Effects of minor Sc and Zr additions on mechanical properties and microstructure evolution of Al−Zn−Mg−Cu alloys

The effects of minor Sc and Zr additions on the mechanical properties and microstructure evolution of Al−Zn−Mg−Cu alloys were studied using tensile tests, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The ultimate tensile strength of the peak-aged Al−Zn−Mg−Cu alloy is improved by about 105 MPa with the addition of 0.10% Zr. An increase of about 133 MPa is observed with the joint addition of 0.07% Sc and 0.07% Zr. For the alloys modified with the minor addition of Sc and Zr (0.14%), the main strengthening mechanisms of minor addition of Sc and Zr are fine-grain strengthening, sub-structure strengthening and the Orowan strengthening mechanism produced by the Al₃(Sc,Zr) and Al₃Zr dispersoids. The volume of Al₃Zr particles is less than that of Al₃(Sc,Zr) particles, but the distribution of Al₃(Sc,Zr) particles is more dispersed throughout the matrix leading to pinning the dislocations motion and restraining the recrystallization more effectively.     

Effects of Li content on microstructure and mechanical properties of as-cast Mg−xLi−3Al−2Zn−0.5Y alloys

The effects of Li content on the microstructure and mechanical properties of the as-cast Mg?xLi?3Al?2Zn? 0.5Y (LAZx32-0.5Y) alloys were investigated by XRD, SEM, TEM, hardness tester and universal testing machine. The results show that the matrix of the alloy transforms from α-Mg to α-Mg+β-Li and then to β-Li when the Li content increases from 4% to 14% (mass fraction). All LAZx32-0.5Y alloys contain AlLi and Al₂Y, while MgLi₂Al appears only in the alloy containing the β-Li matrix. As the Li content increases, the content of AlLi and MgLi₂Al gradually increases, while the content of Al₂Y does not change much. As the Li content increases from 4% to 10%, the ultimate tensile strength and hardness of the as-cast LAZx32-0.5Y alloys gradually decrease while the elongation gradually increases. The corresponding fracture mechanism changes from cleavage fracture to quasi-cleavage fracture and then to microporous aggregation fracture. This is mainly attributed to the decrease of α-Mg and the increase of β-Li in the alloy. When the Li content continues to increase to 10% and 14%, the yield strength, ultimate tensile strength and hardness of the as-cast LAZx32-0.5Y alloys gradually increase, while the elongation decreases sharply, which is mainly attributed to the nano-scale MgLi₂Al uniformly distributed in the β-Li matrix.     

Effect of Al addition on microstructure and mechanical properties of Mg−Gd−Zn alloys

The microstructure evolution and mechanical properties of Mg?15.3Gd?1Zn alloys with different Al contents (0, 0.4, 0.7 and 1.0 wt.%) were investigated. Microstructural analysis indicates that the addition of 0.4 wt.% Al facilitates the formation of 18R-LPSO phase (Mg₁₂Gd(Al, Zn)) in the Mg?Gd?Zn alloy. The contents of Al₁₁Gd₃ and Al₂Gd increase with the increase of Al content, while the content of (Mg, Zn)₃Gd decreases. After homogenization treatment, (Mg, Zn)₃Gd, 18R-LPSO and some Al₁₁Gd₃ phases are transformed into the high-temperature stable 14H-LPSO phases. The particulate Al?Gd phases can stimulate the nucleation of dynamic recrystallization by the particle simulated nucleation (PSN) mechanism. The tensile strength of the as-rolled alloys is improved remarkably due to the grain refinement and the fiber-like reinforcement of LPSO phase. The precipitation of the β′ phase in the peak-aged alloys can significantly improve the strength. The peak-aged alloy containing 0.4 wt.% Al achieves excellent mechanical properties and the UTS, YS and elongation are 458 MPa, 375 MPa and 6.2%, respectively.     

Effect of Y on microstructure evolution and mechanical properties of Mg−4Li−3Al alloys

To obtain magnesium alloys with a low density and improved mechanical properties, Y element was added into Mg−4Li−3Al (wt.%) alloys, and the effect of Y content on microstructure evolution and mechanical properties was investigated by using optical microscopy, scanning electron microscopy and tensile tests. The results show that mechanical properties of as-cast Mg−4Li−3Al alloys with Y addition are significantly improved as a result of hot extrusion. The best comprehensive mechanical properties are obtained in hot-extruded Mg−4Li−3Al−1.5Y alloy, which possesses high ultimate tensile strength (UTS=248 MPa) and elongation (δ=27%). The improvement of mechanical properties of hot-extruded Mg−4Li−3Al−1.5Y alloy was mainly attributed to combined effects of grain refinement, solid solution strengthening and precipitation strengthening.     

