A rival to superalloys at high temperatures

doi:10.1126/science.abd6587

GSTDTAP > 气候变化

DOI	10.1126/science.abd6587
	A rival to superalloys at high temperatures
	Julie Cairney
	2020-10-02
发表期刊	Science
出版年	2020
英文摘要	Although conventional alloys are based mainly on one element, recent design efforts have focused on multiprincipal element alloys (MPEAs) that contain substantial quantities of several elements. Success with this approach requires a robust understanding of the mechanistic origin of MPEA properties. On page 95 of this issue, Wang et al. ([ 1 ][1]) report the deformation behavior in a promising body-centered cubic (bcc) MPEA with good room-temperature plasticity and high strength at the temperatures at which conventional alloys would soften. They observed multiplanar, multicharacter dislocation slip that was not expected in bcc systems. This property is attributed to variations in the glide resistance along the core of dislocations, created by the atomic-scale fluctuations in composition that are characteristic of MPEAs. This mechanism explains the plasticity and could be used to guide the design of new MPEA candidates for high-temperature applications in aerospace and power generation. ![Figure][2] A more equal union The schematics represent the atomic structure of conventional and multifunctional alloys. Conventional alloys are based mainly on a single element, compared with multicomponent alloys that contain several elements in similar proportions. GRAPHIC: KELLIE HOLOSKI/ SCIENCE For centuries, alloy design has involved taking a base metal and adding small amounts of other elements to improve the properties. For example, adding carbon to iron enhances its strength, creating steel, and adding yet another element, nickel, improves its corrosion response, creating stainless steel. Sophisticated superalloys have complex compositions that provide high performance near their melting point, but they are still based on a primary element, usually nickel, cobalt, or iron. To expand the alloy design space, more recent efforts have shifted toward the development of alloys that contain substantial quantities of three or more elements (see the figure). These materials are variously referred to as MPEAs, complex concentrated alloys, or high-entropy alloys (a subset of MPEAs). Some of these new alloys display unprecedented combinations of strength, ductility, high-temperature performance, or functional properties ([ 2 ][3], [ 3 ][4]). Each new composition can result in the formation of different phases and microstructures within the alloy, which can in turn be altered by mechanical deformation and heat treatment. With such an enormous range of possibilities, traditional trial-and-error approaches are ineffective, and researchers are turning to computational and combinatorial approaches to predict the elemental combinations that could lead to alloys with desirable properties ([ 4 ][5], [ 5 ][6]). However, for these approaches to be successful, it is critical that the alloy design process is guided by an understanding of the origins of the specific properties that are desired. Refractory MPEA alloys, with their excellent high-temperature strength, show great promise. They are composed of combinations of three or more of the elements chromium (Cr), molybdenum (Mo), niobium (Nb), vanadium (V), tantalum (Ta), tungsten (W), hafnium (Hf), titanium (Ti), or zirconium (Zr) at nearly equal concentrations. The alloys usually have a bcc crystal structure and, for many of the most desirable combinations, are single-phase solid solutions. However, unlike dilute bcc alloys, many of these alloys show excellent retention of strength up to 1900 K ([ 6 ][7]). Aerospace, petrochemical, and power-generation industries all require tough components that are exposed to high temperatures. Superalloys, the best available option, have an operational limit of around 1100 K ([ 2 ][3]). This material constraint affects the efficiency of potential new technologies for power generation by limiting operating temperatures ([ 7 ][8]). It is also a serious issue in aerospace applications. Presently, aircraft parts that are exposed to hot environments in engines, or components that will be exposed to the high temperatures caused by hypersonic travel, require complex and expensive ceramic thermal barrier coatings to withstand the service environment ([ 8 ][9]). MoNbTaW, MoNbTaVW ([ 9 ][10]), HfNbTaTiZr ([ 10 ][11]), and HfMoNbTiZr ([ 11 ][12]) are all examples of MPEAs that display excellent high-temperature strength ([ 1 ][1]). However, many of these systems have limited