Sessile drop study of Gd–Ti monotectic alloys on ceramic substrates: phase transformations, wetting, and reactivity
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Gd–Ti monotectic alloys were studied in a wide temperature range by the sessile drop technique. The eutectic and monotectic transformations in Gd–Ti alloys upon heating were in situ monitored by a high-speed and high-resolution CCD camera. Our findings suggest that the existing Gd–Ti phase diagram significantly underestimates the monotectic and binodal temperatures in this system. The wetting behavior and the high-temperature interaction of the Gd–Ti melts with yttria and yttria-stabilized zirconia substrates were investigated.
KeywordsY2O3 Sessile Drop Sessile Drop Technique Monotectic Temperature Monotectic Alloy
According to the equilibrium phase diagram , the Gd–Ti system exhibits a miscibility gap in the liquid state. This makes Gd–Ti-based multicomponent alloys appropriate for fabrication of nano- and microphase-separated composite materials. Such composites have a high application potential due to combination of two or more phases with remarkably different properties. The microstructure of solid alloys that exhibit a liquid–liquid miscibility gap is determined by their thermophysical properties and casting conditions (see, for example, refs. [2, 3, 4, 5, 6, 7, 8] and citations therein). The Gd–Ti system also plays an important role in the synthesis of high-performance alloys for magnetic applications by alloying, for example, with Ge, Mn, Fe, and Sn [9, 10, 11, 12]. These materials are usually produced by melting of pure components and subsequent homogenization of the melts at certain temperatures determined from the respective phase diagrams. Therefore, knowledge of thermodynamic and thermophysical properties of the molten alloys is of high practical importance. In this context, accurate data on the binary systems are indispensable.
In this work, we investigated binary Gd–Ti alloys upon heating by the sessile drop technique. Due to the use of a high-speed and high-resolution charge-coupled device (CCD) camera, it was possible to record the wetting behavior and the high-temperature interaction of the Gd–Ti melts with yttria and yttria-stabilized zirconia substrates. Moreover, this technique allowed real-time observation of the eutectic and monotectic phase transformations in the Gd–Ti alloys. The microstructure of solidified Gd–Ti bulk samples as well as the interfaces formed in the sessile drop investigations between Gd–Ti alloys and ceramics were characterized by scanning electron microscopy (SEM) combined with energy-dispersive X-ray (EDX) spectrometry.
Polycrystalline yttria (Y2O3) and two kinds of yttria-stabilized zirconia (ZrO2 stabilized with 3 wt.% of Y2O3 ≡ 3YSZ and ZrO2 stabilized with 5 wt.% of Y2O3 ≡ 5YSZ) were used as substrates in the sessile drop experiments. The substrates were polished with the same oxide powder that was used for the sintering of the ceramics, to avoid their pollution.
The sessile drop investigations were carried out using the experimental setup described in details in ref. . Before loading into the UHV chamber equipped with a vertical resistance heater, the Gd–Ti samples and substrates were cleaned ultrasonically in isopropanol for 5 min. Heating was started when the level of vacuum reached at least 10−6 mbar. At 500 °C, high purity Ar (99.999 %) was introduced into the chamber. The flow rate was 5 cm3 min−1 and the pressure level inside the high-temperature chamber was held in the range of 850–900 mbar. The heating rate was 6 K min−1 and the cooling rate was 18 K min−1.
The behavior of the Gd–Ti samples on ceramic substrates was registered by a high-speed and high-resolution CCD camera. The experiments were performed from room temperature up to some degrees Kelvin above the point, where Gd–Ti droplets looked as being completely melted. At the first stage of heating up to 1000 °C, the drop image acquisition was 1 fps (frame per sec), after that it was increased to 100 fps.
After cooling, the Gd–Ti/substrate couples from the sessile drop experiments were cut in the central part perpendicular to the substrate surface and their microstructure and composition characterization was performed with Hitachi TM3000 SEM microscope equipped with an EDX analyzer.
It is a challenge to find a suitable crucible or substrate material for high-temperature investigations of liquid alloys containing such highly reactive components as Ti and Gd. Y2O3 was reported to be relatively stable against molten Ti ; although small quantities of Y and O could be dissolved from the ceramic [14, 15]. On the other hand, liquid Ti and Ti-based alloys severely react with Al2O3 and ZrO2 ceramics [16, 17]. Despite a lack of information on the interaction between liquid Gd and ceramics, a high chemical aggressivity of rare metals is known. Based on the literature data on reactivity of Ti-based alloys, the polycrystalline Y2O3 substrates were selected for our first sessile drop tests. However, they were quickly penetrated by the liquid Gd phase, as demonstrated in Fig. 2. This well-distinguished infiltration of the first liquid into Y2O3 enabled a visual observation of the two-phase state (Ti-rich solid and Gd-rich liquid) in the Gd–Ti system at the temperatures between 1240 and 1460 °C.
It should be noted that just a very small quantity of Y (0.1–0.3 at.%) was found in the Ti-rich phase solidified on the yttria substrate. We suppose that Y could have appeared there either due to a slight dissolution of the substrate during the sessile drop tests or during the cutting and subsequent polishing of the Gd–Ti/Y2O3 couples for the SEM/EDX characterization, or both.
