Electron microscopy studies of twin morphologies in B19′ martensite in the Ti-Ni shape memory alloy

The microstructure and linear superelasticity of cold-drawn TiNi alloy

“…The substructure of each single martensite variant had been confirmed to be dominantly 011 Type II twin [3][4][5], with a few (111( ) Type I twin observed [5]. The present TEM observations indicate that the coalescence of martensite variants happens after the specimen is subjected to 5% area reduction, as shown in Fig.…”

“…Yet, with the increase of the area reduction, the amount of the 011 Type II twinning gradually reduces. As is well known, the 011 Type II twinning is the most frequently observed twinning mode in the thermal martensite [5], which has also been pointed out theoretically to be a lattice invariant shear [3,4]. Since the microstructure is wholly martensitic at the room temperature for the present alloy before cold-drawing, the appearance of the 011 Type II twinning bands is clearly to provide the necessary lattice invariant shear during the martensitic transformation which had finished before cold-drawing.…”

“…As is well known for the as-annealed and undeformed TiNi alloy, the martensite variants exhibit selfaccommodation morphology, with the various twinning boundaries being straight and well defined [1][2][3][4][5]12]. The substructure of each single martensite variant had been confirmed to be dominantly 011 Type II twin [3][4][5], with a few (111( ) Type I twin observed [5].…”

“…With the increase of the area reduction amount, the probability of random observation of (111( ) Type I twinning mode in the present deformed specimens becomes high. In the thermal Total number 4 ---3 1 5 2 7 3 1 -25 -8 29 23 12 4 5 --32 14 16 8 10 2 -34 12 13 2 6 -22 33 13 10 30 4 3 7 3 7 martensite, the (111( ) Type I twin was mainly believed to be a deformation twin or a variant accommodation twin [5], which implies that it could accommodate the external stress field more easily, especially the shear strains between martensite variants. A recent study [14] on the TiNi alloy due to tension-compression cyclic deformation showed that the (111( ) Type I twin was more frequently observed in the cycled samples.…”

“…The ingot was hot swaged and rolled at 850°C to rods with approximately 8 mm diameter. After annealing at 850°C for 1 h, some specimens were colddrawn at room temperature to the extents of 5,8,12,14,16,22 and 30% reduction in area, respectively. The tensile tests were carried out on an Instron-1186 testing machine with a strain rate of 4.1 × 10 − 4 s − 1 .…”