Physical metallurgy of concentrated solid solutions from low-entropy to high-entropy alloys

The alloy world could be divided into low-entropy (LEAs), medium-entropy (MEAs) and high-entropy alloys (HEAs) based on the configurational entropy at the random solution state. In HEAs, four core effects, i.e. high entropy, sluggish diffusion, severe lattice distortion and cocktail effects, are much more significant than low-entropy alloys in affecting phase transformation, microstructure and properties. In fact, the degree of the influence from these core effects more or less increases with increased mixing entropy. The trend is gradual from low-entropy alloys to high-entropy alloys. In this article, physical metallurgy of HEAs is discussed with the bridge connected to that of conventional alloys. As disordered and ordered solid solutions are the main constituent phases of alloys, the understanding of solid solutions is fundamental for the understanding of alloys. In addition, as dilute solid solutions have been well treated in current physical metallurgy, concentrated solid solutions from low-entropy to high-entropy alloys are focused in this article. Physical properties are especially emphasized besides mechanical properties.     

Documentation of damping capacity of metallic,ceramic and metal-matrix composite materials

High-damping materials allow undesirable mechanical vibration and wave propagation to be passively suppressed. This proves valuable in the control of noise and the enhancement of vehicle and instrument stability. Accordingly, metallurgists are continually working toward the development of high-damping metals (hidamets) and high-damping metal-matrix composites (MMCs). MMCs become particularly attractive in weight-critical applications when the matrix and reinforcement phases are combined to provide high-damping and low-density characteristics. In selecting the constituents for an MMC, one would like to have damping capacity data for several prospective component materials. Based upon data which have been published in the scientific literature, a concise documentation is given of the damping capacity of materials by three categories: (a) metals and alloys, (b) ceramic materials, and (c) MMCs.     

Microstructure and mechanical properties of medium-carbon ferrite–pearlite steel microalloyed with vanadium

Abstract

Microstructural analysis and mechanical testing have been carried out on medium-carbon steels to which additions of vanadium in the range 0·075–0·6 wt-% were made. The steels were either continuously cooled or isothermally heat treated after austenitization. Vanadium carbide precipitation in the proeutectoid ferrite regions of the microstructure and, more unusually, also in the pearlitic ferrite lamellae, were identified by transmission electron microscopy. Moreover, in both ferrite phases the precipitates are aligned in rows, indicative of interphase precipitation at the austenite/ferrite transformation interface. These observations are discussed in terms of the various mechanisms that have been proposed for the interphase precipitation reaction. In the alloys studied the vanadium additions were found to increase the strength of the steels by up to 100%, but to reduce the ductility and notched impact resistance. The most useful combination of increased strength with reasonable ductility and impact toughness was achieved with an addition of 0·15 wt-% V. The vanadium additions contributed to a number of variations in microstructure and therefore in strengthening mechanisms, but the largest effect was the interphase precipitation strengthening of the ferritic phases. The highest strength levels were achieved in fully pearlitic microstructures with the pearlitic ferrite lamellae strengthened by interphase precipitation of the vanadium carbide.

MST/536     

High resolution electron microscopy of alloys

High resolution electron microscopy (HREM) has emerged as a very powerful tool for probing the structure of metals and alloys. It has not only helped in unravelling the structure of materials which have been at the forefront of novel materials development such as quasicrystalline phases and high temperature superconducting compounds, but also is fast becoming a technique for solving some outstanding issues in the case of the commercial alloys thereby helping alloy development. In addition to the determination of the structures of phases, this tool is used for obtaining a first hand information of the arrangement of atoms around the various types of crystallographic defects and interphase interfaces. This mode of microscopy allows direct observation of orientation relationships between two phases across interfaces. HREM can be used for the direct examination of the prenucleation process. Initial stages of nucleation can also be studied readily in amorphous alloys, precipitation hardening alloys like maraging steels and in those systems where the formation of the omega phase occurs. This presentation describes some results of HREM studies on various alloys, commercial as well as alloys of scientific interest, where some of the aforementioned aspects have been examined. The specific examples cited pertain to metallic glasses, NiTi shape memory alloys, Ni-Mo, Zr-Nb and Ti-Al alloys.     

Strengthening of nickel -base superalloys for nuclear heat exchanger application

Strengthening mechanisms of nickel-base superalloys have been discussed with the background of the Japanese research and development activities in this field. As candidates for materials of intermediate heat exchangers which will be used for a future programme of nuclear steelmaking systems, two kinds of alloys have successfully been developed in Japan. The designs of these alloys have been reviewed from metallurgical aspects including their composition and creep properties. In addition to the conventional methods to strengthen these alloys, such as solid solution hardening or particle precipitation hardening, a grain-boundary precipitation strengthening due to tungsten-rich ₂ phase in the Ni-Cr-W system, would be expected as a further advanced method.     

