Dislocation avalanches are like earthquakes on the micron scale.

Compression experiments on micron-scale specimens and acoustic emission (AE) measurements on bulk samples revealed that the dislocation motion resembles a stick-slip process - a series of unpredictable local strain bursts with a scale-free size distribution. Here we present a unique experimental set-up, which detects weak AE waves of dislocation slip during the compression of Zn micropillars. Profound correlation is observed between the energies of deformation events and the emitted AE signals that, as we conclude, are induced by the collective dissipative motion of dislocations. The AE data also reveal a two-level structure of plastic events, which otherwise appear as a single stress drop. Hence, our experiments and simulations unravel the missing relationship between the properties of acoustic signals and the corresponding local deformation events. We further show by statistical analyses that despite fundamental differences in deformation mechanism and involved length- and time-scales, dislocation avalanches and earthquakes are essentially alike.

The Effect of Different Thermal Treatment on the Allotropic fcc↔hcp Transformation and Compression Behavior of Polycrystalline Cobalt.

Pure polycrystalline cobalt is systematically thermally treated in order to assess the effect of the microstructure on the compression behavior. Isothermal annealing of the as-drawn material leads to recrystallization and grain growth dependent on the annealing temperature (600-1100 ∘C). Consequently, the yield strength decreases and the fracture strain increases as a function of rising grain size; the content of the residual fcc phase is ~6-11%. Subsequent thermal cycling around the transition temperature is applied to further modify the microstructure, especially in terms of the fcc phase content. With the increasing number of cycles, the grain size further increases and the fraction of the fcc phase significantly drops. At the same time, the values of both the yield strength and fracture strain somewhat decrease. An atypical decrease in the fracture strain as a function of grain size is explained in terms of decreasing fcc phase content; the stress-induced fcc→hcp transformation can accommodate a significant amount of plastic strain. Besides controlling basic material parameters (e.g., grain size and texture), adjusting the content of the fcc phase can thus provide an effective means of mechanical performance optimization with respect to particular applications.

Effect of Short Attritor-Milling of Magnesium Alloy Powder Prior to Spark Plasma Sintering.

The spark plasma sintering (SPS) technique was employed to prepare compacts from (i) gas-atomized and (ii) attritor-milled AE42 magnesium powder. Short attritor-milling was used mainly to disrupt the MgO shell covering the powder particles and, in turn, to enhance consolidation during sintering. Compacts prepared by SPS from the milled powder featured finer microstructures than compacts consolidated from gas-atomized powder (i.e., without milling), regardless of the sintering temperatures in the range of 400-550 °C. Furthermore, the grain growth associated with the increase in the sintering temperature in these samples was less pronounced than in the samples prepared from gas-atomized particles. Consequently, the mechanical properties were significantly enhanced in the material made of milled powder. Apart from grain refinement, the improvements in mechanical performance were attributed to the synergic effect of the irregular shape of the milled particles and better consolidation due to effectively disrupted MgO shells, thus suppressing the crack formation and propagation during loading. These results suggest that relatively short milling of magnesium alloy powder can be effectively used to achieve superior mechanical properties during consolidation by SPS even at relatively low temperatures.

Damage Characterization during Compression in a Perlite-Aluminum Syntactic Foam.

Aluminum matrix (Al99.5) syntactic foam containing expanded perlite particles was produced using the pressure infiltration technique. The dominant deformation mechanisms during compression of this foam were determined by sequential k-means analysis of the acoustic emission data. Since the different deformation mechanisms were concurrently active even at small strains, successive unloading and reloading measurement was proposed for cluster identification. The repetitive unloading and reloading allowed us to identify two mechanical parameters, namely the unloading modulus and the loss for unloading-reloading cycles. Based on the correlations among the strain localization within the specimen, the acoustic emission results, the changes in these mechanical parameters, and the transition from quasi-elastic deformation to plasticity were revealed in this material.

Mechanical and biocorrosive properties of magnesium-aluminum alloy scaffold for biomedical applications.

