Creep Resistant Magnesium Alloys for Powertrain Applications

Creep resistant magnesium alloys are candidate materials for automotive powertrain applications. Since the 90's, a number of new creep‐resistant magnesium alloy systems have been investigated and developed. These are for the most part based on rare‐earth, alkaline earth, and silicon additions. This paper gives an overview of creep resistance in magnesium and a review of creep resistant magnesium alloys for power‐train applications.     

A review: Hot topics on magnesium technology in China

Magnesium alloys have wide applications in automobiles, aerospace and so on due to many advantages, while a number of undesirable properties including poor corrosion resistance, inferior creep resistance and bad plastic processing ability have hindered their applications. Creep-resistant magnesium alloy design, plastic processing of magnesium alloys and rapid solidification processing of magnesium alloys have become the hot topics in magnesium technology. Other than these, surface modification as well as laser beam welding are also involved. The research progress and development in magnesium technology in China are reviewed in the paper.     

A review: Hot topics on magnesium technology in China

Magnesium alloys have wide applications in automobiles, aerospace and so on due to many advantages, while a number of undesirable properties including poor corrosion resistance, inferior creep resistance and bad plastic processing ability have hindered their applications. Creep-resistant magnesium alloy design, plastic processing of magnesium alloys and rapid solidification processing of magnesium alloys have become the hot topics in magnesium technology. Other than these, surface modification as well as laser beam welding are also involved. The research progress and development in magnesium technology in China are reviewed in the paper.     

Properties of magnesium alloys reinforced with nanoparticles and carbon nanotubes: a review

Magnesium alloys suffer from only moderate high-temperature strength and creep resistance. Aluminium-free magnesium alloys for sand casting or alloys containing aluminium with expensive additional alloying elements may be in use, but only microparticle or microfibre-reinforced magnesium alloys really exhibit satisfactory creep strengths at temperatures up to 250 °C. Reinforcing magnesium alloys with ceramic nanoparticles could be a solution for preserving a low density while increasing the high-temperature performance. When produced using melting processes, nanoparticle-reinforced magnesium composites are expected to enjoy strengthening due to the grain refinement described in the Hall–Petch relation. When an isotropic distribution of nanoparticles is achieved, the composites are additionally expected to be Orowan-strengthened. In this review, a variety of ceramic materials, such as SiC, Al₂O₃, Y₂O₃, SiO₂ and carbon nanotubes were investigated for reinforcement. Pure magnesium and various magnesium alloys were chosen as the matrix material and both powder metallurgical (PM) and melting processes were used for production of the composites. The mechanical properties of the composites were generally enhanced, compared to an unreinforced alloy; not only at room temperature, but also at elevated temperatures. In some cases an increase in strength in combination with increased ductility was also identified.     

ECAP变形对Mg2Si增强耐热镁合金组织及性能的影响

为了提高镁合金的耐热性能,在Mg-Zn合金中加入Si,形成Mg-Zn-Si镁合金.采用ECAP工艺在变形温度为573 K和挤压路径为Bc条件下对Mg-Zn-Si镁合金进行不同道次的变形.运用金相显微镜(OM)、X射线衍射仪(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)等手段对变形后的Mg-Zn-Si镁合金进行了组织表征,对变形后的合金进行了室温拉伸和高温蠕变等力学性能测试.结果表明:随着挤压道次增加,α-Mg基体、Mg Zn相及Mg2Si相均得到细化且分布趋于均匀.1道次挤压后部分基体α-Mg细化,4道次挤压后α-Mg的尺寸减小为5~10μm,且晶粒大小趋于均匀;2道次挤压后Mg2Si相枝晶在原位置破碎为颗粒状,6、8道次挤压后Mg_2Si相呈弥散分布.4道次挤压后合金的屈服强度和抗拉强度均提高120%,伸长率提高353%;8道次挤压后合金的抗拉强度和伸长率与4道次相比变化不大,但屈服强度进一步提高了19%.随着挤压道次增加,高温抗蠕变性能提高,8道次后高温稳态蠕变速率降低5倍.Mg2Si相细化机理为受剪切而机械碎断.     

