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To apply magnesium in biodegradable bone fixation devices, it is favorable to improve its properties through approaches such as by forming composites between magnesium and calcium phosphate. In this study, a powder metallurgical method was used to prepare composites of a magnesium–calcium alloy and α-tricalcium phosphate (α-TCP). Refinement of the α-TCP particle size caused more uniform distribution and microstructure. Consequently, the composite with finer α-TCP particle size had better ductility to the alloy lacking α-TCP. The biodegradation properties were improved by the addition of α-TCP, and the refinement of the α-TCP particles led to a slower degradation rate in electrochemical and immersion tests.

The present study was designed to investigate the biodegradation behavior of Mg alloy plates in the maxillofacial region. For in vitro analysis, the plates were immersed in saline solution and simulated body fluid. For in vivo, the plates were implanted into the tibia, head, back, abdominal cavity, and femur and assessed at 1, 2, and 4 weeks after implantation. After implantation, the plate volumes and the formed insoluble salt were measured via micro-computed tomography. SEM/EDX analysis of the insoluble salt and histological analysis of the surrounding tissues were performed. The volume loss of plates in the in vitro groups was higher than that in the in vivo groups. The volume loss was fastest in the abdomen, followed by the head, back, tibia, and femur. There were no statistically significant differences in the insoluble salt volume of the all implanted sites. The corrosion of the Mg alloy will be affected to the surrounding tissue responses. The material for the plate should be selected based on the characteristic that Mg alloys are decomposed relatively easily in the maxillofacial region.

In recent years, magnesium is expected to be used as a structural material in vehicles. Since limited slip systems in magnesium induces marked yield anisotropy and poor deformability at room temperature, there is a specific demand for developing magnesium alloys with reduced plastic anisotropy. In this study, we focused on the Mg–Al system alloy, which is widely used in commerce, and examined the third element contributing to the reduction of the plastic anisotropy. Generalized stacking fault energy (GSFE), which corresponds to the energy for sliding atomic layer along a slip plane, was calculated in the first-principles calculations to estimate deformability of ternary Mg–Al-X alloys adding the third element X, to the Mg–Al binary alloy. As the third element X, Zn, Zr, and Gd were selected. The slip properties were evaluated using the unstable stacking fault energy (USFE), which represents the maximum value of GSFE. The result suggested that the addition of zirconium contributes most to the reduction of plastic anisotropy.

Addition of Al to equiatomic high entropy alloy CrMnFeCoNi alloy with a single phase FCC structure is known to promote formation of a BCC phase. In addition, tetragonal intermetallic phase σ was also found to form in a (CrMnFeCoNi)92Al8 alloy when subjected to processing by caliber rolling (8 passes) at room temperature followed by annealing at 800 ∘C. Microstructure of the as-rolled alloy is complex, containing a high dislocation density and planar faults (including twins) which can be parallel and perpendicular to the rolling direction. Morphological features of the transformations induced by straining by caliber rolling have been determined by transmission electron microscopy (TEM). Two main morphologies have been determined. In one type of morphology, bundles of elongated domains of B2 ordered BCC phase form preferentially on {111} matrix planes, inside which σ phase particles nucleate on interface with the matrix. In another type of morphology called “aggregate” here, “spines” of elongated grains of σ phase form, over which domains of B2 phase nucleate driven by change in solute concentration ahead of the interface. The σ phase forms by nucleation on planar faults in the matrix with its (001) plane, forming the known orientation relationship (OR) {001}σ ∥ {111}FCC and 〈110〉σ ∥ 〈110〉FCC. Since the planar faults in a caliber rolled alloy form parallel as well as perpendicular to the rolling direction, spines of the σ phase elongated in three mutually perpendicular directions are shown to exist. OR of the B2 phase with the matrix is Kurdjumov-Sachs. The B2 phase matched pairs of close packed planes with σ phase, but no particular or was found. Elemental concentration profiles across interfaces suggests that σ and B2 phases grow into the matrix. All the evidence suggests that the σ phase nucleates on the planar faults in the matrix, and then B2 phase forms by elemental concentration changes in the matrix caused by growth of the σ phase, forming the aggregate morphology.

