Ming Gao

Microconstituent-Induced Pitting Corrosion in Aluminum Alloy 2024-T3

Abstract Free corrosion immersion experiments were conducted on a commercial airframe material, Al 2024-T3 (UNS A92024), in 0.5 M sodium chloride (NaCl) solution to investigate the role of microcon...

Transition from pitting to fatigue crack growth—modeling of corrosion fatigue crack nucleation in a 2024-T3 aluminum alloy

Abstract The nucleation of fatigue cracks from corrosion pits was investigated by conducting fatigue experiments on open-hole specimens of a 2024-T3 aluminum (bare) alloy in 0.5 M NaCl solution at room temperature and different load frequencies from 0.1 to 20 Hz. The maximum cyclic stresses applied at the hole ranged from 144 to 288 MPa and the load ratio, R , was 0.1. A specimen subjected to pre-corrosion in the NaCl solution prior to corrosion fatigue was also investigated. Pitting was found to be associated with constituent particles in the hole and pit growth often involved coalescence of individual particle-nucleated pits. Fatigue cracks typically nucleated from one or two of the larger pits, and the size of the pit at which the fatigue crack nucleates is a function of stress level and load frequency. The observations indicate that the nucleation of corrosion fatigue cracks essentially results from a competition between the processes of pitting and crack growth. Pitting predominates in the early stage of the corrosion fatigue process, and is replaced by corrosion fatigue crack growth. Based on these results, two criteria are proposed to describe the transition from pit growth to fatigue crack growth: (1) the stress intensity factor of the equivalent surface crack has to reach the threshold stress intensity factor, Δ K th , for fatigue crack growth, assuming that a corrosion pit may be modeled by an equivalent semi-elliptical surface crack, and (2) the time-based corrosion fatigue crack growth rate also exceeds the pit growth rate.

In-Situ Monitoring of Pitting Corrosion in Aluminum Alloy 2024

Abstract An in-situ monitoring method was used to observe the initiation sites and the processes of pitting corrosion in aluminum alloy 2024-T3 (UNS A92024) in real time. These observations complemented more detailed information about composition and distribution of constituent particles, pre- and post-corrosion surface morphology, and the internal morphology of corrosion pits. In-situ observations provided a comprehensive view of the development of localized corrosion in real time. Results confirmed the importance of intermetallic constituent particles in promoting initiation and growth of pits in aluminum alloys. Heterogeneous distribution of these particles served to define the location and extent (or severity) of pitting. A conceptual model was proposed as a framework for continued research. The formation of occluded cells under corrosion product domes over severe pits was observed. This formation will be incorporated into an overall reconsideration and modeling of the processes of pitting corrosion o...

Chemical and metallurgical aspects of environmentally assisted fatigue crack growth in 7075-T651 aluminum alloy

A comprehensive study has been carried out on a 7075-T651 alloy to examine the influence of water vapor on fatigue crack growth. The kinetics of fatigue crack growth were determined as a function of water vapor pressure at room temperature and at 353 K. Detailed fractographic analyses and surface chemistry studies were carried out to identify the micromechanisms and to quantify the chemical interactions for corrosion fatigue crack growth in this alloy. Experiments were also carried out in ultra-high vacuum and in oxygen to provide for comparisons. Two regions of fatigue crack growth response were identified. In the low pressure region (below 67 Pa at 5 Hz), crack growth is controlled by the rate of transport of water vapor to the crack tip, and the response can be described by a model for transport controlled crack growth. At pressures above 67 Pa, additional increases in crack growth rate occurred, which are attributed to the further reactions of water vapor with segregated magnesium in this alloy. Different micromechanisms for crack growth have been identified for vacuum, oxygen, and water vapor. These micromechanisms are considered in relation to the environmental parameters through a modified superposition model for corrosion fatigue.

Reconsideration of the superposition model for environmentally assisted fatigue crack growth

Abstract A modification of the superposition model for environmentally assisted fatigue crack growth (corrosion fatigue) is proposed. The modification gives explicit recognition to the fact that mechanical (or “pure”) fatigue and corrosion fatigue can proceed by different micromechanisms and in parallel. The modified model provides a rational basis for considering the role of microstructure in corrosion fatigue. Preliminary fractographic evidence provides support for this modification.

Pitting Corrosion and Fatigue Crack Nucleation

To assess two proposed criteria for the transition from pitting to cracking, corrosion fatigue crack nucleation experiments were conducted on open-hole specimens of 2024-T3 (bare) alloy in a 0.5M NaCI solution at room temperature at different loading frequencies, in conjunction with coordinated studies of pitting corrosion and corrosion fatigue crack growth (CFCG). The proposed criteria are: (1) the stress intensity factor ΔK for an equivalent crack (by modeling the pit as a semi-elliptical surface crack) equaled or exceeded the threshold stress intensity factor (ΔK t h ) for CFCG, and (2) the time-based CFCG rate exceeded the pit growth rate. The nucleation of fatigue cracks was found to be associated with large corrosion pits. The size of the crack-nucleating pits was larger at the lower frequencies. This frequency dependence reflects the competition between pitting and CFCG. Validation of the proposed criteria in terms of the experimental data is presented and discussed.

