Improvement of the shape memory characteristics of a Cu-Zn-Al alloy with manganese and zirconium addition

Copper-based shape memory alloys (SMAs) have good shape memory effect (SME) and have the advantage of lower price than Ti-Ni SMA. However, there are still some problems which should be solved before they can be used widely. Cu-Zn-Al is an important Cu-based SMA that suffers from the martensite stabilization and intergranular cracking in the processing procedures and service. As a modification of Cu-Zn-Al SMAs, the effects of Mn and Zr addition on the structure and martensite transformation behavior of different heat treated Cu-21Zn-6Al-1Mn-0.5Zr (wt.%) SMA have been studied and compared to that of Cu-21Zn-6Al (wt.%) SMA in the present paper. The serious martensitic stabilization problem found in directly quenched (DQ) Cu-Zn-Al alloy is explained mainly by the effects of vacancies pinning. After adding small amount of alloying elements into Cu-Zn-Al alloy, the migration of vacancies in the martensite matrix may be controlled. Therefore, the stabilization of martensite in DQ Cu-Zn-Al-Mn-Zr alloy can be suppressed. Similar effects of alloying elements on the excess vacancies have been observed in Cu-Al-Ni-Mn-Ti and Cu-Zn-Al-Ti SMAs. The step-quenched (SQ) specimens of both alloys exhibited good reverse transformation properties. That was due to the annihilation of supersaturated vacancies in the parent phase matrix in the course of SQ heat treatment. On the other hand, when the specimens were held at parent phase state, the ordering evolution: beta - > B2 and B2- > DO sub 3 occurred. The B2 and DO sub 3 order degree of both SQ Cu-Zn-Al and Cu-Zn-Al-Mn-Zr specimens were all higher than that of DQ ones, respectively. For Cu-Zn-Al alloy, earlier investigations found that the increase of B2 and L2 sub 1 order degree had led to the increase of the splitting of (122)-(202), (1,2,10)-(2,0,10) line pairs because the basal plane of the martensite distorted from the ideal hexagon during the ordering arrangement procedure. That was in agreement with the phenomenon we observed for Cu-Zn-Al alloy. However, for SQ Cu-Zn-Al-Mn-Zr alloy, the increase of B2 and DO sub 3 order degree did not result in the increase of the splitting of (122)-(202) and (1,2,10)-(2,0,10) line pairs. Moreover, the bonding angle phi and the monoclinicity of the martensite did not change apparently in SQ specimens as compared to DQ ones. This may imply that the orthorhombic distortion of martensite phase does not have a simple relationship with the variations of order degree in Cu-Zn-al-Mn-Zr alloy.