塑性成形方法对镁合金耐蚀性的影响及展望

随着镁合金产业的快速发展,如何通过塑性成形方法提高镁合金的耐蚀性成为了重要课题。镁及其合金因具有低密度、高比强度和较好的回收性等优点而受到广泛关注,然而室温变形能力和耐腐蚀性能差等缺点是其广泛应用的瓶颈。在总结镁合金腐蚀特点及面临问题的基础上,综合分析了国内外塑性成形方法对镁合金腐蚀领域的相关研究,综述了不同加工成形方法在提高镁合金耐蚀性应用方面的进展,从腐蚀机理和工艺参数2个方面进行了讨论。介绍了不同塑性成形方法对镁合金耐蚀性的影响机制,其中包括挤压–ECAP、超声滚压处理、等通道转角挤压、热轧处理、触变成形、板材挤压、板材轧制、交叉轧制、异步轧制和异步交叉轧制、压铸、快速凝固、搅拌摩擦焊、增材制造、喷丸等。从成分分布、析出相等微观角度阐述了影响镁合金腐蚀行为的机制,指出了塑性成形方法在提高镁合金耐蚀行为方面存在的问题,为提高镁合金的耐蚀性提出建议。     

A comparison of impression, indentation and impression-relaxation creep of lead-free Sn–9Zn and Sn–8Zn–3Bi solders at room temperature

Creep behavior of Sn–9% Zn and Sn–8% Zn–3% Bi solder alloys was studied by impression, indentation, and impression-relaxation tests at room temperature (T > 0.6T _m ) in order to evaluate the correspondence of the creep results obtained by different testing techniques, and to evaluate the effect of Bi on the creep response of the eutectic Sn–9Zn alloy. Stress exponent values were determined through these methods and in all cases the calculated exponents were in good agreement. The average stress exponents of 8.6 and 9.9, found respectively for the binary and ternary alloys, are close to those determined by room temperature conventional creep testing of the same materials reported in the literature. These exponents imply that dislocation creep is the possible mechanism during room temperature creep deformation of these alloys. The introduction of 3% Bi into the binary alloy enhanced the creep resistance due to both solid solutioning effect and sparse precipitation of Bi in the Sn matrix.     

Sb合金化对AE41镁合金耐热性能的影响

采用X射线、光学显微镜、电子探针、扫描电镜等手段研究了Sb合金化对AE41（Mg-4Al-1RE）镁合金组织和耐热性能的影响．结果表明,Sb取代Al优先与RE形成以RE2Sb相为主的高熔点弥散颗粒质点,而枝条状Al11RE3相数量和尺寸减小．Al11RE3相对基体的割裂作用的减弱,RE～Sb质点的弥散强化作用、以及Sb、RE等元素的固溶强化作用,使合金的常温和高温力学性能尤其是塑韧性显著提高,并且有效地改善了高温抗蠕变能力．过量的Sb反而降低了合金的力学性能和耐热性能．合金的断裂为具有塑性特征的准解理断裂．     

Ti-6Al-4V合金的室温蠕变行为

研究了Ti-6Al-4V钛合金板材的室温蠕变行为及其对合金后续使用性能的影响。结果表明：合金的宏观织构、应力水平以及预塑性应变都显著影响其室温蠕变行为。在加载方向上合金的<0001>峰值极密度越高,则其加工硬化指数越大、蠕变指数越小、室温蠕变性能越好。足够大的应力,是合金发生室温蠕变的必要条件。只有在蠕变应力不小于0.85σ_y的条件下才能观察到较为明显的室温蠕变,且室温蠕变效应随着蠕变应力水平的增大而增大。在室温下无论是蠕变还是单调加载引起的塑性应变,都抑制合金的后续蠕变行为。预加的塑性应变虽然抑制合金的后续蠕变应变,却使合金的后续疲劳性能恶化。     

AZ91镁合金抗高温蠕变性能的研究和发展

综述了国内外AZ91镁合金抗高温蠕变性能的研究进展,重点讨论了其蠕变机理及提高其抗高温蠕变性能的方法,对AZgl镁合金抗高温蠕变性能的研究方向提出了一些看法和展望.     

