Materials Science and Technology

The phase field method has recently emerged as a powerful and versatile tool for mesoscale simulation of microstructure evolution in Mg and Al alloys. It permits the study of the evolution of arbitrary and complex microstructures without any presumption. This article provides a review of applications of the phase-field method in Mg and Al alloys. It covers the evolution of dendrites, the equilibrium shape of some key strengthening precipitates, the effect of pre-existing precipitates and dislocations on the distribution of precipitates, and strengthening effects caused by plate-shaped precipitates that are often encountered in Mg and Al alloys. To further improve the accuracy of the phase field simulation results and to further expand the phase field method to predict mechanical properties, the phase field method needs to be integrated with other methods to establish a multi-scale approach. This integration and its application on Mg and Al alloys are also reviewed.

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A Mg-Al alloy was treated with gaseous carbon dioxide (CO₂) and a refining process, and the grain refinement at each stage was investigated. The results indicated that CO₂ and the subsequent refining process affect the grain refinement in the Mg-Al alloy. Moreover, the average grain size decreased from 254 to 89 µm, and the microhardness increased from 59.7 to 66.0 HV upon the addition of 9 litres of CO₂. Increasing the time of the subsequent refining process to 30 min further decreased the grain size and increased the microhardness. Based on scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffraction analyses, the observed Al₄C₃ particles, which are attributed to the inoculation of CO₂, are responsible for the grain refinement.

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This study investigated the precipitation priority in Al–Mg alloys modified by the addition of 0.8 wt-% Cu or Zn, revealing that both these alloys exhibit increased hardness and decreased corrosion resistance. Scanning electron microscopy, transmission electron microscopy, and high-angle annular dark field imaging show that the precipitation priority changed the main second-phase particles of Cu- and Zn-containing alloys. Additionally, thermodynamic data of these phases were calculated using a semi-empirical model, showing good agreement with the experimental results.

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The effects of Al additions (≤4 wt-%) on the microstructure and mechanical properties of Mg–4Y alloys were investigated. The microstructure of the as-cast Mg–4Y–1Al alloy consisted of an α-Mg matrix, together with Mg₂₄Y₅ and Al₂Y phases. Al₂Y formed preferentially when Al was added, replacing the Mg₂₄Y₅ phase by consuming Y solute. The eutectic reaction occurred, forming α-Mg and Mg₂₄Y₅ phases for Al contents below 2 wt-%; however, the pseudo-binary eutectic reaction occurred, forming α-Mg and Al₂Y phases, for Al contents higher than 2 wt-%. The addition of Al improved mechanical properties. The Mg–4Y–4Al alloy exhibited the optimal ultimate tensile strength and yield strength, with values of 208 and 81 MPa, respectively.

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This study examined the effect of steady magnetic field on the microstructure and mechanical properties of Mg–Zn–Y alloy reinforced by icosahedral quasicrystal phase. Results show that the microstructure of Mg–Zn–Y alloy was further refined as the magnetic field intensity increased. However, the primary α-Mg gradually became coarse over a certain value. No significant effect on the refinement of icosahedral quasicrystal phase was observed. The plane of the alloy treated by steady magnetic field was substituted by the (0002) plane as the primary plane. The plane was intensified gradually as the intensity of the magnetic field increased. The mechanical properties of Mg–Zn–Y alloy also improved with steady magnetic field treatment.

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In this study, the effects of pouring temperature (solid fraction of primary particles) on the structure and liquid segregation behaviour of a cup-shaped semi-solid rheocast processed casting made from the A356 aluminium alloy were investigated. Results showed that the solid fraction of primary particles at the bottom of the castings was greater than in other parts of the castings, and the highest liquid segregation occurred in the mid-height (wall) of the castings. Liquid segregation was increased with an increase in the initial solid fraction of primary particles. The critical solid fraction at pouring for minimum liquid segregation was found to be approximately 25%.

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Double oxide film defects have been associated with reduced mechanical properties and increased variability in properties in cast Al alloys. This paper explores the effects of the addition of around 0.4–0.5 wt-% of the transition metals Mo or W to an Al–7Si–0.3Mg alloy (2L99). The variability of tensile properties was significantly reduced, resulting in an approximate doubling of the values of the Weibull modulus of the ultimate tensile strength, and an increase of 10–20% in the Weibull modulus of the %Elongation. Scanning electron microscopy examination of the fracture surfaces revealed the presence of oxide films, as expected, but also found AlN. The mechanism(s) by which Mo or W additions improve mechanical properties has not been established, but the presence of AlN suggested the accelerated consumption of the internal atmosphere of the double oxide film defects, leading to a reduction in their size and hence their deleterious effects.

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The heterogeneities in tensile properties, microstructure, solute concentrations and defects levels were studied for the castings with different thicknesses made by high pressure die cast Al–Mg–Si alloys. For the local mechanical properties, the casting skin provided the highest, whereas the casting centre provided the lowest yield strength, ultimate tensile strength and elongation. The microstructural heterogeneity was characterised by the segregation of coarse fragmented dendrites, the reduction of solute concentrations of Mg and Si, and the increase of porosity level from the surface to the centre of castings. The growth velocities of eutectic α-Al phase estimated using the Jackson–Hunt model confirm that the growth variation is larger in the thin castings than that in thick castings.

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An Mg–7.2Zn–1.5Cu–1.0Mn (wt-%) magnesium alloy was processed by extrusion and a subsequent solution heat treatment, followed by a two-step aging treatment, with the aim of achieving a high mechanical strength. Results show that a strong age-strengthening response was achieved in the alloy. Compared to the solution heat-treated sample, the increase in the yield strength (YS) of the aged sample was approximately 170 MPa, resulting in a high YS of approximately 370 MPa, which is much higher than that for most high rare-earth-containing Mg alloys. The enhanced age-strengthening response of the alloy is closely related to the high number density of nanoscale rod-shaped precipitates.

