Diffusion Of Solute Atoms Research Articles

Development of a High-Strength Ultrafine-Grained Ferritic Steel Nanocomposite

This article describes the microstructural and mechanical properties of 12YWT oxide-dispersion-strengthened (ODS)-ferritic steel nanocomposite. According to the annealing results obtained from X-ray diffraction line profile analysis on mechanically alloyed powders milled for 80 hours, the hot extrusion at 1123 K (850 °C) resulted in a nearly equiaxed ultrafine structure with an ultimate tensile strength of 1470 MPa, yield strength of 1390 MPa, and total elongation of 13 pct at room temperature comparable with high-strength 14YWT ODS steel. Maximum total elongation was found at 973 K (600 °C) where fractography of the tensile specimen showed a fully ductile dimple feature compared with the splitting cracks and very fine dimpled structure observed at room temperature. The presence of very small particles on the wall of dimples at 1073 K (800 °C) with nearly chemical composition of the matrix alloy was attributed to the activation of the boundaries decohesion mechanism as a result of diffusion of solute atoms. The results of Charpy impact test also indicated significant improvement of transition temperature with respect to predecessor 12YWT because of the decreased grain size and more homogeneity of grain size distribution. Hence, this alloy represented a good compromise between the strength and Charpy impact properties.

High temperature structure stability of GH4169 superalloy

The long-term thermal stability of GH4169 superalloy has been studied as a function of heat treatment and thermal exposure. The GH4169 superalloy was subjected to long-term unstressed heat treatment at 650°C, from 200 to 1000h. The microstructure, tension and stress rupture properties of the samples before and after long-term thermal exposure were characterized. The coarsening of strengthing phase γ′ and γ″ was observed after exposing at 650°C. Coarsening of two different precipitates proceeds by Oswald ripening controlled by volume diffusion of solute atoms in the alloy. The tension strength at room-temperature and 650°C decreases slowly and the stress rapture life decreases remarkably with the increase of thermal exposure time. The decreasing of volume fraction and increasing of mean distance for the precipitates appears to be the main factor in the degradation of the mechanical properties.

Effect of cooling rate on the microstructure evolution and mechanical properties of homogenized Mg–Gd–Y–Zn–Zr alloy

Different cooling processes, such as quenching in warm water and cooling in furnace, were introduced to homogenize Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr (wt%) alloy. Microstructure evolution and mechanical properties of the homogenized alloy were investigated. The as-quenched sample was comprised of α-Mg matrix, Mg5RE phase and 18R LPSO phase distributed at the grain boundaries and a few of RE-rich particles distributed randomly. During the process of cooling in furnace, Mg5RE and 18R LPSO phases were transformed into block-shaped 14H LPSO phase and lamellar-shaped 14H LPSO phase, respectively, due to the diffusion of solute atoms into the α-Mg matrix. Furthermore, the lamellar-shaped 14H LPSO phase grew and ran through the whole grains. The as-quenched sample exhibits tensile yield strength of 130MPa, ultimate tensile strength of 206MPa and elongation to failure of 5.5%, while the sample cooled in the furnace exhibits higher tensile yield strength but lower ultimate tensile strength and ductility due to the coarse grains and formation of block-shaped 14H LPSO phase.

Phase Field Study of Concurrent Nucleation and Growth in a Diffusion-Controlled Solid-State Phase Transformation

The concurrent nucleation and growth in a diffusion-controlled phase transformation is studied using the quantitative phase field method, and the transformation kinetics is obtained for a model alloy. The simulation results show that the simultaneous nucleation and growth of new phase can be described very well in the phase field model, and that the phase transformation is governed by the diffusion of solute atoms. The competition between nucleation and coarsening is also observed. The phase transformation kinetics is found to obey the JMAK equation.

An investigation on the coarsening behavior of γ′ precipitate in GTD-111 Ni-base superalloy

Morphology of γ′ precipitates affects mechanical properties of Ni-base superalloys. In this research study, the coarsening behavior of γ′ precipitates in Ni-base superalloy GTD-111 was investigated. The alloy was solution-treated at 1200°C for 4 h and water quenched to obtain single size distribution of the precipitates. Aging at 1000°C up to 200 h was carried out to examine the effect of aging time on the growth of γ′ precipitates. The shape of γ′ precipitates changed from sphere to oval and then to cube as the aging time was increased. Moreover, the coarsening of γ′ precipitates started fast initially, whereas it slowed down as the growth continued, contrary to the Lifshitz–Slyozov–Wagner theory. The decelerated coarsening of γ′ precipitates was attributed to the effect of elastic interaction energy and decrease in driving force. Finally, the activation energy for diffusion of solute atoms or the growth of γ′ precipitates in GTD-111 alloy was calculated.

