Martin Palm

Reassessment of the binary Aluminum-Titanium phase diagram

All available literature on the constitution of Ti−Al is reviewed. Based on a critical evaluation of these data the phase diagram for this system is assessed.

Experimental determination of phase equilibria in the FeAlC system

Abstract The phase equilibria in the FeAlC system have been determined between 800 °C and the liquidus surface. From cast alloys the liquidus surface was established. From electron microprobe analyses (EPMA) of quenched samples three isothermal sections at 800, 1000 and 1200 °C have been obtained. Additional high temperature X-ray diffraction experiments (HT-XRD) yielded three vertical sections and the temperatures of four invariant reactions. The influence of carbon on the transition temperature between the disordered (A2) and the ordered (B2) α-solid solution was determined from HT-XRD experiments with single crystals. This transition is shifted remarkably to higher temperatures by the addition of carbon. Special emphasis was placed on establishing the homogeneity range of the K-phase and its dependence on temperature. This dependence is discussed in terms of order/disorder. In addition, the properties of the K-phase, lattice constant as function of chemical composition, microhardness and thermal expansion coefficient α, have been determined.

Experimental Determination of Intermetallic Phases, Phase Equilibria, and Invariant Reaction Temperatures in the Fe–Zr System

The phase diagram of the binary Fe-Zr system was redetermined by differential thermal analysis (DTA), electron-probe microanalysis (EPMA), x-ray diffraction (XRD), and metallography in the whole range of compositions. The stable intermetallic phases of the binary system are the cubic and the hexagonal polymorphs of the Fe2Zr Laves phase and the Zr-rich phases FeZr2 and FeZr3. While the cubic polymorph of the Laves phase is the stable structure at the stoichiometric Fe2Zr composition, the hexagonal C36-type polymorph of the Laves phase is a high-temperature phase that is found at Zr concentrations as low as 26.6 at.%. The Zr-rich phases FeZr2 and FeZr3 have small homogeneity ranges of about 0.5 at.%. FeZr2 is a high-temperature phase, stable between 780 and 951 °C. FeZr3 decomposes peritectoidally at 851 °C. The frequently reported phase Fe23Zr6 (Fe3Zr) is found not to be an equilibrium phase of the binary system.

The Fe-A1-Ti system

An extensive experimental investigation of the Fe-Al-Ti system by metallography, microprobe analysis, and XRD on quenched specimens and on diffusion couples is presented. Two isothermal sections at 800 and 1000 °C were established; they differ substantially from the existing (800 °C) or partly determined (1000 °C) diagrams. From these results, existence of the τ1 phase (Fe2AlTi) can be ruled out. Existence of the ternary compounds, τ2 (Al2FeTi) and τ3 (Al22Fe3Ti8), is confirmed. The composition limits of both phases were determined; they differ considerably from those given in earlier reports. The τ2 phase apparently exists in a cubic and a tetragonal polymorph, depending on composition. The cubic form exists at high titanium contents. At 1000 °C, the two polymorphs are separated by a miscibility gap. At compositions where the “X phase” (Al69Fe25Ti6) was previously reported, single-phase samples were obtained at both temperatures. From the present results, there is no evidence to assume that this is a new ternary phase rather than the ternary homogeneity range of the Al3Fe phase. In addition, extensions of the binary intermetallic phases into the ternary system were determined.

Microstructure and mechanical properties of nickel based superalloy IN718 produced by rapid prototyping with electron beam melting (EBM)

Abstract A nickel alloy of a composition similar to that of the nickel based superalloy Inconel alloy 718 (IN718) was produced with the electron beam melting (EBM) process developed by Arcam AB. The microstructures of the as processed and heat treated material are similar to that of conventionally produced IN718, except that the EBM material showed some porosity and the δ phase did not dissolve during the solution heat treatment because the temperature of 1000°C apparently was too low. Mechanical testing of the layer structured material, parallel and perpendicular to the built layers, revealed sufficient strength in both directions. However, it showed only limited elongation when tested perpendicular to the built layers due to local agglomerations of pores. Otherwise, data for the hardness, Young’s modulus, 0·2% yield tensile strength and ultimate tensile strength match those recommended for IN718.

Phases and phase equilibria in the Fe–Al–Zr system

Abstract Isothermal sections at 800, 1000, and 1150 °C as well as a tentative partial liquidus surface of the ternary Fe–Al–Zr system were established by means of electron-probe microanalysis, X-ray diffraction, differential thermal analysis, and light-optical as well as scanning electron microscopy. The most prominent features of the ternary phase diagram are the extended homogeneity ranges of the Laves phases. By continuous substitution of Fe by Al, the structure changes three times starting from the cubic C15 structure of Fe2Zr to hexagonal C14 (λ1) back to cubic C15 (λ2) and again to hexagonal C14 (Al2Zr). The various Laves phase fields are separated by very small two-phase fields. Besides the Laves phases λ1 and λ2, three more ternary intermetallic phases were found, whose homogeneity ranges have been determined for the first time. In addition, new results concerning the homogeneity ranges of intermetallic phases in the binary subsystems Fe–Al and Al–Zr are reported. The solubilities of the third com...

