Journal of Materials Science & Technology  2019 , 35 (6): 957-961 https://doi.org/10.1016/j.jmst.2018.12.002

Superb cryogenic strength of equiatomic CrCoNi derived from gradient hierarchical microstructure

Bin Gana, Jeffrey M. Wheelerb, Zhongnan Bic*, Lin Liua, Jun Zhanga, Hengzhi Fua

a School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an, 710072, China
b Laboratory for Nanometallurgy, Department of Materials Science, ETH Zürich, Vladimir-Prelog-Weg 5, Zürich, CH-8093, Switzerland
c Central Iron and Steel Research Institute, No. 76 Xueyuannan Road, Beijing, 100081, China

Corresponding authors:   * Corresponding author. E-mail address: bizhongnan@cisri.com.cn (Z. Bi).

Received: 2018-07-1

Revised:  2018-08-13

Accepted:  2018-08-27

Online:  2019-06-20

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

More

Abstract

This work demonstrates the effectiveness of a cryogenic torsional pre-straining for significantly improving the cryogenic strength of an equiatomic CrCoNi alloy. The origin of this phenomenon is elucidated by various microstructural characterization tools, which shows that the sequential torsion and tension tests lead to the observed hierarchical microstructure through the activation of different twinning systems and stacking faults. This gives rise to the significant increase in the yield strength from 600 MPa to 1215 MPa, while the fracture strain changes from 68% to 48%. The current study reveals that the incorporation of nanotwins architecture by shear deformation may constitute a viable strategy to tune the mechanical performance and, in particular, to dramatically increase the strength while keeping a good ductility.

Keywords: Medium entropy alloy ; CrCoNi ; Torsion ; Hierarchical microstructure ; Strengthening

0

PDF (2500KB) Metadata Metrics Related articles

Cite this article Export EndNote Ris Bibtex

Bin Gan, Jeffrey M. Wheeler, Zhongnan Bi, Lin Liu, Jun Zhang, Hengzhi Fu. Superb cryogenic strength of equiatomic CrCoNi derived from gradient hierarchical microstructure[J]. Journal of Materials Science & Technology, 2019, 35(6): 957-961 https://doi.org/10.1016/j.jmst.2018.12.002

1. Introduction

Recently, equiatomic CrCoNi medium entropy alloys (MEA) have been found to possess both excellent cryogenic strength and ductility without suffering from the detrimental ductile-to-brittle transition [1,2]. This catalyzed a global interest in delineating the underlying deformation mechanisms operated in cryogenic temperatures, and invigorated teams of people around the world to exploit the full potential of strengthening residing in this new alloy system [2], [3], [4], [5], [6]. For most crystalline metals, plastic deformation is associated with the generation, motion and subsequent pinning or trapping of dislocations within the crystal lattices [7]. The incorporation of various obstacles into the crystal lattice for impeding dislocations is a fundamental strategy to increase the materials strength [7]. Fully recrystallized, ultrafine-grained high entropy alloys are thermally and mechanically stable, and with a combination of high strength and ductility, they are quite competitive for the future applications [8,9]. The magnitude of grain boundaries strengthening increment can be empirically estimated using the Hall-Petch equation [10]. A recent study of the annealing and mechanical behavior of equiatomic CrCoNi processed by high-pressure torsion (HPT) revealed that with a grain size refinement from 111 μm to 200 nm, the tensile yield strength can be increased from 0.6 GPa to 1 GPa [11]. This clearly highlights the beneficial role of grain boundaries in the enhancement of strength. However, the further refinement of grain sizes invariably suffers from an undesirable loss of tensile ductility [11]. CrCoNi alloys have a very low stacking fault energy (SFE) and a high propensity for forming nanotwins, as shown by the prior studies [1,2,6]. Considering that nanotwins can not only impede dislocation motion but are also able to accommodate plastic deformation [1,2], the population of nanotwins in the microstructure may be a superior strategy for achieving high strength and ductility in these alloys.

