발간논문

Home > KJMM 논문 > 발간논문

We have studied the effect of Ag as a substituting element for Cu on class forming ability in Mg_(65)Cu_(25-x)Ag_xY_(10) alloy and the possibility of manufacturing new bulk Mg-based glass material with maximum thickness by the squeeze casting process. Ag was selected as a candidate element due to following reasons : 1) Atomic size difference between Ag and constituting elements is more than 10%; 2) Mixing enthalpies between Ag-Mg and Ag-Y are large; and 3) Ag and Cu belong to a same family in periodic table and number of valence electron of Ag is same as that of Cu. Mg_(65)Cu_(15)Ag_(10)Y_(10) alloy has improved glass forming ability, by partially substituting Cu with Ag in Mg_(65)Cu_(25)Y_(10) alloy. Injection casting produced bulk metallic glass of Mg_965)Cu_(15)Ag_(10)Y_(10) alloy having diameter up to 6 ㎜, which is larger than the maximum diameter of 4 ㎜ in Mg_(65)Cu_(25)Y_(10) alloy. Squeeze casting enabled to produce larger bulk metallic glass with Mg_(65)Cu_(15)Ag_(10)Y_(10) alloy up to 10 ㎜ in diameter due to rapid heat extraction and undercooling of the liquid alloy by applying pressure during solidification.

keyword :

Burst tents of the commercial low tin Zircaloy-4 tubes are performed at room and high temperature(400℃) in order to evaluate the degradation of mechanical integrity of the hydrided tubes. Experimental results show that UHS (Ultimate Hoop Strength) decreases exponentially in both cases as the hydrogen content increases. It is also found that when the hydrogen concentration increases up to 1,000 ppm, UHS decreases to 50% of its initial strength in the room temperature test while it decreases only by 20% in the high temperature test even when the concentration exeeds 2,000 ppm. It is revealed that TCE (Total Circumferential Elongation) and UBE (Uniform Burst Elongation) decreases drastically with increasing hydrogen content. Fractographical analysis with SEM (Scanning Electron Microscope) confirms the ductile-brittle transition in the TCE and UBE measurements. It is concluded that Zircaloy-4 tubes may possibly lose its ductility when hydrogen content exceeds 700 ppm by hydrogen pick-up during normal operation, especially, in the high burn-up and extended cycle.

keyword :

The effect of boron and nitrogen on the low cycle fatigue and creep-fatigue behavior of 316L stainless steel was investigated at 873K. Under low cycle continuous fatigue conditions, 316LN (i.e., nitrogen-alloyed steel) had longer fatigue lives than 316L+B (i.e., boron-alloyed steel). Because 316LN has higher ductility and lower strength at 873K than 316L+B due to the nitrogen addition, 316LN shows better fatigue resistance than 316L+B. When the creep deformation was introduced by imposing tensile hold time, 316L+B had longer creep-fatigue lives than 316LN. From the result of microstructure observation and the fact that the experimental life is in good agreement with the predicted life from the life prediction model which is based on the grain boundary cavitational damage under creep-fatigue condition, the major damaging mechanism is the cavitation for the two alloys. In addition, from observation of the fractured surface and the cavity nucleation factor (P`), which is regarded as a new material constant, 316L+B as a lower grain boundary carbide density than 316LN, which has been known to provide a beneficial site for cavity nucleation. Therefore, the reason for the longer creep-fatigue life of 316L+B than 316LN can be explained by the fact that as the addition of boron significantly retards carbide nucleation and growth near the grain boundary, 316L+B has a lower grain boundary carbide density, which results in a lower cavity nucleation.

keyword :

Al-Mg-Si series aluminum alloys for automobile applications are capable of obtaining high strength due to age hardening during paint bake process. However, auto body sheet should go through storage procedure. During storage, hardness is increased by natural aging. Therefore, it is hard to get formability at pressing process and get the sufficient effect of age hardening at the paint bake treatment because of short process time. In order to prevent an adverse effect of natural aging, the effect of alloying elements and pre-aging treatment on the age hardening behavior was investigated in this study. The hardness increment during natural aging was greatly suppressed by addition of Cu+In, Cu+Cd and Cu+Sn. These results might be explained by the fact that In, Cd and Sn can pick up vacancies during the natural aging. Therefore, the diffusion rate of Mg and Si is slowing down gradually and age hardening is also delayed. Hardness during natural aging was stabilized and Vickers-microhardness over 130 was achieved with short artificial aging(paint baking) time by pre-aging treatment. Elongation was over 30% which gives enough formability at press stage by addition of Cd and Sn. The formability (LDH(Limiting Dome Height) and FLC(Forming Limit Curve)) test was performed by the punch stretch test. It is considered that alloying element of Sn is effective to enhance formability.

keyword :

