The precipitation of the Q phase in an AA6111 Alloy
- PDF / 1,561,054 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 13 Downloads / 189 Views
I. INTRODUCTION
THE quaternary Q phase was first identified some 70 years ago[1] in Al-Mg-Si-Cu alloys from its etching response in optical metallography, at a time when the physical metallurgy community first became interested in precipitation strengthening. A spread of compositions ranging from Al4CuMg5Si4 to Al4Cu2Mg8Si7 was suggested for the Q phase in subsequent studies,[2,3,4] but to this day, there is some uncertainty as to whether the Q phase has a unique composition in the quaternary Al-Cu-Mg Si system, or forms with a range of stoichiometries dependent on the alloy composition or heat treatment. The crystal structure of the Q phase was established by Arnberg and Aurivillius[5] using single crystals grown from a slowly cooled melt. The composition of the crystals was Al4Cu2Mg8Si7, with a hexagonal unit cell, a ⫽ 1.03932 nm, and c ⫽ 0.40173 nm. These same authors suggested that the diffraction data could be fitted to either the P63/m or P6 space group. These two possibilities can be distinguished by the presence or absence of (0002n ⫹ 1) reflections, although X-ray structure factor calculations show that these reflections, if present, would be very weak. However, Arnberg and Auruvillius[5] chose the latter space group, as it led to a better overall fit with their diffraction data, despite the absence of (0001) reflections. There has been a resurgence of interest in the Q phase over the past 10 years with the realization that it precipitates in many of the 6xxx series of age-hardened Al alloys being developed for automotive sheet applications.[6–10] The phase is reported to form with a unique orientation relationship with the matrix, viz. [0001]Q // [001]Al and [1 120]Q // [510]Al giving 12 variants.[6,7] It grows as lath-shaped particles along 具100典 directions in the matrix. This observation has been rationalized by noting that the 具c典 lattice parameter of the Q phase, 0.402 nm, is nearly identical to the lattice parameter
G.C. WEATHERLY, Professor, is with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada L8S 4M1. A. PEROVIC, Research Associate, and D.D. PEROVIC, Professor, are with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada M5S 1A4. N.K. MUKHOPADHYAY, Reader, is with the Department of Metallurgical Engineering, Banaras Hindu University, Varanasi 221005, India. D.J. LLOYD, Senior Scientist, is with the Kingston Research and Development Center, Alcan International Ltd., Kingston, ON, Canada K7L 5L9. Manuscript submitted July 5, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
of Al, 0.405 nm. Indeed, the Q phase is often observed to be fully coherent with the matrix in these directions, leading to the designation of a Q⬘ phase with 具c典 ⫽ 0.405 nm.[7] However, the crystal structure of this phase is probably identical to the Q phase, and it is more correctly described as a coherent or semicoherent precipitate, constrained by its morphology and size to have an identical lattice parameter to Al along the growth d
Data Loading...
