Di Tie

Development and evaluation of a magnesium–zinc–strontium alloy for biomedical applications — Alloy processing, microstructure, mechanical properties, and biodegradation

A new biodegradable magnesium-zinc-strontium (Mg-Zn-Sr) alloy was developed and studied for medical implant applications. This first study investigated the alloy processing (casting, rolling, and heat treatment), microstructures, mechanical properties, and degradation properties in simulated body fluid (SBF). Aging treatment of the ZSr41 alloy at 175 °C for 8h improved the mechanical properties when compared to those of the as-cast alloy. Specifically, the aged ZSr41 alloy had an ultimate tensile strength of 270 MPa, Vickers hardness of 71.5 HV, and elongation at failure of 12.8%. The mechanical properties of the ZSr41 alloy were superior as compared with those of pure magnesium and met the requirements for load-bearing medical implants. Furthermore, the immersion of the ZSr41 alloy in SBF showed a degradation mode that progressed cyclically, alternating between pitting and localized corrosion. The steady-state average degradation rate of the aged ZSr41 alloy in SBF was 0.96 g/(m(2)·hr), while the pH of SBF immersion solution increased. The corrosion current density of the ZSr41 alloy in SBF solution was 0.41 mA/mm(2), which was much lower than 1.67 mA/mm(2) for pure Mg under the same conditions. In summary, compared to pure Mg, the mechanical properties of the new ZSr41 alloy improved while the degradation rate decreased due to the addition of Zn and Sr alloying elements and specific processing conditions. The superior mechanical properties and corrosion resistance of the new ZSr41 alloy make it a promising alloy for next-generation implant applications.

An in vivo study on the metabolism and osteogenic activity of bioabsorbable Mg–1Sr alloy

UNLABELLED Previous studies indicated that local delivery of strontium effectively increased bone quality and formation around osseointegrating implants. Therefore, implant materials with long-lasting and controllable strontium release are avidly pursued. The central objective of the present study was to investigate the in vivo biocompatibility, metabolism and osteogenic activity of the bioabsorbable Mg-1Sr (wt.%, nominal composition) alloy for bone regeneration. The general corrosion rate of the alloy implant as a femoral fracture fixation device was 0.55±0.03mm·y(-1) (mean value±standard deviation) in New Zealand White rabbits which meet the bone implantation requirements and can be adjusted by material processing methods. All rabbits survived and the histological evaluation showed no abnormal physiology or diseases 16 weeks post-implantation. The degradation process of the alloy did not significantly alter 16 primary indexes of hematology, cardiac damage, inflammation, hepatic functions and metabolic process. Significant increases in peri-implant bone volume and direct bone-to-implant contact (48.3%±15.3% and 15.9%±5.6%, respectively) as well as the expressions of four osteogenesis related genes (runt-related transcription factor 2, alkaline phosphatase, osteocalcin, and collagen, type I, alpha 1) were observed after 16 weeks implantation for the Mg-1Sr group when compared to the pure Mg group. The sound osteogenic properties of the Mg-1Sr alloy by long-lasting and controllable Sr release suggesting a very attractive clinical potential. STATEMENT OF SIGNIFICANCE Sr (strontium) has exhibited pronounced effects to reduce the bone fracture risk in osteoporotic patients. Nonetheless, long-lasting local Sr release is hardly achieved by traditional methods like surface treatment. Therefore, a more efficient Sr local delivery platform is in high clinical demand. The stable and adjustable degradation process of Mg alloy makes it an ideal Sr delivery platform. We combine the well-known osteogenic properties of strontium with magnesium to manufacture bioabsorbable Mg-1Sr alloy with stable Sr release based on our previous studies. The in vitro and in vivo results both showed the alloys suitable degradation rate and biocompatibility, and the sound osteogenic properties and stimulation effect on bone formation suggest its very attractive clinical potential.

Microstructures, mechanical properties, and degradation behaviors of heat-treated Mg-Sr alloys as potential biodegradable implant materials

In previous studies, Mg-Sr alloys exhibited great biocompatibility with regard to test animals, and enhanced peri-implant bone formation. The objective of the present study was to investigate the effects of heat treatments on the mechanical and corrosion properties of Mg-Sr alloys. Various heat-treated Mg-xSr (x = 0.5, 1, and 2wt%, nominal composition) alloys were prepared using homogenization and aging treatments. Mechanical tests were performed at room temperature on the as-cast, homogenized, and peak-aged alloys. As the Sr content increased, the volume fraction of Mg17Sr2 phases within the as-cast alloys increased; in addition, the mechanical strength of the alloys initially increased and subsequently decreased, while the ductility decreased. Following the homogenization treatment, the mechanical strength of the alloys decreased, and the ductility increased. Nano-sized Mg17Sr2 phases were re-precipitated during the aging treatment. The age-hardening response at 160°C was enhanced as the Sr content increased. Following the aging treatment, there was an increase in the mechanical strength of the alloys; however, there was a slight reduction in the ductility. Immersion tests were conducted at 37°C for 360h, using Hanks buffered salt solution (HBSS), to study the degradation behavior of the alloys. As the Sr content of the Mg-Sr alloys increased, the corrosion rate (CR) increased owing to the galvanic effect. The homogenization treatment consequently reduced the CR dramatically, and the aging treatment had a slight effect on the CR. The peak-aged Mg-1Sr (wt%) alloy exhibited the best combination of properties. The tensile yield strength (TYS), ultimate tensile strength (UTS), elongation, compressive yield strength (CYS), ultimate compressive strength (UCS), compressibility, and CR of the as-cast Mg-1Sr (wt%) alloy were 56.0MPa, 92.67MPa, 1.27%, 171.4MPa, 243.6MPa, 22.3%, and 1.76mm/year, respectively. The respective results obtained for the peak-aged Mg-1Sr (wt%) alloys were 69.7MPa, 135.6MPa, 3.22%, 183.1MPa, 273.6MPa, 27.6%, and 1.33mm/year. Following immersion in HBSS, the primary corrosion products of the peak-aged Mg-1Sr (wt%) alloy were Mg(OH)2, MgO, MgCO3, Mg3(PO4)2, MgHPO4, and Mg(H2PO4)2, which enhanced the corrosion resistance by forming a composite corrosion film.