A hypereutectic Al-Si-X alloy was fabricated via spray forming & extrusion to judge the feasibility of the alloy for the application to a propeller shaft, which needs high specific strength as well as high specific modulus. The microstructural evolution of spray formed hypereutectic Al-Si-X alloy during extrusion was studied. The effects of the Si contents, additional elements and heat treatments on the Young`s modulus and mechanical properties (UTS, elongation) were also investigated. In additions, the feasibility of the hypereutectic Al-Si-X alloy as a propeller shafts materials was evaluated with respect to S45C steel, 6061 aluminum alloy and Duralcan composite which are being used as a commercialized propeller shaft materials. The hypereutectic Al-Si-X alloy was regarded to have a good competition with S45C steel and 6061 aluminum alloy in reducing a weight and with Duralcan composite in reducing a fabrication cost.

keyword :

An Al_(62)Cu_(26)Fe_(12) alloy was gas-atomized in a protective N₂ atmosphere to prepare powders containing the icosahedral quasicrystalline phase as reinforcing particles. Fabrication of composites was achieved by adding the Al_(62)Cu_(26)Fe_(12) powders into the pure Al molten metal as a matrix material, followed by a quick agitation during conventional casting. The composites showed a relatively good bonding between the particles and molten Al without the formation of any intermetallic compound, despite the partial dissolution of the small satellite particles. Mechanical properties evaluated through compression and non-destructive continuous indentation tests of the composites indicated a significant increase of the yield stress especially in the range up to 10% volume fraction of reinforcing particles within a reliable error range. Strengthening mechanisms were reviewed to estimate the role of the icosahedral and coexisting crystalline phases on the increase of the yield stress.

keyword :

In the present investigation, P/M method and extrusion process have been used to manufacture Al6061/graphite composite materials. The graphite particles were distributed uniformly within the Al6061 alloy matrix. The results showed that tensile properties and hardness of the composites reduced with increasing graphite volume and a comparison of experimental and calculated results was compatible to the tensile strength behavior. The wear behaviour of the 6061/graphite composites was also studied as a function of speed and distance with reference to different values of the matrix hardness. The obtained wear data were interpreted on the basis of the analysis of the surface and subsurface damage to the composites. As the hardness of the materials increased, their specific wear rate also decrease.

keyword :

To examine the effect of acid fog on corrosion fatigue characteristics in SNCM steel and its weldment, fatigue test under acid fog environment was carried out in comparison with distilled water. The corrosive constituents in acid fog piled up the corrosion products on crack face and caused a crack branching and crack tip blunting. Therefore, crack growth rate under acid fog (pH = 5) was reduced by decrease the effective stress range in crack tip rather than that of distilled water. Also, it wart found that corrosion behavior was remarkably dependent upon not only pH and Hz but also constituents of acid fog. When the fatigue cracks are subjected to strong acidic solution and slow testing speed, the crack face dissolution was promoted and therefore crack closure was disturbed just as the stress decrease. Also, due to the detrimental effect of acid fog, the critical crack initiation stress, σ_i and crack growth rate, da/dN in early stage crack growth in weldment are slower than those in distilled water. Under the distilled water and acid fog environment, the weldment shows marked behavior that the crack growth rate is considerably low in heat affected zone but accelerated rapidly in weld metal region.

keyword :

Effects of Ca alloying elements on electrochemical properties of Mg-based sacrificial anodes were evaluated. Potentiodynamic, galvanostatic tests, optical microscopy and XRD analyses were used to investigate the corrosion rate, efficiency, and surface analysis of anodes. Polarization data indicated that the alloying with Ca reduced the corrosion rates of magnesium anodes. The increased corrosion resistance generally improved anode efficiency. All anodes did not undergo passivation but demonstrated only active behavior. Corrosion morphology was changed from localized to uniform attack by the Ca alloying. Addition of Ca to the Mg anodes yielded increased driving potential and efficiency. Mg-Mn-Ca anodes had the highest efficiency and driving potential. The improved performance of Mg-Mn-Ca and Mg-Ca anodes was attributed to : (1) the homogeneous and refined microstructures by the addition of small amount of Ca, i.e., uniformity of anode consumption along grain boundaries (2) extremely electronegative potential of Ca.

keyword :

Effects of final annealing temperature on the corrosion and mechanical behavior, and microstructure were investigated for Zircaloy-4(Zr-1.3Sn-0.2Fe-0.1Cr), Alloy-1(Zr-0.4Nb-0.6Sn-0.4Fe-0.2Cu) and Alloy-2(Zr-1Nb). The corrosion tests were carried out in 360℃ water and 70 ppm LiOH solution in the static mini autoclaves. The mechanical tests were performed using the universal material test machine. The corrosion rate of Zircaloy-4 was not accelerated as the precipitates on Zircaloy-4 grew with the increase of annealing temperature after recrystallization. But the corrosion rate of Alloy-1 and 2 was accelerated due to the formation and growth of β-Zr phases with increasing the annealing temperature. Therefore, the corrosion resistance of the Zr-Nb alloy system could be improved by the control of heat treatment. It was also confirmed that the mechanical properties of Zr-based alloys were affected by the addition of alloying elements, the recrystallization, and the precipitation with annealing temperature.

keyword :