Liquid-Solid Phase Equilibria in Metal-Rich Nb-Ti-Hf-Si Alloys

Abstract

A thermodynamic description of the Nb-Ti-Hf-Si system is extrapolated from descriptions of the constituent Nb-Ti-Si, Nb-Hf-Si, Hf-Ti-Si, and Nb-Hf-Ti systems using the CALPHAD technique. From this thermodynamic description, the liquidus projection at the metal-rich region of the Nb-Ti-Hf-Si system with the Si concentration up to 40% is calculated. The calculated liquidus surface at this region consists of six primary solidification regions: (Nb), Hf2Si, (Hf,Ti)5Si3, (Nb,Ti)3Si, αNb5Si3, and βNb5Si3. Three five-phase equilibria involving these phases are identified on the liquidus surface by this calculation. They are $$ {\text{L}} + {\text{(Nb)}} + \alpha {\text{Nb}}_{\text{5}} {\text{Si}}_{\text{3}} {\text{ }} \to {\text{ Hf}}_{\text{2}} {\text{Si}} + {\text{(Nb,Ti)}}_{\text{3}} {\text{Si at }}1814\,^\circ {\text{C}} $$ , $$ {\text{L}} + {\text{Hf}}_{\text{2}} {\text{Si}} + \alpha {\text{Nb}}_{\text{5}} {\text{Si}}_{\text{3}} {\text{ }} \to {\text{ }}({\text{Hf,Ti)}}_5 {\text{Si}}_{\text{3}} + ({\text{Nb,Ti}})_{\text{3}} {\text{Si at 1739}}\,{{^\circ \text{C}}} $$ , $$ {\text{L}} + {\text{Hf}}_{\text{2}} {\text{Si }} \to ({\text{Nb)}} + {\text{(Hf,Ti)}}_{\text{5}} {\text{Si}}_{\text{3}} + {\text{(Nb,Ti)}}_{\text{3}} {\text{Si at 1400}}\,^\circ {\text{C}} $$ . To validate the calculated liquidus surface, a total of 10 alloys was directionally solidified. The microstructure of these 10 as-cast alloys was examined by using scanning electron microscopy (back scatter electron (BSE) imaging) and energy dispersive spectrometry (EDS). The solidification simulation of these 10 alloys was then performed by using the Scheil model that is integrated in the multicomponent phase diagram calculation software Pandat. The observed phases presented in the as-cast microstructure of these 10 alloys are in good agreement with those predicted from the Scheil simulation.