Nanocrystalline high-entropy alloys
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This article is a review of research on nanostructured high-entropy alloys with emphasis on those made by the severe plastic deformation methods of mechanical alloying and high-pressure torsion. An example of thin film refractory metal alloys made by magnetron sputtering is also presented. The article will begin with a discussion of the seminal research of B.S. Murty and co-workers who first produced nanocrystalline high-entropy alloys by mechanical alloying of powders. This will be followed by a listing of research, in mostly chronological order, of mainly 3d transition metal alloys made nanocrystalline by mechanical alloying. Research on the well-studied Cantor alloy, from the literature and the author’s laboratory will be presented. The author’s and co-worker’s research on a low-density high-entropy alloy with single-phase fcc or hcp structure and an extremely high strength (hardness)-to-weight ratio will be described. I. INTRODUCTION
High-entropy alloys have been defined as multicomponent alloys with equal or near-equal molar ratios of the components (unlike most alloys with one or two major components) with at least 4 or 5 components. Since configurational entropy is not always the determining factor in the structures observed, they are often referred to as multiple-principal element alloys. While there has been intense interest in high-entropy alloys, or more generally, multiprincipal component alloys, it has been estimated1 that only about 5% of those made so far have been processed by the solid-state technique of mechanical alloying that typically results in nanocrystalline microstructures in the as-milled powders. It is materials with the nanocrystalline structure produced by this method, and with the other severe plastic deformation method of high-pressure torsion, that will be the subject of this article. Nanocrystalline structures have also been observed in high-entropy alloy thin films made by various physical vapor deposition methods.2 These materials, with the exception of thin film refractory metal alloys, will not be covered in this review. Mechanical attrition—the ball milling of powders—is often divided into “mechanical alloying” (MA) where compositional changes occur in the milling of dissimilar powders (alloying on the atomic level) and “mechanical milling” (MM) where structural changes can be induced on ball milling of single-component powders
Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2017.341
(elements or compounds) such as solid-state amorphization or production of nanocrystalline grain microstructures. Mechanical alloying (MA) will be one focus of this article. The several advantages of the mechanical alloying process include its versatility. Almost any material can be produced by this method including ductile metal alloys, brittle intermetallic compounds, and composites. Mechanical alloying, which is typically carried out at room temperature or below, by
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