Michihiko Nagumo

Delayed fracture of beta titanium orthodontic wire in fluoride aqueous solutions

Hydrogen embrittlement of a beta titanium orthodontic wire has been examined by means of a delayed-fracture test in acid and neutral fluoride aqueous solutions and hydrogen thermal desorption analysis. The time to fracture increased with decreasing applied stress in 2.0% and 0.2% acidulated phosphate fluoride (APF) solutions. The fracture mode changed from ductile to brittle when the applied stress was lower than 500MPa in 2.0% APF solution. On the other hand, the delayed fracture did not occur within 1000h in neutral NaF solutions, although general corrosion was also observed similar to that in APF solutions. Hydrogen desorption of the delayed-fracture-tested specimens was observed with a peak at approximately 500 degrees C. The amount of absorbed hydrogen was 5000-6500 mass ppm under an applied stress in 2.0% APF solution for 24h. It is concluded that the immersion in fluoride solutions leads to the degradation of the mechanical properties and fracture of beta titanium alloy associated with hydrogen absorption.

Susceptibility to delayed fracture of Ni-Ti superelastic alloy

Abstract The susceptibility of Ni–Ti superelastic alloys to hydrogen embrittlement has been examined by means of a delayed-fracture test and hydrogen thermal desorption analysis. It was found that the time to fracture was drastically reduced when the applied stress exceeded the critical stress for martensite transformation. In the applied stress range lower than the critical stress, the time to fracture lessened in the order of instability of the alloys to undergo reversible martensite transformation. Hydrogen thermal desorption of specimens subjected to delayed-fracture test is classified into two types according to the applied stress level. The amount of desorbed hydrogen was markedly increased when the applied stress was higher than the critical stress. It was concluded that Ni–Ti superelastic alloys transformed to martensite are sensitive to environmental conditions accompanying accelerated hydrogen embrittlement.

Function of hydrogen in intergranular fracture of martensitic steels

Abstract The mechanism of intergranular-mode fracture in hydrogen-related failure of high-strength martensitic steels has been investigated. Pronounced degradation of tensile properties appeared with increasing manganese content in a slow-elongation-rate test under concurrent hydrogen charging. The fracture mode was intergranular with tear traces along martensite lath boundaries. The tear traces disappeared and the average surface roughness decreased with increasing manganese content. Thermal desorption analysis of hydrogen charged to deformed specimens has been conducted using hydrogen as a probe of defects. It was revealed that the density of point defects increased owing to straining and was more noticeable in steels with a higher manganese content. In common with transgranular-mode fracture, the primary function of hydrogen in intergranular-mode fracture is thought to be one of stabilizing and increasing the density of strain-induced vacancies that lead to the formation of microcracks or microvoids in the vicinity of boundaries. The constraint of plastic deformation at grain boundaries due to boundary phases is likely to determine the susceptibility to hydrogen-related failure induced by strain concentration.

Microstructures relevant to brittle fracture initiation at the heat-affected zone of weldment of a low carbon steel

Charpy toughness of the heat-affected zone (HAZ) of weldment of a low carbon steel has been investigated by means of an instrumented Charpy test and fractographic analysis. Microstructures were varied with thermal cycles simulating double-pass welding. The ductile-brittle transition temperature is the most deteriorated at an intermediate second-cycle heating temperature. The origin of the difference in the transition temperatures has been analyzed to exist in the brittle fracture initiation stage. Fractographic examination correlating with microstructural features has revealed that the brittle fracture initiation site is associated with the intersection of bainitic ferrite areas with different orientations rather than the martensite-austenite constituents. The role of the constraint of plastic deformation on the brittle fracture initiation is discussed.

Fatigue damage and its interaction with hydrogen in martensitic steels

Fatigue damage involving creation of point defects has been revealed by means of hydrogen thermal desorption analysis, utilizing hydrogen as a probe of defects. Two types of high-strength martensitic steels were subjected to a rotational bending test for various cycles. The amount of absorbed hydrogen decreased or was almost constant in the early stage of the fatigue process and then began to increase after a substantial number of fatigue cycles. The increase in the final stage was due to the creation of point defects, presumably vacancies, while the decrease in the early stage was ascribed to changes in microstructural constitutions. Hydrogen-precharging drastically reduced the fatigue life, but the effect of hydrogen on the fatigue limit was not significant. The fracture surface of hydrogen-precharged specimens was smooth without cleavage-like crystallographic features. The defect density in fractured specimens in the presence of hydrogen was higher than in specimens fatigued for a similar number of cycles without hydrogen. Involvement of hydrogen in fatigue damage in the crack nucleation and growth stages is discussed.

