铸造镁稀土合金室温损伤累积诱发高周疲劳裂纹微观机制研究

Fatigue crack initiation mechanism study is one important issue of materials fatigue research. Better understanding of damage cumulation induced fatigue crack initiation mechanism in Mg alloys is very necessary before improving their fatigue properties. Though the damage cumulation induced fatigue cracks were often observed to origin from persistent silp bands or twinning boundaries, the damage cumulation process and the critical conditions of fatigue cracks initiation in Mg alloys are still lack of detail investigation and quantitive measurement. High cycle fatigue crack initiation mechanism of namely cast defects free Mg-Nd-Zn-Zr alloy will be focused on in present study by using Confocal Microscope and Electron Backscatter Diffraction (EBSD) Analysis technologies. The 2-D and 3-D damage morphologies variation on the surface of fatigue samples in high cycle tension-compression fatigue test will be investigated. And the influence of slip and twinning deformation mechanisms, grain orientation and grain size on the damage morphology variation and fatigue crack initiation mechanism will also be examined. Especially attention will be focus on Localized Fatigue Damage and its accumulation process. More important, the Critical Localized Fatigue Damage when cracks initiation will be analyzed quantitively and statistically. The influence of Strain Amplitude, Average Grain Size, Precipitates and their distribution on the above fatigue damage evolution will be systematic studied. By the above analysis, high cycle fatigue crack initiation mechanism in namely cast defects free Mg-Nd-Zn-Zr alloy will be revealed, and it will also promote the understanding of fatigue crack initiation mechanism in Mg-RE alloys. Such basic information is necessary to improve the alloy's fatigue performance. Also, in present study, a relative new method to observe the 3D damage morphologies on the surface of fatigue samples, Confocal Microscope, are used, and it is an attempt innovation of investigation method. It is probably a high efficient and relative simple way to study the 3D morphology damage evolution quantitively in fatigue research.

疲劳裂纹萌生机制是材料疲劳性能研究的一个重要方面，理解镁合金损伤累积诱发疲劳裂纹形核机制是改善镁合金疲劳性能的基础，具有重要意义。虽然已经知道镁合金损伤累积诱发疲劳裂纹通常萌生于驻留滑移带与孪晶界面处，但损伤形貌演变规律及裂纹形核的临界条件等微观机制尚不清楚。本研究以名义上无铸造缺陷的铸造Mg-Nd-Zn-Zr镁稀土合金为研究对象，借助共聚焦显微镜和电子背散射技术(EBSD)，通过对合金室温拉压高周疲劳裂纹萌生过程中试样表面两维和三维损伤变形演变规律的表征，重点统计裂纹萌生过程中局部损伤变形的累积过程和裂纹形核时的临界局部损伤变形量，考察不同类型滑移和孪晶等塑性变形机制、晶粒取向及晶粒尺寸的作用，研究应变幅、平均晶粒尺寸、析出相及分布的变化对上述规律的影响，以揭示铸造镁稀土合金室温损伤累积诱发高周疲劳裂纹的萌生机制，为改善铸造镁稀土合金的疲劳性能提供理论基础。

项目通过半连续铸造和低压砂型铸造制备了内部组织致密的Mg-Nd-Zn-Zr(NZ30K)镁稀土合金，采用激光共聚焦显微镜与电子背散射(EBSD)技术相结合的方式，系统研究了室温拉压高周疲劳条件下，试样表面两维和三维疲劳损伤形貌的演变规律，考察了应力大小、疲劳周次、晶粒取向、晶粒尺寸、热处理状态等对NZ30K镁稀土合金疲劳损伤行为的影响。研究结果表明，(1) 基面滑移和孪晶是NZ30K镁稀土合金疲劳损伤的两种主要机制：当外部应力大小与合金疲劳强度相当时，基面滑移是合金主要损伤机制，此时在疲劳试样表面极少能够观察到孪晶变形；而当外部应力较大时(疲劳强度+10MPa)，孪晶损伤机制才变得重要。(2) 合金疲劳过程中，随着疲劳周次的增加，损伤集中发生的位置会发生变化，会在某几个基面滑移条纹(PSM)或者孪晶之间进行切换；当疲劳损伤集中在孪晶内部时，孪晶内部也会形成类似于PSM的挤入挤出条纹，这些条纹也很可能源于基面滑移(有待证实)。(3) 晶粒取向对合金的疲劳损伤影响显著，通常Basal Schmid Factor (BSF)越大，晶粒越容易发生基面滑移，BSF越小，晶粒越容易形成孪晶；随着BSF的增加，发生基面滑移晶粒的比例显著增加，而发生孪晶的晶粒比例则显著减少。(4) 晶粒取向并不能完全决定晶粒是否发生疲劳损伤，实验结果表明，相同取向的晶粒疲劳损伤程度会存在显著差异：有些晶粒已经发生了十分显著的疲劳损伤，而另外一些晶粒内部则完全看不到变形条纹。因此，除了晶体取向外，晶粒的疲劳损伤还受到其他因素的影响。(5) 晶粒尺寸大小会显著影响合金疲劳损伤的均匀性，小晶粒合金在疲劳极限下发生损伤的晶粒比例明显高于大晶粒合金。(6) 热处理能够显著影响合金疲劳损伤过程，研究结果表明，固溶处理态合金疲劳损伤更加均匀，疲劳过程中可以观察到大量的PSM，同时发生疲劳损伤的晶粒比例较高；而时效处理态合金疲劳损伤更为集中，发生疲劳损伤的晶粒比较较低，同时发生损伤的晶粒内部疲劳条纹极少。(7) 共聚焦显微镜可以清晰的表征铸造镁合金疲劳过程三维损伤形貌演变规律，是研究材料疲劳损伤的十分有效观察方法，值得推广。