Papers by Author: Ren Long Xin

Abstract: The high strength of Mg-Y-Nd alloy has been achieved primarily by precipitation hardening. Therefore, it is important to investigate the influence of various precipitate phases on the tensile properties of Mg-Y-Nd alloys. In this study, an extruded Mg-Y-Nd alloy was aged at various temperatures to examine the hardening behaviors. The results showed that the as-extruded alloy exhibited remarkable age hardening response at 210°C due to the precipitation of β’, and slight hardening response at 150°C and 280°C due to the precipitation of β’’ and β, respectively. Furthermore, different precipitates exerted different effects on the tensile properties. In comparison with the as-extruded alloy, the yield strength of the alloys aged at 210 °C and 150 °C was increased by 21 MPa and 8 MPa, respectively, whereas the yield strength of the alloy aged at 280°C was decreased by 30 MPa. The elongation of the alloy aged at 210°C and 150°C was also largely reduced by 3.4% and 2.9%, respectively, while the elongation of the alloy aged at 280°C was only slightly reduced (6.3%). Moreover, compared with the as-extruded alloy, the alloy aged at 210°C and 150°C exhibited lower hardening capacity and higher strain hardening rate at the initial stage, but the strain hardening rate decreased more quickly with the increasing stress. The alloy aged at 280°C exhibited similar strain hardening behavior with the as-extruded alloy. The results in this study provide guidelines for determining the heat treatment parameters for the Mg-Y-Nd alloys to improve their tensile properties.

Abstract: In this study, a well-textured AZ31 Mg alloy sheet was friction stir (FS) processed, and the microstructure and texture evolution in various regions of the processed alloy were examined by optical microscopy (OM) and electron back scatter diffraction (EBSD). The results showed that the grain size in the FS zone was significantly refined compared to that in the base material (BM). The average grain size in the thermomechanically affected zone (TMAZ) and heat-affected zone (HAZ) was comparable with that in the BM. There is a gradual change of local texture from BM to FS zone due to plastic flow together with heating input during the FS processing. The <0002> direction was roughly parallel to the cylindrical pin surface normal of the FS zone. The <0002> direction in the HAZ is similar to that in the BM, but more diffuse. The <0002> direction in the TMAZ was tilted 25~30o away from the ND and there is a distinct boundary between the FS zone and TMAZ at the advancing side which introduced an incompatibility between the FS zone and TMAZ. This might explain the fact that the transverse FS processed Mg alloys commonly fracture at the advancing side during tensile tests.

Abstract: Magnesium alloys are considered as candidate materials for biodegradable implants. However, the key issue is that they corrode too fast in physiological environment. The aim of this study is to investigate the effect of microstructure and texture of magnesium alloys on their corrosion resistance. Magnesium alloy AZ31 extruded rod, hot rolled sheet and extruded sheet with different initial microstructure and texture were prepared. Then they were immersed in conventional simulated body fluid (SBF) for several days for corrosion evaluation. The corrosion products and precipitates on their surfaces were examined by scanning electron microscopy (SEM). The preliminary results showed that the initial microstructure and texture of AZ31 alloys has considerable effect on the weight loss rate of the alloys, suggesting that it is possible to enhance the corrosion resistance of AZ31 alloys through tailoring the microstructure and texture of the alloys.

Abstract: Nanocrystalline Zn-substituted calcium hydroxyapatite (HA) powder was synthesized by wet chemical method. Detailed characterization was carried out with both experimental techniques and numerical simulation method. X-ray diffraction (XRD) patterns show the calcium phosphate maintains as the apatite phase when the atomic ratio of Zn/(Zn+Ca) is less than 17% Zn in aqueous solutions. The calcium phosphate crystallinility decreases with the Zn concentration increase. The morphological changes with Zn substitution in HA were investigated by TEM. Lattice parameters of the apatitic samples were determined by XRD Rietveld refinement method. A computational study using ab initio generalized gradient approximation density functional theory was performed on Zn-substituted HA. Comparison of the experimental and computer simulation results provides our insights of Zn substitution in apatite structure.

Abstract: This study reports a novel method for the preparation of several biologically important calcium phosphate (Ca-P) phases such as hydroxyapatite (HA), dicalcium phosphate dihydrate (DCPD) and dicalcium phosphate anhydrous (DCPA). X-ray diffraction (XRD) results showed that phase pure DCPD, DCPA and HA nano-crystals could be produced in the Ca2+/PO4 3- solutions with the presence of EDTA at 120 oC, 180 oC and 210 oC, respectively. Transmission electron microscope (TEM) micrographs revealed that all the Ca-P precipitates were needle-like or rod-like. Most of the precipitates ranged from 100 ~ 200 nm in length. Selected area electron diffraction confirmed that the longitude direction of the rod-like HA precipitates were along c-axis and the flat surface was (110). Thermal gravimetric analysis of the DCPD precipitates revealed that phase transformations of DCPD to DCPA and DCPA to HA occurred at 139 oC and 195 oC, respectively, which resulted in the different Ca-P phases during hydrothermal synthesis at different temperature ranges.

Abstract: Porous alpha- tricalcium phosphate (α-TCP) was implanted in rabbit and dog models to investigate its induction of calcium phosphate (Ca-P) formation. The morphology and structure of the formed Ca-Ps were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The results showed an animal-dependent behavior of in vivo Ca-P formation. There were flake-like octacalcium phosphate (OCP) precipitates in the rabbit but rod-like hydroxyapatite (HA) precipitates in the dog model. In addition, high-resolution TEM (HRTEM) image revealed that there was OCP structure in certain portions of the rod-like HA precipitates in the dog model. And the orientation relation of OCP/HA was deduced as OCP (010)//HA (0 10) and OCP (001)//HA (00 1) . This implied that the in vivo HA was formed via OCP precursor phase.

Abstract: We have directly observed the transformation from octacalcium phosphate (OCP) to hydroxyapatite (HA) in transmission electron microscope (TEM). The phase transformation was induced by electron beam irradiation in TEM. Several TEM techniques were employed to examine the crystal structure change, including bright field images, electron diffraction, high resolution microscopy (HRTEM) and fast Fourier transformation pattern of HRTEM images. The examinations indicate possible hydrolysis reaction in solid state transformation and crystallographic orientation of OCP (010)//HA (010) and OCP (001)//HA (001) which has not been reported previously.

Abstract: Bioactive calcium phosphate (Ca-P) formation in bioceramics surfaces in simulated body fluid (SBF) and in rabbit muscle sites was investigated. The examined bioceamics included most commonly used bioglass®, A-W glass-ceramics and calcium phosphates in orthopedic and dental applications. The Ca-P cyrstal structures were examined with single crystal diffraction patterns in transmission electron microscopy, which reduced possibility of misidentifying Ca-P phases. The experimental results show that capability of Ca-P formation considerably varied among bioceramics, particularly in vivo. Octacalcium phosphate (OCP) was revealed on the all types of bioceramics in vitro and in vivo experiments. This work leads us to rethink how to evaluate bioactivity of bioceramics and other orthopedic materials which exhibit capability of osteoconduction by forming direct bonding with bone.