6101/A356 bimetal was fabricated by squeeze casting technique and subsequent T6 heat treatment (heating at 540±3 ℃ for 5 h, quenching in cold water and ageing at 175 ℃ for 4 h). This research presents significant microstructure evolution and mechanical properties improvement of the metallurgical bonded 6101/A356 bimetal after T6 treatment. The transition zone between the A356 aluminum alloy and 6101 aluminum alloy is made up of fine equiaxed grain structure. After T6 heat treatment, the morphology of silicon particles in the A356 aluminum alloy changes from long, coarse plate-like to fine spherical, and Zn, Mg diffuse more homogeneously across the interface. The transition zone is broadened from ~110 µm to ~170 µm. Moreover, interfacial shear strength measured by push-out test is improved by about 34%, from 76.7 MPa to 102.7 MPa. In addition, micro-hardness for both the matrixes and transition region is increased simultaneously.

Cyclic extrusion and compression (CEC) was taken to process the carbon nanotubes (CNTs) reinforced AZ91D composites. Effects of CEC passes and CNTs on the microstructure evolution, hardness and tensile properties of the post-processed composites were investigated. Results show that the matrix grain of the 0.5 wt% CNTs/AZ91D composites is greatly refined from ~112 μm to ~126.6 nm after 8 passes of CEC, with Mg17Al12 uniformly distributed along grain boundaries. The addition of CNTs leads to a reduced matrix grain, but exerts a limited effect on the Mg17Al12 precipitates. The initial highly-agglomerated CNTs are gradually dispersed with the progress of CEC, though degrading of CNTs occurs. The implement of CEC markedly enhances the hardness, YS, UTS and elongation to fracture of both the monolithic AZ91D alloy and the CNTs/AZ91D composites, all of which are mainly attributed to the greatly refined matrix grain and the uniformly distributed Mg17Al12 precipitates. Incorporation of CNTs increases the hardness, YS and UTS of the base alloy, but decreases the elongation to fracture, which is closely related with the gradually dispersed but degraded CNTs.

Isothermal hot compression at the temperature range of 573–698 K and strain rates of 0.005–1.0 s−1 was used to investigate the flow behavior and processing characteristics of the nano-SiCp/AZ91 composites. Effects of the incorporated particles and their particulate size on the workability of the base alloy were then compared and discussed. Results show that compared with the monolithic AZ91 alloy, the incorporated nano-SiC particles effectively increase the flow stress of the composites by blocking the strain-induced dislocations, while effect of the micro-SiC particles varies due to the competition between pinning effect and particle stimulating nucleation (PSN) mechanism. Three domains of peak energy dissipation efficiency are identified in the processing map and the corresponding microstructures examined by EBSD indicate that continuous dynamic recrystallization (DRX) occurs during the compression. The instability characteristics at low temperature are severe mechanical twinning and micro-cracks, while that at high temperature is intergranular cracking. The incorporation of SiC particles enhances the high temperature (>655 K) workability of AZ91 by increasing the upper limit of the processing strain rate and enables low temperature processing by decreasing the lower limit of the temperature. However, the added particles impose a side effect by enlarging the instability domain of the base alloy to a lower strain rate and even higher temperature.

The current research investigates the effect of homogenization on the microstructure and mechanical properties of the AZ91D alloy processed by repetitive upsetting (RU). Results show that during RU processing, the initial large Mg17Al12 particles in the as-cast specimen accelerate the dynamic recrystallization (DRX) due to the particle stimulating nucleation (PSN) mechanism. With the progress of RU, the inherent large strain breaks the large second phases into small fragments, which indicates the PSN gradually disappears, while the pinning effect becomes obvious. As for the homogenized specimen, a pre-heat treatment leads to the absence of Mg17Al12 particles but a uniform distribution of Al atoms in the Mg alloy. Though the subsequent RU promotes the precipitation of Mg17Al12 particles, the relatively small particle size and the uniform distribution are more favorable to act as obstacles for grain growth than contributors to PSN. Finally, a more homogeneous and refined microstructure is obtained in the specimen with prior homogenization than the as-cast one.

Cyclic extrusion and compression (CEC) was implemented to process the Mg-Gd-Y-Zr alloy. Microstructure characters, including the matrix grain, precipitates and texture evolution, were tried to correlate with the mechanical performance of the post-processed alloy. Results show that after 14 passes of CEC, the average grain size of GW102K is greatly refined to ~100–200 nm. Secondary-phase particles are broken due to the occurrence of tension tearing and shearing fracture. Quantitative texture analysis elucidates that CEC weakens the initial fiber texture of the extruded GW102K. Unlike most of the severe plastic deformed Mg alloys, the CEC processed GW102K alloy follows the normal Hall-Petch equation, which is closely associated with the disintegrated texture as well as the nanoscale second-phase particles. A considerable increase in elongation is witnessed and the introducing of rare-earth elements (RE) is found to change the deformation mechanisms of the Mg alloy by facilitating non-basal slip systems, twinning and shear bands.

