Effect of Temperature and phase transformation on the Characteristics of Al-Mg alloy

Samples of Al-5wt.%Mg alloy used as sheets, for internal friction Q-1 ,
dynamic elastic modulus Y and wires for stress-strain measurements, were
heated at 823K for 2 hours then one group of the samples was slowly furnace
cooled to room temperature and the second group was rapidly quenched in
iced water. Both groups were tested in the temperature range from 493K to
553K in steps of 10K. The sheets were aged for 2 hours at different
temperatures then quenched in iced water. Q-1 and the dynamic Young’s
modulus, Y measurements by applying resonance technique were obtained at
room temperature. The wires used for stress-strain measurements were tested
in the working temperature range, and the yield stress y at different
temperatures was obtained. The Vickers hardness, Hv, was obtained from its
relation to y. Increasing temperature Q-1 increased showing abrupt increase
at 523K, while Y and Hv decreased showing minima at 523K. Above 533K, Q-1
increased, while Y and Hv decreased. This behavior indicated a thermal
softening in the tested range; structure measurements disclosed phase
transformation from (+ ) phases into the single -phase at 523K. The
quenched samples showed higher hardness levels than the slowly cooled
samples due to the presence of excess point defects and their interaction with
dislocations. The activation energy values were 17.33 and 59.184 kJ/mol
before and after transition, respectively. The microstructure of the aged slowly
cooled samples examined by X-ray diffraction measurements supported the
phase transformation at 523K.