Amplitude Dependent Internal Friction of Magnesium Alloy AZ31 at Room Temperature

The aim of this work is to study the Amplitude Dependent Internal Friction (ADIF) of magnesium alloy AZ31 at room temperature at the frequency 20kHz. The internal friction of AZ31 at room temperature is mostly influenced by mechanical cycling at strain amplitudes in the microplastic deformation region. An excited state of the AZ31 alloy, which can be associated with a higher internal friction and lower dynamic modulus than usual state, was found immediately after mechanical cycling. When the strain amplitude drops, the diffusion of solute atoms restores the Zener atmosphere and the internal friction relaxes exponentially with the second root of time. The measurement methodology and obtained results are presented.

Effects of High Density Electropulsing Treatment on Aging Kinetics of GH4199 Alloy

Abstract GH4199 alloy was treated by high density electropulsing for different time, and the microstructure evolution and tensile properties were examined. The results show that the diffusion of solute atoms in GH4199 alloy can be strongly accelerated by electropulsing. The growth activation energy of γ′ phase of 89.86 kJ/mol in the alloy electropulsing-treated, decreases by 64.31% compared with that normally aged, which accelerates the precipitation and the growth of both the γ′ phase and carbides on the grain boundary of the alloy. The strength of GH4199 alloy increases with increasing of electropulsing treatment time, and the plasticity has no obvious change. The growth of γ′ phase and the precipitation of carbides on the grain boundary is the main reason for obstructing the dislocation motion and improving the plastic deformation resistance of the alloy. The piled-up dislocations can go through the grain boundary across the gap of carbides which is distributed on the grain boundary in chainlike form, and no degradation of the plasticity of GH4199 alloy appears.

Hardness modification of aluminum-alloys by means of energetic ion irradiation and subsequent thermal aging

So far, we have found that the hardness of Al–Cu–Mg alloy (JIS2017, Duralumin) increases by energetic heavy ion irradiation at room temperature. Observations by using the three-dimensional atom probe (3DAP) have revealed that nano-meter sized precipitates are homogeneously distributed in the irradiated specimens, which are produced through the irradiation enhanced diffusion of solute atoms. The small precipitates contribute to the increase in hardness. In this report, we show the results for the hardness modification of Al–Cu–Mg alloy by the combination of energetic ion irradiation and thermal aging treatment. The hardness of the specimens pre-irradiated and thermally aged at 423 K is much larger than that without the pre-irradiation. The present result indicates that the combination of energetic ion irradiation and subsequent thermal aging can be used as an effective tool for the hardness modification of Al–Cu–Mg alloy.

Precipitation and Growth Kinetics in Mechanically Alloyed Ni–Al

The precipitation and growth kinetics ofγ′ precipitates, which are strengthening factors in Ni-base oxide dispersion strengthened (ODS) superalloys, were investigated. The cuboidal-typeγ′ precipitates are formed in conventional arc-melted Ni–Al alloys, whereas spherical-type precipitates are formed in the mechanically alloyed (MAed) specimens. The morphology is controlled by a lattice misfit between theγ′ precipitates and the matrix at the aging temperature of 800°C. The growth kinetics of theγ′ precipitates can be followed by Ostwald ripening. The Arrhenius plot yielded a lower activation energy for the solute atom diffusion in MAed specimens, which is attributed to their high dislocation density and nanosized grains.

Microstructure of AZ91D magnesium alloy semi-solid billets prepared by SIMA method from chips

AZ91D magnesium alloy chips were adopted to prepare semi-solid billets. The chips were subjected to a series of isothermal treatments for various holding times at 783-843 K after being compressed into billet at 523 K. The semi-solid microstructure of AZ91D magnesium alloy containing spherical solid particles was studied. The effects of reheating temperature and holding time on microstructures were investigated. And the semi-solid forming mechanism was discussed. The result shows that semi-solid billets with highly spheroidal and homogeneous grains can be prepared from chips by strain induced melt activation(SIMA) method. Meanwhile, it is found that increasing the heating temperature can accelerate the spheroidizing process and reduce the solid volume fraction. With the increase of the holding time, the solid particles become more globular, the grains grow slowly and the solid volume fraction slightly changes. At the same time, owing to the decrease of interfacial energy, the intragranular liquid phases form by the diffusion of solute atoms, the grain boundaries melt and grains separate from each other during the isothermal treatment. The grains gradually spheroidize and begin to merge with a further increase of the holding time. It is considered that the semi-solid forming process includes three stages: the recrystallization and the growth of grain stage, the semi-solid microstructure forming stage controlled by the diffusion of solute, and the spheroidization of solid particle stage controlled by the liquid-solid interface tension.

