Materials Science Forum Vols. 783-786

Abstract: Single crystals of Magnesium alloys such as Mg-1Zn-0.5Y, Mg-0.1Y, and Mg-0.1Ce alloys were fabricated in this study by employing the modified Bridgman method. To determine the exact orientation of crystals, pole figure measurement using X-ray diffraction was carried out on each single crystal. Hardness and compression tests were conducted followed by subsequent recrystallization annealing. Recrystallization behavior of Mg alloy single crystals has also been investigated. Fabricated single crystals were deformed in compression mode by 30% reduction. Annealing treatment has been conducted on these samples at temperatures of 300°C for various times from 1 to 20 mins. The microstructure observation and hardness measurement conducted on the recrystallized specimens revealed that static recrystallization of ternary alloy single crystal was very slow, while recrystallization behavior of binary alloy single crystals appeared to be very fast.

Abstract: In order to develop a high strength and heat-resistant magnesium alloy, we focused on controlling microstructure of Mg₉₆Zn₂Y₂ (at %) casting alloy by the addition of a 4th element. Initially, we investigated the effects of zirconium addition and cooling rate for grain refinement on microstructure and mechanical properties. Consequently, Mg_95.8Zn₂Y₂Zr_0.2 casting alloy contains fine equiaxed grains (approx. 0.01 mm), and it exhibits tensile and fatigue properties equivalent to or higher than those of commercial aluminum alloys at high temperature above 473 K. At 523 K, this alloy exhibited a tensile strength of 223 MPa nearly twice that of A4032-T6 alloy used in typical automotive pistons. The Mg_95.8Zn₂Y₂Zr_0.2 casting alloy also reveals sufficient ductility and good castability, characteristics not common in current heat-resistant magnesium alloys. Next, we focused on controlling microstructure of Mg₉₆Zn₂Y₂ casting alloy by the addition of Ag. Mg₉₆Zn₂Y₂ cast alloy is composed of alpha-Mg phase, long-period stacking ordered phase and Mg₃Zn₃Y₂ phase; on the other hand, Mg-Zn-Y-Ag cast alloy had 4th phase by an addition of Ag. A substantial increase in yield strength at room temperature, without grain refining, was the result.

Abstract: Commercial MgAlZn alloy AZ31 was processed by two techniques of severe plastic deformation, namely equal channel angular pressing (ECAP) and high pressure torsion (HPT). Microstructure evolution with strain due to ECAP and HPT was investigated by light and transmission electron microscopy (TEM). Significant grain refinement was observed in specimens processed both by ECAP and HPT. Moreover, HPT resulted in radial strain and microstructure inhomogeneity across the diameter of the sample disk. This inhomogeneity was continuously smeared out and almost homogeneous ultra-fine grained structure was observed in specimen subjected to 15 HPT rotations. Dislocation structure changes in individual specimens after different number of ECAP passes and HPT rotations were investigated by positron annihilation spectroscopy (PAS). Sharp increase of dislocation density occurred during the first two passes of ECAP, followed by the saturation and even a decline manifesting the dynamic recovery at higher strains.

Abstract: In this study, the effect of plane strain on the microstructure and texture of AZ31 sheet was investigated with various reduction ratios. To achieve plane strain, that is, to prevent shear deformation during warm-rolling, AZ31 sheet was capsuled by pure Cu tube. The large strain induced by simple one-pass warm-rolling led to the significant grain refinement and texture randomization. The two-dimensional finite-element method showed that the warm-rolling of AZ31 sheet capsuled by Cu tube imposed a uniform plane strain in the whole sheet thickness. The high reduction ratio caused the dynamic recrystallization during annealing of the rolled sheet.

Abstract: The characteristics of AZ91 containing Sn alloy were investigated to understand the compressive behavior with the microstructure of the alloy in elevated temperature state. To change the microstructure, the alloys used for this study were fabricated by the permanent mould casting and cooling slope casting, respectively. The compression test of AZ91 containing Sn alloys was carried out at temperatures from R.T to 400°C and at strain rates from 1.67x10^-5 to 1.67x10^-3/s. Besides, in order to investigate the compressive characteristics of the alloys in the elevated temperature state, this alloys after compression test were performed by optical (OM) and results show that the compressive deformation behavior of AZ91 containing Sn alloy was affected by strain rate and deformation temperature as well as its microstructure under hot forming conditions. In the processing windows for the soundness evolution of the specimen in the compression process, the Mg alloy with a dendrite phase (by permanent mold casting) was narrower than that of the Mg alloy with a globular and fine phase (by cooling slope method).

