Materials Science Forum Vols. 747-748

Abstract: The hot deformation behaviors and microstructural evolution of Mg-3.0Nd-0.2Zn-0.4Zr (wt. %, NZ30K) alloy were investigated by means of the isothermal hot compression tests at temperatures of 350-500 °C with strain rates of 0.001, 0.01, 0.1 and 1s^-1. The results showed that the flow stress increased to a peak and then decreased which showed a dynamic flow softening. The flow stress behavior was described by the hyperbolic sine constitutive equation with an average activation energy of 193.8 kJ/mol. The average size of dynamically recrystallized grains of hot deformed NZ30K alloy was reduced by increasing the strain rate and/or decreasing the deformation temperature. A large amount of fine particles precipitated in the grains interior and at the grain boundaries when heated to the compression temperatures and soaked for 5min below 450 °C. However, the volume fraction of particles decreased significantly when soaked for 5 min at 500 °C, and the coarse particles precipitated mainly at the grain boundaries. Hot deformation at the temperature of 500 °C around and at the strain rate range of 0.1s^-1 was desirable for NZ30K alloy.

Abstract: Carbon fibers reinforced AZ31 composite laminates were fabricated by vibration aided mechanical hot pressing. Optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to analyze their phase components and microstructure. The interfacial bonding strength was measured by uniaxial tension testing. The results showed that Mg-Zn-Al eutectic alloy solder had a strong bonding with carbon fibers and a noticeable diffusion with AZ31 alloy. No debonding was observed on the interface of carbon fibers, and no carbide was detected in this system. The bonding strength of composite laminate reached up to ~17.48MPa which was close to the bonding strength of AZ31/Al composite laminate. It was indicated that vibration assisting hot pressing was a feasible technique for the fabrication of Mg-based composite laminates reinforced with uncoated carbon fibers.

Abstract: By transmission electron microscopy (TEM) and optical microscopy (OM), the isochronal aging hardening law and microstructure have been studied for the Mg-8Gd-3Y-0.5Zr alloy after cold rolling. During isochronal aging process (heated from room temperature to 500 with the heating rate of 1/min), there are two peak hardness temperature points, which are corresponding to the large number of precipitation of β and β phases, and the peak aging time of the alloy substantially decreased with the increase of deformation reduction. The β phases started to precipitate at temperatures of 145, 134, 127 and 124 in the alloy after cold rolling with the reduction of 0%, 8%, 15% and 22%. The heating rate greatly affected the starting precipitation of metastable phases. β phases started to precipitate at temperatures of 109 and 124 with the reduction of 22% and with the heating rates of 3 /min and 30 /min.

Abstract: In this paper, effects of texture on tensile property of extruded AZ31 magnesium alloy are investigated by acoustic emission (AE). The results show that the extruded AZ31 magnesium alloy used in this research has the strong texture of and tensile anisotropy. The yield strength of TD-sample is about 80MPa, which is half of that of ED-sample. AE results show that the AE count of the TD-sample is higher than that of the ED-sample. These AE results suggest that there are more deformation twins in the TD-sample than in the ED-sample. It can be concluded that the deformation twinning can strongly influence the anisotropy, which is mainly due to the flow stress relief induced by twinning and the interaction of twins and dislocations during the strain hardening.

Abstract: In this study, 5052/AZ31B/5052 composite plates were fabricated by two-pass hot rolling, under the rolling parameters 623 K/15 min/40% for the first pass and 673 K/10 min/50% for the second pass. The structure and element change in the zone of the joint interface were studied by means of optical microscope (OM), scanning electron microscope (SEM), and energy diffraction spectrum (EDS). The results showed that the bonding mode of the joint interface between AZ31B and 5052 alloy was metallic bonding after the first pass hot rolling, but the bonding mode of the finished composite plate appeared the distribution characteristic alternating with metallic bonding and metallurgical bonding. Two distinct intermediate layers including Mg₁₇Al₁₂ and Al₃Mg₂ can be clearly observed in the joint interface. During the second pass hot rolling, the AZ31B and 5052 layer were elongated along the rolling direction, while the Al₃Mg₂ and Mg₁₇Al₁₂ layers were up in the joint interface. After the tensile strength testing of the composite plate, the interface bonding appeared very well without any debonding.

