Orientation relationship between β-Mn and L21 matrix in a Cu2MnAl alloy

Abstract

In the as-quenched condition, the microstructure of the Cu 2 MnAl alloy was L2 1 phase containing extremely fine L-J precipitates. This result is different from that reported by other workers in the asquenched Cu 2 MnAl alloy. When the as-quenched alloy was aged at 350 ЊC, ␥-brass precipitates started to appear within the L2 1 matrix. The orientation relationship between the ␥ -brass and the L2 1 matrix was determined to be cubic to cubic. This result is consistent with that observed by other workers in the aged Cu-Mn-Al alloy. When the alloy was aged at 460 ЊC, the ␥ -brass precipitates disappeared and platelike ␤ -Mn precipitates occurred within the L2 1 matrix. As the aging temperature was increased to 560 ЊC, the morphology of the ␤-Mn precipitates changed from platelike to granular shape. Electron diffraction examinations indicated that in spite of the morphology change the same orientation relationship between the ␤-Mn and the L2 1 matrix is maintained, and it could be best stated as follows: (210) ␤-Mn //(100) m , (120) ␤-Mn //(010) m , (001) ␤-Mn //(001) mThis result is in disagreement with that reported by Kuzobski et al. in the aged Cu 2 MnAl alloy. In their study, it was concluded that both the morphology of the ␤ -Mn precipitates and the orientation relationship between the ␤ -Mn and the L2 1 matrix would vary with the aging temperature.

Phase transformations in a Cu–35at.% Mn–25at.% Al alloy

“…1(b) and (c) are two selected-area diffraction patterns (SADPs) of the as-quenched alloy. When compared with our previous studies in the Cu 2.2 Mn 0.8 Al and Cu 2 MnAl alloys [5,6], it is found in these SADPs that the brighter and well-arranged reflection spots are of the ordered L2 1 phase and the extra spots with streaks are of the L-J phase with two variants. Although the brighter and well-arranged reflection spots could be analyzed as a single L2 1 phase, the L2 1 reciprocal lattices contain all the B2-type reflections [7,8].…”

“…4(b), an SADP taken from the precipitate marked as "B" in Fig. 4(a), indicates that the new type of precipitate was ␤-Mn with lattice parameter a = 0.641 nm [6]. Fig.…”

“…Therefore, in order to decide whether the ordered B2-type phase coexists with the L2 1 phase, both electron diffraction method and dark-field technique were performed. In our previous study [6], it was found that the intensity of the (1 1 1) and (0 0 2) reflection spots of a single L2 1 phase should be almost equivalent. However, it is clearly seen in Fig.…”

“…the aged Cu 2 MnAl alloy, no evidence of the ␤-Mn precipitates could be detected at the regions contiguous to the ␥-brass precipitates [4,6]. The reason is probably that along with the growth of ␥-brass precipitates, the Mn concentration at the regions surrounding the ␥-brass may not be sufficient to cause the formation of the ␤-Mn precipitates.…”

“…Recently, we have performed TEM investigations on the phase transformations of Cu 2.2 Mn 0.8 Al and Cu 2 MnAl alloys [5,6]. Consequently, we found that the fine precipitates formed in the Cu 2.2 Mn 0.8 Al alloy aged at 300 • C should belong to the L-J phase, rather than L1 0 phase [5].…”