Hanjung Kwon

A new method for the identification and quantification of magnetite-maghemite mixture using conventional X-ray diffraction technique.

The electrical explosion of Fe wire in air produced nanoparticles containing the binary mixture of magnetite (Fe(3)O(4)) and maghemite (γ-Fe(2)O(3)). The phase identification of magnetite and maghemite by the conventional X-ray diffraction method is not a simple matter because both have the same cubic structure and their lattice parameters are almost identical. Here, we propose a convenient method to assess the presence of magnetite-maghemite mixture and to further quantify its phase composition using the conventional peak deconvolution technique. A careful step scan around the high-angle peaks as (511) and (440) revealed the clear doublets indicative of the mixture phases. The quantitative analysis of the mixture phase was carried out by constructing a calibration curve using the pure magnetite and maghemite powders commercially available. The correlation coefficients, R(2), for magnetite-maghemite mixture was 0.9941. According to the method, the iron oxide nanoparticles prepared by the wire explosion in this study was calculated to contain 55.8 wt.% maghemite and 44.2 wt.% magnetite. We believe that the proposed method would be a convenient tool for the study of the magnetite-maghemite mixture which otherwise requires highly sophisticated equipments and techniques.

Effect of milling on properties and consolidation of AlN by high-frequency induction heated sintering

Commercial AlN powders were high-energy ball milled for various durations, and consolidated without a binder, using the high-frequency induction heated sintering method (HFIHS). The effect of milling on the sintering behavior, crystallite size and mechanical properties of AlN compacts were evaluated. A dense AlN compact with a relative density of up to 96% could be readily obtained within 1 min. The ball milling effectively refined the crystallite structure of AlN powders, and facilitated the subsequent densification. The sinter-onset temperature was reduced appreciably, by the prior milling for 10 h from 750°C to 600°C. Accordingly, the relative density of AlN compact increased, as the milling time increased. It is clearly demonstrated that a quick densification of AlN bulk materials to near the theoretical density could be obtained by the combination of HFIHS and the preparatory high-energy ball milling process.

Deoxidation and subsequent densification of sintered titanium by Ca deoxidizer

In this study, a sintering process involving simultaneous deoxidation and sintering was developed, using the vapor of a deoxidizer (Ca) to produce low-oxygen, high-density titanium. Compared to a series of reference titanium powder samples, the bodies sintered with Ca exposure had an ∼1,400 ppm lower oxygen content, resulting in an ∼2% increase in their density. This appears to be the result of decreased surface oxidation on the titanium, as increased oxidation suppresses densification at high sintering temperatures. The effect of the Ca deoxidizer was a lower oxidation, thereby increasing the sintered density. The Vickers hardness of sintered bodies from both experiments increased from an initial value of ∼249 Hv at 900 °C to ∼286 Hv at 1,400 °C, respectively. Moreover, both experiments revealed a tensile strength of ∼170 MPa at 900 °C that finally increased to ∼218 MPa at 1,400 °C. Meanwhile, the bodies sintered with Ca, the elongation was on average over 1% higher than contrary samples.

Preparation of Ti-Mo-Si alloy powders with enhanced high-temperature oxidation resistance using Ti-10Mo scraps

To recycle Ti alloy scraps and enhance the functionality of the recycled scraps, we manufactured low-oxygen contented Ti-Mo-Si powders using two types of ternary Ti alloy ingots which were prepared by vacuum arc melting of Ti-Mo scrap with different amounts of Si. The weight ratio of Ti-Mo scrap was Ti:Mo=9:1 and Si contents in two alloy ingots were 1 and 2 wt%. The alloy powders were prepared by the hydrogenation-dehydrogenation process and the oxygen atoms were removed by the deoxidation in solid state method. We confirmed that the deoxidation process effectively reduced the oxygen content of Ti-Mo-Si alloy powder from 3,350 ppm to 1,700 ppm. In addition, we confirmed that the high-temperature oxidation resistance was increased by adding Si to Ti-Mo alloys. Thermal gravimetric analysis showed that the weight gain of powders by the oxidation was reduced from 8.85% for Ti-Mo to 7.5% for Ti-Mo-Si at 1,500 °C.

