Katsuhiko Asami

Extremely corrosion-resistant bulk amorphous alloys

The addition of chromium, tantalum and molybdenum was the best combination of small additions in enhancing the corrosion resistance of amorphous Ni-16P-4B alloys in concentrated hydrochloric acid. Preparation of rod-shaped amorphous alloys with small contents of these three corrosion-resistant elements were attempted by copper-mold casting. Ni-5Cr-5Ta-3Mo-16P-4B alloy of 1 mm diameter is identified as a single amorphous phase alloy and spontaneously passive in 6 and 12 M HCl, similarly to melt-spun amorphous counterpart, although high resolution TEM observation reveals the presence of nanocrystalline precipitates of about 2 nm in diameter. Ni-5Cr-5Ta-5Mo-16P-4B alloy is also amorphous by X-ray diffraction but contains nanocrystallites of about 5 nm in diameter, the anodic current in 6 M HCl being more than one order of magnitude higher than that of the melt-spun amorphous counterpart. All other copper-mold cast Ni-Cr-Ta-Mo-16P-4B alloys are composed of crystalline precipitates in the amorphous matrix. Sheath-rolling consolidation of gas-atomized amorphous alloy powders to form the amorphous alloy sheet of about 2 mm was attempted for the Ni-10Cr-5Nb-16P-4B alloy which has a particularly wide temperature interval of the supercooled liquid of 64K. The sheath-rolled alloy specimen are identified as an amorphous single phase by X-ray diffraction and high resolution transmission microscopy. However, because of inclusions of very minor crystalline powders in the major amorphous alloy powders before consolidation, crystalline powder inclusions in the consolidated amorphous matrix was preferentially dissolved by an immersion test in 6 M HCl. In a waste incinerator, the sheath-rolled Ni- 10Cr-5Nb-16P-4B alloy showed no detectable weight loss at 393 and 433K for 20 days, maintaining metallic luster.