NiAl-Cr alloys

The excellent oxidation resistance of pure NiAl decreases with increasing Cr content and increases with increasing temperature (Brumm and Grabke, 1992 Grabke et al., 1992). In any case the oxidation resistance of NiAl-Cr alloys is still acceptable for not too high volume fractions of the a-Cr phase. 

According to the ternary phase diagrams of aluminium-chromium-nickel [7] chromium is soluble in p-NiAl up to about 4 at.% at 1000°C. This is in agreement with the EPMA results for these alloys.The 0 at 1 at.% Cr alloys were homogeneous whereas, segregation was observed in the 5 and 10 at.% Cr alloys. In Table 2 the composition of the alloys, determined with XRF, is reported. In the EPMA-micrographs in Fig. 3 chromium enrichment (a-Cr) can be seen at the grain boundaries. 

In this plot four curves corresponding to the four alloys can be observed. It can clearly be seen that in the 5 and 10 % Cr containing alloys already after short oxidation times (e. g. 2 hours) no 0-phase can be found anymore while the oxide layer on the pure [3-NiAl alloy contains quite some 0-oxide during several hours. The 1 % Cr alloy lies somewhere in between these curves. 

Koiwa, 1992 Bakker et al., 1992). Only recently a model, which is based on a combination of two mechanisms, has been proposed for describing the composition dependence of diffusion in B2 phases (Kao and Chang, 1993). It should be noted that the understanding of the basic diffusion processes for other intermetallics is still less than for NiAl and other B2 phases (Wever, 1992). Diffusion studies of multi-component systems are rare, and with respect to NiAl base alloys only data for the ternary phase (Ni,Fe)Al are available from a systematic study of the system Ni-Al-Fe (Cheng and Dayananda, 1979 Dayananda, 1992). Recently, the effect of Cr on diffusion in NiAl has been studied (Hopfe et al., 1993). 

In these doped alloys, the major phase () had the composition of the Ni Al3 phase and a small amount of dissolved Cr ( -1.5 at %) and Mo ( V 0.2 at X). This primary phase was surrounded by a small amount of a binary phase (Cr Alg) in the case of Cr addition, and of two phases in the case of Mo addition (P., = NiAl + 0.3 % Mo and a ternary phase Pg = (NiMo)Alg). The proportions of the various phases in the alloys varied and the quantity of phase Pj decreased significantly when the Mo content increased. 

Long term behaviour of NiAl in molten carbonate is probably much better than suggested by the short term electrochemical measurements. This is also suggested by the good behaviour of Kanthal-Al (with only 5.8% of aluminium) at long term immersion, which was ascribed also to the formation of an aluminium oxide layer, in that case between the outer Li(Fe,Cr)0 layer and the base alloy. Small amounts of A1 were found to be beneficial also by Uchida [13],... 

It has been found experimentally that alloying NiAl with Cr, which reduces the APB energy, may indeed increase the propensity to <111> slip (Field etal.. 

RNi No. 3, prepared from the commercial NiAl alloy No. 3, containing 2% Cr-i-Ti and small amounts of other additives, contained 6.8% Al, approximately half of the Al in RNi sample No. 2, which was prepared from the NiAl alloy No. 2. Impurities during the preparation of RNi No. 3 were almost completely removed. Table 4.2. and Fig. 4.1. show that decreasing the NisAls phase in the alloy and increasing the contents of NiAU, promoted an increase in both the general aetivity and the enantioselectivity of the resulting RNi catalysts. 

Romeo and McKee (1975), using crucible tests and thermobalance exposures, found that the resistance of NiAl to hot corrosion was excellent, whereas y-Ni and Ni3Al were susceptible to attack, with Ni3Al being the worst. McCarron et al. (1976), found that NiAl was more resistant to hot corrosion than Ni3Al in hot corrosion tests in a burner rig using 1 % S fuel (plus 125 ppm Na, as sea salt) with a specimen temperature of 871 °C. NiAl containing ca. 8-9 at.% Cr was the most resistant alloy -minimal corrosion penetration was apparent after 800 h. A porous corrosion product was almost always formed, and internal sulfides were present. The beneficial effect of Cr was attributed to the stabilization of NiAl. 

In yj-crystallized alloys, a typical microstructure was observed. Grains showed a dendritic structure with a central part constituted of the NiAl phase surrounded by large domains of the NljAlg phase. The composition of the interdendritlc groove depended on the dopant (5). In the case of Cr-doped ju alloys, only Cr segregation was detected at fine-scale observations. 

Total mass gain (specimen + spalled oxide) during lOOh cycles at 1200°C in laboratory air for various Ni-Cr-AI alloys. Generally, the mass gain decreases with Al content due to the formation of a protective alumina scale with NiAl + Hf having one of the slowest growth rates of any alumina-former. Two different specimens are shown for alloy 214 to indicate the reproducibility. 

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