Gerhard Sauthoff

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.

Multiphase intermetallic alloys for structural applications

Abstract Current more or less progressed developments on the base of intermetallic phases usually aim at new materials with the highest possible strength, creep resistance and oxidation resistance at the highest possible temperature and tolerable brittleness at lower temperatures for structural applications at high temperatures. Intermetallic alloys offer advantageous possibilities for reaching these aims by appropriate combination of phases and optimisation of phase distribution. This is exemplified with respect to strength and creep resistance by recent studies on NiAl alloys with strengthening Laves phase and multiphase TiAl alloys. The beneficial effects of additional softening phases on deformability and toughness are demonstrated by the results of recent studies on Laves phase alloys with disordered Fe–Al phase, NiAl alloys with disorderd Ni–Fe phase and partially transformed martensitic NiAl alloys. Mechanisms and problems are discussed and perspectives are outlined.

Deformation behaviour of iron-rich iron-aluminum alloys at low temperatures

Abstract The deformation behaviour of binary monocrystalline and polycrystalline Fe-Al alloys with Al contents up to 18 at.% and only low unavoidable impurity contents—in particular less than 100 wt.ppm C—has been studied at room temperature and −100 °C. The effects of quenching and annealing treatments on the behaviour of as-cast materials were investigated in order to clarify the dependence of strength and ductility on Al content and short-range ordering. It was found that the stress-strain behaviour at low temperatures is controlled primarily by Al solid-solution hardening and quenched-in excess vacancies with only minor effects of short-range ordering.

Deformation behaviour of perovskite-type phases in the system FeNiAlC. I: Strength and ductility of Ni3AlCx and Fe3AlCx alloys with various microstructures

Abstract Some Fe3AlCx and Ni3AlCx alloys with various microstructures were tested with respect to strength and ductility between room temperature and 1200 °C. The studied Ni3AlCx(+ Mn, Fe) alloys show the perovskite-type structure (L′12) with up to 2.5 at% carbon and are single phase. The Fe3AlCx alloys show the same perovskite-type structure, which is stable in this case only for carbon contents exceeding 11 at%. These alloys contain precipitates of graphite or α-FeAl (B2). All alloys were studied by compression tests with constant deformation rate or constant load. Additionally, some hot-hardness tests were carried out. Ductility was characterized by 4-point-bending tests. The properties of Fe3AlCx and Ni3AlCx alloys are compared to each other and to those of Ni3Al.

Deformation behaviour of Al-containing C14 Laves phase alloys

Abstract Various Al-containing ternary Laves phases with the hexagonal C14 structure and two-phase alloys of such Laves phases with the B2 phases NiAl or FeAl were screened with respect to lattice constants, thermal expansion, Youngs modulus, constitution, hardness, yield and creep in compression and bending at temperatures between ambient temperature and 1400 °C. The Laves phases show higher strengths at higher temperatures than the superalloys, whereas their brittleto-ductile transition temperatures are of the order of 1100 °C. The alloying of the Laves phases with the less brittle B2 phases shows promise for developing high-temperature materials for applications above the temperature range of the superalloys.

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...

Structure and properties of Fe–Al–Ti intermetallic alloys

Abstract Structure and mechanical properties of Fe–Al–Ti alloys in the composition range Fe–(10–27.5) at.%Al–(10–30) at.%Ti have been studied. The alloys exhibited a wide range of microstructure comprising α-Fe, Fe 2 Ti (Laves phase) and/or Fe 2 AlTi (Heusler phase) depending on the composition. Their room temperature hardness, Youngs modulus, ratio of plastic to elastic work and yield strength were determined. Elevated temperature yield strength and creep properties of these alloys were also studied. It was observed that significant increase in strength and hardness may be achieved in alloys containing two or more phases. The compressive creep behaviour of these alloys followed the power law in creep in the temperature range 873–1073 K. Creep stress exponents in the range 3–6 were observed, suggesting dislocation creep to be the operative mechanism. Apparent activation energy for creep was found to be in the range of 400 to 600 KJ/mol.

Intermetallic NiAlTa alloys with strengthening Laves phase for high-temperature applications. I. Basic properties

Abstract Various Ta-containing NiAl-base alloys with the Laves phase TaNiAl, with C14 structure, as strengthening second phase were prepared by ingot metallurgy. They were studied with respect to the deformation behaviour at ambient and high temperatures — including elastic deformation and creep — as a function of alloy composition and microstructure. NiAl dissolves up to 0.2 at% Ta in solid solution. NiAl with up to 3 at% Ta forms precipitate particles of Laves phase primarily on grain boundaries, whereas the Laves phase covers the grain boundaries completely to form a continuous skeleton for higher Ta contents. The observed strengthening of NiAl by Ta is a result of both solid-solution strengthening and second-phase strengthening. The latter effect increases with increasing fraction of Laves phase and is accompanied by an increasing brittle-to-ductile transition temperature. The oxidation behaviour was checked. These alloys are promising for applications at temperatures above those of the Ni-base superalloys.

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.

ROLE OF AN INTERGRANULAR PHASE IN RuAl WITH SUBSTITUTIONAL ADDITIONS

Abstract The unusual room-temperature ductility of the binary RuAl (B2) phase is extended by the occurrence of an intergranular eutectic film in a manner seemingly akin to a compliant layer effect. The premise that the controlled deployment of such a film might extend the favourable mechanical properties of RuAl to ternary formulations was examined for substitutional additions of Ni, Co and Ti in RuAl. Whereas single-phase ternary formulations invariably resulted in a sharp loss of ductility, dual-phase alloys in the pseudo-binary series Ru 50 Al 50 -Co 70 Al 30 and Ru 52 Al 48 -Ru 52 Ti 48 were found to support significant plasticity, as determined by room-temperature compression tests, up to alloying additions of the order of 40 at.%. Alloys based on the series Ru 50 Al 50 -Ni 65 Al 35 however, exhibited limited ductility, commensurate with strong solution strengthening effects and only a partially developed intergranular film.