Structural evolutions of metallic materials processed by severe plastic deformation
Introduction
Tremendous efforts have been devoted in the history of mankind to develop structural materials with optimized mechanical properties for applications in areas including civil, medical, transportation, oil, aerospace, and energy industries. Strength and ductility are two of the most fundamental mechanical properties for structural materials. Research over a century on metallic materials revealed a dilemma of strength–ductility trade-off that seemed to be intrinsic [[1], [2], [3], [4]]. In 1989, Gleiter [5] made the visionary argument that significant change of physical and mechanical properties of materials could be realized when the grain size of materials is reduced to the nanoscale regime. Nanostructured (ns) materials usually have very high strength, but ductility is typically low. Recent researches revealed that some ns materials can be designed to have improved ductility without sacrificing their strength [1,[6], [7], [8], [9], [10], [11], [12], [13], [14], [15]]. This triggered unprecedented innovation in the field of ns materials [16] including processing techniques, knowledge of nano/microstructural evolution and knowledge of structure-property relationships. Nanocrystalline (nc) materials have grain sizes smaller than 100 nm. The terminology “ns materials” has often been used to describe materials having structural features, e.g., sub-grains and dislocation cells, smaller than 100 nm but their grain sizes may be larger than 100 nm [17]. It should be noted that the grain size measured by X-ray diffraction (XRD) is actually the crystalline domain size, which makes it consistent with the definition of ns materials. In addition, the term “ultrafine-grained (UFG) materials” has been extensively used for materials processed by severe plastic deformation (SPD), an approach that can produce bulk materials without porosity and large enough for structural applications. UFG materials are defined as materials with grains smaller than 1000 nm, and they are often referred to as ns materials when they have structural features smaller than 100 nm [17].
After decades of development, many methods have been established to synthesize UFG and ns materials. These methods can be classified into two major approaches – “bottom-up” and “top-down” [18]. In the “bottom-up” approach, atoms or nanoparticulate solids are bonded together to form ns bulk solids. Electro-deposition [19], inert gas condensation [20] and ball milling with subsequent consolidation [21] are typical examples of these “bottom-up” methods. In the “bottom-up” approach, the grain sizes, types of boundary and textures of the product materials can be manipulated to a certain extent. Fig. 1 shows transmission electron microscopy (TEM) images of two typical examples of materials synthesized by electro-deposition. Presented in Fig. 1(a) is a nc Ni with an average grain size of ∼30 nm and a narrow grain size distribution [22]. The corresponding selected-area electron diffraction (SAED) pattern indicates small grain sizes and random grain orientations. Fig. 1(b) presents a coarse-grained (CG) Cu containing a high density of nano-twins (the strip-like structures) [23]. However, some “bottom-up” methods are very time consuming and expensive. Materials synthesized by the “bottom-up” methods always have one or more of the following issues: (1) unwanted impurity contents, (2) texture and (3) porosity or incomplete densification [[24], [25], [26]]. Thus, the ns materials synthesized by “bottom-up” methods are often very limited in sizes, and the high impurity and/or porosity content may deteriorate the mechanical properties of the materials [27].
In the “top-down” approach, bulk CG materials are effectively refined by heavily imposed shear strain to form UFG and/or nc materials, by comparing the electron backscatter diffraction (EBSD) images in Fig. 2. The resultant UFG or nc materials are free of porosity and contamination [28,29]. Nowadays, the most successful and popular “top-down” grain refinement approach is SPD. SPD can be achieved by mechanically imposing an extensive hydrostatic pressure and a very high strain to a bulk metallic material. SPD normally does not alter much the shape of the work piece while the grain size can be significantly refined [17]. The most developed SPD techniques include equal channel angular pressing (ECAP) [30], high-pressure torsion (HPT) [29,31], multi-axial forging [32], rotary swaging [33], and accumulative roll bonding [34,35]. Detailed description of these techniques and their development history have been well reported in the literatures [28,31,[36], [37], [38]].