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The microstructure and mechanical properties of hot-extruded Mg-5Sn-4Al-2Ce alloy in the form of rapid solidification ribbons (RS-extruded TAE542) are investigated. The results show that rapid solidification causes ultra-fine a-Mg grains (250 nm) and massive dispersive Mg₂Sn and Al₁₁Ce₃ particles (80–400 nm) in the rapidly solidified (RS)-extruded alloy. As a result, the yield strength and elongation of RS-extruded TAE542 alloy reached 366 MPa and 15.2%, increasing by 72 and 26%, respectively, when compared with that of hot-extruded TAE542 alloy made from a homogenised ingot (HI-extruded TAE542).

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Pure Mg and Mg–Gd alloys with 0.5–2 wt-% gadolinium (Gd) were processed by extrusion and simple shear extrusion (SSE). The strain-rate sensitivity (SRS) was investigated by shear punch testing (SPT) at temperatures ranging from 573 to 723 K and for shear strain rates ranging between 0.016 and 0.13 s⁻¹. The Gd-containing alloys processed by SSE showed small m-values at 573 K, which increased with increasing temperature up to 673 K, and decreased afterwards due to grain growth. The highest SRS index (0.47) was found at 673 K for Mg-2Gd after 4 passes of SSE, coinciding with the finest grain size (2 µm). This implies that the material can behave superplastically via a grain boundary sliding mechanism.

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The dynamic deformation behaviour and dislocation substructure of AZ80 magnesium alloy are investigated at strain rates of 8 × 10², 1.5 × 10³ and 2.2 × 10³s⁻¹ and temperatures of −100, 25 and 300°C using a compressive split-Hopkinson pressure bar system. The flow stress, work hardening coefficient, strain rate sensitivity and temperature sensitivity all increase with increasing strain rate or decreasing temperature. Moreover, the dynamic deformation behaviour is well described by the Zerilli–Armstrong hexagonal close packed (hcp) constitutive equation. Catastrophic failure occurs at all three temperatures under strain rates of 1.5 × 10³ and 2.2 × 10³s⁻¹. Transmission electron microscopy observations show that the dislocation density increases with a higher strain rate or a lower temperature. Finally, the flow stress varies linearly with the square root of the dislocation density in accordance with the Bailey–Hirsch model.

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In this study, a novel approach was taken to investigate plastic anisotropy in coarse-grained (CG) and ultra-fine grained (UFG) AA 5083 aluminium tubes. For this purpose, a parallel tubular channel angular pressing process was used to produce UFG tubes and the related data were collected using uniaxial tensile testing, microhardness measurements, the optical microscopy, X-ray diffraction and scanning electron microscopy, and these data were then analysed. Less anisotropy was found in UFG tubes than in CG tubes for AA 5083 aluminium. The corresponding textures were consistent with the trends for anisotropy in mechanical properties.

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Selective laser melting (SLM) is a form of additive manufacturing technology that makes it possible to create complex parts directly from metal powders. This technology offers a good balance between investment costs, range of materials and part quality. In the work, in order to investigate the structural properties of SLM AlSi10Mg alloys, the microstructure, structural performance and tensile fracture morphology of the alloys are analysed. The sample study results indicate good prospects, and the application of SLM in terms of aeronautics is studied to verify the structural properties. The ambient vibration test results show that SLM AlSi10Mg alloys could have a better performance in structural analysis and vibration test than the conventional manufacturing part and the reasons are analysed.

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The effective Young's modulus of aluminium matrix syntactic foams was determined by modal analysis. Two different matrix materials (Al99.5 and AlSi12) were used, and they were reinforced by Globocer grade ceramic hollow spheres. In order to validate the results, a full-scale finite element model was also created. A new algorithm was developed to place the spheres in a proper, probabilistic spatial distribution. Finite element simulations were carried out in modal analysis and compression test senses. In addition, three different analytical methods were studied to estimate the effective Young's modulus. The measured values were compared with the finite element and analytical results. The determined effective Young's moduli showed good agreement.

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In situ Al₂O₃p/A356 composites with different particle volume fractions were successfully synthesised from the Al-SiO₂ system by a direct-melt reaction. γ-Al₂O₃ was first produced during the reaction between Al and SiO₂, and the phase transformation from γ-Al₂O₃ to α-Al₂O₃ then occurred at higher temperatures (1050°C), resulting in a volume contraction. The α-Al₂O₃p/A356 composite exhibited a reduction in dendrite cell size that was 1.5–2 greater than for the matrix alloy cast using the same conditions. A 10% particle volume fraction yielded optimum mechanical properties, with values for the yield stress, tensile strength, elongation and microhardness of 132 MPa, 268 MPa, 5.1% and 107.2 HV, respectively. These corresponded to increases 50, 31, 19 and 47%, respectively, when compared to the matrix.

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The effects of pulse-impact on liquid-phase pulse-impact diffusion welding of an SiC_p/Al-6061 aluminium matrix composite were investigated. The results show that under the influence of pulse-impacts: (i) the extent of the two-phase liquid–solid region increases with increments in welding temperature; (ii) the internal stresses at the interface are released, resulting in welded joints with a higher strength; (iii) the density of dislocations near to and away from the interface in the matrix are higher than in the parent composite, and the dislocations entangle extensively; (iv) nano-grains form which appear to improve the properties of the welded joints.

This paper is part of a thematic issue on Light Alloys.