V-Zn Experimental results and thermodynamic assessment

Abstract

Experimental investigation followed by thermodynamic assessment of the V–Zn system was carried out in the present study. A series of V–Zn alloys annealed at various temperatures were examined using scanning electron microscopy coupled with energy dispersive spectroscopy/wavelength dispersive X-ray spectrometer, X-ray diffraction and differential thermal analysis. It was confirmed that V Zn16, with a V content of about 5.8 at.%, was indeed an equilibrium phase. DTA results indicated that the peritectic temperature for V Zn16 was about 427 ∘C. Two new metastable compounds, V Zn9 and V 3Zn2, with V contents of 8.5–11.3 at.% and 60 at.%, respectively, were discovered. DTA results together with SEM-EDS examinations revealed that V Zn9 was formed at around 450 ∘C in Zn75V25 alloy with a cooling rate greater than 12 ∘C/min. The V Zn9 phase, however, decomposed into V Zn3 and liquid Zn when the alloy was held above 442 ∘C. The peritectic temperatures for two equilibrium phases, V 4Zn5 and V Zn3, were 651 ∘C and 621 ∘C, respectively. These measurements were slightly lower than the values determined in prior studies. The onset temperature for forming V Zn3 decreased significantly with increasing cooling rate while its exothermic peak widened during fast cooling. These phenomena indicated that both the nucleation and growth processes for V Zn3 were kinetically challenged.

In addition, the solubility of Zn in α-V was measured. It was 2.1 at.%, 2.5 at.%, 2.6 at.%, 2.9 at.% and 3.3 at.% at 450 ∘C, 600 ∘C, 670 ∘C, 800 ∘C and 1000 ∘C, respectively. Based on the results obtained in the present study and previous investigations, the V–Zn system was reassessed thermodynamically. The assessment was in good agreement with experimental results.