Richard E. Ricker

The role of hydrogen in corrosion fatigue of high purity Al-Zn-Mg exposed to water vapor

Corrosion fatigue tests were performed on samples of a high purity Al-Zn-Mg alloy in humid nitrogen gas after pre-exposure to either vacuum or humid air. The results of these tests were compared to the results of fatigue tests performed in dry nitrogen, used as an inert reference environment, after the same pre-exposure treatments. The pre-exposure times were calculated by assuming that bulk diffusion of hydrogen was the rate limiting process in either hydrogen adsorption or desorption. Water vapor in the testing environment resulted in reduced fatigue lives; however, pre-exposure to humid air was just as detrimental as water vapor in the test environment. The pre-exposure embrittlement effect of humid air was found to be completely reversible when the samples were stored in a vacuum long enough to remove hydrogen, assuming a bulk diffusion coefficient of 1 x 10-13 m2/sec. These results confirm the hypothesis that the reduced fatigue lives of Al-Zn-Mg alloys in water vapor is due to hydrogen embrittlement.

SUPPRESSION OF FATIGUE CRACKING WITH NANOMETER-SCALE MULTILAYERED COATINGS

Department of Materials Scienceand Engineering Department, Johns Hopkins University, Baltimore, MD 21218(Received March 1, 2000)(Accepted in revised form May 4, 2000)Keywords: Fatigue; Multilayers; Plating; Surfaces; Crack initiationIntroductionFatigue crack initiation in initially smooth samples of homogeneous ductile metals almost alwaysoccurs at the free surface as a result of surface roughening and the development of a critical surfacemorphology (1–3). For face centered cubic (FCC) metals with high stacking fault energies such as Cu,this critical morphology is the result of irreversible dislocation processes occurring during cyclic slipand consists of notches and peaks (intrusions and extrusions) formed at persistent slip bands (PSBs)(2,4). Recognizing that it should be possible to influence fatigue crack initiation by modifying theproperties or microstructure of the surface, investigators have examined the influence of variouscoatings and surface treatments such as shot peening, anodizing, oxidizing, nitriding, boriding, plasmaspraying, ion beam mixing, and ion implanting (5–10). While hard coatings prevent surface rougheningand crack initiation by the normal roughening mechanism, they usually result in modest improvementsbecause cyclic loading manages to initiate fatigue cracks by another mechanism. For example,Hornbogen and Verpoort (9) examined the influence of boriding, nitriding, and plasma spraying onfatigue crack initiation in austenitic stainless steels and found that while these coatings suppressedsurface roughening, fracture of the films resulted in slip localization and rapid initiation of fatiguecracks which actually degraded performance in some cases (9). Alden and Backofen (5) examined thickanodic films on Al and found that they could extend the fatigue life by periodically treating the surfaceto remove cracks in the films. This review of the literature (1–13) indicates that a surface film shouldhave five properties to maximize fatigue crack initiation resistance: (i) hardness - to prevent surfaceroughening, (ii) ductility (toughness) - to prevent cracking where PSBs intersect the film, (iii) cyclicwork hardenability - to prevent slip localization, (iv) residual compressive stresses - to reduce themagnitude of tensile stresses in the film, and (v) adherence - to stay on the substrate.The mechanical properties of nanometer-scale multilayered thin film coatings can be modified andadjusted by changing the composition and thickness of the layers (14–16). As a result, multilayeredcoatings can be developed that are optimized with respect to the properties identified above as beingdesirable in a coating for fatigue crack initiation resistance. In particular, a multilayer composed of thinlayers of two normally ductile FCC metals may have sufficient hardness to block surface slip yet retainsufficient ductility, toughness, and cyclic work hardening capacity, that cracking of the film or other sliplocalization mechanisms will not prevent this film from suppressing fatigue crack initiation. The

The influence of a multilayered metallic coating on fatigue crack nucleation

Abstract A thorough review of the literature on fatigue crack initiation indicates that for optimum resistance to fatigue crack initiation, a surface coating needs more than just a high hardness and that a combination of properties including toughness, cyclic work hardenability, residual compressive stresses, and adherence, in addition to a hardness higher than that of the substrate are required. Based on this assumption, it was hypothesized that nanometer-scale, multilayer coatings will posses a combination of these required properties enabling significant increases in fatigue crack initiation resistance. To test this hypothesis, fatigue experiments were conducted on Cu samples with different surface treatments including a nanoscale Cu–Ni multilayer. The fatigue lives of the multilayer coated samples were significantly greater than those of uncoated samples or samples coated with a monolithic coating of Cu or Ni indicating that the nanodimensional layering of the multilayer coating is responsible for retarding fatigue crack initiation and failure. The samples were examined with various analytical techniques including scanning and transmission electron microscopy and atomic force microscopy.

