SHS yöntemiyle tufal ve hematit kaynağı kullanılarak Fe3Al intermetalik bileşiğinin üretimi ve Ni, Cr, W ilavelerinin etkileri
Production of Fe3Al intermetallic compound using scale and hematite source by SHS method and the effects of Ni, Cr, W additions
Yazar: GİZEM ÖZÖZTÜRK BARUTÇU
Danışman: PROF. DR. CEVAT BORA DERİN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Malzeme Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Hematit = Hematite ; Metal bileşikler = Metal complexes ; Sıcaklık sentez yöntemi = Temperature synthesis method

Özet:
Bu yüksek lisans tez çalışmasında; kendiliğinden ilerleyen yüksek sıcaklık sentezi yöntemi (SHS) ile demir kaynağı olarak tufalin kullanıldığı Fe3Al intermetalik bileşiğinin üretilmesi, Fe3Al intermetalik bileşiğine Ni, Cr ve W alaşım elementlerinin kütlece % 5, % 10 ve % 15 olacak şekilde eklenmesi ve mekanik özelliklerine etkisinin incelenmesi planlanmıştır. Ayrıca demir kaynağı olarak hematit kullanılan Fe3Al intermetalik bileşiğinin üretilmesi ve kütlece % 10 Ni, Cr ve W alaşım elementlerinin eklenerek, demir kaynağı olarak tufal kullanılan deney sistemleri ile mekanik özelliklerinin karşılaştırılması planlanmıştır. SHS; düşük enerji gereksinimi, düşük üretim maliyeti, kısa işlem süresi, az enerji tüketimi gibi avantajlara sahip olması sebebi ile üretim yöntemi olarak tercih edilmiştir.
İntermetalik bileşikler; iki veya daha fazla metalin uygun stokiyometrik oranda birleşmesiyle oluşur. Yüksek sıcaklıklarda yüksek mukavemet ve sertliğe sahip malzemelere olan talep, intermetalik bileşiklere olan ilgiyi daha da artırmıştır. Demir alüminidler en popüler intermetalik bileşikler arasındadır. Yüksek ergime noktaları, yüksek sertlik değerleri, düşük yoğunlukları, yüksek oksidasyon ve korozyon dirençleri sayesinde yüksek sıcaklıklarda yapısal uygulamalarda kullanılmak için sıklıkla çalışılmaktadır. D03 kristal yapısına sahip olan Fe3Al, en kararlı demir alüminidlerden biridir. Fiziksel ve mekanik özelliklerinden dolayı yüksek sıcaklıklarda kullanılmaya aday malzemeler arasındadır. Ancak oda sıcaklığında düşük sünekliği ve kırılma tokluğu nedeniyle kullanım alanlarında kısıtlamalar vardır. Son yıllarda demir alüminidlerin mekanik özelliklerini iyileştirmeye yönelik başarılı çalışmalar yapılmıştır. Bu çalışmalar, kompozisyon ve mikro yapı kontrolü ile oda sıcaklığında sünekliğin % 10-20'ye, 600 oC'de akma dayanımının ise 500 MPa'ya yükseldiğini göstermektedir. Sisteme eklenen alaşım elementlerinin; D03↔B2 dönüşüm sıcaklığını lineer olarak arttırdığı gözlemlenmiştir. Dönüşüm sıcaklığı arttıkça, yüksek sıcaklık direnci sağlanmaktadır. 
SHS yöntemi, sisteme verilen ilk enerji ile ekzotermik reaksiyonun başlatılmasına ve reaksiyon nedeniyle açığa çıkan yanma dalgası ile kendi kendine yayılmasına dayanır. Sistem, kimyasal reaksiyonlar sonucu oluşan ısı ile kendi enerjisini sağlar. Böylece malzeme üretiminde dış kaynaklardan sağlanması gereken yüksek enerjiye ihtiyaç duyulmaz. Reaksiyonun çok hızlı gerçekleşmesi ve kendi enerjisi ile ilerlemesi nedeniyle ekonomiktir. 
Sürekli döküm tesislerinde gerçekleştirilen çelik üretiminde, çelik külçelerin soğuması sırasında çelik yüzeyinde bir tabaka oluşur. Çelik yüzeylerde oluşan bu oksit tabakasına tufal denir. Tufal yaklaşık % 70 oranında demirden oluşur ve yapısında wüstit ve hematit gibi demir oksitler bulunur. Tufalin yüksek orandaki demir içeriği nedeniyle demir kaynağı olarak geri kazanımı çok önemlidir. 
Bu çalışmada öncelikle FactSage 7.1 termokimyasal modelleme programı ile simülasyon çalışmaları yapılmıştır. Termodinamik incelemeler için program bünyesinde bulunan modüller kullanılarak, SHS deneyleri sonucu üretilecek ürünlerin ağırlıkları ve elemental analizleri, deney esnasında açığa çıkacak ısı miktarları ve adyabatik sıcaklık değerleri hesaplanmıştır ve çıkan tüm sonuçlar deneysel kısımda paylaşılmıştır. Termodinamik incelemeler sonucu deney sistemleri planlanmış ve hammaddeler hazırlanarak deneysel çalışmalara başlanmıştır. 
