Microstructure evolution and age-hardening response in Mg-Sn-Sm alloys under a wide range of Sm/Sn ratio

The microstructure evolution and age-hardening response for different Sm/Sn ratios (0–2.55, in wt.%) of Mg-Sn-Sm alloys were investigated. The second phase formation in as-cast alloys and the Mg₃Sm precipitates formed in aged alloys were characterized using XRD, FESEM and HAADF-STEM with EDS techniques. Results indicate that the Sm/Sn ratio has a great influence on the phase constitution, α-Mg grain size and age-hardening response. With the increment of Sm/Sn ratio, Mg₄₁Sm₅ and thermally stable MgSnSm phases precipitate. When the Sm/Sn ratio is about 1.19, the secondary dendrite arm spacing of α-Mg grains significantly decreases. Furthermore, the alloy with Sm/Sn ratio up to 2.55 exhibits the highest age-hardening response, the hardness value increases from 52 HB at solution-treated condition to 74 HB at peak-aged condition (ageing at 220 °C for a short time of 4 h). This is attributed to the large volume fraction of needle-like Mg₃Sm precipitates formed in the α-Mg matrix during ageing treatment, which results in a significant precipitation strengthening effect.

     

Effects of the lanthanum content on the microstructure and properties of the molybdenum alloy

The effects of the lanthanum content on the microstructure and properties of molybdenum alloy were investigated. The molybdenum powders with various lanthanum contents were prepared by a solid-liquid doping method and reduction under hydrogen atmosphere, which could be processed into sintered molybdenum and rotary swaged molybdenum. The results indicated that the grain sizes of the alloys became finer and the tensile strength increased with increasing La content. The La₂O₃ particles could adsorb impurity elements that existed on the grain boundary and generate the amorphous structure around the particle. The rotary swaged Mo with 0.1 wt.% La was the highest tensile strength, and the rotary swaged Mo with 0.03 wt.% La possessed the highest elongation to failure of 42%. In addition, the electrical resistivity of the rotary swaged Mo increased at first and later decreased with increasing La content.     

Effect of minor Sc additions on structure,age hardening and tensile properties of aluminium alloy AA8090 plate

The grain structure, age hardening response, precipitate structure and tensile properties of AA8090 alloy plate without and with minor additions of Sc were investigated. Primary aluminide particles were present in all alloys. Sc additions ⩾0.43% resulted in refinement of the as-cast grain structure. Further investigations on 0.11% and 0.22% Sc containing alloys showed that the subgrain size was finer while the number density of nano-scale Al₃(Sc,Zr)/Al₃Li composite precipitates and the artificially aged hardness were higher for the Sc containing alloys. There was a marginal increase in the yield strength with a concomitant reduction in ductility with increasing Sc content in the alloys.     

Tetragonal phase stability in ZrO2 film formed on zirconium alloys and its effects on corrosion resistance

A thermodynamic model is developed to understand the origin of variation in the microstructure of ZrO₂ film formed on zirconium alloys and its effects on corrosion resistance. The correlation among the tetragonal phase fraction, the stress (macroscopic and internal one), the ZrO₂ grain size and the microstructural change of oxide film is formulized, and then analyzed. The results show that many complicated factors simultaneously govern the microstructure of oxide film. The tetragonal phase content near the oxide/metal interface, the macroscopic compressive stress near the interface, the decline gradient of macroscopic compressive stress and the internal stress induced by the transformation from the tetragonal to the monoclinic phase have very important influences on the transition from columnar grains to equiaxed grains, the crack formation and the degradation of oxidation resistance. The presence of intermetallic precipitates in oxide film may effectively relax the internal stress caused by transformation strain, stabilize the columnar-grain structure and reduce the probability of crack formation. How to reduce the transformation stress in the oxide film is a key to improve the corrosion resistance of zirconium alloys.     

