Mekanik alaşımlama ile CoCrFeMnNi ve AgCoCrFeNi esaslı yüksek entropi alaşım tozlarının sentezlenmesi ve karakterizasyonu
Synthesization of CoCrFeMnNi and AgCoCrFeNi high entropy alloy powders via mechanical alloying and their characterization
Yazar: EBRU SARIOĞLU
Danışman: PROF. DR. BURAK ÖZKAL
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Malzeme Bilimi ve Mühendisliği Ana Bilim Dalı / Malzeme Bilimi ve Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Mühendislik Bilimleri = Engineering Sciences
Dizin: Mekanik alaşımlama = Mechanical alloying

Özet:
Yüksek entropili alaşımlar (YEA) günümüzde çok ilgi çeken bir araştırma alanı olup bu ilginin nedenleri arasında yüzyıllardır uygulanan geleneksel alaşımlama kurallarının aksine çalışan bir prensibe dayanması gösterilebilir. Geleneksel alaşımlar bir veya iki metal temel alınarak oluşturulurken YEA'lar birçok elementi yakın oranlarda içeren alaşımlardır. Yapısında çok sayıda element bulunmasına rağmen YEA'lar tek bir faz yapısına sahip olma eğilimindedir. Sahip oldukları iyi fiziksel ve kimyasal özellikler sayesinde endüstride birçok alanda ve yeni teknoloji uygulamalarında yüksek performans gösterme potansiyeline sahiptir. YEA çoğunlukla döküm yöntemleri kullanılarak üretilmektedir, toz teknolojisi ile yapılan çalışmalar çok daha az sayıdadır. Ancak, son yıllarda mekanik alaşımlama ile yapılan çalışmalar artmıştır, özellikle nano yapıya sahip YEA üretiminde ve bazı YEA özelliklerini geliştirmede alternatif bir yol olarak öne çıkmaktadır.  
Yüksek lisans tezi olarak sunulan bu çalışmada mekanik alaşımlama (MA) ile yüksek entropili alaşım oluşturma potansiyeline sahip CoCrFeMnNi ve AgCoCrFeNi toz bileşimleri üretilmiştir ve karakterizasyonu çalışılmıştır. 
Cantor alaşımı olarak bilinen CoCrFeMnNi YEA bileşimi hazırlanırken farklı özellikte mangan tozları kullanılmıştır. Mangan tozu olarak saf mangan ve FeMn kullanmanın YEA oluşumuna etkisi incelenmiştir. Ayrıca alaşım oluşumunda başlangıçtaki manganın farklı partikül boyutlarına sahip olmasının etkileri araştırılmıştır. Diğer toz sistemi olan AgCoCrFeNi hazırlanırken termodinamik yaklaşımlardan yola çıkılarak gümüş derişimleri belirlenmiştir. Teorik hesaplanan farklı gümüş içeriklerine sahip tozların MA'lanması sonucu oluşan yapılar incelenmiştir.
Toz harmanlarını alaşımlamak için bir toz metalurjik teknoloji olan MA seçilmiştir. MA ile nano boyutlara sahip, homojen dağılmış partiküller oluşturmak hedeflenmiştir. Tozları öğütmek için iki farklı öğütücü kullanılmıştır ve öğütücü tipine göre farklı öğütme süreleri belirlenmiştir. Gezegen tipi öğütücüde en fazla 40 saat; SpexTM öğütücüde en fazla 10 saat olacak şekilde toz harmanları alaşımlanmıştır. Belli aralıklarda örnek alınıp, zamana bağlı olarak öğütücü tipinin alaşımlamaya etkisi araştırılmıştır. Ayrıca her iki öğütücü tipinde de farklı çaplara sahip bilye kullanmanın zamana bağlı alaşımlama üzerindeki etkileri incelenmiştir.  
Başlangıç tozlarını ve mekanik alaşımlanmış tozları karakterize etmek için X-ışınları difraktometresi (XRD) ile faz analizi, görünür ve gerçek yoğunluk ölçümleri, partikül boyut ölçümleri yapılmıştır ve taramalı elektron mikroskobu (SEM) ile tozlar görüntülenmiştir. Daha sonra, MA'lanan tozlar preslenip sıkıştırılabilirlik özellikleri incelenmiştir. 
Deneysel çalışmalar sonucunda, çalışmaların gerçekleştirildiği koşullarda MA'lanmış AgxCoCrFeNi toz sistemlerinin toz halde basit tek bir yapı oluşturmadığı belirlenmiştir. CoCrFeMnNi bileşimine sahip tozlardan bazılarında MA sonrasında toz halde basit YMK yapısı tespit edilmiş olup sonuçlar MA için seçilen öğütücü tipi ve öğütme süresinin önemini ortaya koymaktadır. Ayrıca iri partikül boyutuna sahip Mn içerikli tozların MA sırasında yapıya daha kolay girdiği görülmüştür.

