合金元素对Mg-Gd系合金组织及时效硬化行为的影响

Effect of Alloying Elements on Microstructure and Aging Hardening Behaviors of Mg-Gd Based Alloys

利用扫描背散射电子像(SEM-BSE)和透射电子显微像(TEM)观察,研究了MgGd3,MgGd3Nd0.3和MgGd3Nd0.3Zn0.33种合金的铸态、固溶态显微组织结构,同时利用显微硬度仪研究了3种固溶态合金在200℃下的时效硬化行为。结果表明:3种合金经过520℃固溶处理后,均能使富稀土的初生相有效地固溶,形成可时效强化的过饱和固溶体合金。其200"C时效后峰值硬度的实现都源于基体中微细β’析出相的形成和弥散分布,而随后硬度随时效时间的变化趋势却有所不同。添加稀土元素Nd,有助于加快合金的时效响应。Nd和Gd共存时的析出强化使MgGd3Nd0.3合金在0～120 h时效阶段具有高于其他两种合金的硬度。过渡族元素Zn的添加,虽然使MgGd3Nd0.3Zn0.3的硬度相比MgGd3Nd0.3略低,但能够在长时间时效中保持硬度不明显下降。基于透射电镜观察,着重分析对比了在200"C 85 h时效条件下的3种合金的显微结构特征,讨论了添加Nd和Zn元素对析出相尺寸、形貌和分布特征的影响。

SEM and TEM observations were applied to investigate the microstructures of as-solidified and solution-treated MgGd3（%,atom fraction）,MgGd3Nd0.3（%,atom fraction） and MgGd3Nd0.3Zn0.3（%,atom fraction） alloys,and microhardness testing carried out to characterize the age-hardening behaviors of the solution-treated alloys at 200 ℃ as well.The results showed that MgGd3,MgGd3Nd0.3 and MgGd3Nd0.3Zn0.3 alloys were all age hardenable,in which the as-cast RE-rich primary phases could be solutionized significantly after heat treated at 520 ℃.For all these alloys,reaching their peak hardness during aged at 200 ℃ was associated with the formation and dispersive distribution of fine β′-phases of high volume fraction,while their hardness variations after the peak aging stage exhibited obviously different tendencies.As a result of introducing Nd to MgGd3 and thus realizing the precipitation strengthening with both Gd and Nd involved,the MgGd3Nd0.3 alloy,with aging response being improved significantly,could have aging hardness higher than the other two alloys during aged for 0～120 h.The addition of Zn could retain adequate high hardness,without decreasing tendency,for the MgGd3Nd0.3Zn0.3 alloy after aged for a long period of time,even though its peak hardness was slightly lower than that of the MgGd3Nd0.3 alloy.By means of TEM observations,the microstructural comparison of the three alloys aged at 200 ℃ for 85 h was focused,through which the formation of precipitates,their sizes,morphology and distribution in these aged alloys would be reported and the effects of Nd and Zn additions on these features would be discussed.