al dissolved in the matrix, which is required for the formation of continuous precipitates (CP). This promotion of DP formation and early termination of CP formation significantly decrease the peak-aging time to one-third. The enhanced precipitation behavior also leads to an additional hardness improvement in the aged alloy, along with an increase in hardness owing to grain boundary strengthening by grain refinement. The amount of increase in hardness changes with aging time, which is determined by the variation of three variables with aging time: DP fraction difference between refined and nonrefined alloys, hardness difference between DP and matrix, and matrix hardness difference between the two alloys. Grain refinement improves both tensile strength and ductility of the homogenized alloy owing to grain boundary strengthening and suppression of twinning activation, respectively. However, the loss of ductility after peak-aging treatment is greater in the refined alloy because of the larger amount of DP acting as a crack source in this alloy.

This study demonstrates that the application of artificial water cooling during extrusion effectively increases the extrudability of the AZ91 alloy and significantly improves the mechanical properties of the extruded AZ91 alloy. The artificial cooling dramatically reduces the actual temperature of the deformation zone, which results in an increase in the maximum exit speed at which the alloy is extrudable without the occurrence of hot cracking from 4.5 m/min to 7.5 m/min. It also promotes dynamic recrystallization and precipitation behaviors during extrusion, which leads to a reduction in grain size and an increase in the amount of fine Mg 17 Al 12 precipitates. As a result, for the AZ91 alloy extruded at an exit speed of 1.5 m/min, the tensile and compressive yield strengths improve significantly by 51 MPa and 114 MPa, respectively, and its tension–compression yield asymmetry reduces from 0.73 to 1.02 owing to the refinement of the grain size by artificial cooling. In addition, the AZ91 alloy extruded at an exit speed of 7.5 m/min with artificial cooling exhibits a finer grain structure than and superior mechanical properties to the AZ91 alloy extruded at a slower exit speed of 4.5 m/min without artificial cooling. This result indicates that the application of artificial cooling can simultaneously improve the maximum extrusion speed and the tensile and compressive properties of Mg alloys.

AZ91 magnesium alloys containing 0.3% Ca and 0.1% Y were cast, and their oxidation behavior was investigated between 425 and 600 °C in atmospheric air to examine roles of Ca and Y during oxidation. During casting, Ca formed Al2Ca particles intergranularly, and reduced the amount of Al12Mg17 particles, while most of yttrium existed as Al2Y particles inter- and intra-granularly in the alloy. The AZ91 alloy oxidized fast above 425 °C, leading to complete ignition. By contrast, AZ91-0.3Ca-0.1Y alloy oxidized very slowly up to 550 °C. Calcium, which is more active than Mg, preferentially oxidized to CaO at the surface of the MgO-rich oxide to suppress the oxidation, evaporation and diffusion of Mg during the initial oxidation stage. Such suppression was due to the quite low vapor pressure and high stoichiometry of CaO. Calcium also suppressed the formation of less oxidation-resistant Al12Mg17 through forming oxidation-resistant Al2Ca in the alloy from the initial oxidation stage.

We report that alloying with Y remarkably enhances the corrosion resistance of extruded Mg–Al–Ca-based alloy. The corrosion rates, measured by immersion tests in 0.6 M NaCl solution at 25 °C, were 0.31 and 1.84 mm y −1 for the Mg–6Al–1Zn–0.5Ca–0.3Mn (wt.%) alloys with and without 0.25 wt.% Y, respectively. The improved corrosion resistance is mainly attributed to a change in the type of Al-containing intermetallic particles toward reducing their electrochemical nobility, resulting in reduced H 2 evolution rate during corrosion.

