Intermetallic Phases & Materials

Intermetallic phases – intermetallics for short – are phases which have different crystallographic structures than the elements they are composed of. Compared to conventional alloys with a solid solution matrix, additional chemical bonding in the intermetallics usually results in higher melting points, significantly increased strength, and high wear resistance, however on the expense of ductility. Because of their low density and good oxidation behaviour, aluminides are of specific interest for the development of novel materials with enhanced properties. Fundamental research for the development of intermetal-lic materials is a long-term topic at the MPIE.

Limited ductility hampered a wider industrial use of intermetallic materials for long time. Breakthrough came with  the  application of  γ-TiAl  alloys for  compressor  blades  in  aero  en-gines.  Due to a fine-scaled lamellar γ-TiAl  +  α2-Ti3Al  microstructure,  they  have  a  high  specific  strength.  Basis for  respective alloy  developments  is  the understanding of the phase transformations leading to the formation of these  microstructures and of the phase equilibria at high temperatures for the long-term stability of these microstructures  during  application. Investigation of phase transformations by differential thermal analysis  (DTA)  or  high-temperature  X-ray  diffraction  (HT-XRD)  and  evaluation of phase equilibria by electron probe  microanalysis  (EPMA)  are  therefore  funda-mental  for  any  aimed  alloy  develop-ment. Such essential  thermodynamic  data will be determined for a number of  Ti–Al–X  systems within the new large-scaled,  collaborative European project ADVANCE. The MPIE not only has the facilities to determine the ther-modynamic data but also a variety of melting devices to produce the necessary high-purity alloys, e.g., by levita-tion  melting.  The  results  are  used  to  generate an advanced ThermoCalc© data base, which is a design tool  for the development of next  generation γ-TiAl alloys.

Iron  aluminides  –  an  economic  alter-native to stainless steels

Fig. 1: High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) picture of a pyramidal fault in off-stoichiometric Nb-rich NbFe2 Laves phase.

Specific interest lies on the development of iron aluminide materials based on the phases FeAl, Fe3Al and the Heusler  phase  (X)2YA1  (X  =  Fe,  Ni,...; Y = Ti, V, Nb, Ta,...). These materials have an outstanding corrosion resistance in a variety of aggressive environments, high wear resistance and low density and are considered as an economic alternative to  stainless steels or Ni- and Co-based superalloys. However, limited strength at high temperatures inhibited any extensive  application. New alloy concepts pursued at the MPIE lead to alloys which partially have much higher strength and creep resistance than advanced Cr steels.The alloy concepts include fine precipitation of  borides or a film of Laves phase along grain boundaries, increasing the D03/B2 transition  temperature, fine dispersion of Laves phase  precipitates  in  the  matrix, or generating coherent  A2/B2/L21  microstructures. These developments have prompted new interest by industry. Casting, forging, and rolling on an industrial scale has been explored  in a series of cooperations and different parts successfully passed tests under application conditions.

Additive   manufacturing   (AM)   is   a   new  technology  by  which  near  net-shape  parts  are  generated  by  layer-wise  melting  of  powders  by  a  laser  or   electron   beam.   As   intermetallic   phases are highly wear-resistant and therefore  difficult  to  machine,  AM  is an  interesting  alternative  for  producing parts from intermetallic materials. Within a large-scale collaboration with research institutes and German industries, AM of advanced iron aluminides was studied. Defect-free samples and parts  were  produced  by  different  AM  techniques and it was shown that basic alloy concepts developed for cast alloys can be transferred to AM. Also the  possibility  of  generating  chemically-graded  samples  by  AM  with  a  continuous  variation  of  the  composition  between  various  stainless  steels  and iron aluminides could be demonstrated.

Laves phases - the most abundant class of intermetallics

Laves  phases  constitute  the  largest  group of intermetallic phases. These very  strong  but  brittle  phases  may  not  be  usable  as  monolithic  materi-als,  but  as  strengthening  phases,  e.g.  in  iron  aluminium  alloys,  Co-based  alloys  or  steels.  They  exist  with  three  different  crystallographic  structures, i.e. polymorphs, two hexagonal  and  a  cubic.  Stability  of  the  different  polymorphs  in  dependence  on  temperature  and  composition  is  another  topic  of  basic  research  at  the MPIE. An important aim is to un-derstand  the  variation  of  properties  in  dependence  of  the  composition,  as many Laves phases do have ex-tended homogeneity ranges and de-viations  from  the  ideal  composition  leading to the occurrence of different types of structural defects (e.g., see Fig. 1).

It has also been shown that novel steels and iron aluminide based alloys are substantially strengthened by micron-thick films of Laves phase on the grain boundaries. Though the Laves  phases  are  inherently brittle,  no  internal cracking of the films is observed  after  plastic deformation, e.g. creep.  Transmission electron   microscopy and  local  strain  mea-surements  by  X-ray  diffraction are currently employed to understand this phenomenon.

Corrosion resistance of iron aluminides

Iron aluminides show an outstanding corrosion behaviour and many investigations have been performed  to  unravel  the  underlying  mecha-nisms. In all cases it has been found, that the ability to form Al2O3  scales  or Al-rich passive films is the key for the excellent corrosion resistance. In oxidizing environments,  thin, dense  and adhesive Al2O3 scales form that also withstand thermal cycling. A minimum of about 15 at.% Al is necessary for their formation. Salt spray tests, corrosion in  water vapour, aqueous corrosion, and hot  corrosion  by  molten  salts  yielded varying  corrosion  resistance.  However,  in  all  cases  an  improvement  was  seen  when  the  Al  content was  high  enough  to  form  protective  scales  or  films. Al2O3 scales can also be generated by a short pre-oxidation  treatment. The scales act as  protecting “isolators”, separating the iron aluminide from the environment. Pre-oxidation has  been  shown to be very effective  to enhance the aqueous corrosion resistance, which otherwise  is only mediocre. The  anodic  current  density in the passive range during immersion in H2SO4 of pH 1.6 is sig-nificantly decreased by two orders of magnitude and also repassivation of smaller defects occurs readily. X-ray photoemission  spectroscopy  (XPS)  revealed that the scale is comprised of an outer layer of mixed Al and Fe oxides and an inner layer of hydrox-ides enriched in Al, which was found to  be  significantly  more  resistant than the oxide layers formed on pure aluminium. Even long-time immer-sing of the scale has no effect on its beneficial performance. Other  intermetallics phases of interest for structural applications studied at the MPIE are silicides,  NiAl  and  N3Al, of which the latter is the strengthening gamma prime (γ ́) phase in Ni-based superalloys.

SN Authors: M. Palm, F. Stein

GO Author: M. Rohwerder

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