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The microstructural evolution of b-NiAl/γ` -Ni3Al two-phase (Ni_(66)Al_(34))_(100-x)X_(X)(X=Ti, Si, Nb) alloys during various heat treatment (quenching and aging) were systematically studied. It was found that the microstructural features of the two-phase NiAl/Ni₃Al alloys could be divided into three types : lamellar, mesh and Widmansttten microstructures. The reason why those three types of microstructure were formed could be explained by the change of austenite start temperature(As temperature) depending on the ternary elements. In case of As>250℃, the lamellar microstructure was formed by following phase transformation: Martensite→Ni5Al3→β+γ` . In this case, the NiAl martensite was quickly transformed into the Ni5Al3 phase at 250℃ by a re-ordering reaction. In case of 20℃
keyword : NiA1/Ni3Al two-phase alloys, Ternary element, Martensitic transformation, Ni3Al3, Microstructural control

The heat affected zone (HAZ) of welded STS304 has been irradiated by using three irradiation modes of electron-beam irradiation, electron-beam irradiation after He-injection and electron/He ion dual-beam irradiation in 1250 kV high voltage electron microscope (HVEM) connected with an ion accelerators to study the effect of Heinjection on microstructural evolution. The size of voids increases considerably and their distribution is a bi-mode shape, when electron-beam is irradiated after He-injection. Void swelling percentage is largest in the case of electronbeam irradiation after He-injection. In cases of electron/He^(+)-ion dual-beam and electron-beam irradiation after Heinjection, the amount of grain boundary segregation is suppressed when compared with the case of electron-beam irradiation.

keyword : Irradiation damage, He-injection, STS304, High voltage electron microscope, Heat affected zone

The effect of S & Mn on the hot ductility of Fe-29Ni-17Co Kovar alloy and the mechanism of high temperature cracking occurrence were investigated. It has been found that the brittle intergranular fracture at high temperature cracking is closely associated with the sulfide along grain boundary. Experimental results showed, especially when the S content was high and the Mn content was free, that small cavities could be nucleated easily on FeS particles along grain boundary and propagated fast at high temperature. The hot ductility of Kovar alloy was drastically increased with the addition of Mn and it was also increased with decreasing S content. SEM and FE-SEM analyses showed that the Mn addition in this alloy could cause ductile intergranular fracture appearance below 1150℃ by the precipitation of MnS. It was proposed from the results that the addition of Mn and optimum S content (below 30 ppm) in Kovar alloy was necessary in order to reduce the hot cracking susceptibility.

keyword : Low thermal expansion, Kovar, High temperature brittle intergranular fracture, Hot ductility, S, Mn, FeS

A series of tensile tests were carried out on a B2-type b-CuZn alloy in the temperature range, room temperature to 400oC at three different strain rates (10^(-5) sec^(-1), 10^(-4) sec^(-1), 5.2x10^(-2) sec^(-1)). The stress-strain curves could be divided into three types. Type I indicates a remarkably linear sustained work hardening to fracture, type II work hardening and subsequent necking, type III yield drop behavior and stress saturation, respectively; corresponding to the insensitive yield stress region, anomalous yield stress region, and yield stress decreasing region, in the yield strength vs. temperature graph. The ultimate tensile strength basically decreased as temperature increased, only showing characterized strength hump-up region just below T_(p). The fracture mode was changed mainly from transgranular at room temperature to intergranular at high temperatures. The elongation increases with increasing temperature in region I, and then slightly decreases in region II (anomalous yield stress region). When the specimens were tensile-fractured at high temperatures (region III, above Tp), the elongation drastically increased with decreasing strain rate, for instance from 12% at 5.2x10^(-2) sec^(-1) to 87% at 10^(-5) sec^(-1). The dependence of ductility on strain rate was mainly attributed to the thermally activated <100> dislocation climb, inducing stress relief at the grain boundaries, as evidenced by microvoids at grain facets.

keyword : β-CuZn alloy, Tensile property, Fracture mode, Yield stress anomaly, Strain rate dependence, Temperature dependence

Tear resistance behaviors of Al-7%Si-0.4%Mg cast alloys were examined using Kahn-type tear test specimens which are distinguished DCC(directional continuous cast) consisting of a directional and ordinary dendrite structure with SLD(semi-liquid die cast) consisting of globular cells and fine grain structure. Restricted plastic deformation occurs near notch tip of semi-liquid die cast characterized by refinement of grain size. Popin for stress was exhibited in semi-liquid die cast but no pop-in was observed in DCC cast alloy. Tear resistance was evaluated by the energies required for crack initiation, UE_(i) and the crack propagation, UE_(p). An unit propagation energy, UE_(p) can be converted into critical stress intensity factor, K_(c), which in directional dendrite structure improves due to a high plane stress state over thickness-direction and the fracture surface of slant type.

