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A Zr–Ti–Cu–Fe quaternary eutectic alloy was employed as a new Be-free brazing filler metal for Zircaloy-4 to supersede physically vapor-deposited Be coatings used conventionally with several disadvantages. The quaternary eutectic composition of Zr 58 Ti 16 Cu 10 Fe 16 (at.%) showing a low melting temperature range from 832 °C to 853 °C was designed by a partial substitution of Zr with Ti based on a Zr–Cu–Fe ternary eutectic system. By applying an alloy ribbon with the determined composition, a highly reliable joint was obtained with a homogeneous formation of predominantly grown α -Zr phases owing to a complete isothermal solidification, exhibiting strength higher than that of Zircaloy-4. The homogenization of the joint was rate-controlled by the diffusion of the filler elements (Ti, Cu, and Fe) into the Zircaloy-4 base metal, and the detrimental segregation of the Zr 2 Fe phase in the central zone was completely eliminated by an isothermal holding at a brazing temperature of 920 °C for 10 min.

Highly dense and homogeneous yttria (Y2O3) thin-wall tubes have been successfully fabricated by applying tape-casting and radial magnetic pulsed compaction (RMPC) followed by thermal sintering. Since a homogeneous density distribution was obtained in the RMPCed compact, by using the Y2O3-based tape instead of a stuck of raw powder, cracking and bending were inhibited throughout the process up to the final sintered body. The relative density of the resultant tube reached 99.9%, and the structure was very fine with grain sizes of 25 mm. Moreover, when the granulated powder was used as the precursor of the tape, the thermal densification was promoted due to its enhanced sinterability as compared to the raw powder. [doi:10.2320/matertrans.M2010113]

In this research, fine-structure TiO bulks were fabricated in a combined application of magnetic pulsed compaction (MPC) and subsequent sintering and their densification behavior was investigated. The obtained density of TiO bulk prepared via the combined processes increased as the MPC pressure increased from 0.3 to 0.7 GPa. Relatively higher density (88%) in the MPCed specimen at 0.7 GPa was attributed to the decrease of the inter-particle distance of the pre-compacted component. High pressure and rapid compaction using magnetic pulsed compaction reduced the shrinkage rate (about 10% in this case) of the sintered bulks compared to general processing (about 20%). The mixing conditions of PVA, water, and TiO nano powder for the compaction of TiO nano powder did not affect the density and shrinkage of the sintered bulks due to the high pressure of the MPC.

Strong bonding between titanium (Ti) and stainless steel (STS) was achieved by employing a commercially available Ag–28Cu (wt.%) filler, and more importantly with a help of an Ag interlayer. A mass transport of the Ti elements from the substrate into the molten filler was completely prevented through the use of the Ag interlayer, so the resultant brazed joint was free from any brittle Ti-based intermetallic compounds. Notably, this Ti–STS dissimilar joint displayed a remarkable improvement in its bonding strength, demonstrating the potential application of an Ag interlayer for joining Ti and its alloys to various structural steels.

In this study, the effects of precompaction on the density, microstructure, mechanical and electrical properties of α-Al 2 O 3 bulks fabricated by the combined application of magnetic pulsed compaction (MPC) and a sintering process were reported. The obtained density of the α-Al 2 O 3 bulks prepared by the combined processes increased with the increasing MPC pressure and precompaction pressure. The resultant higher hardness and breakdown voltage of the consolidated bulks following combined application of the magnetic pulsed compaction, precompaction and sintering process could be attributed to the homogeneously distributed ultra-fine microstructure than that of general processing, suggesting that the grain growth was remarkably reduced during the MPC processes.

The microstructure and mechanical properties of Ti joints brazed with a Zr41.2Ti13.8Ni10.0Cu12.5Be22.5 (at.%) amorphous filler were investigated. With a Zr-based amorphous filler, in this study, Ti joints with a homogeneous composition could be obtained by heating to well below the α-β transformation temperature for a short time, so that the undesirable effects of the high temperature heating are considerably diminished. The joints brazed at 790 °C for 10 min consisted of the coarse acicular structure rather than the fine Widmanstätten structure which generally deteriorates the ductility of the joints. The joints with the homogenous coarse acicular structure, i.e. without a residual liquid region, show almost the same mechanical properties as those required for base metals without heating. Although the residual liquid region in the joints deteriorates the ductility of the joints, this region could be successfully removed by a diminution in the quantity of the filler.

