Designing a wider superelastic window

doi:10.1126/science.abc8244

GSTDTAP > 气候变化

DOI	10.1126/science.abc8244
	Designing a wider superelastic window
	Paulo La Roca; Marcos Sade
	2020-08-14
发表期刊	Science
出版年	2020
英文摘要	Conventional metal alloys can only recover their original shape if subjected to very small elastic deformations. Superelastic alloys (also named pseudoelastic alloys) can recover their shape after deformations as great as 20% ([ 1 ][1]) just by unloading the force on the material. They are part of the larger group of shape-memory alloys but do not require a temperature change for recovery, and they have found applications in areas including robotics, structural engineering of buildings, and aerospace engineering ([ 2 ][2], [ 3 ][3]). A superelastic alloy usually exhibits this property only over a well-determined and often small temperature range normally called a “superelastic window.” On page 855 of this issue, Xia et al. ([ 4 ][4]) describe superelastic “invar” alloys with functional properties that are independent of temperature over the largest superelastic window reported, from 10 to 473 K. ![Figure][5] Widening the superelastic window Superelastic alloys can recover their shape after deformation over a small temperature range. Xia et al. now report an almost temperature-independent response for a superelastic iron alloy. GRAPHIC: C. BICKEL/ SCIENCE Superelasticity is explained by the presence of a stress-induced martensitic transformation. In this solid-solid displacive transition between two crystal structures, usually named austenite and martensite after the phases originally described in steel, much harder martensite forms by rapid quenching of austenite. This transition does not require diffusion because the atoms move over distances much smaller than the interatomic distance. The structure of these phases depends on the specific material ([ 5 ][6]). A schematic plot of a stress-deformation cycle corresponding to a martensitic stress-induced transition obtained at constant temperature (see the figure, left) is accompanied by a schematic of the evolution of a material sample as the transformation cycle goes on. The sample is initially fully austenitic (shown in gray). After a tensile stress is applied, the sample is elastically deformed; when the applied stress reaches a critical value σT, a first martensitic plate forms (shown in blue). A constant-stress plateau in the curve characterizes both the direct and inverse transitions, usually named transformation and retransformation, respectively. The end of the transformation plateau corresponds to the sample completely transformed to martensite. The inverse path is also shown until the material recovers the form when it reaches the original austenitic state. The functional properties of a superelastic alloy that determine its performance are the critical stress to induce the martensitic transformation σT, the retransformation stress σR, the hysteresis width H σ, and the maximum recoverable strain εmax. The dissipated energy in each transformation cycle, determined by the enclosed area of the cycle, increases as the hysteresis width and maximum recoverable deformation enlarge. This energy dissipation makes these alloys potential candidates as damping materials in devices to be used in buildings or aerospace structures ([ 2 ][2], [ 3 ][3]). The variation of functional properties with temperature can be strongly detrimental for several types of applications, such as damping devices in platforms that operate over a wide temperature range. Finding new alloys with functional properties independent of temperature variations would strongly increase the benefit of using superelastic alloys. For example, devices in space applications must function over a wide range of temperature variations ([ 2 ][2], [ 6 ][7]). Unlike a more conventional shape-memory alloy ([ 7 ][8]), the superelastic response of the Fe-Mn-Al-Cr-Ni alloy reported by Xia et al. can be considered independent of temperature (see the figure, right). The family of iron-nickel–based invar alloys already exhibits very low thermal expansion. The addition of Cr led to superelastic alloys in which this mechanical response was independent of temperature. It is known from thermodynamic concepts related to martensitic transformations (first-order transitions) that the variation with temperature of the critical transformation stress needed to induce the martensite phase linearly depends on the entropy change between the involved phases. Thus, there is a thermodynamic magnitude that controls the effect of temperature on the most important parameter that characterizes the superelastic effect—that is, σT. If a specific superelastic alloy is designed to have a negligible amount of entropy change between the austenite and martensite phases, superelasticity independent of temperature should result. Xia et al. showed that it is possible to control the entropy change with the addition of Cr to an Fe-Mn-Al-Ni alloy, and in this way, a composition was found where the entropy change was nearly zero. This remarkable result may be connected with magnetic order transitions that are present in both austenite and martensite phases, and this aspect deserves more attention in future research. Finally, Xia et al. show that the search for superelastic alloys with tunable entropy changes between austenite and martensite phases—for example, by varying composition—constitutes a powerful tool for designing alloys with remarkable properties. 1. [↵][9]1. F. C. Bubani, 2. M. Sade, 3. F. Lovey , Mater. Sci. Eng. A 543, 88 (2012). [OpenUrl][10] 2. [↵][11]1. J. Mohd Jani et al ., Mater. Des. 56, 1078 (2014). 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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/288065
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Paulo La Roca,Marcos Sade. Designing a wider superelastic window[J]. Science,2020.
APA	Paulo La Roca,&Marcos Sade.(2020).Designing a wider superelastic window.Science.
MLA	Paulo La Roca,et al."Designing a wider superelastic window".Science (2020).