keyword : Al-Si-Mg cast alloys, Pop-in stress, Slant fracture surface, Fracture toughness

The effects of the prior austenite grain size on the cleavage fracture toughness were investigated in the transition region of bainitic low alloy steels. Cleavage fracture toughness was evaluated by the ASTM standard E 1921. Yield strengths satisfied the Hall-Petch relationship, but fracture toughness in the transition region did not show significant dependence on the austenite grain size. Fracture surfaces showed that the facet size was related to the austenite grain size, but cleavage initiation distance(CID) from the original crack tip showed no direct relationship to the grain and facet sizes. Therefore, the austenite grain size may have little effects on the fracture toughness of bainitic low alloy steels in the transition region. In the viewpoint of the weakest link theory, particle distribution in front of the crack tip is probably more important than the grain size in the transition temperature range where fracture was controlled by cleavage crack initiation. The experimental results in this study supported the weakest-link-theory.

keyword : Low alloy steel, Transition region, Cleavage fracture, Cleavage Initiation Distance, CID, Fracture toughness, Weakest link theory, Characteristic distance

Multilayer coatings consisting of very thin layers of nitride materials deposited by magnetron sputtering (sometimes called superlattices) exhibit hardnesses over 50 GPa, much higher than that of single layers. Several explanations for this hardness increase have been proposed, including dislocation blocking by layer interfaces, Hall- Petch strengthening, strain effects at layer interfaces, and the supermodulus effect. Among these, dislocation blocking has been generally accepted as a dominant factor in large hardness enhancement. However, the interface between two materials consisting of multilayers is broadened due to interdiffusion. The magnitude of hardness enhancement by the interface has not yet been quantified for real systems. In this study, the concept of composite hardness is introduced to quantify the interface-related hardness enhancement. We suggest a composite hardness equation and quantifythe magnitude of hardness increase by using an equation based on the interface hardness and the interface thickness derived by comparing results derived from this equation and those determined in nanoindentation tests. Three multilayer systems, such as TiN/VN, TiN/NbN and CrN/NbN were used in the equation.

keyword : Multilayer coating, Tribological application, Nitride, Superlattice, Composite hardness, Nanoindentation test, Interface hardness and thickness

In order to develop mechanically alloyed nano-sized ferritic ODS alloys, we have investigated the effect of three kinds of oxide such as Y₂O₃, Al₂O₃ and TiO₂, on the high temperature tensile properties, high temperature oxidation behavior and corrosion resistance. Due to the most distribution of fine and spherical particles, ODS alloys containing Y₂O₃ raise yield strength and tensile strength at high temperatures considerably. The results of high temperature oxidation tests indicate that the ODS alloys containing Y₂O₃, Al₂O₃ and TiO₂ are resistant to oxidation between 823K and 1023K. The ODS alloy containing Y2O3 by attributed by to thermal safety of Y₂O₃ showed an ecellent characteristics in oxidation and corrosion.

keyword : ODS, Ferritic alloys, Mechanical alloying, High temperature oxidation, Tensile properties, Corrosion resistance, Nano-Sized Oxides

Intermetallic compound TiAl was synthesized by using self-propagating high-temperature synthesis(SHS) method. SHS reaction of TiAl was realized by exothermic reaction between molten Al and solid Ti particles. The synthesis product consisted of TiAl(L1_(0) type) matrix which contains small amount of Ti₃Al as a second phase. The reaction temperature increased from 1477K to 1542K with increment of Al contents in mixed powder, but propagation rate decreased from 0.394 cm/sec to 0.238 cm/sec in the range of 52 at%Al to 54 at%Al. Also, both of reaction temperature and propagation rate in the range of 2.24 g/㎤ to 2.93 g/㎤ of green density increased linearly.

keyword : SHS synthesis, TiAl, Green density, Reaction temperature, Propagation rate

Iron and iron nitride nanoparticles were synthesized by chemical vapor condensation (CVC) process, using a precursor of Fe(CO)_(5) and carrier gas of NH₃. Phase change and crystallization behaviors of as received particles were mainly examined by TEM technique. Only α-Fe nanoparticles were formed at the lowest decomposition temperature of 500℃. As the temperature increased, the crystallization of α-Fe nanoparticles was suppressed and Fe₃N nanoparticles were begun to be formed. Finally full crystallization of Fe₃N was observed at 1000℃. Typical size of the α-Fe and Fe₃N particles was less than 30 nm in diameter.

keyword : Chemical vapor condensation, Fe3N nanoparticles, HRTEM microstructure, Crystallization behavior