钢表面热浸渗铝层组织中的调幅分解及其分析

利用透射电镜（TEM）研究了钢表面热浸渗铝层的微观组织,结果表明,热浸渗层组织中出现大量调幅组织,调幅结构形成后,调幅组织经过长大,其波长由表面向内部从3nm逐渐增大到30nm。从渗层内部到表面,调幅组织形貌从网格状向平行交叉条状直到平行直线状过渡,部分区域调幅分解条纹平行于晶界呈环状。晶界对调幅分解的形貌产生明显的影响。通过计算Fe、Al原子尺寸因子,对调幅分解形貌的变化进行了分析。     

渗铝钢中的调幅分解及其形成原因

利用透射电子显微镜研究了经850 ℃×4 h扩散后热浸镀铝层的微观组织.结果表明,在扩散层的Fe2Al5 和 FeAl2 相上存在调幅分解.从扩散层表面向内部,调幅组织形貌从平行直线状向平行交叉条纹状再到网格状过渡.在扩散层的部分区域,晶界对调幅分解形貌有明显的影响.各晶粒内部有各自的调幅分解组织;在整个晶粒内,调幅分解条纹呈片层状.分析了调幅组织的形成原因.     

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利用透射电子显微镜研究了321/Qd370qD爆炸焊接界面附近基板内的组织。结果表明,基板珠光体铁素体内部存在大量的调幅分解组织。调幅分解主要有波长为2 nm和4 nm的波。利用二次衍射分析了调幅分解的电子衍射花样,并且得出调幅分解波上存在两种取向的纳米组织。     

铜钛合金调幅分解之特征

通过透射电子显微分析和Ｘ射线衍射分析对Ｃｕ－５２Ｔｉ（％，原子分数，下同）合金的调幅分解过程进行了研究。确认调幅分解是Ｃｕ－Ｔｉ合金相变初期的过程之一。盐水淬冷不能抑制调幅分解，淬火态调幅波长约为５７ｎｍ。研究表明，调幅分解与调幅组织的长大是两个完全不同的过程，分别属于连续相变和非连续相变。这两个过程的波长长大动力学均符合λ３－λ３０＝ｋｔ定律，长大激活能分别为１７６ｋＪ／ｍｏｌ和１３４ｋＪ／ｍｏｌ。电镜下看到的调幅组织不能作为调幅分解发生的判据。     

Cu—4Ti合金调幅分解的TEM研究

通过透射电子显微镜对Ｃｕ－４Ｔｉ合金的调幅分解过程进行了研究，确认调幅分解是Ｃｕ－Ｔｉ合金相变初期的过程之一。研究的淬火速度（盐水冷却）不能抑制调幅分解的发生，淬火态调幅波长为约为５．７ｎｍ。研究表明，调幅分解与调幅组织的长大是两个完全不同的过程，分别属于连续相变和非连续相变。这两个过程的波长长大动力分别符合λ＾３－λ＾３０＝Ｋｔ定律，长在激活能分别为１７６ＫＪ／ｍｏｌ和１３４ＫＪ／ｍｏｌ。     

铜钛合金调幅分解之特征

通过透射电子显微分析和Ｘ射线衍射分析对Ｃｕ－５．２Ｔｉ（％，原子分数，下同）合金的调幅分解过程进行了研究。确认调幅分解是Ｃｕ－Ｔｉ合金相变初期的过程之一。盐水淬冷不能抑制调幅分解，淬火态调幅波长约为５．７ｎｍ，研究表明，调幅分解与调幅组织的长大是两个完全不同的过程，分别属于连续相变和非连续相变，这两个过程的波长长大动力学均符合λ＾２－λ０＾３＝ｋｔ定律，长大激活能分别为１７６ｋＪ／ｍｏｌ和１３４ｋ     

321/Qd370qD爆炸焊接调幅分解特征

利用透射电子显微镜研究了321/Qd370qD爆炸焊接界面附近基板内的组织.结果表明,基板珠光体、铁素体内部存在大量的调幅分解组织,调幅分解主要有渡长为2 nm和4 nm的渡.电子衍射证明发生了有序化,出现了调幅分解和有序化的共存.探讨了调幅分解波上原住析出纳米颗粒的机制.     

