第二相对Mg-Gd-Y-Nd-Zr合金挤压板材冲击韧性的影响

Impact Toughness of Mg-Gd-Y-Nd-Zr Extruded Plate with Second Phase

通过固溶处理和时效处理,获得了具有不同第二相特征的样品。采用金相显微镜(OM)、扫描电镜(SEM)、电子背散射衍射(EBSD)、硬度测试和冲击韧性测试等方法,研究了第二相对Mg-Gd-Y-Nd-Zr合金挤压板材冲击韧性的影响,结果表明:挤压板材中存在大量微米级动态析出相,经固溶处理可基本消除;时效处理对挤压态样品的冲击韧性无明显影响,挤压态、欠时效态和峰时效态样品的冲击韧性相差不大,冲击韧性值在4.2~4.7 J·cm^(-2)范围内波动,微米级动态析出相是影响合金冲击韧性的主要因素;时效处理对固溶态试样的冲击韧性具有显著的影响,固溶态试样的冲击韧性值最高(14.3 J·cm^(-2)),随着时效时间的延长,冲击韧性急剧降低,峰时效状态下,合金的冲击韧性值为4.9 J·cm^(-2),微米级动态析出相的溶解是固溶态合金冲击韧性提升的主要因素,纳米级时效析出相则是使固溶-时效处理试样冲击韧性降低的主要因素。

The service environment of the spacecraft components is becoming more and more severe as the continuous improvement of spacecraft performance.Magnesium alloys have broad application prospects in the aerospace field due to their advantages of low density,high specific strength and high specific rigidity.Studies have shown that addition of rare earth elements can significantly increase the strength of magnesium alloys,among which Gd is one of the most important alloying elements.Mg-Gd series have a significant precipitation strengthening effect,which makes it to be one of the magnesium alloy series with the highest strength.So,it is an important candidate material for aircraft manufacturing.Grain refinement is obtained when Mg-Gd alloys produced by plastic deformation,due to the dynamic recrystallization behavior of the alloy during deformation.Grain refinement and precipitation strengthening are also considered as the two main strengthening mechanisms of wrought Mg-Gd alloys.The refined grains are usually related to a lower deformation temperature.However,the low deformation temperature will cause dynamic precipitation of Mg-Gd alloys with high Gd content,forming a large number of micron-scale second phase particles in the alloy.The solute atoms in the matrix will be consumed and reduce the precipitation strengthening effect of the alloy.Therefore,in order to improve the mechanical properties of the alloy,the effect of solution treatment and ageing treatment on the microstructure and mechanical properties of Mg-Gd-Y-Nd-Zr alloy extruded plate was investigated,and the mechanism related to the role of second phase in impact toughness was discussed in this paper.Mg-8Gd-4Y-Nd-Zr alloy ingot with a diameter of 400 mm was fabricated by ingot metallurgy.The ingot was extruded into a plate with a cross-sectional dimension of 300 mm×38 mm at 400℃after solid solution and machining.Then,solid solution treatment and ageing treatment were carried out on the samples cut from the extruded plate.The solid solution treatment was performed at 490℃for 1.5 h followed by water quenching.The ageing treatment is carried out at 215℃,and the age hardening curves of the extruded samples and the solid solution samples were obtained through the hardness test of the samples with different ageing time.The samples were machined and tested for impact toughness through GB/T 229-2007 Metallic materials:Charpy pendulum impact test method.The microstructure of the alloy was analyzed by means of optical microscopy (OM),scanning electron microscopy (SEM) and electron backscatter diffraction (EB-SD).The results showed that a large number of fine recrystallized grains with the average grain size of 5.2μm were formed in the extruded plate.In addition,plenty of micron-scale dynamic precipitates were observed.Most of the dynamic precipitates dissolved into the matrix after solution treated at 490℃for 1.5 h,but the average grain size increased to 32.2μm.The hardness of the extruded sample and the solid solution treated sample were both improved after ageing treatment.However,with the extension of the ageing time,the hardness of the extruded sample increased significantly lower than that of the solid solution treated sample,and the peak hardness of the extruded sample was also lower than that of the solid solution treated sample.The extruded sample and the solid solution treated sample reached their peak values of HV 115 and HV 123 after ageing at 215℃for 28 and 12 h,respectively.The impact toughness test showed that ageing had no significant influence on the impact toughness and fracture characteristics of the extruded samples.The impact toughness values of the extruded 215℃/8 h,215℃/14 h and 215℃/28 h samples were 4.7,4.6,4.2 and 4.6 J·cm^(-2),respectively.A large number of dynamic precipitates could be observed in the impact fracture of all the samples.After solution treatment,the impact toughness of the alloy was significantly improved,reaching 14.3 J·cm^(-2).However,the impact toughness of the alloy decreased sharply after solution-ageing treatment.After ageing at 215℃for 4,6 and 12 h,the impact toughness values of the alloy decreased to7.3,4.5 and 4.9 J·cm^(-2),respectively.The variation in impact toughness could be attributed to the influence of the second phase particles.According to the phenomenon that the ageing treatment had no obvious effect on the impact toughness of the extruded samples,it could be concluded that the micron-scale dynamic precipitates was the most important factor affecting the impact toughness of the alloy when the micron-scale dynamic precipitates coexisted with the nano-scale precipitates.After solution treatment,the grain size increased,which was not beneficial for the improvement of impact toughness,but the impact toughness value of the solution treated sample was increased by 204%compared with the extruded sample,indicating that the dissolution of the micron-scale dynamic precipitates led to cracks source reduction was the main factor in improving the impact toughness of the solution treated sample.After solutionaging treatment,the impact toughness of the alloy reduced,indicating that the nano-scale precipitates became the main factor that reduced the impact toughness of the solution-ageing treated samples when the micron-scale dynamic precipitate was absent.