•Slip deformation of a Mg-RE alloy at 250 °C was investigated using in-situ SEM.•The extruded-T5 GW103 alloy did not exhibit a high anisotropic behavior.•Multiple-slip was observed within grains, and basal/prismatic type dominated.•Slip transfer occurred and most of the observations showed basal-basal type.•Slip transfer occurred more easily at LABs and boundaries with misorientations > 75°.The slip activity and slip interaction in tensile deformation of peak-aged cast and extruded Mg-10Gd-3Y-0.5Zr (wt.%) at 250 °C was investigated using in-situ scanning electron microscopy. Basal slip was the most likely system to be activated during the tensile deformation, while prismatic < a > and pyramidal < c + a > slip also contributed to the deformation. No twinning was observed. The number of non-basal slip systems accounted for ~ 36% of the total active slip systems for the cast alloy, while non-basal slip accounted for 12–17% of the total deformation observations in the extruded alloy. Multiple-slip was observed within grains, and the basal/prismatic pairing type dominated the multiple-slip observations. Slip transfer occurred across grain boundaries and most of the slip transfer observations showed basal-basal type. The involved slip systems of slip transfer pairs were always associated with the same < a > direction. The slip transfer occurred more easily at low angle boundaries (LABs) and boundaries with misorientations greater than 75°.

The wrought Al alloy–wrought Al alloy overcast joint was fabricated by casting liquid 6101 Al alloy onto 6101 Al extrusion bars and solidifying under applied pressure. The joint interfacial microstructure was investigated; the effect of applied pressure on the microstructure and mechanical properties was evaluated. The mechanism of joint formation and mechanical behaviors of both squeeze cast 6101 and 6101–6101 overcast joint material were analyzed. The results show that with the application of pressure during solidification process, wrought Al alloy 6101 could be cast directly into shape successfully. Excellent metallurgical bonding was then formed in the overcast joint by electro-plating 6101 solid insert with a layer of zinc coating, and a transition zone formed in the joint region. During the tensile test, the fracture occurs in the 6101 solid insert part with the ultimate tensile strength (UTS) of 200 MPa, indicating that the strength of the overcast joint is higher than 200 MPa, and the tensile strength of overcast joint material is independent on the magnitude of applied pressure. For Al–Al overcast joint material, if a clean and high strength joint is formed, the UTS and yield strength (YS) are determined by the material with the lower value, while for EL, the value is determined by the length proportion and the stress–strain behavior of both components.

The impression creep behavior of Mg–10Gd–3Y–0.5Zr (wt%, GW103) was investigated by flat cylindrical indenter experiments at temperatures ranging from 200 °C to 325 °C. The punching stresses, which were calculated based on the indenter dimensions, varied from 50 MPa to 505 MPa. Samples were examined in the peak-aged cast condition (cast-T6) and in the cast-then-extruded plus peak-aged condition (ex-T5). Using a power–law relationship, the creep stress exponent (n) varied from 1.4 to 5.1 for the cast-T6 alloy, where lower values were obtained at lower temperatures and stresses. The creep activation energy (Q) increased from 87 to 191 kJ/mol with increasing stress. For the ex-T5 alloy, the n values were 2.2, 3.4 and 3.4 at 200 °C, 250 and 300 °C, respectively. The Q value increased from 121 to 165 kJ/mol with increasing stress. However, by using a hyperbolic sine relationship, single activation energies of 93 and 134 kJ/mol were obtained for the cast-T6 and ex-T5 alloy, respectively. The zone at the edges of the indenter underwent the largest stress and strain, resulting in broken grain boundaries. Extension twinning occurred both in the zone at the edges of the indenter and underneath the indenter face for the cast alloy, while no twinning was observed for the extruded alloy. Intergranular cracking was observed in the zone at the edges of the indenter for the tests under high temperature and high stress.

•A novel SPD method for producing large size of plate shaped bulk material.•A systemic investigation of fields of effective strain, microstructure, and micro hardness to illustrate the homogeneity of RU processing.•Microstructure evolution and mechanical properties enhancement of a high performance Mg alloys during RU processing was investigated.A novel severe plastic deformation (SPD) process called repeated-upsetting (RU), which produces homogenous samples of large size, is employed to process a high performance magnesium alloy Mg-9.8Gd-2.7Y-0.4Zr at 350 °C with 1–4 passes. Homogenous microstructure and mechanical properties can be achieved after 4 passes of RU. Experimental and finite-element modeling results show that effective strain accumulation with more passes can improve both grain refinement and micro-hardness of samples.

