Peneliti Kepala : Dr. Ir. Eddy Agus Basuki, M.Sc.
Anggota : Dr. Akhmad Ardian Korda, ST., MT. ; Fadhli Muhammad, ST. MT.
Nuclear reactors have been used to generate electricity by more than 30 countries . Because nuclear is a non-renewable energy, one of global requirement for such facilities is be able to work at high efficiency. This needs operations of the reactors at high temperatures and pressures. Materials that can be used at higher temperatures are demanding for nuclear technology. Recently, oxide dispersion strengthened (ODS) ferritic steels has been developed as material for main wall of modern nuclear reactors of 4th generation . These materials have better properties compared with the conventional materials that are limited temperature of about 600°C . The strength of ODS ferritic is provided by the dispersion of ultra-fine oxide particles dispersed in ferrite α-Fe matrix  that act as obstacles for dislocations movement on the slip planes of the crystal, through strengthening mechanism of dispersion hardening . Moreover, ODS ferritic steels have good thermal conductivity, swelling resistance due to neutron radiation and creep resistance. At high temperatures, the oxides particles are very stable as they have ionic bonding that differ with that of the matrix, so that ODS materials have no coarsening problems that normally occur in non-ODS alloys. Mechanical alloying has been used to prepare the homogeneous mixture of material components, i.e., metals and oxides known as composites, prior to consolidation processes known as sintering to obtain compact materials consisting of disperse tiny oxides in the matrix. The properties of the sintered materials are strongly depend on milling variables, such as powder-ball ratio, time of milling, and sintering process variables. This paper reports the effects of milling time on the density and porosity of compacted and sintered Fe-16Cr-4Al-0.4Y2O3 ODS steel. Effect of sintering temperatures on the microstructures of ODS grains as well as the hardness was also considered to gain better understanding of Fe-16Cr-4Al-0.4Y2O3 ODS steel manufacturing.
Samples were prepared for the alloy model of Fe-16Cr-4Al-0.4Y2O3 by weighing pure Fe, Cr and Al2O3 powders, obtained from Sigma Aldrich, in actual powder compositions shown in Table 1. The average sizes of the powder materials were 150 µm, 74 µm, and 70 nm, respectively. Based on binary phase diagrams of Fe-Cr, this model alloy of Fe-16Cr would give single phase of α-Fe ferrite.
Using a planetary ball mill, shown in Fig.1, each sample and SS316 balls of 8 mm and 11 mm diameter, with powder to ball ratio of 1:10, were filled into a silindrical vial. After sealed and closed tightly, the vial was inserted into planetary ball mill. Milling at different times of 30, 60, 90 and 120 minutes were carried out at rotation speed of 1290 rpm.
To remove the stored energy in the milled composite powders, each sample was annealed at 250 °C for 5 minutes in a tube furnace purged by high purity argon. Compaction was done in a compaction machine under 100 kg/cm2 force or each sample to have tablets of about 11 mm diameter and 3 mm thick. Sintering of the sample tablets was carried out at 800 °C, 900 °C, and 1000 °C in a tube furnace under argon purging. The sintered samples were then cut into several pieces of samples using high speed diamond saw for XRD and SEM analysis.
Figs. 2 and 3 show XRD diffraction patterns and x-ray mapping of Cr for composite powders after milling for 30 min. and 90 min, respectively. The XRD patterns and x-ray mapping of both milling conditions confirmed that fully solid solution of α-Fe was obtained in sample milled for 60 minutes. Some pure Cr powder, however, were still un-alloyed in the sample milled for 30 minutes. Therefore, it is concluded from this experiment that to obtained solid solution of ferrite using mechanical alloying method require at least 60 minutes for milling.
For constant compaction force of 100 kg/cm2, the green density of the green pellet was expected affected by the milling time. Fig. 5 shows the effect of milling time on the green density of the samples. Each value of these green densities was obtained as the average values of three different samples. The highest green density of 4.60 gr/cm3 was obtained in the sample milled in 60 minutes. The density of the green pellet was decreased for longer milling time and reached the lowest density of 4.27 gr/cm3 for milling time of 120 minutes.