# Net-Shape NiTi Shape Memory Alloy by Spark Plasma Sintering Method
## 1 Introduction  
Demands for NiTi alloys for applications in various fields in engineering and medicine make this material one of the most investigated in both academia and industry. NiTi alloys with chemical compositions that are close to equiatomic possess the properties of good corrosive resistance, biocompatible natures, superior mechanical performances, and the functional properties of shape memory and superelasticity (SE) [1; 2; 3]. These properties open up the application areas of NiTi alloys with shape memory effects for various application-orientated industries in material engineering, such as the aerospace industry, actuators, dampers, vibrators, and resonators. The shape memory effects (SMEs) of NiTi alloys relate to the transformation of solid-state phases from the austenite cubic B2 structure to martensitic monoclinic B19\({}^{\prime}\) structure [4; 5]. Ni-Ti offers two candidate successors with lower entropies--a monoclinic B19\({}^{\prime}\) martensite and a rhombohedrally distorted martensite (R phase). The R phase can be imaged as a rhombohedral distortion of the cubic austenitic phase, with one of the four [1 1 1] directions of the cubic austenite being stretched. The transformation of the phase occurs in NiTi and shape memory alloys during cooling and heating cycles [6; 7; 8; 9]. The loading causes the martensitic transformation and then induces a superelastic effect in the martensitic phase and returns to the austenite phase during unloading. The functional behaviors of the SME and SE effect of NiTi alloys allow the material to restore 10% of strain [10]. Based on wide-field applications, there are various methods available for the manufacturing of NiTi alloys. One of the general and commercially approved methods offabrication is the casting method. However, this conventional method is a multistep process that undergoes melting, casting, and postprocessing steps until the final product is achieved [11; 12]. Melting may lead to micro- and macrosegregations that could result in chemical inhomogeneity of elemental composition and oxidization during processing. This may lead to an increase in the level of impurities in the final product. The postprocessing step of the machining could harden the NiTi alloy which may be resistant to deformation. These obstacles and limitations during standard manufacture of NiTi alloys have caused researchers to find other simpler, time-saving, effective steps for the manufacture of NiTi alloys [13; 14; 15]. Recently, researchers have focused on alternative routes to achieve the manufacture of NiTi alloys. Powder metallurgy (PM) is the integrated process of compaction and sintering. PM has been emerging to meet the challenge of conventional casting methods by using powder routes [2]. The PM route offers minimum contamination of impurities such as oxygen and carbon contents, without any segregation of phases, thus avoiding chemical inhomogeneity and a one-step process to reach the stage of the final product [16; 17; 18]. PM parts of NiTi alloys show better physical and machining properties due to their finer and homogenous microstructures. However, one of the limitations lies in the area of manufacture of complex shapes by the PM route. There are various routes of PM methods such as Conventional sintering (CS), Self-propagating high-temperature synthesis (SHS), Metal injection molding (MIM), Hot isostatic pressing (HIP), Spark plasma sintering (SPS), and Plasma melting (PM), microwave sintering (MWS) methods [19; 20; 21; 22]. Out of these sintering processes, the spark plasma sintering method stands out due to the short processing time of the reaction process. The SPS process can be feasible at low sintering temperatures, ranging from 900 to 1100 \({}^{\circ}\)C, which allows uniform sintering and avoids any undesired reaction products and high heating rates, with a high consolidation process for the by-products. However, the only limitation is that the process is unable to produce materials with complex shapes.