Abstract: In this study, the effect of CaO as alloying element on Mg-Zn-Y and Mg-Y-Zn alloys were investigated to develop wrought magnesium alloys available for aeronautic components. The evaluation for the effect of CaO addition in these alloy systems was performed by the processing map development, the tensile test, and the ignition temperature measurement. The addition of CaO changed the hot workability and improved the ignition resistance of both Mg-Zn-Y and Mg-Y-Zn alloys. Although the mechanical properties of CaO added Mg-Zn-Y alloy were lower than those of Mg-Zn-Y alloy, the CaO addition in Mg-Y-Zn alloy enhanced the ductility of the alloy. The effect of metallic Ca addition in these alloy systems were investigated to be compared with the CaO addition.

Abstract: Magnesium-based alloys are promising for various biomedical applications due to their advantageous mechanical and biological properties. In this study, we investigated the potential use of magnesium-based alloys for urological device applications, e.g. a biodegradable and antibacterial ureteral stent. Previous studies showed that magnesium-based samples significantly inhibited bacterial growth and colony formation in artificial urine (AU) solution as compared with the polyurethane-based stent. This current study focuses on long-term magnesium-based sample degradation in AU solution and deionized water. We studied the effects of alloy composition (magnesium alloyed with yttrium or aluminum and zinc) and surface condition (oxide versus metallic surface) on the rate of degradation. Sample degradation was measured by the change in sample mass, pH of immersion solution, and magnesium ion concentration in the solution. Results showed that both alloy composition and surface condition affected the rate of degradation in the AU solution. For instance, magnesium-yttrium alloy degraded the fastest and the presence of the oxide layer increased its degradation rate in the AU solution. The overall degradation rate in the AU solution was in this order (fastest to slowest degrading): MgY_O > MgY > Mg_O > AZ31. Further investigation is necessary to determine the efficacy and safety of magnesium-based biodegradable stents for urological applications.

Abstract: Magnesium alloys containing Rare Earth elements have proven to be suitable candidates for uses at high temperatures due to their good creep resistance as well as for use in biodegradable implants due to their adequate corrosion rate and biocompatibility. This work investigates the fatigue strength and cyclic deformation behavior of an extruded Mg10Gd1Nd in comparison to Mg10Gd and possible benchmark alloys WE43 and AZ31. The influence of the alloying element Nd is remarkable. The finite life fatigue strengths of Mg10Gd1Nd in the SN-diagram (Wöhler curve) are strongly improved compared to Mg10Gd and almost reach the strength values of WE43. Fracture surface morphology and crack propagation are discussed with attention given to low and high cycle fatigue. The very fine grain size, as the result of dynamic recrystallization during extrusion, offers high elongation at fracture. Therefore the residual fracture surface, where rapid failure occurs, is rather small in the high cycle fatigue samples. The size of the slow crack growth area has been determined by the appearance of benchmark ridges and fatigue striations and is discussed in correlation to stress and number of cycles. Scatter behavior of fatigue life was investigated by optical microcopy. The microstructure consists of second phase alignments in the extrusion direction, which differs in length, precipitate size and distance. Crack branching appears depending on microstructure and the load applied.

Abstract: Currently, long period stacking/ordered phases (LPSO phases) are known to reinforceMg97Y2Zn1 type Mg-RE alloys. The LPSO phases are composed of a solid solution of Y and Znatoms placed orderly in long periods along the Mg basal plane. Also, an efficient way to strengthena polycrystalline material is to reduce its grain size. This increases the density of grain boundarieswhich impede the flow of dislocations. In many of the LPSO forming solidification processed Mg-RE alloys, the common practice is to solutionize the ingot, quench in warm water, hot extrude andthermally age. While this practice is suitable for obtaining high strength Mg-RE alloys, itconveniently employs the common idea in conventional metallurgy of fine intermetallicstrengthening while refining the grain size to within the micron regime. In this work, an alternativemethod involving boride nanoparticle addition to obtain a selected solidification processed ultrahighstrength (tensile yield strength > 400 MPa) Mg-RE alloy is discussed. Here, LPSO phaserather than fine intermetallic formation while retaining grain size under the micron regime ishighlighted.

Abstract: Cosma R&D investigated a low temperature warm forming process by which a magnesium ZEK 100 door inner part with a single-stage draw depth of 144 mm was successfully formed. The warm forming process is comprised of three steps: 1) heating pre-lubricated blanks in an oven at temperatures ranging from 215°C to 260 °C, 2) robotic transfer of the heated blank to a mechanical stamping press, 3) forming of the panel in room temperature stamping die at speed of about 160 mm/s. The effect of process parameters on the formability of the part, as well as, the post-forming properties including the mechanical properties, microstructure evolution and deformation thinning are also presented. The result indicates that Magnesium ZEK 100 exhibits superior low temperature warm formability over Magnesium AZ31B, and the developed warm forming process is promising and potential for volume production of magnesium automotive parts.