Effect of heat treatment on microstructures and mechanical properties of sand-cast Mg-10Gd-3Y-0.5Zr magnesium alloy

The microstructure, mechanical properties and fracture behavior of sand-cast Mg-10Gd-3Y-0.5Zr alloy （mass fraction,%） under T6 condition （air cooling after solid solution and then aging heat treatment） were investigated. The optimum T6 heat treatments for sand-cast Mg-10Gd-3Y-0.5Zr alloy are （525 ℃, 12 h＋225 ℃, 14 h） and （525 ℃, 12 h＋250 ℃, 12 h） according to age hardening curve and mechanical properties, respectively. The ultimate tensile strength, yield strength and elongation of the Mg-10Gd-3Y-0.5Zr alloy treated by the two optimum T6 processes are 339.9 MPa, 251.6 MPa, 1.5%and 359.6 MPa, 247.3 MPa, 2.7%, respectively. The tensile fracture mode of peak-aged Mg-10Gd-3Y-0.5Zr alloy is transgranular quasi-cleavage fracture.

Influence of cooling rate on solidification behavior of sand-cast Mg-10Gd-3Y-0.4Zr alloy

The effect of the cooling rate ranging from 1.4 °C/s to 3.5 °C/s on the solidification behavior of the sand-cast Mg?10Gd?3Y?0.4Zr alloy was studied by computer aided cooling curve analysis (CA-CCA). With the increase in cooling rate, the nucleation temperature (Tα,N) increases from 634.8 °C to 636.3 °C, the minimum temperature (Tα,Min) decreases from 631.9 °C to 630.7 °C, the nucleation undercooling (ΔTN) increases from 2.9 °C to 5.6 °C, the beginning temperature of the eutectic reaction (Teut,N) increases, the time of the eutectic reaction shortens, solidus temperature decreases from 546.0 °C to 541.4 °C, and solidification temperature range (ΔTS) increases by 6.1 °C. The increased nucleation rate (N&) is supposed to be the main reason for the increased?TN. Increased value (Teut,N?Teut,G) and shortened time of the eutectic reaction cause the change in the volume fraction and morphology of the second phase.

时效时间对Mg-10Gd-3Y-0.5Zr镁合金力学性能的影响

选取了在工程实践中有广泛应用的Mg-10Gd-3Y-0.5Zr镁合金,在225℃下对合金进行了时效处理,研究了时效时间对合金力学性能的影响。结果表明：在6、8、10、12、14、16 h时效时效范围,合金的硬度、抗拉强度和屈服强度在12 h处理之后达到最大值。

Low temperature mechanical properties of as-extruded Mg-10Gd-3Y-0.5Zr magnesium alloy

Influence of multi-cycle cryogenic treatment and tensile temperature on microstructure, mechanical properties and fracture mechanism of as-extruded Mg-10Gd-3Y-0.5Zr magnesium alloy was investigated. The results show that there have no significant changes in tensile properties of the tested alloy after 10 d in liquid nitrogen immersion or 10 cycles of high-low temperature treatment at all test temperatures. The room temperature ultimate tensile strength increases from 398 MPa to 417 MPa after 20 cycles of high-low temperature treatments. Compared with the room temperature, the tested alloys exhibit higher tensile properties at low temperatures. At -196 °C, the yield strength and ultimate tensile strength of the as-extruded-T5 Mg-10Gd-3Y-0.5Zr alloy are 349 MPa and 506 MPa, respectively, increasing by about 18% and 27%, respectively. The transgranular cleavage fracture mechanism is observed at room temperature, while at low temperatures both ductile fracture and cleavage fracture behaviors coexist.

热轧及退火处理对Mg-10Gd-3Y-0.3Zr合金显微组织及抗压强度的影响

为了探究Mg-10Gd-3Y-0.3Zr高温热轧及退火处理对合金显微组织及抗压强度的影响,文中利用光学金相显微镜观察高温轧制板材的显微组织,分析晶粒尺寸大小和动态再结晶程度.研究结果表明:随着变形量增加,晶粒尺寸显著减小,当下压量为60%时晶粒尺寸大小均匀,约为60μm,孪晶密度高;下压量达到60%之前,晶粒粗大,孪晶密度低,超过60%之后,再结晶晶粒逐步取代变形晶粒,晶粒尺寸减小,孪晶几乎消失.退火处理能够显著提高轧制板材的抗压强度,其中175℃×3h退火处理后强化效果明显,抗压强度达到424MPa,退火温度超过175℃后,强化效果开始减弱.

