Dissolution of iron intermetallics in Al-Si Alloys through nonequilibrium heat treatment
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I.
INTRODUCTION
THE mechanical properties of aluminum-silicon alloys are greatly dependent on the alloy structure, especially on the eutectic silicon, which may assume an acicular form during solidification of "unmodified" melts or may be fibrous in nature during solidification of "modified" melts. In alloys such as 319, which contain major alloying elements such as Si, Cu, and Mg and impurity elements such as Fe, Mn, Ni, and Cr, several brittle intermetallic compounds may form in addition to eutectic silicon particles. Among these brittle compounds, iron intermetallics are considered to be more detrimental to mechanical properties as compared with other second-phase particles, tlj According to Jonason,[2] the eutectic silicon phase seems to be the most important crack initiator in A1Si7Cu3Fe0.2 alloy. However, he claims that for high iron content (>0.6 pct), the iron phases are more fracture initiating than the silicon. The iron intermetallic in the platelike form (/3-phase) is considered detrimental due to its stress raising potential at the plate tips.[~] It is common practice to add a small quantity of manganese (Fe:Mn = 2:1) to the alloy to convert the crystallization of iron intermetallics in platelike form (/3 phase) to Chinese script form (c~ phase). Because of its complex morphology, the t~ phase is considered less detrimental to mechanical properties compared to the/3 phase. However, addition of manganese significantly increases both the volume percent and hardness of iron intermetallic for a given iron level521 Manganese must, therefore, be considered an important iron phase generator. Jonason t2] reports
L. ANANTHA NARAYANAN, Doctoral Student, is with the Department of Mining and Metallurgical Engineering and Dbpartement des Sciences Appliquers, McGiU University-Universit~ du Qurbec Chicoutimi. F.H. SAMUEL, Professor, is with the Drpartement des Sciences Applique~s, Universitab du Qu+bec ~ Chicoutimi, Chicoutimi, PQ, G7H 2B1, Canada. J.E. GRUZLESKI, Professor, is with the Department of Mining and Metallurgical Engineering, McGill University, Montreal, PQ, H3A 2A7, Canada. Manuscript submitted June 20, 1994. METALLURGICALAND MATERIALSTRANSACTIONS A
that the positive effect of manganese on modifying the morphology of the iron phase to make the material more resistant to crack propagation is somewhat offset by the increased brittleness and volume of the phase. The ideal way to neutralize the detrimental effect of iron intermetallics would be to eliminate these phases in the microstructure. Therefore, it is essential to discover new ways of partly/entirely dissolving the harmful iron intermetallics into the matrix through thermal treatment. Under normal heat treatment conditions, the iron intermetallics do not undergo any change. According to Griger et al., [31 the fl-A1FeSi phase (needlelike morphology) that forms in the cast alloys with higher silicon content does not dissolve, but its morphology is changed slightly by heat treatment. With the addition of a small amount of manganese or chromium, the a
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