Ti-Based Solid Solution Carbonitrides Prepared From Ti-Alloy Scraps via a Hydrogenation-Dehydrogenation Process and High-Energy Milling

Ti-based solid-solution carbonitrides (Ti,Al,V)(CN) and (Ti,Al,Mo,V)(CN), were synthesized successfully using Ti-6Al-4V (Ti-64) and Ti-8Al-1Mo-1V (Ti-811) alloy scraps via hydrogenation-dehydrogenation and highenergy milling processes. A single phase of (Ti,Al,V)(CN) could be readily synthesized by the high-energy milling of Ti-64 alloy with graphite in a nitrogen atmosphere regardless of the carbon content. On the other hand, for the Ti-811 alloy, metallic Mo and various Mo-less carbides, in this case Ti2AlC, Ti3AlC2, and Ti3AlC, were also formed in addition to (Ti,Al,Mo,V)(CN) due to the low nitrogen affinity of Mo. The solid-solution carbonitrides consolidated by spark plasma sintering revealed excellent mechanical properties (HV: 19.1-20.6 GPa, KIC: 5.2-6.4 MPa·m1/2) due to the alloying effect of Al, Mo, and V in Ti(CN). These values are superior to those of typical Ti(CN)–based ceramic composites (HV: 16-20 GPa, KIC: 3.2-5.5 MPa·m1/2). We believe that the suggested method would be a valuable option for the production of Ti-based solid-solution carbonitrides with decent mechanical properties economically.

Sintering properties of Ti-6Al-4V-xMo powder prepared by addition of Mo to Ti-6Al-4V scraps and subsequent pulverization

In this study, the sintering properties of Ti-6Al-4V-xMo powder prepared by an addition of Mo to Ti-6Al-4V scraps and subsequent pulverization were investigated. As the content of Mo added to Ti-6Al-4V scraps as a β stabilizer increased, the weight ratio of the α and β stabilizers in the Ti-6Al-4V-xMo changed and the original weight ratio of 6:4 varied to 5.71:8.57 when 5 wt% xMo was added. In order to compare the difference in properties of Ti-6Al-4V-xMo ingots with sintered bodies of the Ti-6Al-4V-xMo powder, we prepared sintered bodies from Ti-6Al-4V-xMo powder with an O content of about 5000 ppm and 325 mesh size. As a result, it was found that the sintered bodies of Ti-6Al-4V-xMo powder showed different properties of density and micro hardness compared to the Ti-6Al-4V-xMo ingots. These differences can be explained by the larger specific surface area of the sintered bodies, which formed a porous oxide layer on the surface due to the increase of Mo in the β zone of the Ti-6Al-4V alloys.

Simultaneous Synthesis and Rapid Consolidation of Nanostructured (Ti,Mo)C and Its Mechanical Properties

Nanocrystalline materials have recently received significant attention in the area of advanced materials engineering due to their improved physical and mechanical properties. A solid-solution nanocrystalline powder, (Ti,Mo)C, was prepared via high-energy milling of Ti-Mo alloys with graphite. Using XRD data, the synthesis process was investigated in terms of the phase evolution. Rapid sintering of nanostuctured (Ti,Mo)C hard materials was performed using a pulsed current activated sintering process (PCAS). This process allows quick densification to near theoretical density and inhibits grain growth. A dense, nanostructured (Ti,Mo)C hard material with a relative density of up to 96 % was produced by simultaneous application of 80 MPa and a pulsed current for 2 min. The average grain size of the (Ti,Mo)C was lower than 150 nm. The hardness and fracture toughness of the dense (Ti,Mo)C produced by PCAS were also evaluated. The fracture toughness of the (Ti,Mo)C was higher than that of TiC.