Ni-Si-B alloys stand out for their high corrosion resistance and mechanical properties at high temperatures, able them to compete with austenitic stainless steels for applications in aerospace, aeronautical, nuclear, chemical, automotive, as well as tools and molds industries. Ni-Si-B alloys can be processed by casting, rapid solidification, brazing, thermal spraying or laser deposition. The casting root is one of the most direct manufacturing processes for producing finished shape components.
The microstructure of Ni-Si-B alloys processed by casting depends essentially on the chemical composition, the inoculation of the melt and the cooling rate. Given the variation in the chemical composition of the melts and the thermal gradients imposed by the wall thickness of the cast component, the microstructural variation is high, and consequently, significant variation in the mechanical properties is expected. Therefore, it is pertinent to study the influence of chemical composition on the volume fraction of phases formed: austenite, borides (Ni3B) and silicides (Ni3Si). While the hard phases provide high hardness and wear resistance, the austenitic matrix is characterized by high ductility and toughness.
This study is focused on the influence of boron on the microstructure and hardness of a Ni-Si-B cast alloy. For this purpose, samples with different boron contents were fabricated by the lost-wax casting process. Differential scanning calorimeter (DSC-TGA) analysis was employed to study the reactions that occur during solidification and the microstructure was characterized using scanning electron microscopy (SEM-BSE) with energy-dispersive microanalysis (EDS), and X-ray diffraction (XRD). The resulting phases were also characterized by hardness tests. Furthermore, the interpretation of experimental results on the kinetics of phases formation under casting conditions was compared and complemented with thermodynamic simulations using the Thermo-Calc software. The results reveal that there is an increase in the percentage of nickel borides (Ni3B) as the boron content increases, which is accompanied by a considerable increase in the hardness of the alloy.
Author Contributions
Conceptualization, G.M.G.; investigation, G.M.G.; supervision, L.M.M.R. and M.F.V.; writing—original draft, G.M.G.; writing—review and editing, L.M.M.R., M.F.V. All authors have read and agreed to the published version of the manuscript.
Funding
This research is sponsored by FEDER funds through the program P2020|COMPETE (project POCI-01-0247-FEDER-039836) and by national funds through the FCT—Fundação para a Ciência e a Tecnologia, I.P., within the scope of the project with the references UIDB/50022/2020 and UIDP/50022/2020.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Not applicable.
Acknowledgments
The authors are grateful to CEMUP (Centro de Materiais da Universidade do Porto) for expert assistance with SEM.
Conflicts of Interest
The authors declare no conflict of interest.
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