Natural and artificial aging in Mg-Gd binary alloys

Early precipitation stages are now frequently observed in Mg alloys with rare earth due to a development of new microscopy techniques. Information about a very early precipitation at room temperature (i.e. natural aging, NA), however, is rare even in Mg alloys with a very well described precipitation sequence. Moreover, there are only few works dealing with an influence of NA on subsequent thermal processes in Mg alloys. The aim of this work was to study kinetics of the NA performed up to 5 years in an Mg-RE system and its influence on a subsequent artificial aging. The solution treated Mg-Gd was chosen as a model binary system where the NA as well as precipitation development during isochronal and isothermal heat treatment was observed. Electrical resistivity measurements at low and well-defined temperature (77 K) and Vickers microhardness were used for monitoring of a solute clustering during the NA. The kinetics of this process is much slower than observed in aluminum alloys and both methods give identical values of kinetics parameters in simple models. The validity of concentration dependences of microhardness and yield stress values supports these models. The NA intensifies precipitation strengthening in solution treated Mg-Gd alloy with 15 wt % Gd isochronally annealed up to 200 degrees C. Formation of short range ordered super hexagons consisting of 6 columns of Gd atoms along the [ 0001] direction resembling the D0(19) structure was revealed by electron diffraction in specimen isochronally annealed up to 200 degrees C. The maximum hardening during isochronal annealing up to 300 degrees C shifts to higher annealing temperatures in the naturally aged specimen. Precipitates of the orthorhombic beta' phase (a(beta') = 2aMg, b(beta') = 4 root 3a(Mg), c(beta') = c(Mg)) were identified after isothermal artificial aging for 165 h at 200 degrees C in the Mg-Gd with 15 wt% in a cell-like arrangement despite of the NA. Since the mesh size is much smaller in the naturally aged alloy, however, it causes the better hardening effect. (C) 2017 Elsevier B.V. All rights reserved.