Magnesium composites containing different amounts of SiC nanoparticles were processed by a two-step cyclic closed-die forging (CCDF), which was carried out at 400 °C for 3 passes (1st-CCDF) and further at 300 °C for 2 passes (2nd-CCDF). Microstructure evolution and mechanical properties of the composites were investigated. After processing, the average grain size is significantly refined to ~2.5 μm, and the morphology exhibits a flow-lined feature. The β-Mg17Al12 phases in the as-cast alloy dissolve completely after 1st-CCDF, and then precipitate out with an average size of ~650 nm during 2nd-CCDF. After CCDF, the as-cast SiC clusters are uniformly dispersed as separate particles, and the yield strength and ultimate strength of the composites reach 258 MPa and 365 MPa, respectively. The ductility of the composites is enhanced after 1st-CCDF but decreased after 2nd-CCDF, which is in accordance with the observed fracture surfaces. From the perspective of strength and ductility, the optimal content of SiC is 0.5 wt%.

Repetitive upsetting (RU), a newly developed SPD technique, was employed to process AZ91D magnesium alloy plates at 400 °C for 0, 2, 4 and 8 passes. The microstructure and texture evolution of the plates after RU were investigated and correlated with the mechanical performances at room temperature. Results show that the initial coarse grains are effectively refined, however an abnormal grain coarsening appears after 8 passes of RU. A compression texture and a shear texture, induced by specific local strains, are observed within the RU-processed plates. Both the ultimate tensile strength (UTS) and elongation to fracture of the RU-processed AZ91D alloy are improved with the increase of RU passes, exhibiting a maximum increase of 35.8% and over 75%, respectively. The yield strength (YS) increases steadily in the first 4 RU passes and then decreases slightly after another 4 passes. The mechanical anisotropy is discussed from the perspective of texture and Schmid factor.

•A NZ30K alloy was processed by RU at 400 °C for 0, 1, 4 and 8 passes.•The microstructure of the alloy was significantly refined with RU passes.•Microstructure homogeneity all over the sample was achieved after RU for 8 passes.•Significant improvement in both strength and ductility was obtained after RU.•Yield phenomenon was observed on the tensile curves.A Mg–3.03Nd–0.24Zn–0.49Zr (wt.%, NZ30K) magnesium alloy was subjected to repetitive upsetting (RU) at 400 °C for 0, 1, 4 and 8 passes, respectively. The influence of RU on the microstructure evolution and mechanical properties was investigated. Results show that with the increase of RU passes, the grains of the alloy are significantly refined and the microstructure homogeneity all over the sample is greatly enhanced. The alloy exhibits a homogenous equiaxed microstructure with an average grain size of 4 μm after RU for 8 passes, compared with the initial 90 μm. Both the strength and ductility are notably improved with RU passes. The yield strength, ultimate tensile strength and elongation of the 8-passes alloy are 208.16 MPa, 248.50 MPa and 30.10%, compared with the 153.28 MPa, 213.99 MPa and 8.43% prior to RU, respectively. Yield phenomenon is observed on the tensile stress–strain curves and it becomes more pronounced with grain refinement.

A solid-recycled Mg–10Gd–2Y–0.5Zr alloy is fabricated by hot deforming of machined chips and conventional hot extrusion. All the recycled specimens exhibit higher yield strength compared with the extruded reference ingots under the same extrusion condition, which is mainly ascribed to fine-grain size, dispersive distribution of oxide and the second phase of recycled specimens. When extruded at 450 °C, the ultimate tensile strength and elongation of solid-recycled specimen are higher than reference specimen. As for the corrosion resistance, the corrosion rate of 450 °C solid-recycled specimen is slightly higher than reference specimen extruded at the same condition. The 400 and 450 °C recycled specimens are corroded uniformly, which is better than the pitting of reference specimens. In 450 °C recycled specimen, the dispersive distribution of oxide induces that the corrosion products could hinder the progress of corrosion effectively. Meanwhile, the second phase precipitates easily concentrate along the grain boundary in the reference specimens. The aggregates of precipitates induce the appearance of pitting and deep etch pits.Graphical abstractHighlights► Yield strength of recycled specimens is higher than that of reference specimens. ► Corrosion rate of 450 °C recycled specimen is slightly higher than references. ► The corrosion mode of 450 °C recycled specimen is uniform, without pitting. ► Fine grains and moderate residual cavities hinders pitting from occurring.

