Grain boundary fracture in molybdenum was investigated by the four-point bending test at 77 K as a function of misorientation angle, by using bicrystals with a 〈110〉 twist boundary. Main results obtained are as follows: (1) Fracture always occurs at the grain boundary. (2) The fracture strength and strain to fracture markedly depend on the misorientation angle, i.e., the bicrystals of misorientation up to 20° and those of around 70° (Σ3) have much higher fracture strength than the others, and some appreciable plastic strain is only observed for these strong bicrystals. (3) Misorientation width of the peak in strength agrees with the predicted value from the Brandon’s equation. (4) There are no peaks in strength around Σ9 and Σ11 coincidence boundaries.
In order to investigate the grain boundary fracture in molybdenum with a 〈110〉 symmetric tilt boundary, the four-point bending test was conducted at 77 K by using bicrystals with various misorientations from 0° to 90°. The stress-strain curve to fracture was measured as a function of misorientation angle \varphi. Two classes of specimens were used to clarify the impurity effect on the grain boundary fracture: One was as-grown specimens which contained carbide precipitates at grain boundaries and the other was purified specimens in which no precipitates were observed. A marked misorientation dependence of both fracture stress and plastic strain to fracture, was observed for the purified specimens; the stress and strain were both much higher for \varphi=0°-10°(Σ1), around 70°(Σ3, (112) twin boundary) and around 87°(Σ17b, (223) twin boundary) than for the other boundaries. The dependence observed for the as-grown specimens was qualitatively similar to but quantitatively much less than that for the purified specimens. The misorientation dependence of the grain boundary energy and the effect of carbon impurity on the grain boundary cohesion were discussed.
In order to get information on the dislocation mechanism of the Bauschinger effect, the Bauschinger strain, γB(n), during compression after tensile prestraining was measured at room temperature in copper in a reversed-stress range of 0≤n≤1 (n=|τ⁄τp|; τ: the reversed stress at which γB(n) was measured, τp: the stress in prestraining at which the stress direction was reversed). The behaviour of increase of γB(n) for n≤∼0.5 (at an early stage of reversed straining) with the progress of work-hardening during prestraining was found to be different from that for n>∼0.5 (at a later stage of reversed straining), that is, the increase was linear over the deformation stages in prestraining I, II and III for n≤∼0.5, but it was rapid, slow and again rapid in stages I, II and III, respectively, for n>∼0.5. In spite of this behavior, however, an amount of γB(n) in a fixed value of n was given as a unique function of an amount of work-hardening, not depending on the specimen orientation. This fact implies that the Bauschinger effect is mainly controlled by the work-hardening phenomena during prestraining. The behaviour of increase of γB(n) with the progress of work-hardening was explained from dislocation structures formed during prestraining.
Determination of elements in Ni-and Fe-base heat resisting alloys by the spark source mass spectrometry using the powdered salt and solid sample electrode was studied. The powdered salts were prepared from synthesized standard solution containing 28 elements, or by dissolving the solid samples in acids. Yttrium or nickel was used as an internal standard, and high-purity graphite and aluminum powder were also used as matrix materials. The relative sensitivity factor (RSF) (Ni=1) obtained by the powdered salts made of synthesized standard solution when aluminum was used as matrix materials, was in good agreement with those obtained by the powdered salts made of the solid samples and by a conventional solid sample. Major, miner, trace and ultratrace elements in the solid samples of Ni-and Fe-base heat resisting alloys were determined by using RSF values obtained from the synthesized solution, and the results were in good agreement with the certified values and those obtained by other methods such as X-ray fluoresence and atomic absorption etc. The present method gives values without the use of a reference standard.
