발간논문

Home > KJMM 논문 > 발간논문

Mechanical alloying in the Fe(1-x)Bx system was performed by ball-Milling. The Mechanically alloyed powders were investigated by X-ray diffraction, differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). In case of Fe(1-x)Bx (x=0.2) specimen, the broad peak of x-ray diffraction and the exothermic peak of DSC indicated the characteristic behavior of amorphous alloy. However, TEM observation revealed the existence of fine bcc Fe phase. On the other hand, the boride of Fe₂B or FeB was formed by mechanical alloying in Fe(1-x) Bx (x＞0.3) specimens.

keyword :

Modification in morpbology of the oxide and sulfide inclusions effected by calcium treatment of the Al-killed steel has been studied. Samples were taken from a 40 ton VAD vessel and examined under an optical microscope and the electron optics. Activities of oxygen measured by oxygen sensors and aluminum contents were utilized in thermodynamic calculations, which lead, in association with plant data, to the following In steels containing more than 0.01% Al, calcium is not so effective deoxidizer as an inclusion modifier. To maximize desulfurization and the conversion of alumina into calcium aluminates, sulfur and aluminum should be below certain threshold levels. It may be added that the total calcium retention is not a good measure of the effectiveness of inclusion modification.

keyword :

Spot, weldability of the galvanized steel, galvanneld steel and cold rolled steel is compared exclusively in this study. In order to investigate the effects of welding parameters on the weld strength of each steel, welding current and welding time were systematically varied under a constant pressure. Spot weldability of surface coated steels was strongly dependent upon types of coating although the compositions of coating (Zn-Fe) and base plate were almost identical. As the welding heat input (current and time) increased, nugget size and strength increased. However, when heat input (especially current) was greater than a critical value, effects of energy input became minor. Results also demonstrate that the tensile shear strength can be expressed by a second order equation of welding current and welding time. Experimental data fall within 5% error limit of the calculated values.

keyword :

The effect of sintering temperature and oxygen partial pressure on the magnetic loss and permeability of MnZn ferrite has been studied. The lowest magnetic loss is obtained at the sintering temperature of around 1280℃ with oxygen partial pressure of 0.04 atm. Permeability increases with increasing sintering temperature, and decreases with oxygen partial pressure at a constant sintering temperature.

keyword :

The fused salt electrolysis of LaCl₃, with molten aluminum cathode and carbon anode was studied to produce the Al-La binary alloy. The decomposition valtage of LaCl₃was determined by two electrodes method. Electrolysis conditions, such as bath temperature, current density and electrolysis times were varied to investigate the effect on the current efficiency. The decomposition voltage of LaCl₃, was found to be 2.67V at 1173±2K. The optimum current efficiency was D_k=0.2∼0.3A/㎠, at the bath temperature of 1173^-K and was decreased by increasing the electrolysis times. Results reveal that Cl₂gas formed by electrolysis of LaCl₃may be responsible for the decrease of the current efficiency due to recombination with reducted La metal.

keyword :

ZrN was deposited on a 304 stainless steel substrate by the reactive sputtering technique, and deposition rate and nitrogen content of deposits were measured as functions of partial pressures of N₂and Ar gases, the input power and the substrate bias voltage, respectively. X-ray diffractometer and scanning electron microscope were used to analyse the preferred orientation and morphology of the deposits. Deposition rate decreased rapidly with increasing the N₂pressure and decreasing the input power, while it decreased slowly with increasing the Ar pressure and decreasing the bias voltage. The N/Zr ratio of the deposts was in the range of 1.6-1.77 for the most deposition conditions. ZrN coatings deposited without any bias voltage showed columnar structure with a strong(111) preferred orientation, and the crystallites were fine and the surface was smooth. Those deposited with bias voltage showed a mixed texture of(111)-(311), where the columnar structure was more or less suppressed.

keyword :

Magnetic measurements on the annealed and rapidly solidified FeAl_(1-x)Ti_x series (x= 0.05-0.40) have been made at temperatures from 77K up to 400K. Annealed specimens showed paramagnetism when x＜0.30, weak superparamagnetism when X=0.35, 0.40. Rapidly solidified specimens appeared paramagnetism when x=0.05, 0.10, superparamagnetism when 0.15＜ x ＜ 0.35 and seemed to be ferromagnetism when x=0.40. The magnetic properties were analyzed on the point of view of the local environmental effect.

keyword :

The effects of temperature and loading rate were investigated on the intiation of plane strain fracture of an HY-100 steel. Fracture toughness tests were conducted using pre-cracked notched round bars loaded in tension to produce two stress intensity rates, viz., K˙₁=1㎫√m/sec and K˙₁=2×10^6㎫√m/sec. Testing temperatures covered the range from -150℃ to 200℃ which encompassed fracture initiation modes involving quasi-cleavage to fully ductile fracture. Fracture processes were also examined by in-situ SEM observation under a wedge loading. The present steel contained the banded structures as well as MnS inclusions in the form of globulars and stringers aligned parallel to the rolling direction. The results show that this HY-100 steel was tougher under dynamic loading than under quasi-static loading, which was interpreted by correlating microstructures including MnS inclusions and the banded structures with fracture toughness and fracture processes.

keyword :

The microstructural aspects of the superplastic behavior in 7475 Al alloy are investigated. After superplastic tests, several specimens revealed large zones free of dispersoid particles-occasionally as large as 5㎛ across. Typical micrograph depicting dispersoid-free zones (DFZ) after 100% tensile strain showed DFZ at grain boundaries lying primarily normal to the tensile stress direction. This result emphasizes the importance of diffusional flow in explaining superplastic deformation of the fine-grained 7475 A1 alloy especially at low elongations, The dissolution and growth processes of dispersoid occured at high elongations and these processes can be responsible for the formation of the dispersoid-free zone.

keyword :

The effects of various molds on solidification structures and mechanical properties were studied in Al-1%Si and Al-10%Si alloys. Green sand mold, COL sand mold, cement sand mold, furan sand mold and shell sand mold which were mixed with different binders and a metallic mold were used with different pouring temperatures of 680℃ and 760℃. The results are as follows ; Solidification variables such as mold binders pouring temperatures and quantities played an important rold in micro-and macro-structures. Fine dendrite arm spacing (DAS) and equiaxed crystals as well as good quality of mechanical properties in Al-Si alloys could be obtained by controlling of the soliddifiation variables. The DAS and mechanical properties in Al-Si alloys are related by nonlinear relation-ships. As a supplemental test method, improved reliability of the casting is possible when DAS measurements are in combintion with nondesturctive inspection method.

keyword :