The reaction of lattice dislocation with twin boundary plays a crucial role in the plastic deformation of magnesium alloys. In this study, we visit the basal dislocation-twin interaction in a hot-rolled AZ31 sheet through pre-compression along rolling direction (RD) and subsequent compression along 45° of RD and normal direction (ND), with focus on the twin boundary (TB) structure evolution and nucleation structure characterization. It is found that basal dislocation slip dominates the compression deformation when the strain along 45° of RD and ND is less than 14%; when the strain reaches 14%, new deformation modes are initiated. When the strain is in the range of 5%–14%, basal-prismatic (BP/PB) boundaries are created by dislocation-twin interaction. Meantime, the number of the BP/PB boundaries increase linearly with strain, leading to an extremely incoherent TB. When the strain reaches 14%, {101¯2} twin nucleates from parent of previous {101¯2} twin and {303¯4} twin, a twinning mode not reported before, nucleates from previous {101¯2} TB. Based on the direct experimental observations, the nucleation mechanisms of the new nucleuses are proposed. Moreover, TBs of these new nucleuses present faceted structures and previous {101¯2} twin-new {303¯4} twin interaction results in a low-angle asymmetrical tilt boundary. These correlations will significantly benefit the development of crystal plasticity modeling and enhance meso-scale understanding of structural evolution in hexagonal close-packed materials.Download high-res image (210KB)Download full-size image

AZ31 alloy ring was processed by hot ring forging at different processing temperatures. Then the relationship between tensile properties and microstructural parameters including grain size and texture of the ring were investigated by using tensile test along three orthogonal directions. After hot ring forging, the grain structure of the AZ31 alloy ring is significantly refined and the grains grow gradually with increasing the finish die forging temperature. The texture feature in the ring changes with the forging temperature, a single peak basal texture is formed after finish die forging at lower temperature, while there exhibits a double-peak texture at higher temperature and the basal poles tilt further away from the center of (0002) pole figure with increasing the finish die forging temperature. Significant mechanical anisotropy is found for the ring and attributed to varied deformation mechanisms along different directions. Effects of grain size and texture on mechanical behavior are discussed.

Two Mg alloys containing 0.03 at% and 0.18 at% Nd were designed to investigate the effects of solute concentration, and thermomechanical processing on the strengths of related deformation modes. Annealing treatments were conducted to stimulate grain coarsening for as-extruded alloys. In view of solute concentration effect, we conducted compressive tests and microstructure characterization to reveal and analyze the activities of the related deformation modes and their corresponding strengths. We find that the solute concentration of Nd does not affect the nucleation CRSS for {10-12} twinning. However, with the increase of Nd concentration, the nucleation CRSS for pyramidal -slip was greatly reduced. We also use a thermomechanical processing route, in terms of pre-compression, unloading, intermediate annealing, and re-compression, to explore the effect of solute segregation on strengths of related deformation modes. It is found that the CRSS for the migration of {10-12} twinning dislocation increased significantly after the intermediate annealing treatment for Mg-0.18Nd alloy. We conclude that when designing high ductility Mg alloys, from the perspective of reducing the strength discrepancies between soft deformation modes and hard deformation modes, important factors including the type of alloying element, solute concentration, and pre-deformation strain need to be taken into account.Download high-res image (130KB)Download full-size image

•The structure of 101¯1 twin boundary is investigated by TEM and HRTEM.•The actual twin boundary deviates from the theoretical twinning plane by about 8°.•Six kinds of boundaries can coexist in one 101¯1 twin system.•The deviation results from strain accommodation rather than steps.Structure of 101¯1 twin boundary in an AZ31 magnesium alloy has been characterized by means of transmission electron microscopy (TEM) and high-resolution TEM. It is found that actual twin boundary entirely departs from theoretical twinning plane in 101¯1 twin system. Furthermore, it is shown that six kinds of facets, including 101¯1 coherent twin boundaries, {0002}‖101¯1¯ basal-pyramidal (BPy), 1¯011¯‖{0002} pyramidal-basal (PyB), 1¯010‖101¯3 prismatic-third pyramidal (P3Py), basal-prismatic (BP/PB) boundaries and 101¯2 coherent twin boundaries, can coexist in one 101¯1 twin system. Based on these structure features, the underling mechanisms responsible for large deviation phenomenon are discussed, with focus on BPy, PyB, P3Py boundaries and strain accommodation.

