Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water
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ALTHOUGH Al-Zn-Mg(-Cu) alloys are immune to sustained-load cracking in dry gases, including diatomic hydrogen, they are potentially susceptible when exposed to water vapor, moist gases, or distilled water, with crack growth reported at relative humidities below 10 pct.[1,2] Published crack growth data as a function of relative humidity level for AA7075-T651 plate loaded in the through-thickness orientation at a stress intensity factor of 20 MNm 3/2 is summarized in Figure 1. Water condensation in the tip region of propagating cracks historically has been considered unlikely unless the relative humidity is above 90 pct.[1–5] The Kelvin equation for water precipitation within a hollow N.J. HENRY HOLROYD, Consultant, is with Riverside, CA 92506. Contact e-mail: [email protected] G.M. SCAMANS, Chief Scientist, is with Innoval Technology Banbury, Oxon OX16 1TQ, UK, and is also a Professor of Metallurgy with Brunel University, Uxbridge, Middlesex UB8 3PH, UK. Manuscript submitted May 24, 2010. Article published online September 13, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
channel[6] was used to estimate the critical minimum radius needed for water to condense within a crack-tip region and the calculated critical radius is below 10 nm,[2–5] considerably smaller than the local conditions at a blunted crack tip, derived from conventional relationships such as those from Dean and Hutchinson.[7] Crack-tip regions exposed to moisture are ‘‘sharper’’ than those typically associated with ‘‘blunt’’ crack tips generated in inert environments as supported by the following experimental observations. (a) In-situ visualization of a crack-tip region using scanning atomic force microscopy during crack propagation for an underaged high-purity Al-6.2Zn-2.1Mg0.4Cu alloy in laboratory air (55 to 60 pct relative humidity, 288 K to 293 K (15 C to 20 C)). Here, Komai et al.[8] showed that a crack tip formed and held in vacuum (10 3 Pa) remained blunt but, following exposure to laboratory air, became sharp and crack growth initiated. The blunting and sharpening of the crack-tip region was found to be reversible in VOLUME 42A, DECEMBER 2011—3979
The validity of using the Kelvin equation to determine whether condensation will occur within a crack has been challenged by recent experimental evidence from in-situ atomic force microscopy studies[10, 11] that demonstrated the nanoscale formation of hydrous capillary condensation inside cracks propagating at a growth rate of around 10 10 to 10 9 m/s in fused silica glass exposed to water vapor. Condensate lengths increased from 50 to 200 nm as the relative humidity rose from 1 to 70 pct,[10] under conditions where calculations based on the Kelvin equation would predict a maximum condensation length of 20 nm at 70 pct RH for a crack-tip radius below ~1.5 nm, whereas experimentally, the measured cracktip radii were in the range 16 to 19 nm.[11] The use of the Kelvin equation is clearly too simplistic an approach to determine whether condensation will occur, as several other phy
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