Formation of 14H long period stacking ordered structure and profuse stacking faults in Mg–Zn–Gd alloys during isothermal aging at high temperature

This paper proposes a time–temperature-transformation diagram of an Mg–Zn–Gd alloy. An Mg₉₇Zn₁Gd₂ (at.%) alloy shows different precipitation sequences at low, medium and high temperatures. Low-temperature aging at <523 K brings about coherent β′-phase precipitation to a -Mg solid solution, resulting in increase in strength of the alloys. However, aging at medium and high temperatures >623 K led to strengthening of the Mg–Zn–Gd alloy, owing to the formation of profuse stacking faults and 14H long period stacking ordered structure from the supersaturated -Mg matrix, respectively.     

Investigation on the microstructure and mechanical properties of a cast Mg–6Zn–5Al–4RE alloy

Mg–6Zn–5Al–4RE (RE = Mischmetal, mass%) alloy was prepared by metal mould casting method. The microstructure and mechanical properties of the as-cast and heat-treated alloys were investigated. The results show that the phase compositions of the as-cast state alloy are supersaturated solid solution -Mg, lamellar β-Al₁₂Mg₁₇, polygonal Al₃RE and cluster Al₂REZn₂ phases. The mechanical properties, especially the ultimate tensile strength and elongation of the alloy were significantly improved by the heat treatment. Fracture surface of tensile specimens was analyzed by optical microscope and scanning electron microscope.     

Zn对Mg-10Gd-2Y-0.5Zr镁合金组织和性能的影响

通过在Mg-10Gd-2Y-0.5Zr合金中添加Zn,采用SEM、XRD及万能拉伸试验机,研究了Zn添加对其铸态组织和力学性能的影响。结果表明,Mg-10Gd-2Y-0.5Zr合金的铸态组织主要由α-Mg、Mg₅(Gd,Y)和Mg₂₄(Y,Gd)5相组成,而添加质量分数为0.5%~1.5%的Zn后,合金的铸态组织主要由α-Mg、Mg₅(Gd,Y,Zn)、Mg₂₄(Y,Gd,Zn)₅及Mg₁₂(Gd,Y)Zn相组成。添加0.5%的Zn后,合金的室温力学性能明显提高,当Zn含量高于1.0%后,镁合金的室温力学性能开始逐步降低。当Zn含量为0.5%时,合金具有较佳的综合力学性能,其抗拉强度、屈服强度和伸长率分别为197 MPa、160 MPa和4.37%。Zn对Mg-10Gd-2Y-0.5Zr合金铸态力学性能的影响与其铸态组织中Mg₅(Gd,Y,Zn)、Mg₂₄(Y,Gd,Zn)₅和Mg₁₂(Gd,Y)Zn第二相及其数量有关。     

The microstructure, tensile properties, and creep behavior of as-cast Mg–(1–10)%Sn alloys

The microstructure, tensile properties, and creep behavior of Mg–(1–10)wt%Sn alloys were studied in this paper. The microstructure of as-cast Mg–Sn alloys consisted of dendrite -Mg and second Mg₂Sn phases and the secondary dendrite arm spacing (DAS) of the -Mg phase was decreased with the increase of tin content. The micro-hardness of the alloys increased when tin content rises, while the greatest tensile and elongation were exhibited by Mg–5 wt%Sn. The indentation creep experiments were conducted at 150 °C for applied loads of 30 kg, it suggested that the indentation creep resistance of Mg–Sn alloys could be obviously improved with the increase of tin content, and Mg–10%Sn alloy had better indentation creep resistance than that of AE42.     

