A Novel High-Strength Zn-3Ag-0.5Mg Alloy Processed by Hot Extrusion, Cold Rolling, or High-Pressure Torsion

Abstract

A novel Zn-3Ag-0.5Mg alloy was plastically deformed using 3 processing paths: hot extrusion (HE), HE followed by cold rolling (CR) and high-pressure torsion (HPT). The processed samples consisted of the η-Zn phase, ε-Zn3Ag precipitates within the matrix, and nanometric Zn2Mg precipitates within the Zn11Mg2 phase located at the grain boundaries. Both the η-Zn phase and Mg-rich phases were enriched in Ag. Electron backscattered diffraction was used to examine the effects of grain size and texture on mechanical behavior with tensile tests performed at room temperature (RT) at different strain rates. The coarse-grained (~ 6 µm) samples after HE exhibited high strength with brittleness due to dislocation interaction with dispersed precipitates and, to some extent, with twinning activation. Significant grain refinement and processing at RT gave an increase in elongation to over 50 pct in CR and 120 pct in HPT. Ductile CR samples with an average grain size of ~ 2 µm and favorable rolling deformation texture gave a yield strength of ~ 254 MPa, a tensile strength of ~ 456 MPa, and a reasonable strain rate sensitivity. These values for the CR samples meet the mechanical requirements for biodegradable stents in cardiovascular applications.

Fine-tuning of mechanical properties in a Zn–Ag–Mg alloy via cold plastic deformation process and post-deformation annealing

“…Moreover, it cannot be ignored that even at relatively slow rolling speeds, the internal friction of the processed material or the frictional heat generated between the rolls and strip may slightly increase the temperature of the process above RT and soften the material between the CR steps due to the dynamic recovery (DR) and/or DRX processes. As shown in our previous studies, CR75% produced a grain size of ~1.8 μm, while high-pressure torsion for 5 turns refined grain size only to ~1.2 μm [ 21 ]. It is apparent that even without high accumulative strain, it is possible to activate DRX in this material.…”

“…The microstructure of the Zn–3Ag-0.5Mg alloy in the as-cast state was characterized in detail in our previous work [ 21 ]. The material is composed of η-Zn(Ag) matrix dendrites and characteristic lamellar Mg-rich eutectic mixture in the interdendritic region after casting.…”

“…The darkest areas correspond to the grains of Mg-rich second phases. According to our previous study [ 21 ] on the same alloy system, these grains contain a mixture of nano-sized Zn 11 Mg and Zn 2 Mg phases enriched in Ag addition. These phases were identified on XRD diffractograms collected for HE, CR, and CR + HT samples.…”

“…However, the highly-soluble Ag addition can provide both solid-solution strengthening and improvement of mechanical properties by tuning the fraction of Ag-rich phases [ 5 , 20 ]. The current study aims to tailor both strength and ductility in the Zn–3Ag-0.5Mg alloy developed recently [ 21 ] to obtain properties significantly exceeding the mechanical requirements for biodegradable implant material using simple processing routes. The novelty in this study is the employment of a hot extrusion (HE) process directly after casting without inter-process annealing (used in our previous study).…”

Controlled Grain Refinement of Biodegradable Zn-Mg Alloy: The Effect of Magnesium Alloying and Multi-Pass Hydrostatic Extrusion Preceded by Hot Extrusion

“…Lately, equal channel angular pressing (ECAP), high pressure torsion (HPT), the KoBo method and hydrostatic extrusion have been proposed. [14][15][16][17] Hydrostatic extrusion is a technique allowing intensive plastic deformation, but is an unconventional SPD technique as the reduction in the cross-section of a material during the process occurs. [18] One of the advantages of the method is a complex stress state due to hydrostatic pressure, which enables even hard-to-deform materials to be processed and crack formation to be suppressed effectively.…”