• Sanamar, Soheil

Abstract

There is still some debate about the nature of the phase transformations that take place in the central region of the Al-Mg binary phase diagram and the formation kinetics of the intermetallic phases that are formed. The purpose of the current work is to characterize the phase formation reaction between Al and Mg and to better understand the formation kinetics of the intermetallic phases that form in the binary Al-Mg system. Therefore, two compositions, Al60Mg40 and Al40Mg60 (wt. %) were investigated, which are very suitable for studying the central part of the Al-Mg phase diagram. Powder metallurgy, including cold extrusion was used to create a large interface between the Al and Mg, which facilitates fast reaction kinetics. In order to observe the phase formation and to analyze their crystal structures, X-ray diffraction using synchrotron radiation was used. This technique offers advantages due to its high brightness and brilliance, enabling the detection of small phase fractions, and has proven to be extremely effective in resolving complex phase studies. Moreover, synchrotron radiation coupled with a fast read-out area detector, makes in situ investigations of phase transformations possible with a high time resolution. The effects of crystal orientation distribution (texture) on the physical and mechanical properties of metallic alloys is well known. Supplementary to the information on phase development, texture at high temperature was also determined to get information on the orientation relationships between the Al and Mg and the intermetallic phases that formed. Texture development and phase analysis were both performed using ex-situ and in-situ experiments. The γ-Al12Mg17-phase is the first phase formed in both the alloy compositions, Al40Mg60 and Al60Mg40. After the γ-Al12Mg17-phase has reached a critical thickness, the β-Al3Mg2–phase is formed. Thus the γ-phase is the first to form in the system. After annealing at 400 °C for 2 h, the Al40Mg60 composition consisted of a very high amount of Al12Mg17 and a small amount of Al3Mg2 while the Al60Mg40 composition consisted of Al12Mg17 and Mg, indicating that thermodynamic equilibrium has been approached. On further annealing at 400°C for 12 h, both compositions formed one phase, this was the Al12Mg17 phase in the Al40Mg60 composition and Al3Mg2 in the Al60Mg40 composition. Due to the extrusion process, a texture gradient was observed over the cross-section of both compositions. The aluminum phase showed a typical texture component of plane-strain deformation in the middle part of the extruded bar and a uniaxial deformation texture near the surface. In the central region of the extruded bars, the (0002) Mg pole figure showed a split along the extrusion direction (±ED), resulted from the activation of c+a glide. These two poles twist towards the transverse direction on moving towards the surface of the extruded bar; one pole moving towards +TD and the other one towards −TD. The angle of twist increased up to 90° towards the TD surface. After annealing at 200 °C for 12 h, a recrystallization texture was observed in both the Al and Mg for both compositions. This recrystallization can be recognized by a 30° rotation of the Mg crystals and by the formation of a cube texture component in the Al phase. After annealing the Al40Mg60 composition at 400 °C for 12 h, a texture transformation occurred. This resulted in the (110) plane of Al12Mg17 being parallel to the (0001) plane of Mg and the (11ത0) plane of the Al12Mg17 being parallel to the (11ത00) of Mg. This indicates that an orientation relationship between the Mg and the Al12Mg17 phases was formed.

Topics

• impedance spectroscopy
• surface
• phase
• x-ray diffraction
• experiment
• aluminium
• composite
• texture
• annealing
• activation
• intermetallic
• phase diagram
• recrystallization
• alloy composition
• cold extrusion