In this study, the microstructural evolution and mechanical properties of a newly developed rare earth free Mg-8Bi-1Al-1Zn (BAZ811, in wt%) alloy were investigated and compared with those of a commercial AZ31 alloy. The as-extruded BAZ811 alloy with much finer grain size shows more homogeneous dynamical recrystallized (DRXed) microstructure and weaker basal texture than those of AZ31 alloy. In addition, compared with bimodal structure AZ31 alloy containing only relatively coarse and sparse Al8Mn5 phases, the coexistence of strip-like fragmented Mg3Bi2 precipitate and nano-size Mg3Bi2 particles in the microstructure was observed in BAZ811 alloy. Moreover, the BAZ811 alloy exhibits a tensile yield stress of 291 MPa, an ultimate tensile strength of 331 MPa, an elongation to failure of 14.6% as well as a reduction in yield asymmetry, which is mainly attributed to the combined effects of grain refinement and micro-scale broken Mg3Bi2 particles together with nano-scale spherical Mg3Bi2 precipitates. The strain hardening behavior of both BAZ811 and AZ31 alloys were also discussed in terms of microstructure variation.

The microstructure and mechanical properties of extruded Mg-xGd binary alloys (x = 1, 5, 10, and 15 wt%) were investigated, and it was found that the grain size, precipitate, texture, and tensile properties of the extruded alloys vary significantly with the Gd content. The size of the recrystallized grains gradually increases with the addition of up to 10 wt% Gd, but then rapidly decreases in the Mg-15Gd alloy due to the formation of numerous fine Mg 5 Gd precipitates and their grain boundary pinning effect. With an increase in Gd content, the texture peak orientation is dramatically changed from the to direction, resulting in an extraordinary rare-earth texture with basal poles parallel to the extrusion direction in the Mg-15Gd alloy. The tensile strength gradually increases with Gd addition up to 10 wt% through solid solution strengthening, and then significantly rises with 15 wt% addition due to the refined grains and abundant precipitates.

The effect of the initial billet size on the microstructural evolution and tensile properties of extruded AZ61 and AZ91 Mg alloys was investigated using homogenized billets with diameters of 50 and 80 mm. All the extruded alloys exhibit a bimodal structure consisting of fine dynamically recrystallized (DRXed) grains with numerous precipitates and relatively coarse DRXed grains with few precipitates. Although the size and hardness of the fine DRXed grains do not vary with the billet size, using a smaller billet results in a significant reduction in the size and area fraction of coarse DRXed grains because of an increase in the quantity of precipitates. This increase in both the fraction of fine grains and the number of Mg 17 Al 12 precipitates improves the tensile strength of the extruded alloys. This size effect can be attributed mainly to the increase in the strain rate with decreasing billet diameter, which promotes dynamic recrystallization and precipitation during extrusion. The effect of billet size is more pronounced in the AZ91 alloy, which has a high concentration of alloy elements, than in the AZ61 alloy.

Non-flammability and high production rate are prerequisite for extending application of Mg extrusion products. In this respect, the present study focuses on microstructure development and mechanical behavior of two novel Mg alloys as compared with a commercial Mg alloy. As a reference, Mg-8Al-0.4Zn (AZ80 bal. wt.%) is extrudable at low speed of 0.5-2.5 m/min. This is due to hot shortness during extrusion, which results from low melting point of secondary phases like Mg 17 Al 12 . Furthermore, Mg alloys are reactive in molten state and rapidly ignited as contacting with oxygen in air. Hence, a non-flammable Mg alloy was developed by a trace addition of 0.3%Ca and 0.2%Y, which form protective oxide layer on Mg grains (AZ80+0.3%Ca+0.2%Y denoted as SEN8). In other way, Mg-5Sn-4Al-1Zn (TAZ541) was developed for high-speed extrusion. Hereby Mg 2 Sn precipitate makes Mg matrix thermally stable due to its high melting point. Three Mg alloys were extruded in the form of 50 mm x 4 mm plate with extrusion ratio of 25:1 at 350 °C with 1.1 m/min. As-extruded microstructures exhibited fully recrystallized structure with equiaxed grains as well as partially elongated grains along the extrusion direction. There was no big difference in grain size with 17.7~19.6 µm. SEN8 showed the weakest texture owing to the effect of Ca and Y. This made yield strength of SEN8 lower and uniform strain higher than AZ80. Mg 2 Sn particles led to higher work hardening of TAZ541 than AZ80 and SEN8. Moreover, SEN8 and TAZ541 had higher hardening capacity due to lower yield strength and higher work hardening rate than AZ80. That is, their relatively weak textures resulted in superior hardening ability. This presented that texture has a more dominant effect on mechanical properties rather than grain size in extruded Mg alloys. Regarding the effects of Ca, Y and Sn, it enables development of next generation Mg alloys with high strength and non-flammability for high-speed extrusion.

