【作者】 褚忠；

【导师】 陈铮；

【作者基本信息】 西北工业大学 ， 材料学， 2006， 博士

【摘要】 以Ni-Cr-Al三元合金为对象，基于微观相场模型，只需输入唯一基本参数——原子间相互作用势，毋需输入无实际物理意义的参数，即可获得原子图像的演化、长程序参数和浓度的变化、平均长程序参数、平均浓度以及体积分数的变化过程，描述合金沉淀全过程的原子簇聚、有序化过程。本文研究了不同合金成分、不同温度下的沉淀序列和沉淀机制，分析了Cr原子替代行为以及分级时效对合金有序相尺寸、体积分数的影响。主要结论如下：当Al、Cr含量较低时，Ni-Cr-Al合金按非经典形核长大机制同时沉淀出L1₂相和D0₂₂相。随Cr含量增加，D0₂₂相沉淀机制由非经典形核长大向混合机制过渡，进一步向等成分有序化＋失稳分解机制过渡；L1₂相沉淀机制由非经典形核长大向混合机制过渡。随Al含量增加，L1₂相沉淀机制由非经典形核长大向混合机制过渡，进一步向等成分有序化＋失稳分解机制过渡，D0₂₂相逐渐消失。当Al、Cr含量都较高时，L1₂相和D0₂₂相沉淀机制都为失稳分解机制。沉淀温度在873K－1373K范围内，随着温度提高，L1₂相和D0₂₂相的沉淀机制由等成分有序化＋失稳分解机制转化为混合机制，进一步向非经典形核机制过渡；沉淀相形貌由片状向球状过渡。非经典形核长大机制的孕育期最长，失稳分解机制的孕育期最短，混合机制居中。随Al、Cr含量增加，孕育期缩短；随着温度的提高，孕育期延长。合金沉淀温度低于1223K时，首先以等成分有序化＋失稳分解机制沉淀出非化学计量比L1₂相。随后，Cr原子在L1₂相有序畴界处偏聚，进一步沉淀出D0₂₂相。部分D0₂₂相在L1₂相内沉淀。温度升高，L1₂相体积分数增加，D0₂₂相体积分数减少。1373K时，首先以非经典形核机制同时沉淀L1₂和D0₂₂相，D0₂₂相逐渐缩小直至消失，最终形成单一L1₂相。Ni₇₅Al_25-xCr_x合金中，Cr原子与Al原子同时发生有序化，共同占据L1₂相的β-格点，形成复合L1₂相(Ni₃Al_1-xCr_x)。在L1₂相畴界，Cr原子逐步取代Al原子位置，最终形成D0₂₂相（Ni₃Cr）。Ni_75-xAl₂₅Cr_x合金沉淀过程中，当Cr原子分数超过3％时，L1₂相内Cr原子在α-格点、β-格点占位几率接近极限值，在L1₂相相界Cr原子浓度逐步提高，形成D0₂₂相。在873K到1173K温度范围内，Ni-Cr-Al合金沉淀温度升高时，在L1₂相内的Cr原子在α-格点和β-格点的占位几率都提高，L1₂相体积分数增加，D0₂₂相体积分数减少。在中间处理阶段，合金沉淀机制为非经典形核长大；时效处理时，沉淀机制为等成分有序化＋失稳分解混合机制。时效温度提高时（973K－1373K），L1₂相尺寸增大，有序相体积分数提高；而降低中间处理温度，时效温度不变，L1₂有序相颗粒数量增多，直径减小，体积分数变化很小。与单级时效相比，二级时效和三级时效工艺可增大沉淀相颗粒尺寸和提高体积分数。更多还原

【Abstract】 The Ni-Cr-Al alloys are studied in this paper. The interaction energies of atoms are only the input parameter, and the evolution of atom morphology, the long range order（lro） parameter and concentration, the averaged lro parameter and concentration and volume fraction can be gotten based on the microscope phase-field model, and atomic clustering and ordering during the precipitation process of alloy could be obtained. The precipitation mechanism and precipitation sequence with different composition and temperature, the substitution behavior of Cr atoms, and the influence of progressive aging to the size and volume fraction of ordering phases are investigated.The L1₂ and D0₂₂ phases are precipitated by non-classical nucleation and growth mechanism during the precipitation process of Ni-Cr-Al alloy with lower Al and Cr concentration at 873K. With the increasing of Cr content, the precipitation mechanism of D0₂₂ phases transforms from non-classical nucleation and growth to the mixture of growth and spinodal decomposition gradually and convert to congruent ordering+spinodal decomposition at last, the precipitation mechanism of L1₂ phases transit to the mixture of growth and spinodal decomposition. When Al content increase, the precipitation mechanism of L1₂ phases change to the mixture of growth and spinodal decomposition gradually and convert to congruent ordering+spinodal decomposition at last. D0₂₂ phases disappear gradually. L1₂ and D0₂₂ phases precipitate by spinodal decomposition mechanism when Al and Cr contents are higher.In the temperature range of 873K to 1373K, the precipitation mechanisms of L1₂ and D0₂₂ phases transit from congruent ordering+spinodal decomposition to mixture mechanism and to non-classic nucleation with increasing of temperature. The shapes of precipitates become spheroid from slice shape.The incubation period of non-classical nucleation and growth mechanism is the longest and that of spinodal decomposition is the shortest, mixture mechanism is mediacy. The incubation period is short with increasing Al and Cr contents and become long gradually with increasing of temperature.Nonstoicheometric L1₂ phases are precipitated with congruent ordering+spinodal decomposition mechanism when the temperature is lower than 1223K, then Cr atoms gather at phases boundaries of L1₂ phases, D0₂₂ phases are formed gradually. The other of D0₂₂ phases appear inside of L1₂ phases. The volume of D0₂₂ phases decrease with the temperature rising. At 1373K, L1₂ and D0₂₂ phases precipitate simultaneously by non-classical nucleation and growth mechanism, the particles of D0₂₂ phases diminish and disappear gradually, L1₂ phases grow and single L1₂ phases are formed at last.The ordering of both Al and Cr atoms takes place simultaneously during the precipitation process of Ni₇₅Al_25-xCr_x alloy. Both atoms occupyβsites together, and complex L1₂ phases(Ni₃Al_1-xCr_x) are formed. Cr atoms substitute Al atoms at the boundary of L1₂ phases, and then the D0₂₂ phases（Ni₃Cr） are formed. The limits of occupation probabilities of Cr atαandβ-lattice in L1₂ phases reached when Cr fraction exceeds 3% in Ni_75-xAl₂₅Cr_x alloy, the concentration of Cr rises at L1₂ phases boundaries, and D0₂₂ phases are formed.The occupation probabilities of Cr atoms atαandβlattice in L1₂ phase increase with the temperature increasing in the range of 873K to 1173K, the volume of L1₂ phase increase and that of D0₂₂ phases decrease.The non-classical nucleation and growth mechanism is present at the stage of middle heat treatment, and precipitation mechanism transit to congruent ordering+spinodal decomposition at the stage of aging（973K～1373K）. The size and volume of L1₂ phases increase with the rising of temperature of aging, and they decrease with the decreasing of temperature of middle heat treatment. The size and volume of ordering phases in re-aging and three step aging are longer than those in single stage aging.更多还原