A study on the severely cold-rolled and annealed quaternary equiatomic derivatives from quinary HfNbTaTiZr refractory high entropy alloy

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Highlights

  • HfNbTaTi and HfNbTiZr equiatomic alloys exhibited great malleability and phase stability.

  • Grain growth activation energies, QG, of the HfNbTaTi and HfNbTiZr alloys are 385.9 kJ/mol and 189.8 kJ/mol, respectively.

  • The low self-diffusion coefficient of Ta would hinder the grain growth and increase the QG value.

Abstract

The malleability, phase stability and grain growth kinetics of the quaternary equiatomic derivatives from quinary HfNbTaTiZr refractory high entropy alloy cold-rolled with 80% thickness reduction and annealed under different conditions were investigated. By excluding the poor malleability of the HfTaTiZr and NbTaTiZr alloys and the appearance of two phase in the HfNbTaZr alloy, the grain growth kinetics of the HfNbTiZr and HfNbTaTi alloys were further studied. The grain growth exponent n, kinetic constant k and activation energy for grain growth QG of the HfNbTiZr and HfNbTaTi alloys are calculated. The kinetic constant can be expressed by the Arrhenius equation with QG, which is attributed to the diffusion coefficient. It is demonstrated that the alloying element Ta with the lowest self-diffusion coefficient among these five elements would remarkably decelerate grain boundary migration, thereby hindering grain growth and increasing QG value.

Introduction

High entropy alloys (HEAs) with multiple principal elements rather than only one dominant element has explored a new path of advanced materials with promising properties. Not only the physical properties of high wear-resistance and great hardness but also the chemical properties of good corrosion resistance and biocompatibility were widely investigated [[1], [2], [3], [4], [5], [6]]. For FeCoNi alloy system, the introduction of Cu and process improvement by rapid cooling could facilitate the formation of ultrafine grain, leading to the high yield strength with good plasticity [3]. The addition of Mo into CoCrFeNiTi alloy could also enhance the corrosion resistance against acidic, marine and basic environments [4]. Besides, due to the vastness of the composition space with multiple constitutional elements in HEAs, many crystalline structures appeared in different alloy systems or in same system with different composition [1,6]. BCC-structured refractory high entropy alloys (RHEAs) are arousing global interests followed by over ten years of development of FCC-structured high entropy alloys (HEAs) [2,7,8]. Literally, RHEAs exhibit remarkable elevated-temperature strength with the primary aim of high-temperature applications [[9], [10], [11]]. It is expected that the RHEAs can have the great phase stability and mechanical properties at high temperatures, suggesting no formation of second phase precipitates and sufficient strength with moderate ductility at high temperatures for long durations [12]. However, compared to the FCC-structured HEAs, although BCC-structured RHEA could reached extremely great hardness of over 5 GPa [2], most of the BCC-structured RHEAs showed higher brittleness with very small plastic strain. In recent years, one of a few exceptions, HfNbTaTiZr RHEA, has rapidly received great attractive attentions [8,[13], [14], [15], [16], [17]]. HfNbTaTiZr alloy possessed excellent phase stability with a single BCC structure at over 1000 °C. Most impressive property of HfNbTaTiZr alloy was its superior malleability at ambient temperature which could reach almost 90% thickness reduction by cold-rolling at room temperature (RT). Therefore, grain refining, regarded as an improvement of mechanical properties, can be further conducted by cold-rolling and annealing. Numerous studies have investigated the kinetics of recrystallization and grain growth in quinary HfNbTaTiZr alloy [[18], [19], [20]]. To further realize the effect of alloying elements on phase stability and grain growth behavior in this alloy, it is essential to study the equiatomic subsystems of HfNbTaTiZr alloy. In this study, the phase stability and grain growth kinetics exhibited in the quaternary derivatives of HfNbTaTiZr RHEA, i.e., HfNbTaTi, HfNbTaZr, HfNbTiZr, HfTaTiZr and NbTaTiZr five alloys, were primarily investigated. Two requirements are set up to screen these five quaternary equiatomic alloys to select the alloys for the further study of grain growth kinetics: (1) good malleability by cold-rolding at RT and (2) phase stability of single BCC phase in the specimen after cold-rolling and annealing. In addition, the factors that affect the grain growth characteristics and the corresponding activation energies of these RHEAs are also discussed.

Section snippets

Experimental procedures

Five equiatomic quaternary derivative ingots of HfNbTaTiZr alloy, namely HfNbTaTi, HfNbTaZr, HfNbTiZr, HfTaTiZr and NbTaTiZr alloys, were fabricated by a vacuum arc remelter (VAR) in a high-purity argon atmosphere. The mixture of raw metals with the purity greater than 99.9 wt% for each metal was remelted and flipped six times to ensure the chemical homogeneity. The ingots were rolled into plates at RT with a thickness reduction of 80%. The specimens for the following measurements were then cut

Malleability by cold-rolling

Table 1 shows the thickness reduction amount of the five equiatomic quaternary derivatives from HfNbTaTiZr RHEA during cold-rolling. The malleability of quinary HfNbTaTiZr alloy could be seen in many previous studies [8,[12], [13], [14], [15],19,20]. Senkov et al. successfully demonstrated the available access to get a thin sheet of HfNbTaTiZr alloy via cold-rolling [15]. The following studies by Juan et al. and Senkov et al. also revealed the excellent malleability of HfNbTaTiZr alloy, which

Conclusions

The grain growth kinetics of BCC-structured quaternary equiatomic HfNbTaTi and HfNbTiZr RHEAs, which were 80% cold-rolled and annealed at 900–1200 °C for various times, were investigated. The five quaternary derivatives of HfNbTaTiZr alloy were first screened by the malleability and the phase stability for the following study of recrystallization and grain growth. Based on these two requirements, the HfTaTiZr and NbTaTiZr alloys were excluded due to the poor malleability and the HfNbTaZr alloy

CRediT authorship contribution statement

Yung-Chien Huang: Conceptualization, Formal analysis, Investigation, Writing - original draft. Yi-Cheng Lai: Conceptualization, Data curation, Investigation, Formal analysis, Methodology, Writing - original draft. Yu-Hsien Lin: Conceptualization, Data curation, Investigation, Methodology. Shyi-Kaan Wu: Conceptualization, Funding acquisition, Project administration, Resources, Supervision, Writing - review & editing.

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.

Acknowledgments

The authors gratefully acknowledge the financial support from the Ministry of Science and Technology (MOST), Taiwan, with the Grants MOST 108-2218-E-002-062, MOST 107-2221-E-002-016-MY2 and MOST 109-2221-E-002-120-MY2.

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