Phase equilibria during solidification in the Ti-Tial-Dy[Al.sub.2]-Dy region of the Ti-Dy-Al system

Phase equilibria during solidification in the Ti-TiAl-Dy[Al.sub.2]-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 Dy[Al.sub.2] undergoes polymorphic transformation at ~1200°C. The [α.sup.l] and [α.sub.2] 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 ([α.sup.h]), low-temperature αTi ([α.sup.l]), [Ti.sub.3]Al ([α.sub.2]), TiAl (λ), [Dy.sub.2]Al, [Dy.sub.3][Al.sub.2], DyAl, /βDy[Al.sub.2], αDy[Al.sub.2], /βDy, and αDy. The solidus surface has elven three-phase fields: β + (βDy[Al.sub.2]) + [α.sub.l], (βDy[Al.sub.2]) + [α.sup.l] + [α.sub.2], β + [α.sub.h] + (βDy[Al.sub.2]) [α.sub.h] + γ + (βDy[Al.sub.2], (βDy[Al.sub.2] + [α.sub.2] + (αDy[Al.sub.2], (DyAl) + ([Dy.sub.3][Al.sub.2] + (αDy[Al.sub.2], (αDy) + β + [α.sup.1], (αDy[Al.sub.2]) + [α.sub.2] + ([Dy.sub.3][Al.sub.2]), ([Dy.sub.3][Al.sub.2]) + [α.sub.2] + ([Dy.sub.2]Al), [α.sub.2] + [α.sup.l] + ([Dy.sub.2]Al), and [α.sup.l] + (αDy) + ([Dy.sub.2]Al). The first two fields result from invariant four-phase peritectic reactions, [L.sub.P1] + β + (Dy[Al.sub.2] [??] [α.sup.l] and [L.sub.P2] + al + (βDy[Al.sub.2]) [??] [α.sub.2] proceeding at 1130 ± 5°C and 1180 ± 7°C, respectively. The next eight three-phase fields result from invariant four-phase transition reactions: [L.sub.U1] + /β [??] [α.sup.h] + (βDy[Al.sub.2]) at 1325 [+ or -] 8°C, [L.sub.U2] + [α.sup.h] [??] γ + (βDy[Al.sub.2]) at 1260 °C, [L.sub.U3] + (βDy[Al.sub.2]) [??] [α.sub.2] + (αDy[Al.sub.2]) at 1060 ± 4°C, [L.sub.U4] + (DyAl) [??] ([Dy.sub.3][Al.sub.2]) + (αDy[Al.sub.2]) at 1010 ± 9°C, [L.sub.U5] + (αDy) [??] β + [α.sup.l] at 970 ± 4°C, [L.sub.U6] + (αDy[Al.sub.2]) [??] [α.sup.2] + ([Dy.sub.3][Al.sub.2]) at 960 [+ or -] 8°C, [L.sub.U7] + ([Dy.sub.3][Al.sub.2]) [??] [α.sub.2] + ([Dy.sub.2]Al) at 955 ± 16°C, and [L.sub.U8] + [α.sup.2] [??] [α.sup.l] + ([Dy.sub.2]Al) at ~930°C. The three-phase [α.sup.l] + (αDy) + ([Dy.sub.2]Al) field results from an invariant eutectic process, [L.sub.E] [??] [α.sup.l] + (αDy) + ([Dy.sub.2]Al), at