Kink-band formation in the directionally-solidified Mg/LPSO two-phase alloys.

The variation in the mechanical properties with the volume fraction of the long-period stacking ordered (LPSO) phase in directionally solidified (DS) Mg/LPSO two-phase alloys was examined. Unexpectedly, the yield stress of the DS alloys increases non-monotonically with an increase in the volume fraction of the LPSO phase. The LPSO phase is considered an effective strengthening phase in Mg alloys, when the stress is applied parallel to the growth direction. Nevertheless, the highest strength was obtained in alloys with 61-86 vol.% of the LPSO phase, which was considerably higher than that in the LPSO single-phase alloy. It was clarified that this complicated variation in the yield stress was generated from the change in the formation stress of kink bands, which varied with the thickness of the LPSO-phase grains. Furthermore, the coexistence of Mg in the LPSO phase alloy induced the homogeneous formation of kink bands in the alloys, leading to the enhancement of the 'kink-band strengthening'. The results demonstrated that microstructural control is significantly important in Mg/LPSO two-phase alloys, in which both phases exhibit strong plastic anisotropy, to realize the maximum mechanical properties.

Ultrahigh specific strength in a magnesium alloy strengthened by spinodal decomposition.

Strengthening of magnesium (Mg) is known to occur through dislocation accumulation, grain refinement, deformation twinning, and texture control or dislocation pinning by solute atoms or nano-sized precipitates. These modes generate yield strengths comparable to other engineering alloys such as certain grades of aluminum but below that of high-strength aluminum and titanium alloys and steels. Here, we report a spinodal strengthened ultralightweight Mg alloy with specific yield strengths surpassing almost every other engineering alloy. We provide compelling morphological, chemical, structural, and thermodynamic evidence for the spinodal decomposition and show that the lattice mismatch at the diffuse transition region between the spinodal zones and matrix is the dominating factor for enhancing yield strength in this class of alloy.

Quantitative ultrasound (QUS) axial transmission method reflects anisotropy in micro-arrangement of apatite crystallites in human long bones: A study with 3-MHz-frequency ultrasound.

Anisotropic arrangement of apatite crystallites, i.e., preferential orientation of the apatite c-axis, is known to be an important bone quality parameter that governs the mechanical properties. However, noninvasive evaluation of apatite orientation has not been achieved to date. The present paper reports the potential of quantitative ultrasound (QUS) for noninvasive evaluation of the degree of apatite orientation in human bone for the first time. A novel QUS instrument for implementation of the axial transmission (AT) method is developed, so as to achieve precise measurement of the speed of sound (SOS) in the cortex (cSOS) of human long bone. The advantages of our QUS instrument are the following: (i) it is equipped with a cortical bone surface-morphology detection system to correct the ultrasound transmission distance, which should be necessary for AT measurement of long bone covered by soft tissue of non-uniform thickness; and (ii) ultrasound with a relatively high frequency of 3 MHz is employed, enabling thickness-independent cSOS measurement even for the thin cortex by preventing guide wave generation. The reliability of the proposed AT measurement system is confirmed through comparison with the well-established direct transmission (DT) method. The cSOS in human long bone is found to exhibit considerable direction-dependent anisotropy; the axial cSOS (3870 ±â€¯66 m/s) is the highest, followed by the tangential (3411 ±â€¯94 m/s) and radial (3320 ±â€¯85 m/s) cSOSs. The degree of apatite orientation exhibits the same order, despite the unchanged bone mineral density. Multiple regression analysis reveals that the cSOS of human long bone strongly reflects the apatite orientation. The cSOS determined by the AT method is positively correlated with that determined by the DT method and sensitively reflects the apatite orientation variation, indicating the validity of the AT instrument developed in this study. Our instrument will be beneficial for noninvasive evaluation of the material integrity of the human long-bone cortex, as determined by apatite c-axis orientation along the axial direction.