The fractre toughness KIC and limiting strength of TiC-base cermets were determined by means of analysis of the effect of fracture-initiating defect-size on fracture stress. The KIC value of TiC-16-44 mass%Ni alloys increases in the range of 6.0-9.5 MPa\sqrtm with increase in nickel content. The value of KIC (8.2 MPa\sqrtm) of a TiC-28 mass%Ni alloy decreases monotonically to 5.8-6.4 MPa\sqrtm with increase in the content of the added carbides (Mo2C, NbC, TaC, VC) up to 18.5 mol% (added carbides in carbide phase), except for WC addition. The decrease of KIC by adding different carbides is due to the embrittlement of binder phase which is caused by dissolution of titanium and other elements be contained in the added carbides. By comparing the KIC of TiC-Ni alloys with that of WC-Co alloys, it was found that the KIC for a cermet composed of carbide and metal phases depends mainly on the elastic moduli (E & ν) of the alloy, and also on the critical strain energy release rate (GIC(B.P.)) and volume fraction (f) of metal phase, according to the following equation. (This article is not displayable. Please see full text pdf.) The limiting strength of TiC-Ni alloys, which is independent of the size of fracture-initiating defect, is raised from 1.98 to 2.57 GPa as the nickel content inceases from 16 to 44 mass%, and is lowered slightly by addition of carbides except tungsten carbide.
A simulation of acoustic emission signals due to fracture events was carried out in order to investigate the temporal inhomogeneity of fracture processes. In the simulation process, two types of source function D(t), the product of crack volume-time history and elastic modulus, were assumed, and output signals from piezoelectric transducer V(t) were calculated from the convolution of D(t) with R(t), which is a combined response function of specimen and transducer obtained experimentally by the use of breaking pencil lead. It was shown that the amplitude of V(t) was proportional to the crack volume and the longer the lifetime of cracking, the smaller the amplitude. From these results, it was proved that only the fracture events with large cracking volume and short lifetime were detectable as an acoustic emission and that burst type emissions during macroscopic subcritical crack growth were attributed to the instantaneous fast fracture in the microscopic sense.
To clarify the correlation between the hydrogen embrittlement and the hydride (β-phase) formation in transition metals, Ni-Cu binary alloys have been investigated by means of electrolytic hydrogen charging and X-ray diffraction methods. The hydride formation was controlled by varying three factors, i.e., copper content, charging current-density and temperature. In any case of these three factors, it was found that independent of hydrogen content the hydrogen embrittlement occurred always only when the hydride was formed. It was concluded that the hydrogen embrittlement can be ascribed to the discontinuous expansion which accompanies the α-β transformation (hydride formation) caused by the change in hydrogen concentration.
In a previous paper, it was reported that the Miharisin’s 3.5 GPa grade maraging steel (8%Ni-14%Mo-18%Co-bal. Fe) had excellent fatigue strength, when it was prepared by a special basic melting and was subjected to thermomechanical treatments followed by cold rolling. In the present work, some problems on the practical use of this steel, that is, mechanical anisotropy, hydrogen embrittlement and the strength of welded joint were examined. The results are as follows: The typical texture as a BCC metal was developed by cold rolling, but the mechanical anisotropy was not so distinct. The conventionally heat treated steel is very sensitive to hydrogen embrittlement, but the cold rolling before age-hardening was effective to mitigate the embrittlement. Electron beam welding reduced the tensile strength from 3.5 GPa to 3.0 GPa, but it is recommended as an available welding process for this steel. The structures of weld metal and heat affected zones were observed by means of transmission electron microscopy.
Fatigue test and fracture toughness test in 780 MPa grade high strength steel weldments in the temperature range from 98 K to room temperature were experimentally carried out. Both the tests were performed with a compact tension specimen by using a program-load-controlled 0.98 MN fatigue testing machine. Main results obtained are summarized as follows: (1) The fatigue crack propagation rate in the base metal shows a similar value in both the weld metal and HAZ. (2) The trend of fatigue crack propagation characteristic in weldments in the temperature range from 223 K to room temperature is in good agreement with the dislocation dynamics theory of fatigue crack propagation. (3) Fatigue fracture toughness, Kfc and fracture toughness, Kc in the base metal are larger than in both the weld metal and HAZ.
