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The effects of different Gd additions on wear behavior of the T6 heat treated Mg-xGd-3Y-0.5Zr alloys were investigated. The wear tests were carried out using a Ball-on-flat type wear apparatus against an AISI 52100 type bearing steel ball counterface in the load range of 3-15 N, sliding speed range of 0.03-0.18 m/s, temperature range of 25-200 °C and at a constant sliding distance of 400 m. The results showed that the wear rate of the tested alloys increased with increasing sliding load. By increasing the wear temperature to 200 °C, the wear rate of the Mg-6Gd-3Y-0.5Zr alloy decreased by about 24%. At higher wear speeds, wear resistance of the alloys increased due to the formation of stable oxide layers on the worn surfaces. The alloy containing 12 wt% Gd exhibited higher wear resistance compared with the alloys containing lower Gd contents under the same conditions. [ABSTRACT FROM AUTHOR]

Current commercial magnesium extrusion alloys do not offer desirable combinations of strength, ductility, and extrusion speed for automotive structural applications. The effect of small additions of cerium (Ce) to pure magnesium (Mg) and Mg-3 pct Al alloy extruded tubes has been studied. The results suggest that 0.2 pct Ce addition can significantly improve the extrudability and mechanical properties of the Mg extrusions. The improvement in mechanical properties is due to grain refinement and dispersion strengthening provided by the MgCe particles and the beneficial texture obtained. Higher Ce contents further increase strength, but significantly reduce ductility and cause excessive surface oxidation during extrusion. The beneficial effect of 0.2 pct Ce on mechanical properties of pure Mg is not observed when it is added to Mg-3 pct Al alloy, due to the higher affinity of Ce to Al to form the AlCe phase in the Mg-Al-Ce ternary alloys. The Mg-0.2 pct Ce alloy is a promising base alloy for further development in automotive applications; however, Al should be avoided in Mg-Ce–based extrusion alloys. [ABSTRACT FROM AUTHOR]

The influence of Cl− and SO on the electrochemical behavior of AZ91D, AZCe2, and AZLa1 was studied. For all alloys, there was a current plateau in the anodic polarization curves in Na2SO4 solutions. In 0.5% NaCl solution, there was a small current plateau, whereas there was none in the 3.5% and 5% NaCl solutions. This indicated that SO is less aggressive than Cl−. The range of the current plateau decreased with increasing SO concentration. For all alloys, the high frequency capacitive loop in the Nyquist plots decreased with increasing concentration consistent with the decrease in corrosion resistance with increasing Cl− and SO concentration. [ABSTRACT FROM AUTHOR]

The grain-refinement effects of titanium (Ti) additions and a low-frequency electromagnetic casting (LFEC) process on the AZ31 magnesium alloy have been investigated. It is shown that Ti has no effect on the formation and distribution of secondary phases in the AZ31 alloy. The results suggests that the grain size decreases with an increasing cooling rate for the AZ31 alloy; it decreases first, however, then increases slightly for Ti-containing AZ31 alloys, indicating that the grain-refinement effect of Ti decreases with an increasing cooling rate. However, the grain size decreases first, and then increases with increasing Ti for a given cooling rate. The lowest grain size is obtained at the addition of about 0.01% Ti and the cooling rate of about 6K/s. The grain refinement effect is explained in terms of the growth restriction factor (GRF) due to the constitutional undercooling generated by the Ti solute elements, which produces grain refinement up to a critical GRF value corresponding to about 0.0 1% Ti addition. The yield strength of a conventional-cast AZ31 alloy can be improved by grain refinement and expressed as σy = 39.31 + 142.53 d-1/2 according to the Hall-Petch relationship. The combination of the Ti grain refiner and the LFEC process during direct-chill (DC) casting of an AZ31 alloy can produce high-quality magnesium alloy billets with uniform and equiaxed grains of less than 40 μm across the billet section up to at least a 100-mm diameter, which further improves the yield strength of the alloy, resulting in σy = 51.53 + 90.85 d-1/2. [ABSTRACT FROM AUTHOR]

