Compaction behavior of water-atomized CoCrFeMnNi high-entropy alloy powders

https://doi.org/10.1016/j.matchemphys.2017.06.013Get rights and content

Highlights

  • The compaction behavior of CoCrFeMnNi high-entropy alloy powders of various sizes is studied.

  • The size and size-distribution of the powder particles are important factors in the tap density.

  • The strength of the matrix is the factor governing the final compact density.

  • The simulation densification behaviors are in good agreement with experimental ones.

Abstract

In this work, compaction behavior of CoCrFeMnNi high-entropy alloy powders with various particle sizes and size distributions, produced by water atomization, was investigated experimentally and theoretically. Theoretical modeling was employed using a pressure-dependent yield function in associated with a phenomenological constitutive model. Results for the quantitative densification behaviors from the experimental and theoretical analyses are in good agreement. We found that the size and size-distribution of the powder particles are important factors in the tap density as with conventional powder compaction. The compact density of large powder particles with coarse dendrite arm spacing is high due to low deformation resistance and low strain hardening (i.e., low evolution of dislocation density).

Introduction

High-entropy alloys (HEAs) have changed the concept of conventional alloys based on principal elements such as aluminum and titanium alloys. The HEAs developed by Yeh et al. are defined as new alloy systems containing at least five principal elements, with the concentration of each constituent element 5–35 at% [1]. Most HEAs form a simple face-centered cubic or body-centered cubic solid solution, without formation of intermetallic compounds or complex phases, due to their high configurational entropy [2], [3], [4]. The HEAs have also received attention for use in a wide range of potential applications such as those in the aerospace and energy sectors [3], [4], [5], [6], [7].

Most HEAs have been synthesized using liquid-phase processing methods (e.g., arc melt casting [8], [9], [10] and induction melting [11]). However, these methods have limitations of phase segregation and formation of shrinkage defects which lead to non-uniform microstructures and mechanical properties. On the other hand, powder metallurgy (PM) processes not only control over phase compositions, but also can prepare fine-grained alloys compared with conventional methods. Therefore, fabrication of bulk HEAs using PM technology has been investigated in many previous studies [12], [13], [14].

In fact, the compaction behavior of powders is influenced by many factors, such as composition, particle size, size distribution, and powder morphology as well as compaction strategy, lubricant, and friction with die walls [15], [16], [17]. The mechanics of powder compaction has widely been studied for the last several decades through several models with the experimental data [18], [19], [20]. However, detailed studies on the compaction behavior of HEA powders have not been reported yet, althoug it significnatly affects the next processing, e.g. sintering and post-sintering. Therefore, understanding the compaction behavior of the HEA powders is necessary as a very basic process in powder metallurgy.

Among various HEAs, CoCrFeMnNi HEA has been widely studied due to its superior combination of high strength and large elongation, as well as its superior fracture toughness at both room and cryogenic temperatures [5], [21], [22], [23], [24], [25]. These alloys also have promising mechanical properties at elevated and at cryogenic temperatures [26].

In this paper, we report a systematic study to understand the compaction behavior of CoCrFeMnNi HEA powders with various particle sizes produced by water atomization. This is a widely used powder making route that provides good homogeneity and potential for mass production. Finally, the compaction behavior of the water atomized HEA powders was analyzed theoretically using a pressure-dependent yield function in associated with a microstructure-based phenomenological constitutive model [27].

Section snippets

Experimental procedure

In order to synthesize the HEA powders (Co, Cr, Fe, Mn, and Ni), high purity Co (>99.95 wt%), Cr (>99.95 wt%), Fe (>99.99 wt%), Mn (>99.9 wt%), and Ni (>99.99 wt%) chips with sizes ≤ 5 mm were melted using high-frequency induction melting at 1873 K. The molten alloy was atomized by a water stream at 30 bar pressure, with a nozzle 7 mm in diameter. The atomized powders were mechanically sieved using a conventional sieving method into four fractions: (<45, 45–75, 75–105, and 105–150) μm; referred

Atomized powders

Fig. 1(a)–(d) show SEM micrographs of the water atomized HEA powders. Small particles were spherical (Fig. 1a and b) and large particles were relatively flat with rounded edges (Fig. 1c and d). These results indicate that the high heat energy of the molten alloy droplets originated from overheating of the liquid metal. This makes the cooling time to room temperature longer, even when being manufactured by water atomization [30]. The particle sizes and distributions illustrated in Fig. 2 exhibit

Experimental compaction behavior

According to the compaction pressure vs. relative density curves (Fig. 6a and b), the tap densities in increasing order were Groups A, B, D, and C. The factors affecting compactibility, including tap density; were microstructure, shape, size, and size distributions of the powder particles. Because the morphology of the particles is mostly spherical, this compaction curve is a direct result of the sizes and size distributions of the powder particles. In Groups A, B, and D, the tap densities

Conclusions

In this study, the compaction behavior of HEA powders with various particle sizes was explored experimentally and theoretically. The following conclusions were drawn.

  • (1)

    The factors determining the tap density of powders are particle size and size distribution. The smaller the size of the powders, the smaller the tap density because the friction in the powders increases as the surface area increases. The size distribution is also the impact on the tap density because smaller particles can fill the

Acknowledgement

This work was supported by the Future Material Discovery Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (MSIP) of Korea (2016M3D1A1023384).

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