Corrosion fatigue behavior of extruded AZ80, AZ61, and AM60 magnesium alloys in distilled water

Rotary bending fatigue tests were conducted in laboratory air and distilled water using three extruded magnesium (Mg) alloys AZ80, AZ61, and AM60 with different chemical compositions. In laboratory air, the fatigue strengths at high stress levels were similar in all alloys because cracks initiated at Al-Mg intermetallic compounds, whereas AZ80 with the largest Al content exhibited the highest fatigue strength at low stress levels, which was attributed to the crack initiation due to cyclic slip deformation in the matrix microstructure. In distilled water, fatigue strengths were considerably decreased due to the formation of corrosion pits in all alloys, and the difference of fatigue strength at low stress levels among the alloys disappeared, indicating that the addition of Al that improved the fatigue strength in laboratory air was detrimental to corrosion fatigue. __________ Translated from Problemy Prochnosti, No. 1, pp. 141–145, January–February, 2008.     

Effects of humidity and temperature on the fatigue behaviour of an extruded AZ61 magnesium alloy

Load‐controlled fatigue tests were performed at 20 and 50 °C using two relative humidity levels of 55 and 80% to characterize the influence of humidity and temperature on the fatigue behaviour of an extruded AZ61 magnesium alloy. Fatigue tests were also conducted at 150 °C. No significant variation in fatigue properties was noticed with respect to temperature over the range from 20 to 50 °C for both the humidity levels. Fatigue limits in the range 140–150 MPa were observed for relative humidity of 55%. Fatigue strength decreased significantly with increase in temperature to 150 °C. Further, a significant reduction in fatigue strength with a fatigue limit of ～110 MPa was observed with increase in relative humidity to 80% at 20 and 50 °C. The crack initiation and propagation remained transgranular under all test conditions. The fatigue fracture at low stress amplitudes and high relative humidity of 80% results from the formation of corrosion pits at the surface and their growth to a critical size for fatigue‐crack initiation and propagation. The observed reduction in fatigue strength at high humidity is ascribed to the effects associated with fatigue–environment interaction.     

Near‐threshold fatigue crack growth properties of wrought magnesium alloy AZ61 in ambient air,dry air,and vacuum

Environmental influences on near‐threshold fatigue crack growth in wrought magnesium alloy AZ61 were investigated. Fatigue tests were performed in ambient (humid) air, dry air, vacuum, and dry nitrogen gas at 19 kHz cycling frequency and load ratio R = ?1. Threshold stress intensity factor amplitudes, K_th, determined for limiting growth rates below 5 × 10^?13 m/cycle were 1.1 MPam^1/2 in ambient air and 1.2 MPam^1/2 in dry air. A much higher K_th of 1.9 MPam^1/2 was measured in vacuum and dry nitrogen gas. This suggests oxygen to be the most detrimental constituent of ambient air that increases near‐threshold crack propagation rates and decreases K_th. The deleterious effect of humidity is comparatively small. Corrosive influences are effective at ultrasonic cycling frequency for growth rates below approximately 3 × 10^?9 m/cycle. The crack propagation curves in ambient and dry air show a plateau‐like regime where the fracture mode changes from purely ductile to a mixed ductile and brittle mode. In vacuum and dry nitrogen gas, a ductile crack path is found for all investigated crack growth rates.     

The effect of contact pad hardness on the fretting fatigue behaviour of AZ61 magnesium alloy

In the present study, the effect of hardness of contact material on fretting fatigue strength was experimentally investigated as a function of stress ratio. AZ61 magnesium alloy used in defense and transportation industries was used as the material for both the specimen and the contact pad. Two levels of hardness of contact material, 55.3 Vickers Hardness (HV) and 83.3 HV, were prepared by heat treatments. According to the results, with increasing hardness, the fretting fatigue strength decreased. The relative slip amplitude increased with increasing hardness, while the tangential force amplitude was not influenced by the hardness. It was speculated that because the local tangential stress at the contact edge increases with increasing hardness, the fretting fatigue strength decreases with increasing hardness.     

Cyclic oxidation behaviour of cerium implanted AZ31 magnesium alloys

AZ31 samples were implanted with 90 keV cerium ions with a dose of 1 × 10¹⁷ ions/cm². Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and Glancing angle X-ray diffraction (GAXRD) were used in order to investigate the characterization of elements in the implanted surface. The results indicate that after cerium implantation a pre-oxidation layer with double structure was formed. The influence of cerium implantation on the cyclic oxidation behaviour of AZ31 samples was studied at 773 K in air for 96 h, and the morphologies of the oxide scales were examined by scanning electron microscopy (SEM). It was found that the oxidation resistance of the implanted sample has been improved. The mechanisms to explain the experimental results were also proposed.     