Microstructure and corrosion properties of extruded AZ61 Mg alloys with varying Ca contents were investigated. Microstructure of the extruded AZ61 alloy consisted of α-Mg grains and Al 8 Mn 5 particles. Alloying with 0.1–0.5 wt% Ca to the AZ61 alloy resulted in the additional presence of (Mg,Al) 2 Ca particles in the extruded microstructure. In 0.6 M NaCl solution at 25 °C, the extruded AZ61 alloys with 0.1–0.5 wt% Ca showed corrosion rates over 2 times faster than that of the AZ61 alloy without Ca addition. Such deteriorated corrosion resistance due to Ca addition is discussed in terms of passivation and microgalvanic corrosion.

In this work, effect of zincate treatment of AZ91 Mg alloy on the following electrodeposition of copper was examined in a non-cyanide bath containing pyrophosphate ions in view of surface morphology and adhesion of the electrodeposited copper layer. Without zincate treatment, the electrodeposited copper layer showed very porous structure and poor adhesion. On the other hand, the copper layer electrodeposited on the zincate-treated surface showed dense structure and good adhesion. The dissolution rate of AZ91 Mg alloy after the zincate treatment appeared to decrease about 40 times in the copper pyrophosphate bath, as compared to that of the surface without zincate treatment. The porous morphology and poor adhesion of a copper layer on the AZ91 Mg alloy surface without zincate treatment were attributed to small number of nucleation sites of copper because of rapid dissolution of the magnesium substrate in the pyrophosphate bath. Based on the experimental results, it is concluded that the zincate treatment to form a conducting and protecting layer on the AZ91 Mg alloy surface is essential for successful electrodeposition of a copper layer on AZ91 Mg alloy with good adhesion and dense structure in the copper pyrophosphate bath.

Investigation of the microstructural characteristics and compressive properties of extruded AZ31 alloy plates with width–thickness ratios (w/t ratios) of 1, 2, and 3 revealed that a higher ratio promotes dynamic recrystallization (DRX) behavior during hot extrusion. This leads to a decrease in both the fraction of non-DRXed grains and the average grain size. Extruded alloy with a w/t ratio of 1 exhibits a ring basal fiber texture that is homogeneously distributed over its entire cross-sectional area, resulting in identical stress–strain behavior under compression along the normal direction (ND) and transverse direction (TD). In contrast, significant anisotropy appears between the compressive behavior along the ND and TD in alloys with w/t ratios of 2 and 3, which is attributed to a difference in the active dominant deformation mechanism (i.e., slip for the ND and twinning for the TD) caused by the strong ND-oriented basal texture. Consequently, the compressive yield strength is up to ~2.25 times higher along the ND than along the TD.

The effects of microstructural factors, including the fraction of second phase particles and solute content in the matrix, on the corrosion behavior of as-extruded Mg-Sn-Zn alloys were evaluated by immersion and salt spray testing. After immersion testing, the average corrosion rate increased with the increase in Sn content and decrease in Sn:Zn ratio. On the other hand, after salt spray testing, the average corrosion rate increased with the increase in Sn content, but it was minimized at the Sn:Zn ratio of 5:2. Hydrogen evolution appeared to be the dominant controlling factor of corrosion during immersion, while the partial protectiveness of the film also contributed to the corrosion during salt spray testing.