Chiral squaramide derivatives are excellent hydrogen bond donor catalysts.

Thioureas represent the dominant platform for hydrogen bond promoted asymmetric catalysts. A large number of reactions, reported in scores of publications, have been successfully promoted by chiral thioureas. The present paper reports the use of squaramides as a highly effective new scaffold for the development of chiral hydrogen bond donor catalysts. Squaramide catalysts are very simple to prepare. The (-)-cinchonine modified squaramide (5), easily prepared through a two-step process from methyl squarate, was shown to be an effective catalyst, even at catalyst loadings as low as 0.1 mol%, for the conjugate addition reactions of 1,3-dicarbonyl compounds to beta-nitrostyrenes. The addition products were obtained in high yields and excellent enantioselectivities.

Unusual dynamic precipitation softening induced by dislocation glide in biomedical beta-titanium alloys.

Softening of metallic materials containing precipitates during cyclic deformation occurs through dissolution of the precipitates, because the to-and-fro motion of the dislocation causes dissolution of the precipitate particles by cutting them. Here, however, we found the completely opposite phenomenon for the first time; a "dynamic precipitation softening" phenomenon. In a Ti-35Nb-10Ta-5Zr body-centered cubic structured ß-Ti alloy single crystal developed for biomedical implant, the to-and-fro motion of the dislocation "induced" the selective precipitation of the ω-phase whose c-axis is parallel to the Burgers vector of the moving dislocation, which led to the significant cyclic softening of the crystal. The formation of the ω-phase is generally believed to induce significant hardening of ß-Ti alloys. However, the present results suggest that this is not always true, and control of the anisotropic features of the ω-phase via control of crystal orientation can induce unusual mechanical properties in ß-Ti alloys. The unique anisotropic mechanical properties obtained by the cyclic-deformation-induced oriented ω-phase formation could be useful for the development of "single-crystalline ß-Ti implant materials" with advanced mechanical performance.

Outstanding compressive creep strength in Cr/Ir-codoped (Mo0.85Nb0.15)Si2 crystals with the unique cross-lamellar microstructure.

A (Mo0.85Nb0.15)Si2 crystal with an oriented, lamellar, C40/C11b two-phase microstructure is a promising ultrahigh-temperature (UHT) structural material, but its low room-temperature fracture toughness and low high-temperature strength prevent its practical application. As a possibility to overcome these problems, we first found a development of unique "cross-lamellar microstructure", by the cooping of Cr and Ir. The cross-lamellar microstructure consists of a rod-like C11b-phase grains that extend along a direction perpendicular to the lamellar interface in addition to the C40/C11b fine lamellae. In this study, the effectiveness of the cross-lamellar microstructure for improving the high-temperature creep deformation property, being the most essential for UHT materials, was examined by using the oriented crystals. The creep rate significantly reduced along a loading orientation parallel to the lamellar interface. Furthermore, the degradation in creep strength for other loading orientation that is not parallel to the lamellar interface, which has been a serious problem up to now, was also suppressed. The results demonstrated that the simultaneous improvement of high-temperature creep strength and room temperature fracture toughness can be first accomplished by the development of unique cross-lamellar microstructure, which opens a potential avenue for the development of novel UHT materials as alternatives to existing Ni-based superalloys.

Isotropic plasticity of ß-type Ti-29Nb-13Ta-4.6Zr alloy single crystals for the development of single crystalline ß-Ti implants.

