ABSTRACT The current research studies the influences of adding yttrium element on the oxidation behavior of NiTi- Shape Memory Alloy system (SMAs). Specimens were manufactured by powder technology method, the blended powder consisting of titanium and nickel with constituents of (45 %wt.) and (55% wt.) correspondingly. Yttrium was added in different weight percentages of (0.5 and 0.9). The constituents of powders were blended for five hours and later compressed with a stable compressing load of (800) MPa to cylinder-shaped specimens. The sintering procedure is done at a specific temperature (950 °C for 6 hours) in vacuum circumstances (10-4) torr. Later, the specimens cool down in vacuum condition till room temperature. The outcomes of X-ray diffraction analysis revealed that entirely specimens (without and with additions) contained [NiTi (B19) and (NiTi (B2)]. Periodic oxidation tests were performed in static air at temperatures (800, 900, and 1000?) for 100 hrs and 10 hours per period to evaluate the behavior of oxidation for all specimens. The results showed that the behavior of oxidation of NiTi (without and with additives) was subjected to parabolic law. Further, the performance of NiTi SMAs was modified after yttrium addition. For the base NiTi alloy, the parabolic rate constant (KP) differs from a KP = 2.4502×10-7 (mg2 /cm4 .s) at 800 ? to a specific value of KP = 10.609×10-7 (mg2 /cm4 .s) at 1000 ?, and the activation energy (Q=297 kJ/mol), while NiTi 0.5 wt.% Y alloy, the (Kp) varies from a Kp = 0.531×10-7 (mg2 /cm4 .s) at 800 ? to a value of Kp = 8.6×10-7 (mg2 /cm4 .s) at 1000 ?and (Q= 187.6 kJ/mol), and for NiTi 0.9 wt.% Y, (KP) differs from a value of KP = 1.0780×10-7 (mg2 /cm4 .s) at 800 ? to a value of KP = 7.2531×10-7 (mg2 /cm4 .s) at 1000 ?. and Q =123.98 kJ/mol. The phases are presented after periodic oxidation at 1000 ? for 100 hrs. and 10 hrs. per period of NiTi 0.9%Y SMA surface as revealed by XRD analysis were NiTi monoclinic, NiTi cubic, Ni3Ti, and TiO2.
CONCLUSIONS
1. Procedure of sintering procedure at 950 ? for six hours (with and without yttrium addition) is sufficient to fulfill the process of transformation of nickel, titanium, and yttrium to alloy construction.
2. The results of scanning electron microscopy showed that the main phase formed for all specimens without and with the addition of yttrium the martensite phase.
3. The porosity decreases significantly with the addition of yttrium.
4. The X-ray examination results indicated the formation of austenite (B2) and monoclinic NiTi (B19) for all specimens.
5. The NiTi alloy showed good periodic oxidation resistance at temperatures 800 °C, 900°C, and 1000 °C.
6. Alloy of the NiTi showed satisfactory resistance for periodic oxidation at 800, 900, and 1000 °C, respectively.
7. The addition of yttrium to the NiTi alloy greatly improved the oxidation resistance in general, and the addition 0.9 wt.% Y gave the best results in particular.
8. All NiTi alloys (without and with additives) showed parabolic oxidation response in all conditions.
9. The energy of activation of NiTi is ~ 297 kJ/mol,(NiTi0.5%Y)~ 187.6 kJ/mol and that for (NiTi0.9%Y) ~ 123.95 kJ/mol. This is a very clear indication of the low activation energy to form a protective layer of titanium oxide with the addition of yttrium to the nickel-titanium alloy.