Brittle fracture initiation in low carbon steels at the ductile-brittle transition temperature region

Abstract The fracture process that determines the temperature dependence of toughness at the ductile-brittle transition was investigated for low carbon steels. It was revealed that the resistance against the stretch zone at the crack tip and the stable crack extensions is temperature independent while it depends on the carbon content. Applicability of elastic-plastic analyses of the crack tip fields was examined in terms of the relationship between J integral value normalized by the flow stress and crack opening displacement. Temperature dependence of toughness is determined primarily by the brittle fracture initiation. Locations of the initiation sites were revealed to coincide with the maximum stress triaxiality rather than the maximum tensile stress. It was suggested that the brittle fracture is caused by the deformation-induced initiation and triaxial stress assisted growth of an incipient crack.

Fracture process of a low carbon low alloy steel relevant to Charpy toughness at ductile-brittle fracture transition region

The fracture process that determines the Charpy energy at the ductile-brittle transition region was investigated by means of the instrumented Charpy test and fractographic analysis with a low carbon low alloy steel subjected to different control-rolling conditions. The decomposition of a Charpy energy into the energies dissipated in the course of the notch-tip blunting, stable crack growth, and brittle crack propagation is unique irrespective of the testing temperatures and specimen series. Toughness level can be divided into four regions according to the pre-dominating fracture process. The temperature dependence of toughness and effects of the an-isotropy of a specimen originates in the brittle fracture initiation stage rather than the resistance against the notch-tip blunting or stable crack growth. From fractographic examination referring to the stress analyses, it is discussed that the brittle fracture initiation is controlled by the local deformation microstructures in the plastic zone together with the stress field ahead of the notch or the stable crack front.

Enhanced susceptibility to delayed fracture in pre-fatigued martensitic steel

Interaction between fatigue damage and hydrogen is the concern of the present study. The susceptibility of a high-strength martensitic steel to delayed fracture has been examined using as-heat-treated and pre-fatigued specimens. The pre-fatigued specimens showed a tendency to fail earlier, but annealing the pre-fatigued specimens at 200 °C recovered nearly all of the delayed fracture life. Production of point defects during fatigue was detected by means of hydrogen thermal desorption analysis (TDA), using hydrogen as a probe of defects. Hydrogen absorption capacity increased in fatigued specimens, but it was reduced to the level of the as-heat-treated specimens when fatigued specimens were annealed at 200 °C, implying that increased hydrogen-trapping defects, presumably vacancies, were produced during fatigue. Hydrogen TDA peak profiles showed alterations that imply agglomeration of vacancies associated with an increase in fatigue cycles. The involvement of vacancies created during fatigue in the enhanced delayed fracture is consistent with a model that proposes strain-induced vacancies and their agglomeration are the primary mechanism of hydrogen-related failure.

Structural relaxation in amorphous Ni-Ti alloys prepared by mechanical alloying

Abstract Structural changes in amorphous states during prolonged milling after the completion of amorphization have been investigated through the crystallization behaviors with Ni-Ti alloys of compositions corresponding to the intermetallic compounds. In 33Ni-67Ti, the crystallization to NiTi 2 takes place in two steps, a higher crystallization temperature dominating after prolonged milling. In 50Ni-50Ti, structural relaxation leading to the crystallization to NiTi 2 and Ni 3 Ti takes place. In 30Ni-70Ti, which is close to the border of the amorphization range, the initially formed amorphous phase turns into a supersaturated solid solution of Ti in Ni.

Characterizations of mechanically alloyed Ti–Zr–Cu–Ni powders

Abstract Influences of Zr and Ti contents on amorphization of Ti–Zr–Cu–Ni alloy induced by mechanical alloying are studied. It is found that by increasing Zr content, the 100 h-milled Cu 58-x Zr x Ti 34 Ni 8 (at %) alloys change from a single fcc solid solution ( x =0) to a partially amorphous state ( x =11) and finally to almost a single amorphous phase ( x ≥31). By changing Ti content, partial amorphization is confirmed to occur in the Cu 47 Zr 11 Ni 42-x Ti x (at.%) alloy with x ≥22. Influences of Zr and Ti contents on the thermal stability of the as-milled Cu 58-x Zr x Ti 34 Ni 8 and Cu 47 Zr 11 Ni 42-x Ti x alloys are investigated by means of a differential scanning calorimeter and X-ray diffraction. Oxidation of zirconium in the above as-milled alloys upon heating is discussed.