The effect of alloying elements on nano-ordered wear properties was investigated using fine-grained pure magnesium and several types of 0.3 at. pct X (X = Ag, Al, Ca, Li, Mn, Y, and Zn) binary alloys. They had an average grain size of 3 to 5 μm and a basal texture due to their production by the extrusion process. The specific wear rate was influenced by the alloying element; the Mg-Ca and Mg-Mn alloys showed the best and worst wear property, respectively, among the present alloying elements, which was the same trend as that for indentation hardness. Deformed microstructural observations revealed no formation of deformation twins, because of the high activation of grain boundary-induced plasticity. On the contrary, according to scratched surface observations, when grain boundary sliding partially contributed to deformation, these alloys had large specific wear rates. These results revealed that the wear property of magnesium alloys was closely related to the plastic deformation mechanism. The prevention of grain boundary sliding is important to improve the wear property, which is the same as that of a large-scale wearing configuration. One of the influential factors is the change in the lattice parameter with the chemical composition, i.e., ∂(c/a)/∂C. An alloying element that has a large value of ∂(c/a)/∂C effectively enhances the wear property.

The effect of grain size and forming speed on room temperature formability was examined using Mg-0.3at%Mn binary alloy with different average grain sizes (2.5 and 18.5 µm). Erichsen tests revealed that the limited dome height (LDH) was clearly influenced by these factors. LDHs increased with finer grain sizes and/or lower forming speeds. The highest LDH was obtained to be 8.2, which is superior to that in the conventional aluminum (A5083) alloy (LDH=4.3), in the present Erichsen testing conditions. The results obtained from the tensile tests under strain rate ranges similar to those of the Erichsen tests showed high strain rate sensitivity (m-value, where m=0.06–0.13 for the meso-grained alloy and 0.11–0.22 for the fine-grained alloy, respectively), irrespective of the extrusion direction vs. the tensile direction. Deformed microstructural observations after tensile and Erichsen tests revealed the existence of deformation twinning; however, the area/volume fraction of its microstructural feature was too small to affect the deformation mechanism. Instead of the deformation twinning-related fracture, cavitation due to grain boundary sliding was the origin of fracture during the present Erichsen tests. The further activation of grain boundary sliding was an effective method to improve the room temperature formability of magnesium alloys.

The effect of alloying elements on grain boundary sliding was systematically investigated using several binary magnesium alloys (X = Ag, Al, Li, Sn, Pb, Y and Zn) via both experimental and numerical methods. The alloying element clearly affected damping properties related to grain boundary sliding, as measured by nanoindentation tests. The properties, such as damping capacity and strain rate sensitivity, apparently depended on grain boundary characteristics, i.e., the grain boundary energy. By increasing and decreasing the grain boundary energy, the alloying element was found to play a role in enhancing and suppressing grain boundary sliding, respectively. First-principles calculations revealed that the lithium element had weak bonding to magnesium due to a few operations of the electric orbit. On the other hand, rare-earth elements exhibited relatively strong bonding to magnesium, because of electron interactions with the first nearest neighbor site, and tended to prevent grain boundary sliding. These results suggest that grain boundary energy is a reliable parameter for predicting grain boundary sliding and developing a magnesium alloy, which has good elongation-to-failure and/or secondary formability at room temperature.

The effect of crystal orientation on incipient plasticity during nanoindentation was investigated by experiments and molecular statics simulation. Pop-in behavior is a result of dislocation activity, and is therefore influenced by crystal orientation. Experimental results using single crystals indicated that indentations on the basal plane had higher pop-in loads and larger pop-in displacements than those on the prismatic plane, an effect also captured by molecular statics simulation. The difference can be traced to the types of activated dislocations, with not only basal but also pyramidal dislocations active for indentations on the basal plane, but only basal dislocations triggered at the first pop-in on the prismatic plane.Download high-res image (508KB)Download full-size image

The effect of solute segregation at deformation twin boundaries on the strength (hardness) and damping capacity was investigated using an extruded Mg-Y binary alloy. The existence of deformation twin brought about an increase in both the hardness and the damping capacity. However, a subsequent annealing had a different effect, in which segregation at twin boundaries led to a decrease in the damping capacity. This is because the segregation of yttrium stabilized the twin boundaries; as a result, these boundaries inhibited alternate shrinkage and growth of deformation twin, by which the vibration energy is absorbed.Download high-res image (309KB)Download full-size image

The impact of alloying elements on crack propagation and atomistic phenomenon at {101¯2}-type twin boundaries in magnesium was investigated via both experiments and calculations. The alloying elements clearly affected the crack propagation behavior. Cracks were difficult to propagate along matrix-deformation twinning interfaces in alloys that had high fracture toughness. In such magnesium alloys, the solute atoms, e.g., silver, manganese and zinc atoms, create adhesive interactions between magnesium atoms. Closed-shell and covalent-like bonding of these types of solute atoms would influence strong adhesion, which impedes the nucleation of a new surface at the twin boundary.Download high-res image (244KB)Download full-size image

