A study on the in-situ powder fabrication and sintering behavior of oxide dispersion-strengthened (ODS) titanium alloy

Abstract

A study on the in-situ powder fabrication and sintering behavior of oxide dispersion-strengthened (ODS) titanium alloy Hyeon Tae IM Department of Materials Science and Engineering Graduate School Hanynag University Oxide dispersion strengthened (ODS) alloys exhibit excellent mechanical properties due to fine oxide dispersion, but they have great limitations due to utilizing powder fabrication via mechanical alloying. Therefore, the first chapter of this study examined the alloy composition and process that can fabricate ODS pure titanium (Ti) / Ti-6Al-4V (Ti64) powder in situ through a gas atomization (GA) method. The composition and content of the oxide, which can dissolve in molten Ti during melting and precipitate inside powders during cooling in the GA, were derived through thermodynamic calculations. As a priority verification experiment for thermodynamic ideas, the ODS pure Ti powder was fabricated. From the thermodynamic calculations, an optimal composition for the ODS Ti alloy was determined, involving the addition of 1 wt% yttrium oxide (Y2O3) to Ti alloy with an oxygen (O) concentration of 0.1 wt%. The ODS Ti alloy was prepared as a powder through an electrode induction melting gas atomization (EIGA) method. First, rod-shaped ingots were prepared through vacuum arc remelting (VAR), and Y2O3 was coarsely precipitated along the grain boundaries due to the slow cooling rate. Then, the ODS Ti powder was fabricated by EIGA using the rod ingot and spherical powders could be continuously fabricated. The cross-section microstructure of the powder was observed, and the Y2O3 particles with several tens of nm were uniformly distributed inside the powder. These thermodynamic calculations and experiments confirmed that ODS Ti powder could be fabricated in situ using the GA method. Based on these validated experiments, the feasibility of in situ fabrication of ODS Ti64 powder was investigated by EIGA method. Thermodynamic calculations were performed to design the ODS Ti64 by deriving compositions of Y2O3, which is completely dissolved into the molten Ti64 and subsequently reprecipitated during cooling in the EIGA process. In addition, the changes in the equilibrium phase fractions based on varying the Y2O3 content and O concentration in the Ti64 matrix were investigated. From the thermodynamic calculations, an optimal composition for the ODS Ti64 alloy was determined, involving the addition of 1 wt% Y2O3 to Ti64 with an O concentration of 0.1 wt%. The resulting ODS Ti64 powder had spherical morphology, with uniformly distributed Y2O3 particles of several tens of nm in size. The hardness of a conventional Ti64 powder was 354 HV, significantly increasing to 485 HV in the ODS Ti64 powder due to the Y2O3 nanoparticles’ dispersion. This study introduces a novel methodology for fabricating high-quality ODS Ti / Ti64 powder by the in situ GA method. The second chapter of this study, the ODS Ti64 powder fabricated using in-situ GA process technology has attracted significant attention due to tens of nm of Y2O3 is uniformly dispersed inside the powder. In this work, ODS Y2O3/Ti64 alloy was fabricated and characterized from these powders using a spark plasma sintering (SPS) method with a sinter heating rate of 100 °C/min and a sintering temperature of 1100 °C. In the X-ray diffraction (XRD) pattern, the Y2O3 peak was not observed in the conventional Ti64 alloy and was confirmed only in the ODS Y2O3/Ti64 alloy. The microstructure of ODS Y2O3/Ti64 alloy showed alpha (α) and beta (β) phases without apparent pores and cracks. ODS Y2O3/Ti64 alloy was also confirmed that tens of nm of Y2O3 particles were uniformly distributed. The grain size of ODS Y2O3/Ti64 alloy was decreased compared to the conventional Ti64 alloy, because the pinning effect of dispersed Y2O3 particles limited grain growth. As a results of the mechanical test, the Vickers hardness of the conventional Ti64 alloy was 374 HV, but the hardness of the ODS Y2O3/Ti64 alloy increased significantly to 494 HV due to the dispersion of Y2O3 nanoparticles. The compressive yield strength and ultimate strength at room temperature improved to 1,021 MPa and 1,488 MPa. The mechanical behavior of ODS Ti64 alloy at 500 °C was also performed with a yield strength of 781.8 MPa and ultimate strength of 913.7 MPa. Compared with conventional Ti64 alloy, ODS Y2O3/Ti64 alloy exhibits higher yield strength and ultimate strength even at high temperatures. The enhanced strength of ODS Y2O3/Ti64 alloy is mainly attributed to grain refinement and dislocation strengthening. This study provides a new methodology for the fabrication of high-performance ODS Ti64 alloy combining ODS Ti64 powder with fine Y2O3 dispersed inside the powder and sintering technology. The third chapter of this study, the experimental sintering relative density and densification mechanism were analyzed to derive the sintering activation energy during sintering of ODS Ti64 powder. The sintering mechanism and behavior of ODS Y2O3/Ti64 powder were investigated at different heating rates (50 °C/min, 100 °C/min, 150 °C/min). The shrinkage behavior of conventional Ti64 and ODS Y2O3/Ti64 powder during SPS showed that as the heating rate increased, shrinkage started at a lower temperature and the relative density increased. On the other hand, the relative density of ODS Y2O3/Ti64 powder was confirmed to decrease more than that of conventional Ti64 powder. The densification behavior presented that the densification rate increased as the heating rate increased. However, ODS Y2O3/Ti64 powder exhibited a relatively continuous decrease due to the influence of dispersed Y2O3. The activation energy (Q) was derived under a theoretical framework based on the instantaneous relative density and densification rate. Compared with conventional Ti64 alloy, ODS Y2O3/Ti64 powder exhibits higher activation energy. Therefore, it has been verified that ODS Y2O3/Ti64 powder requires more energy for diffusion during sintering due to dispersed tens of nm Y2O3 within the Ti64 matrix. This study compared the sintering behavior activated during sintering of Ti64 powder and ODS Y2O3/Ti64 powder and verified the framework by clarifying the underlying mechanism.