The high strain level that is attainable by SPD is far beyond that of conventional plastic deformation methods such as rolling and extrusion. As a result, SPD induced structural evolution processes can be very complex, and may involve various deformation mechanisms including dislocation slip, deformation twinning, elemental redistribution, phase transformation, grain rotation and even grain growth. Each of these deformation mechanisms may contribute or affect the grain refinement to a certain extent. The activation of various deformation mechanisms and the resultant overall microstructural evolution are determined by the processing history and the intrinsic properties of materials such as stacking fault energy (SFE). Microstructures determine the way by which the materials respond to imposed stress; therefore, ultimately microstructures determine the mechanical properties of SPD-processed ns materials. It is of interest and critical importance to understand the evolution of microstructures so that SPD parameters can be tuned to produce microstructures for optimum mechanical properties.
Although there have been a few influential SPD-related review articles [3,17,[28], [29], [30], [31],[36], [37], [38], [39], [40], [41], [42], [43], [44]], they mainly focused on SPD techniques [[28], [29], [30], [31],36] and the properties of processed materials [3,37]. No systematic review on SPD-induced microstructural evolutions has been published. On the other hand, a large number of research papers on SPD-induced microstructural evolutions has been published, and many interesting micro/nanostructures have been revealed in detail, thanks to the fast development of materials and microstructural characterization techniques [[45], [46], [47], [48], [49]]. It is worth to mention that SPD is applicable to a huge variety of materials including both metallic materials and nonmetallic materials [[50], [51], [52], [53]]. Considering majority of the SPD-processed materials are metallic materials, this review will focus on SPD-induced microstructural evolutions of metallic materials, including grain refinement (Section 3), grain growth (Section 4), dislocation density evolution (Section 5), phase transformation (Section 6), structural evolutions in dual-phase and multi-phase materials (Section 7). The effect of SPD-induced structural evolution on the mechanical properties will also be briefly reviewed in Section 8 to correlate microstructure with mechanical properties. At last, concluding remarks and outstanding issues in the field of SPD-induced microstructural evolution are provided in Section 9.
Section snippets
Major deformation modes in coarse grains
Dislocation slip and deformation twinning are the two major deformation mechanisms in CG metallic materials. Dislocations are line defects, which can nucleate and slip under stress to accommodate the applied plastic deformation. In crystalline solids, atoms are arranged in ordered lattice structures. A single dislocation line can glide on a densely packed atomic plane called slip plane along any direction but the resulting crystalline slip is along a particular direction called slip direction.
SPD-induced grain refinement
Dislocation slip and deformation twinning are two major competing plastic deformation mechanisms during the SPD-induced grain refinement processes. The crystalline structures and SFEs of materials play critical roles in determining their deformation modes and therefore SPD-induced grain refinement mechanisms [35,87,[89], [90], [91],94,[101], [102], [103], [104], [105]]. Therefore, this review is organized based on the crystalline structures of materials. For FCC materials, because the grain
SPD-induced grain growth
In recent years, deformation induced grain growth has attracted significant attention. SPD processing can lead to two opposite phenomena – SPD-induced grain refinement and grain growth. The final average grain size of materials processed by SPD is achieved through the dynamic balance between the grain refinement process and the grain growth process. For CG materials, SPD processing leads to grain refinement. Conversely, grain growth has been readily observed in nc materials that were subjected
The dislocation density evolution in SPD-induced grain refinement process
It has been well investigated that deformation induced grain refinement is mainly caused by dislocation activities for materials with medium to high SFEs under low stain rates [92,145,444,460], thereby a deformation-induced grain refinement process is usually accompanied by an evolution in the dislocation density. As discussed earlier, at the early stages of an SPD process, various sub-structures including dislocation cells and cell blocks form via dislocation generation and accumulation [148,
SPD-induced phase transformation
Besides grain refinement and grain growth, SPD can also induce phase transformation. Both diffusive and non-diffusive (martensitic) phase transformations have been reported.
