Ultra-high tensile strength nanocrystalline CoCrNi equi-atomic medium entropy alloy processed by high-pressure torsion

doi:10.1016/j.msea.2018.08.079

Materials Science and Engineering: A

Volume 735, 26 September 2018, Pages 394-397

https://doi.org/10.1016/j.msea.2018.08.079 Get rights and content

Abstract

A nanocrystalline CoCrNi alloy of ~50 nm grain size with the ultra-high ultimate tensile strength of ~2.2 GPa and fracture strain of ~9% was fabricated using high-pressure torsion. The presence of high density of nano-twins, stacking faults, dislocations, and nano-grains is attributed to the superior mechanical properties.

Introduction

It is widely established that nanocrystalline materials possess distinct properties as compared to their coarse-grained counterparts [1], [2]. Among the distinct properties, increase in strength with a decrease in grain size has been of considerable interest to the material science community. In most of the cases, strengthening by grain refinement to nanoscale is often accompanied by a decrease in ductility or brittleness due to plastic instability and artifacts from processing [3], [4]. This strength-ductility trade-off in nanocrystalline materials limits their practical applications. Several attempts have been made to achieve a good combination of strength and ductility in nanostructured materials by several strategies such as careful alloy design, severe plastic deformation, bimodal grain size distribution, nano twinning, second phase particles, heterogeneous microstructure, and lattice softening [4], [5], [6], [7], [8].

In this study, we utilize the advantages of several concepts: high entropy alloy (HEA) concept, alloy with low stacking fault energy, and grain refinement and nano-twin formation by high-pressure torsion (HPT) to produce a nanocrystalline alloy with a high density of nano-twins to achieve superior mechanical properties.

i)
HEA is a new concept of alloy design based on utilizing many principal elements, and it possesses unique and remarkable properties over conventional alloys depending on the elements chosen [9], [10], [11], [12], [13], [14].
ii)
Severe plastic deformation (SPD) is one of the popular top-down techniques to fabricate ultra-fine grained/nanocrystalline material. Among the SPD methods, the HPT method is proven to be a very effective method in grain refinement due to its application of high hydrostatic compressive stress and thereby achieving high strain in the material without undesirable cracking [15], [16], [17]. Recently, nanocrystalline (FeNiCoCu)_1-xTi_xAl_x HEA system with a grain size of ~50 nm processed by HPT is reported to possess superior mechanical properties [18].
iii)
Since nanotwins can accommodate more plastic straining than grain boundaries [19], the design of alloy with nanotwins are expected to improve the mechanical properties. The CoCrFeNi HEA wires fabricated by heavy cold drawing process exhibited high tensile yield strength of 1.2 GPa with tensile ductility of 13.6% at 223 K. Such a remarkable property is attributed to the formation of primary and secondary nano-twins [20]. Among the equiatomic alloys, CoCrNi alloy possesses low stacking fault energy (18–22 mJ m⁻²) [21], due to which the alloy forms deformation twinning during deformation at room temperature and cryogenic temperature and possesses superior fracture toughness [22], [23]. In CoCrFeMnNi alloy (Cantor alloy), the formation of deformation twining at low temperature is mainly attributed to its relatively high shear modulus of ~80 GPa [24]. Wu et al. have reported a shear modulus of ~87 GPa in CoCrNi which is higher than the shear modulus of Cantor alloy [25]. Thus, the CoCrNi alloy processed by HPT is expected to form nanocrystalline grains with a high density of deformation twinning due to its low stacking fault energy.

Accordingly, in the present study, nanocrystalline CoCrNi equiatomic alloy with a high density of nano-twins was fabricated by HPT, and for the first time, ultra-high strength with appreciable strain to failure has been achieved in the equiatomic alloy.