Evaluation of the Propensity of Niobium to Absorb Hydrogen During Fabrication of Superconducting Radio Frequency Cavities for Particle Accelerators.

During the fabrication of niobium superconducting radio frequency (SRF) particle accelerator cavities procedures are used that chemically or mechanically remove the passivating surface film of niobium pentoxide (Nb2O5). Removal of this film will expose the underlying niobium metal and allow it to react with the processing environment. If these reactions produce hydrogen at sufficient concentrations and rates, then hydrogen will be absorbed and diffuse into the metal. High hydrogen activities could result in supersaturation and the nucleation of hydride phases. If the metal repassivates at the conclusion of the processing step and the passive film blocks hydrogen egress, then the absorbed hydrogen or hydrides could be retained and alter the performance of the metal during subsequent processing steps or in-service. This report examines the feasibility of this hypothesis by first identifying the postulated events, conditions, and reactions and then determining if each is consistent with accepted scientific principles, literature, and data. Established precedent for similar events in other systems was found in the scientific literature and thermodynamic analysis found that the postulated reactions were not only energetically favorable, but produced large driving forces. The hydrogen activity or fugacity required for the reactions to be at equilibrium was determined to indicate the propensity for hydrogen evolution, absorption, and hydride nucleation. The influence of processing conditions and kinetics on the proximity of hydrogen surface coverage to these theoretical values is discussed. This examination found that the hypothesis of hydrogen absorption during SRF processing is consistent with published scientific literature and thermodynamic principles.

Origins of the aqueous corrosion and stress corrosion cracking behavior of ductile nickel aluminide

Abstract The stress corrosion cracking resistance of ductile nickel aluminide, Ni 3 Al + B , was evaluated by conducting experiments in solutions with varying pH and ionic concentration. The results demonstrate that the ductility of this material is greatly reduced and the fracture mode changes from ductile transgranular to brittle intergranular cracking when environmental conditions are favorable for hydrogen absorption during the steady state regardless of the solution composition and pH. The results also indicate that, in the absence of cathodic polarization, this material exhibits ductile behavior during free corrosion in solutions of neutral and alkaline pH. Thermodynamic calculations of the activity of aluminum in nickel aluminide indicate that there is sufficient thermodynamic driving force for hydrogen evolution in these environments. Although the presence of surface films and transport through these films prevent this from occurring during steady state free corrosion, the thermodynamic calculations indicate that hydrogen evolution should occur during the transients that follow film rupture in these environments. To evaluate if hydrogen evolution could occur during film rupture and repassivation, nickel aluminide samples were scratched and the resulting potential transient was monitored. The results indicate that the potential drop during the scratch repassivation event will not cause significant hydrogen evolution and absorption. It is postulated that this discrepancy between the thermodynamic calculations and kinetic behavior is due to the ordering of this A 3 B compound into the Ll 2 structure. To test this hypothesis, samples of Ni 3 Fe, which can be easily ordered and disordered, were tested in the ordered and disordered conditions. The results indicate that ordering significantly alters the repassivation transient that follows scratching in A 3 B-Ll 2 compounds where the more active constituent is the B species.

Influence of Postbuild Microstructure on the Electrochemical Behavior of Additively Manufactured 17-4 PH Stainless Steel

The additive manufacturing build process produces a segregated microstructure with significant variations in composition and phases that are uncommon in traditional wrought materials. As such, the relationship between the postbuild microstructure and the corrosion resistance is not well understood. Stainless steel alloy 17-4 precipitation hardened (SS17-4PH) is an industrially relevant alloy for applications requiring high strength and good corrosion resistance. A series of potentiodynamic scans conducted in a deaerated 0.5-mol/L NaCl solution evaluated the influence of these microstructural differences on the pitting behavior of SS17-4. The pitting potentials were found to be higher in the samples of additively processed material than in the samples of the alloy in wrought form. This indicates that the additively processed material is more resistant to localized corrosion and pitting in this environment than is the wrought alloy. The results also suggest that after homogenization, the additively produced SS17-4 could be more resistant to pitting than the wrought SS17-4 is in an actual service environment.

