本文選題：相場方法　切入點(diǎn)：奧氏體/鐵素體相變　出處：《上海交通大學(xué)》2015年博士論文

【摘要】：在新一代高性能鋼鐵材料的研發(fā)中,通過在熱處理加工過程里發(fā)生固態(tài)相變時(shí)進(jìn)行組織調(diào)控,是提高鋼材綜合性能的有效方法之一。奧氏體→鐵素體相變是鋼鐵材料冷卻過程中最重要的組織轉(zhuǎn)變之一,鐵素體晶粒尺寸和形態(tài)對鋼材的力學(xué)性能有重要的影響,基于其理論意義和工業(yè)應(yīng)用價(jià)值,該相變一直是材料研究領(lǐng)域中的重點(diǎn)和熱點(diǎn)。當(dāng)鐵素體從過冷奧氏體中析出時(shí),它可能會具有不同的形態(tài),包括無定形仿晶界鐵素體、塊狀鐵素體、晶內(nèi)等軸鐵素體和針狀魏氏鐵素體等,材料的組織形態(tài)進(jìn)一步地決定了鋼材的各種性能。因此,系統(tǒng)研究奧氏體→鐵素體相變的熱力學(xué)和動力學(xué),深入理解不同鐵素體組織的生長行為,這對于分析鋼材加工工藝與組織形態(tài)間的定量關(guān)系、設(shè)計(jì)合金成分以獲得所需的綜合性能,都有十分重要的意義。本文,通過建立描述奧氏體→鐵素體相變的相場模型,并且考慮因生成環(huán)境各異相變主控因素的不同,系統(tǒng)地研究了不同類型鐵素體新相的形貌特征和生長動力學(xué)行為,其中包括由動力學(xué)系數(shù)的各向異性主控的沿晶界生長鐵素體、由界面能的各向異性主控的晶內(nèi)形核生長鐵素體及由彈性能的各向異性主控的從晶界向晶內(nèi)生長魏氏鐵素體。具體地:(1)模擬研究了在化學(xué)自由能和各向同性界面能的共同作用下,于晶界處發(fā)生奧氏體→鐵素體相變時(shí)新相的生長動力學(xué)行為和形貌特征。在長程擴(kuò)散型相變中,揭示了界面處成分滿足局域平衡條件,新相生長動力學(xué)符合拋物型規(guī)律。若初始兩個(gè)新相核胚同時(shí)生長,會發(fā)生相互作用,伴隨著碰撞加速和碰撞富集等現(xiàn)象。在塊狀相變中,模擬再現(xiàn)了碳原子僅發(fā)生短程擴(kuò)散行為及界面快速遷移等轉(zhuǎn)變特征。此外,通過引入動力學(xué)系數(shù)的各向異性特征,計(jì)算獲得的新相形如扁橢圓狀,與實(shí)驗(yàn)結(jié)果相近,澄清了短路擴(kuò)散行為是決定晶界鐵素體形貌的關(guān)鍵因素。(2)模擬研究了在化學(xué)自由能和各向異性界面能的共同作用下,于晶內(nèi)發(fā)生奧氏體→鐵素體相變時(shí)新相形貌演變的動力學(xué)行為。而且,通過開展耦合各向異性界面能和各向同性相變驅(qū)動力ΔFmc的Allen-Cahn模型的計(jì)算分析,系統(tǒng)地討論了各向異性收縮作用與各向同性長大作用之間的競爭行為及其對新相形貌的影響,并解釋了晶內(nèi)鐵素體穩(wěn)態(tài)形貌與Wulff之間的相似性。當(dāng)ΔFmc的數(shù)量級不斷增大,穩(wěn)態(tài)形貌與Wulff之間存在著三種典型的相似關(guān)系:與Wulff相似地收縮、與Wulff相似地長大及與Wulff不相似地長大。即當(dāng)ΔFmc→0時(shí),穩(wěn)態(tài)形貌與Wulff相似,可由與相場模型等價(jià)的界面模型推導(dǎo)證明;而隨著ΔFmc不斷增大,穩(wěn)態(tài)形貌逐漸偏離Wulff,且不再相似時(shí)對應(yīng)的“臨界”ΔFmc值是與發(fā)生Fisher現(xiàn)象的臨界值ΔFFisher相符的。正是由于Fe-C合金的實(shí)際相變驅(qū)動力遠(yuǎn)小于由相似向不相似轉(zhuǎn)變對應(yīng)的理論臨界值,故模擬獲得的晶內(nèi)鐵素體穩(wěn)態(tài)形貌與Wulff相似,是界面能量最低的構(gòu)型。(3)模擬研究了在化學(xué)自由能、各向同性界面能和各向異性彈性應(yīng)變能的共同作用下,發(fā)生奧氏體→魏氏鐵素體相變時(shí)新相的形貌特征和生長動力學(xué)行為。計(jì)算結(jié)果揭示了彈性能對新相的核胚形狀、生長形貌及不同方向上的長大動力學(xué)行為都有重要影響。當(dāng)體積一定時(shí),橢圓形核胚的形核激活能最低,最易出現(xiàn)。在生長過程中,新相形如針狀,與實(shí)驗(yàn)觀察形貌相一致;且新相沿不同方向上的動力學(xué)特征顯著不同,即沿水平方向呈拋物型增厚,而沿豎直方向呈線性伸長。這是因?yàn)?新相的側(cè)邊可視作“平直界面”的推進(jìn),增厚行為滿足擴(kuò)散型相變中的拋物型長大規(guī)律;而新相的頂部較快地由初始構(gòu)型演變成有穩(wěn)定曲率半徑的尖端,使得其邊緣處濃度梯度保持不變,存在著穩(wěn)恒質(zhì)量流。此外,隨著過冷度或者過飽和度的增大,相變驅(qū)動力增加,新相伸長速度增大。
[Abstract]:In the development of a new generation of high performance steel materials, the organization controlled by heat treatment in the machining process occurred in solid phase, is one of the effective ways to improve the comprehensive performance of steel. The austenite to ferrite transformation is one of the most important cooling process of steel materials in the organization, ferrite has important effects on mechanical properties the grain size and morphology of steel, its theoretical and industrial application value based on the phase change has been an important and hot research in the field of material. When the ferrite precipitated from undercooled austenite when it may have different forms, including amorphous imitation grain boundary ferrite, blocky ferrite in the crystal body, equiaxed ferrite and acicular widmanstatten ferrite materials, the organization further determines the performance of steel. Therefore, the system research of austenite to ferrite transformation thermodynamics and dynamic learning, in-depth Understanding the different ferrite growth behavior, the quantitative analysis on the relationship between steel processing technology and organization form between the design of the components of the alloy to obtain comprehensive performance required, are of great significance. In this paper, through the establishment of a description of the phase field model of austenite to ferrite transformation, and consider the environment due to the formation of different transformation of main control factors, a systematic study of the different types of ferrite phase morphology and growth dynamics, including the main control by the anisotropy of the kinetic coefficients along grain boundary ferrite, the anisotropy dominated by the interfacial energy of intragranular nucleation and growth of ferrite and