Numerical simulation of precipitation kinetics in multicomponent alloys

A universal numerical model based on the particle size distribution(PSD) approach has been developed for the simulation of precipitation kinetics in multicomponent alloys during isothermal ageing. Nucleation was implemented utilizing the classical nucleation theory(CNT). Growth and coarsening were modeled by a single growth kinetics equation, which is constructed based on the interfacial diffusion flux balance and the capillarity effect. Only partial off-diagonal terms in the diffusion matrix(di...     

Periodic precipitation in multicomponent systems

Journal of Materials Science Letters -     

Vacancy effects in multicomponent alloys

A thermodynamic analysis is presented for the anomalies in the physical parameters of tool steels near the Curie point; the energy of vacancy formation is deduced from the temperature dependence of the physical parameters.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 38, No. 4, pp. 599–602, April, 1980.     

Discontinuous precipitation in Cu-Mg alloys

The relation between discontinuous precipitation and continuous precipitation and the cell growth kinetics of the discontinuous precipitation in Cu-Mg alloys containing 2.0, 2.6 and 3.2 wt % Mg have been investigated, mainly by metallographic observation. The volume fraction of cells, the cell width and the interlamellar spacing have been determined by quantitative metallographic measurements. The cell growth rate decreases progressively with ageing time after the initial linear growth of the cells. This may be attributed to the influence of continuous precipitation on the cell growth. The volume fraction of the discontinuous precipitation cells,f, can be represented by the Johnson-Mehl equation: f=1–exp (–bt ⁿ). The value of the parameter n is about 2 and is independent of both the ageing temperature and alloy composition in the ageing range where the cell growth rates are constants. Mass transport of magnesium during the linear growth of cells occurs by grain boundary diffusion in a Cu-Mg solid solution.     

Iron precipitation in Cu-Au-Fe alloys

Interest is focused on the segregation and clustering behaviour of iron atoms in fcc Cu₁₀₀–x–yAu_xFe_y, alloys (x=6 to 50.7 at%, y=0.2 to 1.0 at%) of three different metallurgical states: as-rolled, fast-quenched and melt-spun. The gold concentration was varied to assess the effect of change in lattice parameter. Mössbauer spectroscopy has been used to examine iron clusters and phases in samples as functions of annealing temperature and time. The development of f c c -Fe, segregates with increasing annealing time at temperatures around 410° C has been monitored and the Néel temperature of the antiferromagnetic -Fe precipitates as well as their particle size determined, the latter by transmission electron microscopy. The increase in isomer shift with increasing gold concentration is accounted for mainly by changes in atomic volume.     

An inverse stefan's problem in casting multicomponent alloys

A model is considered that incorporates the feature that melting (crystallization) occurs over a certain temperature range, and it is shown that the solution to the internal inverse problem is unique in one such formulation.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 49, No. 6, pp. 1002–1006, December, 1985.     

Discontinuous precipitation in copper base alloys

Discontinuous precipitation (DP) is associated with grain boundary migration in the wake of which alternate plates of the precipitate and the depleted matrix form. Some copper base alloys show DP while others do not. In this paper the misfit strain parameter, η, has been calculated and predicted that if 100 η > ± 0·1, DP is observed. This criterion points to diffusional coherency strain theory to be the operative mechanism for DP.     

Cluster distribution and long-range ordering in multicomponent interstitial alloys

Formulae determining the long-range order parameters and the population of clusters of interstitial atoms were derived from low-temperature approximation of fundamental principles. The size of the clusters, the range of interactions and the number of the alloy components were not limited. The method proposed is not sensitive to the kind of a crystal structure and thus can be used for real systems.     

Modeling of precipitation hardening in Mg-based alloys

This work deals with the development of Mg-based alloys with enhanced properties at elevated temperatures. This is achieved by precipitation of binary phases such as MgZn₂ and Mg₂Sn during the aging of these alloys. The aim of the present work is to develop and calibrate a model for precipitation hardening in Mg-based alloys, as different types of precipitates form simultaneously. The modified Langer-Schwartz approach, while taking into account nucleation, growth and coarsening of the new phase precipitations, was used for the analysis of precipitates’ evolution and precipitation hardening during aging of Mg-based alloys. Two strengthening mechanisms associated with particle-dislocation interaction (shearing and bypassing) were considered to be operating simultaneously due to particle size-distribution. Parameters of the model, R _N i and k _{σ i} , were found by fitting of calculated densities and average sizes of precipitates with ones estimated from experiments. The effective diffusion coefficients of phase formation processes, which determine the strengthening kinetics, were estimated from the hardness maximum positions on the aging curves.     

