半固态流变压铸Mg-7Zn-X镁合金液相偏析区在时效过程中的GP区和β1'相形成及其对力学性能的影响机制研究

During the studying of Magnesium semi-solid forming, most of object of study is on AZ91D, or the other alloy license. The advantage of heat-treatable of semisolid die-casting can not make full use because of poor heat treatment strengthening function of those alloys. The tensile strength of Mg-7Zn-X, which has been built in early stage, can be raised by 71 percent before and after heat treatment under metal mold forming. However the increasing proportion of tensile strength is decrees contrasting semi-solid die casting and metal mold forming. Based on the previous research, the source of the problems are the liquid segregation models and ripening of the dendrite grain during solid solution. To this end, the applicant comes up with the complex strengthening method making use of GP zones and β1'phase both strengthening heat treatment and the relatively lower processing temperature. The strengthening method is better for semisolid reho-diecasting. This project will be the first to study the effect of the heat treating regime and the solid fraction on element content gradient in the liquid segeregation zones. From the dynamics and the crystallography perspective, the effect of heat treating regime, different position in the liquid segregation zones (cores, middle part and surface layer) of formation condition of GP zones , β1' phase and the second phase are also explored. The controversy of the formation condition of GP zones and β1' phase ingredient both at home and abroad will be solved. The correlative mechanism will be revealed about GP zones and β1' phase compound enhance with mechanical properties. Those are of important values for promoting semi-solid die casting forming technological progress.

镁合金半固态压铸研究多以AZ91D等牌号合金为对象，这些合金热处理强化能力差的缺点使半固态压铸件可热处理的优势无法充分发挥。为此，申请人研发了金属型成形条件下，热处理前后抗拉强度提高71%的Mg-7Zn-X合金。但在流变压铸成形条件下，该合金的抗拉强度提高比例显著降低。研究发现：液相偏析模式和固溶过程中的非枝晶颗粒熟化是问题的根源。申请者提出了更为适合半固态流变压铸件的既有热处理强化作用又可在较低热处理温度下实行的GP区及β1'相复合强化法。本项目将首先研究热处理制度及固相率对液相偏析混合区中的元素含量梯度分布的影响并从动力学及晶体学角度探索热处理制度及液相偏析区(芯部、中部及表层)不同部位对GP区形成条件、β1'相成分及第二相的影响，解决国内外对GP区形成条件及β1'相成分的争议，揭示GP区及β1'相复合强化方法与力学性能的关联机制，为推进半固态流变压铸成形技术的进步奠定理论和技术基础

Mg-Zn-Al系镁合金是开发成本低廉且具有显著热处理强化特征的新型镁合金。目前Mg-Zn-Al和微合金化Mg-Zn-Al系合金在铸态、固溶处理、时效处理和半固态成形后的组织和性能研究还不够充分。另外，合金为非枝晶态时，在热处理条件下研究G.P.区的脱溶析出和G.P.区与析出相强化固相颗粒的机理非常少见。. 鉴于上述，利用光学显微镜(OM)、X射线衍射仪(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、拉伸试验机和显微硬度计等表征方法研究了新型Mg-Zn-Al和元素Sm/Mn微合金化Mg-Zn-Al系镁合金在铸态、固溶态、时效态和半固态的组织和性能。在实验基础上，深入研究分析了Sm/Mn等元素微合金化和热处理对Mg-Zn-Al系合金组织和性能的影响及强化机理。在相同工艺下，Sm微合金化ZA合金后，随其加入量增加，半固态组织中颗粒的尺寸和形状因子趋于增大，究其原因归于铸态发达的枝晶组织和较大原子半径的Sm阻碍了较小原子半径Mg、Zn和Al的扩散。然而，Mn微合金化ZA合金后，在相同工艺下，颗粒的尺寸和形状因子随Mn加入量增加呈先减小后增大的趋势。研究了非枝晶态ZA和ZAM01合金经预时效、单级和双级时效处理的组织和显微硬度。结果表明：ZA和ZAM01合金经350°C固溶12h，随后在90°C时效24 h后，均观察到了形态各异的团簇原子或G.P.区非枝晶态ZA和ZAM01合金中固相颗粒间强化机理主要概括为：G.P.区或团簇原子的内应力强化和位错绕过/切过β1'、β2'、β及Mg32(Al,Zn)49相的协同强化。