Selection of promising quaternary candidates from Mg–Mn–(Sc, Gd, Y, Zr) for development of creep-resistant magnesium alloys

Recently developed Mg–Mn–Sc alloys show a considerable increase of creep resistance at elevated temperature. The endeavor to further improve the properties and to reduce cost of high-price Sc metal initiated a search for additional alloying elements. Gd, Y and Zr were considered for this purpose. The aim is to achieve a large quantity of suitable precipitations to improve mechanical properties using a minimum of expensive alloy element addition. The huge amount of possibilities of combining the elements Mg–Mn–(Sc, Gd, Y, Zr) and the time and cost effort of technological experiments require a preselecting of systems and alloy compositions. Thermodynamic phase diagram and phase amount calculations were performed to give hints for selecting promising candidates. A priority list of three quaternary systems is established: Mg–Mn–Gd–Sc, Mg–Mn–Sc–Y and Mg–Mn–Y–Zr, based on the classification of individual alloys. Most promising is MgMn1Gd5Sc0.8 (wt.%), but the alloys MgMn1Gd5Sc0.3 and MgMn1Y5Sc0.8 are also promising. The entire quaternary Mg–Mn–Y–Zr system is disqualified because of phase diagram features that are detrimental for the required microstructural engineering. The focused alloy development following this approach avoids a waste of time and effort.     

时效处理对Mg-6Al-1Nd-xGd合金组织及高温力学性能的影响

采用光学显微镜(OM)、扫描电镜(SEM)、EDS能谱分析等手段研究了Mg-6Al1Nd-xGd(0,0.5,1,1.5)合金的时效硬化行为及时效处理(T6)对合金高温拉伸性能的影响,结果表明:Gd元素的加入使Mg-6Al-1Nd合金的时效过程延长,随Gd含量的增多,合金在200℃时效时的硬度峰值从28 h延迟到36 h附近,且Gd含量为1%时合金峰值硬度最大,达到HV51.1。T6处理后,合金的强度及塑性都有所提升,Mg-6Al-1Nd-1Gd合金在200℃的抗拉强度为146 MPa,伸长率为22.3%,较铸态分别提高27.5%和29.7%,合金表现出良好的综合高温拉伸性能。     

The effect of Nd content on the electrochemical properties of low-Co La–Mg–Ni-based hydrogen storage alloys

The low-Co content La_0.80−xNd_xMg_0.20Ni_3.20Co_0.20Al_0.20 (x = 0.20, 0.30, 0.40, 0.50, 0.60) alloys were prepared by inductive melting and the effect of Nd content on the electrochemical properties was investigated. XRD shows that the alloys consist mainly of LaNi₅ phase, La₂Ni₇ phase and minor LaNi₃ phase. The electrochemical P–C–T test shows hydrogen storage capacity increases first and then decreases with increasing x, which is also testified by the electrochemical measurement that the maximum discharge capacity increases from 290 mAh/g (x = 0.20) to 374 mAh/g (x = 0.30), and then decreases to 338 mAh/g (x = 0.60). The electrochemical kinetics test shows exchange current density I₀ increases with x increasing from 0.20 to 0.50 followed by a decrease for x = 0.60, and hydrogen diffusion coefficient D increases with increasing x. Accordingly high rate dischargeability increases with a slight decrease at x = 0.60 and the low temperature dischargeability increases with increase in Nd content. When x is 0.50, the alloy exhibits a better cycling stability.     

Electrochemical behaviour assessment of novel Mg-rich Mg–Al–RE alloys (RE = Ce, Er)

The electrochemical behaviour of new Mg–Al–RE (RE = Ce, Er) alloys AE91 was investigated in 0.01 M NaCl electrolyte (pH = 12) and compared with that of the most commonly used Mg alloy in the automotive field, the AZ91D. Scanning electron microscopy and quantitative electron probe microanalysis were used to characterize the samples prior to the electrochemical tests. AE91 alloys showed very similar microstructures characterized by a three-phase appearance: a Mg-based solid solution containing only Al and two intermetallic phases γ(Mg₁₇Al₁₂) and (Al_1 − xMg_x)₃Ce or (Al_1 − xMg_x)₂Er. Free corrosion potential measurements, potentiodynamic polarization curves and electrochemical impedance spectroscopy revealed improved passivity behaviour compared to AZ91D alloy. The apparent presence of trace amounts of rare earth oxides in the passive film is presumed to be the reason for the enhanced corrosion resistance of AE91 alloys in the aggressive environment considered.     

