TZM alaşımının spark plazma sinterleme (SPS) yöntemi ile üretimi ve karakterizasyonu
Production and characterization of TZM alloys prepared by spark plasma sintering (SPS)
Yazar: CAN BURAK DANIŞMAN
Danışman: PROF. DR. GÜLTEKİN GÖLLER
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Refrakter metaller, 1000 °C ve üzeri yüksek sıcaklıklarda çalışabilen, korozyona ve aşınmaya karşı dirençli malzemelerdir. Bu özellikleri ile uçak-uzay, elektrik-elektronik ve nükleer endüstrilerinde kullanılırlar. Bu çalışma kapsamında, refrakter malzeme grubundan TZM (titanyum- zirkonyum-molibden) alaşımının üretimi ve karakterizasyonu gerçekleştirilmiştir. Ön alaşımlandırılmış TZM tozları, toz metalurjisi prosesi olan SPS (spark plazma sinterleme) tekniği ile sinterlenmiştir. Sinterleme sıcaklığı, süre ve basınç gibi parametrelerin TZM numunelerinin rölatif yoğunluğu, ortalama tane boyutu, sertlik ve kırılma tokluğu özellikleri üzerindeki etkileri incelenmiştir. TZM alaşımlarının SPS tekniği kullanılarak üretimi ile ilgili bir çalışmanın literatürde bulunmaması, bu çalışmanın özgün yönünü oluşturmaktadır.
Bu çalışmada; kullanılan ön alaşımlandırılmış TZM tozu, 4 mm yüksekliğinde 50 mm çapında numuneler elde edilecek şekilde SPS cihazı ile sinterlenmiştir. Üretimler; vakum ortamında 1400-1700°C sinterleme sıcaklıklarında, 40-80 MPa basınç altında ve 2,5-5 dk. sinterleme süreleri ile gerçekleştirilmiştir.
Rölatif yoğunluk ölçümleri, Arşimet prensibi kullanılarak belirlenmiştir. Sinterlenen numunlerin yoğunluklarının %92-98 arasında olduğu belirlenmiştir.
Tane boyutu analizi dağlanmış numunelerin optik mikroskop görüntüleri üzerinde lineer kesişme metodu ile yapılmıştır. Deney parametrelerinin ortalama tane boyutunu 6-50 µm arasında değişmektedir.
Taramalı elektron mikroskobu ile yapılan mikroyapı ve faz analizleri sonucunda tane sınırında TiC, ZrC ve Mo2C partikülleri gözlenmiştir. 
Sertlik değerleri, Vickers sertlik cihazında ölçülmüş ve sinterlenen numunlerin sertliklerinin 1,75-2,04 GPa arasında değiştiği sonucuna varılmıştır.
Artan sinterleme sıcaklığı bir birlikte numunelerin rölatif yoğunluklarının arttırdığı belirlenmiştir. 1450°C'de yapılan üretimlerde rölatif yoğunluklar yaklaşık %92 olarak ölçülmüşken, sıcaklığın 1525°C olduğu üretimlerde %95 üzerinde relatif yoğunluk değeri elde edilmiştir. 1700°C'de yapılan üretimde ise %98 relatif yoğunluğa ulaşılmıştır. 250 °C'lik sıcaklık artışının yoğunluğu %6 arttırdığı , tane boyutunu  %80 oranında arttırdığı görülmüştür. Tane boyutu artışının sertlik değerlerini azalttığı fakat önemli bir miktarda değiştirmediği gözlemlenmiştir.
Sinterleme süresi 2,5 ve 5 dakika olmak üzere çalışılmıştır. Sinterleme süresinin rölatif yoğunluk, ortalama tane boyutu ve sertlik üzerinde önemli bir etkisi olmadığı belirlenmiştir.
Basınç değerleri 40 ile 80 MPa arasında değişecek şekilde çalışılmıştır. Sinterleme basıncının 40 MPa artması ile rölatif yoğunluk değerlerinin %4 arttırdığı görülmüştür. Ortalama tane boyutunu ve sertlik değerlerinin artan sinterleme basıncı ile birlikte değişmediği saptanmıştır.
Numunelere kimyasal analiz yapıldığında bileşimlerinin standart'ın belirttiği değer ile paralellik gösterdiği görülmüştür.
TZM alaşımının yüksek sıcaklık davranışları molibden ile karşılaştırıldığında TZM'nin yüksek sıcaklıkta tanelerinin kabalaşmadığı dolayısı ile muadili molibdene göre çalışma şartlarının daha iyi olduğu analaşılmıştır.
SPS ile üretimin sağladığı avantajlar ile, TZM tozları geleneksel sinterleme yöntemleri ve vakum ark ergitme tekniğine göre daha kısa süre ve daha düşük sıcaklıklarda başarılı bir şekilde üretilmiştir. Sonuç olarak tane büyümesi kontrol altına alınmış, yoğun malzemeler üretilmiştir.

Abstract:
Refractory materials exhibit high temperature strength (at 1000°C), high creep resistance, high thermal conductivity(60-130 W/m·K at 500°C) and low coefficient of thermal expansion(4.9-7.3 µm/m·K). Among them, molybdenum has a wide application range such as aerospace, nuclear and electronic industries. Along with the improvements of technology, the demand of using molybdenum has been increased, as consequently TZM alloy has been developed. TZM alloy has been generally desired for high temperature applications such as heat engines, heat exchangers, nuclear reactors, radiation shields, extrusion dies and boring bars.