Effect of Al addition on microstructure and mechanical properties of Mg−Zn−Sn−Mn alloy

The microstructure and properties of the as-cast, as-homogenized and as-extruded Mg−6Zn−4Sn−1Mn (ZTM641) alloy with various Al contents (0, 0.5, 1, 2, 3 and 4 wt.%) were investigated by OM, XRD, DSC, SEM, TEM and uniaxial tensile tests. The results show that when the Al content is not higher than 0.5%, the alloys are mainly composed of α-Mg, Mg₂Sn, Al₈Mn₅ and Mg₇Zn₃ phases_. When the Al content is higher than 0.5%, the alloys mainly consist of α-Mg, Mg₂Sn, MgZn, Mg₃₂(Al,Zn)₄₉, Al₂Mg₅Zn₂, Al₁₁Mn₄ and Al₈Mn₅ phases. A small amount of Al (≤1%) can increase the proportion of fine dynamic recrystallized (DRXed) grains during hot-extrusion process. The room- temperature tensile test results show that the ZTM641−1Al alloy has the best comprehensive mechanical properties, in which the ultimate tensile strength is 332 MPa, yield strength is 221 MPa and the elongation is 15%. Elevated- temperature tensile test results at 150 and 200 °C show that ZTM641−2Al alloy has the best comprehensive mechanical properties.     

Effects of titanium addition on structural,mechanical, tribological,and corrosion properties of Al−25Zn−3Cu and Al−25Zn−3Cu−3Si alloys

To investigate the effect of grain refinement on the material properties of recently developed Al−25Zn−3Cu based alloys, Al−25Zn−3Cu, Al−25Zn−3Cu−0.01Ti, Al−25Zn−3Cu−3Si and Al−25Zn−3Cu−3Si−0.01Ti alloys were produced by permanent mold casting method. Microstructures of the alloys were examined by SEM. Hardness and mechanical properties of the alloys were determined by Brinell method and tensile tests, respectively. Tribological characteristics of the alloys were investigated by a ball-on-disc type test machine. Corrosion properties of the alloys were examined by an electrochemical corrosion experimental setup. It was observed that microstructure of the ternary A1−25Zn−3Cu alloy consisted of α, α+η and θ (Al₂Cu) phases. It was also observed that the addition of 3 wt.% Si to A1−25Zn−3Cu alloy resulted in the formation of silicon particles in its microstructure. The addition of 0.01 wt.% Ti to the Al−25Zn−3Cu and Al−25Zn−3Cu−3Si alloys caused a decrement in grain size by approximately 20% and 39% and an increment in hardness from HRB 130 to 137 and from HRB 141 to 156, respectively. Yield strengths of these alloys increased from 278 to 297 MPa and from 320 to 336 MPa while their tensile strengths increased from 317 to 340 MPa and from 334 to 352 MPa. Wear resistance of the alloys increased, but corrosion resistance decreased with titanium addition.     

High temperature mechanical properties and microstructure of die forged Al−5.87Zn−2.07Mg−2.42Cu alloy

The high temperature mechanical properties (250 °C) and microstructure of a die-forged Al−5.87Zn− 2.07Mg−2.42Cu alloy after T6 heat treatment were investigated. High temperature tensile tests show that as the temperature increases from room temperature to 250 °C, the ultimate tensile strength of the alloy decreases from 638 to 304 MPa, and the elongation rises from 13.6% to 20.4%. Transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) were applied for microstructure characterization, which indicates that the increase of tensile temperature can lead to the coarsening of precipitates, drop of dislocation density, and increase of dynamic recovery. After tensile testing at 250 °C, a sub-grain structure composed of a high fraction of small-angle grain boundary is formed.     

