<p>Ni-based single crystal superalloys have been widely used to produce turbine blades for advanced aero-engines because of the super temperature-related microstructural stability and comprehensive mechanical properties. However, due to effects of the high temperature and complicated stresses in service, the microstructures of superalloys might gradually evolve and fail in different modes. The present paper reviews the progress of microstructural stability and mechanical behavior including the γ’ phase rafting, TCP phase precipitation, high temperature creep, low cycle fatigue and thermomechanical fatigue of single crystal superalloys. The addition of Ru improves the creep life of superalloys, but also indirectly promotes the occurrence of “topological inversion”. On the other hand, with the increase of aging temperature and time, the contents of refractory elements in <i>m</i> phase rise significantly. With the increase of applied tension stress, more <i>m</i> phase precipitate from the γ matrix, whereas inverse tendency is shown under compression stress. Numerous planar defects are formed during precipitation of <i>m</i><i> </i>phase, and these defects promote the nucleation of P and R phases. During high temperature and low stress creep, an important dislocation a<010> superdislocation is observed, which moves in the γ’ phase slowly by a combination of slide and climb. Under very high temperature, incubation with accelerating creep rate occurs before the primary stage, which relates to the extending process of the γ width. At last, the stacking fault energy is significantly reduced after Ru additions, and thus a series of complex deformation mechanisms occur during low cycle fatigue, e.g. stacking faults penetrating γ/γ’ interface, trailing a/6<112> Shockley dislocations shearing into the γ’ phase. During thermomechanical fatigue, the life of superalloys is influenced by the site of crack initiation, microstructural evolution and oxidation resistance.</p>

Ni-based single crystal superalloys are widely used in the manufacture of aero engine turbine blades because of the excellent mechanical properties at high temperature. With the development of single crystal superalloys, the content of refractory elements is constantly increased (especially Re) to improve the high temperature capability, which in turn leads to the decrease in microstructural stability of alloys, such as the TCP phase precipitation. It is important to find one element which not only can maintain high temperature performance but also does not evidently promote TCP phase precipitation and is very cheap in price to replace Re partially. W is one of the most important solution strengthening elements in superalloys, its diffusion rate in Ni matrix is close to Re and far below the other alloying elements, meanwhile, the advantage of low price make it to be the most suitable substitute of Re. However, there is little work about the effect of W on microstructural stability in Re contained third generation superalloys. In this work, the effects of W on the elemental segregation, elemental partitioning ratio of γ /γ′, microstructure evolution and TCP phase precipitation during thermal exposure at 950, 1000 and 1050 ℃ have been investigated in a third generation Ni-based single crystal superalloys with varied contents of W (6%~8%, mass fraction). The results show that the addition of W has no obvious effect on segregation of the alloying elements of as-cast alloys as well as the morphology, size and volume fraction of γ′ phase after heat treatment. During the thermal exposure at 950 ℃, the connection and deformation of γ′ phase are accelerated, but its coarsening rate is decreased with increasing W content. The TCP phases precipitated in three alloys during thermal exposure are mainly μ phase and σ phase. The area fraction of TCP phases is increased slightly with the W addition during thermal exposure, which is the largest at 1000 ℃, less at 950 ℃ and the least at 1050 ℃.

Due to the outstanding creep performance, nickel-based single crystal superalloys (Ni-SXs) are extensively applied in modern aero-engine and industrial gas turbine. Apart from the special single crystal structure which is disadvantageous to extension of creep cracks, Ni-SXs derive the creep strength from intrinsic two-phase microstructure (γphase and γ’ phase). Main microstructural parameters including volume fraction of γ’ phase and the lattice misfit, and the formation and distribution of precipitated phase are determined by the compositions of alloys. Besides, the creep properties are greatly influenced by these microstructural parameters and precipitated phase. This review has summarized the relationships between different alloying elements and microstructures and indicated their influence on creep properties of Ni-SXs. In addition, with the improvements of experimental methods and characterization technique, some recent discoveries have provided additional evidence to support or challenge the pervious creep theories of superalloys. In view of these new discoveries, this review has provided some perspectives which can be referenced in future compositional design of Ni-SXs.

The effect of Mo addition on microstructural characteristics of a nickel-base single crystal superalloy containing 4 wt% Re was investigated. The γ/γ′ partitioning ratios determined by energy dispersive spectrometer attached to a transmission electron microscope showed that the addition of Mo enhanced the partitioning of Re, W and Cr in the γ matrix while decreased the concentration of Ta in the matrix. Synchrotron radiation diffraction was adopted to measure the γ/γ′ lattice misfit at room temperature. The results indicated that Mo addition changed the γ/γ′ lattice misfit towards larger negative as well as increased the tetragonal distortion of the γ lattice. Additionally, Mo addition led to microstructural instability and altered the precipitation behavior of topologically close-packed phases during 1100 °C exposure. Instead of precipitating directly from the matrix, the μ phase was observed to be converted from the σ phase which precipitated preferentially as a metastable intermediate in the alloy with high Mo content.

Oxidation resistant fourth and fifth generation Ni base superalloys were developed. The fourth generation superalloys TMS-138A were modified by increasing the content of Al and Cr and deceasing Mo. Designed alloys were cast as single crystal and examined in cyclic and isothermal exposures at 1100°C and creep rupture test. Under high temperature/low stress condition, 0·5 wt-% of Al addition was effective to improve the oxidation resistance but degrade creep property. Cr addition and Mo and W reduction were effective to improve the oxidation resistance and display creep property almost equal to the original TMS-138A. The fourth generation superalloy TMS-138 and fifth generation superalloy TMS-173 were also modified. By adding Cr and removing Mo and W, their oxidation resistances were dramatically improved. Thus it was found that it is possible to improve the oxidation resistance of Ru containing SC superalloys with keeping the high temperature strength by the increase in Cr and decrease in Mo and W.

