表面超声滚压对Ti-6Al-4V合金多尺度疲劳裂纹扩展行为的影响

发布时间：2018-11-14 13:51

【摘要】：材料的失效行为中,有60%～80%属于疲劳失效,而疲劳裂纹往往萌生于工件表面。采用表面纳米化方法在材料表面制备梯度纳米层,可以有效改善材料的疲劳性能。为澄清疲劳裂纹在梯度纳米层中的扩展过程,选择表面超声滚压作为表面纳米化方法,开展如下研究工作:(1)通过设计特殊的表面超声滚压加工路径,在Ti-6Al-4V合金板材表面制备出厚度达到300 μm的梯度纳米层,随着表面超声滚压处理遍数的增加,梯度纳米层更加均匀稳定。使用透射电子显微镜对梯度纳米层不同深度的组织结构进行观察,发现晶粒细化是孪晶和位错等多种方式共同作用的结果;(2)研究发现表面超声滚压有效提高Ti-6Al-4V合金表层的显微硬度,并在表层引入残余压应力,降低表面粗糙度,并大幅度提升Ti-6Al-4V合金的疲劳寿命;(3)通过预制角裂纹缺陷的方式定位裂纹萌生位置,采用复型法观察裂纹扩展路径,澄清多尺度疲劳裂纹在梯度纳米层中的扩展过程。发现表面超声滚压后Ti-6Al-4V合金的裂纹扩展路径出现裂纹路径改变和裂纹路径偏折行为,与原始样有明显不同。表面超声滚压后Ti-6Al-4V合金的裂纹扩展速率明显降低;(4)随应力比R增大,Ti-6Al-4V合金表面超声滚压样和原始样的疲劳寿命均在不断提高。在试验选择的各个应力比下,表面超声滚压样的疲劳寿命均显著高于原始样,表面超声滚压样的裂纹扩展速率均明显低于原始样的裂纹扩展速率。
[Abstract]:In the failure behavior of materials, 60% of them belong to fatigue failure, and fatigue cracks often occur on the surface of workpiece. The fatigue properties of the materials can be effectively improved by using the surface nanocrystalline method to prepare gradient nanocrystalline layers on the surface of the materials. In order to clarify the propagation process of fatigue crack in the gradient nanometer layer, surface ultrasonic rolling is selected as the surface nanocrystalline method. The following research works are carried out: (1) by designing a special surface ultrasonic rolling machining path, Gradient nanocrystals with thickness of 300 渭 m were prepared on the surface of Ti-6Al-4V alloy sheet. With the increase of the number of times of ultrasonic rolling on the surface, the gradient nanocrystalline layer became more uniform and stable. The microstructure of gradient nanocrystals at different depths was observed by transmission electron microscope (TEM). It was found that grain refinement was the result of twinning and dislocations. (2) it is found that surface ultrasonic rolling can effectively improve the microhardness of Ti-6Al-4V alloy surface, introduce residual compressive stress into the surface layer, reduce the surface roughness, and greatly improve the fatigue life of Ti-6Al-4V alloy; (3) the crack initiation position is located by prefabricated angular crack defects, and the crack propagation path is observed by using the complex mode method to clarify the propagation process of the multi-scale fatigue crack in the gradient nano-layer. It is found that the crack path change and crack path deflection appear in Ti-6Al-4V alloy after surface ultrasonic rolling, which is obviously different from the original sample. The crack propagation rate of Ti-6Al-4V alloy decreases obviously after surface ultrasonic rolling. (4) with the increase of stress ratio R, the fatigue life of ultrasonic rolling specimen and original sample on Ti-6Al-4V alloy surface is increasing. The fatigue life of the surface ultrasonic rolling specimen is significantly higher than that of the original sample under the various stress ratios selected by the test. The crack propagation rate of the surface ultrasonic rolling specimen is obviously lower than that of the original sample.
【学位授予单位】：华东理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.23;TG306

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