Inconel 718 alaşımının katı partikül erozyon davranışının incelenmesi
Solid particle erosion of inconel 718 alloy
Yazar: BERK BİRCAN
Danışman: PROF. DR. HÜSEYİN ÇİMENOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Metalurji Mühendisliği = Metallurgical Engineering ; Mühendislik Bilimleri = Engineering Sciences

Özet:
Teknolojik gelişmelerle paralel olarak elde edilen süperalaşımlar birçok saf metal ve alaşıma göre üstün özelliklere sahiptirler ve bu özellikleri sayesinde stratejik açıdan önem arz eden alanlarda kullanılmaktadır. Bir süperalaşım olan Inconel 718 alaşımı da sahip olduğu üstün yüksek sıcaklık özellikleri sayesinde özellikle havacılık ve uzay endüstrisinde uygulama alanına sahiptir. Bu çalışmada Inconel 718 alaşımının katı partikül erozyonu sonrası davranışlarının incelenmesi amaçlanmıştır. Bu doğrultuda orijinal ve nitrasyon ile sertleştirilmiş numuneler özel olarak tasarlanmış erozif aşınma test düzeneğinde farklı partikül çarpma açıları(30°, 45°, 60°, 90°) ve farklı sıcaklıklarda(20°C, 400°C, 600°C) aşındırıcı partikül(alümina) ile erozyona uğratılmıştır. Deney parametrelerine bağlı olarak Inconel 718 alaşımı numunelerin erozyon oranları hesaplanmıştır. Sıcaklık ve nitrürlemeden bağımsız olarak maksimum erozyon oranı 45° açıda gözlemlenmiştir. Öte yandan orijinal numunelerde sıcaklığın 400°C'ye artmasına bağlı olarak erozyon oranı dar açılarda artarken, dik açılarda kayda değer seviyede bir değişim gözlemlenmemiştir. Ancak sıcaklığın daha da artmasıyla sertlik düşmüş ve buna bağlı olarak erozyon oranı da düşürmüştür. Nitrasyon ile sertleştirilmiş numunelerde ise erozyon oranı tüm deney parametreleri için orijinal numunelerden daha yüksek gelmiştir. Nitrürlenmiş numunelerde sıcaklığın artmasıyla erozyon oranı süneklik artışı ve sertlik düşüşüne bağlı olarak 45°, 60°, 90° için artarken 30° için erozyon oranının da düşüş meydana gelmiştir. Katı partikül erozyon deneyleri sonrası elde edilen ağırlık kayıpları ile oluşturulan, erozyon oranının sıcaklık, açı ve nitrürleme ile ilişkisini açıklayan grafiklerin daha detaylı bir şekilde yorumlanabilmesi adına bir dizi karakterizasyon analizi yapılmıştır. Bu doğrultuda taramalı elektron mikroskobu ile aşınan numunelerin yüzey morfolojileri incelenmiştir. Akabinde optik mikroskop ile farklı büyütmelerde kesiti alınan numunelerde aşındırıcı partikülerin oluşturduğu çukurlar gözlemlenmiştir. EDS analizi ile element pikleri elde edilerek numunelerin içindeki elementlerin yüzdeleri ortaya koyulmuştur. Haritalama yapılarak yüzeyde mevcut alüminyum ve oksijen dağılımı incelemiş ve böylelikle aşındırıcı partiküllerin numune üzerinde dağılımı gözlemlenmiştir.. Orijinal ve nitrürlenmiş numuneler üzerinde indentasyon çalışması yapılarak nitrasyonun elastik-plastik deformasyon oranına etkisi gözlemlenmiştir.

Abstract:
Superalloys have lots of superior properties in comparison with pure metals and alloys due to technological improvements. According to the properties they have, superalloys are used on fields which are critical importance. Superalloys are widely used in aerospace and aircraft industrial applications where high temperature strength and/or corrosion resistance are required. These applications include turbine motor engine, turbine blades and turbine disks and other parts. They are required to exhibit combinations of high strength, good fatigue and creep resistance, good corrosion resistance and the abilitiy to operate at elevated temperatures for extended periods of time. Superalloys is a group of alloys usually based on group VIIA elements and designed for high temperatures where relatively severe mechanical stressing is encountered and where high surface stability is frequently required. In any system where high resistence to under static, fatigue and creep conditions are required, the superalloys are appropriate material group to choice for high temperature applications. The first superalloys are modification of austenitic stainless steels. In 1980s, by developing technology to make alloys for superalloys, some elements have gained excellent mechanical properties, which desired for high temperatures application, to superalloys. Therefore, when the subject is high temperature applications, superalloys are more preferred then other commercial metallurgical materials. However, as high temperature materials are relatively expensive, the superalloys should be employed only after consideration of others that are available. When weight-saving is important, titanium alloys can be preferred instead of superalloys, but their poor oxidation resistence restricts their application to below about 700°C. Nickel based superalloys are the most complex type of superalloys and are used in the hottest parts of aircraft engines, covering over 50% of the engine weight. For lower temperature applications solid solution hardened nickel based superalloys or for higher temperature applications precipitation hardened nickel based superalloys are prefered. Chromium and aluminum are important in providing oxidation resistance by forming the chromium or aluminum oxide films on the surface of superalloy. The nickel base superalloys are applied in several and complexest engineering systems, however there are few data about the erosive behaviour of these superalloys regarding the impact of solid particles in a gaseous flow. The solid particle erosion is a complex phenomenon and it is characterized by the deformation and material removal during the impact of the particles generating high temperatures. Nickel-based superalloys typically constitute 40–50% of the total weight of an aircraft engine and are used most extensively in the combustor and turbine sections of the engine where elevated temperatures are maintained during operation. Creep resistant turbine blades and vanes are typically fabricated by complex investment casting procedures that are essential for introduction of elaborate cooling schemes and for control of grain structure. Such components may contain equiaxed grains or columnar grains, or may be cast as single crystals, completely eliminating all high angle grain boundaries. Because grain boundaries are sites for damage