Nickel aluminides

8.6.2 Nickel aluminides

Ni 3 Al (nickel aluminide) is the ordered fcc γ ′ -phase and is a major strengthening component in superalloys. Ni 3 Al single crystals are reasonably ductile but in polycrystalline form are quite brittle

and fail by intergranular fracture at ambient temperatures. The basic slip system is {1 1 1} has more than five independent slip modes, but still exhibits grain boundary brittleness. Remarkably, small additions of ∼0.1 at.% boron produce elongations up to 50%. General explanations for this effect are that B segregates to grain boundaries, and (1) increases the cohesive strength of the boundary and (2) disorders the grain boundary region so that dislocation pile-up stresses can be relieved by slip across the boundary rather than by cracking. This general explanation is no doubt of significance but, additionally, there are distinct microstructural changes within the grains which must lead to a reduced friction stress and ease the operation of polyslip. For example, the addition of B reduces the occurrence of stacking-fault defects. Addition of solutes, such as B, are not expected to raise the stacking-fault energy and hence this effect possibly arises from the segregation of B to dislocations, preventing the superdislocation dissociation reactions (see Chapter 3, Section 3.6.5).

Microhardness measurements inside grains and away from grain boundaries indeed show that boron softens the grains. The ductilization effect is limited to nickel-rich aluminides and cannot

be produced by carbon or other elements, although some substitutional solutes such as Pd, which

Advanced alloys 469

700 23.5 at.% Al

0.2% flow stress (MN m 100

800 1000 1200 Temperature (K)

Figure 8.15 Effect of aluminum content on the temperature dependence of the flow stress in Ni 3 Al (after Noguchi, Oya and Suzuki, 1981).

Table 8.6 Anti-phase boundary energies in Ni 3 Al.

Alloy

γ 100 (mJ m −2 ) γ 111 /γ 100 Ni–23.5Al

γ 111 (mJ m −2 )

1.17 Ni–24.5Al

1.25 Ni–25.5Al

1.31 Ni–26.5Al

1.51 Ni–23.5Al + 0.25B

substitutes for Ni, and Cu produce a small improvement in elongation. Small additions of Fe, Mn and Hf have also been claimed to improve fabricability. Grain size has been shown to influence the yield stress according to the Hall–Petch equation and B appears to lower the slope k y and facilitate slip across grain boundaries. These alloys are also known to be environmentally sensitive. Hf, for example, which does not segregate to grain boundaries but still improves ductility, has a large misfit

(11%) and possibly traps H from environmental reactions, such as Al +H 2 O →Al 2 O 3 + H. Ti, which has a small misfit, does not improve the ductility. The most striking property of Ni 3 Al is the increasing yield stress with increasing temperature up to the peak temperature of 600 ◦

C (see Figure 8.15). This behavior is also observed in other L1 2 intermetallics, particularly Ni 3 Si and Zr 3 Al. This effect results from the thermally activated cross-slip of screw dislocations from the {1 1 1} planes to the {1 0 0} cube planes, where the APB energy is somewhat lower. The glide of superdislocations is made more difficult by the formation

of Kear–Wilsdorf (K–W) locks (see Chapter 6) and their frequency increases with temperature. Electron-microscopy measurements of APB energies given in Table 8.6 shows that the APB energy on {1 0 0} decreases with aluminum content and this influences the composition dependence of the strength, shown in Figure 8.15. The cross-slip of screw dislocations from the {1 1 1} planes to cube planes also gives rise to a high work-hardening rate.

470 Physical Metallurgy and Advanced Materials Although the study of creep in γ ′ -based materials is limited, it does appear to be inferior to that

of superalloys. Above 0.6T m creep displays the characteristic primary and secondary stages, with steady-state creep having a stress exponent of approximately 4 and an activation energy of around 400 kJ mol −1 , consistent with climb being the rate-controlling process. At intermediate temperatures (i.e. around the 600 ◦

C peak in the yield stress curve) the creep behavior does not display the three typical stages. Instead, after primary creep, the rate continuously increases with creep strain, a feature known as inverse creep. In primary creep, planar dissociation leads to an initial high creep rate which slows as the screws dissociate on {1 0 0} planes to form K–W locks. However, it is the mobile edge dislocations which contribute most to the primary creep strain and their immobilization by climb dissociation which brings about the exhaustion of primary creep. The inverse creep regime is still not fully researched, but could well be caused by glide on the {1 0 0} planes of the cross-slipped screw components.

The fatigue life in high-cycle fatigue is related to the influence of temperature on the yield stress and is invariant with temperature up to about 800 ◦

C, but falls off for higher temperatures with cracks propagating along slip planes. With boron doping the fatigue resistance is very sensitive to aluminum content and decreases substantially as Al increases from 24 to 26 at.%. Nevertheless, crack growth

rates of Ni 3 Al + B are lower than for commercial alloys. Hyperstoichiometric Ni 3 Al with boron can be prepared by either vacuum melting and casting or from powders by HIPing. Fabrication into sheets is possible with intermediate anneals at 1000 ◦

C. At present, however, the application of Ni 3 Al is not significant; Ni 3 Al powders are used as bond coats to improve adherence of thermal spray coatings. Nevertheless, Ni 3 Al alloys have been tested as heating

elements, diesel engine components, glass-making molds and hot-forging dies, slurry-feed pumps in coal-fired boilers, hot-cutting wires and rubber extruders in the chemical industry. Ni 3 Al-based alloys as matrix materials for composites are also being investigated. Nickel aluminide (NiAl) has a cesium chloride or ordered β-brass structure and exists over a very wide range of composition either side of the stoichiometric 50/50 composition. It has a high melting point of 1600 ◦

C and exhibits a good resistance to oxidation. Even with such favorable properties it has not been commercially exploited because of its unfavorable mechanical properties. Because it is strongly ordered, low-temperature deformation occurs by an a a/2 general plasticity criterion and in the polycrystalline condition β-NiAl is extremely brittle. The ductil- ity does improve with increasing temperature but above 500 ◦

C the strength drops off considerably as a result of extensive glide and climb. Improvements in properties are potentially possible by refinement of the grain size and by using alloying additions to promote structure. In this respect, additions of Fe, Cr or Mn appear to be of interest. For high-temperature applications, ternary additions of Nb and Ta have been shown to improve creep strength through the precipitation of second phases and mechanical alloying with yttria or alumina is also beneficial.

A further commercial problem of this material is that conventional production by casting and fabrication is difficult, but production through a powder route followed by either HIPing or hot extrusion is more promising.