Characterization of microstructural evolution pattern toward nano-scale in commercial-purity titanium by incremental sheet forming

In the present investigation, a commercial-purity titanium sheet was formed by a flexible sheet metal forming method, incremental sheet forming (ISF), to investigate its microstructure evolution. The deformation microstructures across the wall of formed parts were systematically examined by optical microscopy (OM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Microstructural evolution from millimeter- to nano-scale was explored, with special attention paid to the refinement below 100 nm. A general pattern of structural evolution begins with the formation of uni- and multi-directional twins, which is accompanied by the initiation and development of ultrafine lamellar structure and final evolution of nano-lamellar structure and nano-equiaxed structure. A twinning-dominated process that was supplemented with dislocation slip-dominated one governed the microstructural evolution toward nano-regime. The strain-induced evolution of nano-lamellar and nano-equiaxed structures were discussed, of which the underlying refinement mechanism was analyzed. This technique was promising to offer an optional route for realizing the strengthening of metal parts via synthesizing nanostructures, showing potential scientific and technological importance in fabricating the high-strength metal formed productions for further meeting industrial applications.


Introduction
The application of titanium and its alloys in biomedical field has a well-established area owing to their excellent biocompatibility, corrosion resistance and low Young's modulus [1][2][3].Commercialpurity titanium (CP Ti) is chemically insert and biologically more compatible than Ti6Al4V, while coarse-grained Ti has low abrasion and wear resistance due to its low strength.Therefore, it is of great significance to enhance the strength of CP Ti via refining the microstructures.As is known to all, heat treatment is not a well option for strengthening Ti, since the strengthening effect of both the second phase and precipitates are relatively weak.However, it is has been demonstrated that plastic deformation-induced grain refinement can greatly improve the strength of pure Ti.A large number of plastic deformation methods have been developed to refine the grain in the bulk Ti, such as cold rolling [4], accumulative roll bonding [5], equal channel angular pressing [6] and high pressure torsion [2], which can reduce the grain size down to the ultrafine-regime by 100-200 nm.Recently, a series of surface plastic deformation techniques like rotationally accelerated shot peening [7], ultrasonic surface rolling process [8] and surface mechanical attrition treatment [9,10], have been proposed to fabricate the nano-sized structures with the size range below 100 nm.However, the detailed process of structural refinement below 100 nm in a wide range of length scale needs to be clarified.
Recently, we have adopted a technique that is able to create gradient plastic deformation in the processing area during shaping the sheet into formed parts at room temperature, i.e. incremental sheet forming (ISF).The surface of processing area was fixed by the surrounding backing plates together with two lower-auxillary sheets and sheared by a WC forming tool with one hemispherical tool tip along the particular toolpath, induced gradient deformation enhanced the grain refinement and the understanding of the structural evolution in a wide range of length scale.The present work is aimed at exploring the successive process of grain subdivision toward the nano-scale and the refinement in the regime below 100 nm was emphasized.

Experimental
A commercial-purity titanium (CP Ti, Grade I) sheet was used in this work with the size of 180 mm × 180 mm × 1.0 mm and the following chemical composition (wt%): 0.2% Fe, 0.08% C, 0.03% N, 0.015% H, 0.18% O and balance Ti.Its microstructure was consisted of equiaxed -grain with a mean size of 17 m.The ISF and toolpath have been described in Figure 1.Ti sheet together with two lower-auxillary sheets (mild steel DC 05, 180 mm x 180 mm x 0.8 mm in size) were fixed by the surrounding backing plates, and tool tip moved at a constant feed velocity of v and feed depth of d along the toolpath that consisted of multilayer processing paths (layer-1, layer-2, layer-3……layer-N) with the similar path lines (spacing size of a) and contour lines in each layer.Until finishing the layer-160 processing, a thinwalled formed part was fabricated with the shape of square cavity.In the present investigation, the ISF process was conducted at room temperature with processing parameters v=2000 mm/min, d=0.2 mm and a=0.1 mm.Microstructural characterization was conducted on the cross section by employing optical microscopy (OM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and high resolution TEM (HRTEM).OM observation was performed on an Axio Imager M 2m light microscope.EBSD examination was operated on a Mira SEM equipped with an EDAX instruments symmetry EBSD system.TEM and HRTEM characterizations were operated in the FEI Talors F2000X at a voltage of 200 KV.The samples for OM and EBSD observations were prepared by the standard metallographic preparation procedures and the following vibratory polishing with a frequency of 70 Hz for 3 hours to obtain a mirror surface, etched by etching liquid composed of 2% HF, 10% HNO3 and 88% H2O.The cross-sectional specimens for TEM and HRTEM characterizations were prepared by the focused ion beam (FIB) milling technique on the Gaia double-beam SEM.

