本文選題：超塑性　切入點(diǎn)：偽彈性　出處：《上海交通大學(xué)》2015年博士論文　論文類型：學(xué)位論文

【摘要】：塑性是材料的重要的力學(xué)性能,決定了材料可實(shí)現(xiàn)的變形量。提高材料的塑性一直是結(jié)構(gòu)材料研究的重點(diǎn)。偽彈性則是形狀記憶材料中的重要性能,高的可回復(fù)應(yīng)變可以使之吸收更多機(jī)械功,故提高可回復(fù)應(yīng)變一直是形狀記憶材料的研究重點(diǎn)。與粗晶材料相比,納米線具有超高的塑性。已報(bào)道通過應(yīng)力誘發(fā)相變或?qū)\生方式變形,納米線的延伸率可以達(dá)到50%左右,即所謂的超塑性。此外,納米線由于表面效應(yīng),變形后可以回復(fù)到初始狀態(tài),實(shí)現(xiàn)偽彈性。因此納米線成為獲得超塑性和超高可回復(fù)應(yīng)變的合適體系。目前納米線的力學(xué)性能研究主要側(cè)重于面心立方和體心立方結(jié)構(gòu)體系,密排六方體系的研究結(jié)果則較少而且存在一些爭議。由于滑移系少,一般認(rèn)為密排六方金屬塑性較差,通常粗晶態(tài)的密排六方金屬延伸率不到10%。但是在納米線材料中,密排六方金屬的力學(xué)性能仍需要進(jìn)一步探索。本工作根據(jù)變形機(jī)制的不同,選取兩大類密排六方金屬的納米線作為研究對象——應(yīng)力誘發(fā)相變類和應(yīng)力誘發(fā)孿生類。前者選取的代表為純鈷和鈷-鐵合金納米線,后者選取的代表為純鎂納米線。通過分子動力學(xué)模擬納米線的單軸拉伸和卸載過程,從而預(yù)測納米線超塑性和偽彈性的可行性,主要的發(fā)現(xiàn)為:(一)在鈷納米線中,通過HCP→FCC→HCP的兩步結(jié)構(gòu)相變,可以達(dá)到80%左右的延伸率,并且變形后的納米線可以回復(fù)至初始構(gòu)型,可恢復(fù)應(yīng)變約為71%。形變過程中的相變可以通過靜態(tài)方法計(jì)算能壘,從而進(jìn)一步解釋相變的合理性。該能壘計(jì)算的方法進(jìn)過進(jìn)一步改進(jìn),將納米線能量表示為應(yīng)變的函數(shù),計(jì)算鈷-鐵合金納米線中各種構(gòu)型之間相互轉(zhuǎn)換的能量關(guān)系,并且用于判斷應(yīng)力誘發(fā)相變的順序以及相變類型。(二)在鎂納米線中,通過應(yīng)力誘發(fā)二次孿晶,使納米線的延伸率可以達(dá)到60%左右,且該形變也能通過去孿晶化回復(fù),即在鎂納米線中發(fā)現(xiàn)超塑性和偽彈性。二次孿晶中的孿生模式為{11(?)1},為首次在鎂中發(fā)現(xiàn),該孿晶的產(chǎn)生是由于初次孿晶界附近的應(yīng)力集中而造成的。此外,鎂納米線中的{11(?)1}孿晶發(fā)現(xiàn)屬于非對稱結(jié)構(gòu)。由此結(jié)果出發(fā)進(jìn)一步拓展,研究了1(?)00對稱傾斜晶界的結(jié)構(gòu)能量,發(fā)現(xiàn)了兩類新的穩(wěn)定結(jié)構(gòu):(1){11(?)3}孿晶確定為一種穩(wěn)定結(jié)構(gòu)且其結(jié)構(gòu)也是非對稱的;(2){11(?)6}孿晶中發(fā)現(xiàn)晶粒的再取向現(xiàn)象。1(?)00對稱傾斜晶界中新結(jié)構(gòu)的發(fā)現(xiàn)對于理解晶界結(jié)構(gòu)及其演化過程提供新的例子�？傊�,本文通過分子動力學(xué)模擬提出在HCP金屬中達(dá)到較高塑性和可回復(fù)應(yīng)變的可能途徑,對材料變形機(jī)制的研究以及高塑性材料的開發(fā)提供新的思路。
[Abstract]:Plasticity is an important mechanical property of materials, which determines the amount of deformation that can be realized. Improving the plasticity of materials has always been the focus of the study of structural materials, and pseudoelasticity is an important property in shape memory materials. High recoverable strain can absorb more mechanical work, so increasing recoverable strain has always been the focus of shape memory materials. Nanowires have super plasticity. It has been reported that by stress-induced phase transformation or twinning deformation, the elongation of nanowires can reach about 50%, which is called superplasticity. In addition, nanowires are due to surface effects. After deformation, nanowires can be restored to the initial state to achieve pseudo-elasticity. Therefore, nanowires are suitable systems for obtaining superplasticity and ultra-high recoverable strain. At present, the mechanical properties of nanowires are mainly focused on face-centered cubic and body-centered cubic structures. Due to the lack of slip systems, the ductility of dense hexagonal metals is generally considered to be poor, and the elongation of dense hexagonal metals in coarse crystalline states is usually less than 10. However, in nanowires, the ductility of dense hexagonal hexagonal metals is less than 10. The mechanical properties of dense hexagonal metals still need to be further explored. Two kinds of dense hexagonal metal nanowires were selected as the object of study-stress-induced phase transition and stress-induced twinning. The former was represented by pure cobalt and cobalt-ferroalloy nanowires. The latter is represented by pure magnesium nanowires. The feasibility of superplasticity and pseudoelasticity of nanowires is predicted by simulating uniaxial stretching and unloading process of nanowires by molecular dynamics. The main findings are: (1) in cobalt nanowires, through HCP. 鈫扚CC. 鈫扵he two-step structural phase transition of HCP can reach an elongation of about 80%, and the deformed nanowires can be restored to the initial configuration, and the strain can be restored to about 71.The phase transition during deformation can be calculated by static method. The energy of nanowires is expressed as a function of strain, and the energy relations between different configurations in cobalt-ferroalloy nanowires are calculated. And it is used to judge the sequence of stress-induced phase transition and the transformation type. (2) in magnesium nanowires, the elongation of nanowires can reach about 60% by stress-induced secondary twinning, and the deformation can be recovered by de-twin crystallization. In other words, superplasticity and pseudoelasticity were found in magnesium nanowires. It is found for the first time in magnesium that the twin is caused by the stress concentration near the primary twin boundary. The twins are found to be asymmetric structures. Two kinds of new stable structures have been found in the structure energy of symmetrical inclined grain boundary. The twin is determined to be a stable structure and its structure is also asymmetric. It is found that the reorientation phenomenon of the grains in the twin crystal is. 1? The discovery of new structures in symmetric inclined grain boundaries provides a new example for understanding grain boundary structures and their evolution. In summary, a possible way to achieve high plasticity and recoverable strain in HCP metals is proposed by molecular dynamics simulation. The research on deformation mechanism of materials and the development of high-plastic materials provide new ideas.
【學(xué)位授予單位】：上海交通大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：O341;TB383.1
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本文編號：1636144