# Net-Shape NiTi Shape Memory Alloy by Spark Plasma Sintering Method
## 4 Discussion on the NiTi Alloy in Terms of Polycrystallinity and Composition  
Since our aim was to study the structure of NiTi alloys, the martensitic transformation and shape memory behavior of the spark plasma sintered NiTi alloy, the relation between  
Figure 10: Tensile behavior of sample 1 shows stress versus strain curve. Figure 9: Sintered NiTi alloy at a constant stress of 300 MPa Sample 1 as frequency Tan (delta) storage modulus (MPa) and loss moduli (GPa) as the function of cooling to the heating range of temperature (\(-150\) to \(+100\)\({}^{\circ}\)C). structure of the sintered NiTi compact alloy, and martensitic transformation were studied using thermal and thermo-mechanical characterizations. DSC curves of the SPS sintered NiTi alloy (Figure 4) show no distinct peaks that may be a result of homogenization. NiTi powder shows two endothermic peaks during heating and two exothermic peaks in the cooling mode. The heating peak indicates a B19\({}^{\prime}\)\(\rightarrow\)R and R\(\rightarrow\)B2 transformations and two-step transformation with cooling as B2\(\rightarrow\)R and R\(\rightarrow\)B19\({}^{\prime}\) for the first and second steps, respectively [22; 23]. The wider distribution of peaks in the SPS sample may relate to the elimination of the dislocation networks during homogenization. The decrease in the transformation temperatures in the SPS sample may result from a local increase in Ni content in the B2 matrix and may be due to some local stress field within grains generated due to the compaction of the applied load in the sintering process [24]. Tensile tests on NiTi alloys show three regions of interest: an elastic deformation region, a plateau region related to detwinning of martensite and, finally, a mixed elastic and plastic deformation region of martensite [25]. The room temperature stress-strain curves of the sintered NiTi alloys show decreases in the strain and stress up to failure on tension related to brittle fracture of the surface. This may enhance the behavior upon aging. The SPS sintered NiTi alloy showed transformation temperature peaks at lower temperatures due to an increase in Ni content, and the functional behavior of the SPS sample shows an SME effect with a strain recovery of 3% at a lower temperature of \(-50\)\({}^{\circ}\)C.  
At high temperatures, the grain boundary mobility might become higher than the pore mobility [24]. It causes some pores to be detached from the grain boundary and move into the grains. As it is known, removal of the pores trapped inside the grains is harder than those attached to the grain boundaries, which results in a decrease in relative density. In other words, as the sintering process starts under high pressure and high temperature, the particles become closer, and neck formation occurs between two adjacent grains due to the diffusion phenomenon, which results in intergranular pores between NiTi particles. These kinds of pores formed during sintering have cause a pinning effect on the grain boundaries (GBs) and inhibit grain boundary movement. As the densification continued at the last stage of sintering, some of the intergranular pores shrank and were removed while some others were separated from the GBs and went into the grains, forming intragranular pores [25]. By the elimination of the intergranular pores, the GBs were freed and could move, which resulted in fast grain growth. Therefore, based on the above discussion, it can be concluded that applied pressure in the SPS method promotes the sintering and rearrangement of the particles. In addition, the pressure can act as an opponent factor against gas pressure trapped in the closed pores. Therefore, the porosities were removed more easily than those in the conventional sintering method. Some GBs and intergranular pores were formed between the particles. By raising the sintering temperature, fewer pores are detected, and grain growth occurs. As mentioned before, the existence of the pores on the GBs pins them and retards fast grain growth.