挤压工艺对Mg-0.6Zr-0.5Y合金力学和阻尼性能的影响

系统地研究了挤压温度和挤压比对Mg-0.6Zr-0.5Y合金力学和阻尼性能的影响规律.采用光学金相显微镜、电子式万能试验机及动态机械分析仪(DMA)等手段分析合金显微组织、力学和阻尼性能的变化,并讨论了晶粒细化与合金力学和阻尼机理之间的内在联系.研究发现,合金在挤压过程中发生了动态再结晶,晶粒显著细化,使综合力学性能得...     

Effects of calcium on the microstructures and tensile properties of Mg-5Li-3Al alloys

The as-cast Mg-5Li-3Al-xCa (x = 0, 0.5, 1, 1.5 wt.%) was prepared with vacuum induction melting furnace, then processed by hot extrusion. The microstructures and tensile properties were investigated. The results show that the grains of as-cast alloys were refined gradually with the increase of Ca content from 0.5 wt.% to 1 wt.%, while the Ca content increases to 1.5 wt.%, the grain size increases. The microstructures of investigated alloys were further refined after hot extrusion. Both as-cast and as-extruded Mg-5Li-3Al-0.5Ca alloys have the highest mechanical properties, which is mainly attributed to the grain refinement caused by the addition of Ca and the formation of strengthening phase, Al₄Ca. When the addition of Ca is up to 1-1.5 wt.%, the tensile properties of alloys are worsened due to the excessive (Mg, Al)₂Ca eutectic phase forming at grain boundary.     

Hot working parameters and mechanisms in as-cast Mg-3Sn-1Ca alloy

The hot working characteristics of as-cast Mg-3Sn-1Ca alloy have been studied using processing-map technique and the kinetic rate equation. The map exhibited two domains; one in the lower strain rate range (0.0003-0.01 s^− 1) and the other in the higher strain rate range (0.1-10 s^− 1)—both falling in the same temperature range of 350-550 s^− 1. Hot extrusion at 500 °C and a speed corresponding to an average strain rate of 3 s^− 1 exhibited dynamically recrystallized microstructure. The estimated apparent activation energy values are higher than those for self-diffusion in pure magnesium suggesting that the large volume fraction of MgSnCa intermetallic particles causes significant back stress.     

Superplasticity of Mg-Zn-Y alloy containing quasicrystal phase processed by equal channel angular pressing

Equal channel angular pressing (ECAP) has been conducted on as-cast Mg-4.3 wt.%Zn-0.7 wt.%Y Mg alloy containing quasicrystal phase at a temperature of 623 K. After 8 ECAP passes, the grain size of the as-cast alloy is decreased from ∼ 120 to ∼ 3.5 μm, and the coarse eutectic quasicrystal phases are broken and dispersed in the alloy. Tensile testing has been performed on the ECAPed Mg-Zn-Y alloy at temperatures of 523 K and 623 K with initial strain rates from 1.5 × 10^− 3 to 1.5 × 10^− 4 s^− 1. The ECAPed alloy exhibits a maximum elongation of about 600% when testing at 623 K using an initial strain rate of 1.5 × 10^− 4 s^− 1. Grain boundary sliding is considered to be the dominant deformation mechanism of the Mg-Zn-Y alloy in the temperature and strain-rate range investigated.     

Improvement of fatigue properties by shot peening for Mg-10Gd-3Y alloys under different conditions

In the present study we investigated the influence of shot peening on the high cycle fatigue (HCF) performance of the Mg-10Gd-3Y magnesium alloys in four different conditions referred to as-cast, cast-T6, as-extruded and extruded-T5, respectively. The results show that shot peening can cause different degree of enhancement of fatigue performance for Mg-10Gd-3Y alloys depending on the Almen peening intensity applied; and that the Almen intensity could always be found that conferred the optimum improvement. The effect of shot peening was quantified, and for the as-extruded and extruded-T5 alloys it was found to be superior to that for the as-cast and cast-T6 alloys. The peened extruded-T5 Mg-10Gd-3Y alloy showed the highest fatigue strength at 10⁷ cycles of 240 MPa. The results of the analyses established a connection between the grain size, ductility and precipitates within the studied alloys. Microstructure affected the magnitude of the surface roughness induced by shot peening and also the maximum compressive residual stress and its relaxation during fatigue, and then determine the beneficial effect of shot peening.     

