HIGH-TEMPERATURE X-RAY-DIFFRACTION INVESTIGATION OF NICKEL-RICH NIAL AND NIAL-TIB2

A high temperature X-ray diffraction investigation of Ni-19.5wt%(34.6 at.%)Al and Ni-20.2 wt%Al-0.9 wt%TiB2 between room temperature and 120 0-degrees-C was carried out. It was found that the NiAl martensite in the alloy retransformed into beta-NiAl first below 410-degrees-C durin g heating then beta-NiAl transformed into the stable Ni5Al3 between 48 0 and 850-degrees-C, which is much easier than that in isothermal agei ng as reported previously. The Ni5Al3 was retained at least until 850- degrees-C which is higher than indicated in the Ni-Al phase diagram (7 00-degrees-C) before transforming into the gamma'-Ni33Al and beta-NiAl . Adding small amount (0.9 wt%) of TiB2 and increasing by a small amou nt (0.7 wt%) of the aluminium content led to obvious effects on transf ormations of Ni-19.5 wt%Al during heating. No obvious change related t o TiB2 in the NiAl-TiB2 was found during the heat cycle and holding. M oreover, the ordered structure of NiAl was retained at least up to 120 0-degrees-C.