Effects of Mg content on microstructure and mechanical properties of low Zn-containing Al−xMg−3Zn−1Cu cast alloys

Effects of Mg content on the microstructure and mechanical properties of low Zn-containing Al?xMg? 3Zn?1Cu cast alloys (x=3?5, wt.%) were investigated. As Mg content increased in the as-cast alloys, the grains were refined due to enhanced growth restriction, and the formation of η-Mg(AlZnCu)₂ and S-Al₂CuMg phases was inhibited while the formation of T-Mg₃₂(AlZnCu)₄₉ phase was promoted when Mg content exceeded 4 wt.%. The increase of Mg content encumbered the solution kinetics by increasing the size of eutectic phase but accelerated and enhanced the age-hardening through expediting precipitation kinetics and elevating the number density of the precipitates. As Mg content increased, the yield strength and tensile strength of the as-cast, solution-treated and peak-aged alloys were severally improved, while the elongation of the alloys decreased. The tensile strength and elongation of the peak-aged Al?5Mg?3Zn?1Cu alloy exceed 500 MPa and 5%, respectively. Precipitation strengthening implemented by T′ precipitates is the predominant strengthening mechanism in the peak-aged alloys and is enhanced by increasing Mg content.     

Sm对挤压Mg-6Al-1.0Ca-0.5Mn镁合金微观组织及力学性能的影响

制备了Mg-6Al-1.0Ca-0.5Mn-x Sm(x=0.5,1.5,4.5,质量分数,%)合金,研究了合金的显微组织和力学性能。实验结果表明,随着Sm质量分数的增加,Al_2Sm相主要在晶内析出且体积分数增加,相反Mg_(17)Al_(12)相的体积分数降低;挤压后合金发生动态再结晶,晶粒细化。在室温条件下,含1.5%Sm合金显示了最佳的力学性能,其极限抗拉强度、屈服强度和伸长率分别为316 MPa,148 MPa和21.3%。该合金优异的力学性能主要是由于晶粒细化、Al_2Sm颗粒的弥散强化和减少Mg_(17)Al_(12)相的析出。     

Microstructure, phase transformation and mechanical properties of Ni-Mn-Ga-Y magnetic shape memory alloys

Influence of rare earth Y addition on the microstructure and phase transitions and mechanical properties of polycrystalline Ni₅₀Mn₂₉Ga₂₁ ferromagnetic-shape memory alloy (FSMA) are investigated. It is shown that microstructure of the Ni-Mn-Ga-Y alloys consists of the matrix and the Y-rich phase. The Y-rich phase firstly disperses homogeneously in the matrix with small amounts and then tends to segregate at the grain boundaries with increasing Y substitution for Ga. The Y-rich phase is indexed to Y(Ni,Mn)₄Ga phase with a hexagonal CaCu₅ type structure. The structural transition from 5 M to 7 M, and then to non-modulated T martensite appears with the increase of Y content. The martensitic transformation temperature increases remarkably with increasing Y content, whereas the Curie temperature almost keeps unchanged. It is revealed that the appropriate addition of Y significantly enhances the yield strength and improves the ductility of the alloys. The mechanism on the influence of Y content on the improved mechanical properties and martensitic transformation temperature is also discussed.     

Gd对ZK60镁合金组织与力学性能的影响

分析了铸态和挤压态ZK60?xGd(x=0~4)合金的组织和相组成,测试了其拉伸力学性能。结果表明,随着Gd含量的增加,铸态组织逐渐细化,Mg?Zn?Gd新相逐渐增多,而MgZn2相逐渐减少直至消失,第二相趋于连续网状分布于晶界处;当 Gd 含量不超过2.98%时,铸态室温拉伸力学性能稍降低。经挤压比λ=40和挤压温度T=593 K的挤压后,组织显著细化,平均晶粒尺寸逐渐减至ZK60?2.98Gd合金的2μm,破碎的第二相沿着挤压方向呈带状分布;挤压态的拉伸力学性能均显著提高：298和473 K时的抗拉强度分别从ZK60合金的355和120 MPa逐渐提高至ZK60?2.98Gd合金的380和164 MPa。挤压态拉伸断口呈现典型的韧性断裂特征。     

Preparation and characterization of porous NiTi alloys synthesized by microwave sintering using Mg space holder