Copper electrodeposition on AZ91 Mg alloy was studied in views of preferential deposition on α- or β-phases and how to achieve uniform deposition over the entire surface on α- and β-phases in a cyanide solution. The inhomogeneous microstructure of AZ91 Mg alloy, particularly α- and β-phases, was found to result in non-uniform deposition of zincate layer, preferential deposition of zincate on β-phases, which leads to non-uniform growth of copper layer during the following electrodeposition process. The preferential depositions of zincate can be attributed to higher cathodic polarizations on the β-phases. Pin-hole defects in the copper electrodeposit were observed at the center of large size β-phase particles which is ascribed to gas bubbles formed at the β-phases. The activation of AZ91 Mg alloy in hydrofluoric acid solution was used to obtain uniform growth of zincate layer on both the α- and β-phases. By choosing an optimum activation time, a uniform zincate layer was obtained on the AZ91 Mg alloy surface and thereby uniform growth of copper was obtained in a cyanide copper electroplating solution.

This investigation into the effect of indirectly extruding Mg–7Sn–1Al–1Zn (TAZ711) alloy at high exit speeds on the microstructure and tensile properties found no evidence of surface cracking, not even at the maximum speed tested of 27 m/min. This high-speed extrudability is attributed to the relatively high incipient melting temperature of the alloy (535 °C), which results from the formation of a thermally stable Mg 2 Sn phase. All extruded samples exhibited a completely recrystallized (DRXed) structure consisting of coarse DRXed grains with few particles in combination with relatively fine DRXed grains containing numerous fine precipitates. With an increase in extrusion speed, the total amount of Mg 2 Sn precipitates decreased and the size and quantity of coarse DRXed grains increased due to a rise in temperature during extrusion. This has the effect of reducing the strength of the extruded alloy through a reduction in precipitation and grain-boundary strengthening, yet the elongation of the extruded alloy remains essentially the same due to a loss of ductility caused by the increase in DRXed grain size.

The effect of Ce addition on the microstructural characteristics and tensile properties of an extruded Mg-7Sn-1Al-1Zn (TAZ711) alloy was investigated, through which it was found that a Ce 3 Sn 5 phase forms along the grain boundaries and within grains; the volume fraction of this phase also increased with Ce content. All of the extruded alloys exhibited a partially recrystallized structure, but the area fraction of dynamically recrystallized (DRXed) grains increased with Ce addition as a result of DRX behavior being enhanced by particle-stimulated nucleation at the Ce 3 Sn 5 particles. With an increase in Ce content, however, the number of Mg 2 Sn precipitates decreased due to a decrease in the Sn content within the matrix, which in turn increased the size of the DRXed grains through a weakening of the boundary-pinning effect. The tensile strength gradually decreased with increasing Ce content, which was mainly attributed to a decrease in the number of precipitates and an increased DRXed grain size. Furthermore, the elongation of the alloy was increased by a decrease in the fraction of coarse unDRXed grains in which micro-cracking can be easily initiated.

The microstructure and mechanical properties of as-extruded Mg–1.3Bi–xCa (x = 0, 0.4, 0.9, and 1.3 wt%) were investigated. All samples exhibit full dynamic recrystallization; the mean grain size first increases slightly and then decreases significantly. Additionally, both Mg2Bi2Ca and Mg2Ca phases are formed with increasing Ca content. In particular, the dynamic precipitation of nanoscale Mg2Ca is enhanced remarkably. The typical fiber basal texture in Ca-free binary alloy transformed into a rare-earth texture with the components of [ 1 ¯ 2 1 ¯ l] and [01 1 ¯ l]//extrusion direction in Ca-bearing ternary alloys. In addition, the tensile elongation increases significantly with a trace amount of Ca addition (0.4 wt%), but an overdose of Ca addition (1.3 wt%) results in the formation of a large number of microscale Mg2Bi2Ca phases. The Mg-1.3Bi-0.9Ca alloy demonstrates the optimal balance of ductility and strength with an excellent elongation of 40% and yield strength of 158 MPa, owing to texture modification and grain refinement. This study may provide insights into the development of high-ductility, low-cost Mg–Bi-based alloys using microstructure tuning and texture modification.