An equiatomic CrMnFeCoNi alloy is usually a single-phase solid-solution with a face-centered cubic (fcc) structure. However, a Cr-rich σ phase with a tetragonal structure is formed in coarse-grained alloys after extended heat treatment between 873 K and 1073 K, which is indicative of metastability of the CrMnFeCoNi alloy. In this study, it was demonstrated that cold-working facilitated σ-phase formation during heat treatment at 1073 K in (CrMnFeCoNi)100−xAlx (x = 8 and 9 at%). More specifically, the heat treatment produced a unique σ–body-centered cubic (bcc) aggregate morphology in the unrecrystallized fcc matrix. Each aggregate was composed of σ-phase spine and bcc particles at the fcc/σ interface, and formed in a short period (0.5 h). It is suggested that aggregates precipitated on dislocations that were introduced by cold-working.

Equal channel angular extrusion (ECAE) was applied to an extruded Mg–1.0mass%Ca alloy to achieve a high strength and high damping capacity. Microstructure observations revealed that the texture and local misorientation distribution depend on the ECAE temperature. ECAE at a low temperature of 260 °C resulted in a high average Schmid factor of basal slip and a high kernel average misorientation in the processed alloy. Consequently, the alloy processed by ECAE showed a high yield strength without sacrificing the damping capacity.

Deformation structures have been examined by transmission electron microscopy after 4% plastic deformation in tension and compression in a Mg-2.2 at%Y alloy extruded at 698 K to obtain a recrystallized grain size of about 20 μ m . The extruded alloy showed similar yield stresses of about 240 MPa in both tension and compression at room temperature. Dislocations formed by tensile stress showed a tendency of 〈 a 〉 type dislocations to cross slip to prismatic planes, resulting in long continuous dislocation loops. Dislocation loops with 〈 c 〉 components were also observed. Occasionally, { 10 1 ¯ 2 } type twinning was also observed in grains with large orientation away from basal texture. These grains contained predominantly basal slip. Compression deformed samples showed a limited number of { 10 1 ¯ 2 } type twins. The matrix contained loops of non-basal 〈 a 〉 type dislocations, together with basal dislocations with 〈 c 〉 component. Inside the twins occurred stacking faults and loops of 〈 c 〉 type dislocations with segments perpendicular to the basal plane. Activation of several deformation modes and complex dislocation structures explains the strain hardening behavior and low anisotropy of the alloy.

Metallic medical devices are typically constructed from non-bioabsorbable metals that remains in the body and causes considerable complications. Particularly in the urinary tract, calculus, intractable infection, and misdiagnosis as calculus are often caused by non-bioabsorbable metals. Here, we developed a zinc–magnesium alloy as a new bioabsorbable metal and sought to evaluate the bioabsorbable behavior of zinc and zinc–magnesium alloy in a rat bladder implantation model. We prepared zinc–magnesium alloy wires with various proportions of magnesium and investigated the strength, shape retention, formability, and absorbability of these novel materials. Then, we implanted zinc and zinc–magnesium alloy rings formed by the wires into rat bladder. Rats were euthanized at the end of the observation period, and the rings were removed for volume evaluation. Extracted bladder tissues were subjected to histological analysis. The strength of the zinc wire was enhanced by more than fourfold upon the addition of magnesium, without loss of ductility. Linear reduction of ring volume in urine was observed based on the concentration of magnesium within the ring. Nearly all rings were covered with a thin layer of calculus. Histological findings of the transected urinary bladder tissues did not differ among groups. Zinc–magnesium alloy is a promising candidate for use as a bioabsorbable medical device in the urinary tract.

An equiatomic high entropy CoCrFeMnNi alloy has been processed by caliber rolling and its microstructure studied by transmission electron microscopy (TEM). The cast dendritic structure and compositional inhomogeneity was reduced to a nanoscale microstructure. Domains as thin as 50 nm elongated in the rolling direction were formed after 19 rolling passes. Boundaries on {111} plane perpendicular to the rolling direction were also observed. FCC twins were observed in the cross-section of the rolled rods, about 50 nm in thickness. A stack of twins and other planar faults result in a periodicity of three times the interplanar spacing of the {111} twinning plane to about 6.2 A. Lattice imaging shows interference fringes and diffraction effects which can be attributed to domain formation at a very fine scale. These give rise to diffraction streaks and additional spots. Extra diffraction spots and associated diffuse intensity also point to an underlying lattice not fully ordered. The nanoscale domain boundaries are often on {111} planes which are not simple stacking faults but complex in structure.

