Strengthening through a homogeneous distribution of a nano-sized second phase is a concept that is proposed to reinforce solid-solution bcc iron for high-temperature application in fossil or nuclear-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. We model this ordering reaction by using ab-initio thermodynamics. An assessment of the phase equilibria in the iron-rich corner of the ternary system allowed us to define a promising composition section. Then, the possible composition range was reduced from observations of the microstructure in different alloys, using the criteria of attaining higher temperature precipitation equilibrium and avoiding the coagulation and coalescence of precipitates.
On the ferritic Fe76Al12V12 alloy, a classical precipitation hardening behavior with time was observed for aging in the range of 600 to 700 °C. At room temperature, the increment of critical resolved shear 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 approximately equal to peak strengthening one, and the Orowan dislocation bypass mechanism for larger sizes.
Main aspects of the high temperature mechanical behavior of the Fe76Al12V12 alloy aged at 700 °C until the maximum hardening were investigated. The brittle ductile transition temperature of the alloy was recorded at 617 °C by using a Charpy impact tester provided with an innovative in-situ heating device. The yield stress at working temperature was characterized by hot compression tests, the values obtained are 815 and 592 MPa at 600 and 650 °C respectively.
As an insight to other hardening mechanisms, we analyzed carbides formation. An addition of 0.07 wt.% C in the ternary Fe-Al-V alloy probed to form vanadium carbides (approximately 5 μm thick) which were located in alignment with the direction of solidification growth of the columnar grains. For different aging times at 1100 °C it was found that the initial carbides are dissolved after 4 hours, and new carbides are formed homogeneously in the cooling at boundaries and interior of the grains. The carbides were characterized as plate type, with sizes of the order of 160 nm and hexagonal structure V6C5 by Transmission Electron Microscopy analyses.
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