The precipitation sequence in a Cu-20 mass%Ni-8 mass%Sn alloy on aging at temperatures between 623 and 723 K after quenching from 1123 K and the equilibrium phase diagram at 823 K of the Cu-Ni rich Cu-Ni-Sn system were investigated by means of optical and electron microscopic observations, X-ray analysis, and hardness measurements. Results obtained are as follows: (1) The equilibrium precipitate phase in a Cu-20%Ni-8%Sn alloy was δ phase (β-Cu3Ti type) with the lattice parameters of a=0.451, b=0.538 and c=0.427 nm. (2) The precipitation sequence of this alloy is as follows; \
oindentα-supersatulated solid solution (This article is not displayable. Please see full text pdf.) (3) The values of apparent activation energy for intragranular and cellular precipitation of this alloy are 170 and 180 kJ/mol, respectively. (4) The phase boundaries among (α+γ(DO3)), (α+γ+δ), (α+δ), (α+δ+θ(DO19)) and (α+θ) regions at 823 K in the Cu-Ni rich Cu-Ni-Sn system were determined.
Austenitic steels for geothermal power plants may meet with failure by stress corrosion in geothermal brines containing Cl−. In this concern, dynamic and static crack propagation properties were investigated for austenitic steels (SUS304 and SUS316) in dilute acidic geothermal brine at Onikobe containing 5400 ppm of Cl−. Experiments were made with apparatuses installed at Onikobe, and results obtained were analyzed especially referring to fractographs. Results were summarized as follows: (1) Dynamic crack propagation of austenitic steels except sensitized SUS304 appeared to be that of corrosion fatigue, for the rate of crack propagation depended on the frequency. (2) As for dynamic crack propagation of sensitized SUS304, the rate of crack propagation increased rapidly in the region of ΔKI≥15 MPa·m1⁄2 at a low frequency (0.002 Hz) by predominant influence of IGSCC. (3) For sensitized SUS304, KISCC was presumed to be about 19 MPa·m1⁄2, and KFSCC (the value of ΔKI at which the influence of SCC begins to appear in dynamic crack propagation) was presumed to be about 15 MPa·m1⁄2. (4) For SUS316, no characteristics of IGSCC were observed even in sensitized specimens.
The relation between the discontinuous precipitation and the continuous precipitation, and the kinetics of the discontinuous precipitation in Cu-Mg alloys containing 2.0, 2.6 and 3.2 mass%Mg have been investigated by metallographic observations and micro-hardness measurements. Variations in cell width and volume fraction of discontinuous precipitation cells with ageing time have been determined in the specimens aged at temperatures from 573 K to 873 K. Results obtained are as follows: (1) The decomposition of Cu2Mg from supersaturated solid solutions of magnesium in copper during ageing is initiated by a discontinuous precipitation and followed by a continuous precipitation. (2) The cell growth is associated with the migration of a grain boundary. (3) The cell growth rate decreases with ageing time progressively after the linear growth rate of cells. This decrease will be attributed to the influence of the continuous precipitation on the cell growth. (4) The volume fraction of the discontinuous precipitation cells, f, can be represented by Johnson-Mehl type equation: f=1−exp(−btn), (where, t is the ageing time and b and n are constants). The value of the parameter n is 2 and independent of ageing temperature and alloy composition in the range of ageing time in which cell growth rates are constant.