Recently, Mg and its alloys have attracted much attention because of their excellent biocompatibility and biodegradability. High anisotropy of Mg crystal structure, however, limits the movement of some slip systems; therefore, pure magnesium possesses poor ductility and/or toughness. A number of studies have revealed that alloying with solute elements and modification of grain structure improved these drawbacks. In this study, to clarify the effect of adding solute elements, e.g., calcium and zinc, impact toughness testing and first-principles calculations of generalized stacking fault energy and grain boundary cohesive energy were conducted. For example, alloying magnesium with calcium and zinc and controlling the microstructure produced a Mg alloy with a high compressive fracture strain of 0.40, which was greater than the estimated maximum strain for fastening a surgical clip. This high fracture strain arose from the enhanced grain boundary cohesive energy and reduced anisotropy of slip systems by solute segregation. As a result, the alloy successfully occluded blood vessels.

Recently, biodegradable bone fixation devices have been demanded when considering the patient’s quality of life (QOL). During the fracture healing, the devices must support the repeated load due to daily performance. At the same time, surface of the magnesium devices was affected by body fluid. Thus in this research, in vitro fatigue properties of biodegradable Mg–0.3at.%Ca alloy was evaluated by using simulated body fluid. Though there was fatigue limit when the test was conducted under the ambient condition, it cannot be confirmed during the test in the simulated body fluid. Inspection of fracture surface revealed that crack propagated along the grain boundary after both the fatigue tests.

The mechanical properties of magnesium alloys under dynamic loading have not been explored in sufficient depth. This research aims to estimate the impact fracture toughness of AZ31 magnesium alloy by finite element (FE) analysis. An FE model of impact three-point bending with three elastic bars is developed. FE analysis is presented for AZ31 magnesium alloy and compared with the results for 7075-T6 aluminum alloy as a reference. The FE analysis showed that the plastic deformation region is wider and the crack propagating rate is lower in the AZ31 specimen than in the 7075-T6 specimen. Moreover, the energy absorption capability and impact fracture toughness of AZ31 alloy were found to be higher than those of 7075-T6 alloy.

Transmission electron microscopic study of crack propagation in a fine-grained Mg–Zn alloy shows a complex interplay of slip and twinning, leading to formation of twin-related nano-sized domains of 50 nm size related by {1 0 1} and {1 0 2} twin orientations. Subgrain formation with low angle boundaries is followed by formation of {1 0 1} twins and twin multiplication by nucleation and growth. Basal and prismatic slip play an important role in the growth and formation of new domains of twins and boundaries. These basal and prismatic slip occur in the matrix as well as inside the {1 0 1} twins. Steps on these twin boundaries indicate interaction with slip, leading to their growth. Evidence of nucleation of twins on grain boundaries is found. Most interesting is the indication of nucleation of {1 0 1}-type twins. String or stacks of nano-sized domains related to each other by {1 0 1} twinning were found in the matrix, where the matrix contained basal, prismatic as well as pyramidal dislocations. It app...

The friction and wear properties of pure magnesium and the Mg–Y alloy were investigated using the pin-on-disk configuration. The friction and wear resistance of the Mg–Y alloy was superior to those of pure magnesium. The wear mechanism was abrasion under all the conditions. The deformed microstructural evolutions near the surface region were observed by transmission electron microscopy and electron backscatter diffraction. The stress and strain states were also evaluated by finite element analysis (FEA). The deformed microstructures of both alloys consisted of the {10–12} twinning formation and the FEA results showed the occurrence of plastic deformation even at the beginning of the test. The formation of low angle grain boundaries was also confirmed with an increase in the applied load in the Mg–Y alloy. On the other hand, grain refinement due to dynamic recrystallization was observed in pure magnesium as the wear test progressed. The different microstructures resulted from difference in the surface temperature during the wear test, which was estimated to be around 393 K and 363 K for pure magnesium and the Mg–Y alloy, respectively. The high increment temperature in the fine-grained alloys brought about the occurrence of grain boundary sliding, i.e., material softening, which led to a decrease in the friction and wear properties. The present results indicated that one of the methods for enhancing the friction and wear properties is to increase the dynamic recrystallization temperature.

Biodegradable magnesium (Mg) alloys are the most promising candidates for osteosynthesis devices. However, their in vivo corrosion behaviour has not been fully elucidated. The aim of this study was to clarify the influence of the physiological environment surrounding Mg alloys on their corrosion behaviour. A Mg-1.0Al alloy with a fine-grained structure was formed into plates using titanium (Ti) as a control. These plates were implanted into the subperiosteum in the head, subcutaneous tissue of the back, and in the muscle of the femur of rats for 1, 2 and 4 weeks. The volumes of the remaining Mg alloy and of the insoluble salt deposition and gas cavities around the Mg alloy were determined by microtomography, and the volume losses were calculated. Then, the tissue response around the plates in each implantation site was examined histopathologically, and its relation to the respective volume loss was analyzed. These analyses determined that the Mg alloy was corroded fastest in the head, at an intermediate level in the back, and slowest in the femur. The insoluble salt deposition at the Mg alloy surface had no influence on the volume loss. Gas cavities formed around the Mg alloy at all implantation sites and decreased after 4 weeks. Histopathological examination revealed that the Mg alloy exhibited good biocompatibility, as was seen with Ti. In addition, vascularized fibrous capsules formed around the plates and became mature with time. Notably, the volume loss in the different anatomical locations correlated with capsule thickness. Together, our results suggest that, to facilitate the successful clinical application of Mg alloys, it will be necessary to further comprehend their interactions with specific in vivo environments.

