3. Phase transformation

Fig. 8 Martensitic and austenitic transformation temperatures of Ni50.5Ti49Co0.5 alloy.

Figure 8 shows the martensitic (M) – austenitic (B2) transformation temperatures of Ni_50.5Ti₄₉Co_0.5 alloy. This transformation of NiTi alloy depends on Ni content and/or Ni/Ti ratio in addition to the stresses conditions [18]. The transformation of B2 to M goes through one stage during cooling as well as the transformation of M to B2 during heating, as shown in Fig. 8. Table 3 shows the transformation temperatures of M and B2 for Ni₅₁-xTi₄₉Co_x shape memory alloys. These temperatures (M_s, M_p, M_f, A_s, A_p and A_f) of the martensitic- austenitic transformation increases as the co content increase.

As a substitute of Ni, Co added to NiTi alloy, this means that as the Co content increase in that alloy the Ni atomic percentage decrease. Moreover, as reported by He and miyazaki [18] the martensitic transformation temperature decreases as the Ni content increase in NiTi alloy. Therefore by increasing Co additions Ni content depress in investigated alloys and hence the transformation temperatures elevates. The latter result, in this study, coincide with the results achieved by Z. Lekston et al [19] where he reported that M_s of Ni-rich NiTi alloy increased from -20 °C to +5 °C by the addition of Co. On the other hand, these results conflict with that obtained by Jing et al [20].

Table 3 Martensitic-austenitic transformation temperatures, °C

Alloy	Ms	Mp	Mf	As	Ap	Af
Ni51Ti49Co0	15	3	-12	21	26	35
Ni50.5Ti49Co0.5	22	9	-5	28	32	43
Ni49.5Ti49Co1.5	31	18	7	39	48	57
Ni47Ti49Co4	42	29	16	48	59	68

4. Mechanical properties

Fig. 9 Hardness measurements versus Co content in NiTi alloy.

To study the influence of Co additions on the mechanical properties of Ni-rich Ni₅₁Ti₄₉ shape memory alloy, hardness property chosen to be the indicator of mechanical properties.

Figure 9 shows the relationship between hardness value and Co content in the studied alloys. The measured hardness of Ni₅₁Ti₄₉Co₀ alloy was about 382 HV₁₀ and increased to 400 HV₁₀ by addition of 0.5 at% Co in alloy Ni_50.5Ti₄₉Co_0.5. By increasing Co content to 1.5 at%, alloy Ni_49.5Ti₄₉Co_1.5, hardness value significantly decreased to 285 HV_10. However the hardness slightly increased again to 295 HV₁₀ in Ni₄₇Ti₄₉Co₄ alloy as the Co content increased to 4 at%.

Nevertheless the addition of Co decreases the V_f of hard Ti₂Ni phase as shown in Fig. 3, the hardness value of Ni₅₁Ti₄₉Co₀ alloy slightly improved by the addition of 0.5 at% of Co. This result could be related to increased martensite volume fraction and/or solution hardening of matrix by the addition of Co.

The dramatically deteriorate in hardness measurements due to the addition of 1.5 at% of Co may originated from the significantly depress in the V_f of Ti₂Ni phase in the microstructure of Ni_49.5Ti₄₉Co_1.5 alloy, as shown in Fig. 3. Moreover, it is believed that the morphology (shape) of Ti₂Ni precipitates that formed as flakes, as shown in Fig. 2, affected negatively the hardness value.

In Ni₄₇Ti₄₉Co₄ alloy the rate determining step is the full martensitic microstructure as shown in Fig. 1 and Fig. 7 in addition to the solution hardening by the addition of 4 at% Co. Moreover the V_f of Ti₂Ni phase increases, as shown in Fig. 3, and support hardness improvements. Therefore the hardness value of that alloy remarkably increased but still lowers than that of Ni₅₁Ti₄₉Co₀ and Ni_50.5Ti₄₉Co_0.5 alloys.