2. Experimental procedures

Ni_51-xTi₄₉Co­_x shape memory alloys (x= 0, 0.5, 1.5 and 4.0 at% Co) were manufactured by melting in induction vacuum furnace (purity more than 99.99%). These alloys were melted four times to ensure homogeneity and cast into an investment casting ceramic mold. Before pouring of the liquid metal, the mold was preheated to 1000 ◦C.

The chemical compositions of the investigated cast Ni_51-xTi₄₉Co­_x shape memory alloys are shown in Table 1.

Table 1. Chemical composition of NiTiCo shape memory alloys, at%

Alloy	Ni, at%	Ti, at%	Co, at%
Ni51Ti49Co0	51.01	48.99	0.00
Ni50.5Ti49Co0.5	50.29	48.88	0.83
Ni49.5Ti49Co1.5	48.79	49.57	1.64
Ni47Ti49Co4	47.4	48.75	3.85

The microstructure of as cast alloys was investigated by Meiji optical microscope (OM) fitted with digital camera as well as JEOL JSM5410 scanning electron microscope (SEM). The specimens for microstructure examination were prepared by standard metallographic procedures then etched in a solution of HNO₃, HF and H₂O in a ratio of 4:1:5, respectively. Another etching solution was used to examine the martensite phase and it consists of HNO₃: HF: CH₃COOH in a ratio of 4:4:2. The different phases existing in the structure were analyzed using energy dispersive X-ray spectrometry (EDS) attached in the SEM operated at 20 kV. Moreover, X-ray diffraction (XRD) was carried out to identify the existing different phases in the structure by using Cu Kα radiation with a step-scanning in 2θ range of 30–70°. The phase transformations of the cast alloys were measured by Netzsch CC 200 F1 differential scanning calorimetry (DSC) with a cooling/heating rate of 10 °C/min in the temperature range from -30 to 150 °C. The hardness property was investigated by using LECO Vickers Hardness Tester LV800AT.

3. Results and discussion

   1. Microstructure

Fig. 1 Microstructure of investigated Ni51-xTi49Cox alloys.

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

3. Ni49.5Ti49Co1.5 alloy (d) Ni47Ti49Co4 alloy

The microstructure of the investigated Ni₅₁Ti₄₉Co₀, Ni_50.5Ti₄₉Co_0.5, Ni_49.5Ti₄₉Co_1.5 and Ni₄₇Ti₄₉Co₄ shape memory alloys (SMAs) is shown in Fig. 1. The microstructure of Ni₅₁Ti₄₉Co₀ alloy consists of austenite (B2) as a matrix, martensite phase (B19') and precipitates of intermetallic compound of Ti₂Ni phase.

As the Co content increases the martensite phase percentage in the microstructure increase, as shown in Fig. 1. In Ni_50.5Ti₄₉Co_0.5 alloy, the parent phase is the martensite phase in addition to austenite and precipitates of Ti₂Ni phases, as shown in Fig. 1 (b). However, the microstructure of Ni_49.5Ti₄₉Co_1.5 and Ni₄₇Ti₄₉Co₄ alloys almost contains martensite phase beside some percentage of Ti₂Ni phase only and there is no existence to austenite phase at all, as shown in Fig. 1 (c) and (d).

Fig. 2 Ti₂Ni precipitated phase in Ni_51-xTi₄₉Co_X alloys.