Tic Ve Zrc İlaveli Tzm Kompozitlerinin Spark Plazma Sinterleme (sps) Yöntemi İle Üretimi Ve Karakterizasyonu

Abstract

Ergime sıcaklığı 1800 °C’den daha yüksek olan malzemeler refrakter malzemeler olarak tanımlanmaktadır. Rekrakter malzemeler, yüksek sıcaklıklarda çalışmaya uygun, aşınmaya karşı dirençli malzemelerdir. Bu özellikleri ile yüksek sıcaklık uygulamaları olan havacılık ve nükleer endüstrilerinde sıklıkla kullanılmaktadırlar. Teknolojinin gelişmesi ile beraber daha yüksek sıcaklık dayanımı ve daha iyi mekanik özelliklere sahip malzemelere yönelik ihtiyaç artmaktadır. Metal matriks kompozitler metallerin mekanik özelliklerini geliştirmek amacıyla metal malzeme içerisine genellikle seramik esaslı malzeme ilave edilerek üretilmektedirler. Titanyum karbür (TiC) ve zirkonyum karbür (ZrC) yüksek sertlik, yüksek elektriksel ve termal iletkenlik ve yüksek aşınma direncine sahip olan refrakter karbürlerdir. Bu çalışma kapsamında refrakter malzemelerden biri olan molibden bazlı TZM (titanyum-zikonyum-molibden) alaşımı matriksli TZM-TiC ve TZM-ZrC kompozitleri üretilerek TZM alaşımının sinterleme davranışı ve mekanik özelliklerinin geliştirilmesi amaçlanmıştır. TZM-TiC ve TZM-ZrC kompozitlerinin toz metalurjisi prosesi olan spark plazma sinterleme (SPS) yöntemi ile üretilmesi ve karakterizasyonu gerçekleştirilmiştir. TiC veya ZrC katkılı TZM kompozitlerinin üretimi ile literatürde herhangi bir çalışma bulunmaması bu çalışmanın özgün yönünü oluşturmaktadır. Ön alaşımlandırılmış TZM tozu, hacimce %0-20 TiC veya ZrC karbürleri ile katkılandırılarak, 4 mm yüksekliğinde, 50 mm çapında silindirik numuneler elde edilecek şekilde SPS cihazı ile sinterlenmiştir. Üretim aşamasında başlangıç malzemeleri olan hacimce %0-20 TiC veya ZrC içeren toz karışımları silistum karbür (SiC) bilyalar kullanılarak turbula tipi mikser aracılığı ile 8 saat süresince karıştırılmıştır. Karıştırılan toz karışımları ile 50 mm çapındakı grafit kalıp doldurulmuş ve 1550 °C’ de 40 MPa basnç altında 5 dk sinterlenme işlemi gerçekleştirilmiştir. Kompozitlerin üretimi sırasında karbür ilavesine bağlı olarak çekilme başlangıç sıcaklıklarında artma gözlenmiş ve maksimum sıcaklık TZM-%5TiC numunesi için 1260 °C , TZM-%20ZrC numunesi için 1200 °C olarak tespit edilmiştir. Üretilen numunelere kumlama yapıldıktan sonra rölatif yoğunluk ölçümleri yapılmış ve sinterlenen Monolitik TZM ve hacimce %5 TiC veya ZrC ilaveli numunelerin en yüksek rölatif yoğunluk değerleri 10T ve 15Z kompozitlerinde sırasıyla %99,6 ve %99,3 olarak ölçülmüştür. Taramalı elektron mikroskobu ile yapılan mikroyapı ve faz analizleri sonucunda TiC, ZrC, Mo2C, (Mo-Ti)C, (Mo-Zr)C, (Mo-Ti-Zr)C karbürleri gözlenmiş ve matriks içerisine homojen bir dağılım sergilemiştir. Üretilen TZM-TiC ve TZM-ZrC kompozitlerinde karbürlerin genellikle homojen dağılım gösterdiği ve ortalama tane boyutunun monolitik TZM numunesi için 43,3 µ, (%5-20) TiC katkılı numuneler için 8,6-6,4 µ arasında, (%5-20) ZrC katkılı numuneler için 11,1-6,6 µ arasında değiştiği gözlemlenmiştir. Sertlik değerleri, Vickers sertlik cihazında ölçülmüş ve sinterlenen numunlerin sertliklerinin en yüksek değerleri 20T ve 20Z kompozitlerinde sırasıyla 4,10 GPa ve 3,88 GPa olarak ölçülmüştür. Üretilen kompozitlerin kırılma tokluğu değerleri ASTM 1820-15a standardına göre 3 nokta eğme testi yapılarak hesaplanmış, elde edilen sonuçlara göre kırılma tokluğu değerleri TiC katkılı kompozitler için TiC ilavesi kırılma tokluğu değerini 17,38 MPa.m1/2 değerinden ~13 MPa.m1/2 düşürmüştür. ZrC katkılı kompozitlerde 10Z numunesi için 22,46 MPa.m1/2 için maksimum değer elde edilmiş ve daha sonra düştüğü gözlemlenmiştir SPS üretiminin sağladığı avantajlar ile, TZM matriks kompozitler kısa süre ve düşük sıcaklıklarda üretilmiştir.

