Effect of Ca on microstructure and high temperature creep properties of AM60-1Ce alloy

A series of AM60-1Ce-xCa(x=0, 0.5, 1.5, 2.5) magnesium alloys were prepared by gravity casting method and analyzed by means of XRD, DSC and SEM. The effects of Ca on normal temperature mechanical properties and high temperature creep behavior of alloys were characterized by tensile and constant creep test.Microstructure analysis indicated that Ca was preferentially combined with Al in the alloy to form the high melting point Al_2Ca phase at the grain boundary. The addition of Ca can refine the crystal grains and reduces the content of β-Mg_(17)Al_(12) phase. With the increase of Ca content in the alloy, Al_2Ca phases at the grain boundary gradually changed to the network of lamellar structure, and replaced the β-Mg_(17)Al_(12) phase as the main strengthening phase gradually. The creep resistance of the alloy continuously increases because the high-temperature stable phase Al_2Ca firmly nailed at grain boundaries impedes the sliding of grain boundaries. However, when the addition of Ca was more than 1.5%, mechanical properties of the alloy started to decrease, which was probably due to the large amount of irregularly shaped Al_2Ca phases at the grain boundary. Experimental results show that the optimal addition amount of Ca is 1.5 wt.%.     

Microstructure and mechanical properties of Mg-Zn-Gd-based alloys strengthened with quasicrystal and Laves phase

One kind of Mg3.5Zn0.6Gd-based alloy strengthened with quasicrystals was designed, and the effect of alloying elements on microstructure and mechanical properties of as-cast Mg-Zn-Gd alloy at room temperature and elevated temperatures were studied. The results indicate that MgZnCu Laves phase, which coexists with quasicrystal at grain boundary, emerges with the addition of copper element in Mg-Zn-Gd alloy. The strength of alloys exhibits the parabola curve with the increase of copper content. The alloy with 1.5% （mole fraction） Cu shows better mechanical properties at room temperature： tensile strength 176 MPa, yield strength 176 MPa and elongation 6.5%. The existence of MgZnCu Laves phase can effectively improve the heat resistance and elevated temperature properties of the alloy. The alloy with 1.5% Cu has better mechanical properties at 200℃ ： tensile strength 130 MPa and elongation 18.5%. The creep test of the alloys at 200℃ and 50 MPa for 102 h indicates that Mg3.5Zn0.6Gd alloy reinforced with quasicrystal has better creep properties than AE42, which can be further improved with the introduction of Laves phase in the alloy.     

Mechanical properties and microstructures of hot extruded AE42 alloy with addition of zinc

Mechanical properties and microstructures of AE42 magnesium alloy with addition of Zn and subjected to hot extrusion at 370 ℃ and an extrusion ratio of 8：1 were investigated. The results show that for the AE42 alloy, the addition of Zn can obviously improve its elongation as well as the ultimate tensile and yield strengths below 150 ℃. The addition of Zn can refine the mierostructure of the AE42 alloy, and result in the precipitation of Mg17Al12 and MgZn2 phases. Due to the addition of Zn to the AE42 alloy, the amount of Al11RE3 phase decreases, while the Al11RE3 phase becomes short rod-shaped from acicular and block, and distributes along the grain boundaries, which will have a stronger effect on the tensile properties of the alloy at elevated temperature. In addition, with the presence of MgZn2 precipitates, the sliding of grain boundaries is restrained and the strength of the alloy gets enhanced.     

Effect of rare earths on phase transformation in as-cast ZA27 alloy dur-ing compressive creep

The effect of the mixed rare earths of Ce on the phase transformation in as cast ZA27 alloy during compressive creep was investigated under 37 MPa and at 160℃ by X-ray diffraction technique and SEM. The results showed that the as cast microstructure of ZA27-RE alloy consisted of a dendritic Al-rich a' surrounded by Zn-rich β' phase, interdendritic ε phase and Zn-rich η phase together with a complex Z phase which was a complex constitute compound, (RE,Cu)Al5Zn16, dispersed in crystal interfaces or branch crystal interfaces and stable during compressive creep test at 160'C. The phase transformations of ZA27-RE alloy, decomposition of β' phase and four transformation, were delayed by the addition of rare earths, also the lamellar structure and the spheroidized structure in ZA27-RE alloy were finer than in ZA27 alloy during compressive creep test at 160℃ at the same creep time, and the compressive creep resistance of ZA27-RE alloy was higher than that of ZA27 alloy.     

