Deformability, NiAl

The intermetallic alloy NiAl is discussed as a potential base alloy for high temperature structural materials. Its use is currently limited by low room temperature ductility and fracture toughness. Consequently, substantial research efforts have been directed towards understanding its mechanical behaviour [1, 2] so that detailed experimental [3, 4, 5] and theoretical [6, 7, 8] analyses of the deformation of NiAl are available today. 

At room temperature, NiAl deforms almost exclusively by (100) dislocations [4, 9, 10] and the availability of only 3 independent slip systems is thought to be responsible for the limited ductility of polycrystalline NiAl. Only when single crystals are compressed along the (100) direction ( hard orientation), secondary (111) dislocations can be activated [3, 5]. Their mobility appears to be limited by the screw orientation [5] and yield stresses as high as 2 GPa are reported below 50K [5]. However, (110) dislocations are responsible for the increased plasticity in hard oriented crystals above 600K [3, 7]. The competition between (111) and (110) dislocations as secondary slip systems therefore appears to be one of the key issues to explain the observed deformation behaviour of NiAl. 

For the deformation of NiAl in a soft orientation our calculations give by far the lowest Peierls barriers for the (100) 011 glide system. This glide system is also found in many experimental observations and generally accepted as the primary slip system in NiAl [18], Compared to previous atomistic modelling [6], we obtain Peierls stresses which are markedly lower. The calculated Peierls stresses (see table 1) are in the range of 40-150 MPa which is clearly at the lower end of the experimental low temperature deformation data [18]. This may either be attributed to an insufficiency of the interaction model used here or one may speculate that the low temperature deformation of NiAl is not limited by the Peierls stresses but by the interaction of the dislocations with other obstacles (possibly point defects and impurities). 

The (110) dislocations are from our calculations not expected to contribute significantly to the plastic deformation in hard oriented NiAl because of the very high Peierls stresses. Experimentally, these dislocations do not appear unless the temperature is raised to about 600 K [18]. At this temperature the experimental data strongly suggest a transition from (111) to (110) slip. 

D.R. Pank, M.V. Nathal, D.A. Koss. Deformation behaviour of NiAl-based alloys containing iron, cobalt and hafnium, in High Temperature Ordered Intermetallic Alloys V, 1. Baker, R.Darolia, J.D.Whittenberger, Man H. Yoo, ed., MRS, (1993), Vol. 288... 

The crystal structure of NiAl is the CsCl, or (B2) structure. This is bcc cubic with Ni, or A1 in the center of the unit cell and Al, or Ni at the eight comers. The lattice parameter is 2.88 A, and this is also the Burgers displacement. The unit cell volume is 23.9 A3 and the heat of formation is AHf = -71.6kJ/mole. When a kink on a dislocation line moves forward one-half burgers displacement, = b/2 = 1.44 A, the compound must dissociate locally, so AHf might be the barrier to motion. To overcome this barrier, the applied stress must do an amount of work equal to the barrier energy. If x is the applied stress, the work it does is approximately xb3 so x = 8.2 GPa. Then, if the conventional ratio of hardness to yield stress is used (i.e., 2x3 = 6) the hardness should be about 50 GPa. But according to Weaver, Stevenson and Bradt (2003) it is 2.2 GPa. Therefore, it is concluded that the hardness of NiAl is not intrinsic. Rather it is determined by an extrinsic factor namely, deformation hardening. 

It may be expected that many intermetallic compounds will behave like metals during plastic deformation. However, some that contain covalent bonds will behave differently. In these, the size ratio tends to be 1.2 or greater. For NiAl the size ratio is 2.86/2.49 = 1.149. This may be compared with TIC (2.89/1.54 = 1.88), or TlB2 (2.89/1.72 = 1.68). The latter are clearly covalently bonded. 

In such models, the bonding is considered to be partially ionic with a charge transfer from A1 to the Ni 3d valence band. To explain the properties of /J NiAl at a more sophisticated level, Fox and Tabernor (1991) measured four low-angle structure factors by the HEED critical-voltage technique. The deformation density based on these four reflections shows a depletion of density around both the Ni and A1 atoms, and a buildup of about 0.13 eA-3 along the [111] direction halfway between Ni and A1 nearest neighbors. 

Oxidised single crystalline NiAl specimens were deformed by four-point bending at elevated temperatures. Bend tests at RT could be performed only with Fe-doped NiAl because of its higher ductility. Typical scale defects which were generated on the side... 

Under compressive load, extensive spallations have been found on Fe-doped NiAl after RT bending (Fig. 6), which were affected by a high intcrfacial void density. On the contrary, despite a rather large deformation (cf. Fig. 7a), no scale defects were detected... 

Fig. 7. Scales on the side face of single crystalline NiAl bending bars after bending at oxidation temperature (Tox 1223 K) and cooling to RT (a) deformed bending bar, (b) adherent scale after slow deformation (tm = 20 h, e — 10 5 s" ), (c) isolated spallations at large interfacial voids (fox — 100h,e= 10 3s ),(d) magnification of spallations together with tensile cracks on the compressive side of the specimen.

In the present analysis, large-scale spallations of alumina scales on NiAl were always connected with the presence of interfacial voids. In the absence of large voids and for small strain rate, it was found that the scale was well adherent under compressive substrate deformation even for large specimen deformations (cf. Fig. 7a).This suggests the presence of an effective stress relief mechanism by a slight scale wrinkling or by oxide Coble creep. 

The softening at high temperatures is related to thermally activated creep processes which are the subject of the next section. Deviations from stoichiometry produce constitutional defects which enhance diffusion and lead to softening at high temperatures, whereas at low temperatures these defects are immobile and act as strengthening deformation obstacles. These different effects of deviations from stoichiometry at low and high temperatures were studied in detail for binary NiAl (Vandervoort etal., 1966). 

As a consequence of the available slip systems, the strength and ductility are highly anisotropic with a hard <100> direction and soft <110> and <111 > directions (Darolia et al., 1992 b Glatzel etal. 1993b Takasugi etal., 1993a). NiAl with the hard orientation shows practically no ductility - in spite of indications of local plastic deformation (Vehoff, 1992) - below the brittle-to-ductile transition temperature (BDTT) which is of the order of 350°C - corresponding to 0.33 (r =... 

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