SPD-induced structural evolution in multi-phase alloys
Recently, multi-phase alloys composed of ultrafine-scale and nano-scale structures have elicited strong interest due to their remarkable enhancement of mechanical properties [9,10,[538], [539], [540], [541], [542]]. SPD, as one of the most effective microstructural refinement techniques, has been employed to tailor multi-phase alloys with nanostructures [10,[543], [544], [545], [546]]. In-depth understanding of the microstructural evolution of multi-phase alloys processed by SPD is necessary.
Effect of SPD on mechanical properties
SPD can impose a shear strain far beyond the strain level attainable by conventional shaping and forming methods such as rolling, drawing and extrusion. As a result, bulk metallic materials can be processed to achieve nanostructures that possess mechanical properties that expand known performance boundaries [7,9,608,651,652], as exemplified in Fig. 67 [653]. Despite of different crystalline structures, both ns Ti and ns Cu possess combined tensile strength and ductility beyond the region (the
Concluding remarks
We have presented a systematic review on SPD-induced grain refinement and other microstructural evolutions of metallic materials with FCC, BCC, and HCP crystalline structures and metallic materials with multiple phases, and the structural effect on mechanical properties. These are summarized below:
- (1)
SFE plays a critical role in the deformation and microstructural evolution of FCC materials. For FCC materials with high SFEs, SPD-induced grain refinement occurs mainly via lattice dislocation
Acknowledgements
We would like to express our sincere gratitude to all researchers who have contributed to the field of SPD processing of metallic materials. We thank the technical support from the Materials Characterization Facility of Nanjing University of Science and Technology. This work is supported by the National Key R&D Program of China (2017YFA0204403), National Natural Science Foundation of China (Grant No. 51601094 (Y.C.) and Grant No. 51771229 (S.N.)), Australian Research Council (DP150101121) and
References (759)
- et al.
Prog. Mater. Sci.
(2006) Prog. Mater. Sci.
(1989)Scr. Mater.
(2004)- et al.
Scr. Mater.
(2004) - et al.
Nanostruct. Mater.
(1993) - et al.
Scr. Metall.
(1989) - et al.
Nanostruct. Mater.
(1992) - et al.
Acta Mater.
(2003) - et al.
Scr. Mater.
(2011) - et al.
Scr. Mater.
(2003)
Acta Mater.
Acta Mater.
Prog. Mater. Sci.
Prog. Mater. Sci.
Prog. Mater. Sci.
Mater. Sci. Eng.: A
Mater. Sci. Eng.: A
Acta Mater.
Mater. Sci. Eng.: A
Acta Mater.
Acta Mater.
Acta Mater.
Acta Mater.
Prog. Mater. Sci.
Prog. Mater. Sci.
Mater. Today
Prog. Mater. Sci.
Acta Mater.
Mater. Sci. Eng.: R: Rep.
Scr. Mater.
Mater. Sci. Eng.: A
Prog. Mater. Sci.
Acta Metall.
Scr. Mater.
Mater. Sci. Eng.: A
Acta Mater.
Acta Metall.
Acta Metall. Mater.
J. Mech. Phys. Solids
Acta Mater.
Acta Mater.
Acta Mater.
Mater. Sci. Eng.: A
Acta Mater.
Acta Metall. Mater.
Acta Metall.
Acta Mater.
Cited by (429)
Mechanical performance and deformation mechanisms of ultrastrong yield strength Fe-Cr-Ni-Mn-N austenitic stainless steel at 4.2 Kelvin
2024, Journal of Materials Science and TechnologyBalanced strength and ductility by asymmetric gradient nanostructure in AZ91 Mg alloy
2024, Journal of Materials Science and TechnologyMicrostructural evolution and multi-mechanism strengthening model of nanocrystalline Al-Mg alloys
2024, Journal of Alloys and CompoundsEnhancing the mechanical properties of high-entropy alloys through severe plastic deformation: A review
2024, Journal of Alloys and Metallurgical Systems