Sintering mechanisms of mechanically alloyed CoCrFeNi high-entropy alloy powders
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ARTICLE Sintering mechanisms of mechanically alloyed CoCrFeNi high-entropy alloy powders Rahul B. Mane and Bharat B. Panigrahia) Department of Materials Science and Metallurgical Engineering, Indian Institute of Technology Hyderabad, Sangareddy, Telangana 502285, India (Received 1 March 2018; accepted 19 June 2018)
High-entropy alloys (HEAs) are receiving considerable attention since last decade because of their ability to give excellent strength with reasonably good elongation during fracture. The mechanical alloying followed by sintering is one of the routes for fabrication; however, there are limited reports on sintering mechanisms of HEA powders. The present investigation studies sintering mechanisms of CoCrFeNi alloy powders in as-milled and annealed conditions using dilatometer experiments. The annealed powder shows slower densification behavior and higher activation energy of sintering, compared to the as-milled powder. Diffusion coefficients were analyzed through sintering models and compared with literature data. The as-milled powder was found to exhibit mixed response, i.e., the grain boundary diffusion seems to be dominating initially due to a large grain boundary fraction but volume diffusion (VD) also contributes significantly, due to high defect concentration and metastable phases. VD was found to be the dominating mechanism during sintering of single phase, stable annealed powder.
I. INTRODUCTION
The development of high-entropy alloys (HEAs) is opening windows for replacement of conventional alloys for various applications in different operating conditions, ranging from cryogenic temperature to elevated temperatures. HEAs are also called multicomponent concentrated alloys (MCCAs).1–4 Initially, the HEA was defined as having at least five or more major elements, where the concentration of elements may vary from 5 to 35%. Later, any quaternary alloys (having four elements), which exhibit behaviors similar to HEAs, were also considered under this class, such as CoCrFeNi,5 AlCoCrFe,6 NbMoTaW,7 etc. Recently, some special variants of high-entropy materials were also reported, such as highentropy superalloys, high-entropy refractory alloys, high-entropy bulk metallic glasses, high-entropy carbides, high-entropy nitrides, high-entropy oxides,8 etc. These new classes of special high-entropy materials are also having potential in applications, such as functional coatings, cutting tools, machine parts, temperature barriers, etc. The four core effects, such as configurational entropy, sluggish diffusion, severe lattice distortion, and cocktail effects, are noteworthy in HEAs than conventional alloys.8,9 The effect of entropy on conventional alloys was previously not given much importance, where prediction of equilibrium phases was done, mostly based on mixing enthalpy. It is now believed that high a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.225 J. Mater. Res., 2018
configurational entropy helps to stabilize the phases in MCCAs.4 The lattice in such an alloy gets d
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