accumulation at high temperatures, the blades in the early stages of the turbine are typically single crystals, whereas the blades in the later (cooler) stages of the turbine are fabricated from equiaxed alloys. Turbine disks are fabricated via wrought processing approaches that either use cast ingots or consolidated superalloy powder performs. Exceptional combinations of strength, toughness, and crack-growth resistance can be achieved in these materials by close control of microstructure through the multiple stages of wrought processing. When aircraft operate in harsh environments where hard particulatematteris entrained by the air flowinto the operating engine, severewear of exposed components may occur through material removal bysolid particle erosion (SPE). This type of damage is most prominent inthe first stage of the aircraft engine, where the compressor blades canbe eroded to such an extent that aerodynamic performance and evenstructural integrity are compromised. Consequently, much work hasbeen done in academia and industry in order to understand thematerialloss mechanisms present in SPE and to develop protective approachesthat will increase component lifetimes. One such technology is the useof hard protective coatings to impede the erosion of the predominantlymetallic engine components. Engineering materials are exposed lots of harmful factors under working conditions. Erosive wear can occur a damage on material when solid particles impact on material with high velocity on dusty working conditions. First studies on erosive wear started on the begining of 1950 and lots of study have been done on the field since that day. Age-hardenable Inconel 718 combines high-temperature strength up to 650°C with corrosion resistance and excellent fabricability. Its welding characteristics, especially its resistance to postweld cracking, are outstanding. With these properties, Inconel 718 is used for part for aircraft turbine engines, cryogenic tankage and components for oil and gas extraction and nuclear engineering. Inconel 718; is an iron-nickel based superalloy that contains a significant amount of iron, nickel and niobium. Due to high content of niobium, it can be strenghtened by precipitation mechanism. Inconel 718 is hardened by the precipitation of secondary phases (e.g. gamma prime and gamma double-prime) into the metal matrix. The precipitation of these nickel- (aluminum, titanium, niobium) phases is induced by heat treating in the temperature range of 600 to 800°C. For this metallurgical reaction to properly take place, the aging constituents (aluminum, titanium, niobium) must be in solution (dissolved in the matrix); if they are precipitated as some other phase or are combined in some other form, they will not precipitate correctly and the full strength of the alloy with not be realized. To perform this function, the material must first be solution heat treated. Inconel 718 alloy is used especially aggresive and hot environments. It can be used in gas turbines, rocket engines, aircraft engines, nuclear reactors and process equipments. As the surface hardening technique, nitriding is the most attractive process for several ferrous and nonferrous alloys. Conventionally nitriding process is mostly performed at relatively high temperatures resulting in tempering effect and/or change in the microstructure via producing non-desirable equilibrium phases. Nitriding of nickel based superalloys has received much less attention as compared with ferrous materials and stainless steels, probably due to the fact that such alloys are well known to be extremely difficult to nitride. Only a few studies have been reported on plasma nitriding of Ni-based super alloys. Nitriding processes was performed in a fluidized bed furnace containing Al2O3 particles as the gas and heat carrier in a atmosphere of 60% ammonia and 40% nitrogen gas mixture at 400°C temperatures for 10 h. Nitriding temperature of 400°C induced a surface layer having thickness of about 5-6 μm thick single-phase nitrided layer. Nitriding at 450°C and 500°C temperatures caused of CrN phase on the surface of AMS 5662 and AMS 5663 Inconel 718 samples It is aim to investigate the behaviours of Inconel 718 after solid particle erosion. Accordingly uncoated and nitration coated samples were eroded by erodent particle alumina, with different impact angles (30°, 45°, 60°, 90°) and temperatures (20°C, 400°C, 600°C) on the specially designed erosive wear test apparatus. Erosion rate of the Inconel 718 alloy samples were calculated for each experiment parameter. Maximum erosion rate was observed on 45° independently of temperature and coating. On the other hand, erosion rate increased when enhanced the temperature up to 400°C for narrow angles while normal angles had negligible variation for uncoated samples. But when the temperature increased from 400°C to 600°C hardness has a dramatic decrease which also caused a decrease on erosion rate. In contrast, coated samples has a decrease on erosion rate when heated up to 400°C due to coating effect. But then erosion rate is started to increase when the temperature rise up to 600°C. Because erodent particulates can penetrate the samples' surfaces due to hardness reduction. Graphics has been drawn to explain temperatures, angles and coating dependence of the erosion rate after solid particle erosion test. A series of characterization test implemented on the specimens to better understand the result obtained with the graphics. Accordingly surface morphology of the eroded samples were investigated with transmission electron microscope. Hereby failure analysis of the surface of eroded material and which wear mechanism effective on what experiment parameter was investigated. In addition eroded sections of the specimens were investigated by optic microscope to see the effect of erodent particles. Element peaks obtained by EDS analysis to show composition of elements. Also mapping was done to observe distribution of aluminium and oxgyen on the surface of the specimens. Indentation test was done to see the effect of the coating on the elastic-plastic deformation behaviour of the material.