Low-Cycle Fatigue Behavior of an As-Extruded AM50 Magnesium Alloy

The low-cycle fatigue behavior of an as-extruded AM50 magnesium alloy has been investigated. The cyclic stress response of the alloy strongly depends on the imposed strain amplitude. It is also noted that at the higher total strain amplitudes, the alloy exhibits a pronounced anisotropic deformation behavior in the direction of tension and compression, where the width of the σ-ε hysteresis loop in the compressive direction is greater than that in the tensile direction. At the total strain amplitude of 1.5 pct, a serrated flow can be observed in both tensile and compressive directions of the σ-ε hysteresis loop. This means that dynamic strain aging takes place during fatigue deformation. The relation between elastic and plastic strain amplitudes with reversals to failure shows a monotonic linear behavior and can be well described by the Basquin and Coffin–Manson equations, respectively. In addition, crack initiation and propagation modes are suggested, based on scanning electron microscopy observations on the fracture surfaces of fatigued specimens. This article is based on a presentation given in the symposium entitled “Deformation and Fracture from Nano to Macro: A Symposium Honoring W.W. Gerberich’s 70th Birthday,” which occurred during the TMS Annual Meeting, March 12–16, 2006, in San Antonio, Texas, and was sponsored by the Mechanical Behavior of Materials and Nanomechanical Behavior Committees of TMS.     

Effects of Rotation Rates on Microstructure, Mechanical Properties, and Fracture Behavior of Friction Stir-Welded (FSW) AZ31 Magnesium Alloy

Three rotation rates were selected for friction stir welded (FSW) rolled AZ31 plates, at a constant welding speed of 100 mm/minutes. As the rotation rate was increased from 800 and 2000 to 3500 rpm, the nugget shape varied from a basin- or ellipse-shaped homogeneous structure to a two-layer structure. The grain characteristic was similar in the nugget zones (NZs) at 800 and 2000 rpm, with the column-rotated basal plane. However, at 3500 rpm, the equiaxed fine grains and column-rotated basal plane were found in the lower zone of the two-layer structure, while elongated coarse grains and the basal plane with little inclination comparable to that in the parent material (PM) were detected in the upper zone. Tensile tests indicated that the fracture behaviors of the FSW AZ31 joints at 800 and 2000 rpm were similar with the weak zones being located at the NZ middle and along the boundary between the NZ and the thermo-mechanically affected zone, while contrasting fracture features were found at 3500 rpm. The ultimate tensile strength (UTS) of the joints increased as the rotation rate increased, with the highest UTS being about 95 pct of the PM at 3500 rpm. The variations in the strength and fracture behavior of the joints with the rotation rate were accounted for by the variation in the texture.     

AZ31镁合金在海洋大气环境中的腐蚀行为

通过室外大气暴露试验,研究了AZ31镁合金在万宁和青岛2个海洋大气环境试验站点1～5年的腐蚀规律,用失重法测定了腐蚀速率,并用SEM和XRD分析了5年后表面腐蚀形貌和腐蚀产物的组成。研究表明,AZ31镁合金暴露在海洋大气环境5年后的腐蚀动力学符合幂函数规律,万宁站的腐蚀速率高于青岛站,且腐蚀速率都随暴露时间的延长而降低,但青岛站腐蚀速率降低幅度更大,腐蚀产物膜对基体保护作用较万宁站强;AZ31镁合金暴露在万宁站和青岛站5年后的腐蚀速率分别为37.6和13.5μm·a-1,腐蚀产物以MgCl2,MgCO3,MgSO3,MgSO4,Mg5(CO3)4(OH)2·8H2O和Mg2(OH)3Cl·4H2O为主;AZ31镁合金在海洋大气环境中不耐腐蚀,表面布满点蚀坑,相对湿度对镁合金较长周期的腐蚀有显著影响。     

Cyclic-Tension Fatigue Behavior in a Rolled AZ31B Magnesium Alloy Studied Using Ultrasonic Shear Waves

Nondestructive evaluation of cyclic-tension fatigue in a rolled magnesium alloy, Mg-3Al-1Zn, was performed using vertically polarized shear wave (SV) reflection and shear horizontal wave (SH) transmission methods. Internal friction measured by SV reflection increased rapidly in the early stages of the fatigue and finally saturated, showing dominating interactions of movable dislocations and twinning boundaries with the waves as acoustic nonlinearities. The propagation time and logarithmic damping ratio in the SH transmission method followed a repeated increase and subsequent sudden decrease pattern, and finally converged toward fatigue failure due to acoustoelasticity, which represents the interaction with residual stresses. The wave and phase data were determined using an optical microscope, a scanning electron microscope, a surface roughness tester, and X-ray diffraction. The results demonstrated that during the fatigue process, residual stress accumulated on the compressive side of the specimen, despite the applied cyclic-tension loading. Brittle cracks that originated in inclusions provided sudden relief from the residual stress.     