Fundamental deformation behavior in high-entropy alloys: An overview

High-entropy alloys (HEAs), as a new class of materials, are nearly equiatomic and multi-element systems, which can crystallize as a single phase or multi-phases. Most of the HEAs described in the literature contain multiple phases (secondary phases, nanoparticles, and so on), rather than a single solid-solution phase. Thus, it is essential to review the typical mechanical properties of both single-phase and multiphase HEAs thoroughly, with emphases on (1) the fundamental physical mechanisms and (2) the difference from conventional alloys. In this paper, mainly based on different mechanical properties, HEAs are classified into four types for the first time, i.e., (a) HEA alloy systems of 3d-transition metals only (Type 1), (b) HEA alloy systems of transition metals with larger atomic-radius elements (Type 2), (c) HEA alloy systems of refractory metals (Type 3), and (4) others (Type 4). Then a number of aspects of mechanical behavior are reviewed and discussed, including the elastic anisotropy, yield strength, high-temperature performance, serration behavior, fracture toughness, and fatigue responses, which may serve as a demonstrative summary for the current progress in the scientific research of HEAs. Several mechanisms that quantitatively explain the mechanical properties of single-phase and multiphase HEAs in terms of basic defects (dislocations, twinning, precipitates, etc.) are discussed. A number of future research activities are suggested, based on the emphasis on developing high-performance structural materials. The review concludes with a brief summary of major mechanical properties and insights into the deformation behavior of single-phase and multiphase HEAs. The comparison and contrast between HEAs and conventional alloys remain the most compelling motivation for future studies. With the integrated experimental and simulation investigations, a clearer picture of the fundamental deformation behavior of single-phase and multiphase HEAs could be explored.     

On the behaviour of body-centred cubic metals to one-dimensional shock loading

The response of metallic materials to shock loading, like all loading regimes, is controlled largely by factors operating at the microscopic or atomic levels. Over the past few years, face-centred cubic (fcc) metals have received a level of attention where the role of features such as stacking fault energy and precipitation hardening have been investigated. We now turn our attention to body-centred cubic (bcc) metals. In the past, only tantalum, tungsten, and their alloys have received significant attention at high strain-rate conditions due to their use by the ordnance community. In particular, this investigation examines the shear strength of these materials at shock loading conditions. Previous results on tantalum, tungsten, and a tungsten heavy alloy are reviewed, and more recent experiments on niobium, molybdenum, and Ta–2.5 wt% W presented. Results are discussed in terms of known deformation mechanisms and variations of Peierl’s stress.     

镁基材料的阻尼性能研究进展与展望

简要介绍了高阻尼镁合金及镁基复合材料的研究进展,叙述了应变振幅、频率、温度、热处理以及合金成分对镁合金阻尼性能的影响,分析了镁基复合材料的阻尼机制及阻尼设计.对高阻尼镁基材料的重要发展方向进行了展望.     

硅合金化ZA27合金的阻尼性行为

硅合金化ZA27合金具有良好的耐磨性能,但其阻尼性能(Q-1)研究尚不深入.本文利用多功能内耗仪研究了铸态ZA27合金和向ZA27合金中加入4%Si的合金(下称ZA27-4%Si合金)的阻尼行为和相对动态模量,分析了两种合金的阻尼机理.结果表明两种合金的阻尼性能随频率降低和温度升高而增大,阻尼值不随应变振幅变化.ZA27合金和ZA27-4%Si合金具有较高的阻尼性能,在0.1Hz下,合金的室温阻尼分别为6.87×10-3与5.83×10-3.合金的相对动态模量随应变频率提高而增大.100℃以下,ZA27-4%Si合金的相对动态模量不随温度变化.研究表明合金的阻尼是由合金晶界和相界面滑动、位错振荡以及各相的热膨胀系数和弹性模量间差造成的微塑性变形共同造成的.认为ZA27-4%Si合金具有良好耐磨性能和减振性能和强度,适合作为滑动轴承合金材料.     

Precipitation of equilibrium phases in vapour-quenched Al-Ni. Al-Cu and Al-Fe alloys

In situ annealing experiments in the transmission electron microscope have been used to investigate the thermal stability and precipitation behaviour of non-equilibrium phases in vapour-quenched Al-Ni, Al-Cu and Al-Fe alloys prepared by co-sputtering. The non-equilibrium phases exhibit surprisingly high thermal stability and the general nature of precipitation is different from that in liquid-quenched or conventional alloys. These phenomena are caused by the very small as-quenched grain size.     