TEM and DTA Study on the Stability of Al5Ti3- and h-Al2Ti-Superstructures in Aluminium-Rich TiAl Alloys

Abstract The phases Al 5 Ti 3 and h-Al 2 Ti, which are superstructures of the L1 0 TiAl structure, are frequently observed in as-cast and low-temperature-annealed aluminium-rich TiAl alloys. The strong decrease of the solubility of aluminium in TiAl with decreasing temperature leads to a supersaturation of the solid solution with aluminium during cooling. The decomposition of the supersaturated TiAl results in the precipitation of the superstructure phases at low temperatures. The evolution of the Al 5 Ti 3 and h-Al 2 Ti phases and the resulting microstructures were studied as a function of time, temperature, and composition by TEM and DTA investigations on Ti–Al alloys with 55 to 64 at.% Al. Both superstructures were found not to be equilibrium phases. Al 5 Ti 3 is metastable below a composition-dependent critical temperature in the range of about 750–900°C with a maximum value reached near the stoichiometric composition. Above this temperature, Al 5 Ti 3 rapidly dissolves. Extended lamellar Al 5 Ti 3 +TiAl microstructures have been found in a Ti–60 at.% Al alloy after low-temperature annealing, whereas in Ti–62 at.% Al large single-phase domains of Al 5 Ti 3 have grown. h-Al 2 Ti is a metastable phase at least up to 1200°C. It slowly transforms into the equilibrium phase r-Al 2 Ti during annealing.

Re-evaluation of phase equilibria in the Al - Mo system

Abstract Phase equilibria and homogeneity ranges of the intermetallic phases in the Al–Mo system have been determined. The microstructures of samples of fixed composition and of diffusion couples were investigated by light-optical and scanning electron microscopy, the crystallographic structures and lattice constants were established by X-ray diffraction and the compositions of the phases were determined by electron probe microanalysis. In addition, transition temperatures were determined by differential thermal analysis. The homogeneity ranges of the Al-rich intermetallic phases are narrow, with about 0.1 to 0.4at.% widths. The phases and phase equilibria as established in previous studies have been confirmed with the exception of the phase Al3+xMo1–x, which has not been detected. The congruent melting point of Al8Mo3 was found at 1546±3°C and the Al-rich phase boundary for AlMo3 has been determined between 800 and 1200°C.

Fe–Al materials for structural applications at high temperatures: Current research at MPIE

Abstract Fe – Al-based materials possess a number of properties which make them highly interesting for the development of new light-weight structural materials. However, lack of strength at high temperatures and limited ductility at ambient temperatures so far has hindered any wider application of these materials. Recent progress achieved within an inter-departmental research initiative at the Max-Planck-Institut für Eisenforschung GmbH is reviewed here. Based on a sound knowledge of physical and thermodynamic properties and careful analysis of the phase equilibria, various alloy systems have been investigated which offer different mechanisms for strengthening Fe – Al-based materials at high temperatures. By applying these mechanisms Fe – Al-based alloys with sufficient strength for structural applications at least between 650 – 800 °C have been developed. For these alloys processing routes such as rolling and forging have been demonstrated. Low ductility is still a crucial issue, but measures exist for improving ductility, e. g. by refining the microstructure through thermo-mechanical treatment. It has also been shown that iron aluminides not only show superior corrosion resistance in oxidising and sulphidising atmospheres but also in other hostile environments like carburising atmospheres and under molten salts.

Formation of lamellar microstructures in Al-rich TiAl alloys between 900 and 1100°C

Abstract The formation of lamellar microstructures in a Ti–62 at.% Al alloy at temperatures of 900–1100°C was investigated by transmission electron microscopy (TEM). The lamellar structure consists of γ-TiAl+r-Al2Ti in the equilibrium state. h-Al2Ti and Al5Ti3 are observed in the as-cast alloy, but Al5Ti3 transforms into γ-TiAl above 900°C. The phase h-Al2Ti transforms into r-Al2Ti by two alternative processes, i.e. by continuous or discontinuous transformation. While the first process is comparably sluggish, areas generated by the second process enlarge quickly, leading to the formation of a fine lamellar microstructure of γ-TiAl and r-Al2Ti. This fine lamellar microstructure coarsens through lamellar fault or/and grain boundary migration during long time annealing. In the equilibrium state, the fully coarsened lamellar microstructure contains regular misfit dislocations, i.e. two types of [110] edge dislocations in the (001) planes at the interlamellar boundaries.