1. Ni51Ti49Co0 alloy (b) Ni50.5Ti49Co0.5 alloy

(c) Ni_49.5Ti₄₉Co_1.5 alloy (d) Ni₄₇Ti₄₉Co₄ alloy

Figure 2 shows the shape, size and distribution of Ti₂Ni phase in different Ni_51-xTi₄₉Co_x alloys. In Ni₅₁Ti₄₉Co₀ alloy, Ti₂Ni phase precipitated in agglomerated non-geometric shapes in addition to small circular shapes, as shown in Fig. 2 (a). In Fig. 2 (b), Ti₂Ni phase in Ni_50.5Ti₄₉Co_0.5 alloy was well distributed and smaller in size in comparison with that in Ni₅₁Ti₄₉Co₀ alloy.

However in Ni_49.5Ti₄₉Co_1.5 alloy, most of Ti₂Ni phase precipitates in another strange flakes shapes in addition to slight precipitation of Ti₂Ni phase in agglomerated non-geometric shapes, as shown in Fig. 2 (c).

As the co content increases to 4 at% in NiTi alloy, the precipitated Ti₂Ni phase shows the best distribution among the microstructures of the four investigated alloys. In comparison with other alloys, this alloy has the finest precipitates of Ti₂Ni phase.

Fig. 3 Ti2Ni ratio versus Co content in NiTi alloy.

The volume fraction (V_f) of Ti₂Ni phase is influenced by the addition of Co at the expense of Ni to NiTi alloy. Ni₅₁Ti₄₉Co₀ alloy has the highest V_f of Ti₂Ni phase among Ni_51-xTi₄₉Co­_x alloys, as shown in Fig. 3. This V_f of Ti₂Ni phase in the former alloy diminished by adding 0.5 at% Co. Further depression in the V_f of Ti₂Ni phase took place reaching its lowest value with increasing Co content to 1.5 at%, as shown in Fig. 3. The addition of 4 at. % Co in Ni₄₇Ti₄₉Co₄ alloy increased a little bit the V_f of Ti₂Ni phase than in Ni_49.5Ti₄₉Co_1.5 alloy, as shown in Fig. 3.

As a result of substitution of Ni, Co suppresses the formation of hard Ti₂Ni phase in the microstructure of both Ni_50.5Ti₄₉Co_0.5 and Ni_49.5Ti₄₉Co_1.5 alloys [17]. Where Ni content in both alloys decreased and consequently a shortage in the free Ni left to combine with Ti to form Ti₂Ni phase took place, leading to lower V_f of Ti₂Ni phase.

Nevertheless as the Co content increased to 4 at% in Ni₄₇Ti₄₉Co₄ alloy, a higher Ni percentage may segregated out of the solid solution phase forming higher V_f of Ti₂Ni phase than in Ni_49.5Ti₄₉Co_1.5 alloy but still lower than that in both Ni₅₁Ti₄₉Co₀ and Ni_50.5Ti₄₉Co_0.5 alloys, as shown in Fig. 3.

Fig. 4 Ni3Ti2 precipitates in the microstructure of Ni51Ti49Co0 alloy.

1. Ni3Ti2 precipitates in small round shape.

2. Higher magnification of (a) Ni3Ti2 precipitates in needle like-shape.

There are two types of NiTi precipitates; Ti₂Ni precipitated in all microstructures of the investigated alloys and Ni₃Ti₂ precipitates formed only in the microstructure of Ni₅₁Ti₄₉Co₀ alloy as shown in Fig. 4. Ti₂Ni phase precipitates in irregular spherical shapes as shown in Fig. 2. However,. Ni₃Ti₂ precipitates formed mainly in tiny size round particles, Fig. 4 (a), in addition to some of it in needle like shape, as shown in Fig. 4 (b).

Fig. 5 Colonies of NiTi phase inside Ti2Ni precipitates in the microstructure of Ni51Ti47Co4 alloy.

Some colonies of NiTi phase were found inside the Ti₂Ni precipitates as shown in Fig. 5. These NiTi colonies sometimes found in a spherical shape as shown in Fig. 5 (a) and other times found in irregular rounded shapes as shown in Fig. 5 (b).