Soft impingement in diffusion-controlled growth of binary alloys: moving boundary effect in one-dimensional system

The impact of soft impingement on the kinetics of diffusion-controlled growth of binary alloys is investigated. An analytical approach is developed which takes into account the process of island growth, that is the time dependence of the position of the nucleus/parent phase interface. The concentration profile, the growth law, and the kinetics of the fraction of transformed phase are computed and compared with those attained for point islands. At odd with the point island approach the local kinetics of growth depends on initial supersaturation. On the other hand, the whole transformation kinetics is in good agreement with that of the point island model with an Avrami exponent close to the theoretical value n = 0.5. The concentration profile is well described by a polynomial function in the whole spatial domain, with an exception for the initial stage of the phase separation. The effect of the spatial distribution of the nuclei on the kinetics is also studied in the model case of hard-core correlation among nuclei.     

An anomalous acoustic effect during martensite transformations in NiTi-based alloys

Martensite transformations proceeding in mechanically loaded TiNi-based alloys account for an “anomalous” character of the acoustic emission from the material, whereby cyclic transformations during the growth of mechanical stress in the course of the direct transition is accompanied by an increase, rather than by a decrease, in the acoustic emission energy. This behavior of the acoustic emission is evidence of a significant influence of the external stresses on the martensite transformations and the related energy dissipation process.     

Modelling of soft impingement during solidifcation

It has been well established that spheroidal grain morphology in the microstructure forms during stir casting (rheocasting) and grain refinement of magnesium alloys by zirconium addition. This curious microstructure has been of interest both commercially from enhanced mechanical properties and also scientific interest in explaining the mechanism of spheroidal grain formation. Vogel and Doherty proposed a model describing the fracturing of dendrite arms during stir casting to produce a high density of nuclei which they presume to give rise to spheroidal grains. They proposed that there is soft impingement of diffusion fields of neighbouring nuclei, which reduces the concentration gradient ahead of the planar solid and liquid interface, which in turn negates shape instability. In this paper, the Vogel and Doherty model is pursued by quantitative modeling of soft impingement problem and related to shape instability by constitutional supercooling theory. This analysis correctly predicts the spheroidal grain formation during stir casting or rheocasting. This model can also be used to explain the grain refinement of magnesium alloys by zirconium addition wherein spheroidal grains are formed.     

Modeling of heat-mass transfer in porous cooling systems during phase transformations

The interaction of a porous cooling system and a high-enthalpy gas flow is considered. The effect of the phase transformation on the heat-mass exchange characteristics in a heat-resisting material is investigated.     

Atomic structure and phase transformations in Pu alloys

Plutonium and plutonium-based alloys containing Al or Ga exhibit numerous phases with crystal structures ranging from simple monoclinic to face-centered cubic. Only recently, however, has there been increased convergence in the actinides community on the details of the equilibrium form of the phase diagrams. Practically speaking, while the phase diagrams that represent the stability of the fcc δ-phase field at room temperature are generally applicable, it is also recognized that Pu and its alloys are never truly in thermodynamic equilibrium because of self-irradiation effects, primarily from the alpha decay of Pu isotopes. This article covers past and current research on several properties of Pu and Pu-(Al or Ga) alloys and their connections to the crystal structure and the microstructure. We review the consequences of radioactive decay, the recent advances in understanding the electronic structure, the current research on phase transformations and their relations to phase diagrams and phase stability, the nature of the isothermal martensitic δ → α′ transformation, and the pressure-induced transformations in the δ-phase alloys. New data are also presented on the structures and phase transformations observed in these materials following the application of pressure, including the formation of transition phases.     

Evaluation of elastic accommodation energies during solid-state phase transformations by the finite element method

The precipitation of a solid phase in a rigid matrix leads to the creation of strain energy in the system as the transformation strain cannot be relaxed in the case of solid state transformations. A finite element model based on the initial strain approach has been presented to evaluate elastic accommodation energies during solid-state transformations. The finite element analysis (FEA) reveals that the total system energy increases with increase in the ratio of the modulus of the precipitate to that of the matrix, and with increase in misfit strain between the matrix and the precipitate. The analysis has been performed by considering the transformation to progress from the centre to the surface and from the surface to the centre of the three dimensional axisymmetric system. The elastic accommodation energies obtained by the FEA are comparable to energies obtained by continuum mathematical models.     

Four-dimensional structural description of phase transformations in titanium alloys

Journal of Materials Science - Titanium alloys display formation of $$\beta$$ , $$\alpha$$ , and $$\omega$$ phases under different processing conditions. Understanding structural transformations...     

