Mechanism of Bainite Nucleation in Steel, Iron and Copper Alloys

During the incubation period of isothermal treatment(or aging) within the bainitic transformation temperature range in a salt bath (or quenching in water) immediately after solution treatment, not only are the defects formed at high temperatures maintained, but new defects can also be generated in alloys, iron alloys and steels. Due to the segregation of the solute atoms near defects through diffusion, this leads to non-uniform distributions of solute atoms in the parent phase with distinct regions of both solute enrichment and solute depletion. It is proposed that when the Ms temperature at the solute depleted regions is equal to or higher than the isothermal (or aged) temperature, nucleation of bainite occurs within these solute depleted regions in the manner of martensitic shear. Therefore it is considered that, at least in steel, iron and copper alloy systems, bainite is formed through a shear mechanism within solute depleted regions, which is controlled and formed by the solute atoms diffusion in the parent phase.     

Continuous Cooling Bainite Transformation Characteristics of a Low Carbon Microalloyed Steel under the Simulated Welding Thermal Cycle Process

Continuous cooling transformation of a low carbon microalloyed steel was investigated after it was subjected to the simulation welding thermal cycle process and the interrupted cooling test.Microstructure observation was performed by optical microscopy and transmission electron microscopy.On the basis of the dilatometric data and microstructure observation,the continuous cooling transformation（CCT） diagram was determined,which showed that the main microstructure changes from a mixture of lath martensite and bainitic ferrite to full granular bainite with the increase in the cooling time t_8/5 from 10 to 600 s,accompanied with a decrease in the microhardness.The interrupted cooling test confirmed that the bainitic ferrite can form attached to grain boundaries at the beginning of transformation even if the final microstructure contains a mixture of granular bainite and bainitic ferrite.     

Nucleation Thermodynamics inside Micro/nanocavity

A thermodynamic approach at the nanometer scale was performed for the heterogeneous nucleation inside nanocavity, and an analytical expression of the critical energy of nucleation was evaluated considering a rough ball nucleus nucleating inside nanocavity. Compared with the case of the nucleation locating on planar or convex substrate, the critical energy of nucleation inside the concave substrate is the smallest. Based on the thermodynamic and kinetic analyses, at low supersaturation, the smaller the curvature radius of cavity and/or the smaller the contact angle, the smaller the critical energy of nucleation, and the larger the nucleation rate. At high supersaturation, the nucleation rate increases with increasing the contact angle and/or increasing the curvature radius of cavity. In this way, at the low supersaturation, the heterogeneous nucleation rate is larger than the homogeneous one, as the nucleation rate is mainly determined by the heterogeneous nucleation. At the high supersaturation, the heterogeneous nucleation rate is smaller than the homogeneous one, as the nucleation rate is mainly determined by the homogeneous nucleation.     

高应变率下金属塑性损伤的形核机理

本文介绍了高应变率下金属塑性损伤形核模型的研究现状，并对现有的几种形核模型作了评述。     

Stress orientation of second-phase in alloys: Hydrides in zirconium alloys

A model for precipitation of the plate-shaped second-phase under applied stress is presented. The precipitates in the matrix-precipitate system are represented by their local volume fraction ζ and an orientation parameter θ that defines the alignment of a precipitate platelet in a given direction. Kinetic equations, based on diffusion theory and classical nucleation theory, are used to describe the time evolution of ζ and θ. The model is used to describe the stress orientation of hydrides in Zr-alloys in light of experiments.     

CdSe 在玻璃中的核化过程

用高分辨电镜、X-射线衍射研究了 CdSe 在玻璃中的成核和生长过程.首次观察到,伴随着这种成核和生长过程,玻璃的光吸收限、光吸收限的温度系数,以及光吸收系数和光子能量的依从关系都发生变化.     

CVD金刚石成核的最新研究

研究了化学气相条件下金刚石在非均匀研磨硅基底表面及镜面基底和均匀研磨基底边缘及角域处的成核行为。发现ＣＶＤ金刚石成核不仅依赖于沉积区缺陷，更主要由缺陷的锐度决定，即缺陷加强ＣＶＤ金刚石成核的锐度效应。在对无序碳上ＣＶＤ金刚石成核研究的基础上，讨论了ＣＶＤ金刚石成核的机理，并由此阐明了各种表面预处理及负偏压等增强ＣＶＤ金刚石成核的微观过程。     

Nucleation Processes Close to the Spinodal

Using an analytical equation of state (EOS) which describes a system that can exist in a liquid or vapor phase, a generalized Landau free-energy expansion around any spinodal point is constructed. For this simple model, the critical exponents associated with the spinodal line are obtained, and the model reduces to the usual expansion around the critical point as a particular case. The approach may become an interesting starting point to study phase separation mechanisms, such as nucleation and spinodal decomposition, far from the critical point. The decay of deeply quenched metastable states close to the spinodal through a nucleation process is analyzed and their mean lifetime is obtained. The results are compared with those obtained through the use of the usual Landau free-energy expansion around the critical point.     