Deneysel çalışmaların ilk aşamasında; demir kaynağı olarak tufalin kullanıldığı deney sistemleri gerçekleştirilmiştir. Atık olarak değerlendiren ve çelik üretimi esnasında tonlarca açığa çıkan tufal; yüksek oranda demir içermektedir. Demir kaynağı olarak tufali kullanmak; geri dönüşüme katkı sağlamanın yanında, hammadde ve üretim maliyetlerini düşürmektedir. Ni, Cr ve W alaşım elementlerinin ve miktarlarının SHS ürünlerine olan etkisini incelemek için saf ve kütlece % 5, % 10 ve % 15 olacak şekilde alaşım elementlerinin eklendiği 10 adet deney yapılmıştır. Bu deneylerde; Fe3Al, Fe3Al-(5Ni), Fe3Al-(10Ni), Fe3Al-(15Ni), Fe3Al-(5Cr), Fe3Al-(10Cr), Fe3Al-(15Cr), Fe3Al-(5W), Fe3Al-(10W), Fe3Al-(15W) elde edilmek istenmiştir. 
İkinci aşamada; demir kaynağı olarak tufal yerine hematit kullanılmıştır. İlk aşamada yapılan deneyler sonucu alaşım elementi olarak eklenecek Ni, Cr ve W için ideal değer % 10 olarak belirlenmiştir. Bu aşamada 4 adet deney gerçekleştirilmiştir. Bu deneylerde; Fe3Al, Fe3Al-(10Ni), Fe3Al-(10Cr), Fe3Al-(10W) elde edilmek istenmiştir. 
Üçüncü aşamada ise, tufal kullanılan deney sistemlerinde alaşım elementi miktarlarının SHS ürünlerine olan etkisini incelemek ve hematit kullanılan SHS ürünleri ile tufal kullanılan SHS ürünlerinin özelliklerini karşılaştırabilmek için analizler yapılmıştır. 
Deneyler sonucu elde edilen ürünler ile farklı analizler gerçekleştirmek için numuneler hazırlanmıştır. Numunelerin; verimlilik analizleri, X-ışınları difraksiyonu (XRD) ile faz analizleri, Vickers ile mikrosertlik ölçümleri, X-ışınları floresans spektrometresi (XRF) ile kimyasal analizleri, SEM ve EDS ile mikroyapı analizleri yapılmıştır. Analiz sonuçları; deney sonuçları ve sonuçların irdelenmesi kısmında detaylıca paylaşılmıştır.

Abstract:
Intermetallic compounds are the materials that have been used since the beginning of metallurgy. Intermetallic compounds are formed by the combination of two or more metals in the appropriate stoichiometric ratio. The demand for materials with high strength and hardness at high temperatures has further increased the interest in intermetallic compounds. Iron aluminides are among the most popular intermetallic compounds. They have frequently been studied for structural applications at high temperatures thanks to their high melting point, high hardness, low density, high oxidation, and corrosion resistance. Fe3Al, which has a D03 crystal structure, is one of the most stable iron aluminides. Due to its physical and mechanical properties, it is among the candidate materials to be used at high temperatures. However, there are limitations in usage areas due to its low ductility and fracture toughness at room temperature. In recent years, there have been successful studies to improve the mechanical properties of iron aluminides. These studies show that with composition and microstructure control, the ductility at room temperature increases to 10-20 %, and the yield strength at 600 oC increases to 500 MPa. The alloying elements added to the system have also been observed to increase the D03↔B2 transformation temperature linearly. As the transformation temperature rises, high-temperature resistance is provided. 
The issue of saving energy is of great importance for the industry as it is in every field, and the advancement of energy-saving technologies has become more critical day by day. The self propagating high temperature synthesis (SHS) method is a unique and straightforward method that does not require additional energy after the start of the process and is used to produce advanced technology materials such as composites, intermetallic compounds, and advanced ceramics. SHS is based on starting the exothermic reaction with the first energy given to the system and self-propagating with the combustion wave released due to the reaction. The system provides its own energy with the heat generated due to chemical reactions. The reaction takes place very quickly and progresses with its energy. These are the essential advantages of the SHS method.
In steel production carried out in continuous casting facilities, a layer forms on the steel surface during the cooling of the steel ingots. This oxide layer formed on steel surfaces is called scale. Approximately 13.5 million tons scale is produced in a year when we look at the whole world in steel production. Leaving these wastes to the environment causes heavy metals with toxic effects to pass into the soil with water by rain, snow and from there it causes damage to the human and animal health. The scale consists of approximately 70 % iron and contains iron oxides such as wurtzite and hematite in their structure. Recovery of scale as an iron source is essential due to its high iron content and environmental damage.
