变形镁合金疲劳性能的研究进展

总结了近年来变形镁合金疲劳性能的最新研究进展,指出夹杂、孔洞和刀痕促进变形镁合金疲劳裂纹的萌生,采用合适的整体(包括合金化、形变强化和热处理)和表面(包括喷丸、滚压和涂层)改性技术是改善变形镁合金疲劳性能的重要途径.此外,加载频率、腐蚀性介质类型及pH值等对变形镁合金的疲劳性能也有重要影响.最后,根据变形镁合金疲劳性能的研究现状指出了今后的研究方向.     

镁合金疲劳性能的研究进展

随着镁合金应用的日益广泛,其疲劳性能也受到人们的重视.对近几年镁合金疲劳性能的研究进行分析,从冶金因素、表面因素、加载方法和温度等方面论述对镁合金疲劳性能的影响.简述了提高镁合金抗疲劳性能的途径:热处理、添加稀土元素、表面处理等.提出了对镁合金疲劳性能研究的意见,希望对镁合金疲劳性能的研究有一定促进作用.     

表面处理镁合金疲劳性能的研究现状

随着镁合金应用的日益广泛,其疲劳性能也越来越受到人们的重视.对近年来有关表面处理镁合金疲劳性能的研究进行了总结,归纳了喷丸处理、滚压强化、阳极氧化、激光熔融等表面处理方法对镁合金疲劳性能的影响及机理.最后对改善镁合金疲劳性能未来应重点发展的表面处理技术进行了展望.     

铸造镁合金及其研究进展

介绍了镁的合金化原理、强化机理和目前所用铸造镁合金的类型及存在的缺点，提出了改善铸造镁合金性能的途径。     

渗注锆酸盐对AZ31镁合金疲劳裂纹扩展速率的影响

研究交变载荷作用下,在AZ31镁合金疲劳裂纹尖端渗注锆酸盐的沉积行为及其对疲劳裂纹扩展速率的影响。用扫描电镜(SEM)、能谱仪(EDS)及X射线衍射仪(XRD)观察分析裂纹尖端的形貌和物相成分,采取贴应变片方法确定渗注锆酸盐前后应力强度因子的变化。结果表明:锆酸盐转化液能在AZ31镁合金疲劳裂纹尖端形成锆酸盐ZrxOyZnxOy覆层;渗注锆酸盐后裂纹尖端应力强度因子降低约40%,能有效增强疲劳裂纹闭合效应,降低或延滞其疲劳裂纹扩展速率。     

轧制组织对镁合金AM60疲劳性能的影响

研究了AM60轧制后挤压镁合金的组织对其机械性能和疲劳裂纹扩展性能的影响。实验表明:轧制使晶粒细化,强度显著提高。沿纵轴轧制方向出现大量等轴李晶组织,而在横向原来的孪晶组织消失。对于存在大量孪晶组织的方向,其抗拉强度明显低于其它方向。轧制AM60的横向疲劳裂纹扩展速度(FCPR)明显地高于纵向。当疲劳裂纹尖端塑性区的尺寸与组织的晶粒度接近时,挤压AM60组织中晶粒大小的不均匀引起裂纹分叉,裂纹分叉和粗糙度诱发的裂纹闭合对疲劳裂纹扩展产生严重的阻滞作用在挤压镁合金AM60的疲劳裂纹扩展速度(da/dN)与应力强度因子范围(△K)的关系曲线上出现拐点(△K=64～7.5 MPa·m~(1/2))。疲劳裂纹扩展为沿晶和穿晶混合方式。     

长周期结构增强镁合金的研究进展

镁合金具有低密度、高比强度、易回收利用等优点,在电子、汽车、航空航天等领域得到了广泛的应用。但是镁合金强度低、变形加工困难等缺点成为阻碍其大规模应用的主要问题。长周期堆垛有序(LPSO)结构增强镁合金具有较高的强度和较好的塑性,引起了人们的广泛关注。列举了镁合金中5种长周期结构类型及其研究进展,分析了常见的4种长周期结构制备方法的特点,阐述了5种长周期结构特征及其表征方法,分析了长周期结构的强化机制,指出了今后的研究方向。     

WE94镁合金长寿命疲劳小裂纹萌生和扩展行为研究

镁合金作为结构材料在汽车、航空、航天等领域有广泛的应用前景,在实际服役过程中存在高频振动荷载的长期作用。为解决镁合金结构件在长寿命服役条件下的安全与可靠性问题,基于旋转弯曲和超声振动加载两种疲劳实验方法,研究WE94镁合金在高周与超高周区间（104～109周次）的疲劳强度与失效机制。结果表明,疲劳裂纹萌生于沿基面的滑移带并在断裂面上形成解理状“小平面”形貌;而在疲劳小裂纹扩展初期,裂纹面存在大量细小平行状条带,该条带于裂纹尖端与孪晶带的交互作用过程中形成。因此,镁合金疲劳小裂纹的萌生与扩展过程对材料局部微结构形态与变形机制表现出密切的相关性。     

镁合金焊接热裂纹的研究进展

镁合金具有很高的使用价值,在工程工业中得到了广泛应用。镁合金零部件的补焊工艺可以实现镁合金铸造缺陷的补救,加强镁合金的耐用性与安全性,减少不必要的浪费与安全隐患。但由于镁合金自身的焊接特性,在补焊过程中或焊补后常会形成热裂纹,严重影响镁合金使用性。结合近几年镁合金常见的补焊工艺,重点介绍了镁合金TIG焊、激光焊、搅拌摩擦焊和电阻点焊的补焊情况。同时,介绍了影响镁合金焊接热裂纹形成的因素。     