We report on a quantitative investigation into the effect of size and distribution of rod-shaped \beta' precipitates on strength and ductility of a Mg-Zn alloy. Despite precipitation strengthening being crucial for the practical application of magnesium alloys this study represents the first systematic examination of the effect of controlled deformation on the precipitate size distribution and the resulting strength and ductility of a magnesium alloy. Pre-ageing deformation was used to obtain various distributions of rod-shaped \beta' precipitates through heterogeneous nucleation. Alloys were extruded to obtain a texture so as to avoid formation of twins and thus to ensure that dislocations were the primary nucleation site. Pre-ageing strain refined precipitate length and diameter, with average length reduced from 440 nm to 60 nm and diameter from 14 nm to 9 nm. Interparticle spacings were measured from micrographs and indicated some inhomogeneity in the precipitate distribution. The yield stress of the alloy increased from 273 MPa to 309 MPa. The yield stress increased linearly as a function of reciprocal interparticle spacing, but at a lower rate than predicted for Orowan strengthening. Pre-ageing deformation also resulted in a significant loss of ductility (from 17% to 6% elongation). Both true strain at failure and uniform elongation showed a linear relationship with particle spacing, in agreement with models for the accumulation of dislocations around non-deforming obstacles. Samples subjected to 3% pre-ageing deformation showed a substantially increased ageing response compared to non-deformed material; however, additional deformation (to 5% strain) resulted in only modest changes in precipitate distribution and mechanical properties., Comment: Author-accepted manuscript. 16 pages, 11 figures

An ultrafine-grained (UFG) ZK60 Mg alloy with an average grain size of ∼1.0 μm was processed by extrusion at relatively low temperature (488 K) with a high area reduction ratio (∼25). The mechanical behavior of the UFG Mg alloy is investigated over strain rates spanning nearly eight decades (10−4–104 s−1). The stress–strain responses in the quasi-static (∼10−4 s−1) and high rate (104 s−1) regimes exhibit the characteristic sigmoidal profile that is a signature of { 1 0 1 ¯ 2 } 〈 1 0 1 ¯ 1 ¯ 〉 extension twinning. Further, this sigmoidal profile is accentuated at high rates, suggesting a rate effect of twinning induced hardening. X-ray diffraction (XRD) and analysis of the as-received and deformed microstructures indicate the occurrence of twinning even at the quasi-static rates of loading. This observation is contrary to some of the theoretical predictions that suggest suppression of twinning in Mg below critical grain sizes much larger than in the present work. From the XRD analysis we infer that the twin volume fraction increases with increasing applied strain rate. Transmission electron microscopy observations of the tested specimens reveal high density non-basal dislocations that may result from the activation of these slip systems following twinning-induced lattice reorientation.

A process to obtain high strength in a Mg–Zn–Y alloy containing quasicrystalline phase is described. The process involves solutionizing at a high temperature, precipitation of the quasicrystal phase during extrusion, followed by ageing. Tensile yield strengths of over 350 MPa are obtained with grain sizes of 14–20 μm.

Very fine grain sizes up to ∼1 μm with yield strengths up to about 400 MPa in tension and compression, with elongations ranging from 12% to 18%, have been achieved by chill casting and direct extrusion of Mg93Zn6Y alloy containing a quasicrystalline phase. These alloys respond to ageing at 150 °C, showing two peaks. The hardness stabilizes at the level of the second peak. The effect of precipitation on the yield strength, ductility and yield asymmetry has been studied.

The ductile fracture mechanism in a fine-grained magnesium alloy has been investigated by transmission electron microscopy-focused ion beam techniques. In coarse-grained or conventional magnesium alloys, twins form at the very beginning of the deformation process, and crack propagation occurs through the twin boundaries. However, in the alloy used in this study, subgrain structures were found instead of twins at the crack tip. Nanoscale twins formed subsequently owing to large stress in the crack propagation route. The fine-grained alloys showed high fracture toughness resulting from resistance to the twins at the beginning of the deformation.