Radiation-induced solute segregation in metallic alloys

The subject of radiation-induced solute segregation (RIS) in metallic alloys is reviewed. RIS manifests itself in several different ways, including diffusion to point-defect sinks (dislocations, grain boundaries, voids, etc.), which can induce precipitation in undersaturated alloys, as well as self-organization of solute clusters and precipitation in defect-free material. Diffusion in dilute and concentrated alloys is highlighted, as are theories of RIS that include new ideas on diffusion of complexes involving coupling between fluxes of point defects and of solute atoms. Many important experimental observations are presented, including up-to-date findings using atom-probe tomography, with special emphasis on solute segregation in austenitic and ferritic steels. Results from computational modeling and theory are also presented and discussed in light of experimental findings. Examples illustrating the factors affecting RIS are shown and some important outstanding issues that impact the current understanding of RIS are described and discussed.     

Influence of solution-aging treatment on damping and tensile properties of Ti-36Nb-2Ta-3Zr-0.3O alloy

High-damping alloys have been used in aerospace, automobile manufacturing and other fields due to its capacity to weaken vibration and noise. The damping and tensile properties of the Ti-36Nb-2Ta-3Zr-0.3O (wt-%) alloy were investigated by dynamic mechanical analysis and tensile tests. The results showed that aging treatment significantly improved the tensile strength. The Snoek-type relaxation was observed in the alloy with and without aging treatment, although the aging treatment exerted a negative influence on damping capacity, especially at high temperatures. This phenomenon could be related to the precipitation of the α phase and the segregation of oxygen in the α phase from the β phase during the aging process. As a result, the alloy aged at 713?K simultaneously presents excellent damping and tensile properties.     

Phase formation under non-equilibrium processing conditions: rapid solidification processing and mechanical alloying

Rapid solidification processing (RSP) of metallic alloys, involving solidification of liquid metals at very high rates, results in the formation of a variety of metastable phases such as supersaturated solid solutions, crystalline intermetallic compounds, quasicrystalline phases, and metallic glasses. Additionally, significant refinement of the grain sizes and segregation patterns also occurs. Mechanical alloying (MA), another powerful non-equilibrium processing technique, utilizes repeated cold welding, fracturing, and rewelding of powder particles in a high-energy ball mill. MA also results in the formation of metastable phases and microstructural refinement similar to what happens during RSP. Consequently, comparisons are frequently made between the phases produced by RSP and MA and the general understanding is that they both result in similar metastable effects. A detailed analysis of the metastable phases produced by RSP and MA is made in the present work, and it is shown that even though the effects may appear similar, the mechanisms of formation and the composition ranges in which particular phases form are quite different. These two methods also have some unique features and produce different phases. The differences have been ascribed to the fact that RSP involves solidification from the melt while MA is a completely solid-state process that is not restricted by the phase diagram.     

Effects of Alloying Elements on the Volume Fraction of Ordered α2 Phase Precipitated in Ti-Al-Sn-Zr Alloys

An ideal method has been established for calculating the precipitation of α2 ordered phase in near-α titanium alloys based on the theory on the critical electron concentration for the precipitation of α2 ordered phase in near-α titanium alloys. With complete precipitation of α2 phase in near-α titanium alloys, the alloys can be considered to be composed of two parts： （1） the α2 ordered phase with the stoichiometric atomic ratio of Ti3X; （2） the disorder solid solution with the critical composition in which the α2 ordered phase is just unable to precipitate. By using this method, the volume fractions of α2 ordered phase precipitated in Ti-Al, Ti-Sn, Ti-Al-Sn-Zr alloys with various AI, Sn and/or Zr contents have been calculated. The influences of AI and Sn on the precipitation of α2 ordered phase are discussed. The calculating results show substantial agreement with the experimental ones.     

Effects of Zn on the microstructure, mechanical properties, and damping capacity of Mg-Zn-Y-Zr alloys

The influences of Zn on the microstructure, mechanical properties, and damping capacity of as-extruded (Mg-5%Y-0.6%Zr)_1−xZn_x (x = 2%, 4%, 6%, mass fraction) alloys were investigated by optical microscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, tensile testing, and dynamic mechanical analysis (DMA). The phase composition and microstructure of the alloys displayed evident variations with changes in Zn content. When the mass fraction of Zn changed from 2% to 6%, the phases mainly consisted of a long period stacking ordered (LPSO) X-phase (Mg₁₂YZn) and a W-phase (Mg₃Y₂Zn₃). Comparison of the mechanical properties and damping capacities of the different phases showed that the X-phase benefits the mechanical properties of the alloy without drastic impairment to their damping capacities. The damping capacities are discussed in terms of the Granato-Lücke theory and G-L plots.     

