Investigation of the metatectic reaction in iron-boron binary alloys

Highlights: • Microstructural evidence of a metatectic reaction is experimentally observed. • The metatectic reaction occurs through two different microstructural changes. • Changes to predicted phase diagrams are proposed based on experimental data. The use of advanced high strength steels, such as boron containing steels, in the automotive industry is enabling lighter weight vehicles leading to increased fuel economies. However, boron containing steels are difficult to commercially produce. Thermodynamic studies of the Fe-B system predict a metatectic reaction, δ → L + γ, to occur on the iron-rich side of the diagram. This would result in solid material remelting as the temperature is decreased, which could be the source of the casting defects. To experimentally investigate the predicted metatectic reaction, levitation zone melting is used to control the boron segregation in two simple Fe-B binary alloys, and confocal scanning laser microscopy allows for in-situ observation of local microstructural changes as temperature is varied. Both experimental methods show evidence of a metatectic reaction providing a good comparison to predicted phase diagrams. Based on the experimental results, the metatectic reaction occurs over a broader range of compositions than predicted with C_s = 0.0025 wt% B, C_L = 2.0 wt% B, and C_R ranging from approximately 0.03 to 0.06 wt% B.