【作者】 赵宇宏；

【导师】 陈铮；

【作者基本信息】 西北工业大学 ， 材料学， 2003， 博士

【摘要】 采用微观相场模型，基于离散格点形式的微观扩散方程和非平衡自由能函数，在国内首先编制了二元、三元镍基合金的原子层面计算机模拟程序，无需预先设定新相结构和转变路径，自动描述可能的非平衡相、原子簇聚、有序化等，适用于沉淀的全过程以及全部成分范围，可获得原子图像、序参数及有序相体积分数等信息。系统地研究了存在有序化和原子簇聚的复杂体系的早期沉淀机制、两种有序相共存的析出序列、共格弹性应变能的影响。为客观地验证模拟结果，与国、内外相关的实验研究和理论进行了对比，主要结果获得印证，并得到了目前实验手段未能或难以发现的现象和规律。 低浓度Ni-Al合金γ′相的沉淀机制为非经典形核长大，有序化与原子簇聚过程同时发生，没有发现GP区。随浓度增加，非经典形核长大的特征增强，有序相临界晶核成分序参数和长程序参数降低，且分布更为平缓，沉淀序列为：过饱和固溶体→非化学计量比有序相→化学计量比平衡γ′相→长大。 高浓度Ni-Al合金γ′相的沉淀机制为：等成分有序化+失稳分解，有序化先于原子簇聚发生。等成分有序化产生非化学计量比的单相有序畴，失稳分解主要发生在反相畴界处，形成无序相，并促进畴内的失稳分解，在畴内形成无序相，但随时间延续，畴内无序相逐渐消失。沉淀序列为： 中间浓度Ni-Al合金的沉淀机制没有发生不连续转变，随浓度增加，逐渐由非经典形核长大的沉淀特征向失稳分解沉淀特征转变，主要表现为：有序化过程提前、加快；有序相形成时浓度降低，尺寸变大直至形成浓度起伏很低的单相有序畴；有序相出现失稳分解等。靠近低浓度区的沉淀机制为非经典形核长大，与低浓度合金的沉淀机制相似；靠近高浓度区的沉淀机制为非经典形核长大+失稳分解，与高浓度合金沉淀机制更接近，沉淀序列为：西北工业大学博士学位论文 低浓度NivsAlxV25一合金的e相先于丫相析出，e相的沉淀机制为:等成分有序化+失稳分解;丫相在e相的反相畴界处非经典形核，二者均先形成非化学计量比有序相，之后向化学计量比有序相转变。随Al浓度增加，e相发生等成分有序化的过程越来越慢，丫相临界核心成分值越来越平缓，尺寸越来越大且所占比例增加。 中间浓度N玩Alxv25一、合金丫相和e相同时析出，随Al浓度增加，丫相所占比例继续增加，e相减少;丫相沉淀方式由非经典形核机制逐渐向等成分有序化+失稳分解机制转变，0相沉淀方式则由等成分有序化+失稳分解逐渐向非经典形核机制转变，e相和丫相都经历了既具非经典形核特征又具失稳分解特征的混合机制的沉淀过程，二者沉淀序列和机制的转变并非在某一点发生突变，而是连续过渡的。 高浓度Ni75AlxV25-x合金沉淀过程中，，·与I区合金相反，丫相先出现，沉淀方式为等成分有序化+失稳分解，e相在丫相的妙BS上非经典形核，二者均先形成非化学计量比有序相，之后向化学计量比有序相转变。随Al浓度增加，丫相发生等成分有序化的过程更快，失稳分解的过程放缓，O相临界核心成分值越来越尖锐，尺寸越来越小且所占比例减少。 研究共格弹性应变能对模型合金沉淀相形貌的影响规律发现:当共格弹性应变能为零时，沉淀相为球状弥散分布的颗粒或“海绵状”结构，呈各向同性特征;随应变能作用的增强，沉淀相分布的取向性越来越明显，趋于沿弹性“软”方向分布，形成类方格阵、纵横层状结构等，呈各向异性特征。更多还原

【Abstract】 With the microscopic phase-field model, the atomic-scale computer simulation programs of the binary and ternary Ni-based alloys were firstly worked out based on the microscopic diffusion equation and nonequilibrium free energy. It can be applied to the whole precipitation process and composition range. Any a prior assumptions on the new phase structure or transformation path was unnecessary, the possible nonequilibrium phases, atomic clustering and ordering could be described automatically, and atomic pictures, order parameters and volume fractions of precipitates were obtained. Computer simulation was performed systematically on the precipitation mechanism, precipitation sequence and influence of coherent elastic strain energy in complicated system with ordering and clustering simultaneously. The main simulation results were confirmed by comparing the experimental results and theories of others, meanwhile, the phenomenon and laws were also obtained, which are difficult or unable to discover in experiments at present. The main conclusions were summarized as follows:Non-classical nucleation and growth mechanism of y’ ordered phase occurs in the lower composition Ni-Al alloys. Ordering and clustering occurs simultaneously and no GP zones are found. With the increase of Al composition, there is more characteristic of Non-classical nucleation and growth, and the order parameter values of critical ordered nucleus decrease. The precipitation sequence is:Supersaturated solid solution Non-classical NG Nonstoicheometric ordered phasestoicheometric ordered phaseGrowthFor higher composition Ni-Al alloys, the precipitation mechanism of y’ phase is congruent ordering followed by spinodal decomposition, and ordering occurs prior to clustering. By congruent ordering, a nonstoicheometric single ordered phase is produced with the ordered domains being separated by antiphase domain boundaries (APBS). Spinodal decomposition occurs predominantly at APBS, which results in the replacement of the APBS by the two-order/disordered interfaces. Spinodal decomposition at APBS promotes the decomposition in domains, however, the disordered phase in domains disappear as time follows. The precipitation sequence is as follows:Supersaturated solid solution Congruent ordering Nonstoicheometric ordered phaseSpinodal decomposition stoicheometric ordered phaseNo incontinuous transition of precipitation mechanism has been found in middle composition Ni-Al alloys. With the increase of Al composition, the precipitation characteristic transforms from non-classical nucleation and growth to Spinodal decomposition. Ordering occurs earlier and quicker than clustering; the composition in critical nucleus is lower and the size is larger; Spinodal decomposition occurs in ordered phase. The precipitation mechanism near lower composition alloys zone is non-classical nucleation and growth mechanism, which is similar to that of lower composition alloys. Non-classical nucleation and growth + Spinodal decomposition mechanism occurs in alloys near higher composition zone, which is similar to that of higher composition alloys. The precipitation sequence is as follows:Supersaturated solid solution Non-classical NG Nonstoicheometric ordered phaseSpinodal decomposition stoicheometric ordered phaseFor Ni75AlxV25-x alloy with lower Al composition, ordered phase precipitated earlier than y’ ordered phase by congruent ordering + spinodal decomposition mechanism and thus produced a nonstoicheometric single ordered phase. Then, the nonstoicheometric r’ phase precipitated by a non-classical nucleation and growth mechanism at the APBS of phases. With the increase of Al composition, the congruent ordering process of phase be more slowly, and the composition in critical r nucleus is lower and the size is larger. Meanwhile, the occupation percentage of r increase. Meanwhile, both of them transformed to stoicheometric ordered phases.For Ni75AlxV25-x alloy with middle Al composition, and y’ ordered phase precipitated at the same time. With the更多还原