| Summary: | 博士 === 國立交通大學 === 材料科學與工程系所 === 94 === Effects of the manganese (Mn) content on the phase transformations of the Cu-Mn-Al ternary alloys have been investigated by means of optical microscopy, scanning transmission electron microscopy and energy- dispersive X-ray spectrometry. On the basis of the experimental examinations, several results can be summarized as follows:
[1].We have studied the Cu3-xMnxAl alloy systems at room temperature. In the as-quenched condition, the microstructure of the Cu2.9Mn0.1Al (Cu-2.7 at.%Mn-25.1at.%Al) alloy was a mixture of (D03 + γ1΄ martensite) phases. However, the as-quenched microstructures of the Cu2.8Mn0.2Al (Cu- 5.1at.%Mn-25.3at.%Al) and Cu2.7Mn0.3Al (Cu-7.6at.%Mn-25.1at.%Al) alloys were found to be D03 phase containing extremely fine L-J precipitates. However, as the X increasing to 0.4, that is Cu2.6Mn0.4Al (Cu-10.3at.%Mn-25.2at.%Al) alloy, it was a mixture of (D03 + L21 + L-J) phases in the as-quenched condition. These results are different from those proposed by Bouchard et al. The D03 phase in the Cu2.9Mn0.1Al, Cu2.8Mn0.2Al and Cu2.7Mn0.3Al alloys was formed by a β→B2→D03 continuous ordering transition during quenching, because of the presence of a/4<111> anti-phase boundaries (APBs). It is a strong evidence to demonstrate that the existing D03 phase was formed by a β→B2→D03 continuous ordering transition during quenching. It is worthwhile to note here also that the a/4<111> APBs have never been found in the Cu-Mn-Al alloy systems before.
[2].The as-quenched microstructure of the Cu2.7Mn0.3Al (Cu-7.6at.%Mn- 25.1at.%Al) alloy was D03 phase containing extremely fine L-J precipitates, where the D03 phase existing was formed by a β→B2→D03 continuous ordering transition during quenching. When the as-quenched alloy was aged at 500℃ for moderate times, the γ-brass particles were found to nuclear preferentially at a/2<100> APBs. However, with increasing the aged times at 500℃, the L-J precipitates started to appear at the regions contiguous to the γ-brass particles. The coexistence of (γ-brass+L-J) phases has never been observed by other workers in the Cu-Mn-Al alloy systems before. As the aging temperature was increased from 500℃ to 700℃, the phase transition sequence was found to be (γ-brass+L-J+D03)→(γ-brass+L-J+B2)→β. This result is different from that reported by previous workers in Cu3-xMnxAl alloys with X<0.32.
[3].In the as-quenched condition, the microstructure of the Cu1.6Mn1.4Al (Cu-35.1at.%Mn-25.1at.%Al) alloy was a mixture of (L21+B2+L-J) phases. This is different from that observed by previous workers in the Cu3-xMnxAl alloys with X<1.0. When the as-quenched alloy was aged at 460℃ for short times, γ-brass precipitates started to occur at APBs. After prolonged aging time at 460℃, the γ-brass precipitates grew and β-Mn precipitates generated at the regions contiguous to the γ-brass precipitates. The orientation relationship between the γ-brass and β-Mn was (001)γ-brass//(012)β-Mn and (011)γ-brass//(031)β-Mn. The coexistence of (γ-brass+β-Mn) has never been observed by previous workers in Cu-Mn-Al alloy systems before. When the as-quenched alloy was aged at temperatures ranging from 460℃ to 700℃, the phase transition sequence was found to be (γ-brass+β-Mn)→(β-Mn+L21)→β.
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