Microstructure and creep properties of Mg-4Al-2Sn-1 （Ca,Sr） alloys

The effects of Ca and Sr addition on the microstructure and creep properties of Mg-4Al-2Sn alloys were examined.Tensile tests at 25℃ and 200℃ and creep tests at 150℃ and 200℃ were carried out to estimate the room temperature and high temperature mechanical properties of these alloys.The microstructure of the Mg-4Al-2Sn alloy showed dendriticα-Mg,Mg17Al12 and Mg2Sn phases.The latter two phases precipitated along the grain boundaries.The addition of Ca and Sr resulted in the formation of ternary CaMgSn and SrMgSn phases within the grain.The grain size was reduced slightly with the addition of Sr and Ca.The tensile strength was decreased by the addition of Ca and Sr at room temperature.However,the high temperature tensile strength was increased.The creep strength was improved by the addition of Ca and Sr.     

Creep resistance of as-cast Mg-5Al-5Ca-2Sn alloy

In the present study, creep properties of as-cast Mg-5Al-5Ca-2Sn(AXT552) alloy were investigated by means of a GWT304 creep testing machine at temperatures of 175 °C and 200 °C in the stress range of 35-90 MPa. Results show that creep rates increase with applied stress at an identical temperature. Creep strain at 100 hours is 0.0518% and 0.083% at creep conditions of 175°C/75 MPa and 200°C/60 MPa, respectively, which is comparable to MRI230 D and much lower than most of AX series alloys. By the observation and analysis for samples before and after creep tests using a Shimadzu XRD-7000 type X-ray diffractometer(XRD) and a Hitachi S-3400 N type scanning electron microscope(SEM), it was found that Al_2Ca(C15) phase precipitated out of C36 phase or matrix. The cavity formation and connection at the interface of soft matrix and hard intermetallics caused the propagation of cracking along the eutectic phase during creep process and dislocation accommodated grain/phase boundary sliding is expected to be the dominant creep mechanism.     

Microstructure and mechanical properties of a new Ti_2AlNbbased alloy after aging treatment

A new Ti_2AlNb-based alloy with the composition of Ti-22Al-25Nb-1Mo-1V-1Zr-0.2Si(at%) was fabricated in the sequence of arc melting, forging, solid solution and aging. Present paper focuses on the effects of aging temperatures(700, 750, 800, 850 and 900 ℃) on microstructure, room-temperature tensile properties as well as tensile properties at 650 ℃ and creep properties at650 ℃/150 MPa. It is obvious that the aging treatment promotes the precipitation of O lathes from B2 matrix.With aging temperature increasing, the content of O phase decreases and its size increases gradually, which leads to the decrease in tensile strengths both at room temperature and 650 ℃ and a little increase in ductility. For creep properties, the sample aged at 700 ℃ shows the best creep resistance due to its extremely high O phase content. Aging at 800 ℃ leads to the decrease in the content of O phase and softening of the alloy significantly. However, with aging temperature further increasing from 800 to 900 ℃,the precipitated O phase will be coarsened gradually, which strengthens the alloy again.     

Influence of microstructure stability on creep properties of single crystal nickel base superalloys

The influence of microstructure stability on the creep properties of single crystal nickel-based superalloys was investigated by means of the measurement of the creep curves and microstructure observation. Results show that the superalloy with 4%（mass fraction）W in Ni-AI-Cr-Ta-Co-5.5%Mo-x%W systems displays a better microstructure stability, but the β phase is precipitated in the superalloy with 6% W during aging. The strip-like μ phase is precipitated to be parallel or perpendicular to each other along the 〈110〉 orientation, and grown into the slice-like morphology along the { 111 } planes. The superalloy with 4%W displays a better creep rupture lifetime under the applied stress of 200 MPa at 982 ℃, but the creep lifetime of alloy is obviously decreased with the increase of the element W content up to 6%. The fact that the μ phase is precipitated in the superalloy with 6% W during applied stress and unstress aging results in the appearance of the poor regions for the refractory elements. This is one of the main reasons for reducing the creep rupture lifetime of the superalloy.     

