Mekanik alaşımlanmış Co18Cr10Fe10Ni47Al10Ti5 yüksek entropili alaşımının spark plazma sinterleme ile üretimi ve karakterizasyonu
Production and characterization of mechanically alloyed Co18Cr10Fe10Ni47Al10Ti5 high entropy alloy by spark plasma sintering
Yazar: AYBÜKE İZCİ
Danışman: PROF. DR. GÜLTEKİN GÖLLER
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Malzeme Bilimi ve Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Yeni malzeme grubu olarak keşfedilen yüksek entropili alaşımlar en az beş farklı elementin yapıya katılmasıyla elde edilerek çok bileşenli alaşımlama ile üretilmektedirler. Yüksek entropi etkisi, yavaş difüzyon etkisi, şiddetli kafes distorsiyonu ve kokteyl etkisi olarak adlandırılan dört temel etki sayesinde yüksek sıcaklıklarda yüksek mukavemet, yüksek korozyon direnci, yüksek aşınma direnci ve yüksek termal direnç gibi özellikleri sayesinde birçok alanda kullanılmaktadır. Jeotermal alanlarda, nükleer uygulamalarda, otomotiv ve motor malzemelerinde üstün özellikleri sayesinde kullanılmaktadır.
Yüksek entropili alaşımların üretilmesinde üretim yöntemi olarak döküm ve toz metalurjisi dikkat çekmektedir. Ancak katılaşma prosesi kontrolünün zorluğu, döküm kusurları, denge fazlarının bastırılması, artık gerilmeler, çatlaklar ve segregasyon nedeniyle istenilen malzeme özelliklerine döküm ile kolaylıkla ulaşılamamaktadır. Bu sınırlamalar, mekanik alaşımlamanın (MA) toz halinde daha iyi kimyasal homojenliğe sahip stabil bir mikroyapı geliştirme ve ardından konsolidasyon yeteneği sayesinde toz metalürjisi yöntemi kullanılarak aşılabilmektedir. Spark plazma sinterleme (SPS) ile daha düşük sıcaklıklarda, kısa sürede, teorik yoğunluğa sahip YEA'lar elde edilen ve mekanik alaşımlama sonrası yapıya karışmayan elementel yapının SPS sonrası tamamen alaşımlanmaya katılmasını sağlaması sebebiyle son zamanlarda ön plana çıkan bir yöntemdir. Ni bazlı yüksek entropili alaşımlar; yüksek sıcaklıkta yüksek dayanım, yüksek mukavemet, yüksek sertlik, yüksek aşınma direnci ve metal alaşımlarından daha hafif olmaları gibi özellikleri sayesinde günümüzdeki en önemli malzeme grupları olarak değerlendirilmektedir. Bu çalışma kapsamında daha önce çalışılmamış olan Ni bazlı Co18Cr10Fe10Ni47Al10Ti5 yüksek entropili alaşım Metalurji ve Malzeme Mühendisliği Bölümü'nde bulunan SpexTM 8000D Mixer/Mill mekanik alaşımlama cihazı ve 20.000 A kapasiteli SPS 7.40 MK VII, SPS Syntex Inc. model spark plazma sinterleme (SPS) cihazı ile üretilmiştir ve konsolide edilmiştir.
Deneysel çalışmalar kapsamında Co18Cr10Fe10Ni47Al10Ti5 YEA mekanik alaşımlama parametreleri optimize edilerek üretilmiştir. Optimize edilen mekanik alaşımlanmış YEA'ın faz analizi ve mikroyapı karakterizasyonu gerçekleştirilmiştir. Daha sonrasında farklı sinterleme sıcaklıklarında (1000°C, 1100°C ve 1200°C) sabit basınç altında (40 MPa) ve sabit ısıtma hızında (100°C/dak) YEA konsolide edilmiştir. Farklı sinterleme sıcaklıklarında konsolide edilen numunelerin karakterizasyonunu yapmak için yoğunluk ve Vickers mikrosertlik değerleri ölçülmüştür ve faz analizi ve mikroyapı karakterizasyonu gerçekleştirilmiştir.  
