## 3 Results and Discussion
### Microstructure  
The optical microstructure of the Ti-6 Al-4V/Nitinol linear friction as-welded joint on the Ti-6 Al-4V side showing three distinct zones, namely the recrystallised zone, deformed zone and base metal zones, of Ti-6 Al-4V are as shown in Figure 6. In contrast, the deformed zone and base metal of Nitinol were observed on the Nitinol side. The recrystallised zone was observed adjacent to the weld interface on the Ti-6 Al-4V side, which is distinguished from all other regions by its whitish appearance at low magnification. The fine grains were observed at high magnification nearer to the weld interface on the Ti-6 Al-4V side due to grain refinement induced by dynamic recrystallisation [25]. During dynamic recrystallization, the material experiences a significant amount of plastic deformation throughout the linear friction welding process. The base metal's low-angle grain boundaries are replaced by high-angle boundaries in the recrystallised zone by means of oscillation during LFW. The fine nuclei of grains begin to develop, eventually forming a microstructure with fine equiaxed grains in the recrystallised zone [25]. Adjacent to the recrystallised zone, there was a deformed zone which is characterised by a highly deformed structure. The base metal Ti-6 Al-4V grains were deformed in an upward flowing pattern showing curved, elongated grains in the deformed zone. The deformed zone undergoes plastic deformation, and in this region, no recrystallisation occurs due to insufficient strain. There is a significant deformed zone adjacent to the Nitinol interface, and the microstructure mainly consists of austenite rather than martensite because of the heating of the material during LFW. Microstructural features present in the deformed zone of the Nitinol side show elongated grains when compared to the Nitinol base metal exhibited fine equiaxed grains. The optical microstructure of Ti-6 Al-4V/Nitinol linear friction welds in the post-weld heat-treated condition (PWHT) is shown in Figure 7. It can be observed from Figure 8 that the welds prepared with PWHT at 400 \({}^{\circ}\)C showed similar microstructural features compared with the as-welded condition except for some dark regions (etch pits) observed on  
Figure 6: The optical microstructure of the Ti-6 Al-4V/Nitinol linear friction welds in the as-welded condition. the Nitinol side. These etch pits have developed during the metallographic etching process. PWHT at 400 \({}^{\circ}\)C, below the beta-transus temperature, results in a duplex microstructure of primary \(\alpha\), and transformed \(\beta\) is generated due to slow cooling. Rapid quenching leads to a martensitic transformation of \(\beta\), resulting in a very fine needle-like microstructure [3]. However, in the present study, rapid cooling is not employed during PWHT at 400 \({}^{\circ}\)C, leading to equiaxed \(\alpha\) in a \(\beta\) matrix. From the optical micrographs, it is difficult to identify the phases and intermetallics present in the microstructure; therefore, further work by XRD is necessary. The SEM images (refer below Figure 8a,b) showed a continuous region of intermediate phases at the interface. This region seems to be around 10 \(\upmu\)m thick. This region could easily have consisted of the oxides of Ti and Ni [26]. Elemental Ti and Ni from both the base metals, which are present majorly composition-wise, would react with the oxygen present on the surfaces during the friction stage. It has to be noted that previous works on friction welding suggest that the chance of this happening--oxygen reacting with the base metal elements--is quite low [27; 28]. This is because the friction stage is very short, and also even if some oxide phases do form, they eventually were pushed out as flash. This reasoning might be valid to a large extent for rotary friction welding, but not so in the case of linear friction welding.