Elsevier

Journal of Alloys and Compounds

Volume 775, 15 February 2019, Pages 742-751
Journal of Alloys and Compounds

Microstructure and mechanical properties of FeCoCrNiMnAlx high-entropy alloys prepared by mechanical alloying and hot-pressed sintering

https://doi.org/10.1016/j.jallcom.2018.10.168Get rights and content

Highlights

  • The FeCoCrNiMnAlx high-entropy alloys were prepared by powder metallurgy.

  • The existing criteria cannot fully predict the crystal structure of the alloys.

  • The addition of Al obviously strengthened the alloys.

  • The high strength was attributed mainly to the finer grains and bcc precipitation.

Abstract

To improve the strength and hardness, Al-containing FeCoCrNiMn high-entropy alloys (HEAs) were fabricated by mechanical alloying (MA) and hot-pressed sintering. The effects of Al concentration on the microstructure and mechanical properties of the alloys were examined. It was found that the Al-containing alloys consisted of the matrix fcc or fcc + bcc duplex solid solution phases and a small quantity of M7C3 + M23C6 (where M = Cr, Mn, Fe) carbides and Al2O3 phases. A high Al concentration induced bcc precipitates in the alloys, and with the increase of Al concentration, the crystalline structure of the matrix solid solution phase changed from fcc to bcc. The addition of Al obviously strengthened the alloys, especially the alloys that contained fcc + bcc duplex solid solution phases. For example, the yield strength, compressive strength and hardness of alloy FeCoCrNiMnAl0.7 reached as high as 2230 MPa, 2552 MPa and 622 HV, respectively. The high strength/hardness was attributed mainly to the finer grains and the bcc precipitation.

Introduction

As a new kind of alloy materials, High-entropy alloys (HEAs) have attracted wide attention because of their unique microstructure and properties [[1], [2], [3], [4]]. Among various HEAs, an equiatomic FeCoCrNiMn HEA is considered to be promising [5]. This alloy has excellent ductility and toughness that exceed those of most other materials [[6], [7], [8], [9]]. However, its low strength and hardness have limited its industrial potential [9].

Currently, conventional melting/casting methods are usually used to fabricate this alloy. However, melting/casting easily cause the formation of a microstructure with coarse grains and composition segregation [9]. Fortunately, this alloy has the high Hall-Petch coefficient [9], therefore, different methods can be applied to obtain fine grains and to improve the mechanical properties, including rolling at different temperatures [[10], [11], [12], [13], [14]], high pressure torsion [15] and swaging [16] after arc melting. However, these fabrication routes are uneconomical for industrial manufacturing, and the shape and size of the final products are limited [17]. By contrast, powder metallurgy is a more convenient approach for industrial use. Preparation of HEAs by powder metallurgy usually consists of two processes: mechanical alloying (MA) and sintering. MA has been widely used to prepare the nanocrystalline HEA powders. Combined with the subsequent sintering, the bulk HEAs with ultrafine or nanosized grains can conveniently be obtained [18,19].

In addition, the alloy also was strengthened by the doping of alloying elements, such as Al [20] or C [21,22]. For example, He et al. [20] used the melting/casting method to prepare alloys with the addition of Al. It was found that the crystalline structure of the matrix solid solution phase in the alloys changed from fcc to bcc with the increase of Al concentration. The addition of Al obviously improved the mechanical properties of the alloy.

By combining the powder metallurgy approach and doping with Al, Al-containing FeCoCrNiMn HEAs were prepared by MA and sintering in order to further strengthen the alloy. Pohan et al. [23] and Wang et al. [17] used MA and spark plasma sintering to prepare FeCoCrNiMnAl0.3 and FeCoCrNiMnAl HEAs with ultrafine and nanosized grains, respectively. Al addition induced bcc precipitates in a soft bcc alloy based on FeCoCrNiMn, thus effectively strengthening the alloy. However, systematic studies on the effects of alloying on the phase evolution and strengthening mechanisms of the alloys prepared by powder metallurgy are still rare. Therefore, in this work, we sought to determine the effects of the Al concentration on the microstructure and mechanical properties of FeCoCrNiMn HEA prepared by MA and sintering and to identify the phase formation and strengthening mechanisms.

Section snippets

Experimental

The average grain sizes of Fe, Co, Cr, Ni, Mn and Al metal powders with high purity (≥99.9%) are approximately 45 μm. The FeCoCrNiMnAlx (x represents atomic proportions, that is, x = 0, 0.1, 0.3, 0.5, 0.7 and 1) HEA powders, which are referred to as alloys Al0, Al0.1, Al0.3, Al0.5, Al0.7 and Al1, respectively, were synthesized by MA in a planetary ball mill (QM-QX4L, Changsha Miqi Instrument Equipment Co., Ltd., China).

The MA process was conducted in the stainless steel vials filled with highly

Microstructure

Fig. 1a shows the XRD patterns of the FeCoCrNiMnAlx HEA powders obtained by milling for 45 h. Alloying of the metal powders was achieved after 45 h of milling. When the atomic proportion of Al is less than or equal to 0.5 (i.e., alloys Al0, Al0.1, Al0.3 and Al0.5), the alloys consist of a fcc phase as the matrix and traces of bcc phase. The diffraction peak of the bcc phase is separated from that of the fcc phase, and the diffraction peak intensity gradually increases as the Al concentration

Phase formation mechanism

In conventional steels, the doping with Al easily destabilizes fcc γ-Fe and induces the formation of bcc α-Fe [33]. Similarly, for HEAs, the doping with Al also leads to the transformation from the fcc to bcc [20,34,35]. This occurs because the Al atom has a larger radius than the other alloy elements, such as Fe, Co, Cr and Ni. The solution of Al into the FeCoCrNiMn alloy can increase the lattice strain effect, thus destabilize the fcc structure [20]. Because the bcc structure has a lower

Conclusions

FeCoCrNiMnAlx HEAs were prepared by MA and hot-pressed sintering. The microstructure and mechanical properties of the alloys were characterized, and the phase formation and strengthening mechanisms were investigated. The main conclusions are as follows.

The bulk alloys consist of the matrix fcc or fcc + bcc solid solution phase(s) and a small quantity of M23C6 + M7C3 carbides and Al2O3 phases. The single fcc solid solution phase was formed in alloys Al0 and Al0.1. With the increase of the Al

Acknowledgements

The research was supported by the Major Industry-academy Cooperation Project of Fujian Province (Grant No. 2014H6005).

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