New (FeCoCrNi)-(B,Si) high-entropy metallic glasses, study of the crystallization processes by X-ray diffraction and Mössbauer spectroscopy.
Introduction
High-entropy alloys (HEA) have attracted significant research efforts in materials science and engineering due to remarkable properties. In particular, their outstanding mechanical properties, such as excellent wear resistance, high strength and exceptional ductility, make them an excellent choice for industrial applications [1]. HEA are multicomponent solid-solutions of several elements, all of them in equal or near-equal atomic percent, this leading to a high entropy of mixing. In some cases, it is found a complex microstructure, including intermetallic compounds which make them brittle and difficult to process. However, for some HEAs only body-centered cubic (BCC) or face-centered cubic (FCC) solid solutions form, without the presence of intermetallic compounds [2]. This type of alloys has led to a new design strategy for the development of new metallic materials, in particular in the field of metallic glasses (MGs). The appearance of some high-entropy metallic glasses (HEMGs) has introduced a new way to obtain new metallic glass-forming compositions. Instead of tailoring the properties of MGs through microalloying, i.e. minor additions of elements with less than 1 or 2 at%, the adequate design of HEMGs can combine the excellent properties of high-entropy alloys with the unique characteristics of metallic glasses. The non-crystalline structure of MGs results in a large elastic region, ultrahigh strength as well as good soft magnetic properties [3]. Moreover, the high-entropy alloy design could lead to enhancements of the glass forming ability of metallic glasses. For example, glass-forming Zr-Ti-Cu-Ni-Be compositions (with or without Hf) have been studied showing an increased glass transition temperature and a slower crystallization process that leads to FCC and BCC solid solutions together with some intermetallic phases [4,5].
The HEA FeCoCrNi has been extensively studied and its strength has been improved through the addition of minor elements that induce the precipitation of second phase grains [6,7]. This composition was used by Ding et al. as a base composition for developing new HEMGs in which 18 to 22 at% of B was added [8], thus combining the characteristics of a well-known glass-forming alloy (Fe80B20) with the HEA design strategy in which the Fe atoms were substituted by equal amounts of Fe, Co, Cr and Ni. These HEMGs show a higher hardness, ductility and improved corrosion resistance as compared with the Fe-B amorphous alloys. On the other hand, Qi et al. produced (FeCoNi)(B,Si) high-entropy bulk metallic glasses with good soft magnetic and mechanical properties [9]. Thus, HEMGs with this set of elements could offer a combination of properties very useful for industrial applications. However, a complete characterization of the microstructural development during annealing is needed in order to be able to tailor their properties by means of thermal treatments and to control the evolution of their properties during working conditions in actual applications.
In this work we report the production of new HEMGs within the (FeCoCrNi)100-x-yBxSiy compositional system. The mechanical and magnetic properties of these alloys will be reported in a following paper. Here, we will focus on the characterization of the several stages of the relaxation of the amorphous structure and the crystallization process. The compositions with x = 20, y = 0 and x = 10, y = 10 were selected with this purpose, as representatives of non-Si and Si containing alloys, in order to perform a detailed study of the microstructural development from the as-quenched samples to the fully crystalline ones. The microstructural characterization was performed using X-ray diffraction and transmission Mössbauer spectroscopy. The use of Mössbauer spectroscopy yields the quantitative atomic percentage of Fe atoms in a particular phase or in a particular environment, thus giving quantitative information of the iron containing crystalline phases at each stage that is difficult to obtain with other techniques. Furthermore, being a local probe, information about the local structural or chemical order can be obtained and in further works correlated to the macroscopic properties. Due to the complexity of the system, with 4 different metal atoms in equal concentration, the characterization of the crystalline route was performed identifying the overall structure of the phases, without determining the exact distribution of metal atoms inside the boride and silicide structures, similarly to other works in this type of materials [9,10]. This strategy is supported by the use of Mössbauer spectroscopy that can identify or discard the presence of pure Fe-based phases through their hyperfine parameters.
Section snippets
Materials and methods
The (FeCoCrNi)(B,Si) master alloys with the different compositions were produced by arc-melting of pure elements: Iron sheet (99.9 wt%), Cobalt lumps (99.5wt%), Chromium lumps (99.95 wt%), Nickel wire (99.98 wt%), Boron lumps (99wt%), and Silicon lumps (99.999 wt%). In all the compositions the master alloy is equiatomic in Fe, Co, Cr and Ni. For comparison purposes the B and Si free FeCoCrNi HEA (in this article labeled as composition A) was also produced. The arc-melting process was performed
Glass formation
Fig. 1 shows the different compositions of the (FeCoCrNi)100-x-yBxSiy system that were produced, indicating the state of the as-quenched ribbons. Glass-forming ability is observed for composition (FeCoCrNi)80B20, as shown previously [8], as well as for the series of compositions substituting B by Si up to (FeCoCrNi)80B10Si10. Within this range of compositions, the partial substitution of B by Si is not detrimental for the formation of an amorphous structure, thus obtaining, as far as we know, a
Conclusions
In the present work, several compositions of high entropy metallic glasses have been produced: (FeCoCrNi)80B20, (FeCoCrNi)80B15Si5, (FeCoCrNi)75B10Si15 and (FeCoCrNi)80B10Si10, the three last ones reported for the first time. The crystallization and microstructural evolution of two of them, (FeCoCrNi)80B20 and (FeCoCrNi)80B10Si10, have been studied by means of X-ray diffraction, Mössbauer Spectroscopy and scanning electron microscopy. In both compositions the crystallization begins with the
Author statement
All authors acknowledge that the material presented in this manuscript has not been previously published, except in abstract form, nor is it simultaneously under consideration by any other journal. This manuscript or a very similar manuscript has not been published, nor is under consideration by any other journal.
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
L. Panahi acknowledges the financial support from Generalitat de Catalunya through a FI grant 2018FI_B_00502. P. Bruna and E. Pineda acknowledge the financial support from MINECO grant FIS2017-82625-P and from Generalitat de Catalunya grant 2017-SGR-42. X-ray diffraction experiments were performed at MSPD beamline at ALBA Synchrotron with the collaboration of ALBA staff. The authors acknowledge the priceless help of Dr. Trifon Trifonov at the facilities of the Barcelona Research Center in
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