非均匀弹性能作用下Fe-Al合金匀相转变动力学的微观相场研究

建立耦合非均匀弹性能的微观相场动力学模型,研究了Fe-Al合金匀相转变过程中微观组织演化和动力学行为。结果表明:在非均匀长程弹性交互作用能的影响下,Fe-Al合金时效过程中发生调幅分解与有序化共存相变,形成细小的调幅分解与有序化共存组织。在时效初期,合金中先迅速发生失稳有序化转变,在这个过程中长程序参数(有序度)逐渐增加,形成单一的粗大有序相并在相界上分布着少量无序相。随着时效时间增加,在有序相内部发生调幅分解,浓度起伏逐渐增大直至平衡,形成原子富集区和贫集区,呈现调幅分解与有序化共存。在这个过程中,长程序参数和浓度序参数连续增加,在时效后期趋于稳定。比较模拟结果和试验结果,发现模拟结果的调幅分解与有序化共存组织形态与试验结果相一致,尺寸略小。     

现代钛合金及其加工工艺

评述了过去２０多年来有关合金调整幅分解和连续有序化过程的研究情况，指出这种普遍化处理实质上澄清了合金中有序／无序反应，浓度波理论和倒易空间的近似方法提供了研究连续相变的基础，运用这些概念结合自由能一成分和相图系统评述了连续相变中的基本规律，最后根据我们的最近研究结果讨论了有序化和调幅分解共存反应的实质，并提出了利用连续相变使合金产生天然纳米材料的新概念以及合金中产生天然纳米组织的成分设计和制订热处     

321钢/Qd370qD钢爆炸焊接界面纳米晶组织

利用透射电子显微镜研究321钢/Qd370qD钢爆炸焊接界面附近的纳米晶组织。结果表明,界面附近出现液态合金的调幅分解现象。过冷度越大,调幅分解的波长越小,形成的纳米晶越小。调幅分解波纹内的有序原子集团通过合并长大,形成初生的小晶体。     

Self-organized layered growth phenomena of diffusion couples with spinodal decomposition in binary alloys

In order to investigate the formation mechanisms of the layered growth phenomena in diffusion couples with spinodal decomposition, a phase field model combined with elastic strain field was employed. Microstructure evolutions of diffusion couple with spinodal decomposition in binary alloys were numerically simulated by considering concentration fluctuation and elastic anisotropy. The simulation results indicate that the number of the periodical layers decreases with the increase of initial concentration fluctuation, even with large elastic anisotropy. The growth of layered microstructures can be attributed to the directional diffusion enhanced by initially discontinuous chemical potential at the interface.     

Al-Zn固溶体价电子结构与Spinodal分解反应

运用固体经验电子理论,采用平均原子模型对Al-Zn固溶体价电子结构进行计算.计算结果表明:Spinodal分解引起合金固溶体的共价键强度发生较大变化,且可以根据各固溶体相的共价键强度的变化对Al-Zn固溶体在室温下发生的Spinodal分解反应所引起的合金硬化以及合金硬度的提高做出合理解释.     

Computer simulation of spinodal decomposition in constrained films

The morphological evolution during spinodal decomposition of a binary alloy thin film elastically constrained by a substrate is studied. Elastic solutions, derived for elastically anisotropic thin films subject to the mixed stress-free and constraint boundary conditions, are employed in a three-dimensional phase-field model. The Cahn–Hilliard diffusion equation for a thin film boundary condition is solved using a semi-implicit Fourier-spectral method. The effect of composition, coherency strain, film thickness and substrate constraint on the microstructure evolution was studied. Numerical simulations show that in the absence of coherency strain and substrate constraint, the morphology of decomposed phases depends on the film thickness and the composition. For a certain range of compositions, phase separation with coherency strain in an elastically anisotropic film shows the behavior of surface-directed spinodal decomposition driven by the elastic energy effect. Similar to bulk systems, the negative elastic anisotropy in the cubic alloy results in the alignment of phases along 〈1 0 0〉 elastically soft directions.     