the anisotropy the main performance of the elastic anisotropy from grain boundary to the grain growth of Widmanstatten ferrite. Specifically: (1) studied in chemical free energy interaction and isotropic interfacial energy, occurs in the grain boundary of austenite to ferrite The growth dynamics of the new phase transformation and morphology. In the long-range diffusion transformation, reveals the composition of the interface to meet local equilibrium conditions, a new phase of growth kinetics with parabolic law. If the initial two new phase and embryo growth, may interact with the collision and collision acceleration enrichment phenomenon in massive transformation, simulation of the carbon atom only short-range diffusion behavior and interface fast migration transformation characteristics. In addition, the anisotropy introduced into dynamic coefficients, obtained the new phase such as flat ellipse shape, similar to the experimental results, clarify the short-circuit diffusion behavior is the key of grain boundary ferrite morphology. (2) studied in chemical free energy interaction and anisotropy of interfacial energy, in the grain dynamic behavior of austenite to ferrite transformation and new phase morphology evolution. And, through the calculation and analysis of Allen-Cahn model of coupled Anisotropic Interfacial Energy and isotropic phase transformation driving force Fmc the competition between anisotropic shrinkage and isotropic growth effect and its influence on the new phase morphology are discussed systematically, and explain the intragranular ferrite between body morphology and steady state similar to that of Wulff. When the number of level Delta Fmc increasing, between the steady morphology and Wulff there are three typical similarity relation: similar contraction and Wulff, similar to Wulff and not grow up grow up with Wulff. That is similar to a Fmc when 0, the steady state morphology similar to Wulff, can be proved by the derivation of the interface model and phase field model equivalent; and with the increase of Fmc, the steady-state morphology gradually deviate from the corresponding Wulff, and no similar "critical" delta Fmc value is the critical value of the phenomenon and the occurrence of Fisher Delta FFisher match. It is from Fe-C The actual transformation of the alloy is much smaller than the critical value of theory driven by similar to similar transitions correspond, so simulation of intragranular ferrite steady-state morphology similar to Wulff, is the lowest energy configuration interface. (3) studied in chemical free energy, isotropic interfacial energy interaction and anisotropic elastic strain energy under the occurrence of austenite to ferrite transformation Widmanstaten new phase morphology and growth dynamics. The calculated results reveal the elastic energy of new phase nucleus shape, have an important impact on the growth kinetics and growth morphologies in different directions. When a certain volume, oval nuclei nucleation minimum activation energy and the most vulnerable. In the growth process, a new phase such as acicular, consistent with experimental observations of morphology; and different dynamic characteristics in the direction of the new phase is significantly different, along the horizontal direction along the parabolic thickening. The vertical direction linear stretch. This is because the new phase side can be regarded as a "flat interface" to promote, thickening of parabolic type diffusion behavior to meet the growth and transformation of new phase; the top quickly evolved into a stable configuration by the initial radius of curvature of the tip of the edge of the concentration gradient remain unchanged there is a steady flow and quality. In addition, with the increase of undercooling or supersaturation, phase change driving force increases, the new phase elongation rate increases.

【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】：TG142.1

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