To improve mechanical properties and particularly corrosion resistance of aluminum high pressure die castings, a new Al–Si based alloy was developed by lowering copper content and optimizing other alloying elements. In comparison with the widely used high pressure die casting A380 and A360 alloys, the newly developed alloy exhibits an improved strength by 20% with respect to A360 and a better corrosion resistance by 45% compared with A380. The improvement in mechanical properties and corrosion resistance of the new alloy is attributed to the dramatic microstructure refinement and eutectic silicon morphology modification with much better homogeneous distribution of the second phase particles in the matrix.Highlights► A new alloy shows strength matching A380 and corrosion resistance similar to A360. ► The corrosion resistance improvement is about 45% compared with A380. ► Mischmetal dramatically refines microstructure and modifies Si morphology. ► Mischmetal improves intermetallic dispersion homogeneity in the matrix.

The Mg–9Gd–1Y–0.5Zr (wt.%) alloy piston was produced by squeeze casting and further heat treated to peak-aged state. The tensile-creep behavior of the specimens taken from the piston top was investigated at temperatures of 200–300 °C (0.54–0.66Tm) and applied stresses of 50–120 MPa. The creep resistance decreases with increasing temperature and applied stress. When tested at 200 °C and 50 MPa, the specimen has a steady-state creep rate of 0.4 × 10− 9 s− 1 and creep strain of 0.014% after 100 h. The creep stress exponent and the activation energy are 1.66–4.47 and 115.9–174.2 kJ/mol, respectively, implying that dislocation creep is the rate-controlling mechanism. At 50–80 MPa the activation energy for creep indicates that dislocation climb is the creep mechanism. At 120 MPa the relatively high activation energy may be associated with the operation of cross slip. After test, the thermal stable β phase appears and its volume fraction increases with testing temperature. The steady-state creep rate and rupture time are modeled by the original and modified Monkman–Grant relationships. The microcracks and cavities preferentially nucleate at the grain boundaries, especially at triple points. Brittle rupture with few plastic tearing ridges is the dominant character of the creep fracture.Highlights► Creep rate and strain at 100 h are 0.4 × 10− 9 s− 1 and 0.014% at 200 °C/50 MPa. ► Creep exponent and activation energy are 1.66–4.47 and 115.9–174.2 kJ/mol. ► Thermal stable β phase appears after creep test at elevated temperature. ► Creep rate and rupture time are modeled by Monkman–Grant relationship.

The sliding friction and wear behaviors of Mg–11Y–5Gd–2Zn–0.5Zr (wt%) alloy were investigated under oil lubricant condition by pin-on-disk configuration with a constant sliding distance of 1,000 m in the temperature range of 25–200 °C. Results indicate that the volumetric wear rates and average friction coefficients decrease with the increase of sliding speeds, and increase with the increase of test temperature below 150 °C. The hard and thermally stable Mg12(Y,Gd)Zn phase with long-period stacking order structure in the alloy presents significant wear resistance. The wear mechanism below 100 °C is abrasive wear as a result of plastic extrusion deformation. The corporate effects of severe abrasive, oxidative, and delaminating wear result in the tribological mechanism above 100 °C.

The tensile creep behavior and microstructure evolution of the extruded Mg–10Gd–3Y–0.5Zr (wt%, GW103) alloy were investigated at temperatures from 523 K to 573 K and stresses from 30 MPa to 120 MPa. The peak-aged extruded GW103 alloy exhibited a minimum creep rate ranging from 3.49×10−8 s−1 to 2.43×10−6 s−1, and the aging treatment exerted limited effect on its creep performance. The measured stress exponent and activation energy of the peak-aged extruded alloy were 2.9±0.5 and 182.5±1.3 kJ/mol, respectively. Increasing precipitates formed during creep, which contributed to improving its creep-resistance. Precipitate free zones (PFZs) were observed in the tertiary stage near the boundaries which were perpendicular to the loading direction, and the formation of specially directional PFZs was demonstrated mainly stress-induced. Fractographic analysis revealed that intergranular ductile fracture was the main fracture mode after creep rupture. The low values of stress exponent and the formation of specially directional PFZs in the extruded alloy indicated that diffusion creep acted as a predominant mechanism.

The deformation behavior of as-extruded Mg–9.8Gd–2.7Y–0.4Zr Mg alloy is investigated by compression test with Gleeble-3500 thermal simulator at temperature of 648–723 K and strain rate of 0.01–5 s−1. It is found that the flow stress behavior is described by the hyperbolic sine constitutive equation in which the determined average activation energy is 229.5 kJ/mol. Through the flow stresses behavior, the processing maps describing the variation of power dissipation efficiency, is constructed as a function of temperature and strain rate. The processing maps exhibit a domain of dynamic recrystallization (DRX) occurring at the temperature of 420–450 °C and strain rate of 0.01–0.1 s−1, corresponding to the optimum hot working window. The instability zones of flow behavior are also recognized from the maps.