In the present study, the effect of various alloying elements on controlling of iron content in magnesium alloys was investigated. A number of alloying elements including manganese, calcium, and yttrium were added separately and simultaneously to the melt then the iron content was determined. The effect of elements on iron removing was evaluated and compared. The results were interesting since it has changed the idea of considering manganese as the best to remove iron from the melt. Other elements would be a better candidate to improve the corrosion resistance of magnesium alloys, especially yttrium due to its double effect on iron removing and forming of a protective film to protect the matrix. This study would contribute effectively to a design of high-performance magnesium alloys in the future.

The corrosion behaviours of commercial AZ91D alloy and Ca–Y-containing AZ91 alloy, fabricated by casting, were evaluated, and their microstructural changes and surface properties were analysed. The Ca–Y-containing AZ91 alloy showed superior corrosion resistance compared with the AZ91D alloy mainly because of the decrease in the Volta potential difference between the matrix and the secondary phases formed in the Ca–Y-containing alloy, such that micro-galvanic corrosion was significantly suppressed. Furthermore, passivity and stability of the surface film improved with the addition of Y to the AZ91 alloy. Thus, the overall corrosion resistance was significantly improved in the Ca–Y-containing AZ91 alloy.

In order to develop low cost Mg alloy with high ductility, a new micro-alloying rare earth free Mg-1.32Bi-0.72Ca (wt.%, BX11) alloy was successfully fabricated by single step extrusion and its microstructure, texture and tensile properties were investigated and discussed in this study. The as-extruded BX11 alloy demonstrates a weakened texture with parallel to the extrusion direction (ED) and a fine dynamic recrystallized microstructure with both micro-scale Mg2Bi2Ca and Mg3Bi2 particles and nano-scale Mg2Ca and Mg3Bi2 precipitates, exhibiting an excellent tensile elongation of 43% at room temperature. This remarkably high ductility is mainly attributed to the refined microstructure as well as weakened ∥ED texture.

Corrosion behavior of extruded Mg and Mg5Sn (in wt%) alloy was investigated, and a passivated surface of the Mg5Sn was examined. Immersion test in a 0.6 M NaCl solution revealed that corrosion of the Mg was significantly faster than the Mg5Sn alloy, and vigorous H2 evolution was accompanied with the severe corrosion of Mg. Polarization tests confirmed that cathodic reaction was inhibited in the Mg5Sn alloy. It was considered that the decreased H2 evolution of the Mg5Sn alloy was attributed to the formation of Sn-accumulated layer between the passive film and matrix, which was examined by an XPS analysis.

The application of artificial cooling to the high speed extrusion (13.5 m/min) of Mg–7Sn–1Al–1Zn alloy has been found to dramatically reduce the size of recrystallized (DRXed) grains, while also increasing the number of fine Mg 2 Sn precipitates. This is attributed to a decrease in the temperature of the deformation zone, and has the effect of improving the strength of the extruded alloy. The increase in yield strength (YS) and ultimate tensile strength (UTS) is enhanced by increasing the feed rate of the cooling water due mainly to a decrease in the DRXed grain size. Significantly, this increase in strength does not come at the expense of a loss of elongation (EL), as the formation of twins that act as a crack initiation site is suppressed during tensile deformation by the grain refinement. As a result, high-strength TAZ711 Mg alloy (210 MPa YS, 303 MPa UTS, and 10.2% EL) can be obtained even with high-speed extrusion by using artificial cooling.