Abstract: Warm-rolled AZ31 alloy sheets were annealed at different temperatures ranging from 150 to 450°C. Effects of annealing temperature on microstructure and properties of warm-rolled AZ31 alloy sheets, especially the formability, were investigated. The results revealed that the Lankford value (r-value) and strain-hardening exponent (n-value) first increased and then became relatively steady with the increase of annealing temperature. The Erichsen value (IE) first increased and then decreased with the increase of annealing temperature and the AZ31 alloy sheets exhibited the highest IE of 3.02 mm when annealing at 250°C, which can be mainly attributed to a larger elongation, a lower r-value and a higher n-value.

Abstract: Microstructures and phase compositions of as-cast and extruded ZK60-xHo (x=0~5) alloys were analyzed by X-ray diffraction, differential scanning calorimetry, optical microscope, scanning electron microscope and transmission electron microscope. Meanwhile, tensile mechanical property and impact toughness were tested. The results showed that as-cast microstructure was refined gradually with increasing the Ho content. Mg-Zn-Ho new phase increased gradually, while MgZn₂ phase reduced gradually to disappear. Second phase tended to distribute along grain boundary by continuous network. As-cast tensile mechanical property remained almost unchanged and impact toughness value α_nK was low (9~17 J/cm²). After extrusion by λ=40/T=593 K, grain size reached the micron level (1~5 μm) and broken second phase distributed dispersedly. Extruded tensile mechanical property was enhanced obviously. Tensile strength σ_b at ambient temperature increased gradually from 355 MPa for ZK60 alloy to 375 MPa for ZK60-4.77Ho alloy, while elongation δ decreased gradually from 19.5% to 12.5%. σ_b at 473 K increased gradually from 120 MPa to 169 MPa. Meanwhile, α_nK was enhanced to 26~54 J/cm². Fine grain and few fine dispersed second phase enhanced the tensile mechanical property and impacted toughness of magnesium alloys effectively.

Abstract: Mg-2.0Zn-0.8Gd (wt. %) alloy was rolled consecutively at different temperatures. The influence of rolling temperature and annealing process on the microstructure, texture and mechanical properties of the sheet were investigated. A deformation microstructure consisting of many intersected twins and a few dynamic recrystallization grains, and a basal texture with basal poles tilting about ± 10-15° from the normal direction towards the rolling direction were observed in the as-rolled sheet after 4 consecutive rolling processes. Static recrystallizaiton took place in the sheet after annealed above 300 °C. The annealed sheet exhibited a uniform microstructure and a non-basal texture with basal poles tilting about ± 38-43° from the normal direction towards the transverse direction. The annealed sheets exhibited higher ductility about 32% along the rolling direction and 40% along the transverse direction comparing with the as-rolled sheets. The static recrystallization during annealing process was helpful to modify the texture as well as the dynamic recrystallization during rolling in the RE-containing alloys.

Abstract: Preheated at 673K for 3h and 6h respectively, as-extruded Mg-9Zn-0.6Zr-2Er (wt.%) slabs with starting thickness of 8mm were hot-rolled at 673K to 2mm. The hot-rolled alloys were then solution treated at 673K for1.5h and aged at 473K for 10h. Microstructure evolution and second phase precipitation behavior during hot rolling and subsequent heat treatments were examined by optical microscopy (OM), scan electron microscopy (SEM) and micro hardness test. Mg-9Zn-0.6Zr-2Er alloy were fully recrystallized with fine equiaxed grains after preheating for 3 hours, prolonging heating time leads to a higher degree of dissolution of secondary phase into Mg matrix but a coarsening of the microstructure. During hot rolling process, the volume fraction of the DRX grains increased gradually with increasing reduction ratio, shearing bands became visble in the final pass. Thermal stable Mg-Zn-Er intermetallic compounds distributed along rolling direction at the first rolling passes and became more homogenous in the final pass. More nanosized MgZn₂ precipitated during hot rolling in the alloy preheated for 6h than that for 3h, leading to an enhanced precipitation hardening effect in the former.

Abstract: The inoculation effect of Al₂Ca intermetallic compound on microstructure evolution was examined performed on as-cast AZ31 and AZ61 alloys. The refinement mechanism of Mg-Al alloys was discussed with the Miedema model and edge-to-edge matching model. Al₂Ca compound has a good crystallographic matching relation with Mg matrix, which formed in the solidification process. It is considered that Al₂Ca can act as a potential and effective heterogeneous nucleation center. The results reveal that the grain refinement effect of Al₂Ca compound has more pronounced in AZ31 alloy compared with in AZ61 alloy.