The Effect of Strontium on the Strength of Layers of Double Oxide Film Defects
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ONE of the most important casting defects in Al alloys, which affects the mechanical properties of the casting parts dramatically, is the double oxide film defect.[1] This defect has been observed on the fracture surfaces of both tensile[2] and fatigue[3] test specimens, and has been clearly identified as an oxide defect by energy-dispersive X-ray (EDX) analysis. Nyahumwa et al.[4] suggested that after an incubation time of about 5 hours, because of the transformation of the oxide layers from c- to a-Al2O3, cracks could be introduced into the oxide which allow the liquid aluminum to come into contact with, and react with, the gases inside the oxide film defect (presumably mainly oxygen and nitrogen). This mechanism could result in the consumption of the atmosphere inside the bifilm and possibly contribute to its deactivation. This hypothesis was confirmed by Raiszadeh and Griffiths,[5] who trapped an air bubble inside the Al melt and monitored its change in volume over time using real-time X-ray radiography. However, they did not find any incubation time required for the cracks to form in the oxide layers and attributed the formation of cracks to the movement of the defects in the liquid metal. They also observed that if the initial hydrogen content of the melt was high, the air bubble expanded initially and then started to shrink. They suggested that the cracks that formed on the oxide layers provided the necessary paths HOSEIN BARTAR ESFAHANI, Ph.D. Candidate, RAMIN RAISZADEH, Professor of Materials Science, and HAMID DOOSTMOHAMMADI, Associate Professor, are with the Department of Metallurgy and Materials Science, School of Engineering, Shahid Bahonar University of Kerman, Jomhoori Eslami Blvd., Kerman, Iran. Contact emails: [email protected]; [email protected] Manuscript submitted June 10, 2015. Article published online December 30, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
for the contact of the internal atmosphere of the bubble and the surrounding melt, and also a penetration path for the diffusion of H atoms into (and from) the trapped atmosphere. Raiszadeh and Griffiths[5] observed that when 0.05 wt pct Sr was added to the commercial purity Al melt, the duration of the air bubble held in the liquid metal increased about four times. They did not offer any explanation for this observation. Iwahori et al.[6] also reported that the hydrogen level in Sr-modified melts was not lowered by vacuum degassing. They suggested that the addition of Sr into the melt increased the strength of the attachment of hydrogen atoms to the aluminum oxide (which was considered inclusions in the melt). The standard Gibbs free energy of oxidation of Sr is more negative than Al (982.46 and 907.55 kJ [7] mol O1 2 , respectively ), hence the Al2O3 layers of a newly formed bifilm begin to transform into an Sr-containing oxide after the oxide defect is submerged into the melt.[8] It is possible that this change in the nature of the oxide layer could cause its strength to change considerably over time, as the defect is submerged in the
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