| 摘要 | 材料的可持续发展需要建立在节约能源和保护环境的基础上，Al-Mg系合金作为轻质、环境友好的结构材料已广泛应用于航空航天，海洋船舶和汽车工业中。近年来，在传统Al-Mg合金基础上发展起来的Al-Mg-Sc-Zr合金，由于低密度、高强度、耐蚀、可焊而倍受关注。
本文采用金相显微镜、扫描电镜、透射电镜、电子探针、X射线衍射等手段，通过不同温度下的拉伸、室温韦氏硬度、腐蚀、动电位极化测试等实验方法，系统研究了Al-6Mg-0.2Sc-0.15Zr-(Ni)(wt.%)合金的显微组织、力学性能、变形行为、腐蚀性能和元素Ni的影响。
实验结果表明，添加0.2Sc和0.15Zr(wt.%)的Al-Mg合金铸态组织为完全等轴晶，凝固冷却过程中会析出两种壳状的初生Al3(Sc,Zr)相，随后继续冷却时会从固溶体中析出共格的L12型Al3(Sc,Zr)相。
利用Gleeble-2000热模拟机研究了铸态Al-Mg-Sc-Zr合金的热变形行为。通过不同温度、不同变形速率下的压缩应力－应变曲线，根据材料的动态模型，绘制出合金的加工图谱和不稳定图谱。确定适合铸态合金热加工的工艺为420ºC－460ºC之间，变形速率不大于5 s-1，变形量小于50%。
由于Al3(Sc,Zr)相的Zener Drag作用，冷变形Al-Mg-Sc-Zr合金的再结晶过程是缓慢进行的，在300ºC以下保持稳定的组织和性能；400ºC加热，硬度开始下降，局部发生再结晶；500ºC加热时，Al3(Sc,Zr)沉淀相迅速粗化，经过100 h后合金已经发生完全再结晶。
Ni含量小于1.0wt.%对Al-Mg-Sc-Zr合金铸态的等轴晶粒没有明显影响。当Ni含量增加到2.0wt.%时，合金的组织为完全树枝晶。这是因为在Al和Al3(Sc,Zr)相的界面上产生的错配位错，影响了Al3(Sc,Zr)的沉淀析出行为，导致类不连续沉淀的发生。
添加0.1wt.%Ni的Al-Mg-Sc-Zr合金具有较好的室温、高温性能和持久寿命，这与弥散细小的不共格含Ni相的作用有关。当Ni含量增加时，粗大的Al3Ni相对合金的力学性能影响不大。
在225℃/50MPa持久拉伸时，由于Al3(Sc,Zr)沉淀相对位错的钉扎和拖拽作用，显著地提高Al-Mg合金的持久寿命。复合添加0.1wt.% 的Ni，形成了不共格的Al(Mn,Ni,Fe)、Al(Mn,Ni,Cr)和Al3Ni相增加合金的临界应力和蠕变抗力，降低蠕变速率，使Al-Mg-Sc-Zr合金的持久寿命提高了一倍多。另外，具有再结晶组织的合金比形变态的合金具有更高的持久寿命。
在NaCl-HCl溶液中，Al-Mg-Sc-Zr-Ni合金中的Al3Ni相充当阴极。Al3Ni颗粒周围的阳极Al3Mg2相和Al-Mg固溶体容易优先被腐蚀，成为裂纹的萌生源。Ni含量的增加，加剧了合金的腐蚀行为。 |
| 其他摘要 | The sustainable development of materials should to be based on the energy saving and environment protection. As the environment-friendly material, Al-Mg alloy is widely used in aeronautic and astronautic structure, marine equipment and automobile industry. It is attractive due to their low density, high strength, superior corrosion resistance and good weldability.
The microstructure, mechanical properties, deformation behaviour, corrosion-resistance performance of Al-6Mg-0.2Sc-0.15Zr-(Ni)(wt.%) alloy have been studied by optical microscopy (OM), canning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe microanalysis (EPMA), X-ray analysis, and different temperature tensile tests, Vicker’s hardness measurements, corrosion test and potentiodynamic polarization measurements. The role of Ni in this alloy is also investigated.
The microstructure of cast Al-Mg alloy with 0.2%Sc and 0.15%Zr show equiaxed grain feature. During the solidification, there are two type of shell structure of primary Al3(Sc,Zr) phase, after that, the coherent L12-Al3(Sc,Zr) phases precipitate from the supersaturated solid solution.
The hot-working characteristics of cast Al-Mg-Sc-Zr alloy were evaluated using Gleeble-2000 simulator. The processing and instability maps were established on the basis of true stress-true strain data at different strain rate and temperature, following the dynamic materials model. The optimum condition of hot deformation for the alloy is that the strain rate is less than 5 s-1 and the reduction is below 50% in the temperature range of 420ºC-460ºC.
The Zener pinning of thermal stability Al3(Sc,Zr) dispersoids preserve the stable microstructure of cold worked Al-Mg-Sc-Zr alloy up to 300ºC, as a result, the recrystallization proceeds slowly. At 400ºC, the Vickers hardness begin to degrade. Especially at 500ºC, Al3(Sc,Zr) dispersoids coarsen rapidly and the alloy fully recrystallize after holding at this temperature for 100 h.
It is found that the Ni content less than 1.0wt.% has little influence on the grain size, whereas the alloy with 2.0wt.%Ni is composed of dentritic grains as the occurrence of quasi-discontinuous precipitation of Al3(Sc,Zr) phases, resulted from the misfit dislocation on the interface of Al and Al3(Sc,Zr) phase..
Al-Mg-Sc-Zr alloy with 0.1wt.%Ni, show better mechanical property at ambient and elevated temperature and rupture life, which is related to the small Ni-bearing dispersoids. When Ni content is further increased, the effect on mechanical property would be little due to the coarse Al3Ni phases.
During 225℃/50MPa creep rupture test, the life of Al-Mg alloy is improved greatly by Sc and Zr because the dislocation is pinned and dragged by Al3(Sc,Zr) precipitates. The addition of 0.1wt.% Ni to Al-Mg-Sc-Zr alloy results in the formation of Al(Mn,Ni,Fe), Al(Mn,Ni,Cr) and Al3Ni phases, which increase the threshold stress and creep resistance. Consequently, the creep rate is reduced and the rupture life is more than twice of that of Al-Mg-Sc-Zr alloy. In addition, the durable time of the recrystallized alloy is longer than that of the deformed one.
Al3Ni particle acts as cathode with respect to the Al-Mg-Sc-Zr-Ni alloy in the NaCl-HCl solution. The anodic Al3Mg2 phases and Al-Mg solid solution in the vicinity of Al3Ni region preferentially eroded and induce the crack initiation. With the improvement of Ni content, the corrosion resistance of Al-Mg-Sc-Zr deteriorate drastically. |
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