Section snippets

Experimental method

The CoCrNi alloy was fabricated using a vacuum induction melting of pure metals in a water-cooled copper hearth in Ar atmosphere. The ingot was homogenized at 1100 °C for 6 h in Ar atmosphere, followed by water quenching. The homogenized sample was cold rolled with thickness reduction from 7 to 1.5 mm (~80% reduction). The disc-shaped samples (10 mm dia and 1.5 mm thick) were cut from the rolled sample and subjected to recrystallization at 900 °C for 1 h in Ar atmosphere, followed by water

Results and discussion

Fig. 1a shows the EBSD inverse pole figure map, and Fig. 1b shows the XRD pattern of the initial microstructure after recrystallization. The EBSD image shows fully recrystallized microstructure with profuse annealing twins. The mean grain diameter estimated from EBSD analysis is ~19 µm (excluding twin boundaries). The XRD pattern of the initial condition and after HPT indicates the presence of single face-centered cubic (FCC) phase. The XRD pattern of the HPT-processed sample shows broadening

Summary

In summary, the nanostructured (nano-grains and nano-twins) CoCrNi alloy with ultra-high tensile strength and appreciable strain to failure was developed by combining the advantageous of HEA design concept, low stacking fault energy material, and SPD process. The presence of nano-grains, high density of nano-twins, stacking fault, high shear modulus, and dislocations led to very high tensile strength in CoCrNi alloy.

Acknowledgments

This research was supported by Future Materials Discovery Project through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF- 2016M3D1A1023383). Dr. S. Praveen is supported by Korea Research Fellowship program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2017H1D3A1A01013666).

Originality statement

We confirm that the article is original. The article has not been published previously, and the article is not under consideration for publication elsewhere. The article has been written by the stated authors who are aware of its content and approve its submission. No conflict of interest exists.

References (35)

H. Gleiter
Acta Mater.
(2000)
M.A. Meyers et al.
Prog. Mater. Sci.
(2006)
C. Koch
Scr. Mater.
(2003)
K. Edalati et al.
Scr. Mater.
(2012)
C.W. Shao et al.
Acta Mater.
(2018)
D.B. Miracle et al.
Acta Mater.
(2017)
S. Praveen et al.
Mater. Sci. Eng. A
(2018)
A. Zhilyaev et al.
Prog. Mater. Sci.
(2008)
D.J. Lee et al.
Acta Mater.
(2014)
R. Zheng et al.
Intermetallics
(2016)

W. Huo et al.

Scr. Mater.

(2017)

S.F. Liu et al.

Intermetallics

(2018)

G. Laplanche et al.

Acta Mater.

(2017)

Z. Wu et al.

Acta Mater.

(2014)

Z. Lee

Scr. Mater.

(2004)

H. Shahmir et al.

Mater. Sci. Eng. A

(2016)

S. Ni et al.

Scr. Mater.

(2011)

Cited by (94)

