Effects of cold rolling and subsequent annealing on the microstructure of a HfNbTaTiZr high-entropy alloy
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The microstructural evolution of a HfNbTaTiZr high-entropy alloy subjected to cold rolling and subsequent annealing was investigated. The dislocation activity dominates the deformation process. The microstuctural evolution of the alloy during cold rolling can be described as follows: (i) formation of dislocation tangles, (ii) formation of microbands, (iii) formation of thin laths and microshear bands containing thin laths, (iv) the transverse breakdown of the lath to elongated segment, and (v) formation of fine grains. During annealing at 800 and 1000 °C, decomposition of the metastable high-temperature body-centered cubic phase proceeded through a phase separation reaction. Annealing at 800 °C resulted in a nonrecrystallized microstructure with abundant second-phase particles distributed randomly. The second-phase particles with an average size of ;145 nm were enriched in Ta and Nb, while the chemical composition of the matrix was close to the average composition of the alloy. Meanwhile, an unknown phase slightly enriched in Hf, Zr, and Ti was detected in the interfacial region between the second-phase particles.
I. INTRODUCTION
High-entropy alloys (HEAs), first introduced by Yeh et al.1 and Cantor et al.2 in 2004, have received significant interest recently due to their unique composition and microstructure, and excellent mechanical properties, including high strength and hardness,3 remarkable structural stability, 4 exceptional lowtemperature strength,5,6 as well as good wear, corrosion and oxidation resistances.7–10 HEAs are a new class of materials that are usually composed of five or more than five metallic elements in equimolar or near-equimolar ratios with content ranging from 5 to 35 at.%.1 HEAs may have great potential for functional and structural applications because of their single solid solution structure (face-centered cubic, FCC, body-centered cubic, BCC, or hexagonal close-packed, HCP), severe lattice distortions and sluggish diffusion.11–16 Over the past years, a lot of researches have been carried out to describe the microstructure and mechanical properties of various HEA systems. The FCC type of HEAs, represented by equiatomic CoCrFeNiMn that were first reported by Cantor et al., exhibit high thermodynamic stability, good ductility but medium tensile strength.2,17,18 On the contrary, the BCC type of HEAs, such as NbMoTaW19 and HfNbTiZr,4 shows high hardness and strength. Right now, there is still limited knowledge about the microstructure features of HEAs due to the fact that the Contributing Editor: Mathias Göken a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.445
high-entropy solid solutions consist of high contents of multiprincipal elements. It is well known that the deformation mechanisms for conventional alloys can be roughly categorized into two types, either by the slip of dislocations or by twinning. However, due to high chemical disordering of HEAs and lack of thermodynamic and kinetic data for multicomponent systems, a series of fundamental is
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