Microstructure and mechanical properties of ZA62 Mg alloy by equal-channel angular pressing

The Mg-6Zn-2Al alloy was processed by ECAP and microstructure and mechanical properties of the alloy before and after ECAP were studied. The results revealed that the microstructure of the ZA62 alloy was successfully refined after two-step ECAP (2 passes at 473 K and 2-8 passes at 423 K). The course bulk interphase of Mg₅₁Zn₂₀ was crushed into fine particles and mixed with fine matrix grains forming “stripes” in the microstructure after the second step of ECAP extrusion. A bimodal microstructure of small grains of the matrix with size of ∼0.5 μm in the stripes and large grains of the matrix with size of ∼2 μm out of stripes was observed in the microstructure of samples after 4-8 passes of ECAP extrusion at the second step. The mechanical properties of the alloy studied were significantly improved after ECAP and the highest yield strength and elongation at room temperature were obtained at the samples after 4 and 8 ECAP passes at the second step, respectively. Tensile tests carried out at temperature of 473 K to 573 K and strain rate of 1 × 10⁻³ s⁻¹ to 3 × 10⁻² s⁻¹ revealed that the alloy after 8 ECAP passes at the second step showed superplasticity and the highest elongation and strain rate sensitivity (m-value) reached 520% and 0.45, respectively.     

Internal friction of Cu-13.5Al-4Ni shape memory alloy measured by dynamic mechanical analysis under isothermal conditions

Cu-13.5Al-4Ni shape memory alloy (SMA) exhibits a β₁(DO₃) → β₁′ (18R) internal friction peak with high damping capacity and elevated martensitic transformation temperature in a dynamic mechanical analysis tan δ cooling curve. When the specimen is isothermally maintained at peak temperature, the damping capacity decreases significantly and reaches a steady value. The inherent and intrinsic internal frictions of Cu-13.5Al-4Ni SMA are extremely low because the β₁′ (18R) martensite has an ordered 9R structure with stacking faults rather than twinning with movable twin boundaries.     

Preliminary investigations about effects of Zr, Sc and Ce additions on as-cast microstructure and mechanical properties of Mg-3Sn-1Mn (wt.%) magnesium alloy

In this paper, the effects of Zr, Sc and Ce additions on the as-cast microstructure and mechanical properties of Mg-3Sn-1Mn (wt.%) magnesium alloy were preliminarily investigated and compared. The results indicate that adding 0.36 wt.% Sc and 0.87 wt.% Ce to the Mg-3Sn-1Mn alloy, respectively leads to the formation of the extra phases of Mg-Sn-Sc and Mg₁₂Ce while adding 0.43 wt.% Zr does not cause the formation of any new phases. At the same time, adding 0.43 wt.% Zr or 0.87 wt.% Ce can refine the grains while adding 0.36 wt.% Sc coarsens the grains. Among the Zr- and Ce-containing alloys, the grains of the latter are relatively finer than those of the former. In addition, adding 0.43 wt.% Zr, 0.36 wt.% Sc and 0.87 wt.% Ce to the Mg-3Sn-1Mn alloy can improve the tensile and/or creep properties of the alloy. However, the addition of 0.43 wt.% Zr is not beneficial to the creep properties. Among the Zr-, Sc- and Ce-containing alloys, the alloy with the addition of 0.87 wt.% Ce exhibits the optimal tensile and creep properties.     

Formability,mechanical and corrosive properties of Mg–Nd–Zn–Zr magnesium alloy seamless tubes

Some large Mg–3.0Nd–0.2Zn–0.4Zr (NZ30K) magnesium alloy seamless tubes were prepared by forward extrusion. The as-extruded tubes were cooled in the air or by spraying liquid N₂ after extrusion. The formability, mechanical and corrosive properties of the NZ30K magnesium alloy seamless tubes were investigated. The experimental results show that seamless NZ30K tubes with an outer diameter of 110 mm and inner diameter of 90 mm can be produced by forward extrusion and the tubes have good roundness, concentricity and straightness even without any straightening. The tensile results show that the maximum ultimate tensile strength, yield strength and elongation of the extruded tubes cooled in the air and by spraying liquid N₂ are 306.3 and 314.6 MPa, 250.4 and 270.3 MPa, 14.2% and 15.6%, respectively. The corrosion rates of the as-extruded tubes cooled in the air and by spraying liquid N₂ immersed in 5% NaCl solution for 3 days are 0.225 and 0.234 mg cm⁻² day⁻¹, respectively, which are a little inferior to the as-cast, T4 and T6 NZ30K alloys, but much lower than that of AZ91 alloy. Localized corrosion is suggested to be its corrosion pattern.     