room-temperature ductility, which is characteristic of bcc alloys. The lack of ductility in conventional bcc systems is related to the mobility of defects (dislocations), but dislocations could behave differently in bcc MPEAs because of local variations in composition along their core ([ 2 ][3], [ 6 ][7]). Local compositional fluctuations are intrinsic to MPEAs, in which the elements that surround each individual atom vary (see the figure, right). The high-temperature strength in MPEA alloys has been attributed to solid-solution strengthening by regions of concentrated solute, the mobility of certain dislocation structures, or both effects ([ 12 ][13]). Understanding the details of the dislocation structure and motion is crucial for a mechanism-guided search for the best refractory bcc alloys across the immense range of possible compositions. Wang et al. present a new MPEA MoNbTi alloy with good room-temperature strength but considerably lower density than many of the other refractory MPEA options. Density is important for transport applications, especially in rotating parts, where lower density increases the allowable service temperature by decreasing the stress caused by self-loading. The alloy displays homogeneous plasticity in microscale tension tests performed at room temperature in a scanning electron microscope. To understand the deformation process, Wang et al. used a sophisticated experimental setup that combines microscale mechanical testing and advanced microscopy. They used a focused ion beam (FIB) to prepare cross sections underneath nano-indents and performed in situ deformation on a single-crystal specimen, again prepared with a FIB. The tensile axis was aligned with the [001] direction so that the four 1/2〈111〉–type Burgers vectors are equally stressed. They observed unexpected nonscrew dislocation structures. Multiple slip systems appear to be operative in addition to those expected for a bcc alloy at room temperature. Wang et al. attribute this multiplanar, multicharacter dislocation slip to variations in the glide resistance for dislocations caused by the atomic-scale chemical fluctuations in composition along the core of the dislocation. Atomistic simulations show that the plane that has the lowest stress required for the movement of dislocations can vary in this system depending on the local atomic configuration. The implication of this finding is that there are additional pathways for dislocation slip, which is desirable for plasticity and toughness. This observation explains the plasticity and supports an explanation for high-temperature strength based around the slip mechanism instead of solid-solution strengthening. Deformation was studied at room temperature, and future work at high temperature may reveal more details about the active slip systems. Activation of additional slip pathways as a design goal will require renewed effort to understand how atoms are arranged along the core of the dislocation. Despite suggestions of local chemical order in MPEAs, experimental verification has been ambiguous ([ 3 ][4]). In Wang et al. 's work, atom probe tomography suggested that the atoms were randomly distributed. A smaller number of species in this alloy, as compared with many other MPEAs, has reduced the complexity of analysis. Further atom probe work could be carried out on carefully chosen model MPEAs to minimize overlapping peaks in data. Last, if bcc MPEA alloys are truly to rival superalloys for high-temperature use, consideration must be given to factors beyond the strength, ductility, and toughness. This includes oxidation resistance, creep strength, fatigue strength, and routes for manufacture, offering directions for future research. 1. [↵][14]1. F. Wang et al ., Science 370, 95 (2020). [OpenUrl][15][Abstract/FREE Full Text][16] 2. [↵][17]1. D. B. Miracle, 2. O. N. Senkov , Acta Mater. 122, 448 (2017). [OpenUrl][18][CrossRef][19] 3. [↵][20]1. E. P. George, 2. D. Raabe, 3. R. O. Ritchie , Nat. Rev. Mater. 4, 515 (2019). [OpenUrl][21] 4. [↵][22]1. A. Abu-Odeh et al ., Acta Mater. 152, 41 (2018). [OpenUrl][23] 5. [↵][24]1. T. Borkar et al ., Acta Mater. 116, 63 (2016). [OpenUrl][25] 6. 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领域	气候变化 ; 资源环境
URL	查看原文
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/298051
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Julie Cairney. A rival to superalloys at high temperatures[J]. Science,2020.
APA	Julie Cairney.(2020).A rival to superalloys at high temperatures.Science.
MLA	Julie Cairney."A rival to superalloys at high temperatures".Science (2020).