Effects of boron on microstructure and mechanical properties of Fe-6.5 wt.%Si alloy fabricated by directional solidification

Effects of boron additions on the microstructure, elemental distribution and tensile properties at an intermediate temperature of the Fe-6.5 wt.%Si alloy fabricated by directional solidification were investigated. The results showed that the undercooling degree of the liquid phase at the frontier of solid–liquid interface was increased with boron additions, leading to significant refinement of the columnar grains of the alloy. When the boron content was increased from 0 to 0.058 wt.%, the width of the columnar grains was reduced from 2157 to 514 μm. Boron facilitated to homogenize silicon in the Fe-6.5 wt.%Si alloy because the solidification segregation level of silicon was decreased due to the interaction between boron and silicon. In the same range of boron content (0–0.058 wt.%), the equilibrium partition coefficient of silicon was increased from 0.961 to 0.982. Meanwhile, both the strength and ductility of the Fe-6.5 wt.%Si alloy at 400 °C could be improved due to the boron addition. Compared with that of the alloy without boron, the ultimate tensile strength of the alloy containing 0.040 wt.% B was increased from 712 to 792 MPa and its elongation was further increased from 38.5 to 54.4%.