The Effect of the solidification conditions and subsequent extrusion of a Mg-3.0Zn-0.5Y (at. %) alloy containing quasicrystalline icosahedral (i-) phase was studied. Solidification was carried out by three methods using a chill casting mold, a conventional steel mold and a water-cooled mold. Subsequently, castings were extruded in the temperature range of 235–270 ∘ C at an extrusion ratio of 25:1. The solidification molds showed different characteristics. The water-cooled mold was most effective in cooling through the walls, but least effective at the center of the mold. The conventional cast mold was the most effective in cooling at the mold center. All the castings had an interdendritic eutectic structure of the i-phase, and a supersaturation of the matrix in zinc. As a result, all the extrusions had similar grain size close to 1 μ m and very fine nano-size precipitation. Yield strengths in tension were in the range of 376 and 404 MPa, and from 300 to 330 MPa in compression. All elongations to fracture were about 13%. It is concluded that supersaturation of the matrix during solidification is the main factor, resulting in the dynamic precipitation of very fine precipitates and fine grain size during extrusion.

Superplastic behavior was investigated using an extruded Mg-Zn-Y alloy with the dispersion of the quasicrystal phase particle in fine-grained matrix. Tensile tests showed that the low temperature superplasticity was behaved at a temperature of 473 K and maximum elongation was 462 % at 573 K in 1  10-5 s-1. The deformed microstructure observation showed that the dominant deformation process was grain boundary sliding. The present alloy also demonstrated a high possibility for secondary forming, such as superplastic forge forming. Furthermore, the forged alloy had a homogeneous microstructures, no mechanical anisotropy and uniform micro-hardness properties in any portion of a forged product.

Grain refinement of an Mg–3Al–1Zn–0.2Mn (wt.%) alloy was conducted by severe plastic working with caliber rolling. The alloy’s yield strength was increased to more than 400 MPa, which is twice that of an initially extruded alloy, and its yield asymmetry in compression against tension was reduced by the weakened basal texture caused by the shear deformation.

The texture and tensile properties of a fine-grained Mg–Al–Zn alloy sheet at room temperature prior to and following superplastic deformation were examined. The pre-existing basal texture of the sheet drastically weakened after superplastic deformation, and thus the geometric conditions for basal slip, characterized by the Schmid factor distributions, became more favorable. Consequently, relatively high ductility was maintained after superplastic deformation in spite of cavity formation.

An Mg-Zn-Y alloy with a dispersion of quasicrystal phase particles showed low-temperature superplastic behavior, and the dominant deformation process was grain boundary sliding. Observations of the deformed microstructure showed that the quasicrystal phase particles were an obstacle to dislocation movements; the kinetics of the superplastic behavior were lower than those of a conventional magnesium alloy. This alloy also demonstrated a high possibility for secondary forming - i.e., superplastic forging.

Quasicrystal and related phase particles were successfully dispersed in wrought-processed Mg–7.6 wt.% Zn–3.9 wt.% Al alloy with homogeneous and fine-grained structures. This rare-earth-free alloy showed high strength and ductility balance, which was superior to that of the well-known quasicrystal phase dispersed in Mg–Zn–RE alloys, with a weak texture and a reduction of mechanical asymmetry in yield strength.

Ultrasonic vibration was introduced into the solidification of AZ91 alloy. Various microstructures were produced in this alloy using ultrasonic vibrations at different temperatures of the melt. The coarse dendrite microstructures were obtained with ultrasonic vibrations at temperatures below the liquidus temperature. The fine uniform grains were achieved under ultrasonic vibrations during the nucleation stage, which was mainly attributed to the cavitation and the acoustic flow induced by the ultrasonic vibration.

Four kinds of extruded Mg–X at.% Zn binary alloys (X = 1.9, 2.4, 3.0, and 3.4) were used to examine the effect of precipitate volume fraction on fracture toughness. All the alloys had fine grain sizes of 1–3 μm and fine sphere-shaped precipitates of 50–60 nm. The volume fraction of precipitates increased with additional zinc content. The results of mechanical property tests showed that the extruded Mg–2.4 at.% Zn alloy exhibited the best balance of strength and fracture toughness. One of the reasons was the different volume fraction of precipitates at the grain boundaries, which was the source of void formation. According to the fracture surface observations and ductile fracture model analysis, the volume fraction of precipitates of 2%–4% was shown to be enough to improve the fracture toughness for the fine-grained magnesium alloys; i.e., higher contents of zinc atoms were not needed to enhance the mechanical properties.

