M. Meshii

Solid solution softening and solid solution hardening

Abstract A mechanism explaining both solid solution softening and solid solution hardening has been developed by examining the motion of a screw dislocation through a combined field of Peierls potential and misfit strain centers. It was found that the strain centers, which assist the double kink nucleation by a couple force exerted on the screw dislocation, impede the sideward motion of the kinks. Therefore, the energies for these two processes must be considered in order to describe the total dislocation motion. The results of the present calculation generally predict a hardening effect at the higher temperature range and a softening effect at the lower temperature range in the same solid solution specimen.

Effect of sulfur segregation and hydrogen charging on intergranular fracture of iron

Abstract The effect of sulfur segregation and hydrogen charging on the grain boundary strength of iron was investigated by a slow strain-rate tensile test at 77 K. The amount of sulfur segregation at grain boundaries was controlled by varying the initial purity of specimens and quenching temperature and was determined by Auger electron spectroscopy after in situ fracture under ultra-high vacuum. The effect of hydrogen on grain boundary strength was examined at various grain boundary sulfur and/or hydrogen contents. The deleterious effects of sulfur and hydrogen on grain boundary strength were found to be independent and additive. The reversible effect of hydrogen is attributed to the reduction in the cohesive strength of grain boundaries due to atomic hydrogen whereas the irreversible effect is attributed to the damage due to molecular hydrogen precipitation.

Electron microscopic investigation of prismatic slip in Mg-10.5 at.% Li single crystals☆

Abstract Mg-10.5 at. % Li alloy single crystals oriented for prismatic slip (1010)[1210] were deformed in tension at 4.2, 77, 198 and 298°K. Slip markings and dislocation structures were studied by transmission electron microscopy. When the crystal was deformed at 77°K, dislocations were predominantly in screw orientation and many small randomly located dislocation loops were present with Burgers vector = a 3[ 1 2 1 0] . The loops are thought to arise by a cross slip mechanism. The screw dislocations are helical presumably from annealing of point defects to the screws after deformation. Tension at 298°K resulted in a cell structure consisting of dislocations and small loops. The surface slip markings became coarser as the test temperature was lowered.

Plastic deformation of quench-hardened aluminum single crystals

Abstract Deformation morphology of quench-hardened aluminum single crystals was studied on the surface and internally with transmission electron microscopy. It was shown that plastic deformation took place only in distinct slip bands and that these slip bands corresponded to vacancy loop free channels observed in transmission electron microscopy. The empirical equations of microscopic parameters describing the behavior of the slip bands were determined. Then a description of the macroscopic plastic deformation was developed from these parameters instead of discussing the macroscopic plastic deformation in terms of a particular dislocation mechanism. Further, these parameters were qualitatively interpreted in terms of dislocation kinetics.

An elasticity calculation of the interaction between a misfit defect and an extended dislocation in f.c.c. metals

Abstract A rigid dislocation model was used to calculate the effect of the extension of a dislocation on the yield stress in a f.c.c. metal. In this calculation a generalization was made to include all dislocation characters from pure edge to pure screw orientations. Three types of point defects were considered as obstacles to dislocation motion: two produce a 〈100〉 distortion and the third a spherical distortion of the lattice. The resistance to dislocation motion due to these obstacles was generally reduced as the width of the extended dislocation increased. The magnitude of the resistance for a widely extended dislocation was about 60% of that for a perfect (unextended) dislocation, which was expected considering the magnitude of Burgers vectors of the two cases. The effect of the extension on the resistance to dislocation motion saturated at a relatively small dislocation width (a few atomic distances). This means that, even in a metal like aluminum which possesses a high stacking fault energy, a dislocation cannot be treated as a perfect dislocation when the obstacles are atomistic point defects. For such a slightly extended dislocation, the maximum resistance due to a 〈100〉 defect was found near the screw orientation rather than the edge orientation. The resistance to dislocation motion due to a defect with spherical symmetry was as large as that due to a defect with 〈100〉 tetragonality.