Mg–Gd–Y is a new type of high performance magnesium alloy with high specific strength at both room and elevated temperature. A recently developed multiple compression severe deformation, repeated-upsetting (RU) processing is applied to further improve both strength and ductility. The microstructure and texture evolution of Mg–9.8Gd–2.7Y–0.4Zr Mg alloy large plate processed by RU are investigated, which are critical to provide a fundamental understanding of the improvement. Reasonably equiaxed spatially uniform microstructure is obtained with a grain size of 2.5–3.0 μm. Randomized texture is achieved through 3D material flow by grain rotation due to imposed shear strain and Mg alloys specific tensile twinning. Reasonably stable {0002} basal texture along horizontal directions is related to tensile twinning mechanism.Highlights► A novel severe deformation technique, repeated-upsetting (RU) was reported. ► Mg–9.8Gd–2.7Y–0.4Zr (wt.%) alloy does not show the yield asymmetry after RU. ► A homogeneous microstructure is reached with a mean grain size of ~ 2.8 μm. ► Randomized texture is achieved through 3D material flow during RU.

The effect of microstructure on the tensile–creep behavior of Mg–11Y–5Gd–2Zn–0.5Zr (wt.%) (WGZ1152) at 573 K (0.64Tm) and stresses between 30 MPa and 140 MPa was investigated. The minimum creep rate of the peak-aged (T6) alloy was almost two orders of magnitude lower than that for a WE54-T6 (Mg–5.2Y–3.6RE–0.5Zr (wt.%)) alloy. The peak-aged condition (T6) exhibited slightly greater creep resistance than the as-cast condition. The solution treated (T4) material exhibited the lowest creep resistance. The creep stress exponent (∼5) suggested that dislocation creep was the dominant secondary creep mechanism. The minimum creep rate and time-to-fracture could be described by the Monkman–Grant equation. An in-situ creep experiment indicated that intergranular cracking was prevalent in the tertiary creep regime and the crack propagation path tended to follow the grain boundaries.Highlights► The alloy exhibited excellent creep resistance at high temperature up to 573 K. ► Creep stress exponent (∼5) suggested dislocation creep was dominant mechanism. ► Minimum creep rate and fracture time followed the Monkman–Grant relationships. ► Microstructure evolution and its effect on creep behavior were investigated. ► In-situ creep experiment highlighted deformation/intergranular cracking evolution.

The tensile-creep and creep–fracture behavior of as-cast Mg–11Y–5Gd–2Zn–0.5Zr (wt%) (WGZ1152) was investigated at temperatures between 523 and 598 K (0.58–0.66Tm) and stresses between 30 and 140 MPa. The creep stress exponent was close to five, suggesting that dislocation creep was the dominant creep mechanism. The activation energy for creep (233 ± 18 kJ/mol) was higher than that for self-diffusion in magnesium, and was believed to be associated with cross-slip, which was the dominant thermally-aided creep mechanism. This was consistent with the surface observations, which suggested non-basal slip and cross-slip were active at 573 K. The minimum creep rate and fracture time values fit the original and modified Monkman–Grant models. In situ creep experiments highlighted the intergranular cracking evolution. The creep properties and behavior were compared with those for other high-temperature creep-resistant Mg alloys such as WE54-T6 and HZ32-T5.

An as-extruded Mg–1 wt%SiC nanocomposite was processed by cyclic extrusion compression (CEC) at 350 °C. The homogeneity of grain and SiC nanoparticle (∼50 nm average diameter) distribution during the processing was investigated. With the increasing number of CEC passes, a finer grain size and more uniform particle distribution are obtained along with significant improvement in hardness. The matrix grain size is reduced remarkably from ∼27.6 μm to ∼6.5 μm after 8 passes of CEC. Nanoparticle declustering occurs through a mechanism of kneading caused by the intense turbulent flow of the Mg matrix during CEC, and the SiC nanoparticles are dispersed into the original particle-free regions. The property improvement is mainly attributed to Orowan strengthening and the Hall–Petch effect.

