Solid State Phenomena Vols. 124-126

Abstract: Fatigue crack propagation (FCP) behavior of friction stir welded (FSWed) 6061-T651 Al alloy was examined with the fatigue crack growing either along the dynamically recrystallized zone (DXZ) at variable
K or perpendicular to the DXZ at a constant
K value of 7.5, 10 and 15 MPa√m, respectively. The FCP rates in the DXZ tended to be significantly lower than those in the PM particularly in low
K regime. Compressive residual stress reducing effective
K cannot be solely responsible for the enhanced FCP resistance. The fine recrystallized grains in the DXZ causing intergranular fatigue failure appeared to be detrimental to the FCP behavior of FSWed 6061-T651 specimen. The constant
K fatigue test across the weld zone showed that substantial crack retardation occurred far beyond the heat affected zone (HAZ) at low
K regime. The FCP behavior of FSWed 6061-T651 is discussed based on residual stress measurement and fractographic observation.

Abstract: Ultrafine grained materials fabricated by severe plastic deformation exhibit both superior and inferior mechanical properties, as the prominent structural materials, compared to coarse grained counterparts. The superior mechanical properties are ultrahigh strength and exceptional ductility at high temperatures (i.e., superplasticity). The inferior mechanical properties are lack of strain hardenability and room temperature ductility. In this study, the relationship between microstructure and mechanical properties of ultrafine grained materials fabricated by severe plastic deformation is investigated in order to provide insight broadening their future applicability.

Abstract: During repair welding of steel bridge under service load, vibration occurs due to the dynamic loads like wind and vehicles, etc. However, the effect of vibration during welding on the fatigue strength of weldments is not yet clearly understood. In this paper, the effect of vibration during welding on the fatigue strength of structural steel weldments was investigated in order to improve reliability in the repair welded joints of steel bridge. The base material used in this investigation was SM 490A steel of weldable grade. Flux Cored Arc Welding (FCAW) process was used to fabricate the single ‘V’ butt joints. Welding was performed on the steel under the mechanical vibration of given frequency. The applied frequency was resonant frequency. Also, weldments under no vibration during welding were prepared. Fatigue tests were conducted using a servo hydraulic controlled 50tonf capacity UTM with a frequency of 5Hz under constant amplitude loading. Effect of vibration during welding on the fatigue strength of weldments was analyzed in detail.

Abstract: Deformation behavior of high nitrogen austenitic Fe-18Cr-18Mn-2Mo-0.9N stainless steel was investigated utilizing electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). During deformation, the <110> grains rotated and the trace of these grains moved towards into the dodecahedral plane (the line connecting (001) and (111) planes). Misorientation mapping in EBSD showed that the special boundaries (almost Σ3) gradually diminished whereas the low-angle boundaries were developed. TEM observation showed that (i) the low-angle boundaries developed corresponded to the deformation twinning with {111}<112> component, (ii) the deformation twinning showed the strong orientation dependence relative to tensile axis, and (iii) the deformed microstructure was characterized by extended stacking faults, planar dislocation array in low strain regime, and by well-developed deformation twinning in high strain region, respectively.

Abstract: Surface roughness is main source of error in instrumented microindentation when it is not negligible relative to the indentation depth. The effect of a rough surface on the results of instrumented microindentation testing using spherical indenter was analyzed by applying the contact depth model, which takes surface roughness into account. Improved variations in hardness and Young’s modulus were shown for W and Ni when the results were analyzed by this rough-surface model, while these values were underestimated with increasing surface roughness when analyzed by the flat-surface model. The deformation state of asperities underneath spherical indenter was also discussed.

Abstract: In the API X65 steel, effects of rolling and cooling conditions on microstructure and mechanical properties were studied. In the case of accelerated cooling after multi-pass rolling in the high/low unrecrystallized range, the tensile strength was 574-670 MPa and the impact toughness was 74-109 J. In the case of accelerated cooled to 550°C and then interrupted by air cooling, on the other hand, those values were 524-538 MPa and 100-135 J, respectively. Whereas the former exhibited the continuous yielding, the latter showed discontinuous yielding. In addition, yield ratio increased from 0.59-0.67 to 0.85-0.87, accompanied with the enhancement of yield strength. Ultrafine ferrite grains formed by the strain induced dynamic transformation during the severe rolling and second phases formed during cooling were observed. In accelerated cooling and interrupted cooling conditions, main second phases formed after cooling were martensite and pearlite, respectively. Separation cracking mostly observed at interfaces of ferrite matrix and second phases, may be attributed to the intrinsic interfacial weakness.