The microstructures and mechanical properties of CoNiCr<inf>x</inf> medium-entropy alloys
2024, Materials Science and Engineering: A
The CoNiCr_x (x = 1, 1.4, 1.8, and 2) medium-entropy alloys were synthesized by vacuum arc-melting method to systematically investigate the roles of Cr on microstructures and mechanical properties. The results of microstructural characteristics indicated that both CoNiCr and CoNiCr_1.4 alloys have a single face-centered cubic (FCC) structure, while CoNiCr_1.8 and CoNiCr₂ alloys consist of FCC plus body-centered cubic (BCC) hypoeutectic and eutectic. The formation of FCC and BCC phases could be probably evaluated using the VEC criteria. The tested mechanical properties demonstrated that CoNiCr_1.8 alloy exhibited the best comprehensive properties at room and liquid nitrogen temperatures (RT and LNT), whereas the yield strength and hardness of CoNiCr₂ alloy were higher than those of other alloys, which may be mainly attributed to the second-phase strengthening and the fine and lamellar microstructure. The yield strength of CoNiCr₂ alloy at RT and LNT was 3.8 and 1.9 times (514 MPa and 825 MPa) that of CoNiCr alloy. In addition, the precipitation of dense and ordered FCC (L1₂) phase could primarily contribute to the enhance of yield strength of CoNiCr_1.4 alloy.
Revealing the deformation mechanisms of 〈110〉 symmetric tilt grain boundaries in CoCrNi medium-entropy alloy
2023, International Journal of Plasticity
The synergy of multiple deformation mechanisms responsible for the excellent mechanical properties attracts an increasing interest in CoCrNi medium entropy alloy (MEA). In this study, we employed molecular dynamics simulations to investigate the chemical properties of grain boundaries (GBs) and their influence on deformation mechanisms in CrCoNi MEA, with particular emphasis on role of lattice distortion and short-range order (SRO) in dislocation plasticity. After aging, we found Ni element segregate at GB through short-range diffusion driven by a more negative segregation enthalpy. The degree of SRO within the GB region is significantly correlated with the Ni concentration, and it markedly influences the structure and local stress distribution of the GB. Compared to the lattice distortion, SRO can effectively suppress the GB dislocations nucleation and slip, thereby increasing yield strength of the material. During the plastic stage, although the deformation microstructure is intimately linked to the active slip systems and grain orientation, the presence of SRO, as well as the resultant rugged dislocation pathways and increased slip resistance lowers the propensity for stacking faults and phase transformation. The current findings can enhance a fundamental comprehension of diverse deformation mechanisms in CoCrNi MEA and suggest that the chemical characteristics of GB could serve as a pivotal approach for modifying the mechanical properties of MEAs.
Effects of nanotwins and stacking faults on the mechanical properties of CrCoNi medium-entropy alloys
2023, Journal of Materials Research and Technology
Experiments have shown that planar defects play an important role in strengthening and toughening medium- and high-entropy alloys (M/HEAs). Understanding the underlying defect dependent deformation mechanisms is helpful in designing strong and ductile M/HEAs. In this work, the compressive mechanical responses of columnar-grained CrCoNi MEAs are investigated via molecular dynamics simulations, considering the interspacing effect and deformation difference of pre-existing nanotwins (NTs) and stacking faults (SFs). It is found that the strength of MEAs is enhanced by the introduction of planar defects through impeding dislocation motion. “Smaller and stronger” is found between the NT/SF interspacing and strength, which is attributed to the stronger obstruction of dislocation motion for the higher fraction of planar defects at smaller interspacing. During compression, as compared with NTs, dislocations are easier to penetrate the SF planes, and partial SFs are annihilated due to the reaction between partial dislocations and SFs. Thus, at an identical NT/SF interspacing, the stacking faulted specimen manifests lower strength than the nanotwinned counterpart. The strain localization and shear deformation are observed in the specimen with a small NT/ST interspacing but dislocation motion dominates plastic deformation. Only at sufficiently small NT interspacing, less than 1 nm, the dominant deformation of the nanotwinned specimen is transformed into strain localization deformation, leading to softening. Our finding provides further atomic insights for understanding the strengthening mechanisms of M/HEAs with pre-existing planar defects.
Influence of annealing temperatures on the microstructure and deformation behavior of a CrCoNi based medium-entropy alloy
2023, Journal of Alloys and Compounds