Refractory materials have the highest melting point (>1800 °C) and have wear resistance at elevated temperature. Recractory materials are preffered in high temperature applications such as aerospace and nuclear industries due to their outstending properties. Thank to development of technologhy, the demand of material which has better mechanical properties and high temperature resistance. Metal matrices composites has developed to improve mechanical properties of material generally via reinforcing with ceramic based materials. Titanium carbide (TiC) and zirconium carbide (TiC) are refractoy carbides have superior properties such as high strength, high electrical and thermal cunductivity and wear strenghth which makes these carbide preferred additive in metal matrix composites. TZM (titanium-zirconium-molybdenum) alloy is a refractory material which is molybdenum based alloy with Mo–0.5Ti–0.08Zr–0.01 (mass %) nominal composition. During production of TZM alloy, TiC and ZrC particles occur in grain boundaries and inhibited grain growth. When TZM alloy compare with molybdenum, even though their compositions close to each other, recrystallization temperature of TZM alloy is 500 °C above than pure molybdenum. This difference in recrystallization temperature provide better mechanical properties such as higher strenght and wear resistance in elevated temperature. In this study, pre-alloyed TZM powder was used as a matrix material and TZM-TiC or TZM-ZrC composites were produced and was aimed to enhance densitification behaviour and mechanical properties of TZM alloy. The purpose of these experimental studies is producing an alternative material instead of TZM alloy. TZM-TiC and TZM-ZrC composites were produced by using spark plasma sintering (SPS) tecnique which is a powder metallurgy methodology and the produced composites were characterized. There is no literature study about producing TiC or ZrC reinforced TZM composites is makes this thesis is unique. Under this experimental Monolithic TZM alloy and TZM based TiC or ZrC reinforced composites were prepared by using an SPS apparatus with a capacity of 20.000 A in Department of Metallurgical and Materials Engineering, Istanbul Technical University. Two different additive were used in experiments to investigate the effects of type of ingredient in TZM based composites. Prepared powder mixtures were sintered in 100 °C/min sintering rate, under 40 Mpa and 5 minute holding time at 1550 °C. TZM, TiC and ZrC powders was used as a starting material. The powder characterizetion steps was carried out to be informed. The phase analysis of each powder was performed with XRD device with the range of 2Ө = 0-90°. The ICDD and JCPDS number of phases was determined. The atomic absorption spectrometry and particle size analyzer is used to investigate chemical composition and paticle size distrubition of TZM alloy. Scanning electron microscopy (SEM) device was used to analyze of grain size. The powder mixture was measured accoring to size of die which has a cylindrical shape with 50 mm inner diameter and 4 mm height. A graphite die was used due to graphite has high electrical conductivity. The powders which have blended and mixed by turbula mixer for 8 hours with SiC balls have placed into die between graphitic sheets. Using graphitic sheets get an advantage since it is easy to remove and better conductivity. A uniaxial pressure of 40 MPa and a pulsed direct current (12 ms/on, 2 ms/off) sintering were applied during the entire SPS process. Microstructure may be controlled and grain growth can be inhibited by rapid heating rate and shorter processing times. Optical pyrometer is used to measure the temperature of the die and sintering of composites was conducted under temperature controlled mode by monitoring the shrinkage behavior of the specimens during the SPS process. Linear shrinkage of the specimens during SPS process was continuously monitored by displacement of the punch rod and the current was controlled manually. TZM composites containing 0-20 volume% of TiC or ZrC and other sintering parameters like sintering rate, pressure, temperature and sintering time were same for all processes. After sintering process, densification behavior, mechanical properties and microstructural characterization of monolithic TZM alloy and TZM composites were investigated. The bulk densities of the specimens were determined by the Archimedes’ method and converted to relative density using theoretical densities of TZM and TiC. The highest relative densities for TZM-TiC samples was achieved in 10T as 99,5% and for TZM-ZrC samples was achieved in 15Z as 99.3% as 15Z which are produced by SPS. TiC and ZrC additives was increased beginning of shrinkage and sintering temperature of TZM alloy. The highest beginning of sintering temperatures was observed 1260 °C in TZM-TiC composites for 5T sample and , 1200 °C in TZM-ZrC composites for 20Z sample. The crystalline phases were identified by X-ray diffractometry in the range of 0-90˚ with Cu Kα radiation in 2°C/min. The ICDD number of phases was determined. The microstructure of polished surfaces of the sintered specimens were observed by scanning electron microscopy (SEM). In composites, TiC, ZrC, Mo2C carbides and (Mo-Ti)C, (Mo-Zr)C and (Mo-Ti-Zr)C complex carbide structures were observed in composites distrubuted homogenously. EDS analysis were carried out to specimens. EDS anaylsis showed that black phases are TiC and light grey phases are ZrC and their compositions were determined. Grain size was analyzed for TZM-TiC and TZM-ZrC composites and average grain sizes were determined between 43,3-6,4 µ for TiC reinforced composites and 43,3-6,6 µ for ZrC reinforced composites. In order to determine hardness of specimens, Vickers hardness (HV) was measured under load of 200 gf (1,96 N) and applied for 12 seconds. The TZM-TiC composites have higher hardness value than TZM-ZrC composites. Vickers micro hardness was 4,10 GPa for 20T sample and 3,88 GPa for 20Z sample. Due to calculate fracture toughness according to ASTM 1820-15a standard, 3 point bending test was carried out and determined, for TZM-TiC composites TiC additives decreasing fracture toughness, for TZM-ZrC composites the maximum value was measured as 22,46 MPa.m1/2 for 10Z and then decrease with increasing ZrC amount. As a conclusion with different material as additive was affected sintering and densitification behaviour of TZM alloy. The addition of TiC or ZrC increased beginning of shrinkage temperature due to TiC and ZrC have covalent bonds in their structure. Covalent bonds makes harder to sintering process than monolthic phase. TZM-TiC composites have higher displacement rate than TZM-ZrC compozites although displacement rate decreases by addition of carbides. TiC and ZrC content increased relative density and hardness. However, while relative density increased by amount of ZrC, hardness increased by amount of TiC.