Phase equilibria and phase transformations in the Ti-rich corner of the Fe–Ni–Ti system
While the main features of the Fe-Ni-Ti system are well known at low Ti content, literature review of the Ti-rich corner revealed inconsistencies between experimental reports. This investigation presents new experimental results, defined to remove the uncertainties concerning melting behavior and solid-state phase equilibria of the (Ni,Fe)Ti2 phase with the adjacent (Fe,Ni)Ti (B2, CsCl-type structure) and Beta-Ti (A2, W-type) phases. Six samples have been prepared and examined by differential thermal analysis performed in yttria and alumina crucibles, and by scanning electron microscopy in the as-cast state as well as equilibrated at 900°C.
Iron – Nickel – Titanium
A fairly large number of experimental studies have been carried out to establish the ternary phase equilibria [1938Vog, 1941Vog, 1963Spe, 1967Dud, 1981Loo, 1999Abr]. The first comprehensive study of phase equilibria was carried out by [1938Vog]. They used metallography, thermal analysis and X-ray diffraction techniques. [1963Spe] reported partial isothermal sections, representing phase equilibria of Fe corner, at 700 and 1100°C. [1967Dud] established a pseudobinary section, TiFe-TiNi, using twenty-one ternary alloys prepared using elements of purity greater than 99.94%. They used metallography, thermal analysis, X-ray diffraction techniques to determine the phase equilibria. [1981Loo] employed diffusion couples technique to determine the isothermal section at 900°C. They prepared alloys using elements of following purity: 99.95% Fe, 99.99% Ni and 99.97% Ti. Three types of diffusion couples, element/element, element/alloy, alloy/alloy, and fifteen in total, were prepared by solid state resistance welding. The couples were then sealed in evacuated silica tubes and annealed at 900°C for up to 900 h. Except [1981Loo], other experimental studies were restricted to alloys containing less than 50 at.% Ti. The results of these phase equilibrium studies were reviewed earlier [1985Gup, 1991Gup]. More recently, [1994Ali1] reported the phase equilibria of alloys containing more than 50 at.% Ti. They used Armco Fe, N-00 grade electrolytic Ni, and iodide Ti to prepare alloys by arc melting in an inert atmosphere. They prepared a number of ternary alloys in the Ni and Fe atomic ratios of 1:3, 1:1, and 3:1. The phase equilibria were determined using thermal analysis, metallography and X-ray diffraction. [1994Ali2] carried out rapid solidification of ternary alloys containing up to 33.85 at.% Fe and 26.8 at.% Ni. The alloys were subsequently annealed at 900°C for 25 h, and the microstructures were compared. [1994Jia] measured the partitioning of Fe and Ti between (Ni) and TiNi3 (or the tie-lines) at 1000, 1100 and 1200°C. They prepared diffusion couples between 5Fe-Ni (mass%) and 21Ti-1Fe-Ni (mass%) alloys. The couples were annealed for up to 300 h followed by quenching into iced brine. The compositions of the phases were measured by electron probe microanalysis. [1999Abr] determined the isothermal section at 1000C using both bulk alloys and diffusion couples. They prepared bulk alloys using iodide grade Ti, electrolytic grade Ni and carbonyl grade Fe in an arc furnace. The bulk alloys were equilibrated at 1200°C for 150 h. They also prepared a large number of couples using Fe-Ni, Fe-Ti and Ni-Ti alloys that were welded at 1200°C and at a pressure of 19.6 MPa. The couples were subsequently annealed at 1200°C. The diffusion paths and phase compositions were established by means of electron probe microanalysis. Besides graphical representation, [1999Abr] tabulated the tie-line and tie-triangle compositions. Based on diffusion couple results, [1999Abr] identified six two-phase regions (TiNi-TiFe2, TiNi-TiNi3, TiNi3-TiFe2, ((Fe,Ni)-TiNi3, ((Fe,Ni)-TiFe2) and three three-phase regions (((Fe,Ni)-TiFe2-TiNi3, TiFe2-TiNi-TiNi3, ($Ti)-TiFe-Ti2Ni). [1999Efi] studied microstructures of TiFe/Ni diffusion couples annealed at 1200°C for up to 1.5 h. Based on the dynamics of interfacial microstructure, they derived the interdiffusion coefficients. Besides phase equilibria studies at high temperatures, the effect of Fe on the low temperature martensitic transformations of TiNi has also been studied extensively [1982Hwa1, 1982Hwa2, 1982Nis, 1985Goo, 1986Edm, 1992Shi, 1993Mat, 1995Gue, 1997Har, 2000Chu, 2000Vor, 2000Xu]. Some of the results of recent phase equilibria studies were reviewed by [2001Gup].
Correction to solid-liquid partition coefficients in Fe-Cr-C alloys
Abstract It has been established by the direct determination of phase boundaries that the addition of 1.5%Cr to Fe-C alloys has only a small effect on the liquidus and austenite solidus curves compared with the binary system, contrary to the predictions from earlier partition coefficient measurements. These coefficients have been redetermined from specimens prepared In a modified equilibration furnace having better atmosphere control. The new values of equilibrium partition coefficient for Cr and C are lower than the previous measurements, and agree well with estimates from available thermodynamic data. It appears that the presence of spurious solid particles in some specimens gave rise to the erroneous partition coefficients in the earlier work.