Effect of I-phase morphology and microstructure transformation in biomedical Mg-3Zn-1Mn-1Y alloys on vitro degradation behavior in dynamic simulated body fluid

The corrosion mechanism of as-cast, heat-treated (H400) and extruded (E30, E60, E90) Mg-3Zn-1Mn-1Y alloys with different microstructure is investigated by scan electron microscope (SEM), scan Kelvin probe force microscope (SKPFM), X-ray photoelectron spectroscopy (XPS), electrochemical impedance analysis and immersion experiments equipped with a dynamic corrosion device. The relevant results are as follows: continuously strip-like I-phase (Mg₃Zn₆Y) in as-cast alloy distributed along the grain boundary played a significant obstacle impact during corrosion, whereas this capability is weakened after heat treatment and large plastic extrusion deformation. However, extrusion deformation significantly improved alloy corrosion performance, the extruded E30 alloy performed superior anti-corrosion behavior among the three extruded alloys owing to the smaller potential difference between I-phase (2.59 V) and DRXed (2.51 V) or un-DRXed (2.54 V) grains. In addition, the corrosion obstacle effect of grains boundaries (the grain boundary has higher potential than the Mg substrate), dense corrosion products film protection (isolate the substrate from contact with SBF) and typical basal texture (lower reactivity of base atoms) have great influence on corrosion behavior.

     

Flexural damping of a P55 graphite/magnesium composite

Recent studies have shown that when magnesium is alloyed with particular solute species having very low solid solubilities (< 1%), such as aluminium, copper, tin, zirconium, manganese or silicon, the characteristically high damping is preserved while the mechanical properties are enhanced. Moreover, both damping, and the amplitude dependence of damping, increase with decreasing solute atom concentration. Accordingly, these materials are considered candidates in the fabrication of metal matrix composites (MMCs) for use in large space structures. This paper presents damping data on two magnesium alloys, Mg-0.6%Zr and Mg-1.0%Mn, and a recently developed magnesium MMC, a [0₈] P55Gr/Mg-0.6%Zr. The alloy data demonstrate the increase in damping and amplitude dependence which accompanies a decrease in alloy concentration. A comparison between the damping of the Mg-0.6%Zr alloy and the [0₈] P55Gr/Mg-0.6 %Zr composite shows that the addition of the strength-enhancing fibres reduces the high damping properties of the matrix.     

Ambient effect on damping peak of NiTi shape memory alloy

We report the effects of ambient condition and loading rate on the damping capacity of a superelastic nickel-titanium shape memory alloy during stress-induced martensitic phase transformation with release and absorption of latent heat. The damping capacity was measured via a tensile loading-unloading cycle in the strain-rate range of 10^− 5-10^− 1/s and three ambient conditions: still air and flowing air with velocities of 2 m/s and 17 m/s. It is found that, for each ambient condition, the maximum damping capacity (damping peak) is achieved at the strain rate whose loading time (t_T) is close to the characteristic heat-transfer time (t_h) of the ambient condition.     

Corrosion properties and corrosion evolution of as-cast AZ91 alloy with rare earth yttrium

The corrosion resistance property and the corrosion evolution of as-cast AZ91 alloy with rare earth Y addition are investigated by using immersion tests, electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). The results show that the proper amount of Y in the alloys can improve the corrosion resistance of AZ91 alloys effectively. With the increment of Y, the corrosion rate of the modified AZ91 alloys by Y addition was markedly less than that of AZ91 alloy. The corrosion rate of AZ91 alloy with 0.3 wt.% Y was the slightest, but further addition of Y content over 0.3 wt.% make the corrosion heavier. The XPS analysis suggests that the compound film of AZ91 alloy with 0.3 wt.% Y is mainly composed of Mg(OH)₂ and MgCO₃ without any Al(OH)₃ and Al₂O₃, in addition, Y₂O₃ phase is found in the compound film of AZ91 alloy with 0.3 wt.% Y, which benefits to stabilize the surface film.     