不同热处理下Mg-10Gd-3Y-0.5Zr合金拉伸力学性能统计分析

在不同热处理工艺参数下研究了Mg-10Gd-3Y-0.5Zr合金力学性能的统计学变化规律。结果表明,Mg-10Gd-3Y-0.5Zr合金在高温服役时拉伸力学性能的稳定性变差。525℃×12 h+225℃×14 h处理工艺比525℃×12 h+250℃×12 h处理工艺更能使合金满足性能要求。

Microstructure evolution of semi-solid Mg-10Gd-3Y-0.5Zr alloy during isothermal heat treatment

In this study,the microstructure evolution of semi-solid Mg-10Gd-3Y-0.5Zr alloy during isothermal heat treatment has been investigated.The results show that primary particles coarsen continuously during the holding.Coarsening rate decreases with the increase of isothermal temperature.When isothermal temperature increases from 600℃ to 620℃,the dominant mechanism for coarsening changes from particle coalescence to Ostwald ripening.Equiaxed as-cast microstructure is beneficial to the semi-solid microstructure after isothermal heat treatment,which brings about the refinement and spheroidization of primary particles,and shortening of holding time.Significant modification of second phases can also be achieved after isothermal heat treatment,due to its unique solidification process.The optimum processing parameters for Mg-10Gd-3Y-0.5Zr alloy in isothermal heat treatment are isothermal temperature of 610℃-620℃ and holding time of 20-40 min.

挤压态Mg-10Gd-3Y合金动态维氏硬度的研究

研究了挤压态Mg-10Gd-3Y合金的动态维氏硬度,分析了压痕周围的微观组织,并与静态维氏硬度值进行对比。结果表明:挤压态Mg-10Gd-3Y合金动态维氏硬度高于静态维氏硬度,表现出明显的应变率正效应,并且其动态维氏硬度随应变量的增加出现一个极大值,压痕周围出现孪晶组织,随应变量的增加孪晶数量减少。

Creep behavior of ageing hardened Mg-10Gd-3Y alloy

Tensile creep behaviors of the ageing hardened Mg-10Gd-3Y alloy(referred to GW103)were investigated at temperatures up to 300℃.The extruded-T5 specimen exhibited high creep resistance,i.e.the low steady-state creep rate and long creep rupture time,while the better creep properties were observed in the cast-T6 one.The low steady-state creep rate of 1.71×10- 9s -1is obtained at 200℃and 80 MPa for the extruded-T5 GW103 alloy.In addition,the microstructure development of GW103-T5 alloy was also examined after creep exposure at different temperatures.On the other hand,the stress exponent and activation energy were studied in the temperature range of 200-300℃for the extruded-T5 specimens,and the creep mechanism was also discussed.

Effect of different casting methods on microstructure and mechanical properties of Mg-10Gd-3Y-0.6Zr alloy

The effects of two different casting methods on the microstructures and mechanical properties of as-cast and T6-cast states of Mg-10Gd-3Y-0.6Zr alloy were studied by using metal mold casting and squeeze casting.The results show that the microstructure of Mg-10Gd-3Y-0.6Zr alloy is mainly composed ofα-Mg primary phase and Mg 24(Gd,Y)5 eutectic phase.The squeeze cast grains are small with a dendrite like morphology,and the tensile strength of the alloy in T6 state can reach 285 MPa.While the metal grains are coarse,the eutectic phases are distributed in the grain boundary,and the tensile strength of the alloy in T6 state is only 250 MPa.

Effect of heat treatment on corrosion behavior of low pressure sand cast Mg-10Gd-3Y-0.5Zr alloys

The corrosion behaviors of low-pressure sand cast Mg-10Gd-3Y-0.5Zr(wt.%)alloys in as-cast,solution treated(T4)and aged(T6)conditions were studied by means of immersion test and electrochemical measurements in 5wt.%NaCl solution saturated with Mg(OH)_(2).It was observed that the corrosion rate in the T4 condition was lower than that of the as-cast and T6 conditions by both sand casting and permanent mold casting with the same order of as-cast>T6>T4;while the corrosion resistance of the permanent mold casting is superior to the sand casting.The morphologies of the corrosion products are similar porous structures consisting of tiny erect flakes perpendicular to the corroded surface of the alloy,irrespective of the heat treatment conditions.Especially,the corrosion film in T4 condition is more compact than that in the other two conditions.In addition,the severer corrosion happening to the as-cast condition is correlated with the galvanic corrosion between the matrix and the eutectic compounds;while improved corrosion resistance for the T4 and T6 conditions is ascribed to the dissolution of the secondary eutectic compounds.The measured corrosion current densities of Mg-10Gd-3Y-0.5Zr alloys in as-cast,T4,and T6 conditions are 36μA·cm^(-2),10μA·cm^(-2),and 33μA·cm^(-2),respectively.The proposed equivalent circuit[Rs(CPE_(1)R_(t)R_(f)CPE_(2))]by Zview software matches well with the tested electrochemical impedance spectra(EIS)data.

