Key Engineering Materials Vol. 795

Abstract: Based on research of the low temperature fracture property of high grade steel pipe, it shows that X70, X80 steel pipe and X80 tee have high Charpy impact toughness. However, as the wall thickness increases, the shear area of DWTT decreases rapidly, and the thickness effect is significant. The research results show that the original wall thickness impact specimen fracture of steel pipe may not be ductile, for design temperature less than -30°C and wall thickness greater than 40mm. The brittle fracture was caused by structural factors. The Charpy impact energy, which just reflects the toughness of materials, does not show the fracture appearance as it would occur in service, because of the different specimen geometry and high rate of impact. The brittle fracture can occur at low temperature and low stress even with a high Charpy impact energy, the conditions of brittle fracture should be established under combination of the wall thickness, temperature and other factors. In this work, it is clarified that measurement of the fracture toughness under service temperature should be used to control low stress brittle fracture, besides the Charpy impact energy to ensure the material toughness.

Abstract: The initiation and propagation behaviors of small fatigue crack in TC4 were investigated in the present work. Surface replication on the basis of a two-part silicon mixture and confocal laser scanning microscope were used to record and observe the small crack initiation and growth processes at room temperature in air. Results showed that surface cracks initiated from the interfaces between α and β phases. When the crack lengths were below ~200 μm, the crack growth rates exhibited large oscillations and temporary retardations due to the presence of α/β interfaces. The corner crack propagated much faster and might have shielding effect on the surface crack.

Abstract: A model has been developed to predict the interdiffusion behaviour of elements between a substrate and a coating. This model, however, relies on knowing accurate diffusion coefficients. However, only limited diffusion data are available in the literature. Recently, it has been demonstrated that Density Functional Theory (DFT) can be used to calculate relevant diffusion coefficients with reasonable accuracy. According to the vacancy diffusion mechanism , diffusion coefficient has an Arrhenius form. The diffusion activation energy can be written as a sum of the diffusion energy barrier and the vacancy formation energy adjacent to a solute.

Abstract: A small punch testing (SPT)-related stress-strain relation (SPT-SR) model is used to obtain the stress-strain curve of DP600 according to Chen-Cai equivalent energy method. And then the SPT and notched small punch testing (NSPT) specimens were simulated in order to determine the critical fracture criterion of DP600 on the basis of the stress-strain curve obtained by SPT-SR model. Lastly, the J resistance curve of small C-shaped inside edge-notched tension (CIET) specimen for DP600 dual-phase steel was successfully predicted based on the aforementioned fracture criterion.

Abstract: Low cycle fatigue (LCF) tests are performed on CP-Tiat different temperatures (293K,423K and 523K). It is found that the fatigue life of CP-Tidecreases with temperature. A short cycle hardening phenomenon occurs at the beginning of cyclic deformationat 293K and 423K, followed by cyclic softening untilfailure. At 523K, cycle hardening isexhibited throughout the entire cycle until thefracture. The fatigue-life curves obtained from the tests are constructed using Coffin-Manson-Basquin model. According to the relationship between the four parameters of Coffin-Manson-Basquin model and temperature, the temperature-based life prediction model is further proposed. Scanning electron microscopy observation of fatigue fractures showsthat the fatigue cracks of CP-Tiat 423K and 523K under different strain amplitudes initiate on the surface of fatigue specimens and extend to the fracture zone by the transgranular mode.

Abstract: The deformation and damage features of a high Mo single crystal Ni-based superalloy during creep at high temperature are investigated by means of measuring creep properties and observing microstructure. Results show that, compared to 4%Mo single crystal nickel-based superalloy, the 6%Mo superalloy displays a better creep resistance, and the creep life of 6%Mo single crystal superalloy at 1040°C/137MPa is measured to be 556 h. In the ranges of applied temperatures and stresses, the creep activation energy of the alloy is measured to be 484.7kJ/mol. Wherein, the deformation mechanisms of the 6%Mo superalloy during steady state creep are dislocations slipping in ϒ matrix and climbing over the rafted ϒ' phase. In the later stage of creep, the deformation mechanism of alloy is dislocations shearing into the rafted ϒ' phase, the alternate activation of dislocations slipping results in the twisted of the rafted ϒ'/ϒ phases, as the creep goes on, to promote the initiation and propagation of cracks along the interface of the twisted ϒ/ϒ' phase perpendicular to the stress axis, up to creep fracture, which is thought to be the damage and fracture features of the alloy during creep at high temperature.

Abstract: 4043 aluminum alloy was printed by cold metal transfer (CMT) additive manufacturing (AM) technology. The microstructure and mechanical properties were analyzed. The effect of ratcheting behavior was analyzed by the tensile test after ratcheting. The results indicate that the dendrite structure of 4043 aluminum alloy has obvious directivity. The binary eutectic structure of α (Al) + Si is mainly distributed at the grain boundaries and the interior of grain is mainly α (Al). The increase of stress amplitude and mean stress leads to ratcheting strain, which can cause plastic damage of AM aluminum alloy. This is related to holes aggregation and dislocation slip caused by ratcheting behavior. Compared to the aluminum alloy of un-ratcheted test, the tensile and yield strength increased and the elongation decreased, but the change of tensile and yield strength are not obvious between the s specimens of different ratcheting.

Abstract: In this paper, 6013 aluminum alloy with the thickness of 2.5mm was overlap welded by fiber laser. The microstructure, mechanical properties and fracture morphology of the joint was tested and observed by Optical Microscope, material testing machines and Scanning Electron Microscope, thus the failure and fracture mechanism of the welded joint are analyzed. The results showed that good shape of weld was achieved under the optimal welding parameters. Equiaxial as-cast microstructures exist in the welding center and the columnar grains are formed near the fusion line in the WZ. The hardness of weld zone is the lowest in the joint, which is about 72 HV, about 57% of that of BM. The tensile shear strength of the joint is 96Mpa, about 25% of tensile strength of BM. The fracture is happened in WZ and the brittle fracture mode is dominated with shear dimples and shear planes.

Abstract: Effects of thermal aging on tensile and Charpy impact properties in 16MND5 steel was investigated, which were aged at 500°C for 0 h, 1000 h, 3000 h, 5000 h. A significant decrease in the yield stress and ultimate tensile strength was observed after thermal aging, while the elongation exhibited a slight decrease follow by an increase aged for 5000 h. What's more, the ductile-to-brittle transition temperature (DBTT) showed a remarkable increase with the prolongation of thermal aging duration. These facts indicate thermal aging caused embrittlement of the steel, which was further investigated by microstructure observation of SEM. The results show cleavage fracture after thermal aging. Furthermore, experimental results at 350°C thermal aging temperature originated from the previous literature were used to analysis the effect of thermal aging temperature. Thus, thermal embrittlement should be taken seriously.