Four kinds of L1 2 -type Ni 3 (Si,Ti) intermetallics alloyed with a quaternary element X (X: Al, Cr, Co and Mo) were warm rolled accompanied by intermediate annealing and then cold rolled to thin foils. The effects of the alloying elements on microstructure, tensile properties and oxidation resistance of the cold-rolled Ni 3 (Si,Ti) foils were investigated. The Al-alloyed Ni 3 (Si,Ti) showed an L1 2 single-phase microstructure, while the Cr-, Co-and Mo-alloyed Ni 3 (Si,Ti) exhibited a two-phase microstructure consisting of L1 2 and fcc Ni solid solution. Room-temperature strength of the Ni 3 (Si,Ti) foils was slightly enhanced by the addition of the quaternary elements, whereas high-temperature strength was significantly enhanced especially by the addition of Mo and Co. High-temperature tensile elongation was remarkably improved by the additions of all the elements investigated. On the other hand, the oxidation resistance was improved by the addition of Al and Cr.
Methanol decomposition tests were carried out for the first time on cold-rolled Ni 3 (Si,Ti) foils in a temperature range of 513-793 K to investigate their potential catalytic properties for hydrogen production. The catalytic activity was observed at temperatures above 713 K. At 793 K, the catalytic activity changed with the reaction time in three stages: low-activity incubation, rapid spontaneous activation and highactivity state. Surface analysis revealed an intensive formation of fine Ni particles on the foil surfaces after the second stage where rapid spontaneous activation was observed. The formation of the fine Ni particles was considered to be induced by the selective oxidation of Si in Ni 3 (Si,Ti). The catalytic activity in the second and third stages was due to the fine Ni particles formed by the spontaneous activation.
The effect of Al addition on microstructures and tensile properties of cold-rolled Ni3(Si,Ti) intermetallic alloys with L12 ordered structure, which were fabricated through thermomechanical processing from arc-melted ingots, were investigated. Addition of 4 and 8 at.% Al to Ni3(Si,Ti) was conducted in two ways that Al substituted for Ti site and both for Ni and Ti sites, respectively. The alloys made by the former way showed a two-phase microstructure consisting of disordered fcc Ni solid solution dispersions in the L12 matrix, irrespective of Al contents, while the 4 at.% Al alloy made by the latter way exhibited an L12 single-phase microstructure. These alloys were successfully cold-rolled to thin sheets with a thickness of 200 μm except the 8 at.% Al alloy made by the latter way. For the thermomechanically processed 4 at.% Al alloys, high-temperature yield stress was higher in the alloy made by the latter way than in the alloy made by the former one, suggesting that the single-phase microstructure consisting of whole L12 ordered structure is favorable for high-temperature tensile property.
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