Mg-10Gd-3Y-Zr镁合金的热处理工艺

针对Mg-10Gd-3Y-Zr高强韧镁合金,采用相图热力学计算方法（CALPHAD）、相平衡热力学计算软件（PANDAT）,根据扎克哈罗夫经验公式等初步确定合金的固溶、时效温度范围,研究了合金时效硬化曲线、时效温度及时间对合金力学性能及显微组织的影响。结果表明：Mg-10Gd-3Y-0.6Zr镁合金的最佳热处理工艺为225℃×20 h直接时效,此时组织中强化相析出完毕,且均匀分布,对应的抗拉强度为490 MPa,断后伸长率为11%,拉伸断裂为韧性断裂。

时效处理对Mg-10Gd-3Y-0.5Zr合金组织和性能的影响

通过X射线衍射、光学显微镜、扫描电镜、极化曲线、阻抗谱等对铸态、时效态Mg-10Gd-3Y-0.5Zr镁合金的显微组织、力学性能及耐蚀性能进行了研究。结果表明：铸态Mg-10Gd-3Y-0.5Zr合金由α-Mg、树枝状相Mg（24）（Gd,Y）5及颗粒状相Mg5（Gd,Y）组成,合金经515℃×8 h固溶＋225℃×36 h时效处理后,树枝状Mg（24）（Gd,Y）5转变为大量薄片状及细长条状的Mg5（Gd,Y）相。室温力学性能测试结果表明,经时效处理后的合金显微硬度增加27.8%,达到101.97 HV0.1,抗拉强度增加52.5%,达到305 MPa,规定塑性延伸强度增加121.3%,达到239 MPa。极化曲线及阻抗谱表明,时效处理后合金的腐蚀电流密度减小,容抗弧半径增大,电荷传递电阻增大,合金耐腐蚀性能增强。

电磁搅拌辅助流变挤压铸造Mg-6Gd-3Y镁合金的显微组织和力学性能

通过电磁搅拌制备了Mg-6Gd-3Y(VW63)稀土镁合金半固态浆料,研究了电磁搅拌工艺参数对合金半固态浆料组织的影响,探讨了半固态浆料初生相的形成机理;并对半固态浆料进行了流变挤压铸造成形,研究了压力对流变挤压铸造合金组织和性能的影响。结果表明:电磁搅拌制备VW63合金半固态浆料的最佳工艺参数为浇注温度620℃、搅拌电压350 V、搅拌频率15 Hz,VW63合金半固态浆料的初生相形成机理为枝晶熔断。当压力为120 MPa时,流变挤压铸造合金有着更好的组织与性能,平均晶粒尺寸43.57μm,其屈服强度、抗拉强度和伸长率分别为119 MPa、156 MPa和6.9%,均优于重力铸造和液态挤压铸造合金。

离心铸造和热轧对Mg-10Gd-3Y-1Sn合金组织和机械性能的影响

采用离心铸造及热轧工艺制备Mg-10Gd-3Y-1Sn合金,利用X射线衍射、光学显微镜、扫描电子显微镜和拉伸试验对该合金的组织和力学性能进行了研究。结果表明:离心铸造Mg-10Gd-3Y-1Sn合金由α-Mg、Mg_(24)(Gd,Y)5、Mg_(2)(Sn,Y)3Gd2和Mg_(3)(Gd,Y)相组成。随着离心半径和离心转速的增大,Mg-10Gd-3Y-1Sn合金的晶粒尺寸逐渐减小,抗拉伸强度逐渐增大。在700 r/min下制备的热轧试样在室温下极限抗拉伸强度为304 MPa,在300℃下极限抗拉伸强度为296 MPa。Mg_(24)(Gd,Y)_(5)、Mg_(2)(Sn,Y)3Gd2和Mg_(3)(Gd,Y)相具有优异的热稳定性,因而Mg-10Gd-3Y-1Sn合金具有优异的高温抗拉伸强度。

