Effect of aluminum and silicon on magnetic properties and microstructures of Fe-Cr-Ni-C-Al and Fe-Cr-Ni-C-Si alloys

The magnetic properties and microstructures of Fe-17.5mass%Cr-2.0%Ni-0.5mass%C-a mass%Al (a=0.04-2.38) and Fe-17.5mass%Cr-2.0mass%Ni-0.5mass%C-a mass%Si (a= 0.18-1.94) alloys were investigated in order to develop a magnetic material exhibiting high electric resistivity and softmagnetism when annealed at low temperatures, and paramagnetism when dissolution treated at high temperature. The addition of Al or Si was effective in increasing the electric resistivity, and improving the softmagnetism of the alloys step-annealed at 1053K and 923K. The improvement of the softmagnetism was discussed from the increase in the transformation temperature and variation of the microstructures. In the lower Al or Si content (less than or equal to0.7% Al or 1.6% Si), the alloys consisted of (alpha+gamma+M23C6) phases at 1053K, the first annealing temperature. The mole fraction of the gamma phase decreased with increasing Al or Si content because of the increase in the A(cl), temperature. This change caused the decrease in the dislocation density due to the y --> alpha' transformation, and resulted in the improvement of the softmagnetism. On the other hand, most of the alloys were transformed to the paramagnetic y phase by dissolution treatment at 1473K. However, the higher Al content alloys ( greater than or equal to 1.91% Al) consisted of (gamma + alpha) phases, and exhibited higher relative permeability than the others. This phenomenon was also interpreted from the increase in the transformation temperature with increasing Al content.