Papers by Keyword: Fracture Morphology

Abstract: Titanium scaffolds produced by additive manufacturing were studied using the scanning electron microscopy (SEM) and the confocal optical microscopy (COM). The previous research has shown that the titanium scaffolds with porous filaments (14 % porosity) exhibited markedly better fatigue resistance than those with compact filaments (6 % porosity). This article is devoted to macroscopic and microscopic images of fracture surfaces of both types of scaffolds after cyclic compression (CC) tests and after cyclic three-point bending tests (CTPB). A high density of cracks and broken filaments was indicated particularly in scaffolds with porous filaments. The fatigue crack growth was highly affected by the microporosity. Fracture facets were smaller and rougher for the porous filaments compared to the compact ones. Values of roughness parameters S_a and S_v for porous filaments were significantly higher than those for compound fibres. Both SEM and COM studies confirmed an important role of crack-pore interactions especially in the porous filaments.

Abstract: In the current paper, the effect of two different coating techniques of boronizing and tungsten carbide (WC) coating on the room and high temperature tensile behavior of the AISI 321 stainless steel were investigated. Consequently, the fracture morphology observations were conducted via scanning electron microscopy (SEM) to inspect the variation of fracture mechanisms after implementing different coating methods. The results of tensile tests at room temperature revealed that despite boronizing reduced the yield strength of the sample due to the softening and grain growth at high coating temperature, the dispersion of boron particles improved the work hardening and ductility of boronized AISI 321. In contrast, the strain to failure of the WC coated sample was decreased due to the fast fracture of the ceramic WC layer at both room and high tensile tests. Furthermore, results of SEM revealed that particle decomposition occured on the fracture surface of the boronized 321 stainless steel represented by dispersed boron particles on the edges of the dimples after failure at high temperature.

Abstract: In order to analyze the influence of neutron irradiation on the critical event of cleavage fracture for the Chinese A508-3 (RPV) steel, the fracture morphology, microstructure and grain size of the irradiated specimens are observed and analyzed by optical microscope (OM) and scanning electron microscope (SEM). The results show that with the increase of neutron doses, the fracture mode of Chinese RPV steel material changes gradually from ductile fracture to ductile-brittle mixed fracture transformation. The brittle part for fracture mode of ductile-brittle fracture is cleavage fracture, and the non-metallic inclusions, microstructure and grain size grades are not significantly different from those of the specimens without cleavage fracture. In the same material, with increasing of neutron doses, the critical event size of the cleavage fracture will be reduced, and the percentage of the grain that satisfies the grain size of the cleavage critical event will increase correspondingly, resulting in an increase of the probability of cleavage fracture of the material. The fine grain with uniform distribution can improve partly the fracture toughness of the material and withstand higher neutron doses under the same conditions.

Abstract: Two states of aluminum alloy material 7B04 T651 and 7B04 T74 using C-ring specimen were selected to carry out stress corrosion simulation test with different stress levels, corrosion concentrations and time, and the fracture morphology of the crack was observed and analyzed by optical microscope and scanning electron microscope (SEM). The results showed that 7B04-T74 alloy was insensitive to stress corrosion and was not prone to stress corrosion cracking under constant tensile stress lower than 432MPa; The stress corrosion cracking time of 7B04 T651 alloy under three different concentrations has no significant difference, and the stress corrosion cracking occurs within 7 days under the stress of 180MPa-432MPa. The time of stress corrosion cracking increased with the decrease of stress. Stress corrosion cracking (SCC) was very sensitive to Cl element, and it was also easy to produce SCC when the concentration of corrosive medium was low, the threshold value of corrosion cracking was about 108 MPa. SEM and EDS analysis showed that the fracture surface was intergranular, mud-like corrosion products, and secondary cracks. At the same time, the matrix grain boundaries were weakened by Cl element.

Abstract: Depending on the composition and structure of metallic glasses cells with the dimensions in the range from tenths nanometers to tenths micrometers were observed on the ductile fracture surface. The variation in dimple size was compared with the serrations presented on the loading curve at the nanoindentation of the metallic glasses with different compositions. Higher instantaneous deformation can be connected with simultaneous shearing at more suitable shear band configurations. The cell morphology with the various cell sizes is observed at the failure of the metallic glasses. At the failure of high strength metallic glasses, the cells are formed in short time due to the release of high amount of stored elastic energy. In this case the uniform cell morphology with the cell size of about 20 nm is observed.

