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The elements of Sc and Zr were added to pure Al for improving thermal stability and refining grain size. Sc was presented on boundaries and Zr dominantly on matrix in as cast state. After homogenization treatment, Sc was dissolved mostly but Zr retained in matrix; though the two elements were presented on the same location in Al-Sc-Zr alloy. Semi-coherent Al₃Sc in Sc added alloy was precipitated during severe plastic deformation, and shown (001)_(matrix)//(001)_(Al₃Sc) orientation relationship with matrix. Al₃Zr in as cast Zr added alloy was shown the orientation relationship (011)_(matrix)//(100)_(Al₃Zr), and partially retained and dissolved with increasing deformation. In the Al-Sc-Zr alloy, retained Al₃Zr had the role of nucleation site for Al-Sc precipitates so that Al₃Zr mainly coexist with Al-Sc precipitates.

keyword : Accumulative Roll Bonding, ARB, Scandium, Zirconium, Precipitate, Orientation relationship

Non-destructive instrumented indentation is proposed as a promising alternative of the conventional stress-measuring methods based on already-reported its sensitivity to applied stresses. In this study, we tried to expand the current indentation model on equi-biaxial stress to the characterization of general surface stress in nonequi-biaxial state. A contribution of the surface stress on indentation plasticity was theoretically extracted from the non-equi-biaxial stress matrix. And, a newly derived stress equation was expressed as a multiple of stress non-equibiaxiality and residual-stress-induced normal load. An experimental tests for confirming the model were also conducted on artificially strained API X65 steel plate, and produced the non-equi-biaxiality factor consistent with that from theoretical analysis.

keyword : Residual stress, Instrumented indentation, Curve shifting, Stress-proportional factor, Stress-directionality coefficient

Tensile behavior of friction stir welded Al 6061-T651, Al 7075-T6 and Al 5083-H32 alloy plates joined with different tool rotating and welding speeds was examined in the present study. Typical microstructure of friction stir welded Al alloy consists of three zones, including dynamically recrystallized zone (DXZ), thermomechanically affected zone (TMAZ) and heat affected zone. The tensile strength decreased by approximately 15 to 20% for age-hardened Al 6061 and Al 7075 accompanied with a slight increase in tensile elongation. Al 5083-H32 showed an approximately 20% increase in tensile elongation with no notable change in tensile strength. The tensile behavior of friction stir welded Al appeared to be strongly related to not only the frictional heat and microstructural evolution but also the amount of plastic flow and clustering of coarse precipitates.

keyword : Friction stir welding, Al 6061-T651, Al 7075-T6, Al 5083-H32

In the present study, the effect of different processing routes, including rheocasting and low-pressure casting, on the mechanical and corrosion behaviors of A356-T6 alloys was examined. It was found that the tensile elongation and the impact energy value of A356-T6 alloy increased substantially with the rheocasting process compared to the low-pressure cast process, with the sacrifice of yield and tensile strength. The resistance to high cycle fatigue was, however, similar with each other. The observed trends in the mechanical properties with different processing routes appeared to be largely related to the shape and size of silicon particles in the eutectic phase, as well as the volume fraction of primary α phase. The resistance to corrosion in 3.5% NaCl solution was also significantly higher for the rheocast A356-T6 than the low-pressure cast counterpart. The corrosion behavior of A356-T6 with different processing routes was also believed to be related to the area fraction between Si particles and the matrix.

keyword : A356-T6 alloy, Rheocasting, Low-pressure casting, Mechanical behavior, Corrosion behavior

High temperature mechanical properties, especially tensile properties, are basic data to determine safety and integrity of materials and structure used at the high operating temperature. High temperature tensile properties of materials and structures in use cannot be generally obtained from conventional tensile test. Continuous indentation technique has been researched as an alternative to conventional tensile test and optimum experimental conditions were reported for the continuous indentation testing at room temperature recently. In this study, optimum experimental conditions were suggested for the testing at elevated temperature. Especially, the parameters related to the time such as loading rate and holding time were given priority contrary to their negligible effects at room temperature. As a result, loading rate should be selected to enable to reflect the work-hardening characteristics of testing object. Since holding time is related to indentation creep phenomena causing the change of indentation load-displacement curve shape by load relaxation during holding at maximum load, it should be selected to enable to minimize load relaxation.

keyword : Continuous indentation test, Tensile properties, Elevated temperature, Experimental conditions, Loading rate, Holding time, Indentation creeps