The enhancement method of room temperature formability was investigated by controlling the rate-controlling mechanism. Wrought processed pure magnesium and its alloys (Mg-Al-Zn and Mg-Mn alloys), which have a basal texture, were used in this study. Pure magnesium had two kinds of average grain sizes (~7 µm and ~30 µm), and the other alloys were 20–30 µm in average grain size. The forming load vs. displacement curves in Erichsen tests were not influenced by the forming rate in the Mg-Al-Zn alloy; however, the fine-grained pure magnesium and Mg-Mn alloy showed a high forming rate dependence. The limited dome heights were 4.7, 2.0 and 5.2 for the fine-grained pure magnesium, Mg-Al-Zn and Mg-Mn alloys, respectively. Deformed microstructural observations revealed that, in the meso-grained pure magnesium and the Mg-Al-Zn alloy, deformation twins formed and were closely related to crack propagation. On the other hand, the formation of such deformed structures was difficult to identify in the fine-grained pure magnesium and meso-grained Mg-Mn alloy. The good stretch formability and high rate dependence of the fine-grained pure magnesium and Mg-Mn alloy resulted from the major contribution of grain boundary sliding to deformation during Erichsen testing. The addition of an alloying element, which plays a role in the enhancement for grain boundary sliding, is quite effective to improve the room temperature stretch formability.

•Grain boundary migrations occurred due to the reduction in the internal energy.•The grain boundaries with high energies enhanced the grain boundary migration.•The origin of this behavior was {1 0−1 1} twinning induced grain boundary migration.•The addition of solute atoms (Al or Ag) suppressed the grain boundary migration.The deformation behavior at the grain boundary was investigated by the molecular dynamics simulation using two models based on different kinds of [1−100] symmetric tilt boundaries (Σ25 with a tilt angle of θ = 23° and Σ10 with θ = 78°) in magnesium. Grain boundary migrations occurred in both models due to the reduction in the internal energy during deformation. The deformation mechanism at the grain boundary was shown to be the twinning induced grain boundary migration. The grain boundary migration was affected by the grain boundary structures, and it was enhanced in the grain boundaries with high energies. On the other hand, the grain boundary migration was suppressed by the addition of solute atoms, i.e., aluminum and silver. The silver atoms were found to be more effective for suppression than the aluminum atoms. These behaviors occurred in both the molecular dynamics simulation and the experiments.

The effect of grain size and alloying elements on the yield strength in compression was investigated using pure magnesium and its diluted solid solution alloys (Mg–Zn and Mg–Y alloys). The yield strength in compression was closely related to the {10–12} twinning, and followed the Hall–Petch relation in all the alloys. The value of σ0 increased in the following order, Mg–Y>Mg–Zn>pure magnesium, indicating solid solution strengthening; however, the slope in the Hall–Petch relation was not influenced much by the alloying elements. The twinning formed frequently at the grain boundaries with a low misorientation angle, and even occurred in the parent grains with a low Schmid factor in all the alloys. These results indicated that one of the most influential parameters for the Hall–Petch slope was not the Schmid factor but the grain boundary characteristics. All the stress and strain curves in the compression showed a plateau region referring to the expansion/propagation of {10–12} twinning after the yielding behavior. The length of the plateau region increased with a decrease in the grain size and the Schmid factor of the basal slip in the parent grains.

The friction and wear properties of pure magnesium and the Mg–Y alloy were investigated using the pin-on-disk configuration. The friction and wear resistance of the Mg–Y alloy was superior to those of pure magnesium. The wear mechanism was abrasion under all the conditions. The deformed microstructural evolutions near the surface region were observed by transmission electron microscopy and electron backscatter diffraction. The stress and strain states were also evaluated by finite element analysis (FEA). The deformed microstructures of both alloys consisted of the {10–12} twinning formation and the FEA results showed the occurrence of plastic deformation even at the beginning of the test. The formation of low angle grain boundaries was also confirmed with an increase in the applied load in the Mg–Y alloy. On the other hand, grain refinement due to dynamic recrystallization was observed in pure magnesium as the wear test progressed. The different microstructures resulted from difference in the surface temperature during the wear test, which was estimated to be around 393 K and 363 K for pure magnesium and the Mg–Y alloy, respectively. The high increment temperature in the fine-grained alloys brought about the occurrence of grain boundary sliding, i.e., material softening, which led to a decrease in the friction and wear properties. The present results indicated that one of the methods for enhancing the friction and wear properties is to increase the dynamic recrystallization temperature.