The distribution of obstacles can also have a significant influence on the mechanical properties of materials. Recently, gradients or heterogeneous distributions of strengthening defects (nanotwins, dislocation substructures, high-angle boundaries, low-angle boundaries, etc.) have been reported to possess an excellent combination of high strength and good ductility [12], [13], [14], [15]. Several novel gradient deformation approaches (high energy shot peening, surface mechanical grinding treatment (SMGT), surface mechanical attrition treatment (SMAT) and simple torsion have been developed to produce a spatial variation of grain sizes or twin sizes in the surface layers of bulk metals [12], [13], [14], [15].Simple torsion can also generate a linearly changed density of twins in high Mn steel, leading to a good combination of strength and ductility [14]. The recent study of the influence of torsional pre-straining on the tensile behavior of CoCrFeNiMo0.3 alloys also reveals the beneficial effect of pre-torsion on the enhancement of strength [16]. In this work, the role of nanotwin gradients induced by cryogenic pre-torsion on the strengthening of CrCoNi alloys is investigated. The influence of the applied torsional strains in cryogenic environments on the microstructure and hardness of CrCoNi is probed using electron microscopy and nanoindentation, and their subsequent impacts on the corresponding tensile behavior and deformation mechanisms are investigated.

2. Experimental

An equi-atomic CrCoNi alloy was produced by vacuum induction melting using commercially pure elements (purity of 99.95 wt%) as the starting materials. The cast ingot was homogenized at 1200 °C for 6 h and then hot swaged into a square bar with side lengths of 20 mm using a four-die rotary swaging machine. Subsequently, these bars were recrystallized for 1 h at 850 °C followed by air-cooling.

Torsional sample with a gage length of 50 mm and a diameter of 9 mm and a square head was machined and was then subjected to torsion tests at 77 K using a MTS model 809 axial/torsional testing system with an angular loading rate of 0.05°s-1 and a torsional angle of 360°. After the torsional pre-straining tests, the square grip regions of the samples were machined down into threaded rods for tension testing. The pre-torsioned sample was then subjected to tension tests with a strain rate of 10 -3 s-1. At 77 K, the extensometer could not be used, so strains were determined indirectly from cross-head displacements that were corrected for compliance. After testing, transverse sections of torsionally pre-strained rod and fractured tensile rods were metallographically prepared, finished with a final polishing using a vibratory polisher (Buehler Vibromet 2) with 0.04 μm colloidal silica.

Texture was determined by electron backscatter diffraction (EBSD) in a Tescan Mira Scanning Electron Microscope (SEM), equipped with a HKL EBSD, at an accelerating voltage of 25 kV, a working distance of 11-15 mm and step sizes between 0.3 and 1 μm. Pattern analysis was performed using the Channel 5 Analysis software, so as to determine the mean grain size. Hardness profiles across the torsional strain gradient were acquired using a high speed mechanical property mapping technique, NanoBlitz, in an iNano® nanoindenter from Nanomechanics Inc. (Oak Ridge, USA). A long grid of 2000 indentations was performed to a maximum load of 15 mN with a spacing of 20 μm to characterize the variation in hardness across the gradient. To investigate the microstructural after the tensile straining, slices for TEM were cut from the gauge sections of deformed tensile specimens, followed by mechanical grinding and a double-jet electrochemical thinning at 20 V using an electrolyte consisting of 90 vol% of ethanol and 10 vol% perchloric acid at -30 °C. TEM analyses were performed on a FEI Talos F200X instrument operating at 200 kV.

3. Results

3.1. Microstructural characterization

The chemical composition of the CrCoNi alloy as determined by inductively coupled plasma (ICP) atomic emission spectrometry, 32.53Cr-33.32Co-32.85Ni (at.%), is close to the targeted equiatomic composition. Additionally, energy dispersive spectrometry (EDS) measurements performed at different locations revealed a uniform chemical composition. Fig. 1(a) shows a grain orientation map of the core region of sample after the pre-torsion, where the individual crystallites are colored according to their crystallographic orientation relative to the rod axis, as shown by the inverse pole figure (IPF). The grain size is uniform along the radius of the recrystallized rod and has a mean value of 16 ± 3 μm, while annealing twin boundaries are not considered as grain boundaries. The texture along the rod axis is almost random. Fig. 1(b) shows a grain orientation map of the edge of the same sample after the torsion treatment. The gradient in color within one grain indicates the strain induced by the pre-torsion. Due to the resolution limit of EBSD and the tiny twin spacing of nanotwins, the density gradients of nanotwins in the radial directions are not so evident and statistical analysis is not possible.