Multilayered WC-Cr_(1-x)Al_(x)N coatings were deposited on AISI D2 steel by cathodic arc ion plating process (CAIP). The Al concentration was controlled by using evaporation source for Al and fixing the evaporation rate of the other metals (WC alloy and Cr). Five types of WC-Cr_(1-x)Al_(x)N were deposited, i.e., WC-Cr_(0.6)Al_(0.4)N, WC-Cr_(0.57)Al_(0.43)N, WC-Cr_(0.53)Al_(0.47)N, WC-Cr_(0.48)Al_(0.52)N, and WC-Cr_(0.45)Al_(0.55)N. The corrosion behavior in deaerated 3.5 wt.% NaCl solution for the prepared samples was investigated by electrochemical tests (potentiodynamic polarization test, galvanic corrosion test, electrochemical impedance spectroscopy (EIS)) and surface analyses (scanning electron microscopy (SEM), X-ray diffractometry (XRD), glow discharge optical emission spectroscopy (GDOES), and electron probe micro analyzer (EPMA)). The results from potentiodynamic polarization test showed that the WC-Cr_(0.48)Al_(0.52)N coating with the lower porosity (0.144) enhanced the corrosion resistance. Galvanic corrosion test was accomplished using the galvanic couple between the substrate and the WC-Cr_(1-x)Al_(x)N coating. The WC-Cr_(0.6)Al_(0.4)N showed a higher galvanic current density. In EIS measurements, the WC-Cr_(0.48)Al_(0.52)N coating showed an increased charge transfer resistance (R_(ct)). The measured electrochemical tests indicated that the WC-Cr_(0.48)Al_(0.52)N coating with the lower corrosion current density (3.023 μA/㎠) and higher charge transfer resistance (158.7 kΩ㎠) enhanced corrosion resistance.

keyword : Multilayerd coating, CAIP, Galvanic corrosion test, EIS, Porosity, Scratch adhesion test

Low carbon 12Cr steel used as a nuclear steam turbine blade material have been surface-modified by the flame hardening process and the properties of hardness, hardening depth, and residual stress have been studied. For this, the thermal cycles of 12Cr steel have been controlled precisely as a function of the surface temperature, the exposed height from the water surface, and the cooling rate. The application of flame hardening increases the hardness of 12Cr steel (base value, 250 HV) to 420~550 HV considerably, forming relatively sharp hardening boundaries. Both the hardening depth and the austenitization period are linearly correlated well within the range of processing conditions employed. The rapid cooling rate also increases the hardening depth as well as the surface hardness. The properties of residual stress are discussed in terms of contributions of both the thermal and transformation stress components. The optimum processing conditions for the desirable residual stress state in the material are as follows : less than 1200℃ for the 18 mm-exposed and less than 1150℃ for full-exposed conditions by the criteria of Siemens AG-KWU, and on the basis of the specification of GE Power Generation Engineering, 944~1050℃ for the 10 mm-exposed, 883~1150℃ for the 14 mm-exposed, 833~1134℃ for the 18 mm-exposed, and 785~1097℃ for the full-exposed.

keyword : Surface modification, Flame hardening, 12Cr steel, Hardness, Residual stress, Martensitic transformation

The effect of dissolved hydrogen on the corrosion fatigue crack growth(FCG) behavior of 3.5NiCrMoV steel in pure water at 25℃ and 150℃ was investigated. The corrosion fatigue crack growth rate measurements employing fracture mechanics specimens were performed in pure water. The water circulating loop system was used to control corrosion environment. The FCG rates highly increased in pure water with the large amount of the dissolved hydrogen (35 cc/kgH₂O) at 25℃. However, it made no difference to FCG in pure water at 150℃ regardless of the contents of the dissolved hydrogen(35 cc/kgH₂O; 0.1 cc/kgH₂O). The hydrogen embrittlement significantly influences the corrosion fatigue crack growth rate of 3.5NiCrMoV steel in pure water of 35 cc/kgH₂O at 25℃.

keyword : Corrosion fatigue, 3.5NiCrMoV steel, Crack growth rate, Hydrogen embrittlement

Amorphous Fe_(73.5)Si_(13.5)B_(9)Cu₁Nb₃ alloy was crystallized in nitriding atmosphere of mixture gas of NH₃ and hydrogen. The crystallization at 823K showed the high electrical resistivity ranging from 494 to 538 mW-cm, which was higher level compared with nano-crystalline Fe_(73.5)Si_(13.5)B_(9)Cu₁Nb₃ alloy with a resistivity of about 110 mW-cm. Through the nitridation treatments, the amorphous alloy was transformed into nano-sized crystalline Fe3Si including iron nitrides such as γ` -Fe₄N phases. EPMA-concentration depth profiles confirmed an existence of nitrogen-rich region on the surface of the alloy ribbon, leading to the preferential formation of g -Fe4N phase. The formation of γ` -Fe₄N phase leaded to an increase of the resistivity and provided stable permeability and a low core loss at high frequency.

keyword : Amorphous, Crystallization, Nitridation, Nitrides, Nano-crystals, Soft magnet

Mechanical properties and weldability such as HAZ hardness, cold cracking susceptibility and hot ductility of two differently heat treated 10Cr-1Mo-VNbN cast steels were measured and compared. Because of high hardenability of the cast steel, as-annealed cast steel showed martensitic microstructure and thus had higher hardness than annealed-normalized-tempered cast steel which had tempered martensite. Because the welding electrode used resulted in a high hardness weld metal, both cast steels showed same weld metal cold cracking susceptibility even though the as-annealed cast steel had higher HAZ hardness than the annealed-normalizedtempered cast steel. Both cast steels had excellent hot ductility in high temperature range, indicating no risk of grain boundary liquation cracking in the HAZ. However, the as-annealed cast steel showed an inferior ductility in the intermediate temperature range of 1000~1150℃ because of larger unrecrystallized grain size.

keyword : 10Cr-1Mo-VNbN cast steel, Heat treatment, Hardness, Microstructure, Weld cold cracking, Hot ductility