Microstructural studies of a Cu-Zn-Al shape-memory alloy with manganese and zirconium addition

Mn and Zr were added to improve the shape-memory characteristics of a Cu-Zn-Al shape-memory alloy (SMA). The microstructure of a Cu-19.0Zn-13.1Al-1.1Mn-0.3Zr (at. pct) alloy was examined using a transmission electron microscope (TEM). The structure of the parent phase and martensite phase are DO₃ (or L2₁,) and M18R₁, respectively. Two kinds of Zr-rich precipitates formed in the alloy. Energy-dispersive X-ray spectroscopy (EDXS) analysis with a TEM indicates that the two precipitates are all new phases and have the compositions of Cu_50.2Zr_24.6Al_17.3Zn_7.9 (at. pct) (Z₁ phase) and Cu_57.4Zr_20.4Zn_10.3Al,_11.9 (at. pct) (Z₂ phase), respectively. The volume ratio of Z₁ phase in the alloy is about 70 pct of the total precipitate volume. The structure of Z₁ phase was studied in detail using TEM electron diffraction analyses. The lattice parameter of fee Z₁ phase is a = 1.24 nm, and the space group of the phase is F432 (No. 209). The Z₁, phase possesses an incoherent interface with the parent-phase matrix. The lattice correspondence of the Z₁ phase and parent-phase matrix is as follows:
                        [111]_β1 ∼ [110]z₁; [1̄10]_β1 ∼ [1̄10]z₁; [112̄]_β1 ∼ [001]z₁
The effect of precipitate formation on the shape-memory characteristics of the Cu-Zn-Al-Mn-Zr alloy is discussed and compared to some other Cu-based SMAs.