The effect of grain boundary structures on the deformation behavior at the grain boundaries in magnesium was examined by the nanoindentation creep test. The results of the nanoindentation creep test showed that the dominant deformation mechanism around the grain boundary was grain boundary sliding; however, the occurrence of grain boundary sliding was closely related to the grain boundary energy. The grain boundary with high energy showed high strain rate sensitivity, which was the same tendency as that of the other metallic materials. Furthermore, the addition of aluminum atoms into magnesium tended to prevent the grain boundary sliding due to the decrease in grain boundary energy.Highlights► The grain boundary structures in Mg affected the deformation mechanism. ► The grain boundary with high energy indicated high strain rate sensitivity. ► The addition of Al atoms into Mg decreased the grain boundary energy. ► Mg–Al alloy prevented the grain boundary sliding to compared that with Mg.

The effect of the interface structure between the matrix and the particle on the damping capacity was investigated using Mg–Zn and Mg–Zn–Y alloys in this study. The damping capacity was not affected by the interface structure at room temperature. However, the onset of temperature, which was higher in the Mg–Zn–Y alloy than in the Mg–Zn alloy despite their similar grain sizes, increased the damping capacity through grain boundary relaxation by grain boundary sliding. Compared to the Mg–Zn alloy, the existence of the quasicrystal phase particles, which had the coherent interface with low interface energy, was likely to have suppressed and delayed the grain boundary sliding in the Mg–Zn alloy.Highlights► The damping capacity in magnesium alloy was not affected by the interface structure between the matrix and the particle at room temperature. ► The loss factor increased in the high temperature region due to the viscous manner though grain boundary sliding. ► The onset temperatures that caused an increase in the loss factor were ~ 400 K and ~ 460 K for the Mg-Zn and Mg-Zn-Y alloys. ► The quasicrystal phase had a role of suppressing the occurrence of grain boundary sliding.

The microstructure evolution of Mg–2.4 at.% Zn binary alloy was examined during extrusion at low temperature. Fine-grained structures were formed near the grain boundary due to dynamic recrystallization. Rod-shaped precipitates were also formed before the extrusion and were elongated and/or torn off by the strain; the precipitate morphology changed from rods to spheres. The spherical precipitates pin the dislocations; they are the nucleation site for new grains and promote recrystallization.

Cavities were formed in the random boundaries of superplastic magnesium alloys due to higher boundary energy. The cavity growth rate of the fine-grained alloy (dominant diffusion process: grain boundary diffusion) was lower than that of the coarse-grained alloy (lattice diffusion) during superplastic flow.

The fracture toughness of an extruded Mg–2.6 at.% Zn–0.4 at.% Y alloy with a grain size of 1 μm and containing quasicrystalline icosahedral phase was determined to be 32.5 MPa m1/2, which is much higher than conventional wrought magnesium alloys. Microstructural observations showed that the quasicrystalline phase pinned many dislocation movements and prevented void nucleation because of its coherent interface with the matrix.

The mechanical properties were investigated by Mg–0.3 at.% Ca–1.8 at.% Zn alloy, which was produced by hot extrusion, having an average grain size of about 1 μm and spherical precipitates in the matrix. The extruded alloy showed a good balance of the yield strength (σys = 291 MPa) and plane-strain fracture toughness (KIC = 28.3 MPa m1/2), obtained by the stretched zone analysis, which were higher than those of conventional wrought magnesium alloys. The microstructure control of both the grain refinement and the dispersion of precipitates in the matrix was a possible method for improvement of the mechanical properties in magnesium alloys.

The effect of texture on fracture toughness was investigated for a wrought AZ31 magnesium alloy, which was a commercial rolled plate having strong basal texture. The value of plane-strain fracture toughness, KIC = 17.6–20.7 MPam1/2, was obtained from the stretched zone (SZ) analysis. The value of KIC varied with the distribution of basal texture; the sample having a pre-crack normal to rolled direction was the highest value of KIC in all present samples. The crack-tip having parallel to the rolled direction was easily able to propagate and/or proceed with applied load.

For a commercially extruded AZ31 alloy, the basal plane in most grains was distributed parallel to the extruded direction. The sample having a pre-crack normal to the basal plane had a higher value of plane-strain fracture toughness than that having a pre-crack parallel to the basal plane.

The fracture toughness of extruded pure magnesium increased with grain refinement due to the effect of the plastic zone, which is a sensitive factor related to the mechanical properties of yield strength, elongation-to-failure and strain hardening exponent.

The climb-controlled dislocation creep behavior was investigated in Mg–Al–Zn alloys of AZ31, AZ61 and AZ91 with different aluminum contents. The flow stress increased with the increase of aluminum content under the same deformation conditions. At high temperatures, the stress exponent was 5 and the activation energy was close to that for lattice diffusion of magnesium, whereas at low temperatures the stress exponent was 7 and activation energy was close to that for pipe diffusion for all alloys. By incorporating the effective diffusion coefficient and stacking fault energy into a constitutive equation, the dislocation creep behavior of magnesium alloys can be described by a single relation.