B4c ilaveli tzm alaşımlarının spark plazma sinterleme (SPS) yöntemi ile farklı tasarımlarla üretimi ve karakterizasyonu

Abstract

Pure molybdenum (Mo) is one of the most important refractory metals, and it is demanded material for high temperature applications which are required high melting point, good thermal conductivity and low thermal expansion coefficient. Furnace and glass melting components, power semiconductor heat sinks, isothermal forging dies and x-ray targets are the main applications of Mo and Mo-based alloys. In addition, high temperature structural applications in military, aerospace and nuclear industries are the targeted usage areas for Mo-based alloys. Applications of pure Mo are generally limited due its relatively low recrystallization temperature compare to its melting temperature, high ductile-brittle transition temperature and poor mechanical properties at elevated temperature. Mo-based alloys are more suitable than pure Mo for high temperature applications above 1000 °C. Alloying is mostly preferred method for molybdenum to increase the performance during the application. Titanium-Zirconium-Molybdenum (TZM) is one of the most important Mo-based alloys which has a composition containing 0.40–0.55 wt% titanium, 0.06–0.12 wt% zirconium and 0.010–0.040 wt % carbon. The alloying elements in TZM form TiC and ZrC within the grains and at grain boundaries. Thus, they provide solution hardening and particle hardening, and stabilize the wrought grain structure to inhibit recrystallization. Homogeneously dispersed carbides stabilize dislocation array, and increase the thermal energy to cause recovery and recrystallization. While the temperature for 100% recrystallized structure (t = 1 h) is 1100 °C for pure Mo, it reaches to 1400 °C for TZM. Besides the recrystallization temperature higher creep and tensile strength are obtained by TZM at elevated temperature. The main limitation for Mo and Mo-based alloys is poor oxidation resistance above 650 °C. Generally they are exposed to high temperature and oxidizing atmosphere during the possible applications. There is no restriction to use in air or oxidizing atmosphere without any protection below 400 °C. However, mass gain rapidly increases between 400 and 650 °C due to formation of oxidation products such as MoO2 and other oxides (MoOZ), where 2 ≤ z < 3. Formation and rapid vaporization of MoO3 result in heavy mass loss and increase in oxidation rates above 650 °C. Dramatic mass loss will cause catastrophic failure of the structure irreversibly. Oxidation of molybdenum and molybdenum based alloys have been investigated for many years. There are many studies and protection methods tried to protect molybdenum against to oxidation. It is clear that Mo is not suitable to be used in practical applications without protection system in oxidizing atmosphere at elevated temperature. The protection systems applied to Mo and Mo-based alloys can be considered as two groups, mainly alloying and coating. Alloying could provide a formation of protective layer on the surface of base metal during oxidizing conditions. In addition to alloying, studies about protective coatings over TZM mainly focused on the methods such as pack cementation, halide activated pack cementation, chemical vapor deposition, physical vapor deposition, thermal plasma spray coating, vacuum plasma coating, laser surface alloying etc. Production methods for TZM alloy are also restricted due to oxidation problem and high melting point. There are two main methods to produce TZM alloy generally used in the literature. They are vacuum arc remelting (VAR) and powder metallurgy including hot press and hot isostatic press. The common side of these methods is that they are under vacuum or inert gas atmosphere. Desired properties could not be achieved by VAR method in single step due to the formation of segregation. Stirring is not possible during VAR process, so segregation of alloying elements (Ti, Zr and C) is inevitable. Therefore, it is required to crush and re-melt the alloy several times which bring time and energy consuming processes to TZM alloy production. Furthermore, to give the TZM alloy a final shape as a product hot forging or extrusion is needed after VAR process. Grain coarsening is also another drawback for this technique. In this study, spark plasma sintering (SPS) is used as production method. SPS has been utilized both to overcome the difficulties faced in VAR. SPS has been used as a sintering method to densify TZM alloys at relatively lower temperatures for shorter holding times compared to the conventional ones. A pulsed direct current passes through the graphite punch rods and dies simultaneously with a uniaxial pressure to sinter powders or pre-mixed