Jeoung Han Kim

High-temperature oxidation behaviour of low-entropy alloy to medium- and high-entropy alloys

The high-temperature oxidation behaviour of CoCrNi, CoCrNiMn, and CoCrNiMnFe equimolar alloys was investigated. All three alloys have a single-phase face-centred cubic structure. Thermogravimetric analyses (TGA) were conducted at temperatures ranging from 800 to 1000xa0°C for 24xa0h in dry air. The kinetic curves of the oxidation were measured by TGA, and the microstructure and chemical element distribution in different regions of the specimens were analysed. The oxidation kinetics of the three alloys followed the two-stage parabolic rate law, with rate constants generally increasing with increasing temperature. CoCrNi displayed the highest resistance to oxidation, followed by CoCrNiMnFe and CoCrNiMn exhibiting the least resistance to oxidation. The addition of Mn to CoCrNi increased the oxidation rate. The oxidation resistance of CoCrNiMn was enhanced by the addition of Fe. Less Mn Content and the formation of more Cr2O3 were responsible for the reduction in the oxidation rates of CoCrNiMnFe. The calculated activation energies of CoCrNiMn and CoCrNiMnFe at 800, 850 and 900xa0°C were 108 and 137xa0kJxa0mol−1, respectively, and are comparable to that of Mn diffusion in Mn oxides. The diffusion of Mn through the oxides at 800–900xa0°C is considered to be the rate-limiting process. The intense diffusion of Cr at 1000xa0°C contributed to the formation of CrMn1.5O4 spinel with Mn in the outer layer of CoCrNiMn and Cr2O3 in the outer layer of CoCrNiMn.

Effect of yttrium on martensite-austenite phase transformation temperatures and high temperature oxidation kinetics of Ti-Ni-Hf high-temperature shape memory alloys

The effect of yttrium (< 5.5 at%) on the martensite-austenite phase transformation temperatures, microstructural evolution, and hot workability of Ti-Ni-Hf high-temperature shape memory alloys is investigated. For these purposes, differential scanning calorimetry, hot compression, and thermo-gravimetric tests are conducted. The phase transformation temperatures are not noticeably influenced by the addition of yttrium up to 4.5 at%. Furthermore, the hot workability is not significantly affected by the yttrium addition up to 1.0 at%. However, when the amount of yttrium addition exceeds 1.0 at%, the hot workability deteriorates significantly. In contrast, remarkable improvement in the high temperature oxidation resistance due to the yttrium addition is demonstrated. The total thickness of the oxide layers is substantially thinner in the Y-added specimen. In particular, the thickness of (Ti,Hf) oxide layer is reduced from ~200 µm to ~120 µm by the addition of 0.3 at% Y.

Evolution of Microstructure and Mechanical Properties of Oxide Dispersion Strengthened Steels Made from Water-Atomized Ferritic Powder

Nano-structured oxide dispersion strengthened (ODS) steels produced from a 410L stainless steel powder prepared by water-atomization was studied. The influences of Ti content and milling time on the microstructure and the mechanical properties were analysed. It was found that the ODS steels made from the Si bearing 410L powder contained Y–Ti–O, Y–Ti–Si–O, Y–Si–O, and TiO2 oxides. Most nanoparticles produced after 80xa0h of milling were aggregated nanoparticles; however, after 160xa0h of milling, most aggregated nanoparticles dissociated into smaller individual nanoparticles. Perfect mixing of Y and Ti was not achieved even after the longer milling time of 160xa0h; instead, the longer hours of milling rather resulted in Si incorporation into the Y–Ti–O rich nanoparticles and a change in the matrix morphology from an equiaxed microstructure to a tempered martensite-like microstructure. The overall micro-hardness of the ODS steel increased with the increase of milling time. After 80 and 160xa0h, the microhardnesses were over 400 HV, which primarily resulted from the finer dispersed nanoparticles and in part to the formation of martensitic phases. Tensile strength of the 410L ODS steels was comparable with that of ODS steel produced from gas-atomized powder.

Diffusion pack cementation of hafnium powder with halide activator on Ni–Ti shape memory alloy

The effect of a Hf chloride activator on the pack cementation of Hf powder on a Ni–Ti shape memory alloy wire was investigated. For this purpose, a Ni–Ti wire with a diameter of 0.5xa0mm was pack cemented in a powder mixture consisting of Hf and HfCl4 powders at 1000xa0°C for 24xa0h. It was observed that Hf noticeably diffused into the Ni–Ti matrix with the aid of the HfCl4 activator. The diffusion distance significantly increased as the amount of HfCl4 activator increased. By the addition of 10xa0mass% HfCl4, the martensite-to-austenite phase transformation start and finish temperatures increased from 12 to 142xa0°C and from 28 to 200xa0°C, respectively. The diffusion kinetics model was established based on Fick’s first law. It is suggested that 48xa0h of halide-activated pack cementation with 10xa0wt% HfCl4 is necessary to increase the overall Hf content above 15xa0at.% throughout the Ni–Ti wire.

Stability of Y-Ti-O nanoparticles during laser deposition of oxide dispersion strengthened steel powder

This study investigated the feasibility of a direct energy deposition process for fabrication of oxide dispersion strengthened steel cladding. The effect of the laser working power and scan speed on the microstructural stability of oxide nanoparticles in the deposition layer was examined. Y-Ti-O type oxide nanoparticles with a mean diameter of 45 nm were successfully dispersed by the laser deposition process. The laser working power significantly affected nanoparticle size and number density. A high laser power with a low scan speed seriously induced particle coarsening and agglomeration. Compared with bulk oxide dispersion strengthened steel, the hardness of the laser deposition layer was much lower because of a relatively coarse particle and grain size. Formation mechanism of nanoparticles during laser deposition was discussed.