Analyses of eutectoid phase transformations in Nb-silicide in situ composites

Area of Science:

• Materials Science
• Metallurgy
• High-Temperature Materials

Background:

• Niobium-silicide composites are candidates for high-temperature turbine applications due to their combination of ductility and strength.
• Alloying elements like Ti, Hf, Cr, and Al can improve fracture toughness, creep resistance, and oxidation resistance.
• In binary Nb-Si alloys, Nb3Si is unstable and decomposes, but Ti addition can stabilize it.

Purpose of the Study:

• To investigate the effect of combined Ti and Hf additions on phase stability in Nb-silicide composites.
• To identify and characterize a novel low-temperature eutectoid phase transformation in these quaternary alloys.
• To understand how bulk composition influences phase formation and eutectoid decomposition.

Main Methods:

• Systematic variation of Ti (21-33 at.%) and Hf (7.5-33 at.%) concentrations in quaternary Nb-Si alloys.
• Analysis of phase composition and identification of phase transformations using advanced characterization techniques.
• Investigation of the crystal structure of the resulting silicides.

Main Results:

• Discovery of a low-temperature eutectoid transformation where Nb3Si decomposes into Nb and Nb5Si3.
• The Nb5Si3 formed in this transformation exhibits the hP16 crystal structure, differing from the tI32 structure in binary alloys.
• The study mapped the phase landscape as a function of Ti and Hf content.

Conclusions:

• The stabilization of Nb3Si and suppression of eutectoid decomposition are influenced by specific Ti and Hf concentrations.
• A new hP16 Nb5Si3 phase has been identified, impacting the understanding of Nb-silicide phase diagrams.
• This research provides critical insights for designing advanced Nb-silicide composites for demanding high-temperature applications.