powders in SPS sintering. During the process grain coarsening can be suppressed by rapid heating and the densification of the powders are accelerated at higher temperatures. The process is also occurred under vacuum atmosphere. The main goal of this study is improve the high temperature performance and mechanical properties of TZM alloy. Experimental studies regarding the aim and targets of the thesis are performed by using SPS method. The powder compositions prepared by mainly boron carbide (B4C), silicon (Si) and alumina (Al2O3) additions to premixed TZM powder were molded with three different designs and then sintered. These molding designs, which also constitute the original aspect of the study, can be summarized as follows: I. Varying amount of B4C powders were dispersed into TZM powder, than sintering was occurred with constant pressure (40 MPa) and holding time (5 min). II. TZM powder was molded between B4C powders. Both sintering and boriding process were performed in a single step at constant temperature of 1420 °C in various pressures (40–60 MPa) and holding times (5–15 min) under vacuum. III. TZM powder was molded between B4C-Si or Al2O3-Si powder mixture with various holding times (5-10 min) and constant pressure (40 MPa) Process was performed by spark plasma sintering equipment (SPS-7.40 MK-VII, SPS Syntex Inc). Direct current (12 ms/on, 2 ms/off) was applied during the entire SPS process under vacuum. Temperature was measured by an optical pyrometer (Chino, IR-AH) focused on the surface of the graphite die. Although the sample is exposed to higher temperature, it is not known exactly. The sintering process was controlled by monitoring the shrinkage behavior of the powders rather than the temperature measured by pyrometer. Different types of samples were obtained in all three designs. In the first design, Mo-B and Mo-C based compounds were obtained in the matrix of TZM in addition to TiC and ZrC. Amount of additional phases formed in the matrix changed by amount of B4C addition. In the second design, a Mo-B based layer was formed on the surface of the TZM matrix with varying thicknesses and morphologies. In the third design, B4C-Si and Al2O3-Si powders placed on the top and bottom of TZM powders were not separated from the surface after sintering and sandwich-type samples were obtained in ceramic-metal-ceramic structure. As a result of the optimization studies, the densification behavior of the samples produced in different molding designs and parameters were determined on the withdrawal curves obtained during sintering of the powders. Bulk density of the specimens was determined by Archimedes' method. Phase analyzes of the stuructures formed on the surface or in the matrix of the samples were identified by X-ray diffractometry (XRD; MiniFlex, Rigaku Corp.) in the 2θ range of 10–90° with Cu-Kα radiation (λ=1.54 Å). Vickers hardness measurements were performed from surface or cross section depending on the design in order to investigate the effect of phases formed on the surface orin the matrix of TZM alloy after sintering. Microstructural characterization of the sintered composites were conducted with scanning electron microscopy (FESEM; JSM 7000F, JEOL Ltd.). Energy dispersive X-ray spectroscopy (EDS; Oxford INCA) was also used to investigate the composition of phases formed after sintering process. In order to determine oxidation behavior which is the most important part of the thesis, oxidation tests were carried out at different temperatures (600-1000 °C) in air atmosphere. Prepared samples were placed into alumina crucible and put in a laboratory muffle furnace after the surface areas were measured to calculate mass change in mg/cm2. In addition, flame tests under dynamic conditions have been performed to simulate the conditions in potential uses. Aa a conclusion, three different designs for sample production have been studied in order to increase the performance of TZM alloy especially during the use of oxidation resistance. Before starting the experimental studies, possible reactions and products were calculated thermodynamically by FactSage© software. After the sample productions, the phases determined by the phase analyzes were consistent with the thermodynamic data calculated before experimental studies. When examined in terms of mechanical properties, composite samples produced with B4C, Si and Al2O3 additives had higher hardness values than monolithic TZM. According to the results of the oxidation tests which constitute the most important part of the study, all three designs contributed to increase the oxidation resistance. The results of the dynamic flame test also showed that the samples produced could operate under conditions of similar difficulty in practice.