Sangshik Kim

Fatigue crack propagation behavior of Fe24Mn steel weld at 298 and 110 K

The fatigue crack propagation (FCP) tests were conducted on Fe24Mn steel in the region of base metal (BM), weld metal (WM) and fusion line (FL) at 298 and 110 K. The FCP rates of Fe24Mn specimens in the region of BM, WM and FL were greatly decreased, while no notable difference in the fracture mode was observed, with decreasing temperature from 298 to 110 K. The FCP rates of Fe24Mn WM and FL specimens were slightly lower than those of BM specimen at both room and cryogenic temperatures. The SEM fractographic analyses suggested that each specimen showed the transgranular facets at both temperatures in low and intermediate ΔK regimes. However, the morphological details varied depending on the region of weld and testing temperature. Relatively large sized facets were observed for the WM specimen with the columnar grain boundary playing the same role as the grain boundary in the BM and the FL specimens in the near-threshold ΔK regime. The FCP behavior of Fe24Mn steel in the region of BM, WM and FL is discussed at 298 and 110 K based on the fractographic and micrographic observation.

Mechanical properties of friction stir welded Al alloys with different hardening mechanisms

The mechanical properties of precipitation hardened Al 6061-T651 and Al 7075-T6 and strain hardened Al 5083-H32, friction stir welded with various welding parameters, were examined in the present study. 4 mm thick Al 6061-T651, Al 7075-T6, and Al 5083-H32 alloy plates were used for friction stir welding (FSW) with rotating speed varied from 1000 to 2500 rpm (rotation per minute) and welding speed ranging from 0.1 to 0.4 mpm (m/min). Each alloy displayed slightly different trends with respect to the effect of different welding parameters on the tensile properties of the FSWed Al alloys. The tensile elongation of FSWed Al 6061-T651 and Al 7075-T6 tended to increase greatly, while the tensile strength decreased marginally, with increasing welding speed and/or decreasing rotating speed. The tensile strength and the tensile elongation of Al 6061-T651 decreased from 135 to 154 MPa and 10.6 to 17.0%, respectively, with increasing welding speed from 0.1 to 0.4 mpm at a rotating speed of 1,600 rpm. Unlike the age-hardened Al 6061-T651 and Al 7075-T6, the strain-hardened Al 5083-H32 showed no notable change in tensile property with varying welding parameters. The change in the strength level with different welding parameters for each alloy was not as significant as the variation in tensile elongation. It was believed that the tensile elongation of FSWed Al alloys with varying welding parameters was mainly determined by the coarse particle clustering. With respect to the change in tensile strength during friction stir welding, it is hypothesized that two competing mechanisms, recovery by friction and heat and strain hardening by plastic flow in the weld zone offset the effects of different welding parameters on the tensile strength level of FSWed Al alloys.

Ambient and cryogenic S-N fatigue behavior of Fe15Mn steel and its weld

The S-N fatigue behavior of Fe15Mn (Fe-0.7C-15Mn-2Al) austenitic steel, including base metal and butt-welded joint, was investigated at 298 K and 110 K, and the results were compared to those of STS304L (Fe-1Si-2Mn- 20Cr-10Ni) counterparts. Both specimens showed improved resistance to S-N fatigue with decreasing temperature from 298 K to 110 K. The resistance to S-N fatigue of Fe15Mn steel was greater at 298 K, while it was lower at 110 K, than STS304L steel. Unlike STS304L, Fe15Mn steel did not show any notable transformation induced plasticity and twining-induced plasticity effect under fatigue loading at ambient and cryogenic temperatures. The S-N fatigue behavior of Fe15Mn steel was strongly dependent on tensile strength at both ambient and cryogenic temperatures. Similar S-N fatigue behavior was also observed for the butt-welded joints of Fe15Mn steel. The S-N fatigue behavior of Fe15Mn steel and its weld was discussed based on the fractographic and microscopic observations.

Factors influencing fatigue crack propagation behavior of austenitic steels

In the present study, the fatigue crack propagation (FCP) behaviors of austenitic single phase steels, including STS304, Fe18Mn and Fe22Mn with different grain sizes ranging from 12 μm to 98 μm were investigated. The FCP tests were conducted in air at an R ratio of 0.1 using compact tension specimens and the crack paths and fracture surfaces were documented by using an SEM. The highest ΔKth value of 9.9MPa·m1/2 was observed for the Fe18Mn specimen, followed by 5.2MPa·m1/2 for the Fe22Mn specimen and 4.6MPa·m1/2 for the STS304 specimen, showing a substantial difference in the near-threshold FCP resistance for each microstructure. The crack path and fractographic analyses suggested that the near-threshold FCP behavior of these austenitic steels was largely influenced by the degree of slip planarity, as determined by stacking fault energy and grain size, rather than the tensile properties. In the Paris’ regime, the slip planarity still played an important role while the tensile properties began to affect the FCP. The FCP behavior of austenitic steels with different microstructural features are discussed based on detailed fractographic and micrographic observations.

