H. H. Johnson

Sub-critical flaw growth☆

Abstract The major evidence bearing upon sub-critical flaw growth in structural materials is reviewed and discussed. Attention is focused upon the growth of pre-existing flaws at operating stresses less than the net section yield strength, from both the separate and combined effects of fatigue and aggressive environments. In Section 1, the applicability of fracture mechanics concepts to flaw growth is considered, and it is demonstrated that the stress intensity factor may be viewed as the driving force for both fatigue crack growth and environmental cracking under static load. This allows a correlation of test results from different specimen geometries, and also a correlation between test results and service failures. Environmental cracking under static load is considered in Section 2. Steels and titanium alloys in various environments of water, water vapor, hydrogen, and oxygen are discussed. For steels, crack growth in water and saturated water vapor is a thermally activated process. The crack growth activation energy agrees well with the measured value for diffusion of hydrogen. The role of oxygen in inhibiting sub-critical flaw growth in vapor environments is discussed. Fatigue crack growth is considered in Section 3. It is shown that the stress intensity range is the major factor governing crack growth rates and fatigue life, with frequency and mean load as secondary variables. Crack growth rate laws are considered, and it is demonstrated that the fourth power law holds over the widest range. For an astonishing variety of steels, the fatigue crack growth rate is insensitive to composition, microstructure, and strength level, when the growth rate is plotted as a function of the stress intensity range. The combined effects of fatigue and aggressive environments are considered in Section 4. Some of the environmental behavior patterns in static loading are observed to carry over to fatigue loading. A lack of data for stress intensity ranges less than the threshold is noted. In Section 5, engineering applications are emphasized. Relationships among flaw size, threshold stress intensities in different environments, and proof and operating stresses are presented graphically and their interpretation is discussed. For 4340 steel, the role of yield strength level is summarized in a diagram which indicates that environmental cracking is the major problem at yield strengths in excess of 180 ksi, while fatigue crack growth is the major problem at lower strength levels.

Internal hydrogen supersaturation produced by dislocation transport

A quantitative model is presented for the generation of internal supersaturations of hydrogen by kinetic rather than thermodynamic means. The maximum brittleness and slow strain rate limit is treated. The process invoked assumes hydrogen transport to deposition sites by dislocation drag, and reemission of the hydrogen by diffusion. The kinetic supersaturation is calculated from a balance between hydrogen arrival and departure rates, for internal and external hydrogen sources, and for smooth section and crack geometries. When applied to ferritic and austenitic steels, the model predicts that the kinetic supersaturations are extremely small in all realistic situations.

Steady state hydrogen transport through zone refined irons

Hydrogen permeation from the gas phase through zone refined iron was measured with the electrochemical technique over 0 to 60°C and 0.01 to 1 atm. The surface impedance problems normally encountered in this pressure-temperature range were eliminated by a thin layer of electroplated palladium on the input surface of the permeation membrane. The steady state flux was propprtional to the square root of the pressure and inversely proportional to the membrane thickness. The permeability coefficient was 2.6 × 1017 exp ((-8500 ± 600 cal/mole deg)/RT) atom H/cm s atm1/2, in good agreement with earlier results at higher temperatures. These results can be used to calibrate electrochemical charging experiments in terms of an effective hydrogen pressure.

Permeation and diffusion of hydrogen and deuterium in 310 stainless steel, 472 K to 779 K

The permeabilities and true lattice diffusivities of hydrogen and deuterium in 310 austenitic stainless steel were measured over 472 to 779 K with an ultrahigh vacuum, monopole gas analyzer system. The permeation activation energies for hydrogen and deuterium were virtually equal at 56,560 ± 2800 and 55,840 ± 3300 J/mol, respectively. Likewise, the diffusion activation energies for hydrogen and deuterium were essentially equal at 48,820 ± 920 and 48,070 ± 710 J/mol, respectively and were indicative of true lattice diffusion. Evaluation of the permeability coefficients and the true lattice diffusivities gave isotope ratios, 1.44 ± 0.14 and 1.38 ± 0.14, respectively, which indicates the mass effect to be confined to diffusivity and in agreement with the isotope ratio of 1.41 as predicted by classical theory.