Texture Development in a Friction Stir Lap-Welded AZ31B Magnesium Alloy

The present study was aimed at characterizing the microstructure, texture, hardness, and tensile properties of an AZ31B-H24 Mg alloy that was friction stir lap welded (FSLWed) at varying tool rotational rates and welding speeds. Friction stir lap welding (FSLW) resulted in the presence of recrystallized grains and an associated hardness drop in the stir zone (SZ). Microstructural investigation showed that both the AZ31B-H24 Mg base metal (BM) and SZ contained β-Mg₁₇Al₁₂ and Al₈Mn₅ second phase particles. The AZ31B-H24 BM contained a type of basal texture (0001)〈11 \( \overline{2} \) 0〉 with the (0001) plane nearly parallel to the rolled sheet surface and 〈11 \( \overline{2} \) 0〉 directions aligned in the rolling direction. FSLW resulted in the formation of another type of basal texture (0001)〈10 \( \overline{1} \) 0〉 in the SZ, where the basal planes (0001) became slightly tilted toward the transverse direction, and the prismatic planes (10 \( \overline{1} \) 0) and pyramidal planes (10 \( \overline{1} \) 1) exhibited a 30 deg + (n ? 1) × 60 deg rotation (n = 1, 2, 3, …) with respect to the rolled sheet normal direction, due to the shear plastic flow near the pin surface that occurred from the intense local stirring. With increasing tool rotational rate and decreasing welding speed, the maximum intensity of the basal poles (0001) in the SZ decreased due to a higher degree of dynamic recrystallization that led to a weaker or more random texture. The tool rotational rate and welding speed had a strong effect on the failure load of FSLWed joints. A combination of relatively high welding speed (20 mm/s) and low tool rotational rate (1000 rpm) was observed to be capable of achieving a high failure load. This was attributed to the relatively small recrystallized grains and high intensity of the basal poles in the SZ arising from the low heat input as well as the presence of a small hooking defect.     

Fatigue and Tensile Behavior of Cast,Hot-Rolled,and Severely Plastically Deformed AZ31 Magnesium Alloy

The work focuses on experimental examination of the fatigue behavior of magnesium alloy AZ31 produced by three different procedures: squeeze casting (SC), hot rolling (HR), and equal-channel angular pressing (ECAP). The microstructures produced were studied by light and transmission electron microscopy (TEM). Squeeze-cast AZ31 had low porosity and coarse grains, while hot-rolled material showed microstructure with grain size of 3 to 20 μm. The finest grain structure with the average grain size of about 1 to 2 μm was found in the material pressed 4 times at 200 °C using the ECAP technique, route B _c . It was shown that low- and high-cycle fatigue behavior under symmetric loading at room temperature and with loading frequency of 20 Hz is strongly dependent on the technique employed in producing the alloy. The ECAP was shown to improve the fatigue life of the material in the low-cycle region over that of the squeeze-cast material. However, the fatigue life of AZ31 after ECAP was slightly lower than that of the hot-rolled material. In the high-cycle region, the hot-rolled material and the material that underwent ECAP exhibit the same fatigue strength, which is superior to that of the squeeze-cast alloy. Fatigue crack initiation and the character of fracture were examined by means of scanning electron microscopy. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application,” which occurred September 25–27, 2006 in Kloster Irsee, Germany.

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AZ31镁合金的研究现状

综述了国内外对AZ31镁舍金的研究现状。讨论了主要合金元素对AZ31镁合金的组织和性能的影响,介绍了AZ31镁合金的晶粒细化、塑性成形技术的研究现状。对AZ31镁合金的发展前景进行分析。     

AZ31变形镁合金及其挤压工艺的研究现状

本文根据文献资料总结了AZ31变形镁合金的性能特点、高温流变应力方程及其挤压工艺的研究和应用现状，为进一步开展AZ31镁合金挤压工艺的研究，特别是计算机模拟提供了参考依据。     

Low-Cycle Fatigue Behavior of Die-Cast Mg Alloys AZ91 and AM60

The influence of microstructure and artificial aging response (T6) on the low-cycle fatigue behavior of super vacuum die-cast (SVDC) AZ91 and AM60 has been investigated. Fatigue lifetimes were determined from the total strain-controlled fatigue tests for strain amplitudes of 0.2?pct, 0.4?pct, 0.6?pct, 0.8?pct, and 1.0?pct under fully reversed loading at a frequency of 5?Hz. Cyclic stress?Cstrain behavior was determined using an incremental step test (IST) and compared with the more traditional constant amplitude test. Two locations in a prototype casting were investigated to examine the role of microstructure and porosity on fatigue behavior. At all total strain amplitudes microstructure refinement had a negligible impact on fatigue life because of significant levels of porosity. AM60 showed an improvement in fatigue life at higher strain amplitudes when compared with AZ91 because of higher ductility. T6 heat treatment had no impact on fatigue life. Cyclic stress?Cstrain behavior obtained via the incremental step test varied from constant amplitude test results due to load history effects. The constant amplitude test is believed to be the more accurate test method. In general, larger initiation pores led to shorter fatigue life. The fatigue life of AZ91 was more sensitive to initiation pore size and pore location than AM60?at the lowest tested strain amplitude of 0.2?pct. Fatigue crack paths did not favor any specific phase, interdentritic structure or eutectic structure. A multistage fatigue (MSF) model showed good correlation to the experimental strain-life results. The MSF model reinforced the dominant role of inclusion (pore) size on the scatter in fatigue life.     

