Rapid solidification (RS) of Al-Cu alloys has attracted considerable attention due to the possibility of obtaining unique microstructures and improved mechanical properties. Control over the morphology and grain size in rapidly solidifying Al-Cu alloys is key to optimizing their performance. This work is devoted to the experimental determination of a microstructure selection map in Al-Cu alloys obtained by spinning.
Laser accelerated solidification studies were carried out for hypereutectic Al-Cu alloys with copper contents of 36, 40 and 44 wt%. By extracting thin layers from the laser-treated surface, it was possible to study the resulting growth morphology using transmission electron microscopy (TEM). The orientation of the microstructure made it possible to establish a relationship between the observed shapes and the corresponding local growth rates.
Orientations of microstructures covering growth rates in the range of 0.01–2.0 m/s were investigated. Different types of solidification front growth were observed: eutectic (with and without oscillatory instability), dendritic, cellular, banded and flat. Comparison of the obtained data with previous results related to alloys with lower copper content allowed the creation of a microstructure selection map for Al-Cu alloys.
This map establishes a correspondence between microstructure, growth rate and composition in the range of 0.01–2.0 m/s and copper concentrations from 0 to 44 wt.%, which covers a significant part of the binary eutectic Al-Al2Cu (0–54 wt.% Cu). The presented map not only provides valuable information on the solidification structures, but also allows one to obtain information on the phase diagram of Al-Cu.
During the study, Al-Cu alloys of various compositions were synthesized, from pure aluminum to eutectic composition. The spinning process was carried out at different rotation speeds of the copper drum, which allowed the cooling rate to be varied from 10^4 to 10^6 K/s. The resulting foils were analyzed using optical and electron microscopy, as well as differential scanning calorimetry (DSC) to determine the temperatures of phase transitions.
The results showed that the microstructure morphology strongly depends on the cooling rate and alloy composition. At low cooling rates, a coarse-grained structure with dendritic morphology is formed. As the cooling rate increases, the grain size decreases and the dendritic morphology is replaced by a cellular one. At the highest cooling rates, a microcrystalline or even amorphous structure is achieved.
Based on the obtained data, a microstructure selection map was constructed, reflecting the relationship between the alloy composition, cooling rate and the forming microstructure. This map can be used for the targeted production of Al-Cu alloys with specified microstructural characteristics and, therefore, with optimized mechanical properties. Further research will be aimed at studying the effect of microstructure on the mechanical properties of rapidly solidifying Al-Cu alloys.
Author: S.C. Gill, W. Kurz
Institute: Department of Materials Science, ETH Zurich, 1015 Lausanne, Switzerland