To obtain the lightweight, high strength, and high damping capacity porous NiTi alloys, the microwave sintering coupled with the Mg space holder technique was employed to prepare the porous NiTi alloys. The microstructure, mechanical properties, phase transformation behavior, superelasticity, and damping capacity of the porous NiTi alloys were investigated. The results show that the porous NiTi alloys are mainly composed of the B2 NiTi phase with a few B19′ NiTi phase as the sintering temperature is lower than or equal to 900 °C. With increasing the sintering temperature, the porosities of the porous NiTi alloys gradually decrease and the compressive strength increases first, reaching the maximum value at 900 °C, and then decreases. With increasing the Mg content from 1 wt.% to 7 wt.%, the porosities of the porous NiTi alloys increase from 37.8% to 47.1%, while the compressive strength decreases from 2058 to 1146 MPa. Compared with the NH₄HCO₃ space holder, the phase transformation behavior of the porous NiTi alloys prepared with Mg space holder changes, and all of the compressive strength, superelasticity, shape memory effect and damping capacity are greatly improved.     

Al含量对铸态V-Alx合金组织和力学性能的影响

V-Al合金膜有着良好的抗氢脆性和高的氢渗透率,有望成为未来取代Pd合金膜的替代材料。V-Al合金的显微组织不仅影响氢分离性能,还将影响强度和塑性成形性能。本研究通过真空非自耗电弧炉熔炼制备了氢分离用V-Alx(x=5,10,20,30,at%,下同)合金。利用扫描电镜/能谱仪、X射线衍射仪及拉伸试验机和维氏硬度仪等手段,研究了Al含量对铸态V-Alx合金微观组织和力学性能的影响。结果表明:V-Alx合金凝固组织均由V基固溶体及Al2O3粒子组成。Al元素的添加对合金有细化晶粒的作用,随Al含量增加,合金晶粒尺寸减小。当Al元素含量为5at%时,合金抗拉强度达到最大,为236MPa,比纯V提高了12%,这是细晶强化、固溶强化和析出强化的共同结果,此时延伸率相比纯V略有降低。当Al元素含量达到10at%及以上时,Al2O3粒子发生粗化导致合金抗拉强度下降,延伸率急剧下降,合金失去塑性。     

Effect of Spraying Powders Size on the Microstructure,Bonding Strength,and Microhardness of MoSi2 Coating Prepared by Air Plasma Spraying

Molybdenum disilicide (MoSi₂) coatings were deposited on carbon steel by air plasma spraying technology with different feedstock powder sizes (i.e., powder A: ?15 + 2.5 μm, powder B: ?30 + 15 μm, powder C: ?54 + 30 μm, powder D: ?74 + 54 μm and powder E: ?106 + 74 μm). Phase composition and microstructure of coatings were characterized by x-ray diffraction (XRD) and scanning electron microscope. The bonding strength and microhardness of coatings were also evaluated. The XRD results show that there exists mutual transformation between T-MoSi₂ and H-MoSi₂ phase and part of Mo-rich phases are formed because of oxidization during the spraying process. With the increase of spraying powders size, the content of Mo-rich phases (Mo or Mo₅Si₃) and molybdenum oxide (MoO₃) in coatings decreases, and that of disilicide-rich phase (MoSi₂) in coatings increases. The oxidation degree of MoSi₂ particle gradually decreases during the spraying process with the increase of spraying powders size. The MoSi₂ is the main phase of the as-sprayed coatings when the spraying powders size is beyond 30 μm. With the increase of spraying powders size, the porosity of the as-sprayed coating increases, and the bonding strength of the coating gradually decreases. The hardness of coatings first increases and then decreases with the increase of spraying powders size.     

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.     