Friction Stir Welded AZ31 Magnesium Alloy: Microstructure, Texture, and Tensile Properties

This study was aimed at characterizing the microstructure, texture and tensile properties of a friction stir welded AZ31B-H24 Mg alloy with varying tool rotational rates and welding speeds. Friction stir welding (FSW) resulted in the presence of recrystallized grains and the relevant drop in hardness in the stir zone (SZ). The base alloy contained a strong crystallographic texture with basal planes (0002) largely parallel to the rolling sheet surface and $ \langle {11\bar{2}0} \rangle $ directions aligned in the rolling direction (RD). After FSW the basal planes in the SZ were slightly tilted toward the TD determined from the sheet normal direction (or top surface) and also slightly inclined toward the RD determined from the transverse direction (or cross section) due to the intense shear plastic flow near the pin surface. The prismatic planes $ (10\bar{1}0) $ and pyramidal planes $ (10\bar{1}1) $ formed fiber textures. After FSW both the strength and ductility of the AZ31B-H24 Mg alloy decreased with a joint efficiency in-between about 75 and 82 pct due to the changes in both grain structure and texture, which also weakened the strain rate dependence of tensile properties. The welding speed and rotational rate exhibited a stronger effect on the YS than the UTS. Despite the lower ductility, strain-hardening exponent and hardening capacity, a higher YS was obtained at a higher welding speed and lower rotational rate mainly due to the smaller recrystallized grains in the SZ arising from the lower heat input.     

磁场作用下合金元素在AZ31镁合金中的分布

研究了镁合金AZ31分别在无磁场及磁场作用条件下凝固的微观结构及合金元素的分布；对凝固过程中磁场的分布进行了数值模拟。结果表明，与无磁场处理凝固的镁合金组织比较，镁合金AZ31的凝固过程在磁场作用条件下合金元素在晶界上和晶内的分布有较大的变化，在较强静磁场作用下聚集在晶界上的共晶体组织明显减少，共晶组织形成的网络变得不连续，同时在晶内和晶界附近出现了大量近似球状的共晶质点，提高了合金元素在晶内的固溶度，有利于改善镁合金的综合性能。     

Specific Character of Material Flow in Near-Surface Layer during Friction Stir Processing of AZ31 Magnesium Alloy

Microstructure and texture evolution in the near-surface layer during friction stir processing (FSP) of AZ31 magnesium alloy was studied. Material flow was found to be a very complex process consisting of several stages. The material in front of the friction stir tool was first deformed by the rotating shoulder. Then, approaching the tool, it experienced a secondary deformation caused by the rotating pin, and finally, behind the tool, it again underwent a tertiary deformation induced by the shoulder. The texture evolution was shown to dictate the grain structure development.     

镁合金微弧氧化研究

本文介绍以硅酸盐溶液为氧化液对AZ31镁合金进行的微弧氧化实验。讨论了氧化膜的性能，并采用正交试验的方法分析了主要成膜物质对膜性能的影响。     

温变形对AZ31镁合金组织的影响

对铸态AZ31 镁合金在210℃温变形时的组织演变和力学性能进行分析,为进一步开发镁合金的成形工艺提供实验依据.     

AZ31镁合金型材挤压生产工艺的研究

采用卧式挤压机,对AZ31镁合金型材挤压工艺进行了生产跟踪试验研究,确定了AZ31镁合金型材在挤压操作时的最佳挤压筒温度、镁棒温度、模具预热温度和挤压速度。分析了型材产品的力学性能和微观组织结构,结果表明在给定操作工艺下,可得到高强度、细晶粒度、良好塑性和表面光洁的挤压型材。     

Low-Cycle Fatigue of Ultra-Fine-Grained Cryomilled 5083 Aluminum Alloy

The cyclic deformation behavior of cryomilled (CM) AA5083 alloys was compared to that of conventional AA5083-H131. The materials studied were a 100 pct CM alloy with a Gaussian grain size average of 315 nm and an alloy created by mixing 85 pct CM powder with 15 pct unmilled powder before consolidation to fabricate a plate with a bimodal grain size distribution with peak averages at 240 nm and 1.8 μm. Although the ultra-fine-grain (UFG) alloys exhibited considerably higher tensile strengths than those of the conventional material, the results from plastic-strain-controlled low-cycle fatigue tests demonstrate that all three materials exhibit identical fatigue lives across a range of plastic strain amplitudes. The CM materials exhibited softening during the first cycle, similar to other alloys produced by conventional powder metallurgy, followed by continual hardening to saturation before failure. The results reported in this study show that fatigue deformation in the CM material is accompanied by slight grain growth, pinning of dislocations at the grain boundaries, and grain rotation to produce macroscopic slip bands that localize strain, creating a single dominant fatigue crack. In contrast, the conventional alloy exhibits a cell structure and more diffuse fatigue damage accumulation.