Mekanik alaşımlama süreçleri ile Al-20Si esaslı toz ve sinter kompozitlerin geliştirilmesi ve karakterizasyon çalışmaları
Studies of development and characterization of Al-20Si based powder and sintered composites by mechanical alloying processes
Yazar: HASAN GÖKÇE
Danışman: PROF. DR. MUSTAFA LUTFİ ÖVEÇOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İleri Teknolojiler Ana Bilim Dalı
Derece: Doktora, 2013
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Alüminyum endüstriyel uygulamalarda en çok tercih edilen malzemelerden biridir. Ancak bu malzemeler tek başlarına sahip oldukları özelliklerle kullanım koşullarının gerekliliğini her zaman karşılayamamaktadır. Yeni malzemelere olan bu ihtiyacı karşılamak amacıyla kompozit malzemelere ihtiyaç duyulmuştur. Metal matriks kompozitler diğer kompozit malzemler gibi ana matriks yapısı içine farklı özelliklere sahip ikincil fazların ve pekiştiricilerin ilave edilmesi ile üretilmektedirler. Kompozit malzemelerin ilk malzeme örneklerinde fiber pekiştiriciler daha fazla uğraşılan bir sınıf kompozit iken üretim proseslerinin zor ve karmaşık olmasından kaynaklanan üretim maliyetlerinin yüksek olması nedeniyle daha düşük maliyetli olan partikül pekiştiricili kompozitlere olan ilgiyi arttırmıştır. Partikül takviyeli metal veya  seramik  matriksli kompozitler tozlarının elde edilmesinde son zamanlarda ilgi çeken yöntemlerden biriside mekanik alaşımlama tekniğidir. Bu yöntem ile oldukça ince hatta gevrek yapılı tozlarda nano boyutlu ve yapıda düzgün dağılmış tozlar üretilebilinmektedir. Bu yapılan çalışmanın amacı yukarıda bahsedilen toz metalurjisi metodu kullanılarak aluminyum-silisyum esaslı alaşımlardan geleneksel üretim yöntemlerinden farklı olarak daha üstün özellikli Al-Si kompozitlerinin üretilmesidir. Buradaki temel noktalardan biriside yüksek enerjili öğütücü sistemleri içinde mekanik öğütme işleminin yapılmasıdır. Mekanik öğütme işlemi ile nanokristalin tozların üretilmesi Al ve Si fazlarının birbiri içerisinde homojen olarak karışması sağlanmıştır. Üretim kademelerini takip ederek mekanik öğütme işleminin oluşturulacak kompozit malzemeye etkisinin incelenecektir. Bu çalışmada ayrıca yapıya katılacak olan TiB2, ZrB2, Y2O3, La2O3 ve CeO2 pekiştirici fazlarının miktarının değişiminin ve kullanılan farklı bileşimlerin yapıya etkisi incelenecektir. 
Partikül esaslı metal matriksli kompozit  malzeme üretiminde kullanılan malzemelerin rahatlıkla temin edilebilmesi ve toz metalurjisi gibi geleneksel bir üretim yönteminin kullanılabilmesi, düşük maliyet ve üretim kolaylığı açısından da bu malzemeleri ilgi çekici bir konuma getirmiştir.
Bu çalışmada, TiB2 ve ZrB2 pekiştiricileri ile geliştirilen alüminyum ve silisyum bileşiminden oluşan ana yapıya sahip metal matriks kompozitler, mekanik alaşımlama yöntemi ile üretilmiştir. Değişen oranlarda TiB2 ilavesinin ve buna ek olarak değişen oranlarda ZrB2 ilavesinin yanında farklı oranlarda Y2O3, La2O3 ve CeO2 pekiştirici fazlarının yapıya olan etkisi incelenmiştir. Ayrıca mekanik alaşımlama süresinin değişmesiyle yapıda meydana gelen farklılıklar gözlemlenmiştir. Mekanik alaşımlama süreçleri sonrası tozlar ile sinterleme sonrası numunelerin faz ve mikroyapı analizleri yapılmıştır. Ayrıca mekanik alaşımlanmış tozların termal analiz yöntemleri kullanılarak termal karakterizasyonları yapılmıştır. Son olarak karşılaştırma amaçlı sinterlenmiş numunelerin yoğunlukları, sertlikleri ve aşınmaya karşı dirençleri ölçülmüştür. 
