高强高韧Al-Zn-Mg-Cu合金疲劳断裂性能以及微观组织的研究
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摘要
本文通过扫描电子显微技术(SEM)、透射电子显微技术(TEM)以及电子背散射衍射技术(EBSD)的观察和分析,研究了Al-Zn-Mg-Cu系合金的疲劳、断裂的微观特征、基本规律以及裂纹萌生与扩展的微观机制,并从理论上对其进行了分析和讨论,得到了如下的主要结论：
1.过时效状态Al-Zn-Mg-Cu合金的断裂韧性主要受到第二相粒子的影响,其断裂方式以粗大第二相粒子引起的韧窝断裂为主。由合金基体的强度和晶界沉淀相的变化所引起的基体与晶界的差异会对合金的断裂方式和断裂韧性起到决定性的作用。
2. Al-Zn-Mg-Cu合金的疲劳损伤一般是由试样自由表面及其附近的富Fe相粒子所引起的,引起疲劳损伤最初形成的粒子大小一般约为10×12～17μm2。微裂纹一般在粒子中间断开处、粒子与基体的界面处、晶界以及表面凹坑等缺陷处萌生,裂纹形成后,继续向纵深扩展,最后发生瞬断。
3.试样在330Mpa的应力幅值下进行疲劳拉伸,其裂纹源区附近约170×190μm2的区域内没有观察到疲劳辉纹,随着裂纹的进一步扩展,可以逐渐观察到二次破裂、疲劳辉纹等典型的断口形貌,在可以观察到疲劳辉纹的区域内,疲劳辉纹的间距随着裂纹扩展的进行而逐渐加宽。
4. Al-Zn-Mg-Cu合金疲劳裂纹的扩展可分为三个阶段,当△K<16MPa×m1/2时,为裂纹扩展的第一阶段,此阶段断口上有很多微小的峭壁；当16MPa×m1/2 <△K<33MPa×m1/2时,为裂纹扩展的第二阶段,疲劳辉纹是该阶段的主要特征形貌；当△K>33MPa×m1/2时,为裂纹扩展的第三阶段(瞬断阶段),断口上可以观察到大量的韧窝和破碎粒子,类似于静载拉伸的形貌。
5. Al-Zn-Mg-Cu合金疲劳裂纹的扩展呈穿晶与沿晶混合的方式,相邻晶粒的晶粒结构与裂纹扩展之间的关系遵循于裂纹尖端的晶体塑性变形机制。当相邻两晶粒的有效滑移面间的面间角较大时,裂纹不能直接穿越晶界,倾向于沿晶扩展。
6.当疲劳裂纹穿越晶界进入下一个新的晶粒时,相比于扭转,裂纹尖端的简单偏转更容易发生。
7.对于不同应力幅值下进行疲劳试验的合金,应力幅值越大,裂纹源距材料的自由表面越近,无辉纹区域的面积也越小；裂纹扩展区中疲劳辉纹的间距、裂纹扩展速率以及疲劳断口上瞬断区占总断面面积的比例也越大。
The fatigue and fracture properties and microstructure of the fatigue specimen have been investigated by using SEM (including EBSD (Electron Back-scattering Patterns) analysis techniques) and TEM techniques, which reveals the micromechanisms of fatigue crack initiation and growth. After further analysis the conclusions come as follow:
1. The fracture toughness of overaged Al-Zn-Mg-Cu alloy is mainly affected by the second phase particles and the fracture mode of this kind of aluminum alloy is dimple fracture caused by coarsed second phase particles. The variation of strength of the alloy matrix and precipitation at grain boundaries give rise to a discrepancy between matrix and grain boundaries, which has an extreme impact on fracture mode and fracture toughness of alloys.
2. The fatigue damage of Al-Zn-Mg-Cu alloys are usually caused by Fe-rich intermetallic particels at or near the surface of the specimen, which are approximately 10×12～17μm2 in size. Micro cracks usually initiate in defects such as fracture particles, interface between partiles and matrix, grain boundaries or surface pits. After initiation fatigue cracks propagate further towards the center of the specimen, and finally fatigue failure occurs.
3. The specimen was subjected to a tensile test with stress amplitude of 330Mpa. In a region of approximately 170×190μm2 no striations were observed. As the fatigue cracks propagate secondary some typical fracture morphology like cracks and striations could be observed. In regions where striations are visible a uniform increase in striation distance as the crack length increased.
4. There are 3 stages of the crack propagation, when△K<16MPa×m1/2, it is the first stage, during which a great number of micro cliffs on the fracture surface can be observed. Afterwards when 16MPa×m1/2<△K<33MPa×m1/2, striation is the characteristic morphology. Thirdly, under the condition of△K> 33 MPa×m1/2, fracture failure occurs, lots of dimples and fracture particles are visible, the fracture surface is familiar with that of static tention.
5. The propagation mode of Al-Zn-Mg-Cu alloys is a mixed pattern of intergranular and transgranular ones. The relationship between misorientation of adjacent grains and propagation of fatigue crack obeys the crystal deformation mechanism of crack tips. When the interfacial angle of adjacent grains is relatively wide, fatigue cracks can't propagate through the grain boundaries and is apt to be intergranular.
6. When fatigue cracks cross the grain boundaries and enter an adjacent grain, compared with twist, deflection of the crack tip is more likely to happen.