Effects of Technical Parameters of Semi-Solid Stirring and Rheo-Rolling on Microstructure of A356–5wt.%B4C Composite Strip

Aluminum boron carbide composite strip was prepared by semi-solid stirring and rheo-rolling, and the effects of process parameters on microstructure of A356–5wt.%B4C composite strip were studied. The results showed that the distribution of B4C particles in the matrix became homogeneous, and the average diameter of α-Al and its roundness decreased with the increase of the stirring speed and the stirring time as well as the decrease of the stirring temperature. The average diameter of the primary α-Al grain increased with the increase of the roll speed. When the stirring speed was 500 rpm, the stirring time was 20 min, the stirring temperature was 853 K (580 °C), and the roll speed was 0.2 m/s, the microstructure of A356–5wt.%B4C composite strip was mainly composed of spherical or rosette α-Al grains, and the distribution of B4C particles was homogeneous. The hardness of A356–5wt.%B4C composite strip was 98 HV, and the ultimate tensile strength and elongation were 186 MPa and 1.8%, respectively. The hardness was improved by 25% comparing with casting alloy A356. The ultimate tensile strength and elongation were improved, respectively, by 28.3% and 5.9%.

Optimisation of composition and cast temperature for continuous semisolid extruded Al–Sc–Zr electrical conductor

Abstract Three aluminium alloy conductive wires, Al–0·16Sc (wt-%), Al–0·16Zr (wt-%) and Al–0·12Sc–0·04Zr (wt-%) were designed and produced by continuous rheo-extrusion process. The influences of composition and pouring temperature during cast process on the microstructure, mechanical and conductive properties of the wires were studied. Results show that compared with the other two alloys, both mechanical and conductive properties of Al–0·12Sc–0·04Zr alloy after aging treatment was significantly higher: the ultimate tensile strength is 160 MPa and the conductive rate is 66·99% IACS. The optimised pouring temperature during the cast process was 690°C, and the most favourable mechanical and conductive properties were achieved at this temperature owing to the alloy’s smallest grain size.

Shear Model of Metal Melt Flowing on Vibration Wall and Effect of Shear Stress on Solidification Microstructure

In this work, the shear model of metal melt flowing on vibration surface is established, and coupling effects of vibration and shear on the distribution of shear stress in melt and melt solidification microstructure are analyzed. Calculation results show that the transition of melt from laminar flow to turbulent flow occurs earlier with increasing vibration frequency and vibration amplitude. In the laminar flow melt, shear stress in melt decreases with increasing vertical length, but it decreases firstly and then stabilizes with increasing flow length. In the turbulent flow melt, shear stress decreases firstly and then stabilizes with increasing vertical length, but it increases with increasing flow length. With the increase in vibration frequency and amplitude, shear stress along flow direction in laminar flow melt increases, while shear stresses along both flow direction and vertical direction in turbulent flow melt increase. Shear stress in melt decreases with increasing length along vertical direction. With the increase in flow length, shear stress decreases firstly and then stabilizes in laminar flow melt, while it increases in turbulent flow melt. With the increase in vibration frequency and amplitude, shear stress increases in laminar flow melt, while it stabilizes in turbulent flow melt. Based on theoretical calculation, the maximum shear stress in melt during vibration shear flow is always much lower than the yield strength of α-Al grain, so the shear stress induced by vibration shear flow cannot break columnar crystal, which agrees with the experiment result. So, the model can explain the shear constitutive relation of melt flow on vibration surface relatively well.

A Novel Process for Grain Refining and Semisolid Processing

A novel process for grain refining and manufacture of high quality semisolid slurries ofalloys was developed. The process was proven to refine metal grain remarkably, and the grain sizeof pure aluminum can be refined to the first grade of Chinese refining standard of pure aluminumGB/T 7946.4-1999 ref) . P rimary silicon and eutectic silicon in the hypereutectic Al-Si alloy can bealso effectively refined. This process was used to prepare the billets with small spherical grains orequiaxed grains of Al-6Si-2Mg, AZ91, AZ31 and hypereutectic Al-Si alloys successfully. The p ip es ,profi le s and wire s of 6201, AZ31 and AZ61 alloys were produced by continuous rheo-extrusion.The strips of AZ91, AZ31, and Mg-Sn alloys were prepared by rheo-rolling. As an innovativeprocessing technology with low cost and high efficiency, vibrating sloping plate melt treatment hasgood prospective application in many fields such as rheo-casting, rheo-extrusion, rheo-rolling,metal microstructure refinement, etc.