The situation has been essentially changed in the case of yttria-stabilized zirconia. Neither Ti nor Gd was found in the drop-side region of the 3YSZ substrates after the sessile drop tests. Pollution of the Gd–Ti sample due to interaction with the ZrO2 substrate was very small (about 0.5–1 at.% Zr in some solidified drops was detected by EDX). It has been reported in ref.  that the high-temperature (1400 °C) solid-state reaction and interdiffusion between yttria-stabilized zirconia and GdCeO is suppressed due to the formation of a thin (≤1 μm) interaction zone. A similar phenomenon might occur at the contact of Gd-rich liquid and YSZ ceramic. It is plausible that a thin GdZrO layer is formed at the interface and prevents interdiffusion of components and further growth of the reaction zone. At the same time, Gd atoms may diffuse from the Gd liquid phase along the ceramic surface feeding the growth of GdZrO crystals in the vicinity of the drop, as observed in Fig. 6c. Obviously, GdZrO grows on the expense of Zr and O from the 3YSZ substrate. The loss of oxygen resulting in the formation of a nonstoichiometric ZrO2−x  could explain gray coloring of the substrates, noted during the high-temperature tests.
The melting temperature for the Gd-rich phase in the Gd40Ti60 and Gd60Ti40 alloys (1220–1250 °C) determined upon heating in the present sessile drop studies on all substrates shows a good agreement with the eutectic isotherm (T E = 1240 °C) from the Gd–Ti phase diagram . However, the tests carried out on the Y2O3 substrates revealed that the monotectic temperature (T M = 1275 °C) in the phase diagram (Fig. 1) is significantly underestimated. Indeed, the Ti-rich phase remained solid until 1450–1460 °C in those experiments. As it has been established in the sessile drop measurements performed on the 3YSZ ceramics, the Ti-rich phase melts at about 1620–1650 °C. Comparison of the images of the Gd40Ti60 and Gd60Ti40 sessile drops (Figs. 3e, 4d, e) with the experimental and calculated sessile drop profiles for the Ga–Pb phase-separated melts studied close to the respective monotectic temperature in ref.  suggests that the Gd–Ti alloys at 1620–1650 °C are rather in a two-phase liquid state than a single-phase liquid. The Gd40Ti60 and Gd60Ti40 samples after the sessile drop investigations on the 3YSZ substrates exhibit macroscopic phase separation (Fig. 6a, b). The oval-like shape of the Ti-rich phase, distorted during crystallization, and its location inside the Gd-rich phase at the top of the samples also indicates that the alloys were completely molten and both fall into the miscibility gap.
The melting temperature for the Ti-rich phase following from our study is about 350–400 K above the calculated monotectic temperature  and about 190–240 K above the melting temperature reported in ref. . Such a big difference cannot be an effect of the applied heating rate, which is rather moderate (6 K min−1), since the liquid phase is already present (recall that the Gd-rich phase melts at 1240 °C). Also, the melting temperature of the Ti-rich phase is unlikely to be influenced by a very small quantity of either Y or Zr found mainly in the Gd-rich phase after the high-temperature tests. The large disagreement of the experimental results can be explained by the different techniques applied in the present work (sessile drop method) and in ref.  (thermal-gradient method ). The thermal-gradient method, in which melting is detected by optical observation of a cavity in the sample, is rather uncertain in two-phase melting alloys, as the cavity is usually filled by the first liquid or the specimen collapses before the second phase completely melts . It should also be noted that the melting data reported in ref.  suffered large scattering and were severely reinterpreted during calculations . Our findings suggest that the liquid state equilibria in the Gd–Ti phase diagram  should be reassessed.
The Ti-rich phase in the Gd–Ti samples solidified after the sessile drop measurements was completely covered by about 5–40 μm thick layer of the Gd-rich phase as it can be seen on the SEM/EDX image in Fig. 6d. Earlier, such surface wetting layers have been observed in other monotectic alloys; for example in Ga–Bi , Ga–Pb , Cu–Co , and Al–In . Calculations showed that complete wetting is preserved in many immiscible systems far below the monotectic or even eutectic temperature [27, 28, 29], and this has been related to the high surface tension σ of metallic liquids. Bearing in mind that σ of the liquid Gd is ~800 mN m−1 at 1600 °C  and σ of the liquid Ti is ~1450 mN m−1 at 1700 °C , it is reasonable to suppose that the Gd-rich phase completely wets the Ti-rich phase in the liquid Gd–Ti alloys and the equilibrium wetting layer grows by diffusion up to a micrometer thickness during solidification as it has been suggested in ref. .
The sessile drop technique equipped with a high-speed and high-resolution CCD camera was used for in situ observation of the eutectic and monotectic phase transformations in the Gd–Ti alloys as well as of the high-temperature interaction of the melts with yttria and yttria-stabilized zirconia substrates.
The observed eutectic transformation in the Gd40Ti60 and Gd60Ti40 alloys at about 1220–1250 °C is in a good agreement with the experimental  and calculated  Gd–Ti phase diagram. In contrast, the monotectic temperature determined by the analysis of the Gd–Ti sessile drop images taken by a high-speed/high-resolution CCD camera lies between 1620 and 1650 °C, which significantly exceeds the monotectic temperature (1275 °C) calculated in ref. . After the high-temperature tests, the surface of the Ti-rich phase in the solidified Gd–Ti samples is covered by a micrometer-thick layer of the Gd-rich phase suggesting that complete wetting in the Gd–Ti system occurs in a wide temperature range above and below the monotectic temperature.
The Gd-rich liquid phase of the semi-solid Gd40Ti60 and Gd60Ti40 alloys wets Y2O3 at about 1350 °C and penetrates the ceramic at 1430–1450 °C. The Gd–Ti melt reacts relatively strongly with the ZrO2 substrates containing 5 wt.% Y2O3. The substrates made of the ZrO2 stabilized with 3 wt.% Y2O3 are stable at contact with the Gd–Ti melts over a wide temperature range up to about 1650 °C.
This study has been carried out in the frame of the German–Polish collaboration program between the German Academic Exchange Service DAAD (Project No. DAAD50755060) and the Ministry of Science and Higher Education of Poland (Project #002/2011/2012). DAAD and the Foundry Research Institute in Cracow are acknowledged for the financial support.
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