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Min-Ten Jahn Mechanical Engineering Department, California State University, Long Beach, California 90840-8305 (Received 9 December 1999; accepted 28 June 2000)

The effects of beryllium (Be) and solution temperature on the morphologies of iron intermetallics, silicon particles, and copper intermetallics, relative to the mechanical properties of 319.0 alloys, were investigated. The experimental results indicated that adding Be to the alloy can raise the Al–Al2Cu eutectic melting temperature, change some plateletlike shape (␤–Al5FeSi) of iron intermetallics to comparatively harmless Chinese-script morphologies (␣–Al8Fe2Si), and reduce the amount and average length of ␤–Al5FeSi platelets. During high-temperature solution treatment (>500 °C), the thinner and smaller the ␤–Al5FeSi platelets were, the faster they dissolved and fragmented. However, where the solution temperature exceeded the Al–Al2Cu eutectic melting point, the Al–Al2Cu eutectic melted and resulted in an ultrafine eutectic phase on quenching, which deteriorated mechanical properties. The fracture behavior of 319 alloy was affected by the morphologies of the iron intermetallics, silicon particles, and copper intermetallics. Fractographic analysis of tested compact tension specimens revealed that the fracture processes were mainly initiated by void nucleation at ␤–Al5FeSi platelets as a result of their cracking and decohesion from the matrix. Adding Be to the 319.0 alloy and optimizing the solution temperature could significantly decrease the number of fracture-initiation sites of ␤–Al5FeSi platelets and improve the tensile properties and fracture toughness.

I. INTRODUCTION

Aluminum–silicon casting alloys such as A356, 319 have been widely used in automotive components due to their excellent castability, strength-to-weight ratio, and heat conductivity. The 319 casting alloy is one of the alloys in the Al–Si hypoeutectic system, which contains copper and magnesium as the main alloying elements and impurity elements such as Fe, Mn, Cr, and Ni. These elements partly go into solid solution in the matrix and partly form intermetallic particles during solidification. Among these brittle intermetallic compounds, iron intermetallics are considered to be more detrimental to mechanical properties than other second-phase particles.1 According to Backerud et al.,2 the two main types of Fe–Si bearing intermetallic phases occurring in 319 alloy are Al5FeSi (␤ phase) and Al15(Mn,Fe)3Si2 (␣ phase). The first phase (␤ phase) with acicular or plateletlike form is considered detrimental to the ductility of alumia)

Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 15, No. 9, Sep 2000

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num alloys due to its stress-raising potential and poor binding strength with the matrix.3 The second phase (␣ phase), with Chinese-script form, has a compact morphology, which is considered less harmful to the mechanical properties than the ␤ phase.2,3 Under normal heat-treatment conditions,