Observation of boron enrichments in a heat treated quasibinary 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 NiAlHfB2 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 NiAlHf0.6B1.2 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.