挤压AZ31B镁合金圆棒不同取向的疲劳裂纹扩展

使用紧凑拉伸(CT)试样,研究了挤压AZ31B镁合金圆棒三个方向的组织及疲劳裂纹扩展性能。结果表明,疲劳裂纹以穿晶为主的混合方式沿滑移带扩展。T-L方向裂纹呈直线扩展,扩展速率最高,T-R方向孪生和滑移协同塑性变形,裂纹局部偏转,呈波浪形沿径向扩展,扩展速率较低,强织构和不均匀组织引起L-T方向裂纹分叉和偏离,降低了裂纹尖端有效驱动应力强度因子幅,裂纹扩展速率最低,在疲劳裂纹扩展速率(da/dN)与应力强度因子幅(ΔK)的关系曲线上出现水平段。在ΔK较小时加载频率对裂纹扩展速率的影响不大,ΔK>3 MPa m时裂纹扩展速率随着加载频率的提高而减小。     

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.     

Very-high-cycle fatigue crack initiation and propagation behaviours of magnesium alloy ZK60

The aim of this paper is to assess the very-high-cycle fatigue (VHCF) behaviour of a magnesium alloy (ZK60). Results indicate that the fatigue crack initiates from an area consisting of many distributed facets, while the region of early crack propagation is characterised by parallel traces, based on a fractographic analysis. The significant differences in morphology around the crack initiation area result from the interaction between the deformation twinning and the plastic zone at the crack tip. In addition, the fatigue crack propagation rate around the crack initiation site is also estimated based on a modified Murakami model. It is found that the formation stage for the fatigue crack is of great importance to the fatigue failure mechanism in the VHCF regime.     

Effect of thick DLC coating on fatigue behaviour of magnesium alloy in laboratory air and demineralised water

Rotating bending fatigue tests have been performed using Diamond‐like carbon (DLC) coated specimens of a wrought magnesium alloy, AZ80A, in laboratory air and demineralised water and the effect of DLC coating on fatigue and corrosion fatigue behaviour was studied. Three film thicknesses of 3.5 μm, 13 μm, and 25 μm (two‐layer film) were evaluated and particular attention was paid to the role of thick DLC coating. In laboratory air, the fatigue strengths of the DLC‐coated specimens were higher than that of the substrate specimen and increased with increasing film thickness. This was because hard DLC coating with good adhesion suppressed the crack initiation due to cracking of inclusions or cyclic slip deformation on the substrate surface. In demineralised water, the fatigue strength of the 3.5‐μm DLC‐coated specimen was the same as that of the substrate specimen due to the penetration of the water through pre‐existing film defects, while the 13‐μm and 25‐μm DLC‐coated specimens showed increased corrosion fatigue strength with increasing film thickness and also exhibited nearly the same fatigue strength as in laboratory air except for a few premature failed specimens, indicating a potential of thick DLC coating or two‐layer coating for complete improvement of corrosion fatigue strength in aqueous environments.     

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 cerium on high-cycle fatigue properties of die-cast magnesium alloy

The effect of cerium (Ce) on high‐cycle fatigue behaviour of die‐cast magnesium alloy AZ91D was investigated. Mechanical fatigue tests were conducted at the stress ratio, R= 0.1 on specimens of AZ91D alloys with different Ce additions. The microstructure and fatigue fracture surfaces of specimens were examined using a scanning electron microscope (SEM) to reveal the micromechanisms of fatigue crack initiation and propagation. The results show that the grain size of AZ91D is refined, and the amount of porosity decreases and evenly distributes with the addition of Ce. The fatigue strength of AZ91D evaluated by the up‐and‐down load method increases from 96.7 MPa to 116.3 MPa (1% Ce) and 105.5 MPa (2% Ce), respectively. The fatigue cracking of AZ91D alloy initiates at porosities and inclusions of the alloy's interior, and propagates along the grain boundaries. The fatigue fracture surface of test specimens shows the mixed fracture characteristics of quasi‐cleavage and dimple.     

The plastic zone and growth of fatigue cracks in magnesium MA12 alloy at room and low temperatures

The plastic zone formed at the fatigue crack tip and the fracture topography in MA12 magnesium alloy samples, tested at 293 and 140 K in air and in vacuum, were analysed. It was found that the plastic zone formed in vacuum is characterized by a greater size ($\text{h}$) and degree of plastic strain that in air, and the crack growth rate ($\text{d}\text{l/}\text{d}\text{N}$) is lower. Temperature reduction leads to a decrease in $\text{h}$, while $\text{d}\text{l/}\text{d}\text{N}$ and the fracture mechanism are affected by temperature ambiguously, depending on the alloy microstructure and the $\text{K}{}_{\mathrm{<mtext>max</mtext>}}$ value. It was established that the size of the plastic zone can be described by the equation:

$\text{h=A}\text{(}\text{K}{}_{\max }\text{σ}{}_{\mathrm{0.2ps}}\text{)}{}^2$

where $\text{A}$ is a coefficient dependent on the alloy structural state, environment and test temperature. Evaluation of the cyclic plastic zone size at $\text{K}{}_{\mathrm{<mtext>max</mtext>}}$, corresponding to the transition from a low temperature region to a ‘Paris’ region, showed that this transition occurred when the cyclic plastic zon reached the structural parameter of the material.     

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.     

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).