【摘要】：提高微晶、納米晶材料的熱穩(wěn)定性是發(fā)揮其力學(xué)性能優(yōu)勢以及實現(xiàn)其廣泛工程應(yīng)用的關(guān)鍵。因而對微晶,納米晶在成型過程和處理過程中的各項穩(wěn)定性特征的研究具有重要意義。本文以低固溶度的Ni-B合金為研究對象,從實驗出發(fā)并結(jié)合理論模型深入研究了其亞穩(wěn)晶粒的形成和穩(wěn)定化的機(jī)制。得出的主要結(jié)論如下:(1)利用循環(huán)過熱和熔融凈化的深過冷法制得了微晶Ni-1at.%B合金,其凝固組織形貌在ΔT80 K前經(jīng)歷了一個由枝晶向條狀晶再到等軸細(xì)晶的過程。通過對不同過冷度下Ni-B合金的多項微觀結(jié)構(gòu)進(jìn)行分析和表征,我們分析了不同過冷度的合金組織在凝固的不同階段的晶粒細(xì)化機(jī)理。同時以合金中溶質(zhì)拖拽力,溶質(zhì)的釘扎作用以及初始過冷度的相互作用為基礎(chǔ),分析了Ni-B微晶合金凝固過程中固態(tài)變化過程中的晶粒生長機(jī)理,即隨著過冷度的增加(80-230 K),在快速凝固之后到淬火之前,Ni-B合金微晶組織的生長方式經(jīng)歷了一個由正常到異常再變?yōu)檎５倪^程。(2)立足于有關(guān)激活能和晶界移動性的Arrhenius方程,建立了一個合金穩(wěn)定化過程中的激活能變化的新模型,模型的驗證和分析以納米RuAl合金基礎(chǔ)。同時,我們以晶粒生長的拋物線模型為基礎(chǔ),將熱力學(xué)模型與動力學(xué)模型相結(jié)合,通過多種方法對晶粒生長的熱動力學(xué)模型進(jìn)行了建立和討論。(3)通過對等溫退火處理后的Ni-B微晶合金的晶粒、晶界形貌以及合金中溶質(zhì)成分、組成物相的分析,合金的穩(wěn)定性得到了研究。在長時間的退火過程中合金的組織經(jīng)歷了一個生長到初始穩(wěn)定和再生長到最終穩(wěn)定的過程,在退火過程中晶粒的二次生長與晶界處第二相沉淀的產(chǎn)生密切相關(guān),其對晶粒的二次生長具有一定的促進(jìn)作用。通過對Ni-B微晶合金晶粒生長進(jìn)行擬合計算和穩(wěn)定化過程中晶界能變化的分析,整個退火過程中晶粒的生長機(jī)理得到了研究。退火過程中,溶質(zhì)原子B在晶界處的偏聚和富集使得合金組織得到了初始的穩(wěn)定;但隨著第二相沉淀在晶界處形成,其使得原先減小的晶界能又重新增加,進(jìn)而為晶粒的二次生長提供了新的驅(qū)動力;同時沉淀相也具有拖拽力的作用,當(dāng)拖拽力PZ逼近與驅(qū)動力PD時,微晶合金晶粒達(dá)到最終的穩(wěn)定。
[Abstract]:Improving the thermal stability of microcrystalline and nanocrystalline materials is the key to give full play to their advantages in mechanical properties and to realize their extensive engineering applications. Therefore, it is of great significance to study the stability characteristics of microcrystals and nanocrystals in the process of forming and treatment. In this paper, the mechanism of metastable grain formation and stabilization of Ni-B alloy with low solid solubility was studied by experiments and theoretical model. The main conclusions are as follows: (1) Microcrystalline Ni-1at.%B alloy was prepared by means of cyclic superheating and melting purification. The solidification microstructure of the alloy experienced a process from dendrite to striped grain to equiaxed fine grain before 螖 T _ (80 K). By analyzing and characterizing the microstructure of Ni-B alloy with different undercooling degree, we have analyzed the grain refinement mechanism of the alloy with different undercooling degree at different stages of solidification. At the same time, based on the solute drag force, solute pinning and the interaction of initial undercooling degree, the grain growth mechanism of Ni-B microcrystalline alloy during solidification was analyzed. That is, as the degree of undercooling increases (80? 230 K), after rapid solidification to pre-quenching, The growth mode of microcrystalline structure of Ni-B alloy has undergone a process from normal to abnormal and then to normal. (2) based on the Arrhenius equation concerning activation energy and grain boundary mobility. A new model of activation energy variation during alloy stabilization was established. The verification and analysis of the model were based on nano-RuAl alloy. At the same time, based on the parabola model of grain growth, we combine the thermodynamic model with the kinetic model. The thermodynamic models of grain growth were established and discussed by various methods. (3) the grain size, grain boundary morphology, solute composition and composition of Ni-B microcrystalline alloy after isothermal annealing were analyzed. The stability of the alloy has been studied. During the long annealing process, the microstructure of the alloy has undergone a process of growth to initial stability and re-growth to final stability. During annealing, the secondary growth of grains is closely related to the formation of the second phase precipitation at the grain boundary. It can promote the secondary growth of grain. The grain growth mechanism of Ni-B microcrystalline alloy during annealing has been studied by fitting calculation and analyzing the variation of grain boundary energy in the process of stabilization. During annealing, the segregation and enrichment of solute atom B at the grain boundary makes the initial microstructure of the alloy stable. However, with the formation of the second phase at the grain boundary, the original reduced grain boundary energy increases again, thus providing a new driving force for the secondary growth of the grain. At the same time, the precipitate phase also has the effect of drag force. When the drag force PZ approaches to the driving force PD, the grain size of the microcrystalline alloy reaches the final stability.
【學(xué)位授予單位】：中國礦業(yè)大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2016
【分類號】：TG146.15

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