【摘要】：鎂合金作為一種輕質(zhì)結(jié)構(gòu)材料,在汽車和航空等領(lǐng)域的應(yīng)用前景廣闊。然而,室溫環(huán)境中較強(qiáng)的各向異性限制了鎂合金的廣泛應(yīng)用。為克服這一缺點(diǎn),許多學(xué)者開展了大量宏觀力學(xué)行為和微觀結(jié)構(gòu)演化方面的研究,但微觀結(jié)構(gòu)演化如何影響鎂合金的宏觀力學(xué)行為仍不清楚,缺乏聯(lián)系宏觀與微觀變形的細(xì)觀尺度的測量。有關(guān)鎂合金各向異性變形的原因眾說紛紜,尚未達(dá)成共識。本文利用原位實時同步輻射X射線相襯成像和衍射多尺度測量方法研究不同應(yīng)變率加載下鎂合金的各向異性變形。實驗設(shè)計兩種加載方式,加載方向垂直或平行于變形鎂合金的c軸,分別記為LA⊥c和LA‖c。在鎂合金變形過程中,實現(xiàn)宏觀應(yīng)力—應(yīng)變曲線、細(xì)觀應(yīng)變場和微觀衍射圖譜的同時采集。電子背散射衍射技術(shù)用于回收樣品中的變形孿晶的表征。1、室溫環(huán)境中準(zhǔn)靜態(tài)壓縮加載,應(yīng)變率5XI0-4 s-1,LA⊥c和LA‖c樣品的應(yīng)力—應(yīng)變曲線、應(yīng)變場和衍射圖譜演化均呈現(xiàn)明顯的差異。對于LA⊥c樣品,{1012}拉伸孿生主導(dǎo)塑性變形,應(yīng)力梯度通過孿生被迅速釋放并使應(yīng)變場變得均勻化,應(yīng)變局域化程度的降低有效激發(fā)了應(yīng)變硬化率的增加。然而LA‖c樣品中的塑性變形主要依賴位錯運(yùn)動,位錯在缺陷處形核并纏結(jié)引起應(yīng)變集中,細(xì)觀非均勻變形導(dǎo)致應(yīng)變硬化率的降低。2、高溫環(huán)境中準(zhǔn)靜態(tài)壓縮加載,應(yīng)變率10-3 s-1,鎂合金的宏觀力學(xué)性能、細(xì)觀應(yīng)變場和微觀晶格變形呈現(xiàn)明顯的各向異性。由于初始織構(gòu)的差異,在室溫和高溫下,{1012}拉伸孿晶主導(dǎo)LA⊥c樣品的塑性變形,而位錯運(yùn)動則在LA‖c樣品中較為盛行。隨著溫度的升高,LA⊥c樣品中激發(fā)的{1012}拉伸孿晶的數(shù)量逐漸減少,使得應(yīng)變場的均勻化程度降低;高溫下,LA‖c樣品中{1122}c + a錐面滑移較易啟動,使得LA‖c樣品在高溫下呈現(xiàn)更加均勻的變形。{1012}拉伸孿晶和錐面c + aa滑移均能通過協(xié)調(diào)垂直和平行于加載方向的變形使塑性變形變得均勻化。應(yīng)變場不均勻度的差異導(dǎo)致LA⊥c和LA‖c樣品的應(yīng)變硬化率呈現(xiàn)明顯的差異。3、分離式霍普金森壓桿加載,應(yīng)變率約為5.5×103 s-1,鎂合金的動態(tài)響應(yīng)同準(zhǔn)靜態(tài)加載類似呈現(xiàn)較強(qiáng)的各向異性。{1012}拉伸孿晶使得應(yīng)變場的不均勻度減小,應(yīng)變場局域化程度的減小有效激發(fā)了應(yīng)變硬化率的增加;位錯運(yùn)動引起非均勻變形,應(yīng)變場不均勻度的升高導(dǎo)致應(yīng)變硬化率的減小。然而,在塑性變形的初期,動態(tài)加載過程中孿生主導(dǎo)塑性變形,而準(zhǔn)靜態(tài)加載下主導(dǎo)塑性變形的則是位錯運(yùn)動,表明加載應(yīng)變率在一定程度上影響著鎂合金的變形模式。4、平板撞擊加載,應(yīng)變率0.92～1.35×105s-1 LA⊥c和LA‖c樣品的彈塑性轉(zhuǎn)變呈現(xiàn)明顯的各向異性,但二者的雨貢紐彈性極限基本相同,約為0.32 GPa。低速撞擊下LA⊥c樣品的層裂強(qiáng)度略高于LA‖c樣品,隨著撞擊速度的增加,二者的層裂強(qiáng)度均隨撞擊速度的增加而增加,且差異減小;當(dāng)撞擊速度增加至400 m/s時,LA⊥c和LA‖c樣品的層裂強(qiáng)度基本一致。LA⊥c樣品中形成大量的{1012}拉伸孿晶,LA‖c樣品中孿晶數(shù)量較少。
[Abstract]:As a lightweight structural material, magnesium alloy has a wide application prospect in automobile and aviation. However, the strong anisotropy in the room temperature environment limits the wide application of magnesium alloys. In order to overcome this shortcoming, many scholars have carried out a lot of research on macroscopic mechanical behavior and microstructure evolution, but how microstructure evolution affects the macroscopic mechanical behavior of magnesium alloy is still unclear, and lack of micro-scale measurement of macroscopic and microscopic deformation. There is no consensus on the reasons for the anisotropy deformation of magnesium alloys. In this paper, the anisotropic deformation of magnesium alloy under different strain rate is studied by using in-situ real-time synchronous radiation X-ray phase contrast imaging and diffraction multi-scale measurement method. In the process of deformation of magnesium alloy, the macroscopic stress relaxation strain curve, micro-strain field and micro-diffraction pattern are simultaneously acquired. The results show that the stress relaxation strain curve, strain field and diffraction pattern evolution of the specimen under quasi-static compression loading, strain rate 5X0-4s-1, LA-c and LA-c in room temperature environment show significant difference. For