Li对快速凝固Mg96.5Gd2.5Zn1合金微观组织和力学性能的影响

本文考察了快速凝固条件下不同含量Li元素添加对长周期有序结构相增强Mg-Gd-Zn合金微观组织和力学性能的影响。结果表明,随着Li元素的添加,铸态合金中Gd、Zn溶质原子在镁基体晶粒中的过饱和度降低、(Mg,Zn)₃Gd晶界析出相增加、镁基体晶粒尺寸减小。而固溶处理后,随着Li含量的增加,合金中14H型长周期堆垛有序结构相(LPSO)的形成受到抑制,同时纳米/亚微米(Mg,Zn)₃Gd颗粒相大量析出,当Li为7.6at. %时合金中无LPSO形成。经热挤压变形后,合金中块状14H相发生扭着分层开裂、层片状14H相发生不同程度溶解、(Mg,Zn)₃Gd相破碎细化、基体发生不同程度再结晶;不加Li的Mg_96.5Gd_2.5Zn₁表现出最佳的力学性能(UTS=325,δ=9.5%),而含Li合金则随Li含量的增加,力学性能逐步下降。合金在不同条件下的组织转变机理及力学行为变化被进行了分析。     

Effects of silicocalcium on microstructure and properties of Mg-6Al-0.5Mn alloy

1 Introduction Magnesium alloys are attractive for applications in the automobile, aerospace and electronic industries due to their light mass, high stiffness, high specific strength, good dimensional stability and damping capacity. It is the lightest sp…     

Microstructure and phase composition of as-cast Mg–9Er–6Y–xZn–0.6Zr alloys

The microstructure and phase composition of as-cast Mg–9Er–6Y–xZn–0.6Zr (x=1, 2, 3, 4; normal mass fraction in %) alloys were investigated. In low Zn content, aside from the major second phase of Mg₂₄(Er, Y, Zn)₅, there are a few lamellar phases that grow parallel with each other from the grain boundaries to the grain interior. With Zn content increasing, the Mg₂₄(Er,Y,Zn)₅ phase decreases, but the Mg₁₂Zn(Y, Er) phase and lamellar phases continuously increase. When Zn content reaches 4% (normal mass fraction), the Mg₁₂Zn(Y,Er) phase mainly exists as large bulks, and some α-Mg grains are thoroughly penetrated by the lamellar phases. Moreover, the crystallography structures of the Mg₁₂Zn(Y,Er) and Mg₂₄(Er,Y,Zn)₅ phases are confirmed as 18R-type long-period stacking ordered structure and body-centred cubic structure, respectively.     

Influences of the substitution of Fe for Ni on structures and electrochemical performances of the as-cast and quenched La0.7Mg0.3Co0.45Ni2.55−xFex (x = 0–0.4) electrode alloys

In order to improve the cycle stability of the La–Mg–Ni system PuNi₃-type hydrogen storage electrode alloys, Ni in the alloy was partially substituted by Fe. The La_0.7Mg_0.3Co_0.45Ni_2.55−xFe_x (x = 0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys were prepared by casting and rapid quenching. The effects of the substitution of Fe for Ni on the structures and electrochemical performances of the as-cast and quenched alloys were investigated in detail. The results of the electrochemical measurement indicate that the substitution of Fe for Ni obviously decreases the discharge capacity, high rate discharge capability (HRD) and discharge potential of the as-cast and quenched alloys, but it significantly improves their cycle stabilities, and its positive impact on the cycle life of as-quenched alloy is much more significant than on that of the as-cast one. The microstructure of the alloys analyzed by XRD, SEM and TEM show that the as-cast and quenched alloys have a multiphase structure which is composed of two major phases (La, Mg)Ni₃ and LaNi₅ as well as a residual phase LaNi₂. The substitution of Fe for Ni helps the formation of a like amorphous structure in the as-quenched alloy. With the increase of Fe content, the grain sizes of the as-quenched alloys significantly reduce, and the lattice constants and cell volumes of the alloys obviously increase.     

