【摘要】：材料的失效行為中,有60%～80%屬于疲勞失效,而疲勞裂紋往往萌生于工件表面。采用表面納米化方法在材料表面制備梯度納米層,可以有效改善材料的疲勞性能。為澄清疲勞裂紋在梯度納米層中的擴(kuò)展過程,選擇表面超聲滾壓作為表面納米化方法,開展如下研究工作:(1)通過設(shè)計(jì)特殊的表面超聲滾壓加工路徑,在Ti-6Al-4V合金板材表面制備出厚度達(dá)到300 μm的梯度納米層,隨著表面超聲滾壓處理遍數(shù)的增加,梯度納米層更加均勻穩(wěn)定。使用透射電子顯微鏡對(duì)梯度納米層不同深度的組織結(jié)構(gòu)進(jìn)行觀察,發(fā)現(xiàn)晶粒細(xì)化是孿晶和位錯(cuò)等多種方式共同作用的結(jié)果;(2)研究發(fā)現(xiàn)表面超聲滾壓有效提高Ti-6Al-4V合金表層的顯微硬度,并在表層引入殘余壓應(yīng)力,降低表面粗糙度,并大幅度提升Ti-6Al-4V合金的疲勞壽命;(3)通過預(yù)制角裂紋缺陷的方式定位裂紋萌生位置,采用復(fù)型法觀察裂紋擴(kuò)展路徑,澄清多尺度疲勞裂紋在梯度納米層中的擴(kuò)展過程。發(fā)現(xiàn)表面超聲滾壓后Ti-6Al-4V合金的裂紋擴(kuò)展路徑出現(xiàn)裂紋路徑改變和裂紋路徑偏折行為,與原始樣有明顯不同。表面超聲滾壓后Ti-6Al-4V合金的裂紋擴(kuò)展速率明顯降低;(4)隨應(yīng)力比R增大,Ti-6Al-4V合金表面超聲滾壓樣和原始樣的疲勞壽命均在不斷提高。在試驗(yàn)選擇的各個(gè)應(yīng)力比下,表面超聲滾壓樣的疲勞壽命均顯著高于原始樣,表面超聲滾壓樣的裂紋擴(kuò)展速率均明顯低于原始樣的裂紋擴(kuò)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.
【學(xué)位授予單位】：華東理工大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：TG146.23;TG306

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