Characterization and hydrogenation of CeNi5−xCrx (x = 0, 1, 2) alloys

The effect of partial substitution of Ni by Cr in CeNi₅ intermetallic compound has been studied by pressure–composition isotherm measurements for different temperatures. The samples were prepared of high purity materials using the standard arc melting technique in argon atmosphere. The structure and the elemental composition of different alloys have been investigated by means of XRD, SEM and EDX techniques. The unit cell volume of the alloy was found to increase with increasing Cr content. In order to calculate the hydrogen storage capacity pressure–composition isotherm has been investigated for CeNi_5−xCr_x (x = 1, 2) alloys in the temperature and pressure ranges of 293 ≤ T ≤ 333 K and 0.5 ≤ P ≤ 35 bar, respectively. The P–C–T isotherm for different alloys clearly shows the presence of three regions ,  + β and β. The enthalpy and entropy for the systems has also been calculated using Van’t Hoff plot. The variation of enthalpy and entropy with hydrogen content has also been studied.     

Evolutions of Microstructure and Mechanical Properties in Mg-5Li-1Zn-0.5Ag-0.5Zr-xGd Alloy

The evolution of the microstructure and mechanical properties of alloy system with nominally composition Mg-5Li-1Zn-0.5Ag-0.5Zr-xGd (x = 0, 1.2, 2.4, 3.6, 4.8, 6) is evaluated based on computational phase diagram and corresponding experimental studies. The results show that grains are significantly refined with the increase of Gd content. The main phases of as-cast alloys are α-Mg, β-Li, AgLi₂Mg, and Mg₃Gd. With the increase of Gd content, the amounts of Mg₃Gd phase and β-Li phase have been increased. When the Gd content exceeds 3.6 wt%, Mg₃Gd phase precipitates in a form of the network at the grain boundaries. The precipitation of β-Li can be attributed to the competitive dissolution of Zn, Gd, and Li in Mg. Meanwhile, γ″ is formed after the addition of Gd, which grows and transforms into γ′ with the increase of Gd content. In solidification process, stacking faults are formed by solid transformation of partial α-Mg and Mg₃Gd. Eventually, with the synergistic effect of Mg₃Gd, β-Li, and γ″ (or γ′), as the Gd content increasing, the tensile strength of the alloy first increases, then decreases, and the elongation decreases. When the content of Gd is 4.8 wt%, the ultimate tensile strength and yield strength reach the maximum values of 227 MPa and 139 MPa, and the elongation is 18.1%, respectively.     

Effects of Ce and Mm additions on the glass forming ability of Al–Ni–Si metallic glass alloys

The effects of Ce and Mm contents on the glass forming ability (GFA) of melt-quenched Al_89−xNi₈Ce_xSi₃ and Al_89−xNi₈Mm_xSi₃ (x = 0, 1, 3, 5, 7 at.%) alloys have been systematically investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). According to the XRD and DSC results, both Ce and Mm elements can enhance the GFA and thermal stability of the Al–Ni–Si alloys. Moreover, only the x = 5 and x = 7 alloys are totally amorphous in both systems quenched at the wheel speed of 36.6 m/s. Compared with amorphous Al₈₄Ni₈Ce₅Si₃ alloy at different cooling rates, amorphous Al₈₄Ni₈Mm₅Si₃ alloy has higher GFA which is considered to have relation to the different atomic structure of the amorphous alloy.     

Effects of cold roll and heat treatments on anomalous thermal behavior in Fe100−xAlx(x = 5–30) alloys

DSC measurements were carried out for various Fe_100−xAl_x(x = 5–30 at%) alloys to clear the effects of cold roll and quenching rate from 1173 K. In the case of cold roll free specimens, an exothermic peak was observed at around 530–560 K in quenched specimens and no peaks in slowly cooled specimens. The peak temperature and its exothermic heat depended on the alloy composition. The maximum exothermic heat was obtained for a 25 at% Al alloy and its value were about 1200 J/mol. The peak in a 5 at% Al alloy was remained as a future work. The exothermic heat was affected by the quenching temperature in alloys above 15 at% Al. The peak temperature was decreased by decreasing the quenching temperature. In a 15 at% Al alloy, the peak became negligibly small by quenching from 1023 K. The activation energies in cold roll free specimens were evaluated from the Kissinger analysis and they were 134, 108, 133 and 110 kJ/mol for 15 at% Al, 20 at% Al, 25 at% Al and 30 at% Al alloys, respectively. On the other hand, cold rolled specimens showed an exothermic peak at around 470 K, independently of the cooling rate. Their exothermic heats and temperatures were comparable order to those of furnace cooled and water quenched specimens. The present results suggested that origin of exothermic peaks of all alloys were same in nature and atomic ordering may be related to the exothermic behavior at relatively low temperatures.     

