Kendiliğinden ilerleyen yüksek sıcaklık sentezi (SHS) ile şekil hafızalı Ni-Ti-X (X= Cu, Nb, Fe vb.) alaşımların üretimi ve karakterizasyonu
Production and characterization of shape-memory Ni-Ti-X (X= Cu, Nb, Fe etc.) alloys by self propagating high temperature synthesis (SHS)
Yazar: BERK KESKİN
Danışman: PROF. DR. CEVAT BORA DERİN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Metalurji ve Malzeme Mühendisliği Bilim Dalı
Derece: Doktora, 2022
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Akıllı malzemeler = Intelligent materials ; Malzeme üretimi = Material production ; Metalik malzemeler = Metallic materials ; Pirometalurji = Pyrometallurgy ; Sıcaklık sentez yöntemi = Temperature synthesis method ; Toz metalurjisi = Powder metallurgy ; Şekil hafızalı alaşımlar = Shape memory alloys

Özet:
Şekil hafızalı alaşımlar, deformasyona uğratıldığında, uygulanan sıcaklık ve gerilmelere bağlı olarak daha önceki şekil veya boyutuna geri dönebilme özelliği gösteren bir grup metalik malzemedir. Şekil hafıza özelliği ve süperelastik özellik, şekil hafızalı alaşımların en önemli iki termomekanik özelliğidir. Şekil hafızalı alaşımların temel karakteristiği, alaşıma giren elementlerin oranlarıyla belirlenebilen bir dönüşüm sıcaklığının üzerinde ve altında farklı iki şekil ve kristal yapısına sahip olabilmeleridir. Bu belirlenebilen sıcaklığın üzerindeki sıcaklıklarda östenitik yapı (ana faz), altındaki sıcaklıklarda ise martenzitik yapı oluşmaktadır. 
İlgili çalışmada şekil hafızalı alaşım özelliklerine sahip Ni-Ti-Cu, Ni-Ti-Nb ve Ni-Ti-Fe sistemlerinin çok hızlı, düşük maliyetli ve yüksek katma değere sahip bir üretim yöntemi olan kendiliğinden ilerleyen yüksek sıcaklık sentezi (Self-Propagating High-Temperature Synthesis - SHS) ile üretimi ve bu ürünlerin morfolojik ve termal özelliklerin incelenmesi amaçlanmıştır.
Bu bağlamda öncelikle referans oluşturulacak şekilde Ni-Ti alaşımlarının üretimi ve üçüncü bir element (Cu, Nb ve Fe) ilavesinin (Cu için at. %25'a kadar, Fe ve Nb için at. %10 a kadar) SHS sentezine ve nihai ürüne olan etkilerinin sistematik şekilde araştırılması hedeflenmiştir. İlk defa SHS senteziyle üretilmesi ve bakırın nikel yerine yüksek miktarlarda kullanımının (at.% 25 e kadar) ürün üzerindeki etkilerinin araştırılmıştır.
Çalışmada mikroanaliz, faz değişimi ve diğer kimyasal kompozisyon belirleme işlemleri XRD, DSC, SEM/EDS ve TEM cihazlarında gerçekleşmiştir.

Abstract:
Shape Memory Alloys (SMAs) are a group of metallic materials, which recover their shapes and dimensions after deformation according to the temperature and strain applied. Shape memory effect and super elasticity are the main two termomechanic properties of shape memory alloys. Fundamental characteristics of shape memory alloys are achieving two different shape and crystalline structures above a transformation temperature that is determined by the proportion of the alloying elements. At the temperatures above this specific temperature: austenitic structure (matrix phase); above this specific temperature: martentisic structure is formed.
In this study; in which, Ni-Ti-Cu, Ni-Ti-Nb ve Ni-Ti-Fe systems that have shape memory alloy properties are planned to be produced by self-propagating high temperature synthesis (SHS) process which is a rapid, low cost and high efficiency production method. Morphological and thermomechanical properties of the ultimate products will be investigated via different characterization methods. Firstly, it is planned to produce various composition of the ternary Ni50-xTi50Cux, Ni50Ti50-xNbx and Ni50-xTi50Fex alloys. Secondly, the effects of addition of a ternary element to the SHS process and to the ultimate products will be systematically investigated. 
Microstructural, phase and other chemical composition analysis of the ultimate products will be examined using XRD, SEM/EDS techniques. Moreover, in order to determine the inner structures and to examine the structural, chemical and morphological distribution of the products at micro and nano scale, it is planned in the proposed project to carry out FIB slice and view and SEM/TEM analyses respectively. 
The SHS experiments were carried out with a specially designed reactor. Before the SHS process started, in order to minimize the oxygen partial pressure, the reactor chamber was vacuumed to a pressure of 10-3 mbar and then flushed by high purity argon two times. Two manually-controlled variacs were employed for the process. While the first one was used to keep the preheating temperature of the samples stable, the second one was used to ignite the SHS reaction. 