Yapılan karakterizasyon çalışmaları sonucunda mekanik alaşımlama süresinin artması ile hem toz haldeki hemde sinterlenmiş numunelerin mekanik özelliklerinde önemli oranda artış olduğu gözlemlenmiştir. Buna ek olarak artan TiB2 ve ZrB2 miktarı ile mekanik özelliklerin daha da iyileştiği gözlemlenmiştir. Son olarak en iyi mekanik özelliklerin ağırlıkça %10TiB2 ve %10 ZrB2 ilave edilmiş numunelerde olduğu görülmüştür. 
Sonuç olarak TiB2, ZrB2, Y2O3, La2O3 ve CeO2 sert seramik partikül pekiştiricilerin Al-20Si ana matriksine ilave edilmesiyle elde edilen yeni kompozit malzemeler ana matriksin özellikle mekanik özelliklerini çok iyi derecede iyileştirmiştir. Bu malzemeler üstün aşınma ve sertlik özellikleri ile pekçok endüstriyel uygulamada kullanılabilir.

Abstract:
Metal matrix composites (MMCs) reinforced with short fibers, whiskers, and particulates reinforced have good wear resistance stiffness, low coefficient of thermal expansion and advanced strength values at, both room temperature and at elevated temperatures. Ductile and formable metals such as Al and Cu combined with abrasion and corrosion resistant ceramic reinforcement phases are very attractive candidates for tribology applications. In comparison to the matrix alloy, Al composites  comprising hard reinforcement particles (SiC, TiB2, B4C, TiC and Al2O3 and etc.) mostly show high wear resistance with regards to volume fraction, size and type of reinforcements. and they are particularly used in aerospace, defense, automotive industries and in structural applications due to their light weight and high performance. A commercially important class of Al-based metal matrices are the Al-Si alloys which are widely used in automotive, aerospace and electronic applications due to their higher strength and thermal conductivity, good wear resistance and low coefficient of thermal expansion. The high strength and wear resistance of these alloys is mainly associated with the presence of hard silicon particles (both primary Si and eutectic Si).
As-cast hyper eutectic Al-Si alloys include large and brittle primary silicon crystals in an eutectic matrix which operate as stress concentrators to cause premature crack initiation and fracture which limits most of their applications in potential fields. To improve mechanical properties of hypereutectic Al-Si alloys, large Si crystals must be modified and refined. There is an increasing interest in the use of TiB2 particles because of their ultra-high hardness, elastic modulus, relatively high temperature stability and good wettability with molten Al matrix. Despite this, only a small number of wear and tribology investigations could be found on the eutectic Al-Si alloys reinforced with TiB2 and ZrB2 particles. There are several production techniques of MMCs such as powder metallurgy, molten metal, semi-solid casting, pressure infiltration, and spray deposition. Among the manufacturing process for MMCs, a lot of studies have been conducted on powder metallurgy. The techniques and especially high energy ball milling (HEBM) have been developed to produce Al-based MMCs with hard and strengthening additives. Among the powder metallurgy routes, rapid solidification (RP) and high energy ball milling or mechanical alloying (MA) are two enhancing methods for the refinement of coarse Si particles RS and HEBM/MA techniques are very important for good microstructure refinement, solid solubility extension and chemical homogeneity of alloys.  Mechanical alloying (MA) is a solid state reaction which is based on the continuous deformation of powders particularly during welding, fracturing, and rewelding in a high-energy ball mill. Compared to other methods, MA has the ability to demonstrate superior features and higher performance of discontinuously reinforced MMCs during decrease in particle size and work hardening. In the present work, TiB2, ZrB2, Y2O3, La2O3 and CeO2 were chosen as a reinforcement to integrate its superior properties with Al-20 wt.% Si (Al-20Si) matrix alloy. Mechanical alloying, uniaxial pressing and conventional solid state sintering were applied to produce Al-20Si matrix composites containing , the reinforcement particles at various amounts. For this purpose, microstructural, physical characterization and mechanical behaviour investigations were carried out on mechanically alloyed and sintered Al-20Si-TiB2, ZrB2, Y2O3, La2O3 and CeO2 composites as a function of the reinforcement amount and milling durations and compared with those of Al20Si matrix alloy.
