This study achieved the controllable preparation of L12 precipitates based on phase diagram calculations. Combined with the thermodynamic potential-pH diagram of corrosion and TEM observations of passive films, the influence mechanism of the content of nanoscale L12 phase on the corrosion behavior of high-entropy alloys was systematically investigated. It was clarified that the precipitation of nanoscale L12 phase helps to enrich Cr elements in the FCC matrix, thereby improving the stability of passive films and the resistance to pitting growth. A new strategy was proposed to significantly increase the pitting potential of high-entropy alloys by precipitating nanoscale L12 phases.
Illustrated Explanation
The phase diagram calculation of L12-high entropy alloys provides accurate guidance for the controllable preparation of precipitation-strengthened high entropy alloys. As shown in Figure 1a, the Co20Cr15Fe20Ni33Al6Ti6 alloy exhibits a simple FCC + L12 structure within a wide temperature range of 800-1100 °C, avoiding the formation of B2 and Sigma phases. By combining Figures 1a and 1b, the size of the L12 phase can be precisely controlled while varying its content. Figures 1c and 1d predict the changes in elemental composition within the FCC and L12 phases as a function of temperature, which may be an important reason for the alteration of the alloy's corrosion resistance.
Key Point 2: Corrosion Behavior of L12 Phase Reinforced High-Entropy Alloys. Figure 2a-b shows that as the content of the L12 phase increases, the pitting corrosion potential of the alloy significantly rises, reaching up to approximately 600 mV SCE. Compared with other multiphase high-entropy alloys or traditional alloys, this alloy exhibits significant advantages in terms of its resistance to uniform corrosion and pitting corrosion (Figure 2c).
Point 3: The thermodynamic stability of the oxide on the two-phase surface is analyzed through the potential-pH diagram. Figures 3a-b respectively show the thermodynamic stability of oxide formation on the surfaces of the FCC phase and the L12 phase. The oxide formed on the surface of the FCC phase is mainly Cr2O3, while the oxide formed on the surface of the L12 phase is mainly Al2O3. Cr2O3 has a stronger resistance to pitting corrosion, while Al2O3 is easily adsorbed and eroded by Cl- in the chloride-containing solution, thus having a weaker resistance to pitting corrosion. Therefore, it can be concluded that the L12 phase is more susceptible to corrosion compared to the FCC phase.
Point 4: The TEM observation of the passivation film confirmed the thermodynamic prediction. Figures 4a-f indicated that the L12 phase was preferentially corroded during the corrosion process, but the passivation film rapidly grew along the lower FCC matrix, forming a curved but continuous and uniform passivation film. This result was consistent with the prediction of the potential-pH graph. Moreover, the stability of the alloy passivation film is mainly related to the properties of the FCC matrix, and the higher the Cr element content in the FCC matrix, the more stable the passivation film. Therefore, by increasing the content of the L12 phase and promoting the enrichment of Cr elements in the FCC matrix (Figure 1c), the stability of the passivation film can be effectively improved.
Point 5: Analysis of Growth Stability of Pitting Corrosion Assuming that the L12 phase is completely dissolved, will the sub-stable "pits" left after dissolution continue to grow or undergo recrystallization? Figure 5a shows the electrochemical process occurring within the sub-stable pitting corrosion pits. Here, whether the pitting can grow stably depends on the competition between dissolution kinetics and diffusion kinetics. For the same size sub-stable pitting corrosion pits, idiff,crit should remain constant, as shown by the blue line in Figure 5b, while FCC matrixes with different Cr contents will lead to significant changes in dissolution kinetics, as shown by the red line in Figure 5b. The higher the Cr content in the FCC matrix, the lower the slope of the current density versus potential, thus a higher critical condition is required for the stable growth of pitting corrosion, which increases the resistance to the stable growth of pitting corrosion.
This work significantly enhanced the corrosion resistance of the precipitation-strengthened high-entropy alloy by regulating the content of L12 phase. The distribution characteristics of elements during the precipitation of L12 phase were understood, the influence of the content of the precipitated phase on the corrosion behavior of the high-entropy alloy was clarified, and the mechanism of the impact of L12 phase content on the stability of the passivation film and the growth process of pitting corrosion was elucidated.