Intermetallics
 
 
Intermetallics constitute a new important class of materials.  These are mostly compounds of metals whose crystal structures are different from those of constituent metals.  They are formed when the bonding strength between unlike atoms (e.g. Ti-Al) is larger than that between like atoms (Ti-Ti, Al-Al).  They possess crystal structures with an ordered atom distribution, and due to their properties they occupy the intermediate position between metals and ceramics.  The intensive research of thee materials started in mid-1970s in view of expectation that they may become prime candidates for high temperature structural application, especially in jet engines in order to increase service temperature and reduce weight.  The best known representatives of Intermetallics are shape memory alloys (SMA), e.g. Nitinol which have gained a growing importance due to the effect of shape memory having its origin in martensitic transformation.  The other important intermetallic compound is γ-Ti-Al based alloy, in view of its low weight and high temperature resistance.  The examples of applications are in the space shuttle of NASA, turbine blades, valves in combustion engines.  The beneficial properties include low density (~3.8g/cm3), high specific yield strength (yield strength/density), high specific stiffness, good oxidation resistance and good creep resistance at high temperatures.  However, low ductility and fracture resistance have limited the wide application of intermetallics Ti-Al.  The intermetallics γ-Ti-Al are called ‘microlaminates’ in view of the lamellar structure of two phases γ and α2-Ti3Al so their mechanical response resembles that of a layered composite material.  Therefore, the analysis of ceramic composite materials and intermetallics can be carried-out within the same formalism.

Intermetallics that are of interest for the contemporary industries include:
  • Iron aluminides (cost attractive for automotive applications to replace stainless steel).
  • Titanium aluminides for aerospace applications
  • Nickel aluminides for gas turbine applications
  • Nb3Al for its superconductivity and certainly a material for the future
  • Smart materials (NiTi, Cu-Al-Ni, Cu-Zn-Sn) for sensors, actuators etc.
  • Magnetic and superconducting materials (Fe-Al, Nb-Al, Heusler alloys)
Intermetallics are known for high temperature properties including melting points e.g. NiAl 1663C, TiAl 1440C,  Nb3Al 2060C, and high elastic modulus.  Typically they manifest high strength at temperature, creep and environmental resistance (e.g. oxidation, sulfidation) and relatively low densities.  Intermetallics have strong potential for replacing superalloys and stainless steels in moderate and high temperature structural applications.  Their exceptional properties lead to increased operating temperatures, efficiency, reduced maintenance.

Iron aluminides may be used as low-cost coatings particularly for corrosion resistance in sulphur-containing applications.

Shape memory alloys (Ni-Ti, Cu-Al-Ni, Cu-Zn-Sn) are well suited for actuators, couplings, electrical connectors, sensors, cardiovascular stents, spectacle frames, adaptive (smart) structures. Nitinol (NiTi) has long fatigue life, relatively high ductility and corrosion resistance in addition to displaying two unusual phenomena: the pseudoelastic effect and the shape memory effect due to reversible martensitic transformation.  For example, if NiTi encounters a constraint during reverse transformation, it can generate extremely large recovery stresses which provide a unique mechanism for the actuation.  Biocompatability of NiTi resulted in numerous medical applications.

Titanium aluminide
(Ti-Al) is the proper material if light parts subjected to a high stress shall work at high temperatures – that means hot rotating parts.  The low weight allows an easier computer control of the motion process.  Since Ti-Al is ductile at high temperatures, it sustains some 100 MPa of stress before it gets to be irreversibly deformed and this before it breaks.  A very important cost-controlling feature is the possibility to produce Ti-Al in the classical way by casting it in an ingot and then forging it.  Since a lamellar microstructure has proven to be favourable, an initially lamellar, nearly unidirectionally oriented composite must be forged.  This process of forging is not really understood.  Obviously the lamellae tend to deform strongly showing partial rotations of the material of 90º. The microstructure gives the impression of the buckled substructure.  The local concentration of strain energy drives a dynamical recrystallisation process which is mesoscopically represented by a softening.  The consequence is a very strong concentration of the strains showing a partially rotated material nearly undeformed by the neighbourhood of heavily deformed shear concentration zones. Therefore, modelling is an important tool to understand the developing microstucture in a macroscopic heavy deformation process like forging.  This is a new subject and much research is necessary.

The main drawback to the use of intermetallics is their lack of toughness at room temperature and the high cost of processing.  However, it has been shown that FeAl is intrinsically much more ductile than previously realised and that the low ductility, commonly observed at room temperature, is the result of extrinsic factor, namely moisture-induced hydrogen embrittlement
 
   
 
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