This master's thesis is aimed to produce Fe3Al intermetallic compound in which scale is used as an iron source by self-propagating high-temperature synthesis method, to add Ni, Cr, and W alloy elements to Fe3Al intermetallic compound as 5 %, 10 %, and 15 % by mass, and to examine the effect on its mechanical properties. In addition, it is planned to produce a Fe3Al intermetallic compound in which hematite is used as the iron source and to compare its mechanical properties with experimental systems in which scale is used as an iron source by adding 10 % Ni, Cr, and W alloying elements by mass. Self propagating high temperature synthesis method has been preferred as a production method because it has advantages such as low energy requirement, low production cost, inexpensive equipment, short processing time, and low energy consumption.
In experimental studies, scale and hematite were used as iron sources, Al as reducer, Ni, Cr, and W as alloying elements. The most important purpose of the test systems in which scale is used as an iron source is scale recycling. The most important purpose of the experimental systems in which hematite is used as an iron source is to examine the effect of impurities from the scale on SHS products by making comparisons.
In the first stage of experimental studies, experimental systems were carried out in which scale was used as an iron source. Ten experiments were conducted in which pure and 5 %, 10 %, and 15 % alloying elements by mass were added to examine the effect of Ni, Cr, and W alloying elements and their amounts on SHS products. In these experiments, Fe3Al, Fe3Al-(5Ni), Fe3Al-(10Ni), Fe3Al-(15Ni), Fe3Al-(5Cr), Fe3Al-(10Cr), Fe3Al-(15Cr), Fe3Al-(5W), Fe3Al-(10W), Fe3Al-(15W) was desired to be obtained. 
In the second stage, hematite was used instead of scale as an iron source. As a result of the experiments carried out in the first stage, the ideal value for Ni, Cr, and W to be added as alloying elements was determined as 10 %. At this stage, four experiments were carried out. In these experiments, Fe3Al, Fe3Al-(10Ni), Fe3Al-(10Cr), Fe3Al-(10W) were desired to be obtained.
In the third stage, different analyses were made to examine the effect of alloying element amounts on SHS products in scaled test systems and compare the properties of hematite SHS products and scale SHS products.
Before starting the experiments, thermodynamic simulation studies were carried out with the FactSage 7.1 thermochemical database program. As a result of these studies, the amount of raw materials, adiabatic temperature values, and the products to be formed were modeled for the planned experimental systems. Also, it has been observed that the test systems can progress spontaneously. All the results were shared in the experimental part.
During the experiments studies, powders were used as raw materials in experiments were weighed with precision balance according to the amounts in the planned experimental systems. The weighed powders were kept in an oven at approximately 105 oC for 30 minutes to be dried. Then they were mixed with a mixer for 30 minutes to become homogeneous. Raw materials ready for SHS experiments; The raw materials that were ready for the SHS experiments were poured into fireclay crucibles and thoroughly compacted with the help of a mallet. The winding made of tungsten (W) resistance wire is connected to the end of the copper cables connected to the power source, which provides the necessary energy for the SHS experiments. This winding fireclay was placed in the crucible to touch the powder, and the mouth of the crucible was closed with a graphite cover. 
The power source was turned on to start SHS reactions with the transmitted energy, and the first ignition was carried out. With the initiation of the reaction, the power source was turned off, allowing the reaction to proceed with self-propagating. After the reaction was completed, the ventilation system was opened, and the crucible was left to cool under normal conditions. SHS products were removed from the crucible by breaking the cooled fireclay crucible with the help of a hammer.
For the results of experiments studies, samples were prepared from the products obtained as a result of SHS experiments. Analyzes made on samples were efficiency analysis, phase analysis with X-ray diffraction (XRD), microhardness measurements with Vickers, chemical analysis with X-ray fluorescence spectrometry (XRF), microstructure analysis with SEM, and EDS. 
According to efficiency analysis, the alloying element amounts did not cause a significant change in metal gains, and the efficiency in scale test systems is higher than the efficiency in hematite test systems. Ni, Cr, and W alloying elements added to the Fe3Al intermetallic compound increase the hardness values of the compound according to the microhardness test. Ni was the alloying element that increased the hardness the most, Cr increased it the least. 
The chemical analysis results made with XRF, and the elemental analysis values obtained according to the modeling made in the FactSage 7.1 program were compared. According to the result of XRF analysis, it was predicted that relative values were obtained, and the experiments were successful. 
XRD analysis showed that samples have contained ordered BCC-D03, ordered BCC-B2, and disordered BCC-A2 crystal structures of Fe-Al intermetallic compounds and  phases. That µ phase is formed by including the W resistance wire in the system. It is thought that the acicular light-colored structures seen in SEM/BEI images belong to this µ phase. In XRD analysis of some samples, a phase containing Fe, Al, and O was found. On the SEM/SEI images of these samples, it was observed that there were light-colored bubbles on their surfaces. It was predicted that there was no excellent metal-slag separation in these samples and that spinal phases were formed as a result. XRD and SEM-EDS analyzes are consistent with each other. 
According to the analysis results, Fe3Al intermetallic compound could be produced successfully by the SHS method. These analysis results are shared in detail in the experiment results and the analysis of the results part.