The microstructure and mechanical properties of Mg–11Y–5Gd–2Zn–0.5Zr (wt.%) (WGZ1152) alloy during different heat treatments were investigated. Almost all the Mg24(GdYZn)5 eutectic phases dissolved into the α-Mg matrix after solution treatment at 535 °C for 20 h. After ageing at 225 °C for 24 h (T6 state), a great amount of fine β′ precipitates formed. Both the 18R-type long period stacking ordered (LPSO) Mg12YZn phase and 6H′-type LPSO phase exhibit good thermal stability during the high-temperature heat treatments process. The 18R-type LPSO Mg12YZn phases are much harder than α-Mg matrix and have a volume fraction of ∼16%. The ultimate tensile strength at the room temperature of the peak-aged alloy (T6 state) is 307 ± 6 MPa and elongation is 1.4 ± 0.3%. The alloy in T6 state shows anomalous positive temperature dependence of the strength from room temperature to 250 °C, and maintains a strength of more than 260 MPa up to 300 °C (0.64Tm). The excellent strength of the WGZ1152 alloy at both room and elevated temperatures is mainly attributed to the solid solution strengthening, β′ precipitates strengthening and LPSO strengthening. Slip line observations suggest a transition from basal to non-basal slip with increasing temperature.Research highlights▶ High strength can be obtained after optimization of heat treatment parameters. ▶ Morphology and microhardness of phases during heat treatments were determined quantitatively. ▶ Slip lines observations reveal a transition from basal to non-basal slip with increased temperature. ▶ The long period stacking ordered structures have good thermal stability and high hardness. ▶ Solution, precipitate and LPSO strengthening are all effective strengthening sources.

The microstructure and mechanical properties of Mg-Zn-Ho-Zr alloys have been investigated in detail. The grain size of the as-cast Mg-Zn-Ho-Zr alloy was greatly decreased by the addition of Ho, and the grain growth during solution treatment was suppressed by Mg-Zn-Ho phases formed at grain boundaries. These thermally stable Mg-Zn-Ho phases could not completely dissolve into the matrix during solution treatment, and the strengthening effect of solution-plus-ageing treatment weakened. The addition of Ho can greatly enhance the high-temperature elongation of the Mg-Zn-Ho-Zr alloy, but the increase of high-temperature tensile strength was just a little.

Unlike that of sintering of fine powders, chips are consolidated by hot deformation in the solid-state recycling. In this work, conventional extrusion (CE) and cyclic extrusion compression (CEC) are used to investigate single and multi-passes shear deformation on the consolidation of chips during solid-state recycling. The results show that utilization of Mg chips with smaller specific surface area (i.e. coarser powder) contributes to easier solid-state bonding because of the decrease in specific surface area which promotes suppression of oxide contamination in the recycled specimens. Enhanced consolidation of chips is not only ascribed to the physical mechanisms caused by plastic deformation, but also to atom diffusion between chips triggered by shear plastic deformation at elevated temperatures. Multi-passes shear deformation breaks the oxide films easier into small particles which are dispersed within the grains or at grain boundaries. It is postulated that the shear deformation of chips is responsible for the better consolidation of chips by high-temperature deformation.

Solid-state recycling by hot extrusion is a new recycling method for machined chips. There are many factors which contribute to the complexity of hot compaction technology of chips. It is very important to obtain relative high-density blocks from chips and optimize these process parameters during the solid-state recycling. However, the process parameters are interdependent, and optimization of the combination of processes is time-consuming. In this work, the nonlinear relation of the temperature, the press and deformation velocity was established according to the rheology of the matrix material using a thermal simulation machine and mathematic regression analysis. Based on the experimental results of densities of blocks from chips, the hot-compacted model was built. The lowest energy consumption as criterion was also introduced to further optimize hot compaction parameters in both direct and indirect solid-state recycling means. The approach was found not only to obtain high-density blocks from chips, but make estimate on energy consumption during the hot-compacted stage. Especially, when the work velocity of the hydrostatic machine has severe influence on rheology of powders and phase transformation doesn't happen in high temperature environment, the method can quickly help engineers make the optimum hot-compacted technology of the powders.Graphical abstractThere are many factors which contribute to the complexity of hot compaction technology of chips. Based on the rheology of the matrix material and the experimental results, the hot-compacted model was built. The lowest energy consumption as criterion was also introduced to further optimize these parameters. The approach was found not only to obtain high-density blocks from chips, but make estimate on energy consumption during the hot-compacted stage.

The microstructure and texture development of an extruded ZK60 Mg alloy during cyclic extrusion and compression (CEC) at 230 °C was investigated. Tensile tests were performed at room temperature at a strain rate of 1 × 10−3 s−1. The results show that the microstructure was effectively refined by the CEC processing. The initial fiber texture became disintegrated during CEC processing and developed a new texture. After CEC 4-passes, the elongation-to-failure significantly increased almost three times compared with that of as-extruded alloy. Texture change and grain refinement were considered to be responsible for the large tensile ductility in the CEC processed ZK60 alloy. Furthermore, the smaller intensity of texture of CEC processed specimen may be another reason for the large ductility.