Elemental aluminum (Al), silicon (Si) and titanium boride (TiB2), zirkonium boride (ZrB2), yttrium oxide (Y2O3), lanthanum oxide (La2O3) and cerium oxide (CeO2) powders were used as starting materials.  Particle size measurements of the starting powders were performed using a laser particle size analyzer. A lot of different batches of starting powders were prepared to constitute the following compositions: Al–20 wt.% Si, Al–20 wt.% Si–x wt.% TiB2, ZrB2, Y2O3, La2O3 and CeO2. The starting powders were mixed with 2 wt% zinc stearate used as process control agent (PCA) to prevent excessive cold welding and agglomeration of the powders and were placed into a hardened steel vial in a glove-box filled with high purity argon using a ball-to-powder ratio of 7:1.  Mechanical alloying (MA) experiments were carried out in a mill for durations of 1, 2, 4, and 8 h for TiB2 and ZrB2 reinforcement, 4 h for Y2O3, La2O3 and CeO2 reinforcements and from 1 to 20 h for main matrix composite powders. Iron contamination amounts in the processed powders during mechanical alloying were determined using a x-ray fluorescence (XRF) spectrometer. Microhardness measurements of the as-blended and MA'd powders were made in a microhardness tester. Microstructural characterization investigations of the MA'd main matrix alloy, and carbide and oxide reinforced Al-20Si composite powders were carried out using a scanning electron microscope (SEM) and X-Ray Diffractometer (XRD). All MA'd powders were consolidated using a one-action hydraulic press. Green densities of the compacts were calculated dimensionally. Prior to sintering, debinding of consolidated samples was carried out in a tube furnace under Ar atmosphere at 400 °C for 1 h using a heating rate of 1 °C/min. The green bodies were sintered at 570 ◦C for 2 h in a high temperature controlled atmosphere furnace with a heating and cooling rate of 8 oC/min, under vacuum up to 400 ◦C and under Ar gas flowing conditions between 400 and 570 oC. Densities of sintered samples were measured using the Archimedes method. Microstructural characterizations of sintered samples were carried out using scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. XRD investigations of the sintered samples were conducted in a powder diffractometer. SEM investigations were performed in a SEM equipped with an energy dispersive spectrometer (EDS). Vickers microhardness measurements were carried out on the sintered samples using a microhardness tester. Bulk densities of the sintered samples were measured according to the Archimedes' method in ethanol used as the immersion medium. Wear properties were determined using a dry sliding reciprocating wear instrument. All wear tests were performed using a fresh Al2O3 ball based on ISO 3290. The worn surfaces were examined in a scanning electron microscope to identify the wear mechanism. Results of the wear tests were evaluated according to volume of the material loss and wear rate of the material measured by a profilometer. 
Al, Si and TiB2, ZrB2, Y2O3, La2O3 and CeO2 peaks can still be observed after MA'd  at  different  durations,  respectively. Also evident that  no  intermetallic phase formation exists  between  Al, Si, TiB2, ZrB2, Y2O3, La2O3 and CeO2 during MA process. 
Additionaly, it is also obvious that Al and Si peaks gradually decrease in height and peaks broaden due to crystallite refinement and increase in lattice strain with increasing MA durations. With increases in  MA duration and the reinforcements amounts, crystallite size decreased and lattice strain increased. After MA the Al20Si/10TiB2 sample for 8h, the smallest crystallite size value of 32 nm  and the highest lattice strain value of 0.8 were observed.
