Ultrafine-grained Zn Research Articles

Hall-Petch strengthening in ultrafine-grained Zn with stabilized boundaries

Hall-Petch strengthening in ultrafine-grained Zn with stabilized boundaries

Suppression of mechanical instability in bioabsorbable ultrafine-grained Zn through in-situ stabilization by ZnO nanodispersoids

The issue of intrinsic microstructural and mechanical instability of Zn-based metals limits their expansion in potential applications of bioresorbable stents and orthopedic fixators. A new concept of stabilization of Zn microstructure by a small fraction of nontoxic nanometric ZnO dispersoids is proposed for the first time and demonstrated on the particular bioabsorbable model material. The effect of the ZnO dispersoids on post-processing microstructural stability, deformation and strengthening mechanisms, corrosion, and in-vitro biological behavior are pursued. The ZnO dispersoids arise in situ within deformed Zn structure during the consolidation of fine atomized Zn 99.99wt.% powder by hydro-extrusion. ZnO nanodispersoids (4.75 vol.%; ∼136 nm) form from passivating films present on Zn. They allow formation of ultrafine-grained Zn structure with an average grain size of ∼750 nm and its retention by Zener pinning action during annealing held at 100 °C. The model Zn + ZnO composite shows the superior mechanical properties than those reported for pure Zn materials. The utilized stabilization concept doesn't compromise corrosion and biological responses. Immersion of the Zn + ZnO in DMEM results in a corrosion rate, which complies with the desirable standard rate for biodegradable materials. Electrochemical tests suggest that the Zn + ZnO reaches a similar degradation rate after the first week of immersion and a more uniform corrosion behavior compared to the cast Zn reference. In-vitro cyto/genotoxicity assays performed using DMEM diluted extracts of the Zn + ZnO and cast Zn incubated with L929 cells yield in comparable and non-toxic responses. The presence of ZnO dispersoids induces a small but still significant bacteriostatic activity.

Lithium-Induced Optimization Mechanism for an Ultrathin-Strut Biodegradable Zn-Based Vascular Scaffold.

To reduce incidences of in-stent restenosis and thrombosis, the use of a thinner-strut stent has been clinically proven to be effective. Therefore, the contemporary trend is toward the use of ultrathin-strut (≤70µm) designs for durable stents. However, stents made from biodegradable platforms have failed to achieve intergenerational breakthroughs due to their excessively thick struts. Here, microalloying is used to create an ultrathin-strut (65µm) zinc (Zn) scaffold with modified biodegradation behavior and improved biofunction, by adding lithium (Li). The scaffold backbone consists of an ultrafine-grained Zn matrix (average grain diameter 2.28µm) with uniformly distributed nanoscale Li-containing phases. Grain refinement and precipitation strengthening enable it to achieve twice the radial strength with only 40% of the strut thickness of the pure Zn scaffold. Adding Li alters the thermodynamic formation pathways of products during scaffold biodegradation, creating an alkaline microenvironment. Li2 CO3 may actively stabilize this microenvironment due to its higher solubility and better buffering capability than Zn products. The co-release of ionic zinc and lithium enhances the beneficial differential effects on activities of endothelial cells and smooth muscle cells, resulting in good endothelialization and limited intimal hyperplasia in porcine coronary arteries. The findings here may break the predicament of the next-generation biodegradable scaffolds.

Ultrafine-Grained Zn-Mg-Sr Alloy Synthesized by Mechanical Alloying and Spark Plasma Sintering.

Zinc materials are considered promising candidates for bioabsorbable medical devices used for the fixation of broken bones or stents. Materials for these applications must meet high mechanical property requirements. One of the ways to fulfil these demands is related to microstructure refinement, particularly the decrease in grain size. In the present work, we combine two powder metallurgy techniques (mechanical alloying-MA, and spark plasma sintering-SPS) to prepare Zn-1Mg-0.5Sr nanograin material. The microstructure of compacted material consisted of Zn grains and particles of Mg2Zn11 intermetallic phases from 100 to 500 nm in size, which resulted in high values of hardness and a compressive strength equal to 86 HV1 and 327 MPa, respectively. In this relation, the combination of the suggested techniques provides an innovative way to form extremely fine microstructures without significant coarsening during powder compaction at increased temperatures.

Softening Behaviors of Severely Deformed Zn Alloy Studied by the Nanoindentation

Zamak 3 alloy treatment by sliding-friction treatment (SFT) was investigated by nanoindentation to explore the influence of microstructure and strain rate on nanoscale deformation at room temperature. The results show that obvious material softening occurs in the ultrafine-grained (UFG) Zn alloy and strain-hardening happens in the twinning-deformed layer, respectively. It can be concluded that almost constant values of V in the UFG Zn alloy contribute to the dislocations moving along the grain boundary (GB) not cross the grain interior. In the twinning-deformed layer, the highly frequent dislocation–twinning boundary (TB) interactions are responsible for subsequent inverse Cottrell–Stokes at lower stress, which is quite different from dislocation–dislocation reaction inside the grain in their coarse-grained (CG) counterpart.

Indentation creep study on ultrafine-grained Zn processed by powder metallurgy

Ultrafine-grained Zn (UFG-Zn) with the grain size of about 200nm was processed by Spark Plasma Sintering at 300°C from fine Zn powder. The grain boundaries in the consolidated material were decorated by ZnO dispersoids with a mean thickness of ~20nm. The creep behavior was studied by indentation tests in the homologous temperature range of 0.87–0.91. The activation energy of the creep for UFG-Zn was found to be much larger (211–252kJ/mol depending on the oxide content) than the value determined previously for coarse-grained Zn (152–159kJ/mol). The activation energy increased with increasing ZnO content in UFG-Zn. X-ray line profile analysis revealed that the population of the different dislocation slip systems changed during creep deformation, indicating a considerable dislocation activity.

