Mg-Zn Binary Alloys Research Articles

Effect of Zn Content on Aging Behavior of Binary Mg-Zn Alloys

The Vickers micro hardness measurements and high resolution transmission electron microscopy (HRTEM) observation were carried out to investigate the effect of Zn content on aging behavior of Mg-Zn alloys. The hardness increased drastically after aging at 473 K in the Mg-Zn alloys with higher Zn concentration. And higher maximum hardness was obtained at the earlier aging time in the alloys with higher Zn concentration. In bright field image and HRTEM, the particle shaped contrasts of the precipitates were not appeared in 3.8%Zn. The particle shaped contrasts of the precipitates were appeared as indicated by arrows in 4.7 and 6.5%Zn. The contrasts of row on {110}Mg planes and the β1’ phase appeared in the water quenched 4.7 and 6.5%Zn alloy age at 473 K for 3.6 ks.

Grain Size Hardening in Mg and Mg-Zn Solid Solutions

Cast specimens of Mg and of several Mg-Zn binary alloys with a wide range of grain sizes were deformed in tension and compression. The k values calculated from the Hall–Petch (H-P) plots of the tensile 0.2 pct proof stress increased with the Zn content, from 0.24 MPa m1/2 for pure Mg to ~0.66 MPa m1/2 for the 2.3 at. pct Zn alloy; k values measured from compression tests were larger, typically by 0.05 MPa m1/2. When the strength measurements were corrected for the pseudoelastic strain resulting from elastic twinning, the k values generally increased, and the difference between tension and compression was eliminated. This showed that the larger k values obtained in compression using uncorrected data were an artifact of the pseudoelastic effect. The apparent friction stress varied between about 14 MPa for pure Mg to very low or negative values for the most dilute alloy, increasing again to about 8 MPa for the most concentrated alloy. The use of strength data corrected for pseudoelasticity effects is necessary for a consistent analysis of the grain size hardening.

Effect of Quenching on Aging Behavior of Binary Mg-Zn Alloys

The effects of the Zn content on the aging behavior of Mg–Zn alloys was examined by means of Vickers microhardness measurements and high-resolution transmission electron microscopy (HRTEM). The hardness of Mg–Zn alloys with high Zn concentrations increased markedly on aging at 473 K. A higher maximum hardness was obtained at an earlier aging time for alloys with high Zn concentrations. The aging response of Mg–4.7mass%Zn alloy aged at 473 K and quenched in chilled water differed from that of a specimen both aged and quenched at 473 K up to the maximum hardness. HRTEM images of the monolayers on the { }Mg planes were recorded for specimens quenched in chilled water; ' and ' phases were found to exist in the maximally aged specimen. Compared with the specimen quenched in water, the number of ' phases was lower in the specimen that was both aged and quenching at 473 K. It was suggested that the difference in the aging response is related to the formation of monolayers in the early stage of aging.

Existing form and effect of zirconium in pure Mg, Mg-Yb, and Mg-Zn-Yb alloys

The existing form and grain refining effects of small zirconium addition in pure Mg, Mg-Yb and Mg-Zn binary alloys, and Mg-Zn-Yb ternary alloy (ZK60-Yb) were investigated. The results show that Zr element exists mainly in single and cluster particles of pure α-Zr or Zn-Zr compounds inside grains and at grain boundaries. Only the particles located in the interior of grains can act as the nucleus for α-Mg growth and effectively promote the formation of fine equiaxed grains. The broken and dispersed Zr-rich particles produced during the hot extrusion process can form nebulous banded structure in which these fine particles may act as obstacles to dislocation motion in wrought magnesium alloys.

Precipitation Hardening of Mg-Zn and Mg-Zn-RE alloys

The age-hardening response of two cast Mg-Zn-RE alloys has been investigated and compared with that of a binary Mg-Zn alloy. The microstructures of the aged specimens were examined by analytical electron microscopy. Formation of a fine dispersion of rodlikeβ1′ precipitates is the main cause for age hardening, while extensive precipitation of disc-shapedβ2′ coincides with the onset of overaging. Rare earth additions retard the formation ofβ2′ precipitates and thus postpone overaging. Four different orientation relationships betweenβ2′ precipitates and matrix were found and explained in terms of the near-CSL model.