•Segregation tendency of solutes into {101¯1} twin boundary or free surface was evaluated.•The effect of segregated solutes on the cohesion of {101¯1} twin boundary was predicted.•The most effective alloying elements were selected according to a “design map”.First-principles calculations were performed to investigate the effect of segregated solutes on the cohesion of {101¯1} twin boundary. The twin boundary cohesion strengthening/embrittling tendency was evaluated, considering segregation ability of randomly distributed solutes into twin boundary and free surface. The most effective solutes were selected according to a “design map” based on the interaction between solutes and twin boundary. The results provided a basis for future investigations on tailoring mechanical behavior through solutes and the development of new high-performance Mg alloys.

Multi-directional forging (MDF) was carried out on AZ31 magnesium alloy up to 36 passes in the temperature range of 300 °C to 400 °C. Microstructure evolution at the center and edge regions of the specimen during the MDF process was investigated and correlated with the mechanical properties. Upon MDF, the material undergoes pronounced grain refinement but an inhomogeneous grain structure is formed due to the inhomogeneous strain distribution between the center and edge regions. After 36 passes, the microstructure at the center region is composed of finer and more homogeneous grains than that at the edge region. Tensile tests show a significant improvement of mechanical properties after MDF. With the increase of MDF passes, the yield strength both at the center and edge regions first increases and then reduces due to twin induced dynamic recrystallization. Compared to the edge region, the mechanical properties at the center region are superior and less affected by the forging temperatures after 36 passes. Among the experimental forging temperatures, 350 °C is favored to get a more uniform mechanical properties across the specimen.

•A large database of solute–solute binding energies for Mg alloys was presented.•Effects of solute–solute binding on Mg properties were inferred and discussed.•Effects of physical factors on solute–solute binding energies were investigated.•Solute pairs such as Bi–Yb are promising in modifying properties of Mg.Solute–solute interactions play a major role in the properties of materials. In this work, we present an extensive database of solute–solute binding energies that captures the detailed interactions in Mg-based alloys from first-principles calculations based on density functional theory. The effects of solute–solute binding energies on magnesium properties, precipitation hardening responses and stacking fault energies in particular, are inferred and discussed. The results of our calculations regarding bindings between solutes with different chemistries, including Al–Sn, Al–Ca, Ca–Zn, Ca–In, and Sn–Zn, were validated using available experimental investigations. Solute pairs that were predicted to show large positive (e.g., Yb–Bi/Sn/Pb and Ca–Bi/Sn/Pb) and negative (e.g., Bi–Sn/Pb/Al) values of binding energies exhibited potential in modifying the precipitation sequence and stacking fault energy. Moreover, alloys added with these alloying elements may exhibit unique mechanical properties, which await experimental verification. Finally, the effect of physical features, including atomic radius and electronegativity, on the solute–solute bindings was investigated.

•Effects of structural relaxation on the GSFE of HCP system were investigated.•{0 0 0 1}〈1 −1 0 0〉, {1 −1 0 0}〈1 1 −2 0〉 and {1 1 −2 2}〈1 1 −2 3〉 slip systems were considered.•Out-of-plane (N-direction) and in-plane (P-direction) relaxing were considered.•N-direction relaxing is essential for the high-index slip plane.•P-direction relaxing is needed for the {1 1 −2 2}〈1 1 −2 3〉 slip system.In this paper, the effects of relaxation parameters on the first-principle-calculated generalized stacking fault energy (GSFE) were investigated. Two relaxing directions were considered, out-of-plane (N-direction) and in-plane (P-direction). N-direction is normal to the slip plane. P-direction is parallel to the slip plane and perpendicular to the slip direction. The results show that relaxation along the N-direction is essential, especially for the high-index slip plane; relaxation along the P-direction is needed when the atoms on the two sides of the slip direction are unsymmetrical. Discussions were made based on the first-principle calculated forces and the geometry of the atomic configurations of different slip systems.

The effects of Er addition on the microstructure and mechanical properties of ZK60 alloy were investigated. Cast ZK60-Er (0, 0.5, 1, 2 and 4 wt%) ingots homogenized at 350 °C were extruded at 400 °C. It was found that Er primarily existed in the form of Mg–Zn–Er intermetallic particles instead of Er-containing α-Mg solid solution. Due to the formation of Mg–Zn–Er particles, the dissolution of Zn solutes in homogenized billets decreased with increasing Er modification. Typical fiber textures along with dynamically precipitated spherical Mg–Zn compounds were developed during the extrusion process. Mg–Zn precipitates gradually decreased with the increasing Er level. Er addition slightly decreased the elongation and scarcely increased the tensile strength of the extruded bars. The extruded alloys with different Er contents shared similar microstructural evolutions in the subsequent heat treatment. No Er-bearing precipitates were found in the heat treated bars. Mechanical strengths decreased but the plasticity increased by increasing the solutionization time from 1.5 h to 12 h at 400 °C. Upon further aging at 200 °C for 10 h, there was no significant change in the optical microstructure while Mg–Zn compounds statically precipitated. The plasticity could be compensated by the decrease of defects caused by the static precipitation of Mg–Zn compounds.