An AZ91 alloy pretreated by ultrasonic vibration (UV) was subjected to extrusion to refine the microstructure. The results indicated that the UV pretreatment had a strong influence on the microstructure evolution and mechanical properties of the extruded AZ91 alloy. With UV pretreatment, a fine and uniform microstructure with an average grain size of 13.5 mm was obtained, and the fraction of the fiber-like, partiallyrecrystallized, structure was markedly reduced. In addition to the superior mechanical properties such as a yield strength of 256 MPa, an ultimate tensile strength of 320 MPa and a fracture elongation of 9.7% were achieved in the extruded AZ91 alloy with UV pretreatment. [doi:10.2320/matertrans.MC200741]

Cavities were formed in the random boundaries of superplastic magnesium alloys due to higher boundary energy. The cavity growth rate of the fine-grained alloy (dominant diffusion process: grain boundary diffusion) was lower than that of the coarse-grained alloy (lattice diffusion) during superplastic flow.

Fracture toughness and its deformed structures were investigated on a Mg–3Al–1Zn (AZ31) alloy processed by direct extrusion. The average grain sizes of the alloy after extrusion at temperatures of 473 and 573 K were 4.0 and 14.5 μm, respectively (i.e., fine-grained and coarse-grained alloys). The plane-strain fracture toughness,KIC, as determined by stretched zone analysis in fine-grained and coarse-grained alloys was estimated to be 24.9 and 22.7 MPam1/2, respectively. Microstructural observations of the fine-grained alloy after fracture toughness testing showed that non-basal slip and grain-boundary sliding was activated even at room temperature. In addition, the {10-12} deformation twins were observed despite the fine-grained structures. However, the fraction of deformation twins was reduced with grain refinement, which related to the enhancement of fracture toughness with grain refinement.

A bulk nanocrystalline Al–5 at.% Fe alloy was synthesized by mechanical alloying and spark plasma sintering. The alloy exhibited a very high compressive yield strength of 1 GPa with a plastic strain of 0.3. The alloy consists of coarse α-Al grains that form from powder boundaries and nanocrystalline regions composed of α-Al and Al6Fe phases. The combination of the coarse and nanoscale grains are considered to be the reason for the large plastic strain in such a high-strength material.

Two alloys, Mg96.9Zn2.7Ho0.4 and Mg93Zn6Ho, containing a quasicrystal phase were extruded at various temperatures to produce different grain sizes and tested at room temperature in tension and compression. For larger grains the strength of the Mg93Zn6Ho alloy was almost twice that of the Mg96.9Zn2.7Ho0.4 alloy. Yield stresses approached 300 MPa as the grain sizes decreased to 1 μm. In the case of the Mg93Zn6Ho alloy, at grain sizes less than about 2 μm, yield stresses in compression become higher than in tension.

The mechanical properties were investigated by Mg–0.3 at.% Ca–1.8 at.% Zn alloy, which was produced by hot extrusion, having an average grain size of about 1 μm and spherical precipitates in the matrix. The extruded alloy showed a good balance of the yield strength (σys = 291 MPa) and plane-strain fracture toughness (KIC = 28.3 MPa m1/2), obtained by the stretched zone analysis, which were higher than those of conventional wrought magnesium alloys. The microstructure control of both the grain refinement and the dispersion of precipitates in the matrix was a possible method for improvement of the mechanical properties in magnesium alloys.

A commercial Mg–6 wt% Zn–0.5 wt% Zr (ZK60) alloy was used to investigate the effect of precipitate shapes on fracture toughness. The ZK60 alloy was extruded at a temperature of 653 K (extruded alloy). The extruded alloy was annealed at 633 K for 86.4 ks (annealed alloy), and then the annealed alloy was aged at 448 K for 100 ks (aged alloy). The average grain size in all the conditions was about the same, 13.5 ± 1.0 μm. The extruded and aged alloys had different shaped precipitates: spherical and rod shaped precipitates, respectively. The plane-strain fracture toughness KIC of the extruded, annealed, and aged alloys were estimated to be 22.4, 20.2, and 21.0 MPam1/2, respectively, by the stretched zone analysis. Transmission electron microscopy (TEM) observations showed that the deformation during the fracture toughness test was dominated by a dislocations on the basal slip planes in all the conditions. Such dislocations are commonly activated in magnesium alloys during the tensile and compression tests. The spherical shaped precipitates were found to be more effective than the rod shaped precipitates for improving the fracture toughness in the magnesium alloy.