Effect of Pd on microstructures and tensile properties of as-cast Mg-6Al-1Zn alloys

The effects of Pd on the microstructure and mechanical properties of Mg-6Al-1Zn alloys were investigated. Mg-6Al-1Zn-xPd (x = 0-6 wt.%) alloys were prepared using a permanent mould casting method. The microstructure of the as-cast alloys was characterized by the presence of Mg₁₇Al₁₂ and Al₄Pd phases. The volume fraction of the Al₄Pd phase was increased by the addition of 1-6 wt.%Pd but the volume fraction of the Mg₁₇Al₁₂ phases decreased. At room temperature, the tensile strength increased with increasing Pd addition up to 2 wt.%Pd, and the elongation to fracture decreased with a concomitant increase in the aggregation of the coarse Al₄Pd phase. At 150 °C, the tensile strength increased with the addition of Pd. Therefore, the room and elevated temperature tensile properties of as-cast Mg-6Al-1Zn alloys can be improved by Pd addition.     

Strength and ductility of novel Mg-8Sn-1Al-1Zn alloys extruded at different speeds

A novel Mg-8Sn-1Al-1Zn alloy developed for high-speed extrusion was successfully extruded at speeds in a range of 2-10 m/min at a temperature of 250 °C. The effect of extrusion speed on the microstructure and tensile properties of the extruded alloys was investigated. Grain size, recrystallization fraction and texture were found to be greatly affected by the extrusion speed, resulting in tensile properties showing lower strength and ductility as the extrusion speed increased. The strength and ductility of the extruded alloys are also discussed in terms of the formation of double twins during the tensile test.     

Effects of Zn on the microstructure, mechanical properties, and damping capacity of Mg-Zn-Y-Zr alloys

The influences of Zn on the microstructure, mechanical properties, and damping capacity of as-extruded (Mg-5%Y-0.6%Zr)_1−xZn_x (x = 2%, 4%, 6%, mass fraction) alloys were investigated by optical microscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, tensile testing, and dynamic mechanical analysis (DMA). The phase composition and microstructure of the alloys displayed evident variations with changes in Zn content. When the mass fraction of Zn changed from 2% to 6%, the phases mainly consisted of a long period stacking ordered (LPSO) X-phase (Mg₁₂YZn) and a W-phase (Mg₃Y₂Zn₃). Comparison of the mechanical properties and damping capacities of the different phases showed that the X-phase benefits the mechanical properties of the alloy without drastic impairment to their damping capacities. The damping capacities are discussed in terms of the Granato-Lücke theory and G-L plots.     

Microstructure and mechanical properties of Mg -2Gd -3Y -0.6Zr alloy upon conventional and hydrostatic extrusion

The Mg-12Gd-3Y-0.6Zr (wt. %) alloy was subjected to conventional and hydrostatic extrusion in two subsequent steps. The best combination of mechanical properties (strength and ductility) was achieved by RT hydrostatic extrusion following conventional extrusion at 430 °C, with the ultimate tensile strength (UTS), tensile yield strength (TYS) and elongation being 485 MPa, 413 MPa and 5.2% at room temperature. The texture results of extruded rods indicate that the c-axis of most grains was aligned preferentially perpendicular to the extrusion direction, forming a typical extrusion Mg fiber texture.     

Damping capacities and microstructures of magnesium matrix composites reinforced by graphite particles

Magnesium matrix composites reinforced by graphite particles were fabricated using stir casting with graphite particle size of 50 μm and graphite particle volume fractions of 5%, 10%, 15% and 20%, respectively. A dynamic mechanical analyzer was used to measure the damping capacities of as-cast composites. The experimental results reveal that the graphite particles play an important role on the damping capacities of as-cast composites. The strain amplitude independent damping of as-cast composites increases significantly as the graphite particle volume fraction increases from 0% to 15%, but decreases when the volume fraction exceeds 15%. The damping values of as-cast composites rise slowly with increasing temperature from room temperature to 125 °C, and have no obvious change with increasing temperature from 125 °C to 250 °C, but rise rapidly with increasing temperature from 250 °C to 400 °C.     

Microstructure and mechanical properties of Mg-4.0Zn-0.5Ca alloy

A new kind of Mg-4.0 wt.%Zn-0.5 wt.%Ca alloy is fabricated by casting and hot extrusion for used as a high performance structure material as well as a biomaterial. In the as-cast alloy, the average grain size of the α-Mg is 120-150 µm and the precipitated second phases are distributed uniformly in α-Mg grains. The as-cast Mg-4.0 wt.%Zn-0.5 wt.%Ca alloy shows a good balance between the tensile strength (211 MPa) and ductility (17% in elongation). After hot extrusion at 593 K, the second phase is greatly refined and the average grain size of the α-Mg is reduced to 8-12 μm which is resulted from dynamic re-crystallization during hot extrusion. In this case, it exhibits a high tensile strength (273 MPa) and a high ductility (34% in elongation) at room temperature.