Michelle G. Koul

Improvements in Strength and Stress Corrosion Cracking Properties in Aluminum Alloy 7075 via Low-Temperature Retrogression and Re-Aging Heat Treatments

Abstract The susceptibility of aluminum alloy 7075 (AA7075 [UNS A97075]) to intergranular stress corrosion cracking (SCC) in the peak strength T6 temper is alleviated through the use of the T73, or over-aged temper, which provides improved SCC resistance with a 10% to 15% strength loss compared to the T6 temper. Previous research has indicated that retrogression and re-aging (RRA) heat treatments reduce the trade-off between T6 strength and T73 SCC resistance. The short-term heat treatment they used, however, limited the applicability of RRA to thin sections of material. The primary goals of this research effort were to determine if lower retrogression temperatures could be used in the RRA process to extend the applicability of this heat treatment to thick section aircraft components and to quantify any observed improvements. Alternate immersion (AI) and double-cantilever beam (DCB) tests were conducted in a 0.6-M sodium chloride (NaCl) solution to evaluate the SCC resistance of various tempers. Improveme...

Hydrogen Desorption Mechanisms in Metastable β Ti-3 Al-8 V-6 Cr-4 Mo-4 Zr

The mechanisms by which hydrogen desorbs from Ti-3 Al-8 V-6 Cr-4 Mo-4 Zr in the presence of a native oxide film under vacuum were examined over a range of temperatures using thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). Hydrogen release was extremely limited and not measured below 260°C due to the thermal stability of the surface oxide. XPS experiments indicate that hydrogen release is not observed in vacuum until after the oxide film is completely dissolved. However, in the temperature range 325-490°C, TDS measurements indicate that hydrogen desorption is bulk diffusion controlled with a high desorption activation energy (135-144 kJ/mol), XPS measurements in conjunction with H desorption measurements indicate a slow but measurable desorption rate following oxide dissolution, with the presence of a high relative O content (25-50 atom %) at the metallic surface. H desorption is rationalized to be diffusion-limited by an O-enriched Ti surface layer at these temperatures. Surface recombination controlled desorption was observed by TDS at temperatures exceeding 490°C, XPS results indicate that the transition from diffusion control to surface recombination control (and an increased desorption rate) is likely coincident with a reduction of the relative surface O content in the alloy to below 25 atom %. Qualitative desorption simulation results indicate agreement with a model that includes H egress by diffusion and surface recombination coupled in series. However, quantitative agreement cannot be obtained without accounting for the effect of the O-enriched surface layer on bulk H diffusion parameters.

A Comparison Study of Stress Corrosion Crack Growth Rates for AA7XXX Alloys as a Function of Bulk Aqueous Chloride Concentrations

Abstract Alternative aluminum alloys are under consideration for drop-in replacement and refurbishment of corrosion and stress corrosion cracking prone AA7075-T6 (UNS A97075) components on aging military aircraft. An assessment of new materials for refurbishment and replacement of older generation, aging components is necessary for a viable life extension program. A quantitative database of stress corrosion cracking (SCC) properties of old generation materials compared to currently available materials is needed as a guideline/justification for part replacement. This effort evaluates AA7150-T7751 (UNS A97150) and AA7040-T7651 (UNS A97040), both which meet mechanical property requirements and are thought to be superior to AA7075-T6 in terms of corrosion performance. Specifically, stress corrosion crack growth rates for these plate alloys were documented and found to be similar in various bulk aqueous chloride environments and to represent a significant improvement over measured rates for AA7075-T651.

Effect of Zn-Rich Coatings on the Corrosion and Cracking Resistance of High-Strength Armor Steel

Scribed panel testing under cyclic salt fog conditions indicates that Zn-rich coatings reduce the corrosion damage observed in armor steel as compared to traditional coatings. Constant extension ra...