Discontinuous Dissolution Reaction in a Fe-13.5 at. % Zn Alloy

The dissolution process of a lamellar structure with α and Γ phases formed during a discontinuous precipitation reaction is investigated here with a Fe-13.5 at. % Zn alloy by means of optical microscopy and scanning and transmission electron microscopy. The α phase is a solute-depleted solid solution and the Γ phase is the intermetallic compound Fe3Zn10. The examination reveals that the dissolution occurs in a discontinuous mode by a receding of the former reaction front of the discontinuous precipitation towards the position of the original grain boundary. A new solid solution in the post-dissolution area is especially inhomogeneous and reflects the former locations of the Γ lamellae (“ghost images”) and the receding reaction front (“ghost lines”). A simulation procedure is applied to determine the Zn concentration profiles left in the post-dissolution region. Their shapes are mostly affected by the Zn content at the positions where the Γ lamellae have just been dissolved, which was also confirmed by the quantitative microchemical analysis.

Effect of Mg contents on the mechanical proprieties and precipitation kinetics in Al–3.3 wt.% Cu alloy

The effect of additional Mg on the microstructure, mechanical properties, and transformation kinetics during aging in Al–3.3 wt.% Cu alloy was studied. The compositions and microstructure were examined by X-ray diffraction, Differential scanning calorimetry (DSC) and scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS). The results show that the Mg in the Al–Cu alloy mainly precipitated to the grain boundaries during the process of transformation and formed a ternary Al2CuMg metallic compound and the rate of discontinuous precipitation reaction decreases with increasing concentration of Mg. The activation energy of crystallization was evaluated by applying the Kissinger equation.

On the Effect of Bulk Diffusion on the Initiation of the Discontinuous Precipitation Reaction: Phase-Field Simulations

Discontinuous precipitation is a solid-state transformation involving the decomposition of a supersaturated matrix into two phases arranged periodically as alternate lamellae or rods, which is accompanied by a grain boundary migration. The rate-limiting step of this process is supposed to be boundary diffusion of solute along grain boundaries. However, volume diffusion is generally present as well, and its influence on the occurrence of the discontinuous precipitation reaction is at present not well understood. We investigate this problem using a phase-field model in which the bulk diffusivity, surface diffusivity and grain boundary mobility can all be varied independently. The main results are that (i) when volume diffusion is the dominant mechanism, a close analogy is observed between the precipitate growth and the growth of a crystalline finger in a channel, and (ii) both the geometry of the precipitate’s tip and the growth velocity are strongly influenced by the relative magnitudes of the bulk and surface diffusivities as well as by the grain boundary mobility. Steady-state growth is possible only for a finite range of precipitate spacings, which is limited for low spacings by a fold singularity and for large spacings by an oscillatory or a tip-splitting instability. The values of these limits are found to depend on the supersaturation as well as on the ratio of bulk and surface diffusivities.