Spinodal Domain Research Articles

Mediating phase decomposition to avoid thermal aging embrittlement in a duplex stainless steel

Due to the presence of miscibility gap in the body-centered-cubic structure iron‑chromium alloy system, the ferrite will decompose into the Fe-rich α phase and Cr-rich α´ phase, resulting in the inherent “475 °C embrittlement”. In this study, a pulsed electric field is introduced to mediate the phase decomposition for avoiding aging embrittlement. Combining nanoindentation testing and atomic-scale characterization, the results show that almost no aging hardening and phase decomposition behaviors occur with the pulsed electric field intervening, and the thermodynamic analysis indicates that the pulsed electric field can shrink the spinodal domain. Further, the life evaluation shows that the equivalent aging time of the duplex stainless steel is 18.2 years without the application of the pulsed electric field, while the equivalent aging time of the duplex stainless steel is 1.2 years under the action of the pulsed electric field, which is equivalent to an extension of the service life by 17 years. This study offers an opportunity for resisting the aging embrittlement process to prolong the longevity of components. • The ferrite decomposition is suppressed by pulsed electric field during thermal aging. • Spinodal decomposition plays the major role in thermal aging embrittlement. • Pulsed electric field induces the spinodal domain to shrink for avoiding phase decomposition. • The equivalent service life of duplex stainless steel is extended by 17 years.

Kinetics of electrochemically induced phase transitions in ion-insertion electrodes and the chemical diffusion coefficient

In this paper, we report on the study, by numerical simulations, of the effect of slow external kinetics steps on the dynamics of electrochemically induced phase transitions in ion-insertion electrode materials. Two alternative kinetics steps such as slow interfacial ion transfer and slow small droplet formation were chosen to evaluate the errors in the calculation of the chemical diffusion coefficient, D, induced by incorrect application of non-equilibrium differential intercalation capacitances, C dif. Comparison between simulated and experimental curves shows that for a moderate limitation of phase transition by Butler–Volmer kinetics, the exact values D can be obtained up to the composition of the binodal point, and then errors increase towards the spinodal domain. Within the spinodal domain, the potential dependence of D obtained formally by application of the Cottrellian model, entirely relates to the potential dependence of C dif. The relevant description of phase transition dynamics in this region should include consideration of moving boundary between the coexisting phases formed via coalescing of growing nuclei.

Comparison between Cottrell diffusion and moving boundary models for determination of the chemical diffusion coefficients in ion-insertion electrodes

Using PITT technique, the various shapes of the Cottrellian diffusion plots, It 1/2 versus log t, were studied as a function of the electrode potential for the Li ions insertion into graphite electrodes. Single maximum appeared on the It 1/2 versus log t plots for stoichiometries close to pure staged phases, whereas two maxima were always observed in the two-phase coexistence domains. Slow nucleation rate during phase 2/phase 1 transition was revealed by the characteristic shape of the initial time domains of both chronoamperometric and chronovoltammetric curves. Corrected for nucleation, the longer time domain of the response was treated chronocoulometrically in the framework of two alternative approaches using the moving boundary and the classical Cottrellian models. The alternative approaches resulted in virtually the same value of D. The diffusion of Li ions during phase 2/phase 1 transition is largerly affected by slow interfacial charge transfer. In this case, the spinodal domain in the free-energy curve is effectively degenerated even at small overvoltages. This substantiates the formal application of the Cottrellian diffusion approach to determine D during the processes of intercalation and deintercalation, accompanied by first-order transitions; however, involvement of the slow interfacial kinetics in the measured response should be adequatly taken into account.

Comparison between potentiostatic and galvanostatic intermittent titration techniques for determination of chemical diffusion coefficients in ion-insertion electrodes

Using well-cycled, thin composite graphite electrodes we analyze carefully the limitations of potentiostatic and galvanostatic intermittent titration techniques (PITT and GITT, respectively) for determination of the differential (incremental) intercalation capacitance, C dif, and the chemical diffusion coefficient, D, of Li ions in these ion-insertion electrodes (IIEs). We demonstrate the superiority of the GITT over PITT to determine these quantities as the former technique allows for a more accurate determination of C dif and hence D which closely approach to the spinodal domain related to the first-order phase transition during ion-insertion. We show that GITT is also more effective in eliminating the parasitic contributions of background currents to the total measured response. A pronounced difference in the initial, intrinsic kinetics of formation of a new phase in the bulk of the old one has been observed depending on the direction of titration (phases less saturated with Li are formed faster during deintercalation than the Li-rich phases in the course of intercalation).

Intrinsic patterns in anisotropic multicomponent diffusive systems at equilibrium

A new class of intrinsic concentration patterns in anisotropic multicomponent fluids is described. These patterns occur in closed, isothermal, isobaric, non-chemically reacting mixtures. They correspond to thermodynamic equilibrium states and appear outside of the spinodal domain. Their appearance involves at least three pairs of unlike molecules (ternary mixtures).