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

Abstract

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

Properties of Cu-Based Shape-Memory Alloys Prepared by Selective Laser Melting

“…2b) are almost equiaxed, and their size is comparable to those of the suction-cast alloy without Zr ( Table 2). The addition of Zr, thus, has a pronounced grain-refining effect as it is also known from other studies [13,14,19,20]. The martensite laths are not as clearly visible as in the case of the Zr-free alloy (Fig.…”

“…For this reason, there is major interest in another group of HTSMAs derived from Cu-based alloys (e.g. Cu-Zn-Al and Cu-Al-Ni) due to their relatively low manufacturing costs, good processability as well as good shape-memory properties [1][2][3][4][5][6][8][9][10][11][12][13][14][15][16][17][18][19].…”

“…The propensity to early failure in coarse-grained Cubased SMAs can be avoided by selecting the proper processing conditions or alloy composition. It has been shown that the poor workability of Cu-Al-Ni SMAs caused by brittle intergranular fracture can be reduced by refining the microstructure through the addition of Co [10], Mn [9], Ti [2, 10-13, 19, 20] or Zr [2,13,14]. When Ti or Zr is dissolved in the melt, it reduces the grain growth rate and thus refines the microstructure [19,21,22].…”