Solid state phase transformations in aluminium alloys containing lithium

Abstract

The microstructure and solid state phase transformations which take place in aluminium alloys containing lithium have been assessed. In addition to the binary Al–Li system, the Al–Li–Cu, Al–Li–Mg, and Al–Li–Cu–Mg systems are discussed in detail together with the role of dispersoid forming elements. The physical metallurgical basis of current alloy development is thereby established.

MST/595     

Modeling of explosive acoustic emission accompanying martensitic transformations in alloys

The acoustic emission accompanying the stimulation of macroexplosive martensitic transformation kinetics in Ti-51.0 at. % Ni alloy has been confirmed. The macroexplosive kinetics were stimulated both by repeated cycling and by incomplete quasicycling of martensitic transformations. Pis’ma Zh. Tekh. Fiz. 24, 31–38 (January 12, 1998)     

Nanodimensional effects in the phase composition of binary alloys

We consider the problem of phase equilibrium in a two-component solid solution, in which small-size clusters consisting of the atoms of both kinds can form. Analytical expressions are obtained, which describe the phase composition of binary alloys containing such clusters. Features of the behavior of the phase composition depending on the cluster size are established, and criteria for the homogeneous and heterogeneous nucleation of a new phase are formulated.     

Impact of soft impingement on the kinetics of diffusion-controlled growth of immiscible alloys

The effect of the soft impingement mechanism on the kinetics of diffusion-controlled phase transitions of binary alloys is investigated. Soft impingement arises from the interference of the diffusional fields and implies a different morphology of the product phase when compared to that attained in the case of genuine impingement. Mean field rate equations for nucleus radius and supersaturation are coupled with the mass balance equation in terms of the geometrical parameter (time dependent) which defines the soft impingement process. The kinetics is studied as a function of the initial supersaturation and it is compared with that attained in the case of genuine impingement. Solutions of the kinetics are obtained in the model case of time dependent supersaturation which scales according to Ham’s law and in the case of simultaneous nucleation. The phase transformation of immiscible alloys in one-dimensional system is also investigated. In this case and for point islands, the exact solution of the kinetics is obtained and allows one to gain an insight into the validity of the Kolmogorov–Johnson–Mehl–Avrami model and Avrami’s exponent for one-dimensional growths.     

Structural transformations produced during tempering of Fe-Ni-Co-Mo alloys

In an attempt to resolve the difficulties in interpreting the behaviour of the industrial maraging steel of VASCOMAX 300 type, we have studied the influence of separate or simultaneous additions of Co and Mo on the structural transformations taking place during the heating cycles at moderate rates (300° Ch^–1) for the Fe-Ni base alloys. An interpretation of the mechanisms of the M transformation, implying preponderantly diffusional mechanisms, has been proposed for the alloys Fe-Ni, Fe-Ni-Co, Fe-Ni-Mo and Fe-Ni-Co-Mo for which the Ni/Fe, Mo/Fe ratios are equal to those of the reference maraging steel. The ageing phenomena in the martensite of the alloys containing Mo have been studied and show the formation of G.P. zones having a diameter of a few tens of Å and spaced at an average distance of 120 Å, in the case of the quaternary alloy, at temperatures of 400 to 450° C.     

Dynamic effects during thermoelastic martensitic transformations in TiNiCu alloys

Spontaneous periodic oscillations in the phase transformation rate during a system evolution toward a stationary state are observed in TiNiCu alloys. The conditions of manifestation and amplification of these oscillations are established and possible applications of this phenomenon are considered.     

Affect of shock-induced phase transformations on dynamic strength of titanium alloys

A series of complex alloyed (+β) VT-16 Titanium alloy targets were subjected to shock loading under uniaxial strain conditions within impact velocity range of 276–600 m/s. The tests reveal a presence of forward (→ω) and (β→ω) phase transitions at the load front and reverse (ω→β ) transition at the release front of compressive pulse. Duration of (β→ω) and (ω → β ) transitions is approximately 0.5 μs. When spallation happens after reverse (ω →β) phase transition, the spall-strength of alloy increases by 25%. Oscillating regime of that transformation proves to widen the impact velocity range where the spall-strength is maximum.     

Interface conditions during mixed-mode phase transformations in metals

A fast three-dimensional phase transformation model is formulated for the transformation from ferrite to austenite in low-carbon steel. The model addresses the parent microstructure, the nucleation behaviour of the new phase and the growth of the new phase. During the growth, the interface velocity of the ferrite grains is calculated using a mixed-mode growth model. The simulated transformation kinetics is compared with experimental kinetics for an Fe–C–Mn steel for four different cooling rates. In general, the model predicts the kinetics quite well. In addition, the mixed-mode character of the transformation is shown for the different cooling rates.