TZM alloy is a Molybdenum based alloy with the small amounts of Ti, Zr and C additives. The main purpose is to form carbides in alloy and disperse them homogeneously. Carbides  located at grain boundaries inhibit the grain growth, increase the recrystallization temperature and improve the creep properties. 
Due to the high melting temperature of TZM alloy (2620 °C), there are only two production techniques, (i)vacuum arc melting, (ii)powder metallurgy. When comparing these two methods with each other, sintering comes forward with the consistency of alloy composition, homogeneity and phase distribution properties.
TZM alloy has been prepared by several researchers using vacuum arc melting and powder metallurgy such as hot isostatic pressing. However, comparatively, there has not been any study on SPS'ed TZM alloy. SPS makes possible to densify materials at a lower temperature and in a shorter time when compared with the conventional techniques. In this method, a pulsed direct current passes through graphite punch, dies and powders simultaneously with an uniaxial pressure. Thus, the grain growth can be suppressed by rapid heating and the densification is accelerated at high temperature. Furthermore, the microstructure can be controlled by a fast heating rate and shorter processing times. In this study, TZM alloys were produced using SPS method at different sintering temperature, holding time and pressure. Densification, microstructure, average grain size, hardness and fracture toughness of samples were characterized.
Pre-alloyed TZM alloys were sintered using SPS. The sintered specimens were in the form of pellets with 50 mm diameter and 4 mm thickness. Production was carried out under vacuum with variable parameters as 1400-1700 °C, 2.5-5 minute holding time and under uniaxial pressure of 40-80 MPa.
Gibbs free energy needed for formation of TiC and ZrC were calculated with Factsage program. For these calculations sintering temperature were estimated as 1600 °C. Temperature and Gibbs free energy diagram were examined at 1600 °C, -157 and -181 kJ was needed for TiC and ZrC, respectively. In this study, formation of these particles was observed in different sintering conditions such as temperature, holding time and pressure. Since the Gibbs free energy was negative, carbide structures have been formed by itself. 
Relative densities were measured and calculated by Archimedes method. Consequently results were between 92-98%. In experiments completed at low temperature relative density were measured 92%, at high temperatures relative density was measured 98%. It was observed that according to increment at sintering temperature from 1450 to 1700 °C with same holding time and sintering pressure, relative density increased about 2.46% from 95.57% to 98.03%. It was understood that, relative density increased with increasing temperature. It was observed that 150 second difference at holding time did not make a significant change in  the relative density. 40 MPa increment at sintering pressure from 40 to 80 MPa with the same sintering temperature and the holding time was increased relative density about 3.76% from 91.81% to 95.57%. So relative density increased with increasing pressure.
The linear intercept method was applied for average grain size measurement for chemically etched, polished surfaces. Eventually as received grain sizes, which were seen at optical microscope, were between 6-50 µm. Effects of the temperature, holding time and pressure were observed upon average grain sizes. As seen in results there was 80% increment from 6 to 50 µm within increasing 300 °C sintering temperature at constant holding time and pressure. Decreasing the holding time to 150 second, did not make a significant change and set values at 30 µm. And also, increasing sintering pressure about 40 MPa, there was not a significant change in average grain size.
Microstructure and phases were analyzed with scanning electron microscope. Lastly TZM alloy's characteristic carbide structures, whose Gibbs free energy were calculated with Factsage program, TiC and ZrC were monitored at grain boundaries. To see the distribution of phases elemental mapping were completed, and it was understood that phases distributed homogeneously.
Sample's hardness and fracture toughness values were investigated to see the mechanical properties. Hardness was measured by Vickers hardness test method and the hardness values were between 1.77-2.04 GPa. It was concluded that changing sintering parameters such as temperature, time and pressure did not affect micro hardness values significantly. In fracture toughness test, values were obtained with three point bending test. It was understood that, increasing the temperature about 300 °C fracture toughness values decreased about 9.5 MPa·m1/2 from 25.9 to 16.4 MPa·m1/2 . Decreasing the holding time to 150 second, did not make a significant change in fracture toughness. İncreasing the sintering pressure about 40 MPa, increased the fracture toughness values about 7 MPa·m1/2 from 18.9 to 25.9MPa·m1/2. 
The sample sintered at 1450°C-150min-80MPa were defined the sample with optimum mechanical properties. This sample had small average grain size (8 µm), highest hardness (2.04 GPa) and highest fracture toughness value (25.9 MPa·m1/2). Chemical Analysis was applied upon this sample and it was understood that the analysis result was suitable with standard's composition limit. There is only difference at carbon composition between analysis and reference standard. At further investigation it was seen that, received starting powder was the main reason of this phenomenon.
High temperature behavior of TZM was observed. In this observation the samples with optimum mechanical properties of sintered TZM alloy and molybdenum were used. The microstructure of these two alloy were examined after the annealing process at 1200 °C for one hour and six hour. Molybdenum sample's grains were coarsened but there was not any significant change in TZM grains. So the microstructure properties of TZM were better than molybdenum. And because of this event led to prediction which consist mechanical properties of TZM were higher than molybdenum prediction.
To conclude with, production of TZM alloy by SPS, advance relatively more easy way, instead of conventional powder metallurgy techniques and vacuum arc melting. The aim of this study TZM alloys were obtained homogeneous, pore-free, structure with stabilized grain size at low temperatures and at short time. TiC, ZrC and complex carbides were seen on its microstructure. The main aim was accomplished. TZM alloy which had good relative density, hardness and fracture toughness values were sintered with SPS. And also this sample's composition were suitable with standards. High temperature behavior of TZM was better than molybdenum as predicted.