Effects of Zn content on as-cast microstructure and mechanical properties of Mg-xZn-4A 1 alloys containing TiC and rare earth elements

The effects of Zn content on the as-cast microstructure and mechanical properties of Mg-xZn-4Al alloys containing TiC and rare earth elements were investigated by optical microscopy（OM）, scanning electron microscopy（SEM） analysis, X-ray diffraction （XRD） analysis and tensile test. The results show that Zn content which increased from 8% to 12% does not obviously influence on the alloy phase type of the Mg-xZn-4Al experimental alloys containing 0.25%RE and 1%TiC, but with Zn content increasing from 8% to 12%, the amount of Mg32（Al,Zn）49 phase in the as-cast microstructure of the experimental alloys increases and its distribution becomes more continuous. In addition, the Mg-10Zn-4Al alloy containing 0.25%RE and 1TiC has the highest ultimate tensile strength at room temperature and 150 ℃ and highest yield strength and elongation at 150 ℃ Furthermore, with Zn content increasing from 8% to 12%, the yield strength and elongation of Mg-xZn-4Al experimental alloys containing 0.25%RE and 1%TiC increase and decrease at room temperature, respectively.     

Effects of nano-sized TiB2 particles and Al3Zr dispersoids on microstructure and mechanical properties of Al–Zn–Mg–Cu based materials

The effects of TiB₂ and Zr on the microstructure, aging response and mechanical properties of hot-extruded Al–Zn–Mg–Cu based materials were investigated and compared by multi-scale microstructure characterization techniques. The results showed that proper addition of TiB₂ particles could refine grain size during solidification, promote dynamic recrystallization during extrusion, and inhibit grain growth during solution treatment. Meanwhile, Zr addition had minor influence on the grain refinement during solidification, but could effectively suppress recrystallization and grain growth compared with the Zr-free alloy. Furthermore, the TiB₂ addition could simultaneously enhance the aging kinetics and peak-aged hardness of the materials. Comparatively, Zr addition could also improve the peak-aged hardness with minor effect on the aging kinetics of the materials. Finally, the quench sensitivity, elastic modulus and tensile properties of the materials were compared and studied. Specifically, the relationship between the microstructure and mechanical properties, and the strengthening mechanisms were discussed in detail.     

Microstructure and mechanical properties of squeeze-cast Al−5.0Mg−3.0Zn−1.0Cu alloys in solution-treated and aged conditions

The microstructure and mechanical properties at different depths of squeeze-cast, solution-treated and aged Al−5.0Mg−3.0Zn−1.0Cu alloy were investigated. For squeeze-cast alloy, from casting surface to interior, the grain size of α(Al) matrix and width of T-Mg₃₂(AlZnCu)₄₉ phase increase significantly, while the volume fraction of T phase decreases. The related mechanical properties including ultimate tensile strength (UTS) and elongation decrease from 243.7 MPa and 2.3% to 217.9 MPa and 1.4%, respectively. After solution treatment at 470 °C for 36 h, T phase is dissolved into matrix, and the grain size increases so that the UTS and elongation from surface to interior are respectively reduced from 387.8 MPa and 18.6% to 348.9 MPa and 13.9%. After further peak-aging at 120 °C for 24 h, numerous G.P. II zone and η′ phase precipitate in matrix. Consequently, UTS values of the surface and interior increase to 449.5 and 421.4 MPa, while elongation values decrease to 12.5% and 8.1%, respectively.     

Phases and microstructures of high Zn-containing Al–Zn–Mg–Cu alloys

Phases and microstructures of three high Zncontaining Al–Zn–Mg–Cu alloys were investigated by means of thermodynamic calculation method, optica microscopy(OM), scanning electron microscopy(SEM)energy dispersive spectroscopy(EDS), X-ray diffraction(XRD), and differential scanning calorimetry(DSC) analysis. The results indicate that similar dendritic network morphologies are found in these three Al–Zn–Mg–Cu alloys. The as-cast 7056 aluminum alloy consists of aluminum solid solution, coarse Al/Mg(Cu, Zn, Al)_2 eutectic phases, and fine intermetallic compounds g(MgZn_2). Both of as-cast 7095 and 7136 aluminum alloys involve a(Al)eutectic Al/Mg(Cu, Zn, Al)_2, intermetallic g(MgZn_2), and h(Al_2Cu). During homogenization at 450 °C, fine g(MgZn_2) can dissolve into matrix absolutely. After homogenization at 450 °C for 24 h, Mg(Cu, Zn, Al)_2 phase in 7136 alloy transforms into S(Al_2Cu Mg) while no change is found in 7056 and 7095 alloys. The thermodynamic calculation can be used to predict the phases in high Zncontaining Al–Zn–Mg–Cu alloys.     