This study investigates the morphology, microstructure, compressive behavior, biocorrosion properties, and cytocompatibility of magnesium (Mg)-aluminum (Al) alloy (AE42) scaffolds for their potential use in biodegradable biomedical applications. Mg alloy scaffolds were successfully synthesized via a camphene-based freeze-casting process with precisely controlled heat treatment. The average porosity was approximately 52% and the median pore diameter was ∼13 µm. Salient deformation mechanisms were identified using acoustic emission (AE) signals and adaptive sequential k-means (ASK) analysis. Twinning, dislocation slip, strut bending, and collapse were dominant during compressive deformation. Nonetheless, the overall compressive behavior and deformation mechanisms were similar to those of bulk Mg based on ASK analysis. The corrosion potential of the Mg alloy scaffold (-1.44 V) was slightly higher than that of bulk AE42 (-1.60 V), but the corrosion rate of the Mg alloy scaffold was faster than that of bulk AE42 due to the enhanced surface area of the Mg alloy scaffold. As a result of cytocompatibility evaluation following ISO10993-5, the concentration of the Mg alloy scaffold extract reducing cell growth rate to 50% (IC50) was 10.7%, which is higher (less toxic) than 5%, suggesting no severe inflammation by implantation into muscle.

Acoustic emission analysis of the compressive deformation of iron foams and their biocompatibility study.

We synthesized Fe foams using water suspensions of micrometric Fe2O3 powder by reducing and sintering the sublimated Fe oxide green body to Fe under 5% H2/Ar gas. The resultant Fe foam showed aligned lamellar macropores replicating the ice dendrites. The compressive behavior and deformation mechanism of the synthesized Fe foam were studied using an acoustic emission (AE) method, with which we detected sudden localized structural changes in the Fe foam material. The evolution of the deformation mechanism was elucidated using the adaptive sequential k-means (ASK) algorithm; specifically, the plastic deformation of the cell struts was followed by localized cell collapse, which eventually led to fracturing of the cell walls. For potential biomedical applications, the corrosion and biocompatibility characteristics of the two synthesized Fe foams with different porosities (50% vs. 44%) were examined and compared. Despite its larger porosity, the superior corrosion behavior of the Fe foam with 50% porosity can be attributed to its larger pore size and smaller microscopic surface area. Based on the cytotoxicity tests for the extracts of the foams, the Fe foam with 44% porosity showed better cytocompatibility than that with 50% porosity.

Micron-Scale Deformation: A Coupled In Situ Study of Strain Bursts and Acoustic Emission.

Plastic deformation of micron-scale crystalline materials differs considerably from bulk samples as it is characterized by stochastic strain bursts. To obtain a detailed picture of the intermittent deformation phenomena, numerous micron-sized specimens must be fabricated and tested. An improved focused ion beam fabrication method is proposed to prepare non-tapered micropillars with excellent control over their shape. Moreover, the fabrication time is less compared with other methods. The in situ compression device developed in our laboratory allows high-accuracy sample positioning and force/displacement measurements with high data sampling rates. The collective avalanche-like motion of the dislocations is observed as stress decreases on the stress-strain curves. An acoustic emission (AE) technique was employed for the first time to study the deformation behavior of micropillars. The AE technique provides important additional in situ information about the underlying processes during plastic deformation and is especially sensitive to the collective avalanche-like motion of the dislocations observed as the stress decreases on the deformation curves.

The Effect of Matrix Composition on the Deformation and Failure Mechanisms in Metal Matrix Syntactic Foams during Compression.

The influence of the matrix material on the deformation and failure mechanisms in metal matrix syntactic foams was investigated in this study. Samples with commercially pure Al (Al) and Al-12 wt % Si (AlSi12) eutectic aluminum matrix, reinforced by hollow ceramic spheres, were compressed at room temperature. Concurrently, the acoustic emission response and the strain field development on the surface were monitored in-situ. The results indicate that the plastic deformation of the cell walls is the governing mechanism in the early stage of straining for both types of foams. At large stresses, deformation bands form both in the Al and AlSi12 foam. In Al foam, cell walls collapse in a large volume. In contrast, the AlSi12 foam is more brittle; therefore, the fracture of precipitates and the crushing of the matrix take place within a distinctive deformation band, along with an occurrence of a significant stress drop. The onset stress of ceramic sphere failure was shown to be not influenced by the matrix material. The in-situ methods provided complementary data which further support these results.