The present work investigated the composition evolution of the TMS series of Ni-base single crystal (SC) superalloys in light of the cluster formula approach systematically. The cluster formula of SC superalloys could be expressed with [Formula: see text], in which all the alloying elements were classified into three groups, Al series ([Formula: see text]), Cr series ([Formula: see text]), and Ni series ([Formula: see text]). It was found that the total atom number (Z) of the cluster formula units for TMS series of superalloys varies from Z ~ 17 to Z ~ 15.5, and then to Z ~ 16 with the alloy development from the 1st to the 6th generation, in which the superalloys with prominent creep resistance possess an ideal cluster formula of [Formula: see text] with Z = 16. Similar tendency of composition evolution also appears in the PWA and CMSX series of SC superalloys. Typical TMS series of superalloys with prominent creep properties generally exhibit a moderate lattice misfit of γ/γ' which could render alloys with appropriate particle size of cuboidal γ' precipitates to acquire a maximum strength increment by precipitation strengthening mechanism. More importantly, the relationship between the lattice misfit (δ) of γ/γ' and the creep rupture lifetime (t) of superalloys was then established, showing a linear correlation in the form of lgt-lg|δ| at both conditions of 900 °C/392 MPa and 1100 °C/137 MPa. Combined with the lattice misfit, the cluster formula approach would provide a new way to modify or optimize the compositions of Ni-base superalloys for further improvement of creep property.

Senkov, O. N.; Miller, J. D.; Miracle, D. B.; Woodward, C. Air Force Res Lab, Mat & Mfg Directorate, Wright Patterson AFB, OH 45433 USA.

Alloy design based on single-principal-element systems has approached its limit for performance enhancements. A substantial increase in strength up to gigapascal levels typically causes the premature failure of materials with reduced ductility. Here, we report a strategy to break this trade-off by controllably introducing high-density ductile multicomponent intermetallic nanoparticles (MCINPs) in complex alloy systems. Distinct from the intermetallic-induced embrittlement under conventional wisdom, such MCINP-strengthened alloys exhibit superior strengths of 1.5 gigapascals and ductility as high as 50% in tension at ambient temperature. The plastic instability, a major concern for high-strength materials, can be completely eliminated by generating a distinctive multistage work-hardening behavior, resulting from pronounced dislocation activities and deformation-induced microbands. This MCINP strategy offers a paradigm to develop next-generation materials for structural applications.Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

Stoichiometric precipitates owe their fixed composition to an ordered crystal structure. Deviations from that nominal value, however, are encountered at times. Here we investigate composition, structure and diffusion phenomena of ordered precipitates that form during heat treatment in an industrially cast Al-Mg-Sc-Zr alloy system. Experimental investigations based on aberration-corrected scanning transmission electron microscopy and analytical tomography reveal the temporal evolution of precipitate ordering and formation of non-equilibrium structures with unprecedented spatial resolution, supported by thermodynamic calculations and diffusion simulations. This detailed view reveals atomic-scale spinodal decomposition to majorly define the ongoing diffusion process. It is illustrated that even small deviations in composition and ordering can have a considerable impact on a system's evolution, due to the interplay of Gibbs energies, atomic jump activation energies and phase ordering, which may play an important role for multicomponent alloys.

Many load-bearing industrial settings require light-weight structural materials with adequate strength. Although commercial aluminum (Al) alloys are suitable for room-temperature applications, their strength at elevated temperatures (300-500oC) is largely reduced by coarsening of the strengthening precipitates. However, high-temperature alternatives such as titanium alloys are much heavier and more expensive than Al alloys. Creating microstructures that remain stable over 300oC is an important goal of the aluminum-manufacturing community. This article focuses on the recent development of high-temperature resistant Al-based alloys. Especially, it discusses the unique microstructural features, selection criteria of the strengthening phase, alloying effects, and microstructural stabilization of aluminum. The strategies summarized in this review are expected to realize the new microstructural architectures of light-weight alloys, which are currently limited to low-temperature service.

工业选材在满足强度要求以保证结构安全之余，通常也需要尽可能兼顾轻量化以使机械结构更加轻盈。当仅仅考虑室温服役性能时，大量的商用铝合金材料因其优异的室温力学性能可供备选，并通常能很好地契合“轻质-高强”这一目标。然而，对于需要在300~500℃中温区间服役的构件，铝合金的上述优势则荡然无存。其主要原因在于传统高强铝合金赖以强化的析出相在200℃以上便将发生快速失稳粗化，导致材料丧失强化效果并快速软化失效。而若以更耐热的钛合金、高温合金等作为替代，则随之带来的结构增重与成本提高又令设计者甚觉“重材轻用”。目前，在>300℃温度区间难以实现安全服役仍是轻质铝基材料开发的难点所在，而其关键的解决方案在于如何寻求微观组织的高温稳定化。本文从耐热铝合金中特异性的微观组织特点出发，综述近期业界在耐热铝基合金开发过程中关于强化相选择、合金化策略以及微观组织热稳定化的进展与内在构建机理，以期能够提供一些耐热轻质合金材料微观组织设计过程的普适性基本理论依据。