Ab initio-based prediction and TEM study of silicide precipitation in titanium

Abstract

a b s t r a c tThe literature contains many contradictory data about the most thermodynamically favorable structure of precipitates in a-Ti matrix of the hypoeutectoid Ti-Si alloys. In this work we applied our recently developed thermodynamic model to predict the structure of Ti-Si precipitates in a-Ti matrix of the TiSi alloy with total Si concentration of 0.7 wt.%. We considered all prominent Ti-Si phases such as Ti 3 Si; Ti 5 Si 3 ; Ti 5 Si 4 ; TiSi and two TiSi 2 phases and discovered that formation of the Ti 5 Si 3 phase is more favorable than that of the Ti 3 Si in contradiction with the known phase diagrams. Theoretical result was confirmed by experimental investigation of microstructure and phase composition of the model Ti-0.7Si alloy annealed at 873 K for 10 h. Indeed, the only observed phase has hexagonal Ti 5 Si 3 structure. To ensure the completeness of the results we calculated ab initio elastic constants for all considered Ti-Si phases.

Assessment of the Ti-Rich Corner of the Ti-Si Phase Diagram: The Recent Dispute About the Eutectoid Reaction

“…The predicted ground-state structures were consistent with those known to be stable at low temperature, except for the Ti 3 Si phase with Ti 3 P (tP32) structure, whose enthalpy of formation at 0K (-47.11 kJ/mol of atoms) was slightly above the ground-state line, indicating that the question of stability of the Ti 3 Si phase shown by previous experimental 5 and calculated 9,11 Ti-Si phase diagrams was in fact "controversial". More recently, Poletaev et al 21 indicated that Ti 5 Si 3 phase was actually more stable than Ti 3 Si phase at 0 K by ab initio calculations. They also confirmed the presence of Ti(α) and Ti 5 Si 3 phases by electron diffraction and microanalysis after the heat-treatment of a Ti-0.7Si alloy at 873K for 10 hours.…”

“…In the light of the dispute over the stability of Ti 3 Si phase in Ti-X-Si engineering alloys [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] , the present work will calculate and compare the Ti-rich corner of the "stable" and "metastable" Ti-Si phase diagrams, by assuming that Ti 3 Si is the stable phase in the eutectoid reaction, Ti(β)  Ti(α)+ Ti 3 Si. These results will be discussed in order to inspire further experimental work on critical regions of the Ti-Si phase diagram 11 .…”

“…More recently, Colinet and Tedenac 20 indicated by ab-initio calculations that the question of the stability of the Ti 3 Si phase was in fact "controversial"; while Poletaev et al 21 showed by ab-initio calculations that the Ti 5 Si 3 phase was essentially more stable than the Ti 3 Si phase at 0 K, adding a new light on the dispute over the stability of Ti 3 Si phase in the Ti-Si and Ti-X-Si systems. In this sense, the present assessments aim to inspire further experimental work in the eutectoid region of the Ti-rich corner of the Ti-Si phase diagram in order to investigate the stability of Ti 3 Si and Ti 5 Si 3 phases; and the effect of interstitials on the stability of the Ti 3 Si phase.…”

Controllable two-scale network architecture and enhanced mechanical properties of (Ti5Si3+TiBw)/Ti6Al4V composites

“…According to the Ti-Si phase diagram 19 , the silicide in the equilibrium microstructure of Ti-Si should be Ti 3 Si. However, there are many contradictions in previous literatures, about the most thermodynamically favorable structure of precipitation at low concentrations of Si 26 27 28 . Poletaev et al .…”

“…Poletaev et al . 26 applied a recently developed thermodynamic model to predict the structure of Ti-Si precipitations in the α-Ti matrix. They discovered that formation of the Ti 5 Si 3 phase is more favorable than that of the Ti 3 Si phase in contradiction.…”