EMF Studies of Ti based Alloy Systems

There is an ever-growing demand for high temperature materials, which are lightweight and can perform in aggressive environments in the field of aerospace and automotive industry. As a result intermetallic compounds were developed. The most important intermetallic compounds in the Ti-Al system are a-Ti3Al, g-TiAl and TiAl3. These intermetallic compounds have low density, excellent high temperature strength and creep resistance. However, they have poor ductility at room temperature and poor oxidation resistance at elevated temperatures. Hence these materials need to be alloyed with elements such as Nb, V, Mo, Ta and Cr in order to improve their ductility and oxidation resistance. Nb is found to improve the high temperature oxidation resistance and ductility of these intermetallics and hence considerable interest has been developed in the detailed study of Ti-Al-Nb ternary system. Thermodynamic data is very important for efficient alloy development. However, very few studies have been conducted on the respective binary systems (Ti-Al, Al-Nb and Ti-Nb) and on the Ti-Al-Nb ternary system. EMF studies are even more scarce in these systems. The present study will undertake the evaluation of the thermodynamic properties of the relevant systems using the EMF technique employing the CaF2 solid electrolyte. EMF measurements using the solid state galvanic cells is one of the best methods to measure the activities of the components of the alloys and subsequently to evaluate the free energy of formation of materials at high temperatures. Solid-state galvanic cell consists of a working electrode, a reference electrode and an ionically conducting solid electrolyte. Solid electrolytes can be anion or cation conducting. Anion conducting electrolytes, especially oxygen ion conducting electrolytes (CaO stabilized ZrO2) are used widely for the measurement of activities in the solid alloy systems. A typical cell configuration suitable for alloy (A-B) employing oxygen conducting solid electrolyte is given by

Pt, A, AO | Oxygen conducting electrolyte | AO, A-B, Pt

The activity of the component A, for example, is evaluated from the cell emf in the following manner

DG = -nFE = RT ln (aA)

where a mixture of A and AO acts as the reference electrode and the working electrode is a mixture of the alloy A-B and its least noble metal oxide (AO). The measured EMF is due to the difference in oxygen chemical potential, which is established at each electrode and electrolyte interface. The easiest way to relate the measured EMF of the cell with the activity of the least noble of the alloy components (A) is to regard the virtual cell reaction. The virtual cell reaction for the above mentioned cell is given by

A = [A]alloy

The meaning of the virtual cell is, one assumes that when certain number of Faradays (in the above case it is 2) is passed through the cell, half a mole of oxygen is transported from the right hand electrode to the left hand electrode. EMF is measured using a potentiometer placed in the external circuit, which stops the flow of electrons and correspondingly stops the movement of oxygen ions through the electrolyte so that the EMF measured is that under the open circuit (reversible). In the present research, thermodynamic properties of the Ti-Al and Al-Nb systems would be evaluated using the EMF technique employing CaF2 as the solid electrolyte in the temperature range of 800-920 K. The configuration of the cell is given by

Pt, gr | Al, Ca2AlF7 || CaF2 || Al(Al-Nb/Ti), Ca2AlF7 | gr, Pt

The experimental setup consists of a vertical resistance furnace which acts as the main furnace. High purity Argon gas is used throughout the experiments after purification in a train consisting of dry CaSO4, anhydrous Mg(ClO4)2 and Titanium sponge maintained at 800C. The cell compartment consists of electrode and electrolyte sandwiched between two alumina plates, which is suspended by two alumina rods. The two alumina rods are attached to top brass flange by stainless steel springs. The cell assembly is kept in a water cooled alumina tube which is closed at one end. The top flange has a proper feed-through for argon gas inlet, outlet, thermocouple and the two platinum leads.