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    Synthesis and microstructure investigation of Ni40Ti50Cu10 intermetallic shape memory alloys by self-propagating combustion method
    (Springer India, 2022) Keskin, Berk; Bassani, Paola; Bakan, Feray; Sezen, Meltem; Derin, Bora
    This study concerns the substitution of copper for nickel in the Ni-Ti system in order to obtain a molar ratio of Ni40Ti50Cu10. Effects of preheating temperature were studied to understand the morphology, phase transformation, and microstructure of the samples by using self-propagating high-temperature synthesis. Therefore, three distinct preheating temperatures (230 degrees C, 320 degrees C, and 410 degrees C) were used for the study. The thermochemical calculations performed with FactS age presented similar results with the experimental data in terms of solid-liquid ratios and adiabatic temperature during the reactions. An increase in the preheating temperature very slightly changed the transformation temperature, but it was shown to be insignificant. B2 crystal structure was found as the main phase besides a small amount of martensite, Ti2Ni(Cu), and Ni(Cu)(2)Ti-Ni(Cu)(3)Ti by various characterization methods. The monoclinic twinned martensitic (B19') structure was encountered in transmission electron microscopy analyses.
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    The efficiency assessment of Ni(40)Ti(50)Cu(x)Fe(10-x) shape memory alloy produced by combustion synthesis through thermal analysis
    (2024) Keskin, Berk; Derin, Bora
    In this investigation, Ni(40)Ti(50)Cu(x)Fe(10−x) (x = 2.5, 5, 7.5) alloys were synthesized through the self-propagating high-temperature synthesis (SHS) process, employing preheating temperatures of 240 and 450 °C. To optimize thermal analysis, samples were drawn from the middle and upper sections of specimens exhibiting superior propagation results at a preheating temperature of 450 °C. Omitting additional heat treatment ensured a precise result interpretation. While the Ti50Ni40Fe2.5Cu7.5 sample displayed minimal variations in martensite and austenite transformation temperatures, others exhibited diverse transformation profiles. Apart from the B19' martensitic phase, distinct B19 martensitic phases were identified in Ti50Ni40Fe5Cu5 and Ti50Ni40Fe2.5Cu7.5 alloys, each showcasing unique transformation temperatures and full-width at half-maximum (FWHM) values. The inquiry unveiled that an increased Fe content prompted the segregation of Fex(Ni)Ti-based intermetallic phases from Cu-based phases, with this effect intensifying in alloys featuring higher Fe atomic ratios. This influence significantly impacted transformation temperature outcomes, overshadowing the inherent impact of the synthesis method.
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    Thermodynamic analysis of production parameters and microstructural evolution in shape memory Ni(50-x)Ti(50)Fe(x) (x = 5, 10) alloy synthesized by combustion synthesis
    (Springer Science and Business Media Deutschland GmbH, 2024) Keskin, Berk; Derin, Bora
    This study represents an initial effort to produce NiTiFe shape memory alloys via the self-propagating high-temperature synthesis (SHS) process. The synthesis successfully yielded Ni45Ti50Fe5 and Ni40Ti50Fe10 alloys from elemental Ni, Ti, and Fe powders at three distinct preheating temperatures (240, 330, 420 °C). To support empirical findings, thermodynamic analysis using Factsage Thermodynamic Software was employed to correlate reaction propagation behavior with chemical composition. The calculation showed that the addition of 5 at.% Fe to the B2 phase did not hinder reaction self-propagation. This conclusion was supported with the ?Hf/Cp ratio and transient liquid ratio, computed using the sub-lattice model, which closely resembled that of NiTi. Whereas 10 at.% Fe that synthesized at preheating temperature of 240 °C exhibits struggle, thus an increase in triggering time causes an effect on crystallite size and a decrease in porosity. Empirical results confirmed these findings, albeit influenced by ignition times. An increase in the liquid ratio due to the adiabatic temperature rise can also result in a reduction of NiTi content when the Fe ratio is increased, consequently diminishing the driving force for the reaction. In the sample containing – 5% Fe, the main phase is B2, and the R martensitic phase is also present. Ms is determined to be – 38.7 °C. SEM analyses revealed the presence of Ti2Ni and Ti2Ni3 phases in the upper and lower regions, while the distribution in the middle section is more homogeneous. No martensitic transformation was observed in 10% Fe. Additionally, nanocrystalline regions were detected within the samples by transmission electron microscopy, contributing to a nuanced understanding of their structural properties. Graphic abstract: (Figure presented.).