Hot deformation behavior of a homogenized Mg–8Sn–1Al–1Zn (wt.%) (TAZ811) alloy was investigated at the temperature range of 523–673 K and the strain rate of 0.33–10 s−1 to explore its potential as a wrought alloy. The kinetic analysis indicated that dislocation climb mechanism was likely to be an operative deformation mechanism for TAZ811 alloy. To obtain the optimum hot working parameters of the alloy, processing map consisting of a power dissipation map and an instability map was constructed. The practical processing window could be determined as T = 623–673 K and ε˙ = 0.33–3.3 s−1 with a peak efficiency of approximately 20% occurring at 673 K and 0.33 s−1. The flow instability region occurs at around the temperature of 523–598 K and stain rate of 0.33–0.66 s−1. In addition, dynamic recrystallization (DRX) mechanisms were analyzed with respect to its microstructure, which was consistent with the predicted results from the processing map. Keywords: Mg alloys, Kinetic analysis, Processing map, Recrystallization

In this study, the effects of solid tin dissolved in the α-magnesium matrix of magnesium-tin alloys on the characteristics of surface films of the alloys were investigated using immersion and electrochemical tests, which were performed in a sodium chloride solution. The macroscopic in-situ and anodic potentiostatic tests showed that the breakdown resistance of the surface films could be arranged in the following order: pure magnesium

This paper seeks to find an appropriate alloying method for improving the mechanical properties of wrought Mg alloys that have a bimodal structure of fine and coarse recrystallized grains created through insufficient precipitation. The addition of an alloying element with a high solid solubility in the a-Mg matrix of the base alloy is found to result in a fine, homogeneous grain structure of wrought material through a restriction of grain growth by newly formed fine precipitates. This consequently leads to a significant improvement in strength via enhanced precipitation/grain-boundary strengthening, without any major loss of ductility. However, excessive addition above the maximum solid solubility greatly deteriorates the material's ductility due to the creation of large, undissolved particles. The validity of this approach is demonstrated by varying the addition of Sn to AM80 alloy.

The effect of initial grain size on the microstructural evolution and tensile properties of an extruded Mg-9Al-0.6Zn alloy was investigated using homogenized billets with grain sizes of 411 and 87 μm. It is found that although dynamically recrystallized (DRXed) grains remain the same size regardless of the initial grain size, a finer-grained billet results in a significant reduction of the size and area fraction of un-DRXed grains through an increase in grain boundaries capable of acting as nucleation sites for DRX during hot extrusion. This increase in the fraction of DRXed grains, combined with more precipitates, improves the tensile yield strength of the extruded alloy. The elongation is also significantly improved, as the reduction in unDRXed grains suppresses the formation of twins that cause micro-cracks. This increased ductility subsequently results in an increase in ultimate tensile strength through continuous strain hardening.

Steering wheel and headlamp housing scrap are chosen as targets for the development of a refining technology for highly contaminated magnesium melt, because they contain abundant foreign materials, which have high potential to form non-metallic inclusions and to deteriorate the corrosion resistance of recycled alloys. A sequential melt treatment technology has been investigated for refining old scrap. The headlamp housing scrap composed of AZ91D magnesium alloy covered with phosphating and Al-Si deposition layers was successfully refined by repeating gas bubbling without flux addition, whereas prefiltering and fluxing were necessary to refine the steering wheel scrap composed of AM50A magnesium alloy containing 1% polyurethane. However, the corrosion rates of the recovered ingots from the headlamp housing were relatively higher than those from the steering wheel, likely due to the greater concentration of iron with the increased amount of flux.

