Evolution of substructure in a CuZnAlMnNi shape memory alloy

Abstract

Martensite transformation is the basic transition in shape memory alloy (SMA), which has been investigated by many researchers [1–5]. It is well known that the dominant substructure in martensite plates is stacking faults in CuZnAl-based SMA [6, 7]. However, there are few reports on the formation of the substructure. In our recent investigation on a CuZnAlMnNi SMA, we observed the detailed substructure in martensite plates by transmission electron microscope, and discussed the formation process of the stacking faults. The material investigated is a polycrystalline Cu23.6Zn-4.47Al-0.23Mn-0.17Ni (wt.%) alloy. The sample was solution treated at 840 ◦C for 20 min, quenched into boiling water for 30 min and then cooled to ambient temperature in atmosphere. The transformation temperatures are as follows: Ms= 39 ◦C, Mf= 22 ◦C, As= 45 ◦C and Af= 63 ◦C. The substructure in martensite plates was observed on a H800 transmission electron microscope, whose specimen was jet-polished with 33.3% nitric acid and 66.7% methanol solution. During quenching, the vacancies gather together and form the vacancy pieces, whose lattice planes collapse and become dislocation loops. The presence of dislocation loops is favorable to the nucleation of martensite plates, but prevents the plates from broadening. Fig. 1 shows the fine martensite plates in the as-quenched specimen. The substructure of dislocation loops can be observed distinctly in the plates, they arrange in a row orderly. Because the loops have not spread, they look like dots. In another plate, we observed the spreading dislocation loops, as shown in Fig. 2. The magnitude of dislocation loop decreases, instead, the stacking faults appear in the plate. Near the plate boundary, the dislocation lines have contacted with the boundary and mix together with it. Meanwhile, the dislocation lines between the neighbor loops change into stacking faults. Fig. 3 shows that the size of dislocation loop depends upon the wideness of the fine plate. The larger loops can be observed at the site with wider part in the same plate, but with less magnitude. On the contrary, the magnitude of stacking fault increases in the identical site. Fig. 4 shows the substructure of two different plates, which is the combination of dislocation loops and stacking faults. When the sample was sloped slightly, the dislocation loops disappear and only the stacking faults can be seen, as shown in Fig. 5. This further verifies the existence of the dislocation loops. With the broadening of the fine plate, the dislocation loops spread and change into stacking faults constantly, leading to the reduction of dislocation loop and the increment of stacking fault. Fig. 6 shows the substructure in the plates with variety of wideness. It can be found that the substructure of the wider plates is dominantly stacking faults, whereas in the thinner plates exists a lot of dislocation