Instable Plasticity and Precipitation in AZ91D Magnesium

Instable plasticity, which is normally a consequence of interactions between diffusing solute atoms and dislocations, is observed in AZ91D magnesium at room temperature. Experimental measurements of different heat treatment of AZ91D magnesium were taken to investigate different influence on PLC effect by the presence and structural state of second phase in this study. Tensile specimens were tested at room temperature at a crosshead speed of 0.6 mm/min. The results indicate that some number of large,coherent precipitates prevents serrated flow by way of trapping vacancies, but small precipitates content do not supress it.

Effect of local solid reaction on diffusion-induced stress

Incorporating the volumetric change due to local solid reaction in the theory of diffusion-induced stress, a general relation is derived among concentration of solute atoms, local reaction product, and mechanical stress. This relation describes the dependence of local stress on the local dilatation created by diffusion of solute atoms and local solid reaction. Assuming that the elastic properties of an isotropic thin plate are constants independent of reaction product and concentration of solute atoms, closed-form solutions of the diffusion-induced deformation fields in the plate are obtained when the plate is free of external stress and subjected to a constant concentration of solute atoms on surface. Local solid reaction significantly increases the stress on the surface of the plate, which can potentially cause structural degradation.

Formation of the Modulated Structure in a Steel by the Addition of a Strong Carbide Forming Element

The modulated structure characterized by periodic concentration fluctuations is caused by the up-hill diffusion of solute atoms due to the spinodal decomposition, when the Gibbs free energy surface of the solid solution has a convex curve. This convex shape in the free energy surface will appear in multi-component systems, even though the energy in each binary system does not show the convex curve, when a binary system exhibits a strong attractive interaction, i.e., the Gibbs free energy shows a deep concave curve. This may be the case that steel containing an element having a strong tendency of carbide forming. The Fe-Ni-V-C system was selected in this study. V is a strong carbide forming element and forms VC carbide based on f.c.c. lattice. A series of experiments was carried out using Fe-25Ni-3V-3C (mol%) alloy after aging at elevated temperatures. A modulated structure was observed in specimens aged at temperature range of 550 °C to 650 °C, and the maximum temperature occurring the spinodal decomposition was estimated to be 715 °C in Fe-25Ni-3V-3C alloy. Furthermore, the coefficient of the gradient energy caused by composition fluctuations was estimated to be 2.8 x 10-15 J.m2/mol.

Coarsening of cuboidal Al3Sc precipitates in an Al–Mg–Sc alloy

The coarsening of {001}-faceted cuboidal Al3Sc precipitates in an Al–Mg–Sc alloy during ageing at 698, 723 and 748 K have been examined by the transmission electron microscopy and the electrical resistivity. Application of the theory of Kuehmann and Voorhees for the coarsening of second-phase precipitates in ternary systems, generalised to any centro-symmetric precipitates, has enabled the independent derivation of the {001} interface energy without assuming the values of the diffusivity of solute atoms (Sc and Mg) in the Al matrix. The {001} interface energy is estimated as 0.19 J/m2, which is in excellent agreement with the value of 0.192 J/m2 obtained using the first-principles calculation method. The sphere-to-cuboidal shape change of the Al3Sc precipitate during ageing is discussed by means of a simple energy consideration.

Diffusion-controlled phase growth on dislocations†

We treat the problem of diffusion of solute atoms around screw dislocations. In particular, we express and solve the diffusion equation in two dimensions with radial symmetry in an elastic field of a screw dislocation subject to conservation of flux at the interface of a new phase. We consider an incoherent second-phase precipitate growing under the action of the stress field of a screw dislocation. The second-phase growth rate as a function of the supersaturation and a strain energy parameter is evaluated in spatial dimensions d = 2. Our calculations show that an increase in the amplitude of the dislocation force, e.g. the magnitude of the Burgers vector, enhances the second-phase growth in an alloy. Moreover, we calculate the reduction in concentration of solute atoms as a function of radius around a second phase which grows cylindrically (in the radial direction) so that its radius varies as the square root of time for various levels of the dislocation force amplitude.

Modeling the Effect of Active Fiber Cooling on the Microstructure of Fiber-Reinforced Metal Matrix Composites

A modified pressure infiltration process was recently developed to synthesize carbon-fiber-reinforced aluminum matrix composites. In the modified process, the ends of carbon fibers are extended out of the crucible to induce selective cooling. The process is found to be effective in improving the quality of composites. The present work is focused on determining the effect of the induced conductive heat transfer on the composite system through numerical methods. Due to the axisymmetry of the system, a two-dimensional (2-D) model is studied that can be expanded into three dimensions. The variables in this transient analysis include the fiber radius, fiber length, and melt superheat temperature. The results show that the composite system can be tailored to have a temperature on the fiber surface that is lower than the melt, to promote nucleation on the fiber surface. It is also observed that there is a point of inflection in the temperature profile along the particle/melt interface at which there is no temperature gradient in the radial direction. The information about the inflection point can be used to control the diffusion of solute atoms in the system. The result can be used in determining the optimum fiber volume fraction in metal matrix composite (MMC) materials to obtain the desired microstructure.

