Coarsening process and precipitation hardening in Fe₂AlV-strengthened ferritic Fe₇₆Al₁₂V₁₂ alloy

Strengthening through a homogeneous distribution of a nano-sized second phase is a concept that is proposed to reinforce solid-solution body centered-cubic iron for high-temperature application in fossil-energy power plants. It was shown that these microstructures can be obtained in the Fe-Al-V system with L2₁-ordered Fe₂AlV precipitates in a ferritic matrix. The effect of aging in the range 600–700 °C on the ferritic Fe₇₆Al₁₂V₁₂ alloy was investigated using Vickers micro-hardness test and transmission electron microscopy. The diffusion screening coarsening theory is used to analyze the ripening kinetics. When volume fraction and mobility of the components in the ternary alloy are considered, the interfacial energy between the matrix and the precipitate was determined as (18±3)×10⁻³J/m² at 700 ºC but increases strongly when the temperature decreases. A classic precipitation hardening behavior has been observed along the time for each aging treatment. At room temperature, the increment of flow stress has a peak of about 450 MPa for a precipitate radius of 10 nm. Quantitative agreement is found with strength values predicted from order strengthening theory, predicting that strength is controlled by a precipitate shearing mechanism for sizes around that of peak strengthening, and the Orowan dislocation bypass mechanism for larger sizes. The APB energy of Fe₂AlV precipitate was estimated to be (27±4)×10⁻²J/m².