Deneysel çalışmalar sonucunda artan sinterleme sıcaklığının relatif yoğunlukta artış (96.13%, 97.01%, 97.32%) meydana getirdiği gözlenmiştir. Relatif yoğunluktaki artış, porozite miktarının artan sıcaklıkla birlikte azalması ile meydana gelmiştir. Artan sinter sıcaklığıyla birlikte yapılan sertlik ölçümlerinde; oluşan intermetalikler ve tane büyümesi (0.123 mm, 0.147 mm, 0.573 mm) ile ilişkili olarak sırasıyla 724.05HV, 691.3HV ve 657.26HV değerleri elde edilmiştir. Yapılan mikroyapı incelemelerinde intermetalik yapı, gözenekli yapı ve tane büyümesinin mikrosertlik ve yoğunluk değerlerindeki artış ve azalışa neden oldukları belirlenmiştir.

Abstract:
High-entropy alloys, discover as a new material group, are produced by multi-component alloying by adding at least five different elements into the structure. Depending on the ductile - ductile, ductile - brittle, brittle - brittle combinations of the elemental powders used, changes occur in the cold welding, breaking and re-welding processes. It is used in many areas thanks to its properties such as high strength at high temperatures, high corrosion resistance, high wear resistance and high thermal resistance, thanks to four effects called high entropy effect, slow diffusion effect, severe lattice distortion and cocktail effect. It is used in geothermal areas, nuclear applications, automotive and engine materials thanks to its superior properties.
Casting and powder metallurgy attract attention as production methods of high entropy alloys. However, the desired material properties cannot be easily achieved by casting due to the difficulty of solidification process control, casting defects, suppression of equilibrium phases, residual stresses, cracks and segregation. These limitations can be overcome using the powder metallurgy method, thanks to the ability of mechanical alloying (MA) to provide a stable microstructure with better chemical homogeneity in the powder form and subsequent consolidation such as spark plasma sintering (SPS). Mechanical alloying is the process of crushing powder particles in high energy mills. It is a cost-effective solid-state powder production method that is alloyed by sequential cold welding, breaking and re-welding processes. With the high mechanical energy applied, energy is transferred between two or more amounts of powder and the diffusion distances decrease. In this way, it is a process that can be carried out by reducing grain and particle sizes with a homogeneous distribution. For this reason, it is aimed to obtain homogeneous powder by mechanical alloying. SPS is preferred because it produces HEAs with theoretical density in a short time at lower temperatures. It is a method that has emerged recently because it ensures that the elemental structure, which does not mix with the structure after mechanical alloying, is completely included in the alloying after SPS.
Ni-based high entropy alloys; they are considered to be the most important material groups, thanks to their properties such as high strength at high temperatures, high strength, high hardness, high wear resistance and being lighter than metal alloys. Within the scope of this study, the Ni-based Co18Cr10Fe10Ni47Al10Ti5 high entropy alloy, which has not been studied before, was produced by the SpexTM 8000D Mixer/Mill mechanical alloying device and the 20000 A capacity SPS 7.40 MK VII, SPS Syntex Inc in the Department of Metallurgical and Materials Engineering, ITU. 
Within the scope of experimental studies, thermodynamic calculations were carried out since it is necessary to understand the phase and structure of the high entropy alloys. As a result of thermodynamic calculations, FCC and intermetallic phases were designed due to the Ω and δ parameters, and the VEC parameter was designed expecting that the FCC phase could be formed. For this reason, as a result of the study, FCC solid solution and intermetallic phases were expected to be present in the structure at the same time. 
