We show by high pressure torsion at room temperature and transmission electron microscopy that nucleation of recrystallized α-Mg grains occurs preferentially over quasicrystalline i-phase particles at an early stage of recrystallization in a Mg-3.0Zn-0.5Y (at%) alloy. α-Mg grains nucleated on i-phase surfaces by torsional rotation N=1/2 (at 5GPa pressure), and show definite orientation relationships with the i-phase. These orientation relationships are asymmetrical variations of those reported for as-cast and extruded alloys. The nucleated grains show a planar interface with the i-phase and are often faceted as they grow into a yet unrecrystallized matrix.Download high-res image (462KB)Download full-size image

Deformation structures have been examined by transmission electron microscopy after 4% plastic deformation in tension and compression in a Mg-2.2 at%Y alloy extruded at 698 K to obtain a recrystallized grain size of about 20μm. The extruded alloy showed similar yield stresses of about 240 MPa in both tension and compression at room temperature. Dislocations formed by tensile stress showed a tendency of 〈a〉 type dislocations to cross slip to prismatic planes, resulting in long continuous dislocation loops. Dislocation loops with 〈c〉components were also observed. Occasionally, {101¯2} type twinning was also observed in grains with large orientation away from basal texture. These grains contained predominantly basal slip. Compression deformed samples showed a limited number of {101¯2} type twins. The matrix contained loops of non-basal 〈a〉 type dislocations, together with basal dislocations with 〈c〉 component. Inside the twins occurred stacking faults and loops of 〈c〉 type dislocations with segments perpendicular to the basal plane. Activation of several deformation modes and complex dislocation structures explains the strain hardening behavior and low anisotropy of the alloy.

We show that, while interfaces formed in a Mg-3Zn-0.5Y (at%) magnesium alloy by severe plastic deformation (SPD) by high pressure torsion (HPT) at room temperature (RT) are segregated with Zn (without aid of any thermal treatment), only a small fraction of grain boundaries are segregated when extruded at 573 K (300 °C). We have examined the effect of strain rate and temperature on the diffusion behavior and segregation of zinc in magnesium alloys. At first, we have established the driving force for segregation by evaluating segregation energy and the interface energy gain by density functional theory calculations. The mechanism of segregation is established through calculation of excess vacancy concentration and critical dislocation velocity as a function of strain rate. It is estimated that Zn atoms are transported by SPD induced vacancy flux in case of HPT at RT, whereas the Zn atoms are dragged by equilibrium vacancies and dislocations on extrusion at 573 K (300 °C). The amount of segregation to the different twins and grain boundaries with different interfacial energies have been calculated by thermodynamic parameters and found to be in the range of 1–8 at% of Zn for the case of HPT process and 0.7–1.6 at% of Zn for extrusion processed specimens. These estimates correspond to the solute concentrations determined experimentally.

Very high strengths, with tensile yield strength from 377 to 405 MPa, combined with elongation to failure of over 12 pct, have been achieved in Mg-Zn-Y dilute alloys by direct extrusion. Alloys Mg-6xZn-xY, where x = 0.2, 0.35, and 0.5 (at. pct) were chill cast in a steel mold and direct extruded at a temperature in the range 508 K to 528 K (235 °C to 255 °C), which produced an average grain size of about 1 μm. Quasicrystalline i-phase particles were dispersed in the matrix with size ranging from 50 nm to 1 μm. In addition, high density of nano-precipitates of average size 15 nm was dispersed in matrix. Thus we have developed magnesium alloys of very high strength combined with ductility by a simple process using extrusion with very little addition of yttrium.

We report on a quantitative investigation into the effect of size and distribution of rod-shaped β1′ 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 β1′ 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.Highlights► Quantitative investigation into the effect of size and distribution of precipitates in Mg–Zn. ► Modification of size and distribution of precipitates via pre-ageing deformation. ► Change in length as well as diameter of β1′ precipitates occurs. ► Tensile tests performed to correlate strength and ductility with precipitation. ► Calculations based on Orowan mechanism compare with experimental results.

 Tin particles of about 1 μm size embedded in Al–Cu–Fe quasicrystal matrix were sharply faceted on close-packed planes of tin and fivefold and twofold planes of the matrix. In order to match close-packed planes, tin was found to make five orientation relationships (ORs) described here. These ORs try to match planes which are closest-packed or close-packed with matching spacings. This demonstrates the ability to form matching interfaces in a variety of orientations.

β1′ precipitates in an Mg97Zn2.5Y0.5 alloy are shown to have a complex domain structure based on the structure of monoclinic Mg4Zn7 phase. All domains are oriented with [0 1 0]∥[0 0 0 1]Mg and one of the four planar matches with the matrix. Two of these orientations are related to two others by twinning on the (0 0 1) plane of the Mg4Zn7 phase. These orientations are brought about by crystallographic constraints from the matrix. An ordering of the domains creates a new structure.