  The effect of pre-ageing deformation on the size and distribution of
beta-prime precipitates and subsequently on the resulting strength and
ductility have been measured in a Mg-3.0at.%Zn-0.5at.%Y alloy. The alloy was
extruded and then subjected to a T8 heat treatment comprised of a
solution-treatment, cold-work and artificial ageing. Extrusion was used to
introduce texture, ensuring that deformation occurred via slip rather than
twinning. Samples were subjected to controlled uniaxial deformation and then
isothermally aged to peak hardness. Precipitate length, diameter and number
density were measured and evaluated in terms of the strength and ductility of
the alloy. The nucleation of the beta-prime precipitates in peak-aged condition
without pre-ageing deformation (i.e.T6 treatment) was poor, with only 0.5%
volume fraction, compared to approximately 3.5% in T6 treated binary
Mg-3.0at.%Zn alloy. The microstructure of the Mg-Zn-Y alloy was less refined,
with larger diameter precipitates and lower beta-prime number densities
compared to a binary Mg-3.0at.%Zn alloy. Deformation to 5% plastic strain
increased the volume fraction of beta-prime precipitates to approximately 2.3%
and refined the beta-prime precipitate length and diameter. The combination of
these effects increased the yield strength after isothermal ageing from 217MPa
(0% cold-work) to 287 MPa (5% cold-work). The yield stress increased linearly
with reciprocal interparticle spacing on the basal and prismatic planes and the
alloy showed similar strengthening against basal slip to Mg-Zn. The elongation
increased linearly with particle spacing. The ductility of Mg-Zn-Y alloys was
similar to that of Mg-Zn for equivalently spaced particles.