Fatigue strength and microstructure evaluation of Al 7050 alloy wires recycled by spray forming,extrusion and rotary swaging

Recycled high-strength aluminum alloys have limited use as structural materials due to poor mechanical properties. Spray forming remelting followed by hot extrusion is a promising route for reprocessing 7xxx alloys. The 7050 alloy machining chips were spray formed, hot extruded, rotary swaged and heat-treated in order to improve mechanical properties. Microstructures, tensile properties and fatigue strength results for a 2.7 mm-diameter recycled wire are presented. Secondary phases and precipitates were investigated by XRD, SEM, EBSD, TEM and DSC. As-swaged and heat-treated (solution and aging) conditions were evaluated. Mechanical properties of both conditions outperformed AA7050 aerospace specification. Substantial grain refinement resulted from the extensive plastic deformation imposed by rotary swaging. Refined micrometric and sub-micrometric Al grains, as well as coarse and fine intermetallic precipitates were observed. Subsequent solution treatment resulted in a homogeneous, recrystallized and equiaxed microstructure with grain size of 9 μm. Nanoscale GP(I) zones and η′ phase precipitates formed after aging at 120 °C, imparting higher tensile (586 MPa) and fatigue (198 MPa) strengths.     

Microstructure and Mechanical Properties of AZ91 Alloys by Addition of Yttrium

Effects of yttrium (Y) on the microstructure and properties of as-cast Mg-Al-Zn (AZ91) alloys were studied. Y additions not only change the microstructure but also influence the mechanical properties of AZ91 alloy. AZ91 unmodified alloys under as-cast state indicate that eutectic phase Mg₁₇Al₁₂ is continuous and reticulated. Yttrium addition to AZ91 casting alloys has an important influence on the primary-phase and precipitation. When the Y content is 0.3 wt.%, no Y-containing compound was observed. When the Y content is 0.6 and 0.9 wt.%, Al₂Y phase formed in the alloy and the growth morphology of eutectic Mg₁₇Al₁₂ phase is modified. When the Y content is further increased to 1.2 wt.%, the Al₂Y phase becomes coarser and Mg₁₇Al₁₂ transforms into a cotton-shape structure. The results showed that Y can improve significantly as-cast microstructure of AZ91 alloys, refining Mg₁₇Al₁₂ phase and increasing in hardness and strength and decreasing in impact toughness and elongation.     

Strengthening CoCrFeNi high-entropy alloy by Laves and boride phases

To strengthen the face-centered-cubic (FCC) type CoCrFeNi high-entropy alloy (HEA) by in-situ reinforced phase, (CoCrFeNi)_100−x(NbB₂)_x (x=0, 2, 4, 6, 8, at.%) alloys were prepared. Phase constitution, microstructure, tensile mechanical properties of the alloys were studied, and the mechanisms were discussed. Results show that the microstructure of all the reinforced alloys consists of the matrix FCC phase, Laves phase, and (Cr₃Fe)B_x phase. The eutectic structure and (Cr₃Fe)B_x phases are formed in the interdendritic region, and the eutectic structure is composed of Laves and FCC phases. When x increases from 0 to 8, i.e., with increase of Nb and B elements, the volume fraction of Laves and (Cr₃Fe)B_x phases increases gradually from 0 to 5.84% and 8.3%, respectively. Tensile testing results show that the ultimate strength of the alloys increases gradually from 409 MPa to 658 MPa, while the fracture strain decreases from 75% to 1.6%. Fracture analysis shows that the crack originates from the (Cr₃Fe)B_x phase. The CoCrFeNi alloys are mainly strengthened by the second phase (Laves phase and boride phase).

     

铝含量和退火温度对锆铝合金相组成和力学性能的影响

利用非自耗电弧炉熔炼了Al含量为6.0%,7.0%,8.0%(质量分数)的锆铝二元合金,通过退火过程中的包析反应得到了不同相组成的Zr3Al基合金,借助光学显微镜、XRD分析研究了合金的金相组织和相组成,进行了显微硬度测定和拉伸试验。结果表明:铸态的锆铝合金显微硬度随着铝含量的增加而增大;退火可得到组织均匀的Zr3Al基合金,其显微硬度和抗拉强度主要与相组成和基体晶粒大小有关,而与第二相的形态无关;合金的显微硬度、抗拉强度随着Zr3Al相的增多而增大,延伸率随着Zr3Al相的增多而减小;合金的显微硬度、抗拉强度和延伸率随着Zr3Al晶粒的细化而不同程度地增大。