Effects of shot peening on internal friction in cp aluminum and aluminum alloy 6008

Abstract The strain-amplitude-dependent damping of bending beams of aluminum alloy 6008 and CP aluminum was measured at room temperature after different heat treatments and after shot peening. Shot peening led to an increase of damping in almost the whole measured amplitude strain range from 10 – 6 to 10 – 3 for CP aluminum. Strong ageing effects at room temperature were observed immediately after the shot peening process, namely an increase of the amplitude dependent part and a decrease of the amplitude-independent part of damping. After about 2700 h, ageing of the samples had saturated. For aluminum alloy 6008 much smaller ageing effects were found being due to compensating effects like formation of Cottrell clouds, precipitation of G.P. – zones, and the reduction of foreign atoms in solid solution. The found amplitude-dependent damping can be explained by the reversible movement of dislocations between strong pinning points like, e. g., precipitates and weak pinning points like solid solute atoms as proposed by the dislocation damping theory of Granato and Lücke. Using this model the found ageing effects can be explained by the diffusion of solid solute atoms to the dislocations.

Behavior of (Ti, Nb)(C, N) complex particle during thermomechanical cycling in the weld CGHAZ of a microalloyed steel

This study aims to investigate the effect of deformation (stress and strain) on the coarsening kinetics of a (Ti, Nb)(C, N) complex particle during isothermal and continuous thermomechanical cycles in the weld coarse-grained heat-affected zone (CGHAZ) of a Ti + Nb microalloyed steel. Results in isothermal coarsening tests showed that the coarsening rate was enhanced by deformation under both compressive and tensile stress: it was enhanced mainly owing to the increased diffusivity of solute atoms. Then the compressive stress was found to be more effective than tensile stress. Interestingly, despite the increase in the coarsening rate, the mean particle size in the weld CGHAZ, after continuous thermomechanical cycle, decreased more than it did in the simple thermal cycle. This was because of enhancement of the precipitation of fine particles by increased precipitation kinetics under deformation.

Kinetics of coupled ordering and segregation in antiphase domains

AbstractWe study a multi‐domain ordering kinetics in solid solutions under simultaneous diffusion of solute atoms. By the example of a binary bcc alloy a system of kinetic equations is derived, describing the coupled relaxation of occupancies of the two sublattices, building a bcc lattice, by A and B atomic species. Such an approach supplemented by the simplest mean‐field approximation proves sufficient to describe both the establishing of long‐range order and the segregation processes occurring in antiphase domains. An interaction and interrelation between evolution of the conserved and non‐conserved order parameter fields are elucidated. Asymptotical and numerical analysis of the obtained evolution equations reveals a multi‐stage scenario of the alloy relaxation: first comes the quick development of long‐range order which is then followed by the slow redistribution of local alloy concentration, so that the majority atoms segregate towards the region of an antiphase boundary. The alloy exhibits a distinct tendency to form a multi‐domain structure out of a solitary long‐range order parameter fluctuation of a certain sign. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Atomistically informed solute drag in Al–Mg

Solute drag in solute-strengthened alloys, caused by diffusion of solute atoms around moving dislocations, controls the stress at deformation rates and temperatures useful for plastic forming processes. In the technologically important Al–Mg alloys, the solute drag stresses predicted by classical theories are much larger than experiments, which is resolved in general by eliminating the singularity of the dislocation core via Peierls–Nabarro-type models. Here, the drag stress versus dislocation velocity is computed numerically using a realistic dislocation core structure obtained from an atomistic model to investigate the role of the core and obtain quantitative stresses for comparison with experiment. The model solves a discrete diffusion equation in a reference frame moving with the dislocation, with input solute enthalpies and diffusion activation barriers in the core computed by or estimated from atomistic studies. At low dislocation velocities, the solute drag stress is controlled by bulk solute diffusion because the core diffusion occurs too quickly. In this regime, the drag stress can be obtained using a Peierls–Nabarro model with a core spreading parameter tuned to best match the atomistic models. At intermediate velocities, both bulk and core diffusion can contribute to the drag, leading to a complex stress–velocity relationship showing two peaks in stress. At high velocities, the drag stress is controlled solely by diffusion within and across the core. Like the continuum models, the drag stress is nearly linear in solute concentration. The Orowan relationship is used to connect dislocation velocity to deformation strain rate. Accounting for the dependence of mobile dislocation density on stress, the simulations are in good agreement with experiments on Al–Mg alloys over a range of concentrations and temperatures.