Do thermal spikes contribute to the ion-induced mixing of Ni into Zr, Ti, and Pd

Low‐temperature ion beam mixing rates for Ni‐Ti, Zr‐Ni, and Pd‐Ni bilayers significantly exceeded binary collision estimates, and appeared quite sensitive to thermodynamic driving forces. In the absence of a temperature dependence such a behavior is commonly ascribed to interdiffusion within thermal spikes. However, the Ni‐Ti mixing rate was seen to vary linearly with nuclear damage energy for irradiation with 600 keV Xe, Kr, or Ar, 300 keV Ne or N, or 200 keV N ions, or 1 MeV Au ions (literature value). This excludes overlapping thermal spikes. An expression was derived for mixing due to nonoverlapping thermal spikes, but this could also not explain our results.

Hydrogen trapping mechanisms by permeation techniques

Trapping phenomena may be well characterized by transport measurements carried out as a function of lattice concentration of the species undergoing diffusion and trapping. Although the reasoning in general, applications are probably most feasible for gas-solid systems, and in particular for hydrogen in metals.

Hydrogen permeation in the metallic glass Fe40Ni40P14B6

Hydrogen permeation from the gas phase through the metallic glass Fe40Ni40P14B6 (Metglas 2826) was measured with the electrochemical technique over 313 to 353K and 105 to 103 Pa. Specimen surfaces were coated with a thin layer of electrodeposited palladium to eleminate surface impedances to hydrogen entry and exit. Sieverts law behavior was observed over the temperature and pressure ranges studied. An excellent fit to the permeation transients as obtained from Ficks diffusion theory. Diffusivities derived from least squares fitting of permeation trasients were in good agreement with those obtained from time lag measurements. Within the concentration range studied, diffusivity was found to be independent of input hydrogen concentration. The expressions for the permeability coefficient, 2.24 × 1016 exp{[(48.99 ± 2.93) kJ/mol]/RT} atom H/ms Pa12, the diffusivity 8.52 × 10−7 exp{[(−47.07 ± 1.63) kJ/mol]/RT} m2/s, and the solubility, 2.62 × 1022exp{[(−1.92 ± 4.56) kJ/mol]/RT} atom H/m3Pa12, were determined. An annealing treatment at 573K for one hour decreases bot the permeability and diffusivity of hydrogen in the metallic glass.

Ion beam mixing of aluminum and titanium

The ion beam mixing of Al and Ti by 600 keV Xe ions was studied at room temperature and near 80 K. In view of recent observations of large differences between bilayer and multilayer mixing rates in the Fe‐Ti, Ni‐Ti, and Cu‐Ti systems, both bilayer and buried‐layer samples were investigated. Results obtained with the various sample configurations were in good mutual agreement. Comparison to literature data on marker experiments in Al suggested no significant dependence on layer thickness above ∼5 A. At room temperature, the mixing rate is in excellent agreement with previous multilayer mixing experiments. The initial mixing rate was found to vary by less than 20% between 80 and 300 K. The results are discussed in terms of theoretical models for the mixing mechanisms together with published data on comparable systems. In spite of the low Z values involved, the mixing rate is in good agreement with an expression based on a thermal spike mechanism. At large fluences, a 1260‐A‐thick surface layer of Al would suddenly start to degrade quite rapidly.

The quasicrystalline transformation in the AlCr system

Amorphous Al/sub 80/Cr/sub 20/ films were made by coevaporation and by room temperature ion irradiation of the coevaporated films. The amorphous phase was transformed into the quasicrystalline state through two routes: thermal and ion beam assisted anneal. The intensity of the quasicrystalline electron diffraction pattern increases continuously within the annealing temperature range from 547 to 607 /sup 0/C. The starting state of the films (as-deposited or ion-irradiated codeposited) had no effect on the thermal transformation to the quasicrystalline state. Ion irradiation of the amorphous phase at 200 /sup 0/C produces a more complete set of icosahedral diffraction lines. Icosahedral AlCr has the same reciprocal lattice spacings as icosahedral AlMn.

Phase transformation of Ni2Al3 to NiAl. I. Ion irradiation induced

An ion‐irradiation‐induced phase transformation from Ni2Al3 to NiAl has been investigated by transmission electron diffraction. Compound samples of Ni2Al3 were irradiated at liquid nitrogen, room temperature, and 100 °C with 300‐keV Xe and 60‐keV Ne ions. Transformation to NiAl was observed in all cases. Detailed examination of the closely related equilibrium NiAl and Ni2Al3 structures permits explanation of this transformation based on the structural similarity of the two phases and using a model built around radiation disordering of vacancies.