I. M. Bernstein

The Role of Metallurgical Variables in Hydrogen-Assisted Environmental Fracture

This chapter focuses on two kinds of environmental fracture, stress corrosion cracking (SCC) and hydrogen embrittlement, and discusses the role in such failures of a number of metallurgical variables. These include chemical composition; microstructural components such as precipitate type and structure, and grain size and shape; crystallographic texture; heat treatment and its effect on the foregoing variables; and processing, particularly the thermomechanical treatments (TMT), which are attracting increased attention for property optimization. These variables are expected to be of great importance in the development of new engineering materials to meet demanding service conditions.

An example of the effect of hydrogen trapping on hydrogen embrittlement

The role of internal hydrogen in reducing the tensile reduction of area of iron-titanium alloys is examined. The population of hydrogen at potential crack nucleii is shown to be controlled by its dynamic interaction with mobile dislocations and its subsequent transport to fixed traps. Expressions are developed for both the number of hydrogen atoms at a given irreversible trap, as well as the time necessary to reach such a number. Reducing the number below the critical value to nucleate a crack, or increasing the time to reach this value will improve an alloy’s performance, and this improvement is related to controllable external and metallurgical variables. These predictions of the model are shown to be consistent with companion experimental data, and with the trap theory of hydrogen embrittlement.

Evidence for Dislocation Transport of Hydrogen in Aluminum

The use of concurrent plastic straining during cathodic charging of equiaxed-grain, high purity 7075 aluminum has provided evidence that dislocations can transport large amounts of hydrogen deep into the interior of the alloy; as a direct consequence of this, highly brittle intergranular fracture ensues. This effect is most pronounced for heat treatments that produce a microstructure which allows for planar dislocation arrays and long slip lengths. The implications of these findings to the occurrence of hydrogen embrittlement in other alloy systems have been assessed.

Hydrogen assisted ductile fracture of spheroidized carbon steels

The effects of hydrogen on ductile fracture were studied in two spheroidized plain carbon steels, containing 0.16 and 0.79 pct C. A combination of fractography and quantitative metallography on sectioned, deformed specimens permitted separation of the effects of hydrogen on the initiation, growth, and link-up of voids. In both steels, hydrogen was found to have no significant effect on either the initiation of voids at carbides, or early growth of voids, prior to link-up. In the higher carbon steel the fracture surface dimple size increased after hydrogen exposure with no other evident change in the fracture surface appearance; it is therefore inferred that hydrogen primarily assists void growth during link-up in this steel. In the lower carbon steel the fracture appearance changed and a decrease in void size due to hydrogen was found fractographically; thus, both initiation and growth of voids are apparently enhanced during the link-up phase of fracture in this steel. It is hypothesized that these effects may be due largely to a void pressure mechanism if hydrogen is transported by mobile dislocations.

The role of microstructure in hydrogen-assisted fracture of 7075 aluminum

Underaged, peak strength (T6), and overaged (T73) microstructures were studied in 7075 plate material. Hydrogen charged and uncharged tensile specimens of longitudinal orientation were tested between −196°C and room temperature. The results confirm a hydrogen embrittlement effect, manifested mainly in the temperature dependence of the reduction of area loss; a classical behavior of hydrogen embrittlement. The maximum embrittlement shifted to lower temperatures with further aging. The effect of hydrogen was largest for the underaged condition and smallest for the overaged, thus following the pattern found for the sensitivity to stress-corrosion cracking in high strength aluminum alloys. The fracture path was predominantly transgranular, with minor amounts of intergranular fracture.

The effect of defects on the fatigue crack initiation process in two p/m superalloys: part i. fatigue origins

Two high strength P/M nickel-base superalloys, AF-115 and AF2-1DA, with different defect populations, were tested to determine the effect of preexisting defects on the fatigue crack initiation process. Strain controlled continuous cycle fatigue tests were performed at room and at elevated temperature; these were followed by fractographic examination to characterize both the location and character of the fatigue origins. In most cases, particularly at elevated temperature, the initiation process was associated with a large pre-existing defect, either a pore or a nonmetallic inclusion. There was also a change in the location of the crack that caused failure as the strain range varied: at high strain ranges initiation occurred at or near the specimen’s surface, while at the lower strain ranges the failure originated in the specimen’s interior. The initiation mode for both alloys at room temperature was different than at elevated temperature. At room temperature, Stage I crystallographic cracking at or near the surface dominated the process in all strain range regimes. This difference was attributed, in part, to the differences in deformation mode for nickel-base superalloys at room and elevated temperature.

The effect of defects on the fatigue crack initiation process in two p/m superalloys: part ii. surface-subsurface transition

The change in fatigue failure initiation sites from a surface to subsurface location for two P/M nickel-based superalloys is analyzed. In particular the influences of defect size, shape, and population on the elevated temperature fatigue processes are assessed. The analysis shows that at high strain ranges, crack initiation occurs rapidly, and crack propagation rates determine the fatigue life and failure site. As a result, defect location (related to population) and size are the more important parameters. At lower strain ranges, however, crack initiation is critical in determining the failure origin, and this is primarily controlled by defect shape.

The effect of copper content and microstructure on the hydrogen embrittlement of AI-6Zn-2Mg alloys

Two commercially-processed Al-6Zn-2Mg alloys, 7050 and a “low copper” 7050, were tested for susceptibility to embrittlement by precharged hydrogen and by simultaneous cathodic charging and straining (SET procedure). Specimens were heat treated to underaged, peak-strength aged, and overaged conditions. In 7050, the peak strength and overaged conditions were not embrittled by hydrogen, though underaged material showed marked embrittlement. All microstructures tested for the low-copper alloy were embrittled. The results agree with the microstructural rationale established through earlier work on 7075 and 2124 aluminum alloys, particularly with respect to the susceptibility of underaged material to hydrogen. As in earlier work, the extent of dislocation transport of hydrogen, and local hydrogen accumulation at grain boundaries, evidently controlled the extent and degree of brittle fracture. These three important alloys can now be ranked in the order 7050, 2124, 7075 of increasing relative susceptibility to theonset of stress corrosion cracking.

Hydrogen Embrittlement in a 2000-Series Aluminum Alloy

Tensile tests were performed on an Al-Cu-Mg alloy, 2124, in plate form, after aging to seven different tempers. Cathodic charging with hydrogen caused no significant loss of ductility for any temper, in contrast, for example, to 7075. Simultaneous straining and cathodic charging did, however, result in reduction in area losses up to 25 pct. The fracture mode was not altered by hydrogen. In general, the results conformed to the framework established through investigation of 7075 alloys, in that the underaged microstructures were the most susceptible to hydrogen embrittlement; this susceptibility can be explained in terms of microstructure and slip behavior.

Strain-rate effects on hydrogen embrittlement of 7075 aluminum

Abstract Three microstructures of 7075 aluminum alloy were studied: T6, T73, and an underaged temper (UT) equal in strength to T6. Specimens with these structures were cathodically charged with hydrogen and tensile tested at room temperature over a range of four orders of magnitude in strain rate. All three microstructures exhibited a maximum in embrittlement at an intermediate strain rate; this strain increased with degree of aging, from about 0.0005/s for UT, to about 0.01/s for T73. The decreased embrittlement at the lowest strain rate was shown not to be a result of hydrogen loss from specimens.