Microstructure and Room Temperature Mechanical Properties of Different 3 and 4 Element Medium Entropy Alloys from HfNbTaTiZr System

Abstract

Refractory high entropy alloys (HEA) are promising materials for high temperature applications. This work presents investigations of the room temperature tensile mechanical properties of selected 3 and 4 elements medium entropy alloys (MEA) derived from the HfNbTaTiZr system. Tensile testing was combined with fractographic and microstructure analysis, using scanning electron microscope (SEM), wavelength dispersive spectroscope (WDS) and X-Ray powder diffraction (XRD). The 5 element HEA alloy HfNbTaTiZr exhibits the best combination of strength and elongation while 4 and 3 element MEAs have lower strength. Some of them are ductile, some of them brittle, depending on microstructure. Simultaneous presence of Ta and Zr in the alloy resulted in a significant reduction of ductility caused by reduction of the BCC phase content. Precipitation of Ta rich particles on grain boundaries reduces further the maximum elongation to failure down to zero values.Entropy 2019, 21, 114 2 of 17 Different elements are used to fabricate HEAs. Although the number of possible 4 or 5 elements combinations is enormous five basic metallic HEA groups can be distinguished: FCC HEAs based on the Cantor alloy, BCC refractory metal HEAs, light element HEAs, HCP HEAs and precious functional HEAs [7]. Ceramic HEAs have been established as well [8,9].Our attention was attracted by the refractory HEA group, which is composed of elements from IV, V and VI groups of the periodic table of elements [10]. These elements are characterized by prevailing good mutual miscibility and high melting points. The melting point of titanium (1668 • C) is the lowest among them. Therefore HEAs composed of these elements are intended for high temperature applications. Moreover some of these elements are biocompatible [11]. Therefore some of these alloys can be attractive for bioimplant related materials research. Note that β-Ti alloys used in bioimplant research are usually composed of this group of elements [12]. Several biocompatible HEAs have been already studied [13][14][15][16]. Also, hard ceramic composites were produced from the metals from IV, V and VI groups of the periodic table [17].Original definition of HEAs with single phase solid solution microstructure and good room temperature mechanical properties, especially ductility, is beneficial also for use in human medicine. Inspiration can be found also in other materials used for production of bioimplants which are used or have been used in other parts of material research, for example, TiAl6V4 in aerospace, CoCrMo in aero engines.Our first attempt in research of HEAs was therefore related to HfNbTaTiZr refractory metal alloy [18]. Our research was inspired by Senkov [19] and confirmed our expectations. Ingots produced by vacuum arc melting possessed single phase solid solution microstructure, with a high room temperature tensile strength and ductility.Some of the elements used in the HfNbTaTiZr alloy are very similar to each other with very similar chemical behaviour; Nb is chemically similar to Ta and Zr to ...

New approach to model the yield strength of body-centered cubic solid solution refractory high-entropy alloys

“…With changing the alloy √ Table 2 Values of coefficients obtained after fitting the proposed polynomial to the experimental dataset in Table 1 Coefficient The hardest alloy in the Hf-Mo-Nb-Ta-Ti-V-Zr system within the investigated composition range is predicted to be Hf 15 Mo 20 Nb 5 Ta 15 Ti 5 V 20 Zr 20 with the yield stress of 1792 MPa. According to the tensile-ductility data in previous research works [34,[41][42][43][44][45][46][47][48][49][50][51][52], alloys with tensile yield stresses less than 1200 MPa could have tensile ductility higher than 5% (Fig. 8).…”

“…Tensile ductility versus tensile yield stress for single-phase bcc RHEs[34,[41][42][43][44][45][46][47][48][49][50][51][52] …”

Microstructure and Mechanical Properties of Sintered and Heat-Treated HfNbTaTiZr High Entropy Alloy

“…From the study of β-Ti alloys, it is known that oxygen can significantly increase the tensile strength and change the deformation mechanisms [63]. Also, the hardness of hot deformed (without residual porosity) specimens showed significantly higher hardness values (i.e., 450 HV10) than the hardness of arc melted HfNbTaTiZr specimens, which is reported to be 300-350 HV10 [19,44,45,64]. This difference can be ascribed to the strengthening effect of oxygen (interstitial strengthening).…”

“…Only a few works using the powder metallurgy process for the synthesis of HfNbTATiZr (or similar) HEAs have been reported to date (e.g., [9,43]). Our previous works reported on the preparation of the HfNbTaTiZr equimolar alloy via the arc melting [44][45][46] and SPS [47] processes. However, the disadvantage of the SPS technology is complicated powder preparation.…”

New Advances in High-Entropy Alloys

“…The Hf-Nb-Ta-Ti-Zr refractory HEA shows great high-temperature properties and room-temperature properties. Zýka et al [ 44 ] presented investigations of the room-temperature tensile mechanical properties of selected three- and four-element medium entropy alloys derived from the Hf-Nb-Ta-Ti-Zr system, and they found that it is the five-element HEA alloy that exhibits the best combination of strength and elongation. Tseng et al [ 45 ] aimed to reveal the effects of Mo, Nb, Ta, Ti, and Zr on mechanical properties of equiatomic Hf-Mo-Nb-Ta-Ti-Zr alloys.…”