Strain-Controlled Low-Cycle Fatigue Properties of a Newly Developed Extruded Magnesium Alloy

Abstract

To reduce fuel consumption and greenhouse gas emissions, magnesium alloys are being considered for automotive and aerospace applications due to their low density, high specific strength and stiffness, and other attractive traits. Structural applications of magnesium components require low-cycle fatigue (LCF) behavior, since cyclic loading or thermal stresses are often encountered. The aim of this article was to study the cyclic deformation characteristics and evaluate LCF behavior of a recently developed AM30 extruded magnesium alloy. This alloy exhibited a strong cyclic hardening characteristic, with a cyclic strain-hardening exponent of 0.33 compared to the monotonic strain-hardening exponent of 0.15. With increasing total strain amplitude, both plastic strain amplitude and mean stress increased and fatigue life decreased. A significant difference between the tensile and compressive yield stresses occurred, leading to asymmetric hysteresis loops at high strain amplitudes due to twinning in compression and subsequent detwinning in tension. A noticeable change in the modulus was observed due to the pseudoelastic behavior of this alloy. The Coffin-Manson law and Basquin equation could be used to describe the fatigue life. At low strain ratios the alloy showed strong cyclic hardening, which became less significant as the strain ratio increased. The lower the strain ratio, the lower the stress amplitude and mean stress but the higher the plastic strain amplitude, corresponding to a longer fatigue life. Fatigue life also increased with increasing strain rate. Fatigue crack initiation occurred from the specimen surface and crack propagation was mainly characterized by striation-like features. Multiple initiation sites at the specimen surface were observed at higher strain amplitudes.

Cyclic Deformation Behavior of a Rare-Earth Containing Extruded Magnesium Alloy: Effect of Heat Treatment

“…It is seen from Figure 1(a) that uniform equiaxed grains with an average grain size of about 12 lm were obtained in asextruded sample due to the occurrence of dynamic recrystallization (DRX) in the hot extrusion process at 673 K (400°C). [24] The grain size of as-extruded sample was fairly small in comparison with the common extruded Mg alloys, such as AZ31 and AM30, [9][10][11][12][13][14] and the smaller grain size was due to the role of added RE elements and zirconium where Zr mainly restricted the grain growth. [48] Figure 1(b) shows a typical SEM back-scattered electron image of as-extruded sample where several RE containing particles can be seen.…”

“…Further studies in this aspect are needed. However, in comparison with the as-extruded Mg alloy, [10][11][12][13]17] the pseudoelastic behavior reduced to a certain extent in the GW103K alloy, especially in the T5 and T6 conditions. D. Cyclic Deformation Response Figure 7 shows the evolution of cyclic stress amplitude as a function of the number of cycles at different strain amplitudes on a semi-log scale for the GW103K alloy in the (a) as-extruded, [17] (b) T5, and (c) T6 states, respectively.…”

“…[71] This was, however, in contrast to the hysteresis loops in the rare earth-free extruded or rolled Mg alloys, where the hysteresis loops were very asymmetrical due to the presence of strong crystallographic texture. [10][11][12][13] The maximum and minimum peak stresses in the T5 and T6 conditions showed a significant enhancement in different loops as compared with those in the as-extruded GW103K alloy, due to the presence of a large number of precipitates after the T5 and T6 heat treatment ( Figure 3). Similar to the tensile properties shown in Figure 5 and Table I, the T6 alloy exhibited only a slightly higher maximum and minimum peak stresses than the T5 alloy ( Figure 6).…”

“…[8,9] Despite the potential of substantial reductions in weight, most wrought Mg alloys exhibited unusual mechanical properties, e.g., tension-compression yield asymmetry, limited ductility, and pronounced directional anisotropy arising from the presence of strong crystallographic texture related to their hexagonal close-packed (HCP) structure with a limited number of slip systems activated during extrusion or rolling processes. [10][11][12][13][14][15] Indeed, for the vehicle components subjected to dynamic cyclic loading, such mechanical anisotropy and tension-compression yield asymmetry could lead to irreversibility of cyclic deformation which may have an unfavorable influence on the material performance. [15] These problems could be tackled through texture modification.…”

Microstructure and Mechanical Properties of Fiber-Laser-Welded and Diode-Laser-Welded AZ31 Magnesium Alloy

“…The formation of the fatigue striations in the magnesium alloys with a hexagonal close-packed crystal structure was expected to be related to the twinning in the compressive phase and detwinning in the tensile phase. [58][59][60][61][62] Subsequent studies are needed in this aspect.…”