Interface-directed spinodal decomposition in TiAlN/CrN multilayer hard coatings studied by atom probe tomography

Microstructural and compositional changes in TiAlN/CrN multilayered films occurring at temperatures up to 1000 °C were studied at different length scales by a combination of atom probe tomography, transmission electron microscopy and X-ray diffraction. We observe the onset of decomposition of the multilayer structure at 700 °C via the mechanism of interface-directed spinodal decomposition of TiAlN layers, where Al atoms preferentially move toward the nearest interface and segregate there. The interface-directed mechanism later transforms into isotropic spinodal decomposition and is accompanied by intense interdiffusion between the constituting layers. Distinct compositional gradients across columnar grain boundaries (extending perpendicular to the multilayers) are detected at this stage of decomposition. Drastic differences in decomposition behavior across the film depth were observed at elevated temperatures (800–1000 °C): the layered structure completely dissolves in the near-surface part but persists in the regions distant from the surface. The influence of residual stresses caused by the sputter deposition process on the thermally induced evolution of the multilayer thin films is discussed.     

A model-based continuum treatment of ordering and spinodal decomposition

Using the absolute reaction rate theory, a continuum treatment of ordering and spinodal decomposition is derived from the original planar discrete lattice model which was based on a Bragg–Williams type of approach. For small amplitudes the result agrees with Cahn's linearized equation for spinodal decomposition. The kinetic equation for ordering has a factor not included in the Allen–Cahn equation which depends on the local degree of order and may have important consequences.     

Three-dimensional phase-field modeling of spinodal decomposition in constrained films

The morphological evolution during spinodal decomposition in binary alloy thin films elastically constrained by substrates is studied. Elastic solutions, derived for both elastically isotropic and anisotropic thin films subject to mixed stress-free and constraint boundary conditions, are employed in a three-dimensional phasefield model to investigate the effect of coherency strain and substrate constraint on microstructural evolution. The temporal evolution of the Cahn-Hilliard equation under thin film boundary conditions is solved with a semi-implicit Fourier-spectral method. The phase separation with coherency strain in an elastically anisotropic film shows the behavior of surface-directed spinodal decomposition driven by the elastic energy effect. Negative elastic anisotropy in the cubic alloy causes the alignment of the phases along <100> elastically soft directions. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

Modified EPR‐DL method for detection of the spinodal decomposition of 2205 duplex stainless steel

The presented work deals with the possibility of electrochemical detection of the spinodal decomposition of duplex stainless steel's ferritic phase using the modified electrochemical potentiodynamic reactivation double loop (EPR‐DL) method. Specimens of FeCr23Ni6Mo3N (2205) steel exposed to the model low‐temperature annealing at 420 °C for up to 1000 h were used in the experiment. The specimens were evaluated using potentiodynamic curves measurements in chloride solutions and microhardness measurements. Despite hardening the ferritic phase for up to 1 h, a substantial drop of resistance to attack by chlorides was reported only after 1000 h of annealing. The modified EPR‐DL method in 2 M HCl + 0.1 M KSCN solution enables sensitive identification of the spinodal decomposition rate already from early stages.     

Phase field study of grain boundary effects on spinodal decomposition

We have developed a phase field model of a polycrystalline alloy by combining the Cahn–Hilliard model [J Chem Phys 28 (1958) 258] with a model of polycrystals due to Fan and Chen [Acta Mater 45 (1997) 3297]. We have used this model to study grain boundary (GB) effects on spinodal decomposition (SD) in two-dimensional (2D) systems. In binary A–B systems with constant atomic mobility, when the GB-energy (γ_α) of the A-rich α phase is lower than that (γ_β) of the B-rich β phase, decomposition starts by enriching the GB with species A, setting off a composition wave that produces alternating α and β bands near the GB. Simultaneously, the grain interiors undergo normal SD. Thus, when decomposition ends, GB-bands coexist with grain interiors with spinodal microstructure. The number of GB bands is rationalized in terms of (γ_β−γ_α) and the rate of SD in the grain interior. Further, during decomposition, grain growth is effectively suppressed.     

Effect of charge separation on the stability of large wavelength fluctuations during spinodal decomposition

A stability analysis of phase separation of charged species by spinodal decomposition is presented. The charge effects introduce a short wave number cutoff for linear perturbations about the homogeneous, neutral solution. Phase field calculations using a semi-implicit spectral method support this conclusion. This suggests that coarsening is limited in ionic solid systems that are unstable with respect to charged-phase separation.