The effects of Sc on the microstructure and mechanical properties of hypo-eutectic Al−Si alloys

The eutectic Si microstructure in Al-8.5wt.%Si alloy was changed from large flakes to fine lamellar when the Sc amount in the alloy reached 0.2 wt.%. 0.8wt.%Sc was optimal in terms of attaining the best modification effect. Study on the distribution of the modifiers and measurement of the surface tension of Al-8.5wt.%Si alloy melt with added Sr, Na, and Sc modifiers, respectively, reveals that Sc modifies eutectic Si by a decrease of surface tension, while Sr and Na modify eutectic Si mainly by an impurity-induced twinning mechanism. Al-8.5wt.%Si-0.4wt.%Sc alloy displayed approximately 50 and 70% increases in tensile strength and elongation, respectively, over Al-8.5wt.%Si alloy in the cast state. It also presented approximately 65 and 70% increases in tensile strength and elongation, respectively, over Al-8.5wt.%Si alloy at a ppt heat-treated state at 200°C for 3 h.     

Effect of Gd content on microstructure and dynamic mechanical properties of solution-treated Mg−xGd−3Y−0.5Zr alloy

The effect of Gd content ranging from 6.5 wt.% to 8.5 wt.% on microstructure evolution and dynamic mechanical behavior of Mg?xGd?3Y?0.5Zr alloys was investigated by optical microscopy, X-ray diffraction, scanning electron microscopy and split Hopkinson pressure bar. The microstructure of as-cast Mg?xGd?3Y?0.5Zr alloys indicates that the addition of Gd can promote grain refinement in the casting. Due to the rapid cooling rate during solidification, a large amount of non-equilibrium eutectic phase Mg₂₄(Gd,Y)₅ appears at the grain boundary of as-cast Mg?xGd?3Y?0.5Zr alloys. After solution treatment at 520 °C for 6 h, the Mg₂₄(Gd,Y)₅ phase dissolves into the matrix, and the rare earth hydrides (REH) phase appears. The stress?strain curves validate that the solution-treated Mg?xGd?3Y?0.5Zr alloys with optimal Gd contents maintain excellent dynamic properties at different strain rates. It was concluded that the variation of Gd content and the agglomeration of residual REH particles and dynamically precipitated fine particles are key factors affecting dynamic mechanical properties of Mg?xGd?3Y?0.5Zr alloys.     

Effect of Sn content on microstructure and tensile properties of as-cast and as-extruded Mg−8Li−3Al−(1,2,3)Sn alloys

The effects of Sn content on microstructure and tensile properties of as-cast and as-extruded Mg−8Li−3Al− (1,2,3)Sn (wt.%) alloys were investigated by X-ray diffractometry (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and tensile test. It is found that, as-cast Mg−8Li−3Al−(1,2,3)Sn alloys consist of α-Mg+β-Li duplex matrix, MgLiAl₂ and Li₂MgSn phases. Increasing Sn content leads to grain refinement of α-Mg dendrites and increase in content of Li₂MgSn phase. During hot extrusion, complete dynamic recrystallization (DRX) takes place in β-Li phase while incomplete DRX takes place in α-Mg phase. As Sn content is increased, the volume fraction of DRXed α-Mg grains is increased and the average grain size of DRXed α-Mg grains is decreased. Increasing Sn content is beneficial to strength but harmful to ductility for as-cast Mg−8Li−3Al−(1,2,3)Sn alloys. Tensile properties of Mg−8Li−3Al− (1,2,3)Sn alloys are improved significantly via hot extrusion and Mg−8Li−3Al−2Sn alloy exhibits the best tensile properties.     

Simulation on microstructure evolution and mechanical properties of Mg−Y alloys: Effect of trace Y

The influence of trace Y on the microstructure evolution and mechanical properties of Mg_100?xY_x (x=0.25, 0.75, 1.5, 3, 4, 5, at.%) alloys during solidification process was investigated via molecular dynamics (MD) simulations. The results show that the Mg_100?xY_x alloys are mainly characterized by a face-centered cubic (FCC) crystal structure; this is different from pure metal Mg, which exhibits a hexagonal close packed (HCP) structure at room temperature. Among these alloys, Mg_99.25Y_0.75 has a larger proportion of FCC cluster structures, with the highest fraction reaching 56.65%. As the content of the Y increases up to 5 at.% (Mg₉₅Y₅ alloy), the amount of amorphous structures increases. The mechanical properties of the Mg_100?xY_x alloys are closely related to their microstructures. The Mg_99.25Y_0.75 and Mg₉₇Y₃ alloys exhibit the highest yield strengths of 1.86 and 1.90 GPa, respectively. The deformation mechanism of the Mg?Y alloys is described at the atomic level, and it is found that a difference in the FCC proportion caused by different Y contents leads to distinct deformation mechanisms.     