In this study, joining characteristics of dissimilar Ti and Cu metals have been investigated, when using both an Ag-based eutectic alloy as a filler and an Ag layer present on Ti base metal as a diffusion barrier. The observed microstructures were classified into three characteristic types, depending on the presence of a Ag layer at the Ti interface, e.g. first, the sample retaining thick continuous intermetallic layers, e.g. Ti2Cu, TiCu, Ti3Cu4, Ti2Cu3, and TiCu4 by a significant dissolution of the Ti atoms into the molten filler, in the absence of a Ag coating layer onto the Ti base metal, second, the sample with relatively thin Ti-Cu intermetallic layers by the reduced reaction of Ti with Cu due to a prominent decrease in the Ti dissolution, owing to the role of the Ag coating layer as a diffusion barrier, and finally, the sample without any brittle Ti-Cu intermetallics in the joint by a complete suppression of both the dissolution of the Ti atoms and its reaction with the Cu elements in the molten filler due to the presence of a Ag layer.

The fracture property improvement of Zr-based bulk amorphous alloy containing ductile crystalline particles was explained by directly observing microfracture processes using an in situ loading stage installed inside a scanning electron microscope (SEM) chamber. Strength and apparent fracture toughness measured from the in situ fracture test of the amorphous alloy containing crystalline particles were lower than those of the monolithic amorphous alloy, whereas its ductility was higher. According to the microfracture observation, shear bands were initiated from ductile crystalline particles, and the propagation of the shear bands or cracks was blocked by crystalline particles, thereby resulting in stable crack growth which could be confirmed by the fracture resistance curve (R-curve) behavior. This increase in fracture resistance with increasing crack length improved fracture properties of the alloy containing crystalline particles, and could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting, and shear band branching.

Microfracture mechanisms of Zr-based bulk metallic glass (BMG) alloy containing ductile crystalline particles were investigated by directly observing microfracture processes using an in situ loading stage. Strength of the BMG alloy containing crystalline particles was lower than that of the monolithic BMG alloy, while ductility was higher. According to the direct microfracture observation, crystalline particles initiated shear bands, acted as blocking sites of shear band or crack propagation, and provided the stable crack growth which could be confirmed by the R-curve analysis, although they negatively affected apparent fracture toughness. This increase in fracture resistance with increasing crack length improved overall fracture properties of the alloy containing crystalline particles, and could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting, and shear band branching.

In this study, effects of various crystalline particles formed in strip-cast Zr-base amorphous matrix composites on deformation behavior were investigated. The compressive and fracture toughness test results indicated that the ductility of the strip-cast alloys containing hard/brittle crystalline particles as well as containing ductile β-phase particles was higher than that of the monolithic amorphous alloy. From finite element calculations and in situ microfracture observations, this ductility improvement could be explained by the role of the particles to initiate shear bands and to block the shear band propagation. However, this improvement could be obtained only when the size and volume fraction of crystalline particles and characteristics of amorphous matrix were optimized by controlling the strip casting conditions.

Development of wrought Mg alloys, particularly in sheet form, is essential to support the growing interest for lightweight components in the automotive industry. However, development of Mg alloy sheets has been quite slow due to the complexity of sheet production originated from limited deformability of Mg. In this respect, twin-roll strip casting, a one-step processing of flat rolled products, can be an alternative for the production of Mg alloy sheets. In this study, AZ31 and experimental ZM series alloys are twin-roll strip cast into 2 mm thick sheets. The microstructure of the as-cast AZ31 alloy sheet consists of columnar zones near the roll side and equiaxed zones in the mid-thickness region. On the other hand, as-cast ZM series alloy sheets show equiaxed dendritic structure through the thickness of sheet. These alloys were subjected to various thermo-mechanical treatments and their tensile properties were evaluated. Twin-roll strip cast AZ31 alloy in H24 condition has equivalent yield and tensile strengths with similar ductility compared to commercial ingot cast AZ31-H24 alloy, indicating that twin-roll strip casting is a viable process for the fabrication of Mg alloy sheets. The experimental ZM series alloys have a large volume fraction of fine dispersoid particles in the microstructure, resulting from the beneficial effect of twin-roll strip casting on microstructural refinement. It has been shown that the experimental ZM series alloys have superior tensile properties compared to commercial ingot cast AZ31-H24 alloy, suggesting the possibility of the development of new wrought Mg alloy sheets by twin-roll strip casting.