OM observation of deformation layer
Typical deformed microstructures is observed within the thickness of 300 m by OM image in Figure 2, where the depth-dependent microstructure could be divided into three regions including twinning region, twin-dislocation transition region and nanostructured region.Twinning region consists of slightly deformed coarse grains coexisted with a large amount of deformation twins, which locates at the depth range from 200 to 300 m.Twin-dislocation transition region is characterized by severely elongated grains with the low-density of DTs at a depth range of 60-200 m.Nanostructured region is the topmost layer with a depth below 60 m and typical of extended deformation bands parallel with the shear direction, in which DTs are rarely observed and forming extremely fine structures could need to be examined by TEM.

Electron microscopy characterization of deformation layer.
Twinning region at the depth of 220 m is characterized by which the grains are slightly elongated along the shear direction and generate typical deformation microstructures inside the grain interiors in Figure 3.(a), featured by three deformation characteristics as follows: i) orientation changes reflected by color variations in G1 and G2 grains, depending upon the movement and accumulation of dislocations [11]; ii) low-density unidirectional deformation twins expanded the entire G3 grain due to activating monosystem twinning; and iii) multi -directional deformation twins in a high density interserted with each other, ascribed to the activation of multisystem twining.Obviously, here plastic deformation is governed by twinning with different behaviors, and the following accompanied with slip.The twinning-and slippinginduced microstructures could be detailed characterized by TEM as shown in Figure 3(b) and (c).The microstructures are composed of parallel or interlaced bands with the thickness varying from tens of nanometer to a few microns.According to the inserted selected area electron diffraction pattern taken from areas containing two intersected bands in Figure 3(b), mirror spots can be observed with respect to the (10-12) plane, indicative of a {10-12} twin system.These intersected twins subdivide the area into the block bordered by twin boundaries and a number of dislocation piles up in the intersection position.In addition, the intersected bands could be observed in the other forms without the twin relationship as seen in Figure 3(c).Their boundaries are dense dislocation walls (DDWs) and not as straight as those of the twins and dislocation pile-ups are formed near the boundaries, which demonstrate that the formation of bands can be dominated by dislocation slip.Nanostructured region with the structural size below 100 nm at the depth below 60 m is featured by nanolamellar structures (NL) and nano-equiaxed (NE) structures.In the depth of 50 m in Figure 5(a), NL is formed with the interconnecting dislocation boundaries (IDBs, marked by black arrows) and the long and straight lamellar boundaries inside the lamellae, which show a similarity in morphology as those ultrafine lamellar (UFL) structures in pure nickel and IF steel deformed by severe plastic deformation [12,13].However, the difference between them is that NL has low angle boundaries as reflected by the inserted SAED pattern consisted of discontinuous diffraction circles.In addition, NL has the lamellar boundaries roughly parallel with SD having a deviation angle of 14.8°, and the boundary spacing is spanned from 50 nm to 200 nm, having a mean size of 96 nm in terms of the histogram of boundary spacing in  Further reaching the processed surface of 5 m deep as shown in Figure 6(a), the microstructures are composed of NL and NE structures.Here NL becomes extremely thinning.The lamellar boundaries are long and parallel, showing strong deformation texture according to the discontinuous diffraction circles in the inserted SEAD pattern.This indicates that most lamellar boundaries are low angle boundaries, which resembles that reported in the SMGT Ni [14,15] and IF steel [16].Here NL has the lamellar boundary spacing below 50 nm in Figure 6(b), with an average size of 18 nm that is one order of magnitude smaller than the structural size induced by traditional severe plastic deformation [2,[4][5][6].Furthermore, as shown in