采用正交法研究金刚石偏压形核

在微波等离子体化学气相沉积装置中 ,采用正交试验法研究金刚石在镜面抛光的Si( 1 0 0 )面上的偏压形核过程中 ,形核时间、偏压电压、气压及甲烷浓度对形核密度的影响 ,研究结果表明 :形核密度随形核时间的增加而增加 ,适中的偏压电压和沉积气压有利于金刚石的形核 ,而甲烷浓度的影响很小。正交试验所得的最佳形核条件为偏压 -1 5 0V ;时间 1 2min ;气压 4kPa;CH4 比率 5 % ,在该条件下金刚石的形核密度达到 1 0 1 0 个 cm2 。     

Nucleation of CdTe thin films deposited by close-space sublimation under a nitrogen ambient

The early stages of the close-space sublimation growth of CdTe/ITO (indium tin oxide) at 500°C under 26.7 kPa (200 Torr) of N₂ were investigated, the relatively high pressure being used to slow the growth. Film development was monitored over 60 mins by ex-situ AFM (atomic force microscopy), the growth being controlled by a shutter. The films formed by the ‘island’ or Volmer-Weber growth mechanism. Developments in the areal island growth rate, the island density and spatial distribution type were explained using growth mechanisms. Significant changes in these phenomena at t ≥ 10 mins were attributed to a change in growth mechanism from surface migration limited, to a regime in which island coalescence and direct impingement of vapour species on the islands became important. Since the islands are characterised by distinct crystalline facets, this indicates the growth mechanism to be uninterrupted step-flow like addition of material to each island. Arguments are given to relate the final grain size in the films to the density of nuclei that are stable to re-evaporation after t = 10 mins.     

镍衬底上定向金刚石膜的成核与生长

提出了一种包括晶粒接种、高温退火、成核、生长四过程的薄膜沉积新方法 ,用射频等离子体增强热丝化学气相沉积系统 ,在Ni衬底上制备了定向金刚石膜。通过对成核和生长两过程工艺条件的研究 ,掌握了提高成核密度和金刚石定向生长规律。实验还表明 ,膜与Ni衬底之间未见Ni C H界面层的形成     

Nucleation studies in supersaturated aqueous solutions of urea and thiourea doped with inorganic dopants

Induction periods were measured for various supersaturated aqueous solutions of urea doped separately with NH₄NO₃ and NaNO₃ and thiourea doped separately with NaCl and KCI by the direct vision method. Based on classical theory for homogeneous crystal nucleation various critical nucleation parameters were calculated and the results are reported and discussed. In all the cases, the critical nucleation parameters increased with increase in doping concentration.     

Crystal Nucleation and Growth of Al-based Alloys Produced by Electrolysis

The nucleation and growth of grains in a series of Al-based alloys produced by electrolysis are observed under SEM. The atomic Ti/Al ratios of the nuclei and the distribution of Ti at certain points are analyzed by point EDS. The particles in different atomic Ti/Al ratios might act as the nuclei of α-Al. At the early stage of growth, the spherical Ti-enriched regions might form around these particles within very limited temperature ranges in which the reactions such as the peritectic reactions etc occur.At the latter stage of growth, the dendrites freely develop in the radial orientations, and the concentration of Ti decreases linearly along the dendrite arm and becomes negligible in the region near the periphery of the dendrite. It is believed that the nucleation is closely related with the number and dispersion of primary spherical areas in the melts, and the segregation of Ti leads to the free growth of dendrite, which is necessary for the formation of equiaxial grains.     

Effect of Substrate Surface Microstructure on Heterogeneous Nucleation Behavior

The heterogeneous nucleation behaviors of NH4Cl crystal on a rough chilling surface of aluminum immerged in NH4Cl-H2O solution were experimentally analyzed and the relationship between the surface roughness and the nucleation site selection behaviors on polished aluminum substrate was investigated, and it was discovered that the number of nucleation sites decreases significantly with decreasing the roughness of the polished substrate. Further nucleation experiments were carried out on chemically etched aluminum substrate with regular micro-morphology on its surface. It has been shown that both the micro-morphology and the wettability vary with the substrate surface prepared by different etching process. The prepared surface with step-like structures has a strong wettability with NH4Cl-70 wt% H2O solution and the nucleation density of NH4Cl on the its surfaces is significantly higher than that of the reference surfaces, which shows that the geometrical morphology features have important effects on both the wettability and the nucleation behaviors.     