Mg-15Gd-3Y挤压合金的时效强化

研究时效处理对挤压态Mg-15Gd-3Y合金显微硬度及力学性能的影响,采用透射电镜对时效析出相的形态进行分析。结果表明:225℃、14h时效时合金获得最高硬度,为135HV,此时,晶内弥散析出许多纳米β′亚稳相,且与基体有着良好的共格匹配性,使合金获得较高强度和硬度,同时塑性降低;在225℃、70h过时效作用下,纳米β′亚稳相原位长大成β稳定相,且与基体失去共格匹配性,降低合金的强度和硬度,同时塑性有所回升;合金的最高强度出现在200℃、100h时效处,此时,抗拉强度达到446.67MPa,合金中的β′亚稳相呈蠕虫状,沿亚晶界分布,有效地阻碍亚晶界和位错的运动,从而强化合金。

Influence of casting module on corrosion behavior of Mg-11Gd-3Y alloy

The influences of two kinds of casting modules of metal casting （MC） and expandable pattern casting （EPC） on the corrosion behavior of Mg-11Gd-3Y alloy were studied by electrochemical measurements, scanning electron microscopy （SEM） observation, X-ray diffractometry （XRD） and X-ray photoelectron spectroscopy （XPS） analysis. It is found that the quantity of the Mg 24 （Gd, Y） 5 phase in MC is more than that in EPC due to the cooling rate. There is more alloying element dissolved in the matrix compared with MC. For EPC, the galvanic corrosion effect between the matrix and the Mg 24 （Gd, Y） 5 phase decreases and the corrosion resistance increases compared with the MC. The chief corrosion mode for Mg-11Gd-3Y alloy is pitting corrosion because most of the alloying elements are transformed into intermetallic phases. The average corrosion rate of the MC alloy in the immersion test is five times higher than that of EPC alloy and yttrium is present in the product film, which will provide increased protection for Mg-11Gd-3Y alloy. The electrochemical measurements and immersion test show that the EPC process increases the corrosion resistance compared with the MC Mg-11Gd-3Y alloy.

均匀化对Mg-9Gd-3Y镁合金热压缩变形的影响

通过对比不同状态下变形镁合金GW93（Mg-9Gd-3Y-0．5Zn-0．5Zr）热压缩变形下组织与性能的变化关系，从而了解其变化的机理．实验研究铸态和均匀化态下该合金在350～450℃、0．001～0．1s^-1、最大应变70％条件下的压缩行为．分析了两种状态下的应力应变曲线和显微组织演变过程．结果表明：铸态合金显微组织由α—Mg和Mg5Gd，Mg24Y5组成，经过均匀化处理后，α-Mg等轴晶的晶尺寸变大，晶界处的Mg5Gd，Mg24Y5相大部分溶解于基体中，只剩少量的非平衡相未溶解；随应变速率的增加，两种状态下的流变应力均先增加，达到峰值后降低，最后保持不变．两者相比较，均匀化态具有更高的峰值流变应力、稳态应力和发生动态再结晶的临界应变．铸态试样热压缩后基本上发生了完全再结晶，晶粒尺寸明显细化；而均匀化试样热压缩后只在晶界处发生了部分动态再结晶．

Al对Mg-5Gd-3Y铸态合金组织和性能的影响

通过X射线衍射、扫描电镜、金相组织分析和拉伸性能测试等方法,研究了Al对Mg-5Gd-3Y铸态合金组织和性能的影响。结果表明,Mg-5Gd-3Y铸态合金的组织由α-Mg基体和共晶相Mg5Gd和Mg24Y5组成。加入Al元素后,有新相Al2Gd、Al2Y析出,对合金的晶粒起到细化作用,有效提高了Mg-5Gd-3Y铸态合金的力学性能。

微量Sb对Mg-7Gd-3Y铸态合金组织及力学性能的影响

利用光学显微镜、扫描电镜和X射线衍射仪,分析研究了微量Sb对Mg-7Gd-3Y铸态合金组织及其室温和高温力学性能的影响。结果表明,在合金中加入1%的Sb,促进了新的颗粒相Gd5Sb3和Y3Sb产生,使晶粒细化;合金的室温和高温抗拉强度分别提高了3.3%和5.3%;屈服强度分别提高了12.8%和17.0%;伸长率分别降低了15.8%和12.3%。