On precipitation in rapidly solidified aluminium-silicon alloys

The precipitation of silicon in rapidly solidified AlSi alloys was studied. For alloys with 2.4 and 11.0 wt % Si (2.3 and 10.3 at % Si, respectively) the lattice parameters of the Alrich and of the Si-rich phases were measured after ageing at 397,425 and 448 K. For alloys with 2.6 and 13.0 wt % Si crystallite sizes and lattice strains were determined by analysis of the X-ray diffraction line broadening. After ageing the lattice parameters of the Al-rich and the Si-rich phases were influenced by the difference in thermal expansion between both phases. After correction for this effect the amount of silicon dissolved in the Al-rich phase was estimated as a function of ageing time. Quenched-in (excess) vacancies influenced the precipitation kinetics. Activation energies for precipitation appeared to depend on the extent of transformation. Further, quenched-in vacancies caused anomalous maxima in the lattice parameter curves. The behaviour of the lattice microstrains on ageing was explained as a result of the disappearance of stresses due to quenching and the introduction and subsequent dissipation of stresses due to precipitation. After completed precipitation stresses due to the difference in thermal expansion between both phases still exist at room temperature.     

Interrupted aging and secondary precipitation in aluminium alloys

Abstract

It is shown that, if elevated temperature aging of aluminium alloys is interrupted with a dwell period at a low temperature (65 °C), age hardening continues due to so called secondary precipitation. If elevated temperature aging is then resumed, significant improvements can be obtained in mechanical properties compared with those available using a conventional T6 temper. Average increases in 0.2% proof stress and tensile strength of 10%, combined with improved fracture toughness, have been achieved in a wide range of alloys. These effects arise primarily because interrupted aging promotes the formation of more finely dispersed precipitates in the final microstructures. The concept of interrupted aging is described in some detail with respect to the model system, Al-4Cu, and examples are then given of the effects of the treatment on the microstructures and mechanical properties of several wrought and cast aluminium alloys.     

Deformation and recrystallization in cross-rolled Al-Cu precipitation alloys

The deformation and recrystallization behaviour of single-phase f.c.c. metals have been studied mostly after conventional, uniaxial rolling. In this paper, the effect of both the presence of a second phase and the deformation path (i.e. straight rolling versus cross-rolling) on the evolution of microstructures and textures of an Al-3% Cu alloy are presented. The changes in the deformation and recrystallization texture are found to be complex, and it is not the deformation path nor solely the type of precipitate alone which produces the changes.     

Heating-induced transformations of multicomponent alloys prepared by mechanochemical synthesis

Multicomponent (Cr, Fe, Co, Ni, Al, Ti, and Nb) powder alloys prepared by milling five- to seven-component equiatomic mixtures of elemental powders in a Fritsch (P-7) planetary mill have been characterized by differential thermal analysis and X-ray diffraction. The results demonstrate that, if the starting mixture contains Al, the BCC solid solution formed as a result of the milling undergoes heating-induced CsCl-type ordering (β-phase). If not only Al but also Ti are present in the starting mixture, further heating causes the β-phase to convert to an L2₁ phase with the composition (Ni,Co)TiAl. The elements Cr and Fe form a tetragonal σ-phase. The presence of Nb in the starting mixture suppresses the formation of the σ- phase and favors the formation of a hexagonal Laves phase of complex composition: (Fe,Co)CrNb.     

Microstructures and liquid phase separation in multicomponent CoCrCuFeNi high entropy alloys

The microstructures of high entropy alloys of the system CoCrCuFe_xNi and CoCrCuFeNi_x (where x indicates the molar ratio, which, where not specified, is 1) have been investigated. Many Cu rich spheres were evident in the microstructure of CoCrCuFe_0.5Ni and CoCrCuFeNi_0.5 alloys, which indicates that liquid phase separation had occurred before solidification. During liquid phase separation, the original liquids separated into two liquids: Cu rich and Cu depleted. In contrast, in other alloys (x?=?1.0, 1.5 and 2.0), typical dendritic and interdendritic structures are obtained. Cu and/or Cr rich precipitates, with various morphologies, can be seen in the interdendritic region. Additionally, Cu rich nanoparticles and Cr rich bird shaped structures can be observed in the Cu rich spheres. Sluggish cooperative diffusion causes the element segregation and formation of nanoprecipitates in the microstructures. The calculated positive mixing enthalpies of CoCrCuFe_0.5Ni and CoCrCuFeNi_0.5 alloys are likely reasons for their liquid phase separation.