Effect of extrinsic grain-boundary dislocations on M23C6 precipitate nucleation in an austenitic stainless steel

From a comparison of the isothermal precipitation curve for M₂₃C₆ grain-boundary precipitates in an austenitic stainless steel and of the spreading times of extrinsic grain-boundary dislocations (EGBDs), it has been shown that M₂₃C₆ precipitates cannot directly nucleate on EGBD lines lying on random high-angle grain boundaries. This process can occur only on coherent and incoherent twin and other special boundaries.     

The relationship between activation energy and precipitate size for precipitate agglomeration

Superalloys are used in aggressive atmospheres in aero-engines and land-base gas turbines. IN738LC is an important superalloy used in gas turbines. Strengthening Ni₃(Al,Ti,Nb) type precipitate phase in this superalloy goes through size and morphological changes with time and temperature. Experimental observations suggest precipitate motion as a whole in the matrix leading to agglomeration of nearby precipitates for coarsening, which is in contrast to the traditional Ostwald ripening. Likewise, contrary to the conventionally accepted view of constant activation energy, experimental data in this study indicate varying activation energy, which causes coalescence and relates to the precipitate size in the form of an increasing polynomial.     

Non-isothermal calorimetric determination of precipitate interfacial energies

Equations suitable for computing specific interfacial energies of precipitates dissolving by a first order transformation were developed on the basis of the singularities taking place at the critical temperature,T _c, under equilibrium conditions. These equations employ data which can be determined by differential scanning calorimetry. Also the conditions for reaching dynamic equilibrium of particle volume fraction belowT _c are analysed in terms of a maximum permissible experimental heating rate. The results obtained for disperse order inCu-Al alloys are in good agreement with those expected from the observed particle features.     

A precipitate phase in AA2124

A new precipitate phase (designated X phase) other than the S(Al₂CuMg) precipitate phase has been discovered in the 2124 aluminium alloy. Using selected-area electron diffraction analysis, dynamical diffraction simulation, energy dispersion analysis of X-rays and high-resolution electron microscopy, it is suggested that the X phase has an orthorhombic crystal structure witha=0.492 nm,b=0.852 nm andc=0.701 nm. The space group of the phase is Cmmm. There are 10 atoms in a cell with AlCuMg=244. The orientation relationship between the X phase and the matrix is determined as (1–10)_m(010)_x, [–1–1–1]_m [001]_x.     

Stress induced precipitate microstructure in anisotropic alloys

Precipitate microstructure in the anisotropic alloys under applied strain and periodic dislocations were investigated by using phase-field simulation. The results show that the ratio of shear modulus (RSM) between precipitates and matrix influences the formation of rafting structure under the applied strain. The dislocation network can induce the nanoscale precipitate pattern, and the orientation or shape of the precipitates is dominated by the dislocation stress and internal stress together. The novel nanoscale microstructure in anisotropic alloys may be controlled by the combination of internal stress, applied strain and dislocation network.     

铌钒微合金钢中碳化物的析出过程

为了解铌钒微合金钢碳化物析出过程对机械性能的影响,将Nb-V微合金钢在1200℃固溶0.5h后淬火,在500℃回火不同时间,淬火样品的显微硬度最高,随着回火时间延长,显微硬度先快速下降,随后上升,在回火4～10h时出现一个硬度峰,随后又缓慢下降.用三维原子探针(3DAP)研究这些样品中碳化物的析出特征.结果显示,淬火样品中V、Nb元素分布均匀;随着回火时间延长,碳化物的尺寸逐渐增大,数量增加,回火4～10h的样品数达到最多,与硬度峰对应,同时V碳化物依次向V、Nb和V、Nb、Mo的复合碳化物转变.     

Quantum nucleation of solids

Quantum nucleation of solid⁴He from superfluid⁴He is discussed on the basis of a simple isotropic model. The nucleation rate is evaluated in the semiclassical WKB approximation developed earlier by Lifshitz and Kagan. The rate very strongly depends on the magnitude of overpressurization. The effect of a substrate that is characterized by a surface tension is examined; since the substrate reduces the liquid kinetic energy, the nucleation rate is substantially increased when the contact angle is small. The same method is applied to the two-dimensional nucleation of a new atomic layer on a singular face, with the result that the rate is very small under usual conditions.This research was supported in part by the National Science Foundation under the Materials Research Laboratory Grant DMR-80-20250 and the Division of Materials Sciences, Department of Energy under Contract DE-AC02-76ER01198.     

纳米CuO的沉淀转化法制备与表征

采用沉淀转化法制备CuO的前驱体Cu（OH）2，对纳米CuO微晶的不同热分解条件下的样品进行了x射线衍射（XRD）实验研究。结合所获得的晶粒度和XRD微结构参数，发现纳米晶CuO的晶粒组元中存在着晶格畸变；随着温度的升高，纳米CuO晶粒长大且其微结构呈现出明显的变化，其晶格畸变与晶粒度有着比较密切的关系。     

Kinetics of nucleation and growth

A number of solutions have been obtained to describe the size distributions and average particle size for nucleation and reaction controlled growth of precipitates; this includes the solutions for constant nucleation rate, the exponential nucleation law, Kashchiev's equation and some multistep nucleation models. The increase in the average particle size is related to increase in maximum size. Time-dependent effects occur if nucleation is affected by an induction time or a decay time.     

Universal Sequences

 An (n, k)-universal sequence is a binary sequence with the property that each window of size k and span at most n is covered by the sequence, i.e., each sequence of length k occurs as the content of a shift of the window. We derive upper and lower bounds on the minimum length of universal sequences, both for the linear case and the circular case. Received: November 4, 1996     

Stochastic nucleation and growth of islands on surfaces with the theory of non-classical nucleation

In order to study the irreversible growth and dynamics of islands on surfaces in metal-on (1 0 0) metal homoepitaxy at low temperatures and small coverage, a stochastic element in the theory of non-classical nucleation is introduced. The order parameter is the fractional change in the mass density σ=(ρ−ρ₀)/ρ₀ of the system. The local and non-local free energy densities are functionals of σ. The size distribution of the islands is extracted and compared with the experimental distribution and with the distribution obtained from other simulations. The time dependence of the average island size is also studied. The results are in relatively good agreement with the experimental results and with the results obtained from other simulations.     

Features of nucleation in nanovolumes

A precisely solvable model of three-dimensional nanoobject nucleation for a spherical geometry of the medium and nucleus at a constant total volume of the system has been constructed. Activation barriers for the formation of nuclei in various positions are calculated as dependent on the supersaturation and the surface energies of various phases. It is shown that the formation of a metastable crystalline phase can take place due to a gain in the effective surface energy.     

Kinetics of nucleation and growth

The kinetics of nucleation and diffusion controlled growth have been re-examined; this includes the most commonly assumed nucleation laws. The square size distributions and the time dependence for the average square size were obtained from previous solutions for nucleation and reaction controlled growth. The solutions for the size distributions and average particle size were also derived.