7. For experimental alloys in tesile tests, with the increase of stress amplitude, the distance between the crack source and the free surface of the specimen decreases, and the size of the region without striations is smaller. At the same time the striation distance, the growth rate of cracks and the size of the fatigue rupture region increase.
1.过时效状态Al-Zn-Mg-Cu合金的断裂韧性主要受到第二相粒子的影响,其断裂方式以粗大第二相粒子引起的韧窝断裂为主。由合金基体的强度和晶界沉淀相的变化所引起的基体与晶界的差异会对合金的断裂方式和断裂韧性起到决定性的作用。
2. Al-Zn-Mg-Cu合金的疲劳损伤一般是由试样自由表面及其附近的富Fe相粒子所引起的,引起疲劳损伤最初形成的粒子大小一般约为10×12～17μm2。微裂纹一般在粒子中间断开处、粒子与基体的界面处、晶界以及表面凹坑等缺陷处萌生,裂纹形成后,继续向纵深扩展,最后发生瞬断。
3.试样在330Mpa的应力幅值下进行疲劳拉伸,其裂纹源区附近约170×190μm2的区域内没有观察到疲劳辉纹,随着裂纹的进一步扩展,可以逐渐观察到二次破裂、疲劳辉纹等典型的断口形貌,在可以观察到疲劳辉纹的区域内,疲劳辉纹的间距随着裂纹扩展的进行而逐渐加宽。
4. Al-Zn-Mg-Cu合金疲劳裂纹的扩展可分为三个阶段,当△K<16MPa×m1/2时,为裂纹扩展的第一阶段,此阶段断口上有很多微小的峭壁；当16MPa×m1/2 <△K<33MPa×m1/2时,为裂纹扩展的第二阶段,疲劳辉纹是该阶段的主要特征形貌；当△K>33MPa×m1/2时,为裂纹扩展的第三阶段(瞬断阶段),断口上可以观察到大量的韧窝和破碎粒子,类似于静载拉伸的形貌。
5. Al-Zn-Mg-Cu合金疲劳裂纹的扩展呈穿晶与沿晶混合的方式,相邻晶粒的晶粒结构与裂纹扩展之间的关系遵循于裂纹尖端的晶体塑性变形机制。当相邻两晶粒的有效滑移面间的面间角较大时,裂纹不能直接穿越晶界,倾向于沿晶扩展。
6.当疲劳裂纹穿越晶界进入下一个新的晶粒时,相比于扭转,裂纹尖端的简单偏转更容易发生。
7.对于不同应力幅值下进行疲劳试验的合金,应力幅值越大,裂纹源距材料的自由表面越近,无辉纹区域的面积也越小；裂纹扩展区中疲劳辉纹的间距、裂纹扩展速率以及疲劳断口上瞬断区占总断面面积的比例也越大。
The fatigue and fracture properties and microstructure of the fatigue specimen have been investigated by using SEM (including EBSD (Electron Back-scattering Patterns) analysis techniques) and TEM techniques, which reveals the micromechanisms of fatigue crack initiation and growth. After further analysis the conclusions come as follow:
1. The fracture toughness of overaged Al-Zn-Mg-Cu alloy is mainly affected by the second phase particles and the fracture mode of this kind of aluminum alloy is dimple fracture caused by coarsed second phase particles. The variation of strength of the alloy matrix and precipitation at grain boundaries give rise to a discrepancy between matrix and grain boundaries, which has an extreme impact on fracture mode and fracture toughness of alloys.
2. The fatigue damage of Al-Zn-Mg-Cu alloys are usually caused by Fe-rich intermetallic particels at or near the surface of the specimen, which are approximately 10×12～17μm2 in size. Micro cracks usually initiate in defects such as fracture particles, interface between partiles and matrix, grain boundaries or surface pits. After initiation fatigue cracks propagate further towards the center of the specimen, and finally fatigue failure occurs.
3. The specimen was subjected to a tensile test with stress amplitude of 330Mpa. In a region of approximately 170×190μm2 no striations were observed. As the fatigue cracks propagate secondary some typical fracture morphology like cracks and striations could be observed. In regions where striations are visible a uniform increase in striation distance as the crack length increased.
4. There are 3 stages of the crack propagation, when△K<16MPa×m1/2, it is the first stage, during which a great number of micro cliffs on the fracture surface can be observed. Afterwards when 16MPa×m1/2<△K<33MPa×m1/2, striation is the characteristic morphology. Thirdly, under the condition of△K> 33 MPa×m1/2, fracture failure occurs, lots of dimples and fracture particles are visible, the fracture surface is familiar with that of static tention.
5. The propagation mode of Al-Zn-Mg-Cu alloys is a mixed pattern of intergranular and transgranular ones. The relationship between misorientation of adjacent grains and propagation of fatigue crack obeys the crystal deformation mechanism of crack tips. When the interfacial angle of adjacent grains is relatively wide, fatigue cracks can't propagate through the grain boundaries and is apt to be intergranular.
6. When fatigue cracks cross the grain boundaries and enter an adjacent grain, compared with twist, deflection of the crack tip is more likely to happen.
7. For experimental alloys in tesile tests, with the increase of stress amplitude, the distance between the crack source and the free surface of the specimen decreases, and the size of the region without striations is smaller. At the same time the striation distance, the growth rate of cracks and the size of the fatigue rupture region increase.
引文
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