Observation of boron enrichments in a heat-treated quasi-binary hypoeutectic NiAlHfB2 alloy by means of atom probe field-ion microscopy (APFIM)

The mechanical properties of the inter‐metallic phase NiAl with B2 superlattice structure can be improved by strengthening particles of the IV B transition‐metal diborides formed in situ. These compounds form with NiAl quasi‐binary eutectic systems. Among them, the quasi‐eutectic alloy with hafnium diboride exhibits favourable properties owing to its excellent oxidation resistance and high thermal stability up to 1400 °C. The eutectic composition of the quasi‐binary system NiAlHfB2 was determined to be 0.75 at.% Hf and 1.5 at.% B, and the eutectic temperature is Teut = 1640 °C. In the present study, the microstructure of the hypoeutectic NiAlHf0.6B1.2 alloy was characterised after heat treatment at 1400 °C for 100 h by means of SEM, TEM, and atom probe field‐ion microscopy (APFIM). The alloy consists of fine globular and fibrous eutectics HfB2 and primary NiAl, with high thermal stability. APFIM investigations on the heat‐treated alloy indicate a relatively high density of small boron clusters containing a few boron atoms (about 5–15 boron atoms) and line‐shaped enrichments of about 50–200 boron atoms. These are free from hafnium and other impurities of transition metals and exhibit a depletion of aluminium. For an exact determination of the composition even by using high‐resolution APFIM, their sizes seemed to be too small. The appearance of boron‐rich clusters is in contrast to the formation of HfB2 of high thermodynamic stability and the very low solubility of boron and hafnium in B2‐ordered NiAl. Copyright © 2007 John Wiley & Sons, Ltd.