Quinary Zr-based alloy compositions with improved glass forming criteria have been sought and the glass forming ability (GFA), thermal stability and mechanical properties of these alloys have been investigated. Monolithic amorphous structure has been confirmed for all compositions in 5 mm rods prepared by a Cu-mold casting method. They also show large plastic strain maximum of about 12% under uniaxial compression test with yield stress of about 2000 MPa. The compressive plasticity of the cast rods was found to be influenced by the casting temperature to a great extent.

Fracture toughness was examined on a commercial Mg-Zn-Zr alloy, ZK60. The commercial alloy was extruded at a temperature of 493 K to obtain fine grain structures having fine spherical shaped precipitates. The microstructures consisted of equi-axed grains. The average grain size and the precipitate diameter were about 3 μm and 25 ∼ 50 nm, respectively. The yield strength and elongation-to-failure were 287 MPa and 26.7%, respectively. The plane-strain fracture toughness, K 1c , was estimated to be 34.8MPam 1/2 by the stretched zone analysis. These mechanical properties were superior to that of conventional wrought magnesium and magnesium alloys. The deformed microstructure observations showed i) the activation of non-basal dislocations even at room temperature and ii) the pinning of dislocations by the spherical shaped precipitates during the fracture toughness test. Thus, a combination of grain refinement and dispersion of fine spherical shaped precipitates were found to be effective methods for improving the fracture toughness of magnesium alloys.

To develop a biodegradable clip, the equivalent plastic strain distribution during occlusion was evaluated by the finite element analysis (FEA) using the material data of pure Mg. Since the FEA suggested that a maximum plastic strain of 0.40 is required to allow the Mg clips, the alloying of magnesium with essential elements and the control of microstructure by hot extrusion and annealing were conducted. Mechanical characterization revealed that the Mg–Zn–Ca alloy obtained by double extrusion followed by annealing at 673 K for 2 h possessed a fracture strain over 0.40. The biocompatibility of the alloy was confirmed here by investigating its degradation behavior and the response of extraperitoneal tissue around the Mg–Zn–Ca alloy. Small gas cavity due to degradation was observed following implantation of the developed Mg–Zn–Ca clip by in vivo micro-CT. Histological analysis, minimal observed inflammation, and an only small decrease in the volume of the implanted Mg–Zn–Ca clip confirmed its excellent biocompatibility. FEA using the material data for ductile Mg–Zn–Ca also showed that the clip could occlude the simulated vessel without fracture. In addition, the Mg–Zn–Ca alloy clip successfully occluded the renal vein. Microstructural observations using electron backscattering diffraction confirmed that dynamic recovery occurred during the later stage of plastic deformation of the ductile Mg–Zn–Ca alloy. These results suggest that the developed Mg–Zn–Ca alloy is a suitable material for biodegradable clips. Statement of significance Since conventional magnesium alloys have not exhibited significant ductility for applying the occlusion of vessels, the alloying of magnesium with essential elements and the control of microstructure by hot extrusion and annealing were conducted. Mechanical characterization revealed that the Mg–Zn–Ca alloy obtained by double extrusion followed by annealing at 673 K for 2 h possessed a fracture strain over 0.40. The biocompatibility of the alloy was confirmed by investigating its degradation behavior and the response of extraperitoneal tissue around the Mg–Zn–Ca alloy. Finite element analysis using the material data for the ductile Mg–Zn–Ca alloy also showed that the clip could occlude the simulated vessel without fracture. In addition, the Mg–Zn–Ca alloy clip successfully occluded the renal vein. Microstructural observations using electron backscattering diffraction confirmed that dynamic recovery occurred during the later stage of plastic deformation of the ductile Mg–Zn–Ca alloy.

Improving mechanical properties of magnesium and understanding fracture behavior under impact loading are necessary to apply magnesium alloys to structural components of automobiles. We have investigated the fracture behavior of binary magnesium alloys by three-point bending experiment and conducted a first principle calculation to estimate the effect of solute segregation on fracture energy. In this paper, we have focused on experimental result of impact three-point bending test for Mg-0.3at.%Y alloy and the results of the test were compared with that of AZ31 commercially available alloy [1]. As a result, the crack propagation speed of Mg-0.3at.%Y was found to be slower than that of AZ31 alloys. Moreover, the absorbed energy of Mg-0.3at.%Y was more than twice as high as that of AZ31 alloys. These results suggested that yttrium solute in magnesium improved the fracture toughness of magnesium under impact loading. Then, fracture surface was observed by SEM to consider the effect of microstructure on crack propagation speed.