Deformation Mechanisms in Ultrafine-Grained Zn at Different Strain Rates and Temperatures

The deformation mechanisms in ultrafine-grained hexagonal close packed Zn were investigated at different strain rates and temperatures. The influence of grain size on the deformation mechanisms was revealed by comparing the results obtained on ultrafine-grained and coarse-grained Zn. It was found that for coarse-grained Zn at room temperature and strain rates lower than 10-2 s-1 twinning contributed to plasticity besides dislocation activity. For strain rates higher than 103 s-1 the plasticity in coarse-grained Zn was controlled by dislocation drag. In ultrafine-grained Zn the relatively large dislocation density (~1014 m-2) and the small grain size (~250 nm) limit the dislocation velocity yielding the lack of dislocation drag effects up to 104 s-1. For ultrafine-grained Zn, twinning was not observed in the entire strain rate range due to its very small grain size. During room temperature compression at strain rates higher than 0.35 s-1 and in high temperature creep deformation of ultrafine-grained Zn besides prismatic and pyramidal <c+a> dislocations observed in the initial state, <a>-type basal and pyramidal dislocations as well as other <c+a>-type pyramidal dislocations were formed.

Mechanical behaviour and underlying deformation mechanisms in coarse- and ultrafine-grained Zn over a wide range of strain rates

Spark plasma sintering was used to process bulk ultrafine-grained (average grain size of ∼250nm) samples from high purity nanocrystalline Zn powder. The microstructure of the consolidated samples was investigated and the mechanical behaviour was characterised by means of quasistatic and dynamic compression tests at room temperature and compared to that of coarse-grained counterparts. For both materials in the strain rate regime of 10−4–101s−1 a linear relationship between the logarithm of flow stress and strain rate values was observed since room temperature corresponds to a relatively high homologous temperature of 0.43. The strain rate sensitivity was higher for ultrafine-grained Zn (∼0.18) than for coarse-grained Zn (∼0.12) and twinning in the former sample was not observed in the entire strain rate range. For strain rates higher than 103s−1 the plasticity in coarse-grained Zn was controlled by dislocation drag but partial recrystallization was also observed. In the latter regime the mobile dislocation density was about 1011m−2. However, in ultrafine-grained Zn the relatively large dislocation density (∼1014m−2) and the small grain size limit the dislocation velocity yielding the lack of dislocation drag effects up to 104s−1.

LOW CYCLE FATIGUE BEHAVIOR OF Zn-22Al ALLOY IN SUPERPLASTIC REGION AND NON-SUPERPLASTIC REGION

The crack propagation properties for ultrafine-grained Zn -22 wt % Al alloy during low cycle fatigue (LCF) in the superplastic region and the non-superplastic region were investigated and compared with the corresponding results for several other materials. With the Zn - 22 wt % Al alloy, it was possible to conduct LCF tests even at high strain amplitudes of more than ±5%, and the alloy appeared to exhibit a longer LCF lifetime than the other materials examined. The fatigue life is higher in the superplastic region than in the non-superplastic region. The rate of fatigue crack propagation in the superplastic region is lower than that in the other materials in the high J-integral range. In addition, the formation of cavities and crack branching were observed around a crack tip in the supereplastic region. We therefore conclude that the formation of cavities and secondary cracks as a result of the relaxation of stress concentration around the crack tip results in a reduction in the rate of fatigue crack propagation and results in a longer fatigue lifetime.

Low-cycle fatigue properties of ultrafine-grained zinc–22 wt.% aluminum alloy during room-temperature superplastic flow

Low-cycle fatigue behavior during room-temperature superplastic flow was investigated for ultrafine-grained Zn–22 wt.%Al alloy. This alloy exhibited a longer fatigue life and a lower crack propagation rate than other materials at high strain amplitudes above ±5%. In addition, many cavities and secondary cracks were observed near the tips of cracks. These are probably responsible for the apparent reduction in the crack propagation rate and the long fatigue lifetime of this alloy.

The Ultra Fine Grained (UFG) Zn Produced by Ball Milling

Bulk ultra fine grained (UFG) Zn was produced by in situ consolidation of Zn elemental powder with ball milling at room temperature and annealed for 1 hour at 200 after pure Zn milled. The 150 % elongation is obtained in UFG Zn during tensile test at 20 which is equal to 0.43 Tm (Tm is the melting temperature of pure Zn). The elongation of UFG Zn is higher than the 110 % elongation of nanocrystalline Zn ball milled at tensile elongation. The largest tensile elongation (150%) was observed at room temperature in UFG Zn.

Studies of deformation mechanisms in ultra-fine-grained and nanostructured Zn

The temperature, strain rate, grain size and grain size distribution effects on plastic deformation in ultra-fine-grained (UFG) and nanocrystalline Zn are systematically studied. The decrease of ductility with the decrease of average grain size could be an inherent effect in nanocrystalline materials, that is, not determined by processing artifacts. The superior ductility observed in UFG Zn may originate from both dislocation creep within large grains and grain boundary sliding of small nanograins. The stress exponent for dislocation creep is about 6.6. The activation energy for plastic deformation in UFG Zn is close to the activation energy for grain boundary self diffusion in pure Zn.

Tensile elongation (110%) observed in ultrafine-grained Zn at room temperature

Tensile tests were performed for Zn at room temperature, which show elongations of 110%–20% for average grain sizes of 240–23 nm, respectively. The ductility of ultrafine-grained and nanocrystalline Zn was found to decrease with grain size refinement. The deformation mechanisms in ultrafine-grained Zn are believed to be a mixture of grain boundary sliding of small nanograins and intra-grain dislocation creep within the large grains.