Microstructure and mechanical properties of ZA62 based magnesium alloys with calcium addition

As-cast microstructure and mechanical properties of Mg-6Zn-2Al-0.3Mn (ZA62) alloys with calcium addition were investigated.The as-cast microstructure of the base alloy ZA62 consists of the α-Mg matrix and eutectic phase Mg51Zn20.The Mg51Zn20 eutectic was gradually replaced by MgZn phase and Mg32(Al,Zn)49 phase when calcium is added into the base alloy.Further addition of calcium leads to the increase of grain boundary phases and formation of a new quaternary Mg-Zn-Al-Ca eutectic compound.In comparison with the base alloy,the increase of calcium addition to the base alloy results in the reduction of both strength and ductility at ambient temperature,but increase at elevated temperatures due to the thermal stability of Ca-containing phases.At elevated temperatures,the creep resistance of ZA62 based alloys containing calcium is significantly higher than that of AZ91 which is the most commonly used magnesium alloy.     

Microstructure and properties of modified and conventional 718 alloys

Continuing the effort to redesign IN718 alloy in order to provide microstructural and mechanical stability beyond 650 ℃, IN718 alloy was modified by increasing the Al, P and 13 contents, and the microstructure and mechanical properties of the modified alloy were compared with those of the conventional alloy by SEM and TEM. The precipitation of the grain boundaries of the two alloys is different. The Cr-rich phase, Laves phase and α-Cr phase are easily observed in the modified alloy. The γ＂ and γ＇ phases in the modified alloy are precipitated in a ＂compact form＂. The tensile strengths of the modified alloy at room temperature and 680 ℃ are obviously higher than those of the conventional one. The impact energy of the modified alloy is only about half of that of the conventional alloy. Ageing at 680 ℃ up to 1000 h lowers the tensile properties and impact energy of both the conventional and modified 718 alloys, except increasing the ductility at 680 ℃. It is concluded that the modified alloy is more stable than the conventional one.     

AE42合金的抗压入蠕变性能

采用自制实验装置对Mg-Al-2RE(AE42)合金进行压入蠕变实验,利用带能谱(EDS)的扫描电镜(SEM)和X射线衍射(XRD)分析合金蠕变前后的组织和成分的演化.结果表明:随温度或应力的增加,AE42合金的压入蠕变速率和第一阶段的蠕变量逐渐增加;合金在压入状态下的蠕变应力指数和蠕变激活能的均值分别为3.06和72.4 kJ/mol;压入条件下AE42合金的稳态蠕变速率由晶界扩散主导的位错粘滞性滑移控制;铸态AE42合金由α-Mg基体、针状Al11La3和少量颗粒状Al2La组成;固溶处理8 h后,合金中的β-Mg17Al12相溶入α-Mg基体,合金的硬度上升;固溶24 h后,晶粒得到粗化,合金的硬度和抗蠕变性能均下降;固溶处理后再人工时效24 h,晶粒略有细化,但大量β-Mg17Al12相沿晶界不连续析出,合金的硬度和抗蠕变性能进一步下降.     