For applying a magnesium alloy to nails in the guided bone regeneration (GBR) method, sufficient strength and appropriate degradation speed are required to fix a membrane. In the case, the nails are exposed to body fluid after penetrating the alveolar bone. Therefore, in this research, degradation behavior after penetration of magnesium-calcium alloy which is expected to possess high biocompatibility was investigated. As a result, the Mg-Ca alloy nails were degraded inhomogeneously after immersion for 4 weeks in simulated body fluid. The degraded portions corresponded to the distribution of residual strain estimated by finite element analysis. Mg-Ca nails without precipitates possessed comparatively gradual degradation rate. It can be also confirmed that the region with residual strain degraded preferentially when compared the CT images and the residual strain distribution after penetration.

Weight reduction of automobiles and aircrafts improves fuel economy and reduces greenhouse gas emissions. Use of Mg alloys may allow weight reduction because of their low densities, but adoption is hindered because they exhibit limited ductility at ambient temperatures [1]. In a previous study of fracture toughness in a Mg alloy, crack readily propagated near twin boundaries and resulted in poor durability [2]. It has been shown that pile-up of dislocations at the interface between the matrix and deformation twins caused stress concentration to form cracks [2]. Another study suggested that the ductility of Mg alloys is further limited under dynamic loading due to lowered activity of dislocations [3]. It has also been reported that a Mg-Al-Mn alloy had pronounced mechanical anisotropy at high strain rates of around 1.0 ×103 s-1 [4]. Therefore, the mechanical properties of Mg alloys should be evaluated accurately for applications involving possible dynamic loading.

Fracture toughness and deformation structures have been investigated using an AZ31 magnesium alloy processed by equal-channel-angular extrusion (ECAE). The ECAE-processed alloy (as-ECAE) was annealed at 573 K for 24 h (annealed-ECAE). The average grain sizes of as-ECAE and annealed-ECAE alloys were 4.0 and 16.3 µm, respectively. The plane-strain fracture toughness K IC, obtained by stretched-zone analysis in as-ECAE and annealed-ECAE, were estimated to be 27.3 and 23.5 MPa/m1/2, respectively. From optical microstructural observations in samples after the fracture-toughness tests, deformation twins were observed in annealed-ECAE. No deformation twins were observed in as-ECAE. In addition, dislocations on basal planes, as well as on non-basal planes, were activated in as-ECAE. It is concluded that the enhancement of the fracture toughness in the fine-grain structure was related to a reduction of deformation twins and dislocation movement in non-basal planes.

Compressive behavior of a closed-cell aluminum has been investigated at high strain rates, utilizing AZ31 magnesium alloy bars as compared to maraging steel bars, to estimate the validity of the mechanical response of the foam. Apparent strain rate sensitivity of plateau stress has been observed and strain hardening occurred during compression at the dynamic strain rate.

The fracture toughness in Mg–Al–Zn alloy was improved by the equal-channel-angular extrusion process. The improvement was the result of a large plastic zone size, which was sensitive to elongation-to-failure and a high strain hardening exponent, created ahead of the fracture pre-crack, because of the modified distribution of the basal texture.

Examination of fracture toughness has been performed a commercial Mg-Zn-Zr alloy, ZK60, with fine strengthening particles. The commercial alloy was extruded at 633 K, and then heat treated at specific conditions. The microstructures were equi-axed grains, and average grain size and precipitate size were 11.6 μm and 50-150 nm, respectively. The yield strength and elongation-to-failure were 225 MPa and 17.0%. The plane-strain fracture toughness, K IC , was estimated to be 20.6 MPam 1/2 in stretched zone analysis. From ductile fracture model, all the finer particles did not affect the void formation. The precipitates having a large diameter, more than 100 nm, were supposed to be the origin of void formation.

Fully dense bulk nanocrystalline Fe–0.8wt.%C alloy was synthesized by spark plasma sintering of mechanically milled Fe–C nanocrystalline powder. The sample sintered at 600 °C was composed of 150 nm ferrite grains with nanocrystalline cementite dispersoids, whose compression yield strength, fracture strength, and plastic strain were 1900 MPa, 3500 MPa, and 40%, respectively.

The precipitation process of an Mg–0.3Ca–0.3Zn (in at.%) alloy has been investigated by transmission electron microscopy and the chemical compositions of the precipitates have been characterized by a three-dimensional atom probe. The reason for the pronounced precipitation hardening in the alloy compared to the Mg–0.3Ca alloy is discussed.