Investigations of Residual Stresses and Mechanical Properties of Single Crystal Niobium for SRF Cavities

This work investigates properties of large grained, high purity niobium with respect to the forming of superconducting radio frequency (SRF) cavities from such large grained sheets. The yield stresses were examined using tensile specimens that were essentially single crystals in orientations evenly distributed in the standard projection triangle. No distinct yield anisotropy was found, however, vacuum annealing increased the yield strength by a factor 2…3. The deep drawing forming operation of the half cells raises the issues of elastic shape changes after the release of the forming tool (springback) and residual stresses, both of which are indicated to be negligible. This is a consequence of the low yield stress (< 100 MPa) and the large thickness (compared to typical thicknesses in sheet metal forming). However, the significant anisotropy of the transversal plastic strains after uniaxial deformation points to potentially critical thickness variations for large grained / single crystal half cells, thus r...

Measurement and Calculation of Elastic Properties in Low Carbon Steel Sheet

Low carbon steel (usually in sheet form) has found a wide range of applications in industry due to its high formability. The inner and outer panels of a car body are good examples of such an implementation. While low carbon steel has been used in this application for many decades, a reliable predictive capability of the forming process and “springback” has still not been achieved. NIST has been involved in addressing this and other formability problems for several years. In this paper, texture produced by the in-plane straining and its relationship to springback is reported. Low carbon steel sheet was examined in the as-received condition and after balanced biaxial straining to 25%. This was performed using the Marciniak in-plane stretching test. Both experimental measurements and numerical calculations have been utilized to evaluate anisotropy and evolution of the elastic properties during forming. We employ several techniques for elastic property measurements (dynamic mechanical analysis, static four point bending, mechanical resonance frequency measurements), and several calculation schemes (orientation distribution function averaging, finite element analysis) which are based on texture measurements (neutron diffraction, electron back scattering diffraction). The following objectives are pursued: a) To test a range of different experimental techniques for elastic property measurements in sheet metals; b) To validate numerical calculation methods of the elastic properties by experiments; c) To evaluate elastic property changes (and texture development) during biaxial straining. On the basis of the investigation, recommendations are made for the evaluation of elastic properties in textured sheet metal.

On the stress corrosion cracking of Al-Li alloys : the role of grain boundary precipitates

The stress corrosion cracking (SCC) behavior of precipitation-hardened alloys may depend on a large number of microstructural parameters that vary during fabrication and heat treatment, such as grain size, grain boundary (GB) solute segregation, matrix precipitate size, GB precipitate size, precipitate-free zone size, and matrix slip character.[1] Since all of these factors vary simultaneously during normal heat treatments, it is difficult to assess independently the contribution of each microstructural factor to the SCC behavior of an alloy. In particular, a series of experiments was designed which would allow the evaluation of the role of GB precipitates in the SCC behavior of Al−Li and Al−Li−Cu alloys independent of the other factors (such as matrix precipitate microstructure) that normally vary during aging treatments. For these experiments, the matrix precipitates of a binary Al−Li alloy were held constant, keeping the yield strength constant, while the GB precipitate size and volume percent were systematically varied. In contrast, to keep the yield strength of the ternary Al−Li−Cu alloy constant at the same level as the binary alloy, the matrix precipitate size and distribution were varied with the GB precipitate size.

Can corrosion testing make the transition from comparison to prediction

Corrosion testing has evolved to the point where it is possible to compare the relative corrosion resistance of two or more materials in a small number of experiments of relatively short duration. However, this comparison is only valid for the environments used for the experiments, and these experiments are frequently considered poor indicators of ctual in-service behavior. This paper examines the emerging need for better prediction of performance in corrosive environments, the form that a prediction should take, and some of the tools being developed and explored for meeting this need.