Fig. 1.   (a) Orientation imaging microscopy map generated by EBSD showing grain structure of the center of CrCoNi MEA tensile rod, (b) EBSD map of the edge of wrought CrCoNi MEA tensile rod after a pre-torsion treatment.

3.2. Mechanical properties

Fig. 2(e) shows the picture of the pre-torsioned sample with a diameter of 9 mm, and Fig. 2(a) to (d) highlight the microstructural changes from the center to the edge, respectively. Consistent with Fig. 1(a) and 1(b), in the core region of the sample after the pre-torsion treatment, the original microstructure is relatively intact, while an increasing amount of deformation is observed from the center to the edge, which is actualized with a density change of nanotwins. Fig. 2(f) shows the hardness gradient in the radial direction of sample after the pre-torsion. This plot reveals a continuous increase of hardness from the center to the edge in the torsion specimen. In specific, hardness gradient with a variation from 3.0 GPa to 4.3 GPa across the radial direction is obtained, which is mainly caused by the combination of working hardening and Hall-Petch hardening. This is well aligned with the gradient microstructural distribution in terms of dislocations and twin density.

Fig. 2.   (a)-(d), Backscatter Electron Images of the sample with a pre-torsion treatment from the center to the edge, (e) picture of the pre-torsioned sample with a diameter of 9 mm, (f) hardness gradient in the radial direction of sample after the pre-torsion.

Fig. 3 shows the engineering stress-engineering strain curves of CrCoNi alloys with and without the cryogenic pre-torsion treatment, tested in the liquid N2 environments and room temperatures, respectively. This plot reveals that the yield strength and tensile strength of CrCoNi alloys with a grain size of 16 μm at cryogenic temperature is 600 MPa and 1247 MPa, respectively, while the fracture strain is 68%; The corresponding values for room temperature tests is 400 MPa, 800 MPa and 60%, respectively; for sample with the same grain size, the yield strength and tensile strength of CrCoNi alloys with a one torsional revolution pre-torsion treatment at cryogenic temperature is 1215 MPa and 1440 MPa, respectively, while the fracture strain is 48%. There is a double increase in yield strength, and 15% increase in tensile strength. These results reveal that the application of torsional strain can effectively increase the tensile strength of materials while retaining a decent amount of ductility.

Fig. 3.   Engineering stress-engineering strain curves of CrCoNi alloys with and without the pre-torsion treatments in the liquid N2 environments as well as that of the recrystallized CrCoNi alloys tested at room temperature.

4. Discussion

Over the years, the mechanical behavior of metals has been well investigated. The application of stress can produce the following structural changes: slip (by dislocation motion); twinning (which also requires dislocation activity); phase transformation; and fracture [1], [2], [3], [4],10]. Slip and fracture have received the greatest amount of attention from both theoretical and experimental researchers during the past 60 years [10,[17], [18], [19]. Mechanical twinning also constitutes a significant mode of deformation and can dominate under certain conditions [20].

