When a cooling rate is fast enough to prevent crystallization, molten metals are solidified into disordered structures so called amorphous metals. During the cooling process, molten metals become supercooled liquid below the melting point, and dynamic factors such as viscosity and relaxation time rapidly increase, while static factors such as density show no significant change. In the present study, we investigated a temperature dependence of viscosity of Cu-Zr bulk metallic glass above the glass transition temperature Tg using both molecular dynamics (MD) technique and a recently developed energetic technique. A limitation of MD time scale prevents us to calculate viscosity at Tg, because a relaxation time of supercooled liquid becomes significantly long at slightly above Tg. On the other hand, a new method developed by Kushima et. al., analyzes transition state pathway trajectory from an energetic viewpoint and provides viscosity of liquid state at wide temperature range from sufficient high temperature to Tg. We discuss a temperature dependence of viscosity of Cu-Zr bulk metallic glass both from atomistic and energetic viewpoints.
We have performed the indentation simulation to a Ni50Al50 amorphous metal by the molecular dynamics method, and discussed the plastic deformation behavior in amorphous structure by atomic strains and atomic elastic stiffness coefficients, Bαij, or the second-order derivatives of atomic energy. During the indentation, zonal regions like shear bands below the indentation point show large values of local deviatric strains and local volume strains.The number of both positive and negative switching of positivity of det Bαij increases according to the formation of shear bands. On the other hand, the “probabilistic” switching of positivity of det Bαij constantly occurs due to subtle changes in local volume strains even at the equilibrium state. In order to understand the plastic deformation behavior, we should observe the switching of positivity of det Bαij with the exception of small variation of local volume strains. Evaluation of the switching with a large variation of local volume strains reveals occurrence of deformation propagation and extensive rearrangements of atoms around shear bands.
MeV electron irradiation induced Solid-State Amorphization (SSA) in B2 intermetallic compounds was investigated by High Voltage Electron Microscopy (HVEM) and Molecular Dynamics (MD) simulation focusing on the relationship between the SSA tendency and martensite (MS) transformation. There is no relationship between the SSA tendency and the structure of compounds, however, an unique feature of SSA tendency was found in B2 intermetallic compounds ; intermetallic compounds which show MS transformation has a tendency to exhibit SSA.
In this work, the ultra-high cycle fatigue behavior of Zr55Al10Ni5Cu30 metallic glass was investigated by using ultrasonic fatigue testing method. Test alloy rods with a diameter of 8mm were prepared by tilt-casting with copper mold. Test specimens were machined to hourglass shape type (the minimum diameter ; 2.0mm), and after machining the surface of specimens was manually polished with polishing paste. The specimens were tested at a frequency of 20kHz under a stress ratio of -1. The fatigue limit (σw) (half a total stress amplitude) and fatigue ratio (σw/σB) of the alloy showed 893MPa and 0.53, respectively. Furthermore, we did not see any fish-eye like crack-growth morphology on fracture surfaces. The initiation region of fatigue crack was always observed at the side surface of the specimens. A viscous-flow fracture region with large dimples between the fatigue crack-growth region and the final unstable fracture region was observed. This peculiar fracture morphology may be due to adiabatic heating of the metallic glass specimens during the fracture, resulting in transition to supercooled liquid, under extremely high frequency of fatigue stress using ultrasonic loading.
Zr-Cu-Al bulk metallic glass matrix composite with precipitated crystalline phase was fabricated. The major precipitated phase of the composite is B2 type ZrCu. Compressive tests were carried out using the composite specimens with high volume fraction of B2 type ZrCu in order to investigate the mechanical properties and shape memory effect of the composite. As the results, it was found that the composite has not only high strength but also low elastic modulus. In addition, it was confirmed that a stress-induced martensitic transformation in B2 type ZrCu occurs during compressive test. Furthermore, it was confirmed that the composite exhibits shape memory effect.
Low cycle fatigue tests were performed for austenitic stainless steel, JIS SUS316L, used for pressure vessel and piping of engineering plants. This study investigated the effect of variable amplitude strain conditions on the fatigue strength. Fatigue tests were conducted under two-step and repeated two-step variable amplitude strain conditions, in which the influence of straining history on the hardening and softening behavior of the material had an important role on the fatigue strength. The experimental results were compared with predictions of the fatigue lives based on the fracture mechanics approach in conjunction with the hardening and softening behavior. These comparisons were performed for a fatigue usage factor, UF, calculated by the linear damage accumulation law using the fatigue strength diagram obtained under constant amplitude straining tests. The UF value at fracture, UFf, reduced under the variable amplitude straining conditions depending on the straining history. Variations of stress under the strain controlled tests were measured in detail and used for the fatigue life predictions. The results indicate that the minimum value of 0.7 for the UFf gives conservative evaluation of the low-cycle fatigue life.
The corrosion resistance at defect of paint film for Sn-bearing steels exhibiting superior atmospheric corrosion resistance under high chloride content environments was investigated by an accelerated corrosion test. It was found that the delaminated area and corrosion depth around the defect of paint film for the Sn-bearing steels after the test were drastically suppressed, comparing with that for carbon steels. Even on rusted and Cl--contaminated surfaces simulating insufficient surface preparation when repairing the paint film, the Sn-bearing steel showed the superior corrosion resistance. Sn ion dissolved at the defect by anodic reaction can inhibit the anodic dissolution of Fe.
In the present study, a monitoring technique has been investigated in which debonding of adhesive composite joints is detected using Extrinsic Fabry-Perot Interferometric (EFPI) optical fiber sensors embedded in the adherend. First, strains at the end of the bonded part in adhesively bonded aluminum-GFRP single lap joints were measured using the monitoring system with an EFPI sensor constructed in our laboratory. The initiation and development of the debonding was also observed by eyes in the experiment. Strain analysis was performed for the joint specimens by FEM. These results showed that the strain monitoring system had the ability to measure the change in strains caused by the initiation and development of the debonding at the bonded part. Secondly, the strain monitoring using two EFPI sensors having the different gage length was performed for the same aluminum-GFRP joint specimens. The tensile loading experiments and strain analysis were performed by almost the same procedure as those using one EFPI sensor. These result showed that two EFPI sensors could measure the high and low mean strain at the bonded area corresponding to their gage lengths and detect the change in strain distributions caused by the occurrence and development of the debonding. Therefore, it was found that this monitoring system had the detection ability of the debonding in the adhesively bonded FRP joints.
Hydrated hardened cement paste samples using white cement were cured under different relative humidity conditions. After 180-day drying, Adsorption and desorption isotherms as well as chemically bound water of cured hardened cement paste were measured. Based on the adsorption data, it was concluded that drying process has two different effects on a sorption property. One effect was seen in drying process over 70%RH. Capillary pore with more than 2nm size was collapsed and amount of capillary condensation is decreased as it is dried in the lower relative humidity. The second effect was seen in drying process under below 60%RH. The vapor BET surface area was dramatically decreased as cement paste cured in lower relative humidity. From the change of chemically bound water and vapor BET surface area, it was deduced that a reaction of silanol to siloxane bond in C-S-H poses decrease in vapor adsorption sites.