Mg–10 wt.%Y–5 wt.%Gd–0.5 wt.%Zr (WG105) and Mg–10 wt.%Y–5 wt.%Gd–2 wt.%Zn–0.5 wt.%Zr (WGZ1052) alloys are processed via a conventional casting method. By adding 2 wt.% Zn into WG105 alloy, the microstructure changes remarkably. It can be seen that a long-period stacking order structure (LPSO) is formed. The lattice is a 6H′-type (ABCBCB′) which is a distorted stacking order from an ideal hexagonal lattice of 6H-type (ABCBCB). It also can be seen that the growth of some 6H′-phases stop growing at the grain interior and others run through the whole grain. Furthermore, in different grains the growth of 6H′-phase takes on different orientations, but in the same grains the orientation is uniform.

High-pressure die casting is a versatile process for producing engineered metal parts. There are many attributes involved which contribute to the complexity of the process. It is essential for the engineers to optimize the process parameters and improve the surface quality. However, the process parameters are interdependent and in conflict in a complicated way, and optimization of the combination of processes is time-consuming. In this work, an evaluation system for the surface defect of casting has been established to quantify surface defects, and artificial neural network was introduced to generalize the correlation between surface defects and die-casting parameters, such as mold temperature, pouring temperature, and injection velocity. It was found that the trained network has great forecast ability. Furthermore, the trained neural network was employed as an objective function to optimize the processes. The optimal parameters were employed, and the castings with acceptable surface quality were achieved.

Cyclic extrusion compression (CEC) as a new solid-recycling processing was applied to recycle the Mg–10Gd–2Y–0.5Zr alloy. The microstructure and mechanical properties of the recycled alloy were studied. Results showed that after 6-passes CEC processing at 673 K, equiaxed grains of <5 μm were obtained. Meanwhile, deep cracks caused by interfaces decohesion between chips were almost vanished. More second phase particles were observed in the recycled specimens than ingot-processed ones. High temperature CEC of the recycled specimens can accelerate the precipitation of the second phase particles. Ductility of the recycled alloy mainly depended on the degree of elimination of the interfaces between the chips. After 4-passes CEC processing at 723 K, the recycled alloy exhibited a combination of high strength and excellent ductility, and this strength can be ascribed to the grain refinement, uniform distribution of oxide and the precipitation of Mg24(Gd,Y)5, as the second phase particles.

The microstructure and creep behavior of the extruded Mg–4Y–4Sm–0.5Zr alloy were investigated. It was shown that dynamic recrystallization (DRX) took place during extrusion at 673 K and equilibrium β precipitates were observed in the extruded alloys. Numerous fine globular β′ precipitates homogeneously formed within grains when the as-extruded alloy was aged at 473 K for 16 h, leading to effectively strengthening. The as-extruded alloy exhibits good creep resistance. The creep stress exponent n is 4.42 and the creep activation energy Q is 140.6 kJ mol−1 suggesting that dislocation climb plays a dominant role when the alloy is crept at temperatures ranging from 453 K to 493 K and at a stress of 180 MPa.

An extraordinary microstructure with mean grain size of 1.77 μm accompanying fine grains of 150 ± 50 nm was obtained for magnesium alloy AZ31 by cyclic extrusion compression (CEC). The ductility improved 2.2 times while yield strength decreased 50 MPa after CEC 7 passes compared with as-extruded alloy. A compound grain-refining mechanism was proposed to explain the microstructure evolution during CEC. Dislocation density, texture and grain-boundary structure were employed to clarify the relationship between microstructure and mechanical properties of AZ31 alloy after CEC.

The high strain rate superplastic deformation properties and characteristics of a rolled AZ91 magnesium alloy at temperatures ranging from 623 to 698 K (0.67Tm−0.76Tm) and high strain rates ranging from 10−3 to 1 s−1 were investigated. The rolled AZ91 magnesium alloy possesses excellent superplasticity with the maximum elongation of 455% at 623 K and a strain rate of 10−3 s−1, and its strain rate sensitivity m is high up to 0.64. The dominant deformation mechanism responsible for the high strain rate superplasticity is still grain boundary sliding (GBS), and the dislocation creep mechanism is considered as the main accommodation mechanism.

A new severe plastic deformation (SPD) method called C shape equal channel reciprocating extrusion (CECRE) was developed to fabricate fine grained AZ31 Mg alloys. The results show that homogeneous microstructure with mean grain size of 3.6 μm is obtained as the accumulated true strain is increased to 11. Strain localization leading to dynamic recrystallizaion (DRX) occurring is the main reason for grain refinement during CECRE process. At the same time, the hardness of AZ31 alloy increases from 62.6 of as-extruded to 74.6 of CECRE 4 passes.