AZ81镁合金压入蠕变性能

采用自制装置对AZ81镁合金进行压入蠕变实验,通过建立稳态压入蠕变本构模型分析合金的蠕变机制,利用扫描电镜(SEM)和X射线衍射(XRD)等方法研究合金蠕变前后的组织和成分。结果表明:压铸AZ81合金在稳态蠕变阶段的应力指数n为2.08,蠕变激活能QC为87.26 kJ/mol;蠕变诱导β相首先由非连续方式析出,到达一定程度后连续析出;沿晶界析出的β相导致合金抗蠕变性能降低;蠕变温度越高,基体和析出相的晶粒尺寸越大;压铸AZ81合金的压入蠕变机制为晶界扩散主导的位错交滑移运动。     

Effect of Ca addition on microstructure and impression creep behavior of cast AZ61 magnesium alloy

Microstructure and creep properties of AZ61 alloy containing 1 and 3 wt.% Ca were investigated. The creep properties were examined using impression method under different stresses between 200 and 500 MPa at the temperature ranging from 423 to 491 K. The microstructure of AZ61 alloy contains α(Mg) matrix and Mg₁₇Al₁₂ intermetallic phases. It is shown that adding Ca to AZ61 alloy reduces the amount of Mg₁₇Al₁₂ phase via forming (Mg, Al)₂Ca phase; furthermore, increasing the Ca content to 3 wt.% leads to the formation of (Mg, Al)₂Ca phase, as well as the elimination of the Mg₁₇Al₁₂ phase. Creep properties of AZ61 alloy are improved with the Ca addition. The improvement in creep properties is attributed to the reduction in the amount of Mg₁₇Al₁₂ phase and the formation of (Mg, Al)₂Ca phase with high thermal stability. According to the obtained creep data, it is concluded that the pipe diffusion−climb controlled dislocation creep is the dominant creep mechanism and Ca addition has no influence on this mechanism. The effect of pre-deformation on the creep properties of AZ61+3%Ca alloy reveals that the creep resistance of the alloy depends on the continuity of (Mg, Al)₂Ca phase. It is decreased by reducing the phase continuity.     

Ca对Mg-5Al-1Bi合金显微组织和力学性能的影响

使用X射线衍射仪、金相显微镜、扫描电镜、能谱仪及力学性能测试等试验手段,研究了Ca含量对铸态Mg-5Al-1Bi镁合金显微组织和力学性能的影响。结果表明,铸态Mg-5Al-1Bi镁合金由α-Mg基体和β-Mg17Al12相组成,加入Ca后,合金晶粒细化,β-Mg17Al12相的数量减少,由连续变得较为分散。当Ca含量达到3%时,合金中生成新的第二相Al2Ca。高熔点相Al2Ca在高温条件下能钉扎晶界,阻碍晶界滑移,有利于提高合金的高温蠕变性能。合金硬度和屈服强度随着Ca含量的增加而提高,而抗拉强度和伸长率下降。     

Microstructure and creep behavior in AE42 magnesium die-casting alloy

The micro structural analysis of die-cast AE42 reveals a correlation between micro structure and creep strength. A lamellar-phase Al₁₁RE₃, which dominates the interdendritic microstructure of the alloy, partly decomposes above 150‡C into Al₂RE and Al (forming Mg₁₇Al₁₂). The increased solubility of aluminum in magnesium at higher temperatures may also promote the decomposition of Al₁₁RE₃. The creep strength decreases sharply with these phase changes. A mechanism for the decrease in creep strength of AE42 is proposed whereby the reduced presence of lamellar Al1₁₁RE₃ and/or the presence of Mg₁₇Al₁₂ contribute to the observed poor creep strength at higher temperatures.     

Microstructural evolution and creep deformation behavior of novel Ti−22Al−25Nb−1Mo−1V−1Zr−0.2Si (at.%) orthorhombic alloy

The microstructural evolution and creep deformation behavior which were adjusted and controlled by age treatment of a novel Ti?22Al?25Nb?1Mo?1V?1Zr?0.2Si (mole fraction, %) alloy, were investigated. The microstructures were obtained at different heat treatment temperatures and analyzed by SEM and TEM techniques. The creep behavior of the alloy was studied at 650 °C, 150 MPa for 100 h in air. The results showed that the initial microstructure mainly contained lath-like α₂, B2, and O phases. The precipitated O phase was sensitive to aging temperature. With the aging temperature increasing, the thickness of the precipitated O phase was also increased, and the length was shortened. The creep resistance of this alloy was relevant to the morphology and volume faction of the lamellar O phase. The increase of lamellar O phase in thickness was the main reason for the improved creep properties.