Abstract: The temperature dependence of the tensile properties and the fracture toughness of the cold-worked STS 304 stainless steel have been examined in the temperature range of 293 K to 111 K. The tensile strength significantly increases with a decrease in temperature, but the 0.2% yield strength is relatively insensitive to temperature. The total elongation at 193 K abruptly decreases by 50% of that at 293 K, and it decreases slightly at 193 K to 111 K. The strain hardening exponents at low temperatures are about four times as high as that at 293K. Initiation fracture toughness (Jc) and tearing modulus (Tmat) tend to decrease with a decrease in temperature. The Jc values exhibit an inverse dependency on the effective yield strength (σflow) at all the test temperatures. Fractographic examination revealed that the critical stretch zone width (SZWc) at room temperature was about three times as large as that at 111 K. This indicates that the variation in fracture toughness according to temperature corresponds to the decrease in SZWc with decreasing temperature.

Abstract: New gadolinium-yttrium zirconate thermal barrier coating(TBC) material is deposited by electron beam PVD method, as an alternative to YSZ TBC layer for gas turbine blade applications. XRD analysis reveals that the new TBC material consists of thermally stable pyrochlore structure. Hertzian and nanoindentation evaluations reveal that gadolinium zirconate materials show superior properties as a TBC candidate material with high mechanical properties. The Y2O3 doping improved hardness and elastic modulus of TBC layers. The indentation stress-strain curves by Hertzian indentation and the load-penetration depth curves by nanoindentation indicate that the new TBC layer has higher damage resistance combined with superior thermal insulation properties rather than commercial YSZ coatings.

Abstract: Layered structure with composition gradient between inner layer of alumina (Al2O3) and outer layer of zirconia ( ZrO2-3mol% Y2O3 ;TZ-3Y) was fabricated using a gel casting process for the inner layer and a dip coating process for the outer layer in an aqueous system. The interlayer with composition gradient was prepared by mixing the Al2O3 and TZ-3Y slips as a volume ratio. Dried green bodies of the inner layer were pre-sintered at 1100°C for 5hr. After pre-wetting the pre-sintered bodies, the dip coating was preformed with mixed slips. Two kinds of composition gradients in the interlayer were prepared to observe the effect of composition gradient on mechanical properties, with same thickness. One is continuous composition gradient from Al2O3 to TZ-3Y with 9:1 to 1:9 volume ratios, and the other is layered structure with 8:2, 6:4, 4:6, and 2:8 volume ratios of Al2O3 and TZ-3Y. After the dip coating with each mixed slip, the coated bodies were sintered at 1100°C for 1hr to prevent delamination between each layer. The last dip coating was performed with the TZ-3Y slip, and then sintered at 1600°C for 3hr. Microstructure and element analysis of the sintered bodies were observed using SEM. For measurement of flexural strength, a standard four-point banding test was conducted. Also, hardness and modulus were measured using micro- and nano-indentation techniques. The step-like gradient in microstructure and composition is achieved through the gel casting and dip coating processes, including a gradual change in H and E from the inner to outer layers.

Abstract: The effect of prior T4 heat treatment and subsequent microstructural evolution on tensile behavior of friction stir welded (FSWed) AZ31B-H24 alloy was examined in this study. Selected AZ31B-H24 plates were prior T4 heat treated at 400
for 24 hours and subsequently friction stir welded. The tensile properties, optical micrographs and SEM fractographs for FSWed AZ31B-T4 specimens were compared with those for the H24 counterparts. Prior T4 heat treatment tended to decrease the tensile ductility reduction in FSWed AZ31B-H24 specimen. The tensile ductility reduction mechanism as associated with prior T4 heat treatment is discussed based on detailed micrographic and fractographic observations.