For the torsional treatment of CrCoNi alloys, due to the low stacking fault energy of this alloy system [4], both dislocation activity and deformation twinning dominate the deformation process, and the torsional strain is as a function of the radial position [21]. Fig. 4 (a) and (b) shows the SEM images of the edge of the specimen after the pre-torsion followed by tension tests, respectively, while Fig. 5 (a) to (h) shows the hierarchical microstructure of sample after tensile tests, showing the presence of intersecting slip bands with a high density of dislocations, stacking faults and nanotwins. Fig. 4(a) demonstrates that after one torsional revolution, primary nanotwins are activated in some of the grains. Additional SEM images show that there is a gradient in the density of nanotwins in the radial direction of torsion specimen. Fig. 4(b) show that for the sample with pre-torsion, after the tensile deformation, slip bands [22] and high dislocation density walls (HDDWs) could be clearly observed in the deformed microstructure, which testifies the strong planar slip character of this alloy system and is consistent with the prior study of the influence of torsional strain on the development of microstructure of CoCrFeNiMo0.3 alloy [16]. It has been reported that the planar slip in FCC metals can be promoted by short range ordering (SRO), low stacking fault energy (SFE) and large lattice friction stress [17]. In the present study, CrCoNi MEA has a low SFE (∼18 mJ*m-2) at room temperature [4], Given the low SFE of MEA investigated here, full dislocations dissociate into partials with a wide stacking fault between each pair of the dissociated partials, and cross-slip becomes very difficult and therefore slip is planar [23,24]. In addition, as compared to the lattice in pure metals, the MEA lattice is heavily distorted and possesses a remarkable lattice strain energy. Large lattice friction stress values were reported in CrCoNi and thus hindering the cross slip. Therefore, the combined effects of low SFE and large lattice friction stress contribute to the presence of planar slip in CrCoNi MEA. In addition to the HDDWs, a high density of nanotwins is observed in the deformed microstructure. In addition, the TEM micrograph of the sample after the tensile deformation shows that a high density of stacking faults are activated among the nanotwins, as shown by Fig. 5(b). In this work, high dislocation density walls were clearly observed, as shown in Fig. 5(a), which was originated from reorganization of planar dislocations formed at low strains. The concurrent appearance of high dislocation density walls and deformation twins is commonly observed in medium-to low SFE materials.

Fig. 4.   (a) SEM micrograph of the microstructure of the edge of CrCoNi MEA with pre-torsion, (b) SEM image of the edge of CrCoNi MEA after torsion and tension tests.

Fig. 5.   (a) TEM bright field image of the tensile bar with a beam direction of [111], showing the intersections of slip bands with an orientation of <011 > , (b) TEM bright field image showing the presence of stacking faults, (c) TEM bright field image of the slip bands with a beam direction of [001], (d) TEM dark field image showing the high dislocations density in the slip bands, (e) a high resolution image of the region with a yellow square, (f) TEM bright field of the region containing a high density of nanotwins with a beam direction of [011], (g) the corresponding diffraction pattern showing the twinning spots, (h) a high resolution image showing the presence of nanotwins.

As shown in Fig. 5(a) to (h), after the sequential deformation of torsion and tension tests, stacking faults, secondary and tertiary nanotwins are activated and result in an increased strength. As shown in Fig. 3, the strength of CrCoNi MEA can be substantially enhanced without sacrificing ductility, by introducing a gradient twin density with parallel twins in individual grains via pre-torsion. In the subsequent tension, the formation of secondary twins and twin junctions where several secondary twins meet at a primary twin suggest a hierarchical twin deformation mechanism [14]. The overall material hardening due to the hierarchical twin structures and the high density of stacking faults among the nanotwins as well as the resulting distributed plasticity due to twin-twin, dislocation-twin and stacking faults-twins interactions play the dominant role for retaining ductility [25]. Given the general correlation between fatigue limit and yielding strength in most metals and metallic alloys, the enhanced yielding strength in pre-torsioned CrCoNi alloys could result in much better reliability of structures composed of such material. Note that the essence of the present method is to produce non-uniform shear strains in metals prone to deformation twinning, and twins in the pre-processing stage are so oriented that subsequent deformation would trigger different twin systems.

5. Conclusion

In summary, this work demonstrates that with a cryogenic pre-torsion treatment, a cylindrical tensile rod of an equiatomic CrCoNi alloy exhibits a significant increase in tensile yield strength from 600 MPa to 1215 MPa in cryogenic environment. Systematic microstructural characterization reveals that the sequential torsion and tension tests lead to the observed hierarchical microstructure through activation of different twinning systems and stacking faults.

Acknowledgments

The authors thank the fruitful discussion with Dr. Yanzhong Tian, Dr. Stoichko Antonov and Dr. Haibo Long, and gratefully acknowledge the financial support of the project from the National Natural Science Foundation of China (No. 51601147), the Natural Science Foundation of Shaanxi Province (No. 2017JQ5010), and “the Fundamental Research Funds for the Central Universities” (No. 3102016OQD048, 3102017JC11001, 3102017JC01003).

The authors have declared that no competing interests exist.


/