GRAIN BOUNDARY NIOBIUM CARBIDES IN INCONEL 718

A coordinated, multidisciplinary investigation was conducted to determine the mechanisms and rate controlling processes for environmentally assisted crack growth under sustained (static) loading in Inconel 718 at elevated temperatures. The results showed that oxygen had a significant influence on crack growth, increasing the crack growth rate, for example, by nearly four orders of magnitude at 973 K. Based on results from the companion surface chemical and metallurgical studies, it was suggested that the mechanism for crack growth enhancement by oxygen was the formation and fracture of a brittle niobium oxide (Nb{sub 2}O{sub 5}-type) film on the grain boundary surfaces. The Nb came principally from the oxidation and decomposition of NbC (or Nb-rich carbides) at the grain boundaries, and crack growth was controlled by the rate of oxidation and decomposition of these carbides. Because there was extensive oxidation of the primary NbC, these carbides were implicitly considered to be the primary source of Nb. Although these carbides were large and contained substantial amounts of Nb, they were too few and spaced too far apart to be of concern (averaging one NbC for two grain boundary facets and spaced about 25 {micro}m apart). To wit, whether the freed Nb could diffuse overmorexa0» such a large distance and be oxidized to support the postulated mechanism for crack growth? The presence of other Nb-rich carbides elsewhere on the grain boundaries, or other sources for Nb, therefore, needs to be re-examined and is the focus of this study. In this paper, the results of a study of grain boundary niobium carbides and their distribution in Inconel 718 are reported, and are discussed in terms of their contribution to crack growth.«xa0less

Environmentally assisted fatigue-crack growth in 7075 and 7050 aluminum alloys

Abstract Tests of 7050-T7451, 7050-T651 and 7075-T651 aluminum alloys show that these alloys exhibit similar fatigue-crack-growth kinetics in water vapor. The kinetics conform with the model for transport-controlled crack growth. The saturation water vapor pressure for the 7075-T651 alloy is lower than that for the 7050-T651 and 7050-T7451 alloys because tha latter alloys have rougher fracture surfaces. A slight increase in saturation pressure for 7050-T7451 compared to 7050-T651, on the other hand, is attributed to the difference in yield strengths. Good agreement between the experimentally determined saturation pressures and those predicted by the model provides further confirmation for the model for transport-controlled fatigue crack growth and its applicability to high-strength aluminum alloys.

Hydride formation and decomposition in electrolytically charged metastable austenitic stainless steels

An investigation of phase transformations in hydrogen-charged metastable austenitic stainless steels was carried out. Solution-annealed, high-purity, ultralow-carbon Fel8Crl2Ni (305) and laboratory-heat Fel8Cr9Ni (304) stainless steels were examined. The steels were cathodically charged with hydrogen at 1, 10, and 100 mA/cm2, at room temperature for 5 minutes to 32 hours, in an lN H2SO4 solution with 0.25 g/L of NaAsO2 added as a hydrogen recombination poison. Changes in microstructure and hydrogen damage that resulted from charging and subsequent room-temperature aging were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Hydrides from hydrogen charging (hep ε* in 305 SS and fcc γ* and hcp ε* in 304 SS) were observed. The evidence suggests the following mechanisms for hydride formation during charging: (1)γ → ε → ε* hydride and (2) γ → γ* hydride. These hydrides were found to be unstable and decomposed during room-temperature aging in air by the following suggested mechanisms: (1)ε* hydride (hcp) → expanded ε (hcp) phase →α′ (bcc) phase and (2) γ* hydride →γ phase. The transformation from ε* toα′, however, was incomplete, and a substantial fraction of ε was retained. A kinetics model for hydride decomposition and the accompanying phase transformation during aging is proposed.

The role of magnesium in CF and SCC of 7000 series aluminum alloys

Abstract Preliminary fatigue crack growth and surface reaction experiments were carried out on a 7075-T651 aluminum and an AZ31 magnesium alloy to examine the chemical role of magnesium in corrosion fatigue and stress corrosion cracking susceptibility of 7000 series (Alue5f8Mgue5f8Zn and Alue5f8Mgue5f8Znue5f8Cu) alloys. The evidence, although limited, strongly supports the further reactions of magnesium with water as being the cause for environmental cracking (CF and SCC) susceptibility of 7000 series aluminum alloys. This explanation is very different from the current hypotheses that are based on deformation (e.g., slip planarity), and opens up new avenues for improving the environmental cracking resistance of high strength aluminum alloys.