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 DO 3 (or L2 1 ) and M18R 1 , 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 50.2 Zr 24.6 Al 17.3 Zn 7.9 (at. pct) (Z 1 phase) and Cu 57.4 Zr 20.4 Zn 10.3 Al 11.9 (at. pct) (Z 2 phase), respectively. The volume ratio of Z 1 phase in the alloy is about 70 pct of the total precipitate volume. The structure of Z 1 phase was studied in detail using TEM electron diffraction analyses. The lattice parameter of fcc Z 1 phase is a ϭ 1.24 nm, and the space group of the phase is F432 (No. 209). The Z 1 phase possesses an incoherent interface with the parent-phase matrix. The lattice correspondence of the Z 1 phase and parent-phase matrix is as follows: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.
I. INTRODUCTIONCu-Zn-Al is an important Cu-based shape-memory alloy (SMA) which possesses a good shape-memory effect (SME) and has the advantage of a lower price than the Ti-Ni SMA. However, it suffers from martensitic stabilization [1] and intergranular cracking during processing and service because of the large shape-memory anisotropy and large grain size when in the  parent-phase condition. [2] The addition of suitable alloying elements can improve the mechanical properties and stabilize the martensitic transformations and the SME of a Cu-based SMA. For example, with the increase of manganese concentration, the thermoelastic and pseudoelastic behaviors of Cu-Al-Ni-Mn-B SMAs were enhanced and the ductility of the alloy was improved. [3,4] On the other hand, the elements B, V, Zr, and Ti can be added to refine the grain size of the parent phase in Cu-based SMAs. [5,6,7] As a modification of Cu-Zn-Al SMAs, Zr and Mn are added to the Cu-Zn-Al alloy to refine the grain size of the alloy, to increase the ductility of the matrix, and to suppress the stabilization of the martensite. Compared to a Cu-Zn-Al SMA, a significant grain refinement effect was found in the Cu-Zn-Al-Zr and Cu-Zn-Al-Mn-Zr alloys, and the characteristics of martensitic transformation in the Cu-Zn-Al-Mn-Zr alloy are better than those of the Cu-Zn-Al SMA. [8] Zr-rich precipitates formed in a Zr-added Cu-Zn-Al SMA. These precipitates were initially observed in the Cu-Zn-Al-Zr alloy and at the grain boundaries. [5] However, in a Cu-19.0Zn-13.1Al-1.1Mn-0.3Zr (at. pct) SMA, Zr-rich precipitates appeared not only in the vicinity of grain boundaries
The excellent fit obtained in the SSS article for low-carbon steel and CP titanium recrystallization data using SSS Eq.[5] also supports the validity of the overall SSS approach, at least for these two materials.One may also compare the EB analytical result (their Eq. [4]) to the staircase calculation based on the isothermal Avrami equation. The EB expression, however, contains a constant related to the number of nuclei and whose value is therefore not easily determined. Nevertheless, if the value of the constant in the EB relations is determined by ''forcing'' the value of X(T) to agree with that from the staircase calculation for a specific temperature, then the SSS and EB results can be compared. This was done for the case illustrated in Figure 1. It was found that the curves from the two analytical expressions were nearly identical when the EB result was forced to fit the staircase-calculation at X ϭ 0.50.
REFERENCES1. V. Erukhimovitch and J.
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