In the first trials, It has been understood that the mixing process of raw material powders is very important because the powders do not perfectly touch each other. Therefore, the combustion reaction does not take place completely, and besides the formation of high amounts of NiTi2, Ni3Ti, by-products such as unreacted free Ni and Ti or their oxide forms may occur. For this reason, the mixing was carried out using ZrO2 balls. The trigger voltage-current values and duration of the W wire are also very important parameters. Heating the W wire at a slow rate or heating it more or less can cause differences in porosity results and errors in preheating temperature, cause the sample to collapse by melting deform or stick to the sample as a result of rupture. In each experiment, the W trigger wire should be of fixed dimensions (20 turns in 2.5 mm diameter, 28 mm total length), stay 4-5 mm away from the sample in the reactor and in parallel. The power was kept nearly constant (650-700 W) for the ignition wire, until the reaction was started (3-8 seconds for low at.% alloyed NiTiX.).
In the section of NiTiCu; the substitution of copper for nickel in Ni-Ti system in order to obtain a molar ratio of Ni40-xTi50Cux (x=10, 15, 20, 25). Effects of preheating temperature were studied to understand the morphology, phase transformation, and microstructure of the samples by using Self Propagating High-Temperature Synthesis (SHS). Therefore, three distinct preheating temperatures (230 °C, 320 °C and 410 °C) were used for the experiments. The thermochemical calculations performed with FactSage presented similar results with the experimental data in terms of solid-liquid ratios and adiabatic temperature during the reactions. The samples (%10 to %25 (at.) Cu) were successfully produced under preheating temperatures of 230, 320 and 410 °C, and it was observed that all of them had shape memory effects as a result of DSC analysis. 
It was observed that the reaction could not easily propagate with high copper ratio (>%10 Cu, at.), therefore the trigger wire was driven for a longer time. As a result, the structure was denser at the lower preheating temperatures. The propagation rate of the flame was also quite slow. Up to 18 times the rate difference was determined between the sample produced at 230 °C and the sample produced at 410 °C for the %25 Cu atomic ratio.
In the DSC analyses, an increase in the preheating temperature slightly changed the transformation temperature, but it was shown to be insignificant. Martensite start temperature was increased, when atomic ratio of the copper that was substituted for nickel position, was slightly increased. The temperature was not clearly identified after adding %15 (at.) Cu to the B2 phase for the some samples due to the broadening. Because inhomogeneities were occurred in the microstructure. The samples (>%15 Cu) showed nearly no porosities for any preheating temperature. 
In the XRD analyses, the peaks show broadening by increasing Cu ratio in B2 phase. The increase in micro strains may be the reason for it. The propagation rate of the flame was too slow and it was not propagated by itself, therefore the ignition wire was continuously driven. It proved that Cu reduced driven force the propagation of the reaction. B2 crystal structure was found as the main phase besides a small amount of martensite, Ti2Ni(Cu), and Ni(Cu)2Ti-Ni(Cu)3Ti by various characterization methods. In the sample that contains %25 Cu (at.), Ti2Ni phase was increased. The monoclinic twinned martensitic (B19') structure was encountered for Ni40Ti50Cu10 in Transmission Electron Microscopy (TEM) analyses.  
In the section of NiTiFe; two compositions were selected. Ni45Ti50Fe5 and Ni40Ti50Fe10 and successfully synthesized, starting from elemental Ni, Ti and Fe powders. Three preheating temperatures (230, 320, and 410 °C) were investigated. 
The reactions were triggered with W wire in the experiments as the NiTiCu experiments, and it was proved that triggered time was also an important parameter for the sustainability of the synthesis. In the literature, instead of the ignition wire, single shot charged arc or laser equipped system can be used for the NiTi system, but it is not suitable for alloyed NiTi(Fe) system because the reaction is weakly sustained by itself. It needs more energy. Therefore, the triggered wire should be ignited for longer time to supply more heat energy for the propagation. In the Ni40Ti50Fe10 (S230d10) sample operated at 230 °C, the preheating temperature was not somehow sufficient and the system was needed to be triggered twice for 15 seconds each. Therefore, the shrink is greater than the samples with the same iron atomic ratio. It was proved that the trigger wire was very important parameter for the ternary (NiTiFe) alloyed system.
Selected samples were further analysed by various characterization techniques. X-Ray diffraction (XRD) analyses demonstrated the presence of B->R phase for the composition of Ni45Ti50Fe5. In specimens with the lower Fe content, whilst in specimen with higher Fe content, martensitic transformation was suppressed. The Ti2Ni phase was reduced as instantaneous temperature increased, according to scanning electron microscopy (SEM) and XRD analysis.
In the section of NiTiNb; two compositions were selected, Nb was substituted for Ti atomic position, and the samples (Ni50Ti45Nb5  and Ni50Ti40Nb10) were synthesized for the 3 distinct preheating temperatures (230, 320 and 410 °C). In all of these samples, the reaction propagation was smooth and rapid. However, no shape memory effect was observed in any of the %5 (at.) and %10 (at.) addition samples containing Nb. In the SEM analysis, Nb enrichment was found at grain boundaries. Although no clear shape memory effect was found in DSC analyses of Nb-containing samples, B19' was observed in TEM analyses and vaguely XRD analyse.
As a result of this thesis, it has been understood that the production of these ternary systems is possible by SHS. However, the mechanical properties of the products should be examined in detail. This study topic still needs to be developed. In the next study, it is necessary to transfer more extensive information to the literature by obtaining products with optimized ratios and performing detailed tests such as dilatometry and mechanical test.