Critical ausforming temperature to promote isothermal bainitic transformation in prior-deformed austenite

The effects of deformation temperature on phase transformation and microstructure in nanostructured bainite steel were studied. The results indicate that the deformed austenite with a strain of 0.3 at 300°C presents accelerated kinetics of bainitic transformation. However, the amount of bainite in ausformed austenite then reduces with the increase in deformation temperature. A critical deformation temperature, determining whether the bainitic transformation can be promoted, was found in deformed austenite. In addition, the thickness of bainite plate in deformed austenite reduces with the decrease in ausforming temperature. The adjacent bainite ferrite plates grow up interactively, and the intersection angle is about 60–73°. A lower ausforming temperature contributes to a more serious cross-growth phenomenon of bainite plates.     

泡沫塑料成型机理研究

本文对泡沫塑料的成型机理进行了全面综合的分析，指出发泡成型各个阶段的机理及影响泡体性能和质量的因素，还把现有的各种发泡成型方法的成核过程归纳为三大类，指出每一类的成核机理，使用范围及主要影响参数，为进一步的深入研究提供参考。     

对中真空条件下铝蒸气相变形核及冷凝的研究

在中真空条件(18~50 Pa)下,从原子碰撞与碰撞凝聚的角度分析系统温度、铝分压及冷凝区温度梯度对铝蒸气相变形核及冷凝的影响。采用真空蒸发冷凝法(不充入惰性气体)制备出铝珠及铝粉,并利用扫描电镜与能谱进行表征。结果表明:在中真空条件(18~50 Pa)下,蒸气过饱和度增大,利于液相形核,这与温度对单位时间铝原子碰撞次数的影响一致,但当系统温度降到液气转变最低温度后,铝蒸气将冷凝成固态。中真空下,其影响蒸气冷凝方式的重要因素之一是系统中冷凝区的温度梯度。通过真空蒸发冷凝法,在冷凝区温度梯度约1.4 K/mm时制得金属铝珠,温度梯度约19.5 K/mm时制得金属铝粉。     

硬质合金YG8上金刚石薄膜的成核研究

利用调节基底表面碳流量的方法促进了热丝ＣＶＤ中硬质合金ＹＧ８上金刚石薄膜的成核，使成核期大为缩短，根据扫描电镜和拉曼光谱对沉积结果的分析，研究了ＹＧ８上金刚石薄膜成核的机理。     

Study of nucleation,growth and coarsening of precipitates in a novel 9%Cr heat resistant steel: Experimental and modeling

Nucleation, growth and coarsening of three different precipitates (NbC, M₂₃C₆ and V(C,N)) in a novel 9%Cr heat resistant steel designed by the authors were investigated. The microstructure evolution after tempering (780 °C/2 h) and after creep (650 °C/100 MPa) was characterized using transmission electron microscopy in the scanning mode (STEM). Thermodynamic and kinetic modeling was carried out using the softwares Thermo-Calc, DICTRA and TC-PRISMA. The Thermo-Calc software predicted formation of NbC, V(C,N) and M₂₃C₆ carbides at the tempering temperature of 780 °C. STEM investigations revealed that M₂₃C₆ precipitated on prior austenite grain boundaries and lath or block boundaries whereas NbC and V(C,N) were located within sub-grains. Simulations by TC-PRISMA showed that M₂₃C₆, NbC and V(C,N) particles nucleation begins as soon as the tempering treatment starts and it is completed in a very short time, reaching the equilibrium volume fraction after 40 s for M₂₃C₆, 100 s for NbC and 80 s for V(C,N). Best agreement between simulations and experimental investigations was found for low interfacial energy values of 0.1 J m⁻². Both STEM measurements as well as DICTRA simulations indicate very low coarsening rate for both kind of precipitates. Creep tests up to 4000–5000 h suggest that this special combination of NbC, V(C,N) and M₂₃C₆ may provide increased pinning of dislocations reducing boundary migration therefore enhancing creep strength.     

Nucleation studies in supersaturated aqueous solutions of KH2PO4 doped with KBr and K2Cr2O7

Induction periods were measured for various supersaturated aqueous solutions of potassium dihydrogen orthophosphate doped separately with potassium bromide and potassium dichromate by the direct vision method. Various critical nucleation parameters were calculated based on the classical theory for homogeneous crystal nucleation and the results are reported and discussed. The critical nucleation parameters increased with increase in doping concentration for both the dopants considered.