Sintering Mechanisms of Mechanically Alloyed High Entropy Alloy Powders

Mane, Rahul B and Panigrahi, Bharat Bhooshan (2019) Sintering Mechanisms of Mechanically Alloyed High Entropy Alloy Powders. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

The high entropy alloys (HEA) have emerged as a new class of alloy with many attractive properties. Initially, alloy consists of a solid solution of five or more principal elements with almost equiatomic ratio are called HEA, but lately, several alloys with a solid solution of four elements were also included in this class. Composition, mixing enthalpy, configurational entropy, and several other factors affect the formation of a solid solution. Though initially four core effects were proposed to affect the behavior of HEA, such as configurational entropy sluggish diffusion, lattice distortion, and cocktail effect; however, all these are still debatable. It was started with the discovery of Cantor alloy (CoCrFeNiMn) with a single solid solution; since then several alloys were reported, and it has taken huge attention worldwide. The increasing popularity is mainly due to its ultra-high strength, considerable ductility, slow grain growth, good oxidation, and corrosion resistance, etc. Apart from melting route (i.e., induction melting or arc-melting), mechanical alloying followed by sintering, has been one of the widely used methods for producing HEAs. While synthesizing, achieving a fully single phase solid solution, had been a major challenge. Formation of a single-phase solid solution exits in a very narrow zone in the multicomponent alloy system. Sintering studies on a variety of HEA powders are available in the literature; however, these studies have focused mostly on densification process or phase evolutions. Sintering kinetics of high entropy alloy powders have not been fully understood so far. It had been earlier reported that diffusivity on a given system, may be affected by its configurational entropy. This leads to another question, whether the sintering process, which depends on diffusion behavior, could also be affected by increasing entropy or not. The present investigation aims: i) To synthesize the high entropy alloy powders of Face Centered Cubic (FCC) based (CoCrFeNi, and CoCrFeNiMn) and Body Centered Cubic (BCC) based (TiCrFeNi and AlCoCrFeNi) alloys through mechanical alloying. ii) To study the effect of increasing number of elements and configurational entropy, on sintering kinetics of HEA powders. iii) To study the effect of initial conditions of CoCrFeNi and CoCrFeNiMn HEA powders (i.e., as-milled state and phase stabilized powders (through heat treatment) on sintering kinetics, and iv) To determine the sintering mechanisms of CoCrFeNi and CoCrFeNiMn HEA powders through employing existing sintering models. Nanocrystalline powders of two FCC HEAs (equiatomic CoCrFeNi and CoCrFeNiMn alloys, designated as 4F and 5F respectively) and two BCC HEAs (equiatomic TiCrFeNi and AlCoCrFeNi alloys, designated as 4B and 5B respectively) were synthesized through mechanical alloying. For a given phase, crystallite size was found to be decreased with an increasing number of elements. Both non-isothermal as well as iso-thermal sintering were performed on a vertical dilatometer. Non-isothermal shrinkage data were analyzed. It was observed that for a given number of alloying elements, FCC alloys exhibit slightly larger activation energy than BCC alloys. Results suggest a noticeable trend in the estimated activation energy of sintering, with alloying numbers and configurational entropy. For a given phase (FCC or BCC), higher the configurational entropy of the alloy, larger the activation energy of sintering. It was also found that the grain coarsening slows down with increasing alloying elements. It is expected that as-milled powders may contain high defect concentration, large grain boundary fraction and metastable phases, etc. However, through the annealing of the powder, effect of many of these factors can be minimized. A comparative study on sintering kinetics of two different initial conditions of CoCrFeNi and CoCrFeNiMn HEA powders (i.e., asmilled state and phase stabilized powders through heat treatment prior to compaction and sintering) has been made. Dilatometric results show a decrease in densification for annealed powder, compared to as-milled powders. The activation energies evaluated through nonisothermal sintering using grain boundary diffusion (GBD) and volume diffusion (VD) mechanisms show increased values after annealing. The estimated activation energies were compared with the available data in the literature for different mechanisms for constituting elements. Results show, the activation energies of GBD mechanism are close to that of reported data; suggesting the possibility of dominating GBD mechanism in as-milled powders during initial sintering. Whereas activation energy measured for VD in CoCrFeNiMn annealed powders are in close proximity to the literature data; suggesting that in annealed powder VD may be a dominating sintering mechanism during non-isothermal sintering. To get a more clear understanding about sintering mechanisms of HEA powders in initial stage sintering, isothermal sintering data were analyzed through the sintering models (Johnson 1969) for both VD and GBD mechanisms. Employing these models, requires some materials specific data, namely atomic volume () and specific surface energy (s), etc., viii which were not available in the literature. Therefore, values of s were determined experimentally whereas atomic volume () was estimated theoretically. Another issue was, about using the existing models (which were developed for spherical powders) for the current powders (which are highly irregular in shape and varied size). Therefore, to overcome such problems, models were employed in a novel way in the current work, by normalizing with respect to the grain size. With the help of these equations, both coefficient of diffusion as well as activation energy for a given mechanism (VD or GBD) could be obtained from the shrinkage data of HEA powders. In the next step, the set of values (coefficient of diffusion along with respective activation energy) were compared with the available set of values in the literature for various constituting elements in the alloy. In the as-milled 4F and 5F powders, domination of any single sintering mechanism was ruled out. Whereas, in the annealed 4F and 5F powders, volume diffusion was clearly identified as dominating sintering mechanism. The thesis has been organized into five chapters, and a brief overview has been given below: The first chapter mainly consists of three sections. First section presents the historical development of HEA, and some properties. Second section highlights the synthesis routes and sintering of HEAs. Third section deals with the different methods of sintering, sintering mechanisms and relevant sintering models. At the end of the chapter, objectives of the present work, have been listed. Second Chapter deals with the experimental procedure; it includes mechanical alloying, compaction, sintering using dilatometer, etc. It also includes characterization of powders and sintered samples. The third chapter presents the results of powder synthesis, sintering, phase evolution, microstructures and other characterizations. Effect of increasing alloying and configuration entropy on sintering kinetics has been shown for both FCC and BCC HEAs. Further, the effect of initial powder condition of two FCC powders on sintering, have also been shown. In the Fourth Chapter, obtained results at various sections have been discussed and analyzed. In the first section, phase formation has been discussed. In the second section effect of increasing elements in HEA on sintering kinetics, has been discussed. The comparison of diffusion parameters with existing literature to find out controlling mechanisms have been discussed. The last chapter (fifth chapter) summaries major findings of the present investigation, including powder synthesis, phase formation, powder sintering, and sintering mechanisms.

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