Co18Cr10Fe10Ni47Al10Ti5 high entropy alloy (HEA) was produced by optimizing mechanical alloying parameters. Mechanical alloyed powders obtained after optimized mechanical speed and time. Phase analysis and microstructure characterization of the optimized mechanically alloyed HEA was carried out. At the end of 5 hours of mechanical alloying, the XRD peaks of all powders completely disappeared, while the Ti peak intensity decreased to a certain extent as the mechanical alloying time increased, but the Ti peak intensity did not completely disappear. After mechanical alloying, Ni-based FCC solid solution and Ti elemental XRD peaks were detected. 
Then, HEA was consolidated at different sintering temperatures (1000°C, 1100°C and 1200°C), under constant pressure (40 MPa) and constant heating rate (100°C/min). To characterize the samples consolidated at different sintering temperatures, density and Vickers microhardness values were measured, and phase analysis and microstructure characterization were performed. After sintering, XRD and SEM-EDS results clearly were detected FCC solid solution phase, AlFe3, TiC, Al2Ti intermetallic phases at sintering temperatures and pressures of 1000°C, 40 MPa and 1100°C, 40 MPa while FCC solid solution phase, TiC, Al3Ti intermetallic phases were detected at sintering temperature and pressure of 1200°C, 40 MPa. Lattice strain was expected to increase due to mechanical alloying at high mechanical alloying speed and time causing incompatibility between elements of different sizes and increasing crystallite defects. For this reason, the fact that Al and Fe are separated from the structure and easily diffused into each other explains how the intermetallic structure was formed. At 1200°C sintering temperature, the increase in the peak intensities of the FCC phase and the disappearance of the AlFe3 intermetallic phase peak indicate that the FCC solid solution phase, which has a more ordered structure, has been stabilized. In addition, as the AlFe3 intermetallic phase disappears at 1200°C, Al becomes free and the amount of Al increases in the structure, and the Al2Ti structure that occurred at 1000°C and 1100°C may have transformed into the Al3Ti structure. The free energy of carbide formation among the alloying elements is the most negative for Ti, and therefore TiC formation was expected clearly to occur thermodynamically. Stearic acid, which was used as a process control agent to reduce the effect of cold welding, can cause significant carbon pollution due to decomposition after mechanical alloying. In addition, sintered of samples at different temperatures (1000°C, 1100°C and 1200°C) indicates that it may form due to carbon contamination as a result of the decomposition of stearic acid as a result of mechanical alloying. Therefore, Ti easily have reacted with C during sintering and mechanical alloying process.
Phase analysis of the starting powders, mechanical alloying powders and sintered samples was performed using a X-ray diffractometer. Measurements were performed at 10-90⁰ with 2⁰/min scanning rate by using CuKα radiation. The microstructure characterization of the mechanical alloyed powders and consolidated samples was carried out with scanning electron microscopy. The bulk denisities of the sintered samples were measured by the Archimedes' method. The Vickers hardness values of the samples consolidated at different sintering temperatures were measured under a load of 9.8 N and the load was applied in 12 seconds.
As a result of experimental studies, it was observed that increasing sintering temperature caused an increase in relative density (96.13%, 97.01%, 97.32%). The increase in relative density occurred as a result of the decrease in the amount of porosity with increasing sintering temperature. In addition, better consolidation was achieved as increasing sintering temperature reduces the diffusion distance. The Vickers hardness (HV) of HEA' was measured with increasing sinter temperature; the values of 724.05HV, 691.3HV and 657.26HV were obtained in relation to the intermetallics and grain growth (0.123 mm, 0.147 mm, 0.573 mm, respectively). High hardness values of higher sintering temperatures samples indicate the formation of FCC solid solution and intermetallic phases such as AlFe3, TiC, Al2Ti precipitate phases are more abundant than lower sintering temperature. The grain size of the different sintering temperature samples whose grain size was calculated after etching was 0.123 mm after 1000°C, 0.147 mm after 1100°C and 0.573 mm after 1200°C. For this reason, faster grain growth occurring at 1200°C, which is the highest sintering temperature, may have led to the lowest hardness.