Abstract:
Alloying has long been used to improve the weakness of the base metal by adding secondary/alloying elements. However, as technology developed, conventional alloying has been insufficient to meet with advanced engineering requirements such as unique magnetic, electrical properties, resistance to extreme corrosion, strength at high or cryogenic temperatures, resistance to wear.
Indeed by 1970's, the number of alternative alloys use in the majority of engineering applications was around 30 and since then this number has been increasing as a result of advancement of engineering technology. Intermetallic compound may be accepted as one of the first material group which enlarging these alternatives without having a base metal structure. Compounds with intermetallics have a secondary phase that has its lattice other than the main phase. However, intermetallic alloys that were not formed by conventional techniques were also limited to use due to their limited alloying formation.
In the early 2000s, using transition metals, a multi-component alloy structure was introduced to modern engineering technology which can be accepted as completely different from traditional alloys. These multicomponent structures are called high entropy alloys (HEA) and consist of from the many different elements which are aloyyed together in equimolar ratios. HEAs generally comprises five or more principal elements in similar atomic percentages. 
Nowadays, HEAs are a new class of materials showing certain progress based on ongoing research and many of them are the candidates alloy for demanding applications based on their unusual mechanical properties. Also HEAs have particularly advanced physical properties such as thermal, electrical, and magnetic. HEAs can also be used under extreme conditions thanks to their promising corrosion resistance, high wear resistance, high oxidation resistance specialties. Its mechanical properties, especially fracture toughness and hardness, are also very favorable for industrial applications at high and low temperatures.
Ingot metallurgy has been the most widely selected or applied processing method for HEAs. It has been expressed that mainly arc melting used casting way related to HEAs made it possible to alloy more than five metals efficiently. They could be melted despite their high melting temperatures and have been a mixture without segregation. On the other hand, arc melting has some struggles in cases where a complicated content must be accomplished; therefore, it could not be a good option for processing all family of alloys. So, powder metallurgy (PM) has been another alternative route to synthesize HEAs.
In this study, which was presented as a master thesis, CoCrFeMnNi and AgCoCrFeNi powder mixtures were mechanical alloyed with the aim of HEA formation in powder state. For this purpose, pure Ag, Co, Cr, Fe, Mn, FeMn, and Ni powders with particle sizes less than 45 µm and pure Mn powders with particle size 795 µm were used as starting materials.  In addition, FeMn as a different source of manganese was used in the preparation of CoCrFeMnNi HEA composition known as Cantor alloy. The effect of manganese sources on HEA formation has been investigated. While preparing the other powder system of AgxCoCrFeNi (x = 5 and 10), silver concentrations were determined by HEA thermodynamic approaches. Then, the effect of silver contents on HEA formation via alloying has been examined.
As it is mentioned MA is selected fort his study and with this approach it is expected to obtain nanocrystalline materials avoiding segregation. For this purpose, planetary ball mill and SpexTM high energy ball mill as two different milling obtions were selected in order to see the effect of the milling type and experiments performed against different milling times. 
The elemental powders were mixed to constitute target compositions of CoCrFeMnNi and AgxCoCrFeNi, then milled in a planetary ball mill for up to 40 hours at 320 rpm in an argon atmosphere. A Fritsch Pulverisette with four grinding stations was used as a planetary ball mill. Stainless steel vials and balls with diameters of 6 and 12.5 mm were utilized as the milling media with a ball-to-powder mass ratio (BPR) of 10:1. No process control agent (PCA) was used. The processes of packing and sealing powders in vials were carried out in the Pluslabs glove box. High purity Ar gas was used to prevent the powders from oxidizing. Under these conditions, while the powders that belonged to CoCrFeMnNi systems were milled 10, 20, 30, and 40 hours, the powders belonged to AgxCoCrFeNi systems were milled 5, 10, 20, and 30 hours.
However, in one set that belonged to the CoCrFeMnNi system, the vials were not sealed with Ar gas. The system was filled with air was stopped for 10 minutes to reduce the heating and prevent oxidation, then milled for 30 minutes.
