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One of the most important assumptions of conventional dilatometric analysis is an isotropic volume change during the transformation. However, the volume change does not occur isotropically and thus measured dilatation data upon the transformation consists of isotropic and non-isotropic contribution. In the present study, we expand the conventional analysis model to consider the non-isotropic dilatation during the transformation. A procedure to quantify the non-isotropic dilatation upon thermal cycle is suggested and it is implemented into the analysis model. Dilatometric analysis is conducted on measured dilatation data of low carbon steels to validate the proposed model. The phase fractions evaluated with the proposed model show a good agreement with those with the metallographic analysis. The result shows that considering the contribution of non-isotropic dilatation is crucial for quantitative dilatometric analysis.

keyword : steel, phase transformation, dilatometric analysis

Deformation behavior of high nitrogen austenitic Fe-18Cr-14Mn-4Ni-N stainless steels was investigated by means of room temperature tensile test and transmission electron microscopy (TEM). Yield and tensile strength showed almost linear relationship with nitrogen contents, whereas elongation decreased with increasing nitrogen contents. Based on the modified Ludwik model, the stress-strain curves were evaluated, and the calculated deviation parameter (△) increased linearly with decreasing strain as well as increasing nitrogen contents. TEM observation showed that (i) the deformed microstructure was characterized by planar dislocation array, stacking faults, and deformation twinning, (ii) as increasing nitrogen contents, the planar dislocation arrays were more tightly distributed and the onset of deformation twinning was shifted to lower strain, and (iii) the deformation twinning had a {111}<112> crystallographic component.

keyword : high nitrogen steel, HNS, modified Ludwik relation, deformation twin, TEM

The effect of trace elements on tensile behavior of ARB (accumulative roll-bonding) processed commercially pure copper products with nano grain size was examined. The trace elements in commercially pure copper substantially affected dynamic recovery behavior during ARB process, and eventually tensile behavior of pure copper. Various copper products, including OFC (oxygen-free copper), DLP (deoxidized low-phosphorous copper), PMC90 (Cu-0.15Fe-0.02P) and C194 (Cu-2.4Fe-0.02P), were used for 2-layer ARB process. Only PMC90 alloy was used for 3-layer ARB process. The increase in tensile strength for 2-layer ARBed OFC and PMC90 tended to saturate after three ARB process cycles. For DLP, on the other hand, the tensile strength continuously increased with increasing ARB process up to eight cycles. For 3-layer ARBed PMC90, the tensile strength also increased with equivalent strain up to a strain of~5.1. The effect of trace elements on the tensile behavior of ARBed commercially available copper products was discussed based on the thermal analysis, micrographic and fractographic observation.

keyword : accumulative roll-bonding, ARB, tensile behavior, dynamic recovery, pure copper, trace element

Surface yield strength can be estimated by considering the force equilibrium around an indentation-induced plastic zone; the plastic zone dimension is a key information for this analysis. However, conventional approach has analogized the plastic zone radius from few cross-sectional line profiles of the remnant indent and yielded erroneous strength result. Thus, we tried to extract a two-dimensional closed boundary of the plastic zone in this study by plotting a contour graph for the surface pile-up. This image processing was applied to the strength characterization of 1.0 ~μm-thick Au film on Si substrate. However, shallow indentation for film-only deformation brought about weak surface pile-up, thereby producing the strength overestimation compared to the microtensile results. In order to explain this discrepancy we introduced the concept of impression volume conservation and redetermined an extent of the hemispherical elastic/plastic boundary. Based on the modified data, the recalculated yield strength at 1.0 mN was 255.5 ± 37.2 MPa, comparable with the microtensile strength 243 MPa. This testing result confirms validity of the modified characterization method for the indentation-induced plastic zone.

keyword : nanoindentation, yield strength, pile-up contouring, impression volume conservation, matlab(R) visualization tool

The present study was concerned with the effects of over-aging on damping property and fracture toughness in Ti-6Al-4V alloy for biomaterials. Damping property and toughness become important factors for titanium implants, which have big modulus difference between bone and implant, and need high damping capacity for bone-implant compatability. Widmanstatten, equiaxed, and bimodal microstructures containing fine α2 particles were obtained by over-aging a Ti-6Al-4V alloy. Over-aging heat treatment was conducted for 200 hrs at 545℃. Fracture toughness, Charpy impact and bending vibration tests were conducted on the unaged and the over-aged six microstructures, respectively. Charpy absorption energy and apparent fracture toughness decreased as over-aging was done, even if the materials were strengthened by precipitation of very fine and strong α2-Ti3Al particles. On the other hand, damping properties were enhanced by over-aging in Widmanstatten and equiaxed microstructures, but was weakened in bimodal microstructure due to the softening of tempered martensite and the decreasing of elastic difference between tempered martensite and α phase contained α2 particles, etc. These data can provide effective information to future work about internal damping and fracture properties of Ti-6Al-4V biomaterials.