LA-c samples, {1012} tensile twinning dominant plastic deformation, the stress gradient is rapidly released by twinning and the strain field becomes uniformized, and the decrease in strain localization degree effectively stimulates the increase of strain hardening rate. However, the plastic deformation in LA-c samples depends mainly on dislocation movement, dislocation is at the defect and entanglement causes strain concentration, and meso-non-uniform deformation results in a decrease of strain hardening rate. The quasi-static compressive loading and strain rate of 10-3s-1 and the macroscopic mechanical properties of magnesium alloy in high-temperature environment are studied. The micro-strain field and the micro-lattice deformation show obvious anisotropy. Due to the difference in the initial texture, the {1012} tensile uniaxial crystals dominate the plastic deformation of the LA-c samples at room temperature and high temperature, while dislocation movements are more prevalent in the LA-c samples. As the temperature increases, the number of {1012} tensile microcrystals excited in the LA-c sample is gradually decreased so that the degree of homogenization of the strain field is reduced; at high temperatures, the {1122} c + a cone slip in the LA-c sample is easier to start, so that the LA-c samples exhibit more uniform deformation at high temperatures. The {1012} tensile stressor and the cone c + aa slip can uniformize the plastic deformation by coordinating the vertical and the deformation parallel to the loading direction. The difference of non-uniformity of strain field results in a significant difference in strain hardening rate of LA-c and LA-c samples. 3. Split Hopkinson pressure rod is loaded with a strain rate of about 5. 5-103 s-1. The dynamic response of magnesium alloy is similar to quasi-static loading. The increase of strain hardening rate is effectively excited by the decrease of the degree of localization of the strain field and the decrease of strain hardening rate due to the increase of the non-uniform deformation of the strain field and the increase of the non-uniformity of the strain field. However, during the initial stage of plastic deformation, the twin dominant plastic deformation during dynamic loading, while the dominant plastic deformation under quasi-static loading is dislocation movement, indicating that the loading strain rate affects the deformation mode of magnesium alloy to some extent. The elastic-plastic transition of the strain rate of 0. 92 ~ 1. 35-0105s-1LA-c and LA-c-c samples showed obvious anisotropy, but the elastic limit was about 0.32GPa. When the impact velocity increases, the crack intensity increases with the increase of the impact velocity, and the difference is reduced; when the impact velocity increases to 400 m/ s, The fracture strength of LA-c and LA-c samples was consistent. A large number of {1012} tensile microcrystals were formed in the LA-c samples, and the number of crystals in LA-c samples was small.
【學(xué)位授予單位】：中國科學(xué)技術(shù)大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2017
【分類號】：TG146.22

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