Fig. 6 edx of phases in Ni52Ti48Co0

Element	Alloy	Ni, at% Ti, at% Co, at%
Phases		0	0.5	1.5	4.0	0	0.5	1.5	4.0	0	0.5	1.5	4.0
Matrix		52.83	52.63	48.88	48.36	47.17	46.89	49.40	48.64	0.00	0.47	1.72	3.00
Ti2Ni		34.15	34.30	32.49	32.72	65.85	65.20	66.42	64.31	0.00	0.50	1.09	2.96
Ni3Ti2		59.26	0.00	0.00	0.00	40.74	0.00	0.00	0.00	0.00	0.00	0.00	0.00

Table 2. Composition of different phases in as cast Ni₅₂Ti_48-xCo_x SMAs.

Fig. 6 and Table 2 show the microanalysis, by EDS, of different phases found in the microstructure of the four investigated alloys. It can be noticed that, the matrix has higher content of Ni than Ti. Ni₃Ti₂ phase found only in Ni₅₁Ti₄₉Co₀ alloy contains lower Ti contents than Ni. However, Ti₂Ni phase has higher Ti percentage, almost double, than Ni in its precipitates. Ti content in the latter phase reaches around 65%, while Ni content is about 35%. Co content increases as well in Ti₂Ni precipitates with increasing Co additions.

By the additions of Co, Ni₃Ti₂ phase found only in Ni₅₁Ti₄₉Co₀ alloy was eliminated from the microstructure of other Ni_50.5Ti₄₉Co_0.5, Ni_49.5Ti₄₉Co_1.5 and Ni₄₇Ti₄₉Co₄ alloys.

2. X –ray diffraction

Fig. 7 X ray diffraction patterns for NiTiCo shape memory alloys.

Figure 7 shows the XRD patterns of the as cast Ni-rich Ni_51-xTi₄₉Co_x shape memory alloys at room temperature. There are four diffraction patterns according to the atomic percentage of Co added to Ni_51-xTi₄₉Co_x alloy (at% Co= 0, 0.5, 1.5 and 4). In the XRD pattern of Ni₅₁Ti₄₉Co₀ alloy, the main diffraction peak (110) is related to the austenite phase, denoted as B2, which is the parent phase. Moreover, martensite phase, denoted as M, represented in this diffraction pattern by one peak; M (002). For the other three diffraction patterns related to the Ni_50.5Ti₄₉Co_0.5, Ni_49.5Ti₄₉Co_1.5, and Ni₄₇Ti₄₉Co₄ alloys_, the main diffraction peak is shifted to the left in comparison with the main peak in the Ni₅₁Ti₄₉Co₀ alloy, as shown in Fig. 7. This means that the parent phase in these alloys is another phase than that in Ni₅₁Ti₄₉Co₀ alloy. The main diffraction peak in these three alloys is M (ī 11), which indicates that the parent phase in these alloys is the martensite phase not austenite one, as shown in Fig. 7. In addition to the M (ī 11) peak, there is another diffraction peak emphasized the presence of martensite phase in the microstructure of these alloys, which are M (002).

There was not any peak related to Ti₂Ni phase in all diffraction patterns of the investigated alloys.