Marina V. Bulanova

Influence of Heat Treatment on High-Temperature Mechanical Properties of Ti-5Dy-5Si-Sn Alloys

By the methods of X-ray diffraction, metallography, TEM, SEM and mechanical testing for compression, the influence of heat-treatment condition on the structure and properties of Ti-bL)y-5Si-Sn alloys was studied. High dispersity and stability of the microstructure were observed, predetermining a high level of mechanical properties. Annealing by the regime 1350°C, 31.5 h + 1250°C, 32 h + 1200°C, 32 h + 1100°C, 32 h was shown to provide optimum combination of high-temperature strength and roomtemperature plasticity.

Phase Equilibria During Solidification in the Ti–TiAl–DyAl2–Dy Region of the Ti–Dy–Al System

Phase equilibria during solidification in the Ti–TiAl–DyAl2–Dy region of the Ti–Dy–Al system are studied by differential thermal analysis, X-ray diffraction, metallography, and electron microprobe analysis. The liquidus and solidus surfaces, vertical sections, and reaction scheme in the solidification range are presented. No ternary compounds are found in the studied composition range. It is shown that DyAl2 undergoes polymorphic transformation at ~1200°C. The αl and α2 phases that coexist only with solid phases in the binary Ti–Al system participate in equilibria with the liquid phase in the ternary Ti–Dy–Al system. The liquidus surface is characterized by the primary solidification fields of the phases based on βTi (β), high-temperature αTi (αh), lowtemperature αTi (αl), Ti3Al (α2), TiAl (γ), Dy2Al, Dy3Al2, DyAl, βDyAl2, αDyAl2, βDy, and αDy. The solidus surface has elven three-phase fields: β + (βDyAl2) + αl, (βDyAl2) + αl + α2, β + αh + (βDyAl2), αh + γ + (βDyAl2), (βDyAl2) + α2 + (αDyAl2), (DyAl) + (Dy3Al2) + (αDyAl2), (αDy) + β + αl, (DyAl2) + α2 + (Dy3Al2), (Dy3Al2) + α2 + (Dy2Al), α2 + αl + (Dy2Al), and αl + (αDy) + (Dy2Al). The first two fields result from invariant four-phase peritectic reactions, LP1 + β + (DyAl2) ⇄ αl and LP2 + αl + (βDyAl2) ⇄ α2 proceeding at 1130 ± 5°C and 1180 ± 7°C, respectively. The next eight three-phase fields result from invariant four-phase transition reactions: LU1 + β ⇄ αh + (βDyAl2) at 1325 ± 8°C, LU2 + αh ⇄ γ + (βDyAl2) at 1260°C, LU3 + (βDyAl2) ⇄ α2 + (αDyAl2) at 1060 ± 4°C, LU4 + (DyAl) ⇄ (Dy3Al2) + (αDyAl2) at 1010 ± 9°C, LU5 + (αDy) ⇄ β + αl at 970 ± 4°C, LU6 + (αDyAl2) ⇄ α2 + (Dy3Al2) at 960 ± 8°C, LU7 + (Dy3Al2) ⇄ α2 + (Dy2Al) at 955 ± 16°C, and LU8 + α2 ⇄ αl + (Dy2Al) at ~930°C. The three-phase αl + (αDy) + (Dy2Al) field results from an invariant eutectic process, LE ⇄ αl + (αDy) + (Dy2Al), at 910 ± 15°C. The two-phase region in the solidus surface has a temperature maximum at 1343 ± 5°C, corresponding to the invariant three-phase le1 ⇄ β + (βDyAl2) reaction.

Phase Equilibria in the Ti–Ti5Si3–Dy5Si3–Dy Region of the Ti–Dy–Si System

Differential thermal analysis, X-ray diffraction, and metallography are employed to examine the phase equilibria in the Ti–Ti5Si3–Dy5Si3–Dy region of the Ti–Dy–Si system. Isothermal sections at 1100 and 900°C, vertical sections at 5Si, 65Ti, and 65Dy isopleths, and a reaction scheme are constructed. The ternary compound TiDySi (τ) exists at experimental temperatures and has no appreciable homogeneity range. The isothermal sections at 1100 and 900°C are similar and characterized by five three-phase regions (β + (α-Dy) + τ, β + (Ti3Si) + τ, (Ti3Si) + τ + (Ti5Si3), (α-Dy) + τ + (Dy5Si3), and (Ti5Si3) + τ + (Dy5Si3)) and respective two-phase fields. Three invariant four-phase equilibria are found in solid state: β + (Ti5Si3) ⇄ (Ti3Si) + τ (U3), β + τ + (Ti3Si) ⇄ α (P2), and β + τ + (α-Dy) ⇄ α (P3) at ~1150, 900, and 885°C, respectively. There is also a threephase equilibrium, β + τ ⇄ α, at 845°C (p4). The phase equilibria are summarized in the reaction scheme.