SpexTM Duo Mixer/Mill 8000D with two grinding stations were used as another high energy ball miller. The elemental powders were mixed in specified composition for CoCrFeMnNi and AgxCoCrFeNi, then milled in a SpexTM miller for 10 hours at 1200 rpm in the argon atmosphere. Stainless steel vials and balls with diameters of 6 and 12.5 mm were utilized as the milling media with a ball-to-powder mass ratio (BPR) of 10:1. No process PCA was used. The processes of packing and sealing powders in vials were carried out in the Pluslabs glove box. High purity Ar gas was used to prevent the powders from oxidizing. Under these conditions, the powders belonged to powder systems were milled 5 and 10 hours.
The crystal structure of the starting powders and alloyed powders was examined by X-ray diffractometer (XRD, BrukerTM D8-Advance) with CuKα (λ=1,5406) radiation. Bulk and true densities of the starting powders and alloyed powders was calculated using respectively Arnold Density meter, and He gas pycnometer (MicromerticsTM AccuPyc II 1340). Besides, particle size and distribution (PSD) of powders were measured. The morpology of the powder particles and pressed particles determined by using JEOLTM JCM-6000 Benchtop Scanning electron microscope (SEM) and Zeiss™ stereo microscope.
According to the XRD analysis of all CoCrFeMnNi powders milled in the SpexTM high energy ball mill, powders had the FCC ((111), (200)) structure. This structure illustrates that alloying become. However, some of CoCrFeMnNi powders milled in the planetary high energy ball mill had FCC ((111), (200)) CoCrFeMnNi HEA peaks. In the early hours, the diffraction peaks of the Mn (330) can still be detected. As the milling time increases, it can be observed that the diffraction peaks of Mn become invisible. Thus, it has been seen that the miller type has an important effect on CoCrFeMnNi powders gaining the HEA structure. The lower energy planetary mill having 320 rpm requires longer alloying times to give the system the energy required for alloying. Compared to milled SpexTM high energy ball mill, there is no obvious difference between milling time can be recognized. 
With prolonged milling time to 30 h in the planetary ball mill, only the powder composition containing coarse Mn particles (C2) have peaks belongs to FCC structure CoCrFeMnNi. Even throughout the milling, other powders, C1 and C3, the diffraction peaks of the Mn and CoCrFeNi can still be observed. But, at the same time, peaks become broad gradually. It can be thought that the number of collisions with the balls has increased due to the larger surface area of the coarse particles. In addition, coarse particles with a larger surface area have greater values of surface energy, which can be said to facilitate the deformation of the powders during collisions. Besides, compared to the peaks belong to C1-G2 and C3-G2, there is no obvious distinction can be marked. 
According to the XRD patterns of the CoCrFeMnNi powders milled in the planetary ball mill with different sized balls, it has been observed that milling with balls of different sizes affect HEA formation. They (6 and 12,5 mm) have the HEA structure earlier than milling with the balls of the same diameter (6 mm). It is thought that the number of random collisions has increased since the spaces between the balls and the balls will differ in an alloying with balls of different sizes. As a result, the energy in the milling environment will increase, it can be said that high milling energy facilitates the formation of HEA.
When the compressibility of CoCrFeMnNi powders was examined, it was determined that the powders milled by the planetary ball mill and SpexTM mill have nearly the same behavior. As the milling time increases in the planetary mill, the compressibility values generally decreased. However, it is thought that the compressibility of the powders milled in the SpexTM mill changed depending on the porosity of the powders. It has been determined that all powder compositions have the lowest compressibility values in the samples of prolonged milling time to 30 h in the planetary ball mill. There was a difference between the compressibility of the powders alloyed with balls of different sizes in the SpexTM mill and the powders alloyed with the same size balls, It was determined that the compressibility increased as the MA duration increased. These results are consistent with the particle sizes and porosities detected in the SEM images of the powders and pressed samples, and the nanoscale particle analysis results.
According to the XRD results of AgxCoCrFeNi (x = 5, 10) powder systems, the diffraction peaks of the Ag and CoCrFeNi peaks can be seen. These peaks shows that the metals are not being dissolved each other. Consequently, these powders do not form a simple FCC solid solution structures in accordance with the definition of HEA in the literature.
Densities of Ag10 coded powders are higher than Ag5 coded powders since they have higher Ag concentration. However, the true density values of Ag10 coded powders were higher than theoretical densities of the powders. According to the XRD results, it was thought that the sample was taken from the place where Ag concentration was high during the measurement, since Ag did not enter the structure. Finally, it has been determined that powders of both compositions have a similar tendency to decrease with increasing alloying time in their compressibility.
To sum up, as a result of the experimental studies, it was determined that the alloyed AgxCoCrFeNi powder systems under the conditions where the studies were carried out did not form a simple solid solution structure in powder form. In some of the alloyed powders with CoCrFeMnNi composition, simple FCC structure in powder form was determined, and the results reveal the importance of the mill type selected for MA and the milling time.