Since mechanical alloying (MA) is a non-equilibrium high energy milling process, powders fabricated by this technique can undergo several microstructural and/or phase transformations during heating. In order to study possible microstructural changes and thermal stabilities of the Al20Si matrix alloy and Al20Si/10TiB2 composite powders, differential scanning calorimetry (DSC) analyses were carried out on the as-blended Al-20Si matrix alloy powders and those MA'd for 0, 1, 2, 4, and 8 h and on the Al-20Si/x TiB2, ZrB2, Y2O3, La2O3 and CeO2 composite powders MA'd for 4h. With increase in MA duration, the endothermic peak temperatures shift to lower values, i. e. 582 oC for the Al-20Si powders MA'd for 1 h denoting a Si solid solubility of ~ 1.4 wt %Si in α-Al and 579 oC for that MA'd for 2 h corresponding to solubility of 1.6 wt% Si in α-Al..  This indicates that MA enables increased solid solubility of Si in α-Al. Sharp and clear endothermic peaks of MA'd composite powders in the temperature range between 577 and 582 ºC belong to the variation of solid solubility of Si in α-Al in Al-Si systems . On the basis of DSC curve, it can be concluded that liquid phase forming of stable Al and Si alloy phase starts at 577 °C.  Both intermetallics are expected to be stable at temperatures below th eutectic temperature of 577 °C.  Therefore, sintering experiments were carried out at   570 °C below the euetectic temperature to prevent the formation of the liquid phase. 
Microhardness measurements were conducted on the as-blended and MA'd powders to determine the effects of mechanical alloying and TiB2, ZrB2, Y2O3, La2O3 and CeO2 particle reinforcements.  Hardness values of as-blended samples are almost the same, indicating that the presence of the TiB2, ZrB2, Y2O3, La2O3 and CeO2 have had no effect. Increases in the hardness of composite powders with MA durations can be attributed to the combined effects of TiB2, ZrB2, Y2O3, La2O3 and CeO2 particles and the cold deformation of matrix powders. When the MA duration and reinforcement amount increase, the powders exhibit a smaller but an increasing trend in hardness. Thus a maximum hardness value of 302 HV was achieved for the Al-20Si-15ZrB2 composite powders MA'd for 8 h. The higher iron contamination amount observed in the composite powders  compared to that of Al-20Si matrix powder indicates that the presence of hard reinforcement particles also contributed to iron contamination during milling process. The xrd pattern of the Al20Si matrix alloy exhibits strong diffraction peaks belonging to the Al, Si, stable phases and also to the Al9Si stable intermetallic phase. Although any new phase was not found in MA'ed powders, the reactions occurred during sintering and resulted in the formation of intermetallic compounds are totally in accordance with the phase transformations expected by the Al-Si binary phase diagram. The Al-20Si/x TiB2, ZrB2, Y2O3, La2O3 and CeO2 composites consisted of the diffraction peaks belonging to phases of the Al, Si, TiB2, ZrB2, Y2O3, La2O3 and CeO2 and Al9Si. Also, iron impurities were not identified by X-ray diffraction analysis of MA'd powders, after sintering, peaks belonging to a Fe containing phase, Al0.5Fe3Si0.5 phase are present in the xrd patterns of all sintered samples. This observation suggests that the steel balls and vial utilized as the milling media during MA process wear out. Furthermore; Al3Ti, Al3Zr, Al3Zr4, Ce2O7Si, AlLaO3 La2O7Si2 and Y2SiO5  intermetallic  and residual phases identified from carbide and oxide reinforced Al-20Si based matrix alloy composite powders of XRD patterns at small amounts. 
Using the rule of mixtures, the theoretical densities of Al20Si based matrix alloy composites are calculated.  The restraining effect of MA on the packing characteristics of aluminium powders due to the hardening effect of MA was reported elsewhere. Thus, the highest sintered density observed is 99,5% for the sample is achieved for the Al-20Si/10TiB2 MA'd for 8 h 
As expected, microhardness values of all  sintered samples increase with increasing MA durations and the reinforcements contents. A maximum bulk microhardness of  358 HV is reached for the sintered Al-20Si-10ZrB2 composite MA'd for 8 h. the reinforced composites exhibit higher wear resistance respectively compared to that of the Al-20Si matrix alloy. A maximum wear resistance of 9.93 is reached for the sintered Al-20Si-10TiB2 composite MA'd for 1 h. The minimum wear resistance of 1 is obtained for as-blended and sintered Al-20Si matrix alloy.  The wear resistance is observed to increase with increasing the TiB2 particles amount.  Additionally, a tendency deal with increasing in the relative wear resistance with increasing hardness can be observed. 
The wear resistance and tracks are observed to increase and decrease with the addition of the reinforcement phases, respectively.