The effect of homogenization temperature (HT) on the microstructure and tensile properties of extruded Mg–7Sn–1Al–1Zn (TAZ711) alloy was investigated by conducting homogenization heat-treatment at 400, 450, and 500 °C for 24 h, followed by indirect extrusion. The results obtained through this revealed that second phases are nearly fully dissolved by homogenization at 500 °C, but undissolved Mg 2 Sn particles remain after homogenization at 450 and 400 °C. Furthermore, the increase in large undissolved particles with decreasing HT helped to promote dynamic recrystallization (DRX) during extrusion by particle stimulated nucleation (PSN), thus resulting in an increase in the area fraction of DRXed grains in the extruded alloy. A decrease in the amount of Sn supersaturated in the matrix, however, suppressed the formation of fine Mg 2 Sn precipitates during extrusion, which in turn led to an increase in the size of DRXed grains due to a reduction in grain boundary pinning by precipitates. Ultimately, this meant that although the yield and ultimate tensile strengths gradually improved with increasing HT due mainly to a decrease in the DRXed grain size, an increase in texture intensity, and an increased number of fine Mg 2 Sn precipitates, the elongation deteriorated due to crack initiation at twins formed within coarse non-DRXed grains.

Microstructure and corrosion properties of extruded Mg–8Sn–1Zn alloys with varying Al contents were investigated. Microstructures of the extruded alloys mainly consisted of α-Mg grains and Mg2Sn particles in common. Addition of Al to the ternary alloy resulted in the formation of different types of Al–Fe-based intermetallic particles depending on Al contents. In 0.6 M NaCl solution, the extruded alloys with 1–3 wt% Al showed over 30 times higher corrosion rates than those containing less than 0.3 wt% Al; this discrepancy was attributed to the presence of Al5Fe2 particles acting as strong cathodic sites for H2 evolution during corrosion.

The effect of aging prior to extrusion (APE) on the microstructure and mechanical properties of Mg–7Sn–1Al–1Zn (wt.%) alloy was investigated under various extrusion conditions. The extruded alloy exhibited a partially or fully recrystallized (DRXed) structure with fine Mg2Sn precipitates, with the fraction and size of the DRXed grains decreasing with a reduction in extrusion temperature and ram speed. The numerous precipitates formed by APE also reduces the size of the DRXed grains and increases the overall amount of precipitate in the extruded alloy, thereby improving the yield and tensile strength regardless of the extrusion conditions. The ductility of the alloy was also improved by APE when extruded at relatively high temperature and speed, which is attributed to the suppressed formation of coarse banded Mg2Sn; however, a deterioration in ductility was observed at low ram speed and/or low extrusion temperature due to the reduction in the fraction of DRXed grains and increased amount of Mg2Sn precipitates. Furthermore, the extruded alloy with APE showed superior strength and ductility to that aged after extrusion, indicating that APE is the more effective means of improving the mechanical properties of extruded alloy.

An Mg–7Sn–1Al–1Zn alloy known to have excellent extrudability and superior strength was subjected to artificial cooling during indirect extrusion by directly spraying water onto the extruded rod at the die exit. The results obtained revealed that this artificial cooling dramatically reduces the temperature of the deformation zone during extrusion, thereby creating a finer grain size, an intensified texture and a greater amount of precipitates when compared to extrusion without artificial cooling. The yield and tensile strength of the extruded alloy is also significantly improved, which is attributed to the effects of grain refinement in combination with an enhanced texture and precipitate hardening.

The effect of adding Sn to an extruded Mg–8Al–2Zn (AZ82 alloy) was investigated by analyzing its microstructural characteristics before and after extrusion, and by conducting tensile and compressive tests with 2, 4, and 6 wt% Sn addition. This revealed that although second phases formed during solidification are nearly fully dissolved by homogenization treatment in AZ82 alloy with 2 wt% Sn, numerous Mg2Sn particles remain in AZ82 alloys with 4 and 6 wt% Sn due to this concentration being over the solubility limit. All of the extruded alloys were found to have a fully recrystallized structure, yet the addition of 6 wt% Sn created a considerable quantity of large, banded Mg2Sn particles oriented along the extrusion direction. The tensile and compressive yield strength gradually increased with Sn content mainly due to a decrease in the size of recrystallized grains and an increased amount of fine Mg2Sn precipitates, though this came at the expense of a decrease in elongation. It was also found that the ultimate tensile strength improves with Sn addition of up to 4 wt%, but deteriorates beyond that point due to premature fracture caused by crack initiation at large particles.