{1 1 −2 2}〈1 1 −2 3〉 generalized stacking fault energies and {0 0 0 1} surface energies of a wide range of Mg-based binary alloy systems were computed using first-principles methods. The intrinsic ductility of these systems was evaluated, taking into consideration the probability of dislocation emission and crack propagation. A design map based on the intrinsic ductility was constructed. The results provide a basis for the design of high-ductility Mg alloys.

•SFEs of binary Mg alloys at two solute concentrations were calculated.•Supercell structure was optimized based on energy convergence against size test.•All RE elements were found to decrease the SFE.•A correlation between SFE and physical factors of solutes was found.•Majority of RE elements were found to follow an abnormal variation with concentration.The stable stacking fault energies for basal stacking faults I1 in various categories of Mg-based binary alloys have been studied using density functional theory. Two concentrations of alloying atoms, 11 and 25 at.% at the stacking fault interface, were considered in the computations, by constructing different supercells after strict test of energy convergence against the sizes and shapes of the supercells. It has been shown that the stacking fault energy of Mg varies in a broad range with alloying elements. While the influence on the stacking fault energy becomes stronger with the increase of solute concentration for majority of the alloying elements, some elements show an opposite tendency. The effects of solute atoms and their concentrations on stacking fault energy were discussed in view of ionization energy, atomic radius and quantum tunneling effect.

Microstructure stability is essential to maintain a fine grain structure for an alloy throughout its processing. The effects of Er addition and its existing form on the static recrystallization and grain growth during annealing of an extruded Mg–1.5Zn–0.6Zr magnesium alloy were studied in this paper. The results showed that microstructure stability was much improved by Er addition and the best thermability was obtained in 2 wt.% Er-containing alloy. For the incomplete dynamic recrystallization (DRX) microstructures extruded at a lower temperature of 350 °C, Er addition increased the resistance of static recrystallization; and for the complete DRX microstructures extruded at a relatively high temperature of 420 °C, Er addition suppressed grain growth. The difference in microstructure stability was then correlated with the microstructure features. Both the intermetallic phase and the solute atoms of Er in α-Mg matrix contributed to the microstructure stability. Moreover, it is believed that the existing form of Er–Zn atom pairs in the α-Mg solid solution favored the most to improve the thermal stability of the alloy.

Single Er and composite Er/Al additions have been made to Mg–1.8Mn (M2) alloy in order to investigate their influences on the microstructure and resulting mechanical properties of alloy extrusions. Both Er and Er/Al additions suppress the dynamic recrystallization (DRX) during the hot deformation and improve the mechanical properties of the M2 alloy. While single addition of Er significantly suppresses recrystallization and grain growth, composite addition of Er/Al is more effective in grain refinement and microstructural homogeneity of alloy. A versatile mechanical property combination, covering from high plasticity to improved moderate strength, can be achieved by micro alloying. With Er or Er/Al additions, the plasticity substantially increases and generally increases with additional amount. Moreover, composite Er/Al addition is much more effective in improving the plasticity than single Er addition. Comparatively, single Er-containing alloys have considerably higher yield and ultimate strength. Furthermore, deformed microstructures could be preserved in the Er-containing alloys after annealing treatment. The stabilized deformed microstructure is deemed to be the combined effects of the dispersed Mn particles and rare-earth element addition.

Mg–1.8Mn (wt.%) alloy was solution-treated by three regimes to obtain different initial microstructures. Isothermal hot compression tests were conducted in the temperature range of 250–450 °C and strain rate 0.01–1 s−1 for each of the initial states. Constitutive constants for each initial state were obtained and compared. The results showed that homogenization of initial microstructure can significantly decrease the deformation resistance during hot working. It was also revealed that the dispersion of second-phase particles 1–1.5 μm in size with appropriate volume fraction (1–2%) and inter-particle spacing in initial microstructure can impart superior hot work deformation ability to the materials. Excessive amounts of particles resulted in an increase of the flow stresses and working-hardening degree. A large volume fraction of nano-sized particles also imposed difficulty in plastic deformation but with less harmful effects than the larger-sized particles. The influencing mechanism of the particle distribution on the constitutive behavior is discussed.Research highlights► Proper particle dispersion promotes softening process. ► Homogenization enhances the hot workability of Mg–Mn alloy. ► Particle dispersion captures the nature of the effect on the constitutive behavior. ► The alloys exhibit distinct deformation activation energy.