钇含量对Mg-xY-1.5LPC-0.4Zr合金的时效硬化和力学性能的影响(英文)

研究钇含量对Mg-xY-1.5LPC-0.4Zr镁合金的时效硬化、显微组织和力学性能的影响(其中LPC代表富镧混合稀土金属)。当将Y加入Mg-1.5LPC-0.4Zr时,随着Y含量的增加,合金的时效硬化反应相应增强,晶粒尺寸变小,强度增加。当将Y添加到Mg-1.5LPC-0.4Zr合金中时,时效析出相发生改变,由Mg-LPC基合金的稳态Mg12RE相转变为Mg-Y基合金的亚稳态β′相,且随着Y含量的增加,β′相的数量也相应增多。在合金晶界上还发现了稳态立方形的β-Mg24Y5相。对于Mg-Y-LPC-Zr合金,拉伸性能的改善主要归功于均匀、弥散分布的β′相,在晶界上的β-Mg24Y5相对合金的晶界也有明显的强化作用。当Y含量达到6%时,合金的拉伸强度最大,合金在室温和250°C的抗拉强度分别是250 MPa和210 MPa。     

微量Sc对Mg-7Gd-3Y合金组织及力学性能的影响

利用光学显微镜、扫描电镜和XRD,分析研究了微量Sc对Mg-7Gd-3Y铸态合金组织及其室温和200℃力学性能的影响.结果表明,在合金中加入0.5%的Sc,促进了Mg24(Y,Gd)5和Mg5(Gd,Y)相的析出,降低Gd在Mg24(Y,Gd)5相中的相对含量,合金的室温和200℃时的抗拉强度分别提高了25 MPa和18 MPa;屈服强度分别提高了28 MPa和22MPa;伸长率分别提高了18.3%和37.8%.     

Cycle stabilities of the La0.7Mg0.3Ni2.55−xCo0.45Mx (M = Fe, Mn, Al; x = 0, 0.1) electrode alloys prepared by casting and rapid quenching

In order to improve the cycle stability of La–Mg–Ni system (PuNi₃-type) hydrogen storage alloy, Ni in the alloy was partly substituted by Fe, Mn and Al, and the electrode alloys La_0.7Mg_0.3Ni_2.55−xCo_0.45M_x (M = Fe, Mn, Al; x = 0, 0.1) were prepared by casting and rapid quenching. The effects of the substitution of Fe, Mn and Al for Ni and rapid quenching on the microstructures and electrochemical properties of the alloys were investigated in detail. The results obtained by XRD, SEM and TEM indicate that element substitution has no influence on the phase compositions of the alloys, but it changes the phase abundances of the alloys. Particularly, the substitution of Al and Mn obviously raises the amount of the LaNi₂ phase. The substitution of Al and Fe leads to a significant refinement of the as-quenched alloy's grains. The substitution of Al strongly restrains the formation of an amorphous in the as-quenched alloy, but the substitution of Fe is quite helpful for the formation of an amorphous phase. The effects of the substitution of Fe, Mn and Al on the cycle stabilities of the as-cast and quenched alloys are different. The positive influence of the substitution elements on the cycle stabilities of the as-cast alloys is in proper order Al > Fe > Mn, and for as-quenched alloys, the order is Fe > Al > Mn. Rapid quenching engenders an inappreciable influence on the phase composition, but it markedly enhances the cycle stabilities of the alloys.     