Analysis of diffusion controlled release of hydrogen from nickel and inconel 600

Abstract An analysis is given for the release of hydrogen from a thin sheet on heating at a constant rate. Control by lattice diffusion, by release from gas bubbles (H2 or CH4), and by escape from deep traps are treated, as well as the information obtainable from isothermal holds. Analysis of published experiments on pure Ni, allow the determination of the lattice diffusion coefficient, show that methane is formed on annealing samples with pores in 1 atm. of hydrogen, and show that pore surfaces provide a deep trap. Samples of Ni and INCONEL 600 slowly strained to fracture in 350°C water accumulate large amounts of hydrogen and exhibit IGSCC. Highly strained regions contain ten times the hydrogen trapped in the unstrained regions. Most of this is shown to be chemisorbed on internal surfaces (microcracks or pores) where it can aid in the cracking.

Surface observation of nickel-plated iron single crystals deformed at 77 K

Abstract The surface morphology of nickel-plated iron single crystals deformed at 77 K has been investigated to identify the mechanism of surface film softening. Two types of striation were found to form on the specimen surfaces, i.e. cracks approximately perpendicular to the tensile axis on the edge-faces and slip-like lines on the screw-faces. The direction of these lines deviated from the traces of the primary slip planes. Specimens which were electroplated on only one of the edge-faces exhibited a clear correspondence between the cracks on the plated face and the slip steps on the unplated face. Both the cracks and the slip steps were irregularly jogged. The lines connecting the corresponding points on the two faces were parallel to the primary slip direction. The observations indicate that dislocation loops are generated near the cracks below the stress level for screw dislocation motion. As the edge component of these loops can move substantially at the low stress level, the generation of a large number of dislocation loops results in macroscopic deformation. The observed angle of deviation between the direction of the slip-like lines and the trace of the primary slip plane on the screw-face is consistent with this model.

Point defect hardening of aluminum—I

Abstract Aluminum single crystals of 99.999% initial purity were irradiated at 23°K or below with 1.0 MeV electrons to a total dosage of 0.5 ~ 10 × 1016e/cm2. Stress-strain curves of irradiated crystals were drastically different from those of annealed crystals. Yield stress was increased by a factor of about eight by irradiation to a total dosage of 1017 e/cm2. Extention of easy glide was greatly prolonged and slip lines of crystals were coarsened by the irradiation. No appreciable change in the work hardening rate in the easy glide region was found. A sharp yield point was not observed and the transition from elastic to plastic behavior was gradual in all cases following the irradiation. The increase in the yield stress, δτ, was found to exhibit a square-root dependence on the defect concentration, c; namely, δ τ ≅ G c 1 2 , where G was the shear modulus. Under the present experimental conditions, only dispersed interstitials and vacancies were introduced by the irradiation. Several mechanisms of hardening were examined on the basis of the observed effects. Since dispersed vacancies were known to produce little hardening, the present observations were concluded to be due to the dispersed interstitials. At present, no theory is capable of explaining the observations quantitatively. The theories based on the dispersed barrier model appeared to be the most favorable, as they predicted the square-root dependence of δτ on the defect range concentration in the most convincing manner.

Effect of electron irradiation on mechanical properties of aluminum single crystals at 80°K

Abstract Aluminum single crystals of 99.999% initial purity were irradiated at 80°K with 1 MeV electrons to total dose of 1.2 ~ 8 × 1017 el/cm2. The flow stress was measured at 77°K over the strain rate range 5 × 10−4 to 4 × 10−3 and increased with irradiation. A larger increase for the higher strain rate was observed. The increase is interpreted to be due to interstitials which have migrated to dislocations and have been absorbed heterogeneously by making superjogs or kinks, which produce resistance to motion of the dislocations. An interruption of tensile test for a short time resulted in a yield drop which was found only in the irradiated crystals. This yield drop may be due to the Cottrell locking of the interstitials which were freed from impurity trapping sites by glide dislocations. Annealing at room temperature resulted in recovery of the hardening caused by the irradiation. Migration of vacancies or divacancies to the dislocation having interstitial jogs may produce the recovery.

Effects of residual oxygen on mechanical properties of iron

Oxygen was carefully removed from iron by the combination of the floating zone refining in hydrogen plasma and the ZrH2 purification treatment, and an iron containing a very small amount of interstitial impurities (2.6 C, 2.3 N and 3.5 O in at.ppm) was produced. Mechanical properties of the iron were substantially different from those of high-purity irons reported in the literature, indicating the prominence of oxygen in determining mechanical behavior of α-iron. The yield stress of the present iron in the athermal region was only half of those previously reported for high-purity irons. Dynamic strain aging due to interstitial impurities completely disappeared in the present iron. Tensile tests at 77 K indicated that oxygen strongly facilitates intergranular fracture.