The pursuit of strong and ductile synergy is a key objective in the development of metals and alloys. Medium entropy alloys (MEAs) have emerged as a highly promising structural material, with potential applications in high-speed trains, aerospace, and defense fields. This study systematically investigates the effect of annealing temperature on the mechanical properties of a CrCoNi based MEA, namely (CrCoNi)₉₇Al₁Ti₂ MEA. The (CrCoNi)₉₇Al₁Ti₂ MEA was fabricated by cold rolling reduction of 40% and subsequently annealed at 820 ℃ for 1 h to achieve a heterogeneous structure that exhibits an excellent combination of strength and plasticity. Scanning electron microscopy and transmission electron microscopy were used to characterize the samples before and after tensile deformation, and to reveal the underlying deformation mechanisms. The results show that annealing at 820 ℃ for 1 h introduces a large number of precipitation phases, which are not observed at other annealing temperatures. During plastic deformation, the fine recrystallized grains contribute to the good ductility of the (CrCoNi)₉₇Al₁Ti₂ MEA, while deformation twinning and dislocation densities enable it to maintain good strength. The precipitation phase formed by annealing at 820 ℃ also significantly improves the strength of the (CrCoNi)₉₇Al₁Ti₂ MEA. Overall, this study provides insights into the mechanical properties of (CrCoNi)₉₇Al₁Ti₂ MEA, and highlights the importance of annealing temperature in achieving an optimal combination of strength and plasticity. The findings of this study have important implications for the design and development of MEAs for various applications in structural engineering.
Effect of Fe content on structure and mechanical properties of a medium: Entropy Fe<inf>x</inf>(CoNi)<inf>100-x</inf>Cr<inf>9.5</inf>C<inf>0.5</inf> (x = 60 and 65) alloys after cold rolling and annealing
2023, Journal of Alloys and Compounds
In this study two medium-entropy Fe₆₀Co₁₅Ni₁₅Cr_9.5C_0.5 (hereinafter referred to as Fe60) and Fe₆₅Co_12.5Ni_12.5Cr_9.5C_0.5 (Fe65) alloys (both compositions in at%) were studied after thermomechanical processing involving cold rolling and subsequent annealing at 800 and 1000 ℃. The Fe60 alloy was expected to have a higher stability of a fcc phase and to be less prone to the martensite formation than those of the Fe65 alloy as suggested the $∆ G_{fcc \to bcc}$ values calculated using a Thermo-Calc software. Both the Fe60 and Fe65 alloys underwent deformation-induced microstructure refinement upon cold rolling and recrystallization in the fcc phase and precipitation of Cr-rich carbides after annealing. However, the Fe60 alloy had the fcc-based structure, while the Fe65 alloy contained a significant amount (35–60 %) of the bcc martensite. After cold working, the Fe65 alloy was stronger than the Fe60 alloy (yield strength of 1050 MPa and 1390 MPa, respectively). Annealing resulted in considerable softening of the alloys - yield strength was in a range of 205–360 MPa, and no significant difference in strength between Fe60 and Fe65 was found. The latter finding was associated with the plastic flow initiation with the softer fcc phase. Both alloys demonstrated the transformation-induced plasticity (TRIP) effect associated with the bcc martensite formation during tensile deformation. At room temperature, the TRIP effect was more developed in the Fe65 alloy due to which a comprehensive combination of properties after cold rolling and annealing at 800 °C can be attained; i.e. yield strength of 360/1100 MPa, ultimate tensile strength of 1060/1900 MPa, uniform elongation of 20/26 %, and impact toughness (KCV) of 1300/885 kJ/m² at 293/77 K, respectively. Quantitative analysis of the structure-properties relationships in the program alloys was performed to reveal similarities and differences in the behavior of the alloys.
Ultrasonic provoked network-like intragrain carbide precipitations strengthen and ductilize a C-doped CoCrNi medium entropy alloy
2023, Scripta Materialia
A strategy bypassing “rolling+annealing” while involving merely ultrasonic vibration-modulated static compressive loading was established to enhance the strength-ductility synergy in a C-doped CoCrNi medium entropy alloy. Specifically, by superimposing ultrasonic vibration of 20 KHz in frequency and 20 μm in amplitude onto a compressive load of ∼180 MPa (∼60% of yield strength) applied on a coarse grain CoCrNiC_0.1 alloy for 25 min, yield strength, ultimate tensile strength, and total elongation of the alloy were largely improved to 530 MPa, 950 MPa, and 36.5%, showing respectively an increment of 77%, 28%, and 53% from the original state (300 MPa, 740 MPa, and 23.8%). Structure analysis on the ultrasonically processed CoCrNiC_0.1 alloy indicates that intragrain precipitation of Cr7C3 networks of a characteristic size ∼20 μm provoked by ultrasonic vibration was the main cause for the strength-ductility synergy. Our work provides a promising path to engineer the strength-ductility synergy in precipitation-strengthened alloys.

View all citing articles on Scopus

View full text

Short communicationUltra-high tensile strength nanocrystalline CoCrNi equi-atomic medium entropy alloy processed by high-pressure torsion

Abstract

Introduction

Section snippets

Experimental method

Results and discussion

Summary

Acknowledgments

Originality statement

Acta Mater.

Prog. Mater. Sci.

Scr. Mater.

Scr. Mater.

Acta Mater.

Acta Mater.

Mater. Sci. Eng. A

Prog. Mater. Sci.

Acta Mater.

Intermetallics

Scr. Mater.

Intermetallics

Acta Mater.

Acta Mater.

Scr. Mater.

Mater. Sci. Eng. A

Scr. Mater.

Short communication
Ultra-high tensile strength nanocrystalline CoCrNi equi-atomic medium entropy alloy processed by high-pressure torsion