Magnesium-lithium-alloys — constitution and fabrication for use in batteries

Additions of just over 10 wt% lithium to magnesium cause its crystal structure to become cubic and render the alloy readily workable. The present work studies the preparation and fabrication of binary Mg-Li and ternary Mg-Li-Al alloys and makes a preliminary assessment of their suitability as anodes in primary battery systems. The preferred technique is induction melting in a silicon carbide crucible, followed by chill casting, under a protective argon atmosphere. Single-phase alloys, including those with lower lithium content, require intermediate annealing. These alloys have similar corrosion resistance to other magnesium-base alloys such as AZ31 and give consistently higher voltages and better discharge characteristics during battery studies.     

Microstructure and creep properties of a cast Mg-1.7%wt rare earth-0.3%wt Mn alloy

A study has been undertaken to reveal the creep properties of an experimental magnesium alloy, Mg-1.7RE-0.3Mn, and its microstructure after creep. Transmission electron microscopy examinations on the samples over aged during creep showed that extensive precipitation of Mg₁₂RE type intermetallics took place on dislocations in the form of long single rows. Other precipitates of Mg₁₂RE with lozenge morphology were observed to be distributed more uniformly inside the grains. Precipitates types have been identified via X-ray and electron diffraction, and microanalysis techniques. The relatively low creep rate compared to the commercial magnesium alloys was measured. A high value of activation energy, 240 kJ/mol, was calculated. The creep resistance and the activation energy of the alloy were discussed in terms of microstructural observations.     

TiAl合金表面涂层技术研究现状

TiAl合金由于其密度低,比强度和比刚度高,是航空航天工业理想的新型高温结构材料.室温塑性差的问题已通过添加合金元素和显微组织调控等手段基本得到解决,进一步提高其高温抗氧化和耐磨性能已成为需要重点研究的问题.表面涂层技术为这一问题的解决提供了一条有效的途径,为此,综述了国内外TiAl合金表面涂层技术的研究现状,重点介绍了激光技术和热喷涂技术及其应用,并展望了TiAl合金表面涂层技术的发展趋势.     

包铝镁合金的工艺特点及应用前景

传统的镁及镁合金存在着表面耐蚀性能差、冷加工性能不理想等缺点。采用包覆轧制的方法制备包铝镁板Al/Mg/A1层状复合材料,并对其进行适当的扩散退火处理,可有效地改善镁合金的室温塑性低、耐腐蚀性能差等缺点,且显著地提高了镁合金产品的应用与发展,扩大了镁合金的开发应用领域及应用前景等。     

Properties of squeeze cast Mg-10Al-Mn alloy and its alumina short fibre composites

Magnesium alloys are very suitable for applications that require materials with high strength-to-weight ratio. However, the use of magnesium alloys is limited due to their low elevated temperature properties. Magnesium matrix composites are the possible alternatives. The present work involved the production and subsequent property evaluation of AM100 magnesium alloy and its alumina short fibre reinforced composites. Studies on microstructure, hardness, density, stiffness, tensile properties, impact strength, wear resistance and corrosion resistance were carried out. Results indicate the significant improvement in the properties achieved by making composites. The findings also highlight the dominant roles of the base alloy matrix and the fibre volume fraction in determining the above properties.     

Effects of Sn segregation and precipitates on creep response of Mg‐Sn alloys

Mg‐Sn alloys are promising for the development of new cheap creep resistant magnesium alloys. In the present paper, the creep behaviours of Mg‐Sn and Mg‐Sn‐Ca alloys were examined at the constant temperature and different stresses. The measurements of stress exponents indicate that the dislocation climbing is the dominant mechanism during the creep of Mg‐3Sn or Mg‐3Sn‐2Ca alloys. The poor creep resistance of the binary Mg‐3Sn alloy is caused by the easy movement of dislocation and the segregation of Sn at the boundaries. Both T4 and T6 heat treatments improve the creep resistance of Mg‐3Sn alloy due to the alleviation of Sn segregation at grain boundaries and the precipitation of Mg₂Sn particles, respectively. Ca is an effective alloying element to increase the creep resistance of Mg‐Sn alloys. The Ca addition leads to the formation of thermal stable phases Mg₂Ca and CaMgSn in Mg‐3Sn‐Ca alloys. These two phases effectively hinder the movement of dislocations and the sliding of grain boundaries. On the other hand, the addition of Ca alleviates the segregation of Sn by the interaction of Ca with Mg and Sn to form the phase CaMgSn.     