The effects of aging treatment on the microstructures and mechanical properties of extruded AM50 + xCa alloys (x = 0, 1, 2 wt.%) were studied. The results indicated the secondary phase Mg17Al12 precipitated from the saturated α-Mg solid solution while Al2Ca changed slightly when the aging time was increased. The hardness of extruded AM50 + xCa alloys increased initially to its peak, and then dropped to reach its original hardness with the increase in aging time. With the increase in aging temperature, the hardness of the AM50 + 2Ca alloy decreased, whereas the hardness of AM50 and AM50 + 1Ca alloys decreased in the initial stages of aging treatment and increased in the later stages of aging treatment. The tensile strengths of AM50 and AM50 + 1Ca alloys increased after aging treatment for the precipitation of Mg17Al12 phase, which increases the resistance against dislocation movement at the grain boundary; with increase in aging temperature, their tensile strengths increased. For AM50 + 2Ca alloy, the tensile strength declined after aging at 150°C and 175°C, while it increased slightly at 200°C. The ductility of AM50 + xCa alloys (x = 0, 1, 2 wt.%) declined after aging treatment.

Effects of composition, mold temperature, rotating rate and modification on microstructure of centrifugally cast Zn–27Al–xMg–ySi alloys have been investigated. In situ composites of Zn–27Al–6.3Mg–3.7Si and Zn–27Al–9.8Mg–5.2Si alloys were fabricated by centrifugal casting using heated permanent mold. These composites consist of three layers: inner layer segregates lots of blocky primary Mg2Si and a litter blocky primary Si, middle layer contains without primary Mg2Si and primary Si, outer layer contains primary Mg2Si and primary Si. The position, quantity and distribution of primary Mg2Si and primary Si in the composites are determined jointly by alloy composition, solidification velocity under the effect of centrifugal force and their floating velocity inward. Na salt modifier can refine grain and primary Mg2Si and make primary Mg2Si distribute more evenly and make primary Si nodular. For centrifugally cast Zn–27Al–3.2Mg–1.8Si alloy, the microstructures of inner layer, middle layer and outer layer are almost similar, single layer materials without primary Mg2Si and primary Si are obtained, and their grain sizes increased with the mold temperature increasing.

•Tensile deformation modes of a Mg-RE alloy at elevated T was investigated using in-situ SEM.•The extruded GW103 alloy exhibited weak anisotropic behavior.•The non-basal slip contribution to the deformation decreased with increasing T.•Multiple-slip was observed at lower T and the basal-prismatic type predominated.•Slip transfer occurred and the involved slip systems were always with the same vector.The deformation modes and anisotropic behavior during the tensile deformation of peak-aged extruded Mg-10Gd-3Y-0.5Zr (wt.%) at 200, 250 and 300 °C were investigated using in-situ scanning electron microscopy. The samples tested along the extrusion direction (ED) exhibited the highest strength and the samples tested perpendicular to the extrusion direction (TD) exhibited the lowest strength, while the samples oriented 45° to the ED exhibited intermediate strength. Dislocation slip was the main deformation mode during the tensile deformation, while grain boundary sliding contributed to the deformation at 300 °C. No twinning was observed. Non-basal slip accounted for 21%, 7% and 36% of the total active slip systems at 200 °C for the ED, TD and 45° samples, respectively. The non-basal slip contribution to the total active slip systems was 12–17% at 250 °C, and it decreased to 3–5% at 300 °C. Multiple-slip was observed and there were more multiple-slip observations at lower temperatures (200 and 250 °C) compared to those at 300 °C. Basal-prismatic pairing type predominated the multiple-slip observations and the involved slip systems were associated with different slip directions. Slip transfer across grain boundaries occurred at 200 and 250 °C. The slip systems of the observed slip transfer pairs were always associated with the same . Slip transfer occurred more easily at low angle grain boundaries (LABs) and grain boundaries with angles higher than 75°. Intergranular cracking was the main cracking mode. Cracks were more likely to initiate at the grain boundaries where the neighboring grains were in hard orientations for basal slip.

An extraordinary microstructure with mean grain size of 1.77 μm accompanying fine grains of 150 ± 50 nm was obtained for magnesium alloy AZ31 by cyclic extrusion compression (CEC). The ductility improved 2.2 times while yield strength decreased 50 MPa after CEC 7 passes compared with as-extruded alloy. A compound grain-refining mechanism was proposed to explain the microstructure evolution during CEC. Dislocation density, texture and grain-boundary structure were employed to clarify the relationship between microstructure and mechanical properties of AZ31 alloy after CEC.