The effect of strontium (Sr) on the microstructure, mechanical properties, and fracture behavior of AZ31 magnesium alloy and its sensitivity to cooling rate are investigated. Three phases--blocky shaped Mg17Al12, acicular Mg20Al20Mn5Sr, and insular Mg16 (Al, Zn)2Sr--are identified in the Sr-containing AZ31 alloys. With increasing cooling rate. the blocky-shaped Mg16Al12 phase increases, the acicular Mg20Al20Mn5Sr phase diminishes, and the insular Mg16 (Al, Zn)2 Sr phase is refined and granulated. The study suggests that the grain size decreases with increasing cooling rate for a given composition. However. the grain size decreases first, then increases, and finally decreases again with increasing Sr for a given cooling rate. The yield strength (σy) of AZ31 magnesium alloy can be improved by grain refinement and expressed as σy = 35.88 + 279.13d½ according to the Hall-Petch relationship. The elongation increases when Sr is added up to 0.01 pet and then decreases with increasing Sr addition. Grain refinement changes the fracture behavior from quasicleavage failure for the original AZ31 alloy to mixed features of quasicleavage and microvoid coalescence fracture. [ABSTRACT FROM AUTHOR]

The effects of Cerium (Ce) addition on microstructures and mechanical properties of hot-extruded AZ61 alloy were investigated. It is found that Ce refines Mg17Al12 particles and brings about precipitation of a new rod-like phase that is identified as Al4Ce by X-ray diffraction. During hot deformation, Al4Ce particles can refine dynamic recrystallized grains by impeding grain growth. For Ce additions up to 1.0 wt%, the recrystallized grain size decreases; When Ce addition is more than 1.0 wt%, grain size increases inversely. Strength and elongation of extruded or annealed specimens at room temperature increase along with Ce addition up to 1.0 wt%, then decrease. Optimal mechanical properties are correspondent to 1.0 wt% Ce addition. [ABSTRACT FROM AUTHOR]

The elevated temperature deformation behavior of permanent cast magnesium alloy AZ91 with and without Sb addition has been investigated using slow strain rate (5.0 × 10−4s−1) elevated temperature tensile and constant load creep testing at 150°C and 50 MPa. The alloy with 0.4 wt% Sb showed a higher elevated temperature tensile strength and creep resistance due to the formation of thermal stable Mg3Sb2 precipitates and a smaller microstructure as well as the suppressing of the discontinuous precipitation. Plastic deformation of AZ91 based alloys is determined by motion of dislocation in basal plane and non-basal slip systems. The dislocation motion in a slip system is influenced by temperature, precipitates and other lattice defects. Dislocations jog, grain boundaries and/or precipitates are considered as obstacles for moving dislocations. The $$(01\overline 1 2)$$ deformation twinning were founded in the creep process by TEM. Cross slip of dislocations was taken into account as the main softening mechanism for permanent cast AZ91 alloy during elevated temperature deformation process. [ABSTRACT FROM AUTHOR]

The effects of Ca addition on the microstructure and mechanical properties of AZ91 magnesium alloy have been studied. The results show that the Ca addition can refine the microstructure, reduce the quantity of Mg17Al12 phase, and form new Al2Ca phase in AZ91 magnesium alloy. With the Ca addition, the tensile strength and elongation of AZ91magnesium alloy at ambient temperature are reduced, whereas Ca addition confers elevated temperature strengthening on AZ91 magnesium alloy. The tensile strength at 150°C increases with increasing Ca content. The impact toughness of AZ91magnesium alloy increases, and then declines as the Ca content increases. The tensile and impact fractographs exhibit intergranular fracture features, Ca addition changes the pattern and quantity of tearing ridge, with radial or parallel tearing ridge increasing, tensile strength, elongation and impact toughness reduce. [ABSTRACT FROM AUTHOR]

Based on a ternary Mg75Ni15Gd10 metallic glass former, a new Mg80Ni12Gd4Nd4 bulk metallic glass composite (BMGC) was developed by tailoring the compositions of Mg and rare earth (RE) elements. This BMGC displayed compressive ultimate strength over 900 MPa with a total strain to failure of 4.3% and specific strength of 3.12 x 105 Nm/kg. The improved mechanical properties were attributed to a "dual phases" structure consisting of Mg solid solution flakes and glassy matrix in the Mg80Ni12Gd4Nd4 BMGC. The homogeneously dispersed Mg phases reinforcement in the BMGC were characterized as a long period ordered structure (LPOS) with periodic arrays of six close-packed planes distorted from the ideal hexagonal lattice of 6H-type. The LPOS-Mg in the composite can act as a soft media to trap or interact with the unstable shear bands and contribute to plastic strain. The present study may provide a guideline for designing the Mg-TM-RE-based (TM: transition metals) BMGCs with "dual phases" structures. [ABSTRACT FROM AUTHOR]