The investigation of the metallurgical and mechanical characteristics of NiTi shape memory alloys produced with powder metallurgy

In this study, a porous NiTi SMA was produced with by PM and SHS and phase transformations, macro and microstructures and mechanical properties (compression strength) of the specimens were investigated.In the first chapter, brief information about the history of martensitic phase transformation and porous NiTi SMAs is given. In the second chapter phase transformation of SMAs and shape memory phenomenon are dealth with. In the fourth chapter, phase diagrams, crystal structures, martensitic transformations, general properties, use, effect of composition and precipitates on the properties of SMAs and the effect of aging were given. In the fifth chapter, the literature research on porous NiTi SMAs produced by PM technique was presented. In the sixth chapter experimental studies, in the seventh chapter results were given and discussed. In the eight general conclusions and recommendations were given.In the production of SMA, an atomic percent of Ni-49.5Ti was chosen. The mixture of Ni and Ti powders in the given percent was mixed in an apparatus on a lathe with a rotation speed of 16 rpm for 24 hours. The powder mixed was green compacted in a die 10 mm in diameter and 10 mm in hight by 50 MPa, 100 MPa and 200 MPa. Then, the specimens were heated with a heating speed of 15 °C/min up to 100 °C, 200 °C and 350 °C preheating temperatures in a furnace and were ignited by high voltage with a different ignition technique from the literature. To prevent the oxidation, high purity argon was flowed to furnace. Once ignited, combustion wave could self-propagate along the axis to the other end of the compact in a very short time, and then porous NiTi SMA was synthesized. While the specimens green compacted in 100 MPa were synthesized by igniting at every three different preheating temperature, the specimens green compacted in 50 MPa and 200 MPa were synthesized by igniting only at 200 °C preheating temperature. Some of the specimens green compacted by 100 MPa and synthesized at 200 °C preheating temperature were solution treated at 1050 °C under different loads (0 MPa, 3.2 MPa, 6.41 MPa, 9.61 MPa and 12.82 MPa) for 1 h.As a result, it was possible to control the orientation of combustion channel with the applied ignition technique. Thus, it was determined that the highest compression strength was obtained in the specimen in which the combustion channels oriented parallel to the longitudinal axis of the specimens. It was seen that the porosity ratios were reduced when the preheating temperature, the green compact and the loads applied during the solution treatment were increased.Optical microscopy of the synthesized specimen indicated that open and closed pores distributed homogenously within the structure. From EDX and X-ray analyses, it was understood that matrix was mainly (NiTi) austenit, secondary phase was martensite (NiTi) and there were other phase such as Ni4Ti3, NiTi2 and no free Ni, Ti and Ni3Ti phase were determined. DSC measurements indicated martensite in the room temperature and transformation temperatures of austenite and martensite were higher than the human body temperature.It was also understood that the secondary phases decreased after the heat treatment at 1050 °C under various loads. When the load was 6.41 MPa, the structure consisted of only austenite but with removal of the load, austenite and martensite.It was also determined that compression strength increased in case of less porosity and combustion channels aligned longitudinally. Compression strength also increased with the heat treatment under loading.Keywords: NiTi, shape memory alloys, martensite-austenite phase transformation, porous alloys, powder metallurgy, SHS, solution treatment, microstructure, mechanical properties.