Evolution of Nb oxide nanoprecipitates in Cu during reactive mechanical alloying
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2D AND NANOMATERIALS
Evolution of Nb oxide nanoprecipitates in Cu during reactive mechanical alloying Qun Li1,a) , Xuekun Shang2,b), Blanka Janicek1, Pinshane Y. Huang1, Pascal Bellon1, Robert S. Averback1 1
Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana 61801, Illinois, USA Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China a) Address all correspondence to these authors. e-mail: [email protected] b) e-mail: [email protected] 2
Received: 24 August 2019; accepted: 22 November 2019
Microstructural evolution of Cu–Nb oxide nanocomposite alloys during ball milling is investigated using a two-step ball-milling approach. In the first step, Cu and Nb powders are milled to create a two-phase alloy comprising a Cu-rich matrix containing a high density of 20- to 30-nm Nb precipitates. In the second step, this nanocomposite is co-milled with CuO, resulting in the reduction of CuO and the oxidation of the Nb nanoprecipitates. Transmission electron microscopy characterization shows that three distinct types of Nb oxide precipitates evolve at different levels of strain. First, nanocrystalline NbO particles (∼10 nm) are formed by dissolved Nb in Cu reacting with oxygen evolved from the CuO. Next, the Nb nanoprecipitates in Cu further reduce CuO to form Nb/Nb oxide and NbO/Nb oxide core–shell inclusions (20–30 nm). These inclusions coalesce during additional milling to form amorphous Nb oxide agglomerates (>700 nm after 50 h). The growth of Nb precipitates during step-one milling, the initial growth of NbO nanoparticles, and the formation of core–shell Nb oxide precipitates during step-two milling are attributed to the convective transport of atoms and clusters combined with shear-induced agglomeration.
Introduction Nanocomposite materials in bulk form are becoming increasingly important for a wide range of advanced technologies owing to their often exceptional properties, e.g., high strength [1], resistance to radiation damage in nuclear environments [2, 3], and superior electrical charging characteristics in batteries [4], to highlight a few. A significant challenge in developing these materials, however, derives from the difficulty in processing nanocomposites in bulk quantities without greatly increasing their microstructural length scales. One attractive method to overcome this problem involves mechanical alloying (MA), such as ball milling [5] or high pressure torsion (HPT) [6], as these methods are relatively inexpensive and applicable to a wide range of material systems. They enable, moreover, the synthesis of not only nanocomposite metals but also nanocomposite compound phases by chemo-mechanical displacement reactions [7]. While MA has been extensively studied ever since the pioneering work of Benjamin some 50 years ago [8], and many nanocomposite alloys have now been
ª Materials Research Society 2020
synthesized, an understanding of the fundamental MA processes involved in the formation
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