Nucleation Of Laves Phase Research Articles

Fast event-driven simulations for soft spheres: from dynamics to Laves phase nucleation.

Conventional molecular dynamics (MD) simulations struggle when simulating particles with steeply varying interaction potentials due to the need to use a very short time step. Here, we demonstrate that an event-driven Monte Carlo (EDMC) approach was first introduced by Peters and de With [Phys. Rev. E 85, 026703 (2012)] and represents an excellent substitute for MD in the canonical ensemble. In addition to correctly reproducing the static thermodynamic properties of the system, the EDMC method closely mimics the dynamics of systems of particles interacting via the steeply repulsive Weeks-Chandler-Andersen (WCA) potential. In comparison to time-driven MD simulations, EDMC runs faster by over an order of magnitude at sufficiently low temperatures. Moreover, the lack of a finite time step in EDMC circumvents the need to trade accuracy against the simulation speed associated with the choice of time step in MD. We showcase the usefulness of this model to explore the phase behavior of the WCA model at extremely low temperatures and to demonstrate that spontaneous nucleation and growth of the Laves phases are possible at temperatures significantly lower than previously reported.

Improved corrosion resistance of super austenite stainless steel by B-induced nucleation of Laves phase

Cr-depleted zones induced by precipitates in Mo-rich stainless steel will lead to severe local corrosion. In this work, σ/Laves formation and corrosion resistance influenced by B addition was investigated through experimental and theoretical analysis. The results showed that B alloying can significantly inhibit the σ phase formation and induce nucleation of Laves phase. Furthermore, Laves phase that contained B decreased Cr-depleted zones near the grain boundaries and reduced the interface potential difference, which can enhance the intergranular corrosion resistance. Thus, the B-controlled precipitates formation method is conducive to improving the corrosion resistance for Mo-rich austenitic stainless steel.

A first step towards computational design of W-containing self-healing ferritic creep resistant steels

ABSTRACT In this work, we combine a generic alloy-by-design model with a novel concept, the nucleation barrier for the formation of Laves phase to fill the creep cavities, in order to develop multi-component creep resistant steels with kinetically tuned self-healing behaviour. In the model the high-temperature long-term strength is estimated by integrating precipitation strengthening due to M23C6 carbides and solid solution strengthening, while the optimized compositional solutions are determined by employing the coupled thermodynamic and kinetic principles. W-containing Laves phase herein is selected as the self-healing agent to autonomously fill the grain boundary cavities, so as to prolong the creep lifetime. To achieve the effective healing reaction, the nucleation time for Laves precipitates are expected to coincide simultaneously with which creep cavities start to form or reach a healable size. Using experimental data from literature, an empirical relationship to estimate the incubation time for Laves phase formation has been constructed, from which the thermodynamic driving force for onset of precipitation as a function of temperature and intended precipitate nucleation time was derived. Three sample alloys have been selected among the desirable solutions, which are predicted to have the same strength but widely different Laves phase nucleation times. The calculations are also performed for different use temperatures to explore the compatibility between high temperature strength and timely cavity filling behaviour. In its current form the model is not expected to yield the truly optimal composition but to demonstrate how the kinetics of the healing reaction can affect the predicted optimal alloy compositions.

M <sub>23</sub>C <sub>6</sub>-Assisted Laves Phase Growth Mechanism in a Novel 9Cr-3W-3Co-1CuVNbB Steel Under Creep Deformation

Laves phase particles are an important precipitate in tempered martensite ferritic steels that can strongly affect the microstructure and mechanical properties. We systematically investigated the evolution of Laves phase in G115 steel. Experimental results showed that the amount of Laves phases increases, whereas that of M23C6 carbides decreases as creep deformation proceeds. Further characterizations were conducted to thoroughly explore the evolutionary mechanism of Laves phase. Interestingly, a hybrid Laves-M23C6 particle was observed, and Cr segregation occurred near the remaining M23C6 carbides. Further, we found that C and Cr were also segregated near a Laves phase that was formed. The Cr concentration of the Laves phase was much lower than that of M23C6 carbide, but slightly higher than that of the ferrite matrix. The result confirms that Laves phase is transformed from M23C6 carbide, and it can also explain reasonably well the decrease in M23C6 carbide during creep deformation. We also proposed a M23C623C6 carbides, which is considered to be an interface-controlled M23C623C6-assisted Laves phase growth model was established assuming that the Gibbs energy is balanced at the M23C6/Laves phase interface. Furthermore, a guideline for development of novel tempered martensite ferritic steels was proposed. Decreasing the Si content of the material can effectively postpone Laves phase nucleation. In addition, the fraction of the Laves phase decreases, which helps improve the creep strength.