Discussion
The examined microstructural features at different depths with various levels of strain in the ISF Ti sample can be summarized in Table 1.As the strain and strain rate were increased, the general pattern of microstructural evolution toward nano-regime of CP Ti during ISF involved: i) formation of uni-and multi-directional twins; ii) development of UFL structure; iii) evolution of NL structure and NE structure; Table 1.Summary of TEM observations at different depth on the ISF Ti sample.
There are five independent slip systems required for polycrystalline materials to undergo homogeneous plastic deformation, and thus the operations of twin systems are necessary in order to maintain the deformation compatibility because of merely four independent slip systems in Ti [17].The main twinning planes in Ti are {10-12}, {11-21}, {11-2-2} and {10-11} determined by deformation conditions [18].In the present work, twinning-dominated deformation is observed at the low level of Mixed NL and NE NL: preferential orientation with mean boundary spacing of 18 nm; NE: random orientation with structural size of several of nanometers plastic strain, namely twinning region, in which the observed twinning planes is the {10-12} type that is most frequent during room-temperature deformation.As the strain is increased, twinning can rotate the parent grain lattice to an orientation that favors other twinning behavior and dislocation slip, leading to the further deformation.The typical structural features are twin-twin intersection and formation of DDWs.The different twinning systems are activated along the multiple directions in parent grains, leading to twin intersections.Twins with different orientations interact with each other to significantly refine the parent grains, meanwhile slip is active due to a remarkable increase of low angle grain boundary (LAGB) fraction.The formation of LAGBs for instance DDWs is owing to the dislocation tangle, which promote the structural subdivision.
As the strain further increases to a certain level, the dislocation activity will gradually predominant plastic deformation, while the presence of a large number of twins and their intersection will hinder their movement, which results in a high dislocation density occurring at twin boundaries that is observed at the transition region between slip and twinning.At this stage, deformation twins ceases due to the very small grain size that is demonstrated by the previous investigation on the critical resolved shear stress of twinning increased with decreasing grain size in HCP materials [19,20].An increasing dislocation density is observed, which gives rise to the formation of lamellae including DDWs and IDBs.The area was bordered by parallel DDWs with the spacing between 100-1000 nm and vertical IDBs, namely forming UFL structure, and such microstructural feature is very similar to those BCC and FCC metals deformed at large strain [13,14].
Formation of the nanostructures in the topmost surface layer of 60 m (nanostructured region) is the mixture of NL structure and NE structure.The unique microstructures depends on the deformation conditions that facilitate two basic processes including: i) enhanced generation of dislocations and boundaries; and ii) stabilization of boundaries and dislocations generated.ISF process was anticipated to induce a shear deformation with large strain, high strain rate and high strain gradient, which is akin to other surface plastic deformation like SMGT [14][15][16].Among these deformation parameters high strain rate and strain gradient differ greatly from those traditional severe plastic deformation.
The high strain rate facilitates dislocation generation and suppresses dislocation annihilation so that the pump-in dislocation density can be elevated, which enhance grain refinement in the early stage of deformation before the low-angle boundaries changes into high-angle ones.The presence of a high strain gradient needs storage of geometrically necessary dislocations (GNDs) to maintain the crystal continuity.As high strain gradient accumulated a high density of GNDs may influence the accumulation, annihilation and trapping of statistically stored dislocations according to the classical Kocks model [21], so as to enhance total dislocation density and suppress the process of dynamic recovery.Under the conditions, as the high density of dislocation was poured, the microstructures could be significantly refined down to the nano-sized, which is demonstrated by the publication that the fabrication of nanostructure was subjected to the surface plastic deformation [14][15][16].

Conclusion
1) The nanostructures were fabricated on the surface of formed parts during ISF.Typical nanostructure was composed of NL structure with the mean size of 18 nm and NE structure with a diameter ranged from 5 nm to 8 nm;

Figure 1 .
Figure 1.The toolpath set-up.Microstructural characterization was conducted on the cross section by employing optical microscopy (OM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and high resolution TEM (HRTEM).OM observation was performed on an Axio Imager M 2m light microscope.EBSD examination was operated on a Mira SEM equipped with an EDAX instruments symmetry EBSD system.TEM and HRTEM characterizations were operated in the FEI Talors F2000X at a voltage of 200 KV.The samples for OM and EBSD observations were prepared by the standard metallographic preparation procedures and the following vibratory polishing with a frequency of 70 Hz for 3 hours to obtain a mirror surface, etched by etching liquid composed of 2% HF, 10% HNO3 and 88% H2O.The cross-sectional specimens for TEM and HRTEM characterizations were prepared by the focused ion beam (FIB) milling technique on the Gaia double-beam SEM.

Figure 4 .
Figure 4. TEM bright-field image and the inserted SAED pattern at 140 m deep.Yellow dashed line and black dashed line indicates SD and DDWs, respectively.

Figure 5 .
Figure 5. TEM observation (a) and histogram of boundary spacing (b) at 50 m deep.

Figure 6 (
c), NE structures have a random orientation and many high angle boundary reflected by continuous diffraction circles in the inserted SAED pattern.Further observation by HRTEM in Figure6(d), the diameter of NE structure is roughly 5-8 nm that is the attainable minimum structural size in deformed Ti.

Figure 6 .
Figure 6.TEM observation (a), histogram of boundary spacing (b), local-magnified TEM image (c) and HRTEM image (d) from the red dotted rectangle in (a) at the depth of 5 m.