A pathway to achieving strength–ductility simultaneous improvement via solute segregation in gradient-structured Mg–Zn–Gd alloy

Abstract

This study reports a novel dual gradient-structured (GS) Mg–2Zn–0.8Gd (wt.%) alloy fabricated via surface sliding friction and followed by heat treatment. Excellent grain boundary (GB) stability is achieved through the co-segregation of Zn and Gd atoms. The segregated gradient structure produces an excellent strengthening and work hardening effect, which is attributed to the combined effects of prominent GB segregation within the GS layer and the recovery of dislocation storage capacity. Furthermore, a remarkable segregation hardening in the surface layer increases the hard–soft disparity, which arouses more geometrically necessary dislocations that further synergistically strengthen and toughen the alloy. GRAPHICAL ABSTRACTNine visuals including microstructure maps, microscopy images, atom map, 3 line graphs, 1 bar chart, and 1 load-displacement graph.The figure shows a set of nine visuals related to microstructure and mechanical measurements of a material. On the far left, a microstructure map displays many differently colored grains arranged in layers along a vertical depth axis labeled from 200 to 550, with a scale bar labeled 25 micrometers at the bottom. To its right, an electron microscopy image shows bright grain boundaries with text annotations indicating low angle grain boundary segregation and high angle grain boundary segregation, several small arrow markers along the boundaries and a 50 nanometers scale bar. Below, two separate microscopy images labeled gadolinium and zinc present dispersed dark regions; both carry 25 nanometers scale bars and circles marking areas noted as W phase. Next, a tall three dimensional atom map shows a tapering column composed of many purple points with elongated green clusters; a magnified rectangle highlights one region, and colored spheres labeled magnesium, gadolinium, and zinc appear below. To the right, a line graph plots engineering stress in megapascals versus engineering strain in percent for three curves labeled E0, GS, and GS + A; stress rises steeply at low strain and then either drops or levels as strain increases up to about 25 percent. Another line graph plots work hardening rate in megapascals versus true strain in percent for the same three conditions, each curve decreasing with strain from above 2500 to near 0 as strain approaches 25 percent. A smaller chart beneath shows hardness in gigapascals versus distance from the surface in micrometers with discrete data markers for two processing routes labeled surface friction treatment and surface friction treatment + annealing; hardness decreases from near 1.8 at about 100 micrometers toward about 1.1 at 600 micrometers, and a secondary vertical axis on the right indicates a parameter delta in gigapascals declining from about 0.23 to about 0.12. All data are approximate. The final graph at the bottom right shows nanoindentation load in nanonewtons versus displacement in nanometers for four conditions labeled coarse grained, coarse grained annealed, ultrafine grained, and ultrafine grained annealed; each curve increases nonlinearly, reaching maximum loads between about 200 and 300 nanonewtons at displacements up to about 4000 nanometers.