Homogeneous glow plasma at atmospheric pressure without streamers and arcing was generated by making use of a radio-frequency (RF, 13.56 MHz) power supply. Oxygen gas was added to Ar/He gas as reactive agents for photo-resist (PR) ashing. The input power, flow rate, oxygen concentration, treatment time, substrate temperature are controlled for high ashing rate and uniform ashing. Thickness of PR film was measured by NANOSPEC (AFT200) and α -Step (P-10). An unstable discharge occurs destroying the uniformity, when the input power exceeds a threshold value determined from the distance between the substrate and plasma source. An increase of oxygen quantity or temperature increase makes high ashing rate, but the ashing surface is rugged. The PR ashing rate was related to oxygen atom in plasma. The number of treatment may not be important in PR ashing at the atmospheric pressure.

Highly dense Y2O3 ceramics have been fabricated by a magnetic pulsed compaction (MPC) which is capable of reaching a sufficiently high pressure (~1GPa) in a very short duration (a few microseconds), and a subsequent pressureless sintering at 1600°C. The Y2O3 green bodies with a relative density of about 68% were achieved by the application of the MPC process due to the effect of an enhanced rearrangement and a high speed movement of the particles, without the help of ceramic binder. Those compacts showed densities greater than 95%, which is very close to the theoretical density, after the subsequent pressureless sintering process at 1600 oC. The shrinkage rates of the diameter for the samples compacted by the MPC process were markedly reduced, when compared to those for the ones by the conventional compaction (CC) process.

In this study, three Fe-base amorphous alloys with quite different critical cooling rates were subjected to twin-roll strip casting to see the possibility of fabricating amorphous sheet by the same process. Continuous cooling transformation (CCT) diagrams of the alloys were calculated using the heterogeneous nucleation theory coupled with thermal data obtained during cooling to evaluate their critical cooling rates and glass forming abilities (GFAs). It shows that the GFAs calculated by CCT diagram are in agreement with the experimental results, while the well known empirical thermal parameters do not agree with the experimental results. Optimum twin-roll strip casting conditions have been determined based on the calculated critical cooling rates and the simulated thermal behavior of the sheet during twin-roll strip casting.

Twin-roll strip casting has been utilized to fabricate the sheet products of bulk amorphous alloys. Simulation of solidification behavior and actual twin-roll strip casting show that the optimum casting conditions are such that the alloy is in supercooled liquid state at roll nip area rather than fully amorphized.

Effects of crystalline phase particles formed in a strip-cast Zr-base bulk amorphous alloy on strength, ductility, and fracture toughness were investigated by directly observing microfracture processes using an in situ loading stage installed inside a scanning electron microscope chamber. The compressive and fracture toughness test results indicated that strength, ductility, and fracture toughness of the strip-cast amorphous alloy were higher than those of the as-cast monolithic amorphous alloy, although the strip-cast alloy contained a considerable amount (4.5 vol.%) of hard, brittle crystalline particles. According to the in situ microfracture observation, crystalline particles were easily cracked under low stress levels, acted as blocking sites of shear band or crack propagation, and provided initiation sites of multiple shear bands. Thus, the improvement of mechanical properties in the strip-cast alloy could be explained by mechanisms of (1) blocking of crack propagation, (2) formation of multiple shear bands, and (3) crack deflection by crystalline particles.

Magnetic pulsed compaction (MPC) was introduced to consolidate mixed diamond and cobalt (Co) granule powders for the production of drilling segments. The diamond-Co samples prepared by MPC at the compacting pressure of 4 GPa showed a high sintered density of 99.6% as well as a high green density of 86.4%. A fine and homogeneous microstructure and a high hardness were also observed in the sintered bulk. Finally, a perforating test revealed that a higher drilling speed of 11.71 cm/min and a longer tool life of 7.96 m were obtained for the drilling segments prepared by the MPC process, whereas the values for those fabricated by a conventional process were 10.15 cm/min and 6.55 m, respectively. This property improvement of the MPCed segments was attributed basically to the enhanced green density and the homogeneous distribution of the diamonds.