Microstructural evolution of ASTM grade 91 after long-term creep testing

Modified 9Cr–1Mo steel (ASTM grade P/T91) is conventionally applied in the construction of high efficiency supercritical and ultra-supercritical boilers, reaching metal temperatures up to 600–610 °C. As a part of the characterisation process of commercial pipes, Tenaris has performed over the years several long-term creep tests, with duration up to over 100,000 h, for temperatures between 550 and 650 °C. Microstructural characterisation has been performed on broken specimens, including analysis of the state of precipitation after TEM imaging of extraction replicas. The evolution of M23C6 coarse carbides, and MX carbo-nitrides, as well as the nucleation of new phases, such as Laves and Z are described in this paper. Observations reveal Laves phase nucleation and growth peak at 600 °C, whereas almost no particles were observed after ageing at higher temperature; moreover, transformation of MX carbo-nitrides into Z-phase is also faster at 600 °C, and slows at higher temperature.

On the nucleation of Laves phase particles during high-temperature exposure and creep of tempered martensite ferritic steels

This paper reports on the formation of an Mo-rich Laves phase during high-temperature exposure and creep of a tempered martensite ferritic steel with 12wt.% Cr and 1wt.% Mo. The material was exposed to 550°C for time intervals between 864 and 81,984h. For comparison, a few creep tests were carried out at 550°C and 120MPa (duration between 864 and 12,456h). All tests were interrupted after specific time periods and microstructures were investigated using transmission electron microscopy and atom probe tomography. Laves phase formation occurs during both heat treatment and creep. Creep stress and strain have no significant effect on the early stages of Laves phase formation. In the present work we show that prior to Laves phase nucleation Si and Mo segregate to micrograin boundaries, where subsequently Laves phase particles appear next to M23C6 carbides.

Influence of long heat treatments on the laves phase nucleation in a type 444 ferritic stainless steel

Composition, morphology and nucleation of Laves phase particles containing Mo, Nb and Si were investigated in a type 444 (18%Cr–0.4%Nb–0.1%Ti–0.5%Si–2.0%Mo) ferritic stainless steel heat treated at 650°C for up to 120h with and without a heat treatment and dissolution treatment at 1050°C during which all Laves phase dissolve. In situ and extracted precipitate particles were analysed using scanning and transmission electron microscopy, energy-dispersive spectroscopy and X-ray diffraction. An (FeCrSi)2(MoNb) Laves phase, titanium nitrides and niobium carbides were present in the as-received sheet prior to heat treatment but during treatment at 650°C the number density and size of Laves phase particles increased substantially within 120h. The morphology of the hexagonal (FeCrSi)2(MoNb) Laves phase was characterized as either plate-like or equiaxed depending on whether the particle had an orientation relationship with the matrix or not, i.e. whether the precipitate nucleated during the heat treatment or was present in the as-received material.

Microanalysis of two creep resistant 9–12% chromium steels

The ferritic chromium steels P122 and P92 have been investigated using APFIM and TEM. The two steels are similar in composition with the exception of an addition of copper to P122, allowing a higher chromium content. The investigated materials were tempered at 770°C and isothermally aged at 600°C for times ranging from 0 to 10 000 h. The matrix and precipitates of type M23C6, MX and Laves phase have been analysed with APFIM. Steel P122 contains 0.9% copper and it was found that the matrix concentration of copper drops during ageing from 0.4% to an equilibrium level at 0.1%, which is in good agreement with previous thermodynamical calculations. No copper was found in M23C6, MX or Laves phase. Copper instead forms a separate phase. This phase was identified by TEM. During ageing the amount of tungsten in the matrix drops due to formation of Laves phase. This process is faster in steel P122 compared with steel P92, indicating an accelerating effect of copper on the nucleation of Laves phase. In both steels, enhanced concentrations of boron were found inside M23C6 carbides. Phosphorous was found to segregate to a very narrow region at the carbide/matrix interface.