Coarsening of β' Precipitates in an Isothermally-Aged Fe₇₅-Ni₁₀-Al₁₅ Alloy

The coarsening process of the β' precipitates was studied in an isothermally aged Fe₇₅-Ni₁₀-Al₁₅ alloy. The aging treatments at 750, 850, and 920°C caused the precipitation of the β'phase with the B2 type crystalline structure in a bcc ferritic matrix phase. As the aging progressed, the initial rounded shape changed to cuboids aligned in the <100> directions of the ferritic matrix, and finally to rectangular plates also aligned in the same direction. The coarsening process of the β'phase was faster as the aging temperature increased. Nevertheless, the highest hardness and slowest overaging process took place during the aging at 920°C. This increase in hardness seems to be associated with the more rapid formation of the cuboid precipitates aligned in the <100> direction of the ferritic matrix.