| Summary: | 博士 === 國立交通大學 === 材料科學與工程系 === 90 === The effects of the nickel content on the phase transformations of the Cu-Al-Ni ternary alloys have been investigated by means of scanning transmission electron microscopy and energy-dispersive X-ray spectrometry. Based on the examinations, some results can be summarized as follows:
[1].In the as-quenched condition, the microstructure of the Cu — 14.2 wt. % Al — 7.8 wt. % Ni alloy was D03 phase containing extremely fine L-J precipitates. When the alloy was aged at 500℃ for short times and then quenched, γ2 particles started to precipitate within the D03 matrix at the aging temperature and the remaining D03 matrix would transform to γ1' martensite during quenching. However, after prolonged aging at 500℃, the remaining D03 matrix would transform to a mixture of (α + B2 precipitate) phases. When the as-quenched alloy was aged at temperatures ranging from 150℃ to 750℃, the phase transformation sequence as the aging temperature increased was found to be D03 → (D03 + γ2) → (γ2 + B2 precipitate + α) → (β + γ2) → β. It is noted that this transformation is quite different from that observed by other workers in the various Cu-Al-Ni alloys.
[2].In the as-quenched condition, the microstructure of the Cu — 14.2 wt. % Al — 9.0 wt. % Ni alloy was D03 phase containing extremely fine L-J precipitates. During the early stage of isothermal aging at 500℃, a high density of fine B2 precipitates was observed within the extremely thin lamellar γ1' martensite, where the γ1' martensite was formed by a D03 → γ1' martensitic transformation during quenching. With increasing aging time at 500℃, the phase transition sequence was found to be D03 → (D03 + B2 precipitate) → (D03 + α + B2 precipitate) → (α + γ2 + B2 precipitate). Besides, both Kurdjumov- Sachs (K-S) and Nishiyama-Wassermann (N-W) orientation relationships between the B2 precipitate and the α phase could be detected in the aged alloy.
[3].The as-quenched microstructure of the Cu — 14.2 wt. % Al — 12.0 wt. % Ni alloy was D03 phase containing extremely fine L-J precipitates, where the D03 phase was formed by an A2 → B2 → D03 continuous ordering transition during quenching. The a/4<111> anti-phase boundaries (APBs) have never been found by other workers before. The presence of the a/4<111> APBs strongly confirms that the D03 phase existing in the as-quenched alloy was formed by an A2 → B2 → D03 ordering transition during quenching, rather than A2 → D03 transition reported by other workers. Besides, the addition of nickel to the Cu— 14.2 wt. % Al binary alloy would raise the B2 → A2 transition temperature.
[4].When the Cu — 14.2 wt. % Al — 15.0 wt. % Ni alloy was solution heat-treated and then quenched, the microstructure was D03 phase containing extremely fine L-J precipitates. Since both fine D03 domains with a/2<100> APBs and small B2 domains with a/4<111> APBs could be observed, the D03 phase existing in the as-quenched alloy should be formed by an A2 → B2 → D03 continuous ordering transition during quenching. With increasing the aging temperature from 400℃ to 1000℃, the phase transformation sequence was found to be (α + B2 precipitate) → (B2 + B2 precipitate) → B2 → A2. It is worthwhile to note that no evidence of the γ2 phase could be detected in the alloy.
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