ß-type Ti-29Nb-13Ta-4.6Zr alloy is a promising novel material for biomedical applications. We have proposed a 'single crystalline ß-Ti implant' as new hard tissue replacements for suppressing the stress shielding by achieving a drastic reduction in the Young's modulus. To develop this, the orientation dependence of the plastic deformation behavior of the Ti-29Nb-13Ta-4.6Zr single crystal was first clarified. Dislocation slip with a Burgers vector parallel to <111> was the predominant deformation mode in the wide loading orientation. The orientation dependence of the yield stress due to <111> dislocations was small, in contrast to other ß-Ti alloys. In addition, {332} twin was found to be operative at the loading orientation around [001]. The asymmetric features of the {332} twin formation depending on the loading orientation could be roughly anticipated by their Schmid factors. However, the critical resolved shear stress for the {332} twins appeared to show orientation dependence. The simultaneous operation of <111> slip and {332} twin were found to be the origin of the good mechanical properties with excellent strength and ductility. It was clarified that the Ti-29Nb-13Ta-4.6Zr alloy single crystal shows the "plastically almost-isotropic and elastically highly-anisotropic" nature, that is desirable for the development of 'single crystalline ß-Ti implant'.

Degradation behavior of Ca-Mg-Zn intermetallic compounds for use as biodegradable implant materials.

With the goal of developing new biodegradable implant materials, we have investigated the degradation behavior of (Ca, Mg)-based intermetallic compounds. The degradation behavior of the compounds within the Ca-Mg-Zn system was roughly classified into four groups, and their behaviors were strongly influenced by the compositions of the compounds. For example, the Ca3MgxZn(15-x) compound exhibited a large solubility region with varying the Mg/Zn ratio, and the Ca3Mg12Zn3 phase alloy with the lowest Zn content was rapidly broken apart within 6h of immersion. Alternatively, the Ca3Mg4.6Zn10.4 phase alloy with the highest Zn content retained the bulk shape even after 250 h of immersion. These varying degradation behaviors were ascribed to the difference in the formability of Zn oxide as a protective layer against corrosion on the specimen surfaces, depending on the Zn content. The gained results suggest that there is a feasibility on developing new biodegradable materials based on intermetallic compounds in which the degradation rate can be controlled by their compositions.

Possibility of Mg- and Ca-based intermetallic compounds as new biodegradable implant materials.

Mg- or Ca-based intermetallic compounds of Mg2Ca, Mg2Si, Ca2Si and CaMgSi are investigated as possible new candidates for biodegradable implant materials, attempting to improve the degradation behavior compared to Mg and Ca alloys. The reactivity of Ca can be indeed reduced by the formation of compounds with Mg and Si, but its reactivity is still high for applications as an implant material. In contrast, Mg2Si shows a higher corrosion resistance than conventional Mg alloys while retaining biodegradability. In cytotoxicity tests under the severe condition conducted in this study, both pure Mg and Mg2Si showed relatively high cytotoxicity on preosteoblast MC3T3-E1. However, the cell viability cultured in the Mg2Si extract medium was confirmed to be better than that in a pure Mg extract medium in all the conditions investigated with the exception of the 10% extract medium, because of the lower corrosion rate of Mg2Si. The cytotoxicity derived from the Si ion was not significantly detected in the Mg2Si extract medium in the concentration level of ~70 mg/l measured in the present study. For aiming the practical application of Mg2Si as an implant material, however, its brittle nature must be improved.

Diastereoselective and enantioselective Mukaiyama aldol reactions of alpha-ketoesters using hydrogen bond catalysis.

Hydrogen bond catalyzed Mukaiyama aldol reactions of alpha-ketoesters proceed in high diastereo- and enantioselectivities, giving products possessing two chiral centers, of which one is a tertiary alcohol.

Novel 4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)methylbenzofuran derivatives as selective alpha(2C)-adrenergic receptor antagonists.

The synthesis of a series of 4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)methyl-2-arylbenzofuran and 4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)methylbenzofuran-2-carboxamide derivatives as novel alpha(2C)-adrenergic receptor antagonists are described. Their affinity at three different human alpha(2)-adrenergic receptors is reported, and some of these compounds exhibited high affinity for the alpha(2C)-adrenergic receptor with high subtype selectivity. Among them, compound 10e has been found to show the anti-L-dopa-induced dyskinetic activity in marmosets. The structure-activity relationship of these compounds is also discussed.