“… Figure 1. Experimentally observed incubation times for Laves phase precipitation at several temperatures as reported for nine commercial steels [ 39 – 42 ]. …”
Section: Model Description: Alloy By Designmentioning
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.
“… Figure 1. Experimentally observed incubation times for Laves phase precipitation at several temperatures as reported for nine commercial steels [ 39 – 42 ]. …”
Section: Model Description: Alloy By Designmentioning
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.
“…A temperature of 600°C or greater is not desirable for evaluating the microstructural stability under FBR conditions, because the precipitation nose of the Laves phase is between 620°C and 670°C. 27) Metallurgical examination and quantitative analysis were conducted before and after the creep test using transmission electron microscopy (TEM) at 200 kV. Precipitates were observed using extracted replicas, and identification of the precipitates was conducted using electron diffraction patterns and the chemical compositions.…”
Section: Materials and Experimental Proceduresmentioning
The applicability of high chromium (Cr) steel as the main structural material in fast breeder reactors (FBR) has been explored to enhance the safety, the credibility and the economic competitiveness of FBR power plants. Vanadium (V) and Niobium (Nb) are believed to improve the high-temperature strength of high Cr steels by precipitating as carbides and/or nitrides, namely fine MX particles, although the longterm efficiency and stability of such precipitation strengthening mechanisms resulting from the fine MX particles have not been clarified yet. The effects of V and Nb on degradation of creep properties were investigated under FBR operating conditions, e.g., at 550°C for 500 000 h, and the relationship between the long-term creep properties and microstructural changes was investigated considering the MX particles and the Z-phase. It was found that the optimal V and Nb contents for excellent high Cr steel of FBR grade are 0.2 mass% and <0.01 mass%, respectively, under FBR operating conditions.
“…5) They mainly precipitate adjacent to M 23 C 6 on prior austenite grain, packet/block, and subgrain boundaries. [5][6][7][8][9] Several studies [10][11][12][13][14][15][16][17][18] on the relationship between the Fe 2 M Laves phase (M is mainly Mo, W, and Nb) precipitated in ferritic heat-resistant steels and the decrease in creep ductility and toughness of steels have been reported. However, Zhong et al 19) confirmed that the ductility of the specimens embrittled by aging at 600°C was recovered by reheating at 700°C.…”
In this study, the cleavage fracture of the C14 Fe 2 W Laves phase was investigated by first-principles calculations and crystal orientation analysis using scanning electron microscopy. Trace analysis of the orientations of cleavage planes revealed that cleavage fracture occurred in five types of crystal planes: (0001), {1100}, {1120}, {1101}, and {1122}. Among these fractures, the fracture at (0001) is the most preferable. From, the first-principle calculations of the surface energy for fracture, Young's modulus, and Poisson's ratio, the minimum fracture toughness value of 1.62 MPa•m 1/2 was obtained at (0001). The tendency of the calculated fracture toughness to become larger with high indexed planes is almost the same as the frequency of the types of cleavage planes in the trace analysis. It was concluded that the fracture toughness of the C14 Fe 2 W Laves phase is controlled by the surface energy for fracture and Young's modulus.
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