Effect of γ′ and γ (Ag2Al) Precipitates on the Steady State Creep of Al−16 wt% Ag Alloys with and without Zr Addition

Abstract

The steady state creep behaviour of A1-16 wt% Ag and A1-16 wt% Ag-0.1 wt% Zr alloys has been studied at constant load (= 137.2 MPa) and tested at different working temperatures for samples aged at temperatures favouring the formation of y' and y precipitates. It was found that addition of Zr to A1-16 wt% Ag alloy accelerates the precipitations y' and y due to the formation of the metastable phase A13Zr. For both alloys, two values of activation energy have been obtained as (34f0.3) kJ/mol for viscous glide of dislocations due to the existence of precipitates and (68 f 0.3) kJ/mol for grain boundary diffusion due to the existence of y precipitates. TEM investigations of y' and y precipitates confirmed the abovementioned effect of Zr addition.

Transient and Steady State Creep of Al-10 wt% Ag and Al-22 wt% Ag Alloys Containing ??- and ?-Precipitates

“…4). These precipitates with higher density are usually more effective as dislocations locking agents [2,5,12,13,[20][21][22][23]. A higher Ag content in alloy B is assumed to accelerate the rate of dissolution of GP zones and the rate of formation and growth of g 0 -plates as will be discussed later.…”

“…Accordingly, shorter aging time (%40 min) is required to achieve the maximum strength of alloy B, while for alloy A -with lower Ag contentdue to the lower rate of formation and growth of g 0 -plates, the maximum strength is proved after a relatively longer aging time (%90 min). At aging time higher than %90 or %40 min for alloys A and B, respectively, nucleation and growth of g 0 -precipitates have ceased and the coarsening, by a kink-terrace-ledge mechanism [4,20], has become the dominant reaction whereby g 0 -precipitates acquire a semi-coherent nature.…”

“…Such a coarsening results in an increase in the precipitate thickness concurrently with a decrease in their number. So, the number of obstacles for the dislocation motion will extensively be reduced leading to loss of strength [5,12,20,21], i.e. increasing n, b and e st values.…”

“…1 (2001) fraction (see Fig. 7), they are considered to be less effective as impeding agents for dislocation motion [5,20,21].…”

Transformation characteristics of Al–Ag and Al–Ag–Ti alloys

“…Previous studies [11,1] on Al-16 wt%Ag-0.1 wt%Zr and Al-16 wt%Ag-0.28 wt%Fe alloys showed that small additions of Zr or Fe accelerate the precipitation of the equilibrium ␥-phase, during straining, which reduces the intensity of the precipitate-dislocation interaction and hence lowers the strength of the alloy. The effect of additions of small amounts of zirconium and titanium to Al-1 wt%Si alloy via creep test, a sound stabilization effect of Zr and Ti on the ageing characteristics was noticed [12] and generally, Zr and Ti addition reduced the rate of creep.…”