Thermo‐mechanical methods for improving fatigue performance of wrought magnesium alloys

Wrought magnesium alloys AZ80 and ZK60 were extruded at 300 °C with extrusion ratios of ER = 12 and 44. Resulting microstructures, crystallographic textures and mechanical properties were investigated. Extruding led to profound reduction in grain size, which drastically improved yield stress, tensile elongation and HCF performance. Strength differentials in ZK60 after extruding at ER = 12 were more pronounced than after extruding at ER = 44, whereas no such effect of ER was observed in AZ80. Swaging after extruding further increased yield stress and endurance limit, while strength differential increased and ductility was lowered.     

AZ31镁合金棒材循环扭转变形及其对力学性能的影响

在室温条件下,对AZ31镁合金挤压棒材进行循环扭转变形,测试了扭转变形过程的力学性能以及变形后的微观组织和织构特征,并对扭转变形对镁合金棒材的力学性能影响进行了分析。结果表明:镁合金棒材在循环扭转过程中得到了严格对称的应力-应变滞回线,并且随着循环周期的增加,由于加工硬化和内部微裂纹扩展的共同影响,应力-应变滞回线上的应力峰值呈现先增加后减小的特征。在最大扭转角分别为60°和90°条件下,应力峰值出现在第四周期。镁合金棒材扭转变形后的晶粒中出现大量的拉伸孪晶带,孪晶启动使晶粒的 C 轴转向棒材轴线方向。镁合金棒材扭转变形后的力学性能测试结果显示,循环扭转变形明显提高了镁合金棒材压缩变形的屈服强度,其值由扭转前的约100MPa最大提高至约200MPa。     

Fatigue behaviour of bare and pre-corroded magnesium alloy AZ31

Constant amplitude fatigue tests have been performed using smooth specimens of a rolled AZ31 magnesium alloy in order to assess the fatigue behaviour of the material. The tests were periodically interrupted and replicas were taken from the surface of the specimens in order to reveal crack initiation and early crack propagation. Based on the derived S–N curve a very high stress sensitivity of the fatigue life can be concluded; it may be attributed to the inability of the material to accumulate fatigue damage in terms of cyclic plasticity at the early stage of fatigue. Fatigue cracks initiate already after few fatigue cycles between strain incompatibility points (e.g. grain boundaries) due to difficulties in satisfying the von Mises criterion. The initiation and propagation mechanisms of the fatigue cracks are characterized as cleavage. Furthermore, the corrosion susceptibility of the material has been investigated in a salt spray environment. It becomes evident that the presence of corrosion damage, in terms of corrosion pitting, results in the development of stress concentration, facilitating essentially the initiation and propagation of fatigue cracks. Thus, the fatigue limit is reduced to 50% of the respective value of the un-corroded material.     

Indentation creep of the wrought AZ31 magnesium alloy

Creep behavior of a wrought Mg–3Al–1Zn (AZ31) alloy was investigated by long-term Vickers indentation testing under constant loads of 5 and 10 N and at temperatures in the range 423–523 K. Based on the steady-state power-law creep relationship, the stress exponents were determined. The creep behavior can be divided into two stress regimes with different stress exponents and activation energy values. The low-stress regime activation energy of 96.2 kJ mol⁻¹, which can be interpreted as that for the activation energy for Al diffusion in Mg, and stress exponents of about 3.0–3.4 suggest that the operative creep mechanism is dislocation viscous glide governed by the diffusion of aluminum atoms in magnesium. This behavior is in contrast to the high-stress regime, in which the average values of n = 6 and Q = 132.4 kJ mol⁻¹ imply that dislocation climb-controlled creep is the dominant deformation mechanism. Stress exponents and activation energies obtained by different analysis methods of the indentation tests are in good agreement with each other and with those of the conventional tensile creep tests on AZ31 magnesium alloy reported in the literature. The localized indentation creep tests are, thus, considered capable of acquiring reliable information on the creep behavior of wrought magnesium alloys.     

Effects of extrusion and equal channel angular pressing on the microstructure,tensile and fatigue behaviour of the wrought magnesium alloy AZ80

The microstructure, mechanical properties, fatigue life and fatigue crack propagation rate of Mg‐8Al‐0.5Zn‐0.3Mn (AZ80) magnesium alloy were investigated after extrusion and equal channel angular pressing (ECAP). The highest ultimate and yield strengths and a large enhancement in the fatigue lifetime were obtained after two passes of ECAP. These were decreased with further pressing, although the grain size became finer. There was a correlation between the fatigue and ultimate strengths of AZ80 alloy. The transition from twinning to dislocation slip has also occurred at an average grain size of 7.9 μm. Simultaneous influences of the grain size and the yield strength caused an almost the same threshold of the stress intensity ratio for different process conditions. Moreover, the enhanced ductility of the ECAPed alloy resulted in an increase in the crack growth resistance because of its better ability to accommodate plastic strains during cycling.     

