We report on a quantitative investigation into the effect of size and distribution of rod-shaped β ′ 1 precipitates on strength and ductility of a Mg-Zn alloy. Despite precipitation strengthening being crucial for the practical application of magnesium alloys this study represents the first systematic examination of the effect of controlled deformation on the precipitate size distribution and the resulting strength and ductility of a magnesium alloy. Pre-ageing deformation was used to obtain various distributions of rod-shaped β ′ 1 precipitates through heterogeneous nucleation. Alloys were extruded to obtain a texture so as to avoid formation of twins and thus to ensure that dislocations were the primary nucleation site. Pre-ageing strain refined precipitate length and diameter, with average length reduced from 440 nm to 60 nm and diameter from 14 nm to 9 nm. Interparticle spacings were measured from micrographs and indicated some inhomogeneity in the precipitate distribution. The yield stress of the alloy increased from 273 MPa to 309 MPa. The yield stress increased linearly as a function of reciprocal interparticle spacing, but at a lower rate than predicted for Orowan strengthening. Pre-ageing deformation also resulted in a significant loss of ductility (from 17% to 6% elongation). Both true strain at failure and uniform elongation showed a linear relationship with particle spacing, in agreement with models for the accumulation of dislocations around non-deforming obstacles. Samples subjected to 3% pre-ageing deformation showed a substantially increased ageing response compared to non-deformed material; however, additional deformation (to 5% strain) resulted in only modest changes in precipitate distribution and mechanical properties.

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The effect of pre-ageing deformation on the size and distribution of β ′ 1 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 β ′ 1 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 β ′ 1 number densities compared to a binary Mg-3.0at.%Zn alloy. Deformation to 5% plastic strain increased the volume fraction of β ′ 1 precipitates to approximately 2.3% and refined the β ′ 1 precipitate length and diameter. The combination of these effects increased the yield strength after isothermal ageing from 217 MPa (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.

Isothermal ageing of plastically deformed Mg-Zn-Y alloys resulted in precipitation along {1012} twin boundaries. The bulky precipitates formed had structures similar to those recently reported for the rod-like β ′ 1 precipitates, but afforded a more detailed study by high resolution TEM due to their larger size. The core of the precipitates often had the structure of monoclinic Mg 4 Zn 7 phase, and had the orientation [0001]Mg [010]; {1010}Mg (201) with either the matrix or the twin. On this Mg 4 Zn 7 phase, hexagonal MgZn 2 phase grew in two orientations, both with [010]Mg 4 Zn 7 [1120]MgZn 2 . One of these orientations formed a known orientation relationship [0001]Mg [1120]MgZn 2 ; {1120}Mg (0001)MgZn 2 with the matrix. The part of the precipitate with the MgZn 2 structure was usually in direct contact with the twin boundary. Both Mg 4 Zn 7 and MgZn 2 phases have layered structures that can be described with similar building blocks of icosahedrally coordinated atoms. The atomic positions of zinc atoms comprise the vertices of these icosahedra and form "thick" rhombic tiles. Orientation of these rhombuses remain unchanged across the interfaces between the two phases. Near the interface with MgZn 2 , transition structures formed in Mg 4 Zn 7 phase, with Zn:Mg atom ratio between those of Mg 4 Zn 7 and MgZn 2 phases. In these transition structures, the unit cell of Mg 4 Zn 7 phase is extended along [100] or [001] by half a unit cell length by continuation of the rhombic tiling. Structures of these extended unit cells are proposed.Mg-Zn alloys since rare earth elements and yttrium segregate to ternary phase precipitates at the grain boundaries [11] and have not been detected within the β ′ 1 and β ′ 2 precipitates [9, 11]. Among the ternary phases also exists the quasi-crystalline icosahedral phase (I-phase, Mg 3 ZnY 6 ) [12,13]. This phase exists in equilibrium with α-Mg phase, forming a two phase field when the atomic ratio of Zn:Y is near 6, as in the alloy studied here.The monoclinic Mg 4 Zn 7 and Laves phase MgZn 2 phases include zinc lattice sites which possess icosahedral symmetry and are thus are structurally related. The monoclinic Mg 4 Zn 7 phase has been observed to co-exist with a decagonal quasi-crystalline approximant phase [14] and recent work has shown that Mg 4 Zn 7 and MgZn 2 phases co-exist within the rod-like β ′ 1 precipitates in Mg-Zn-Y alloys [15]. There appear to be structural similarities between the binary phases and the icosahedral phase and the monoclinic phase provides a model for the icosahedral structure [16]. A proper understanding of the relationships between the monoclinic and hexagonal phases would assist in clarifying their structural relationship to the i-phase and resolve the uncertainty regarding the phase present in the rod-shaped β ′ 1 precipitates. In the present work, extrusion followed by slight compression has been used to produce twins. Precipitation occurred along twin boundaries upon ageing. Twin boundary precipitates have been identified as belonging to Mg 4 Zn 7 and ...