Effects of extrusion and heat treatments on microstructure and mechanical properties of Mg–8Zn–1Al–0.5Cu–0.5Mn alloy

The effects of extrusion and heat treatments on the microstructure and mechanical properties of Mg–8Zn–1Al–0.5Cu– 0.5Mn magnesium alloy were investigated. Bimodal microstructure is formed in this alloy when it is extruded at 230 and 260 °C, and complete DRX occurs at the extruding temperature of 290 °C. The basal texture of as-extruded alloys is reduced gradually with increasing extrusion temperature due to the larger volume fraction of recrystallized structure at higher temperatures. For the alloy extruded at 290 °C, four different heat treatments routes were investigated. After solution + aging treatments, the grains sizes become larger. Finer and far more densely dispersed precipitates are found in the alloy with solution + double-aging treatments compared with alloy with solution + single-aging treatment. Tensile properties are enhanced remarkably by solution + double-aging treatment with the yield strength, tensile strength and elongation being 298 MPa, 348 MPa and 18%, respectively. This is attributed to the combined effects of fine dynamically recrystallized grains and the uniformly distributed finer precipitates.     

Effects of Fe content on microstructure and properties of Cu–Fe alloy

Cu–Fe alloys with different Fe contents were prepared by vacuum hot pressing. After hot rolling and aging treatment, the effects of Fe content on microstructure, mechanical properties and electrical conductivity of Cu–Fe alloys were studied. The results show that, when w(Fe)<60%, the dynamic recrystallization extent of both Cu phase and Fe phase increases. When w(Fe)≥60%, Cu phase is uniformly distributed into the Fe phase and the deformation of alloy is more uniform. With the increase of the Fe content, the tensile strength of Cu–5wt.%Fe alloy increases from 305 MPa to 736 MPa of Cu–70wt.%Fe alloy, the elongation decreases from 23% to 17% and the electrical conductivity decreases from 31%IACS to 19%IACS. These results provide a guidance for the composition and processing design of Cu–Fe alloys.     

Effect of non-isothermal aging on microstructure and properties of Al–5.87Zn–2.07Mg–2.42Cu alloys

The evolution of microstructure and properties of Al–5.87Zn–2.07Mg–2.42Cu alloys during non-isothermal aging was studied. The mechanical properties of the alloy were tested by stretching at room temperature. The results show that in the non-isothermal aging process, when the alloy is cooled to 140 °C, the ultimate tensile strength of the alloy reaches a maximum value of 582 MPa and the elongation is 11.9%. The microstructure was tested through a transmission electron microscope, and the experimental results show that the GP zones and η’ phases are the main strengthening precipitates. At the cooling stage, when the temperature dropped to 180 °C, the GP zones were precipitated again. Besides, the experimental results show that the main strengthening phase during non-isothermal aging is η’ phases.     

Effects of Sr addition on microstructure,mechanical properties and in vitro degradation behavior of as-extruded Zn−Sr binary alloys

The microstructures, mechanical properties and in vitro degradation behavior of as-extruded pure Zn and Zn−xSr (x=0.1, 0.4, 0.8 wt.%) alloys were investigated systematically. For the microstructure and mechanical properties, SrZn₁₃ phase was newly formed due to the addition of 0.1 wt.% Sr, improving the yield strength, ultimate tensile strength and elongation from (85.33±2.86) MPa, (106.00±1.41) MPa and (15.37±0.57)% for pure Zn to (107.67±2.05) MPa, (115.67±2.52) MPa and (20.80±2.19)% for Zn−0.1Sr, respectively. However, further increase of Sr content led to the deterioration of the mechanical properties due to the stress concentration and cracks initiation caused by the coarsening SrZn₁₃ particles during tensile tests. For in vitro degradation, since micro galvanic corrosion was enhanced owing to the formation of the inhomogeneously distributed SrZn₁₃ phase, the corrosion mode became non-uniform. Corrosion rate is gradually increased with the addition of Sr, which is increased from (11.45±2.02) μm/a (a=year) for pure Zn to (32.59±3.40) μm/a for Zn−0.8Sr. To sum up, the as-extruded Zn−0.1Sr alloy exhibited the best combination of mechanical properties and degradation behavior.