keyword : Ti-6Al-4V alloy, α2-Ti3Al precipitates, Widmanstatten, equiaxed, Bimodal, damping capacity, impulse-frequency response method, fracture toughness

A Cu-based composite, CU86Zr11Al3, was synthesized by copper mold casting. The composite exhibited an unusual microstructure consisting of a ductile Cu crystalline phase and hard superlattice phase (Cu5Zr)0.8Zr0.2. Quasistatic uniaxial compression tests revealed a high strength (1.0~1.6 GPa). In this study, we report the mechanism underlying the superior mechanical properties of the composite in relation with its unique microstructures.

keyword : composite, superlattice, strengthening

Low pressure cold spray deposition using Ni-Al2O3 blending powder was carried out to study the effects of processing parameters such as spray distance, powder feed rate, and scan speed on the coating properties. The test results indicated that the spray distance of 5~20 mm was adequate range for low pressure cold spray showing the best coating at the distance of 10 mm. When the He gas was used as the main processing gas, the coating properties such as hardness, porosity, and tensile bond strength as well as coating deposition efficiency were better than N2 gas. Particle velocity using N2 and He gases during cold spray deposition was measured to be 357 and 577 m/sec, respectively. It was considered to be reasonable compared to the expected value from the theoretical calculation.

keyword : cold spray, composite coating, critical velocity, deposition efficiency

The electrolytic reduction of spent oxide fuel involves the liberation of oxygen in a molten LiCl electrolyte, which is a chemically aggressive environment that is very corrosive for typical structural materials. So, it is essential to choose the optimum material for the process equipment handling molten salt. In this study, corrosion behavior of Haynes 263, 75, 214 and HR-160 in molten salt of LiCl-Li2O under oxidizing atmosphere was investigated at 650℃ for 72~168 hours. Haynes 263 alloy showed the highest corrosion resistance among all the examined alloys. The formed corrosion products were Li(Ni,Co)O2 and LiTiO2 on Haynes 263, Cr2O3, NiFe2O4, LiNiO2 and Li2NiFe2O4 on Haynes 75, Cr2O3, LiNiO2, (Al,Cr)2O3 and LiFeO2 on Haynes 214 and LiCoO2, CoCr2O4, Li(Ni,Co)O2 and (Cr,Ti)2O3 on Haynes HR-160, respectively. Haynes 263 showed a local corrosion behavior and Haynes 75, 214 showed a uniform corrosion behavior. In the case of Haynes HR-160, an intergranular corrosion behavior was observed.

keyword : molten salt corrosion, hot corrosion, molten salt

The elastic modulus of copper thin film with various thicknesses has been analyzed using nanoindentation and micro-cantilever beam bending tests. The specimens were fabricated by electroplating and silicon bulk micromachining processes. The elastic modulus measured by nanoindentation were 112 GPa for 3.0 mm thick and 99 GPa for 12 μm thick. The micro-cantilever beam bending test was conducted to measure the modulus and its result were 121 GPa for 2.8 μm thick and 99 GPa for 10.5 mm thick. The texture of copper thin films was measured quantitatively from the orientation distribution function by pole-figure technique. The calculated modului were 115 GPa for 3 μm thick and 102 GPa for 12 μm thick. Such theoretical estimations based on the texture analysis showed a good agreement with experimental measurements. Therefore, we demonstrated that the elastic modulus of electroplated copper thin films was influenced by the variation of preferred orientation of the films.

keyword : nanoindentation test, micro-cantilever beam bending test, elastic modulus, texture, orientation distribution function

Various Cu-Zr binary amorphous alloys, which exhibit different strains and atomic packing densities, were synthesized to investigate the crucial factor governing the degree of the plasticity. It was found that there is a linear proportion between the atomic packing density and the plasticity such that amorphous alloys with lower packing density exhibited higher plasticity and vice versa. Considering that the plasticity of the amorphous alloy is closely related to the facility of the structural rearrangement during the process of loading, the role played by the atomic packing density on the plasticity can be quantified by measuring the effective activation energy for overall crystallization. Indeed, there was an inverse proportionality between the packing density (i.e. plasticity) and the activation energy for overall crystallization. Herein, we report that the origin of the plasticity exhibited by monolithic amorphous alloys lies in the characteristics of the atomic packing state, which can be quantified by the effective activation energy for overall crystallization.

keyword : ReSi(l.75), thermoelectric properties, seebeck coefficient, electrical properties