Seong Lee

The effect of yttrium oxide on the sintering behavior and hardness of tungsten

In this study, the relative density and hardness of Y2O3 dispersed tungsten alloy were investigated as functions of the Y2O3 content and sintering temperature. The sintering temperature and the amount of the second phase were varied from 1800 to 2500°C and 0 to 2.0 weight pct, respectively. The relative density of the alloys is higher than that of pure tungsten in the range from 2000 to 2500°C, whereas the density is lower at 1800°C. As the Y2O3 content increases, the sintered density increases at a given temperature. The transition temperature (Ttr), where the relative density of the alloys exceeds that of pure tungsten, is reduced with increased Y2O3 particle content. In order to examine the effect of the second phase on the mechanical property, the hardness of pure tungsten and the alloys are measured. The hardness is mainly dependent upon the relative density of the alloys, rather than the amount of the second phase and tungsten grain size. The relationship between hardness and density is discussed according to the plasticity theory of porous materials.

Property Evaluation of Kinetic Sprayed Al-Ni Composite Coatings

Shaped charge(SC) ammunition is a weapon that penetrates directly the target by made jet from metal liner on impacting at a target. In SC, the liner occupies significantly important role causing an explosion and penetration of the target. The Al-Ni composite coating was deposited on copper liner in a solid state via kinetic spraying to improve the explosive force. The mechanical properties, reactivity and microstructure were investigated to confirm the possibility of kinetic sprayed Al/Ni composite coating as a reactive liner material. Reactive liner using Al/Ni composite exhibited much enhanced reactivity than pure copper liner due to Self-propagating High-temperature Synthesis (SHS) reaction with significantly improved adhesive bond strength. Especially, among the Al/Ni composite coatings, AN11 (the Al versus Ni atomic percent ratio is 1:1) showed the greatest reactivity due to its widest reaction area between deposited Al and Ni.

Heat Treatment Behavior of Tungsten Heavy Alloy

This paper focuses on the variations of static and dynamic properties of tungsten heavy alloy with heat treatment. The matrix phase of 93W-4.9Ni-2.1Fe (weight percent) has been penetrated into W/W grain boundaries during a cyclic heat treatment which consists of repeated isothermal holdings at 1150 °C and water quenching between them. By applying the cyclic heat treatment, the impact energy of tungsten heavy alloy is increased about three times from 57 to 170 J. When the tungsten heavy alloy is cyclically heat treated at 1150 °C and then re-sintered at 1485 °C, W/matrix interface is changed from round to undulated shape. The irregularity of the interface is increased with increasing the number of heat treatment cycles. From the measurement of the residual stress of W grains by X-ray diffraction, it is found that the irregularity of the interface is closely related with strain energy stemmed from the difference of thermal expansion coefficient between W particles and matrix phase. From dynamic ballistic test, it is found that the tungsten heavy alloy with undulated W grains forms many narrow fracture bands which are preferential for the self sharpening effect, thus, for the improvement of the penetration performance.

Investigation of powder size effects on sintering of powder injection moulded 17-4PH stainless steel

ABSTRACT Densification behaviours of 17-4PH stainless steel powders (SUS17-4PH) of three different particle sizes during the sintering were investigated. The samples with different mean particle sizes of 3.17, 4.22 and 8.30 µm were prepared through powder-binder mixing, injection moulding, and solvent and thermal debindings. The in situ shrinkage data measured by dilatometry tests were treated to analyse the densification behaviour of each sample. In order to characterise the densification behaviour with a minimum set of experiments, the master sintering curve (MSC), as well-known and straight-forward approach, was employed. After constructing the MSCs for powders of different particle sizes, the effects of particle size on the densification were analysed in many respects including shrinkage, strain rate, apparent activation energy, and work of sintering.

Process optimisation and microstructural evolution of friction stir spot-welded Al6061 joints

The joint strength and subsequent microstructural evolution of a friction stir spot-welded AA6061-T6 alloy was investigated according to the process parameters: tool rotation speed, dwell time and pin angle of the tool. A maximum tensile shear load of 2.78 kN was obtained from the joints generated under a combination of process variables like 1000 and 1500 rev min−1, 5 s, 5°. Under a fixed pin angle of 5° and a rotational speed of 1000 rev min−1, an increase in dwell time from 1 to 5 s resulted in a considerable increase in tensile shear load. An empirical process map under a fixed tool design is proposed to determine a feasible range of process conditions.

Super low work function of alkali-metal-adsorbed transition metal dichalcogenides

Discovering the materials that have work functions less than 1 eV is essential for efficient thermionic energy converter (TEC). The lowest work function of materials reported so far is in a range of about 1 eV. Here, to design low work function materials, we perform first-principles calculations on selected materials of transition metal dichalcogenide as substrates and alkali metals as adsorbates. The work function of our selected materials has a dip ubiquitously independent of the true binding distances of the adsorbates and exhibits contrasting behavior between empty d-shell elements (K, Rb, and Cs) and the others (Li and Na). We show that the interaction of empty d-orbitals of alkali metals and lone pair electrons of chalcogen is a key to the behavior of the work function. From calculated key parameters that determine the work function, we find that, regardless of the amount of charge transfer, K on WTe2 induces the largest surface dipole moment, which consequently makes the surface work function of as small as 0.8 eV, the smallest reported to date, and that the work function is lowered further to 0.7 eV by lattice strains. We demonstrate that the thermal efficiency of TEC using the low work function material exceeds that of thermoelectric materials with figure of merit of 5-10 in temperature range of 880-1200 K.

Oxidation behaviors of the aluminide coated TZM alloy via pack cementation

TZM is a traditional alloy for high temperature applications, in which (Ti, Zr)C particles are dispersed in a Mo matrix. However, due to easy formation of an oxide layer on the TZM alloy under ambient atmosphere, an oxidation protective coating is needed for any high temperature structural applications of TZM. In this study, aluminium pack cementation coatings have been carried out on TZM alloys, resulting in the formation of Al8Mo3 layer on the surface of TZM alloys. In order to examine the oxidation stability, the TZM alloy was exposed in an aerobic atmosphere. For the aluminide coated TZM alloys, an alumina layer was produced at the outer surface layer. The alminide coated TZM alloys showed excellent oxidation resistance. The coating layer kinetics and the corresponding oxidation stability are also discussed in terms of microstructural observations.

A New Insight into the Mechanistic Study on Synthesis of Silica Nanospheres

The physicochemical characteristics (SEM, N2 adsorption, FT-IR, MASNMR) of samples from kinetics (10 min, 20 min, 30 min, 1 h and 3h) of hydrolysis of tetraethyl orthosilicate (TEOS) gives new insight into the synthesis of nanosphere. The particle sizes are increased with time, however, the surface areas decreased. FT-IR investigation reveals the presence of high intensed silanol groups of 960 cm-1 at 10 min, which reduced on progress of the reaction time. This suggests the partial hydrolysis and condensation of alkoxy groups in TEOS. 29Si MASNMR analysis shows the presence three different silica species(Q4, Q3 and Q2) in 10 and 20 min samples. The intensity of Q2 species decreases with time and other species concentration were increased.