Carbide-dispersed layers (CD layers) containing fine particles of TiC or V4C3 were formed by carburizing Fe-Ni-2%Ti and Fe-Ni-4%V alloys at temperatures between 1073 and 1273 K. The microstructures and the formation rate of the CD layers and the particle size in the CD layers were investigated. A boundary between the CD layer and an unreacted region was plane and distinct in Fe-Ni-Ti alloys, but the boundary in Fe-Ni-V alloys was more irregular and less distinct. TiC particles in Fe-Ni-Ti alloys appeared to have no definite shape, but V4C3 particles in Fe-Ni-V alloys were massive and had the cube-cube orientation relationship with the austenite matrix; (100)γ\varparallel(100)V4C3, γ\varparallelV4C3 and γ⁄V4C3. The particle size of V4C3 in the CD layer of Fe-Ni-V alloys increased lineally with the depth from the specimen surface, and the change in particle size with depth was more remarkable in high Ni alloys. The CD layer grew in proportion to the square root of time, and the reaction rate constant A was shown quntitatively as follows; (This article is not displayable. Please see full text pdf.) \
oindentwhere ΔCCγ is the carbon concentration difference between the specimen surface and the reaction front, DCγ is the diffusion coefficient of carbon in austenite, ν is the molar ratio of carbon to the solute (Ti or V) in the carbide MCν formed by carburizing, CM(0) is the initial solute concentration (Ti or V) in the bulk alloy, and MM and MC are the atomic weights of solute atom and carbon atom.
The development of a software was carried out for the solidification phenomenon of rapid cooling. The computer simulation by the software can estimate the effect of the solidification by rapid cooling and the solidified structure for Al-2 to 8 mass%Pb alloys. An outline of the simulation is as follows: The cooling process by successive temperature steps was applied to the continuous cooling process in the solidification. The solidification and reaction process were analyzed by four proposed models. In the phase separation reaction, L1→L2s+(L1), spherical liquid phases L2s of CH%Pb are randomly distributed throughout the liquid phase L1 of CL%Pb, and the liquid L1 around the L1/L2s interface can be supercooled by 1ΔT1. A total amount of the liquid L2s increases as the coarsening and sifting-out growth proceed. In the monotectic solidification, L1→Al+L2m+(L1), the solid phase aluminum is nucleated in the liquid L1 supercooled by ΔTm due to rapid cooling. Then, by the diffusion event between an Al-rich diffusion layer around the liquid L2m and a Pb-rich diffusion layer ahead of the solid aluminum, the solid aluminum is grown in a shape such as an ellipsoid, and dropletes of liquid L2m of CH′%Pb form locally around its solid aluminum. At the completion of the monotectic solidification, the retained liquid phases L2m and L2s are entrapped or isolated by the growing solid phase. Their liquid pools L2 solidify individually, L2→Al+(L2), or grow by sifting-out growth with decreasing temperature. Then, at the supercooling by ΔTd below the eutectic temperature, the solid phase lead form from the retained liquid pools of 100%Pb as divorced eutectics, L2→Pb. In that case, the phase transformation and concentration change in phases were considered to follow the equilibrium phase diagram. For cooling rates of 102 K/s to 103 K/s, the calculated structure was in good agreement with the experimental one.
The programme of this computer simulation was made by the introduction of new numerical calculation method on gravity segregation in the computer simulation on the structure solidified by rapid cooling for Al-Pb alloys. The control of the gravity segregation by rapid cooling and the formation process were investigated by the present computer simulation. A mechanism of the gravity segregation of Al-Pb alloys is considered as the falling phenomenon of growing L2 liquid phase during the separating and coexisting reaction. The falling behavior of L2 depended on density difference between L1 and L2 or growth of L2 at elevated temperatures and the viscosity of L1 at temperatures below about 1123 K. The rapid solidification method could decrease the gravity segregation of high lead alloys. In low lead alloys, the method was rapid in cooling enough to prevent the gravity segregation. For cooling rates of 50 K/s to 103 K/s, the calculated condition for the prevention of the gravity segregation was in agreement with the experimental one.
The solidification problem of a binary eutectic alloy has been treated taking both heat and solute transport into account. A numerical solution for the freezing region is matched at two moving boundaries, through a simplex method for function minimization, with analytical solutions obtainable for a pure solid or a pure liquid region. As the results of calculation, it is possible to obtain the amount of solute diffusing toward the pure liquid region, the temperature profiles within the three regions, the velocities of two moving boundaries and the distribution of solid fraction within the freezing region. The results of calculation are compared with the experimental results of solidification using the H2O-NH4Cl eutectic system.