泡沫金属材料阻尼性能的研究进展

以泡沫金属材料的阻尼性能为主要分析对象,总结比较了使金属泡沫材料阻尼性能逐步提高的方法的发展过程及其相应的阻尼机制,特别介绍了高阻尼多孔Cu基形状记忆合金的发展前景和制约其发展的主要问题,并针对性地提出了解决方法和建议。     

Cavitation in creep

Metals often fail in service under creep conditions because of the formation of cavities on the grain boundaries which are approximately normal to the applied stress. This phenomenon of creep cavitation is becoming of increasing technological importance. As a result a complete understanding of it is desirable so that alloys with improved cavitation resistance can be designed. This paper reviews the development of our present understanding of the phenomenon which is one of nucleation, growth and linkage leading to failure. Several mechanisms of nucleation, such as at grain-boundary ledges or precipitates, have been postulated and experimental evidence in support of each has been cited. Similarly, deformation- or vacancy-controlled growth mechanisms have been discussed. It is apparent from the literature thatno single mechanism is applicable, indeed, the work discussed here suggests that several mechanisms may operate and each may become dominant at different stages of the creep life. Finally, the status of research into nickel-base superalloys is reviewed with reference being made to such effects as regenerative heat-treatment.     

泡沫金属基高阻尼复合材料的研究进展

分析了金属基复合材料和粘弹性材料的阻尼机制,认为金属材料与粘弹性材料复合可开发出高阻尼结构材料。探讨了获得泡沫金属基复合材料的途径,指出了泡沫金属基复合材料是一种具有广阔前景的新型结构功能一体化材料。     

On the shock compression of polycrystalline metals

At the present time, materials are being considered for use in increasingly extreme environments; extreme in terms of both the magnitude of the imposed pressures and stresses they encounter and the speed of the loading applied. Recent advances in understanding the continuum behaviour of condensed matter have been made using novel loading and ultrafast diagnostics. This insight has indicated that in the condensed phase, the response is driven by the defect population existing within the microstructure which drives plastic flow in compression as well as damage evolution and failure processes. This article discusses shock compression results, focusing upon research conducted on cubic-structured metals but also giving an overview of results on hexagonal-close-packed (HCP) metals and alloys. In the past, shock physics has treated materials as homogeneous continua and has represented the compressive behaviour of solids using an adaptation of solid mechanics. It is clear that the next generation of constitutive models must treat physical mechanisms operating at the micro- and mesoscale to adequately describe metals for applications under extreme environments. Derivation of such models requires idealized modes of loading which limits the range of hydrostatic or impact driven experimental techniques available to four principle groups. These are laser-induced plasma loading, Z pinch devices, compressed gas and powder-driven launchers and energetic drives and diamond anvil cells (DACs). Whilst each technique or device discussed brings unique advantages and core competencies, it will be shown that launchers are most capable of covering the spectrum of important and relevant mechanisms since only they can simultaneously access the material microstructural ‘bulk’ dimensions and timescales that control behaviour observed at the continuum. Shock experiments on a selection of metals whose response is regarded as typical are reviewed in this article, and sensors and techniques are described that allow the interpretation of the compression that results from idealized step loading on a target. Real-time imaging or X-ray techniques cannot at present access bulk states at the correct microstructural resolution, over a macroscopic volume or at rates that would reveal mechanisms occurring. It is controlled recovery experiments that provide the link between the microstructure and the continuum state that facilitates understanding of the effect of mesoscale properties upon state variables. Five metals are tracked through various shock-loading techniques which show the following characteristic deformation features; a low Peierls stress and easy slip allow FCC materials to develop dislocation cells and work-harden during the shock process, whereas the higher resistance to dislocation motion in BCC-structured materials and the lower symmetry in HCP metals slows the development of the microstructure and favours deformation twinning as an additional deformation mechanism to accommodate shock compression. Thus not only energy thresholds, but also operating kinetics, must be understood to classify the response of metals and alloys to extreme loading environments. Typical engineering materials possess a baseline microstructure but also a population of defects within their volumes. It is the understanding of these statistical physical relationships and their effects upon deformation mechanisms and defect storage processes that will drive the development of materials for use under extreme conditions in the future.