Microstructural evolution and phase transition dependent on annealing temperature and carbon content for LaFe11.5Si1.5Cx compounds prepared by arc-melting

Three different parts of LaFe11.5Si1.5Cx(x = 0, 0.1, 0.2, 1.0) ingots prepared by arc-melting were determined. The region of the sample close to the surface towards the arc was marked as part 1 and the region of the sample close to the water jacketed copper crucible was marked as part 3. Without affecting the microstructure of part 1, carbon doping could increase the amount of 1:13 phase of the LaFe11.5Si1.5Cx(x = 0, 0.1, 0.2) ingots, which mainly existed in the part 3. The DSC curves of part 1 of LaFe11.5Si1.5Cx(x = 0, 0.2) ingots show a peak at 1445 K moved to 1402 K after carbon doping. The eutectoid reaction: LaFeSi → 1:13 phase + La5Si3 is believed to be around the peak. After annealing at 1353 K, 1443 K and 1473 K for 6 h, the microstructure of LaFe11.5Si1.5Cx(x = 0, 0.1, 0.2) compounds was investigated. Too high annealing temperature could induce the formation of La5Si3 phase and even the growth of α-Fe. The best annealing temperature should be slightly lower than the eutectoid reaction temperature, even if larger atomic diffusion rate can be obtained at higher temperature. Carbon doping could reduce the eutectoid reaction temperature. Energy can be saved for a low annealing temperature. In addition, carbon doping can accelerate the formation of the 1:13 phase by improving the nucleation rate.