Mechanical performance of extruded Ti–46Al–5Nb–1W titanium aluminide

Abstract

Titanium aluminide alloys offer considerable promise for use in high temperature applications, such as gas turbines. In this study an extruded Ti–46Al–5Nb–1W alloy has been examined, in terms of its tensile and creep behaviour. A reasonably fine and uniform microstructure was found in this bar product. This gave excellent properties, with tensile strengths up to ～950 MPa at room temperature, along with 1% elongation. These properties were accompanied by a very good creep behaviour, with low primary strains at the lower stresses and very low secondary creep rates. Comparison of the creep properties of this titanium aluminide alloy with other similar compositions and some typical nickel alloys shows that it is significantly superior to first generation titanium aluminides but also nickel alloys, such as IN718 and Udimet 720Li. However, the strain controlled fatigue performance of the titanium aluminide alloy was significantly poorer than these same wrought nickel alloys.     

The development of Fe-Al intermetallics

The intermetallics based on aluminides have long been known for their excellent resistance to high-temperature oxidation. However, for use in structural components the poor ductility at ambient temperatures has always been felt as a stumbling block. Interest in these materials has been revived recently, after achieving some success in improving the ductility at ambient temperatures and creep at elevated temperatures in titanium aluminides. For the iron aluminides, too, similar methodologies have been attempted, namely alloying with elements such as titanium, boron, molybdenum, chromium, silicon and manganese, as well as grain refinement for improving high-temperature creep and room-temperature ductility. Raising the creep resistance close to 600 °C and improving the ambient-temperature ductility to around 6% have been the major immediate aims. Attempts are also being made to improve the high-temperature fatigue and creep properties in these materials, particularly by pushing the stability temperature of ordered D0₃ upwards. It is now visualized that once the above properties are achieved, the iron aluminides, particularly the types based on Fe₃Al, could offer themselves as excellent candidate materials For structural purposes. Their attractiveness also stems to a large extent from their low cost, as they contain only abundantly occurring materials. The present work examines two routes for introducing ductility in the Fe₃Al-based materials: one by ternary-Quarternary additions and the other by grain refinement. Structural studies have been made on materials obtained through conventional casting as well as through rapid solidification with minor alloy additions. The results confirm that Fe₃Al-based alloys, even when air-melted, are amenable to a high degree of hot working and could be made to display improved ductility at room temperature by a careful control of the chemistry. Rapidly solidified ribbons also show reasonably good bonding during high-temperature compaction. Ordering in these alloys is not suppressed even by rapid solidification.     

Mechanical properties and creep of Mg – rare earth – Sc – Mn squeeze cast alloys

The microstructure developed during the T5 treatment of squeeze cast magnesium alloys containing rare earth (Gd, Y, or Ce, ≈ 3–10 wt.%), Sc (< 1 wt.%) and Mn (< 1.5 wt.%) is responsible for a reasonable age hardening in MgGdScMn alloys. Moderate age hardening is only possible in MgY4Sc1Mn1 or MgCe3Sc1Mn1 alloys. The c‐based centred orthorhombic phase precipitating as fine prismatic plates in a triangular arrangement is the most effective hardening phase. The stability of yield strength up to 250°C–300°C was confirmed in T5 treated MgGdScMn and MgY4Sc1Mn1 alloys. All alloys exhibit a reasonable ductility at room temperature. The precipitation of very fine basal discs of Mn₂Sc phase observed in all T5 treated alloys investigated does not contribute considerably to the hardness, but it is very effective in restricting creep. The creep resistance of all alloys investigated is superior to that of commercial WE54 alloy up to 350°C.     

Microstructure and plasticity of two molybdenum-base alloys (TZM)

In two different commercial Mo-base alloys (TZM), produced by vacuum melting and by powder metallurgy respectively, microstructural differences (particle size and chemistry, grain and subgrain sizes, dislocation density) have been found which affect the observed mechanical properties of the material. The compression-creep properties at 1423 K show a negligibly small creep rate at a stress of approximately 200 MPa. Trapping of dislocations by particles is proposed to be the controlling deformation mechanism during creep. The microstructure and fatigue properties of TZM welds were also investigated. Friction welds showed the best mechanical properties. Fatigue measurements in load control at room temperature and 1123 K show that the endurance limit of the vacuum-melted alloy is higher than that of the powder-metallurgically processed alloy.