Phellinus baumii extract influences pathogenesis of Brucella abortus in phagocyte by disrupting the phagocytic and intracellular trafficking pathway

To clarify the effects of Phellinus baumii ethanol extract (PBE) on Brucella abortus pathogenesis in phagocytes focusing on the phagocytic and intracellular trafficking pathway.

Stress corrosion cracking behavior of X80 steel in artificial seawater under controlled strain rate and applied potentials

The effect of applied potential on the stress corrosion cracking (SCC) behavior of X80 steel was examined in artificial seawater (ASW) at different strain rates of 1×10−4, 1×10−5 and 1×10−6/sec. The controlled potential of −650, −850, −950 and −1,050 mVSCE, respectively, was applied during strainig. It was found that X80 steel was susceptible to SCC in seawater environment under both anodic and cathodic applied potentials and the susceptibility was sensitive to strain rate. The SCC was initiated at the surface pits under an anodic applied potential of -650 mVSCE. The effect of cathodic applied potential on the SCC behavior of X80 steel in ASW was more complex, such that the combined effect of surface damage, including pits and hydrogeninduced cracking, and hydrogen concentration generated on the surface tended to determine the SCC susceptibility. The SCC behavior of X80 steel with different applied potentials in ASW was discussed based on the microstructural and the fractographic observations.

Effect of thermal treatment on the atomic ordering of mechanically alloyed Al3Nb

Abstract The present study was carried out to investigate the effect of thermal treatment on the atomic ordering of mechanically alloyed Al 3 Nb. The partially disordered Al 3 Nb alloy is transformed into an ordered 10-nm sized Al 3 Nb after a following heat treatment. The value of Δ H order for ordering of Al 3 Nb intermetallics is more negative for faster heating rates and prolonged milling time. The average values of Δ H order correspond to about 35% of the Δ H necessary for Al 3 Nb phase formation. The activation energies for ordering of Al 3 Nb at 30 and 40 h of milling are relatively close to the isothermal grain growth enthalpy of nanocrystalline Al 3 Nb intermetallics.

Effect of applied potential on fatigue crack propagation behavior of Fe24Mn steel in seawater

In the present study, the fatigue crack propagation (FCP) tests were conducted on X80 steel in air and artificial seawater (ASW) under various applied potentials to establish optimum and safe working limits of cathodic protection (CP). The slow strain rate test (SSRT) was also conducted on the X80 BM specimens in ASW under CP potential to identify the susceptibility of hydrogen affecting the FCP behavior. The CP potential of −850 and −1,050 mVSCE suppressed the environmental effect of seawater on the FCP behavior of X80 BM and WM specimens, showing almost identical da/dN-ΔK curves for both air and ASW environments. The SSRT in ASW under CP potential of −1,050 mVSCE suggested that the X80 BM specimen steel is susceptible to hydrogen embrittlement, but the effect of hydrogen was believed to be marginal in affecting the FCP behavior of the X80 specimens at a loading frequency of 10 Hz. The FCP behavior of high strength X80 steel is discussed based on the fractographic observation to understand the FCP mechanism in seawater under various CP potentials.

Effects of pre-corrosion and cathodic protection in artificial seawater on S-N fatigue behavior of X80 steel

The effects of pre-corrosion and applied potentials in artificial seawater on the S-N fatigue behavior of X80 steel were examined. The X80 specimens were pre-corroded in a FeCl3 solution for varying immersion times ranging from 0 to 96 h and subsequently S-N fatigued in air. It was found that the resistance to S-N fatigue decreased abruptly with 1 h immersion, while it became saturated with a further increase in immersion time. The trend observed in this study was relatively well explained by the stress concentration effect from the presence of corrosion damage on the surface. The in-situ S-N fatigue tests were also conducted on X80 steel in artificial seawater under applied potentials of ‒600, ‒850 and ‒1,050 mV (SCE). The fractographic and micrographic analyses were conducted on the fatigue specimens to identify the optimum cathodic protection conditions. †(Received December 30, 2013)

Increasing strength and conductivity of Cu alloy through abnormal plastic deformation of an intermetallic compound

The precipitation strengthening of Cu alloys inevitably accompanies lowering of their electric conductivity and ductility. We produced bulk Cu alloys arrayed with nanofibers of stiff intermetallic compound through a precipitation mechanism using conventional casting and heat treatment processes. We then successfully elongated these arrays of nanofibers in the bulk Cu alloys to 400% of original length without breakage at room temperature using conventional rolling process. By inducing such an one-directional array of nanofibers of intermetallic compound from the uniform distribution of fine precipitates in the bulk Cu alloys, the trade-off between strength and conductivity and between strength and ductility could be significantly reduced. We observed a simultaneous increase in electrical conductivity by 1.3 times and also tensile strength by 1.3 times in this Cu alloy bulk compared to the conventional Cu alloys.