Production and characterization of W-Ni matrix composites reinforced with CeB6, NdB6, ErB4 particulates by powder metallurgy methods

Boztemur, Burçak
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Graduate School
Energy is the most important topic for centuries. An alternative energy resource should be considered for the survival of humanity and environment because of decreasing resources of energy. Although, fusion is not commercialized yet, it has the potential to be an excellent alternative even to renewable energy resources due to its lack of radioactive waste and harmful emissions. There are many types of fusion energy. Tokamak where magnets form a donut shape is one of fusion energy type. Many experiments have been carried out in these Tokamaks all over the world, the largest currently being built in France, known as ITER (International Thermonuclear Experimental Reactor). Plasma facing materials (PFMs), which act as the protective armor of fusion reactors and directly face the huge energy from fusion reactions, become one of the main problems that limit the practical application of fusion reactions. Tungsten (W) and its alloys are considered to be the most promising PFM candidates as the armor material in divertor components due to its high melting point, high strength at elevated temperatures, high elastic modulus, low thermal expansion, high thermal conductivity, high sputtering threshold, low hydrogen isotope retention, high resistance to neutron damage and low activation compared to other fusion relevant materials. However, the extremely harsh operational environment in the fusion reactor would inevitably result in serious performance degradation of PFMs. Under helium ion irradiation, the microstructure of tungsten-based materials is obviously changed and causes the brittleness of materials. Under the thermal shock loads, the W-based materials experience severe plastic deformation and crack growth. Therefore, improving the resistance of helium ion irradiation and thermal shock is of vital importance for engineering application. In order to overcome the severe problems of W-based materials which are hard to produce due to its high melting point, researchers investigated grain refinement, second phase addition and deformation. The most valid technique proposed to overcome these difficulties is the method called activated sintering. Activated sintering is the process of improving sinterability as a result of small additives to the powder composition or various changes made in the sintering atmosphere. In order to achieve activated sintering, the metal with a low melting temperature (various transition metals such as Pd, Pt, Ni, Cu, Co and Fe) must be dissolved in the metal with a high melting temperature. In this way, the sintering path is shortened and the process is facilitated. In addition, up to now, various carbides and nitrides such as TiC, ZrC, HfC, TiN, various oxides such as La2O3, Y2O3, HfO2, Sm2O3, ThO2 and various borides such as TiB2, HfB2, ZrB2, CrB2 were incorporated into W matrix to form composites. It has been revealed that composites which are reinforced with oxide, carbide, nitride, boride phases offer better and superior mechanical properties. Moreover, oxide, boride or carbide reinforced and activated sintered W matrix composites such as W-Cu-CeO2, W-Ni-TiC-Y2O3, W-Cu-Ni-Y2O3, W-Ni-La2O3, W-Ni-TiB2, W-Ni-TiB2-La2O3, etc. exhibit enhanced microstructural and mechanical properties. In this study, CeB6, NdB6 and ErB4 rare-earth borides were produced with mechanochemical synthesis. Different milling time (0, 2, 3, 4, 5, 6, 7 and 8 h) and speed (920 and 1188 rpm) were tried for production. Also, 5 h and 920 rpm were used for optimization of milling. For this process, stainless steel balls and vials were used. Then, pure boride powders were synthesized after HCl leaching process. 99 wt% W and 1 wt% Ni were mechanically alloyed for 800 rpm and 6 h. For all process, WC balls and vials were used and a ball-to-powder weight ratio was 10:1. After that, rare-earth boride powders (1, 2, 5 and 10 wt%) were added into W1Ni with 6 h mechanical alloying. For phase characterization of composite powders, XRD was used: four main W peaks were obtained with small amount WC impurity. The lattice strains and crystallite sizes of the milled powders were calculated with TOPAS software based on the XRD patterns. When the lattice strains increased, the crystallite sizes decreased because of the milling process. Then, density of powders was measured using a He gas pycnometer. When amount of rare-earth boride particulate increased, density of W1Ni composite powders decreased. Particle size of powders was measured with ethanol solution and lognormal distribution was obtained. Also, particle sizes were decreasing with increasing amount of reinforcing. Powder morphology, size and distribution were determined with SEM/EDS. TEM analysis was done for NdB6 and ErB4 powders. After powder characterization, two different methods that are pressureless sintering and spark plasma sintering were used for consolidation of powders. Before pressureless sintering, powders were compacted 13 mm diameter and 5 mm height with hydraulic pressing (480 MPa, 1 min) and cold isostatic pressing (390 MPa, 1 min). Then, pressureless sintering was applied to the bulk samples. Samples were holded in vacuum atmosphere until 100 C. Then, Argon gas was given into chamber between 100-750 C. Hydrogen gas was used for getting rid of oxide phases between 750-900 C. After this period, Ar gas was used always and cooling/heating rate is 10 C /min. These samples were holded at 1400 C for 1 h. Producing powders were spark plasma sintered in graphite mold as 20 mm diameter and 2 mm height with graphite foils in the Hefei University of Technology (HFUT). For SPS, cooling/heating rate was 100 C/min until 600 C. Samples were holded 5 min at 600 C. After that, temperature was reached to 1410 C for 1 min with 90 C /min and cooled. In all this process, pressure was started with 4.4 kN and increased to 9.4 kN. After producing of bulk samples, XRD analyses were done for all samples. According to the results, four main W peaks were obtained with other smallest peaks. However, W2B was started to be main phase after adding 5 wt% rare-earth borides. Densities of the sintered samples were measured with Archimedes' principle. The highest density was obtained for W1Ni reinforcing with 10 wt% NdB6 particulates SPS sample that had 100% relative density. SEM/EDS was done for all samples. Three different phases as grey, black and white were determined for samples. Vicker microhardness test was done for samples at 200 gram and 10 sec. The SPS'd W1Ni sample reinforced with 10 wt% ErB4 particulates had the highest value (21.16±0.72 GPa). After that, wear resistance test was done for samples. Wear testing conditions were selected as a sliding speed of 6 mm/s, sliding distance of 20 m and the wear track length of 2 mm. The tests were performed with a normal load of 4 N at room temperature. The SPS'd W1Ni sample reinforced with 10 wt% ErB4 particulates had the lowest wear volume loss (0.341×10-4 mm3). Finally, He+ irradiation tests were applied to samples in HFUT. All samples were exposed to 20-eV He+ ion irradiation with a flux of 1.102×1021 ions/(m2s). Irradiance fluence was set 1.32×1024 ions/m2 with 20 min durations. After this test, XRD peaks of W1Ni and all reinforced with 2 wt% CeB6, NdB6, ErB4 and 5 wt% ErB4 pressureless sintered samples were shifting to right. Also, XRD peaks of reinforced with 1 wt% CeB6, NdB6, ErB4, 5 wt% CeB6, NdB6 and 10 wt% ErB4 pressureless sintered samples were shifting to left side. While 5 wt% NdB6 pressureless sintered sample gave bad results, 2 wt% CeB6 pressureless sintered sample had less surface deformation after irradiation test as SEM images results. As a conclusion, two different production methods and different amounts of rare-earth boride particulates were compared in this study. Different characterization methods were used for analysing samples. Properties of W1Ni composite material was improved with adding rare-earth boride particulates. Enhanced hardness and wear resistance properties were achieved for potential high-temperature applications and fusion reactor applications.
Thesis (M.Sc.)-- ITU Graduate School, 2022
Anahtar kelimeler
Powder metallurgy, Toz metalurjisi, Sintering, Külçeleme