Texture effect on microyielding of wrought magnesium alloy AZ31

Uniaxial tension tests and elastoplastic self-consistent (EPSC) simulations were conducted to study the effect of texture on the microyielding of highly textured AZ31 plate. The onset of microyielding is always associated with the activation of basal 〈a〉 slip irrespective of starting texture, while the value of the microyield stress does depend on the texture. The macroscopic yield stress is largely affected by the critical resolved shear stress of the deformation mode with the highest Schmid factor. An inverse relation is found between the microyield strength normalized by the macroscopic yield strength and the average Schmid factor of prism 〈a〉 slip, which is useful for a rough estimate of the microyield strength.     

Corrosion fatigue of extruded magnesium alloys

Corrosion fatigue tests were carried out on extruded AZ31 (3% Al, 1% Zn, 0.3% Mn, Mg—the rest), AM50 (5% Al, 0.4% Mn, Mg—the rest) and ZK60 (5% Zn, 0.5% Zr, Mg—the rest) Mg alloys in air, NaCl-based and borate solutions. N_sol/N_air ratios (the relative fatigue life) were used for the analysis of the corrosion fatigue behavior of Mg alloys in various environments, where N_sol and N_air are the numbers of cycles to failure in the solution and in air, respectively. Extruded ZK60 alloy reveals very high fatigue and corrosion fatigue properties in comparison with other alloys. However, it has the lowest relative fatigue life (N_sol/N_air 10⁻³–10⁻²) or the highest sensitivity to the action of NaCl-based solutions in comparison with that of AM50 and AZ31 alloys (N_sol/N_air 10⁻²–10⁻¹). Under the same stress, the corrosion fatigue life of extruded alloys is significantly longer than that of die-cast alloys (N_sol for extruded AM50 in NaCl is two to three times longer than that of die-cast AM50).     

Corrosion fatigue behavior of extruded magnesium alloy AZ31 in sodium chloride solution

In the present study, corrosion fatigue experiments were done using the extruded magnesium alloy AZ31 in the 3% sodium chloride solution to clarify the corrosion fatigue characteristics of the material. Corrosion fatigue lives greatly decreased as compared with those in laboratory air. It was also clarified that most of the corrosion fatigue life (70–80%) at the lower stress amplitude is occupied with the period of the corrosion pit growth. Corrosion fatigue lives were evaluated quantitatively by dividing the corrosion fatigue process into the following two periods, i.e. (1) the corrosion pit growth period preceding the crack initiation from the pit and (2) the crack growth period before the specimen failure. In the analysis, the law of the corrosion pit growth proposed by authors was used to deal with the above first period. The evaluated results corresponded well to the experimental results.     

The semi-solid tensile deformation behavior of wrought AZ31 magnesium alloy

The semi-solid tensile deformation behavior of wrought AZ31 magnesium alloy has been studied through applying a set of low strain rate (0.001 s⁻¹) hot tension tests at temperature range of 300–500 °C. The results indicated a ductility drop at ∼450 ± 25 °C. This was attributed to the occurrence of eutectic reaction (L → α + γ) and the partial melting of intermetallic γ phase. The ductility was started to improve by increasing the temperature to 500 °C. The latter was explained considering the effect of liquid phase on stress relaxation through accommodation of the grain boundary sliding phenomena. To further investigating the semi-solid tensile deformation behavior of the experimental alloy, the cavitations characteristics of the alloy were also examined.     

Effect of cerium on microstructures and mechanical properties of AZ61 wrought magnesium alloy

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.     

The cyclic strain resistance, fatigue life and final fracture behavior of magnesium alloys

This paper summarizes the results of a comprehensive study on the cyclic strain resistance, low-cycle fatigue life and fracture behavior of three rapidly solidification processed magnesium alloys. Test specimens of the magnesium alloy were cyclically deformed under fully-reversed total strain amplitude control straining, over a range of strain amplitudes, giving less than 10⁴ cycles to failure. The cyclic stress response characteristics, strain resistance and low-cycle fatigue life of the alloys are discussed in light of alloy composition. All three alloys follow the Basquin and Coffin-Manson strain relationships, and exhibit a single slope for the variation of cyclic elastic and cyclic plastic strain amplitude with reversals-to-fatigue failure. The cyclic stress response characteristics, fatigue life and final fracture behavior of the alloy are discussed in light of competing and synergistic influences of cyclic total strain amplitude, response stress, intrinsic microstructural effects and dislocation-microstructural feature interactions during fully-reserved strain cycling.     

镁合金疲劳的研究进展

对近年来有关镁合金疲劳的研究进行了总结,考察了材料性质、腐蚀介质、高温环境等多种因素对镁合金疲劳性能的影响,并对裂纹萌生及扩展方式进行了分析,归纳了提高镁合金疲劳性能的方法--改善材料质量及表面强化处理或涂层处理,对镁合金疲劳性能的研究进行了展望.