Abstract: In order to characterize the effect of solution treatment on the properties of Selective laser melting (SLM) 316L material for nuclear power field, the effects of different solution treatment conditions on the microstructure evolution and mechanical properties of 316L material were studied. The results show that 316L material is arc-shaped fish scale structure for as-deposited condition. After solution treatment at 1050 °C and 1150 °C respectively, the material microstructure recrystallizes and the arc boundary between layers disappears. The fracture characteristics is typical ductile fracture characteristic. Compared with temperature of 1050 °C, the material is fully recrystallized at 1150 °C and the microstructure uniformity is improved together with the anisotropy is obviously reduced, meanwhile, the formation of twin structure will be beneficial of improving the mechanical properties which has been demonstrated through metallographic analysis combined with characterization of mechanical performance.

Abstract: TA29 titanium alloy forging was heated by duplex annealing. The fatigue crack growth behaviour of the alloy at room temperature, 400 °C, 500 °C and 600 °C was studied. The relationship between the fatigue crack growth rate (Da/dN) and the stress intensity factor amplitude (△K) of TA29 alloy at different temperatures was revealed by Paris formula. The fatigue fracture morphology of fatigue crack growth specimens was analysed by scanning electron microscopy (SEM). The results showed that the TA29 titanium exhibited good crack growth resistance at room temperature. With the increase of test temperature, the C value of the Paris formula increased, the m value decreased, and the fatigue crack growth rate increased. From the fatigue fracture morphology of the specimens, it was found that the fractures of fatigue crack growth specimens at different temperatures exhibited typical pre-splitting zone, steady-state expansion zone and rapid expansion zone. As the temperature increased, the range of the pre-cracking zone was larger, the range of the steady-state extension zone and the rapid expansion zone were smaller. At room temperature, there was no obvious fatigue strips in the steady-state expansion zone, and the transient fracture zone exhibited the characteristics of cleavage fracture, while the steady-state expansion zone at high temperature showed obvious fatigue strips and secondary cracks, and the transient fracture zone was a typical ductile fracture mechanism.

Abstract: The life extension of the reactor requires the improvement of the material performance, especially the radiation resistance. At present, some data about the irradiation hardening and embrittlement behavior of reactor pressure vessel steel have been obtained, mainly the standard tensile properties, standard impact properties and so on. However, to understand the service behavior of materials at higher doses, higher neutron fluence radiation tests are needed. The problem is that because of the large size of the standard sample, if the irradiation dose continues to increase, the surface dose of the irradiated sample will be larger, and it is difficult to test after irradiation. In addition, if the irradiation sample can be replaced by smaller sample, the utilization space of the radiation channel will be greatly improved. The high-throughput characterization for irradiation performance of reactor pressure vessel steel can be achieved. Therefore, in this work, the small punch test of reactor pressure vessel material A508-3 steel was carried out at the temperatures ranging from -150 °C to room temperature. The performance data, such as tensile characteristics, yield strength and fracture energy, were obtained from load-deformation curves. Comparing the results of small punch test with the data obtained by standard tensile and impact test, the correlation on the performance data was established for A508-3 steel.

Abstract: Forging processes, such as liquid forging, rheoforging and thixoforging process, were used in this study as an effective method for manufacturing high-strength, finely-dispersed and highly-uniform AZ91D alloy parts. It has been found that although the microstructures of the three forging parts are all composed of α-Mg and β-Mg₁₇Al₁₂ phases, their shapes, sizes, numbers and distributions are all different, which also influences their mechanical properties. The tensile fracture morphologies were observed and their energy spectrums were analyzed by scanning electron microscopy. The results show that the cracks are mainly originated from the brittle fracture of the β-Mg₁₇Al₁₂ eutectic phases, whereas their fracture morphologies are greatly different, which indicate the direction for providing a reasonable forging process.

Abstract: This study deals with the influence of thickness of copper foil on its tensile property. Tensile tests were carried out on copper foils with various thickness ranging from 5 μm to 2 mm in air. Then, the tests on copper foil with the thickness of 20 μm and copper sheet with the thickness of 2 mm were also conducted in a scanning electron microscope, and their deformation was examined during testing using an electron backscattered diffraction method. The influences of thickness of copper foil on its deformation, tensile strength, and fracture morphology were investigated. As a result, the tensile properties of copper foil depend on its thickness, and the tensile strength decreased when the foil thickness was thinner than the grain diameter.