The elevated-temperature plasticity and flow behavior of an Er-modified, heat-resistant ZA73 alloy was evaluated by thermal simulation. The results showed that the addition of Er to ZA73 alloy notably improves the deformability and higher strain rate and temperature favors hot deformation. Bars with sound surface quality were successfully extruded at 350 °C and a strain rate of ~ 0.1 s− 1. Furthermore, dynamic precipitation of nano-sized spherical τ phase was found to occur uniformly in the α-Mg matrix during hot extrusion, which is considered helpful to both strength and plasticity enhancement. The yield strength and ultimate tensile strength of the as-extruded bars reached 240–265 MPa and 355–360 MPa, respectively, while maintaining a large elongation rate of 18–19.5%.

The effects of trace Er addition on the microstructure in Mg–9Zn–0.6Zr alloy during casting, homogenization, pre-heating, and hot extrusion processes were examined. The mechanical properties of alloys with and without Er were compared. The results showed that Er exhibited a lower solubility in solid magnesium and formed thermally stable Er- and Zn-bearing compounds. The Er-bearing alloy exhibited a considerably improved deformability, as well as a fine and uniform microstructure. Moreover, dynamic precipitation of fine MgZn2 particles with a modified spherical morphology occurred during hot extrusion, resulting in a tensile yield strength of 313 MPa and a high elongation to failure value of 22%. Further aging of the Er-bearing alloy led to an increment of another 30 MPa in yield strength. In addition, Er markedly increased the thermal stability of the alloy structure.

The solid solubility of elements Fe and Si in aluminium matrix, and the amount of precipitation of intermetallic compounds bearing Fe and Si from the matrix are crucial microstructural parameters, which are found to have marked effects on the formability of aluminium sheets and the mechanical properties of final foil products. The low temperature precipitation in commercial-purity aluminium sheets for foils was investigated by XRD, TEM, optical microscopy, and quantitative metallography. A method was proposed to quantify the precipitation fraction and solid solubility from matrix lattice parameter based on X-ray diffraction analysis. The results showed that the fine rod-like phase, β-FeSiAl5, with average size of about 0.2 μm, precipitates out from the Al matrix during low temperature additional annealing. The precipitation of β-FeSiAl5 was found to lead to an increase in the lattice constant of the Al matrix. Annealing at 210 °C is favourable to the precipitation of the compound containing Si and Fe from the matrix. After 15 h annealing at 210 °C, the solid solubility of Si is reduced by about 50%. Appropriate pre-anneal deformation promotes the precipitation and helps microstructure improvement.

Magnesium is receiving great attention for transport applications, particularly its cast alloys. This investigation focuses on the as-cast microstructure and mechanical properties of permanent-mould cast Mg–Zn–Al alloys with typical compositions within the high zinc castable domain. Three types of alloys were identified and characterized by Mg32(Al, Zn)49, also known as the τ phase; MgZn phase, also known as the ɛ phase; and a ternary icosahedral quasi-crystalline phase, denoted as the Q phase, respectively. A schematic phase diagram is proposed to show the change of microstructral constituents with element content and the Zn/Al ratio. The diagram reveals that the microstructral constituent is dominated by both the content of Zn or Al and the Zn/Al mass ratio; alloys with a high Zn/Al ratio and a low Al content fall into the ɛ-type; alloys with an intermediate Zn/Al ratio and an intermediate Al content favour the τ-type; and those with a low Zn/Al ratio and a high Al are dominated by the icosahedral quasi-crystalline phase. No Mg17Al12 (γ) phase was found in those ZA series alloys. The solidification process and its effects on the phase constituents were discussed. Preliminary mechanical property testing showed that all the experimental alloys possess comparable ultimate strength and yield strength with the AZ91 alloy at ambient temperature, but show far superior creep resistance at elevated temperatures. Moreover, while ambient-temperature properties solely depend on the total element contents, the τ- and the Q-type alloys show greater potential than the ɛ-type alloys on the improvement of elevated temperature properties.