Influence of deformation on precipitation process in Mg–15 wt.%Gd alloy

Mg–Gd is a promising light hardenable alloy with a high creep resistance at elevated temperatures. The supersaturated solid solution of Gd in Mg decomposes in a sequence of the following phases: β″ (D0₁₉) → β′ (c-base centered orthorhombic-c-bco) → β (fcc) stable. Formation of the metastable β′ phase causes a strong hardening. Dislocations facilitate nucleation of precipitates. Dislocation density is, therefore, an important parameter which influences the precipitation process. This effect was examined in the present work by comparison the decomposition sequences in Mg–15 wt.%Gd alloy cold rolled to various thickness reductions. It was found that precipitation of the β′ phase starts at lower temperatures in the cold rolled specimens.     

Hydriding combustion synthesis of Mg–CaNi5 composites

The composites of Mg–x wt.% CaNi₅ (x = 20, 30 and 50) were prepared by hydriding combustion synthesis (HCS) and the phase evolution during HCS as well as the hydriding properties of the products were investigated. It was found that Mg reacted with CaNi₅ forming Mg₂Ni and Ca during the heating period of HCS. Afterwards, the resultant Mg₂Ni and Ca as well as the remnant Mg reacted with hydrogen during the cooling period. The lower platform in the P–C isotherms corresponds to the hydriding of Mg, and the higher one corresponds to Mg₂Ni. With the increase of the content of CaNi₅ from 20 to 50 wt.%, the hydrogen content of the HCS products increases at first and then decreases. The Mg–30 wt.% CaNi₅ composite has the maximum hydrogen capacity of 4.74 wt.%, and it can absorb 3.51 wt.% of hydrogen in the first hydriding process without activation.     

CrFeCoNiB0.05Tix高熵合金的显微组织和力学性能

采用机械合金化和放电等离子烧结工艺制备了CrFeCoNiB_0.05Ti_x(x=0.2、0.4、0.6、0.8、1.0)高熵合金材料,通过X射线衍射分析、扫描电镜观察和能谱分析以及维氏硬度测试和压缩强度测试等,研究了Ti含量对高熵合金微观组织和力学性能的影响。结果表明,CrFeCoNiB_0.05Ti_x(x=0.2、0.4、0.6、0.8、1.0)高熵合金由FCC、BCC和α相组成。当x=1.0时,合金由BCC结构转向HCP结构并析出新相Laves相,其具有最高硬度416.54 HV0.2。当x=0.8时,合金达到最大抗压强度586.3 MPa。     

Ir–Nb–Si ternary refractory superalloys with a three-phase fcc/L12/silicide structure for high-temperature applications

Ir–Nb binary alloys doped with silicon have been used in this work to attain a three-phase fcc/L1₂/silicide structure. Typical Ir–Nb binary alloys, including a hypoeutectic Ir–10Nb, an eutectic Ir–16Nb, and a hypereutectic Ir–25Nb, were used as alloy bases, and Ir was further replaced by 5 at% Si. With the addition of Si, the microstructures of the Ir–(10–25)Nb–5Si ternary alloys contained three phases: fcc, L1₂, and compounds of Ir and Si (referred to silicide hereafter). Compressive tests from room temperature to 1500 °C showed that the Ir–10Nb–5Si alloy, with a predominant fcc microstructure, always had the highest deformation hardening rate, strength, and ductility; on the other hand, the Ir–25Nb–5Si alloy showed the worst performance. With the silicide in the microstructures, the damage sustained by the Ir–Nb–Si alloys at both room and high temperatures was dominated by interface debonding, which occurred between the fcc and the silicide or the L1₂ and the silicide. It is believed that the interface debonding is an instinct failure mechanism of Ir-based alloys. Additionally, a strong solid-solution hardening effect of Si acting on the fcc phase was found to occur without loss of ductility. A principle in the composition and microstructure design is proposed in this paper for further development of Ir-based alloys with Si addition. This principle is to saturate the fcc phase with Si and other alloying elements so as to achieve maximum solid-solution hardening and tie-in fine silicides homogenously distributed within the fcc by elimination of the grain boundary concentration of silicides.