Microstructures and strengthening mechanism of 2D laminated structures in Ni balls after high‐speed collisions with 316 L stainless steel

In many cases, due to high surface stress, stress concentration and wear, the metal material starts to fail from the surface. It is necessary to investigate interface evolution characteristics under high‐speed collisions for widely used metal materials. The microstructures and strengthening mechanism of the Ni balls after high‐speed collisions with 316 L stainless steel were investigated by a self‐designed high‐speed collisions test system. It was found that ultrafine twins and α'‐martensite were formed in 316 L stainless steel (316 L SS) near the interface region. The elongated nano/ultrafine lamellar structures with high‐angle grain boundaries in Ni ball were formed near central interface region, while more equiaxed nano/ultrafine grains with random orientations and high‐angle grain boundaries were formed near edge interface region due to friction process and high shear stress. The gradient structure in the Ni ball near central interface region was formed due to normal stress, while the high shear strain could promote the formation of finer equiaxed grains in the Ni ball near edge interface region. Moreover, in addition to grain refinement strengthening, nano/ultrafine lamellar structures with sufficient intragranular recrystallization facilitated the activation of more dislocations and increased the strengthening of the Ni ball near central interface region.

mechanism of metallic materials at a high-strain-rate. 6 It is well known that the SPD at very high strain rates is a complex process, at the same time, it should be important to study dislocation configuration, nanotwin distribution and phase transformation during the SPD at high strain rates. 7 It was found that the SPD at high-strain-rate was potential to induce numerous defects and interfaces, which resulted in the inhomogeneous microstructure along with large strain gradients. 8 Lu et al. 9 suggested that an ultra-high strain rate was a solution for the grain refinement in the laser shock processing. Moreover, multidirectional intersections of mechanical twins facilitated grain subdivision during multiple laser shock processes. Therefore, severe plastic strain with an ultra-high strain rate plays a crucial role in the grain refinement and the strengthening. In addition, the high strain rate and high strain gradient in surface mechanical grinding treatment (SMGT) promoted the formation of fine-grained Ni with 2D laminated structures, which was attributed to the enhanced accumulation of geometrically necessary dislocations (GNDs). 10 The stabilization of the nanolaminated structures with very fine size can enhance the hardness of the metals. 11 Although obtained 2D laminated structures showed ultrahigh strength, the deformation and strengthening mechanisms of the metals under high velocity impact were more complex. 12 Since the strain induced phase transformation, deformed twin and dislocation motion all determined minimum grain size and metal strength, especially in the case of high strain rate and large deformation. Therefore, the formation and reinforcing mechanisms of the 2D laminated structures obtained by the high-speed collision need to be further revealed.
It was suggested that critical dislocation density was necessary to achieve grain refinement during the SPD, 13 and the high-strain-rate should be a dominant factor to obtain this critical dislocation density. 14 In the present work, the effect of the high-speed collision (velocity was over 610 m/s) on microstructures near the interface between metastable stainless steel and pure Ni was implemented by a self-designed high-speed collisions test system. At the same time, the strength of the Ni ball in the vicinity of two interface regions was also evaluated. This investigation can provide a theoretical basis for future material design.

EXPERIMENTAL
The square sample of 316 L SS bulk (5 mm × 5 mm) has a chemical composition (in wt%) of C-0.022, S-0.002, P-0.04, Cr-16.2, Mo-2.05, Ni-10.09, Mn-0.98, Si-0.36, Fe balance. In order to obtain a single austenitic phase, the 316 L SS sample was first subjected to annealing treatment at 1100 • C for 1 h before quenching in water, and then solid solution 316 L SS bulk was fixed in an airtight equipment. A pure Ni ball with a diameter of 1 mm was annealed at 1200 • C for 1 h to produce a coarse-grained structure and then put on one end of a fine stainless steel tube, as shown in Figure 1A, while the other end was connected with an argon cylinder. The Ni ball can be emitted by adjusting the pressure of high purity argon. In addition, the impact velocity of Ni ball can be calculated by the two sensors (the distance between the two sensors is a constant value 10 cm) which can record the time of a pure Ni ball moving in the airtight equipment. The calculated collision velocity of a pure Ni ball in this study is about 610 m/s. The energy disperse spectroscopy (EDS) near the interface between 316 L SS and Ni ball was examined. Electron Backscatter Diffraction (EBSD) analysis was performed at the interface after a high-speed collision test. Initially, the samples were prepared by grinding the cross section using SiC papers (P2500 abrasive) and diamond suspension (1 μm). Then the samples were further polished continuously with alumina suspensions (0.3 and 0.1 μm) for 15 min, respectively, and finally polished with colloidal silica solution (0.02 μm) for 30 min. Crystal orientation maps and IQ (image quality) value data were obtained using a scanning electron microscope (SEM) operating at 20 kV. The collected EBSD data was analyzed by TSL OIM Analysis 6 × 64 software. The strength of typical regions was evaluated by the nanohardness technique. Figure 1B shows the morphology near the interface between 316 L SS and the pure Ni ball after a high-speed collision. Obviously, macroscopically SPD occurs, and the microstructure with the gradient characteristic is expected for 316 L SS and pure Ni ball. Severe normal stress induced compression deformation of the Ni ball near central interface region, and the strong shear stress and friction also resulted in serious deformation of the Ni ball at the edge of the contact surface.  Figure 2A shows the orientation map of deformed 316 L SS near central impact interface region. It was obvious that the internal orientation of coarse grains changes greatly due to SPD in high strain rates. In the local region where dislocations and substructures were concentrated, therefore, the corresponding IQ value in these local areas was very low in Figure 2B. The number of deformation bands should increase due to the large strain in a high strain rate for austenitic SS during high-speed collision, 15 and then strain induced α'-martensite transformation occurred in the deformation bands and intersection of deformation bands, as shown in Figure 2C. The ratio of the high-angle grain boundary (HAGB) was high in Figure 2D, which should be attributed to the high strain rate during the high-speed collision. High shock induced storage energy may facilitate the formation of HAGBs, which needs further extensive investigations in the future. Some twisted twin boundaries were activated in these areas with a high IQ values in Figure 2E, since the dislocations were not easily activated in these areas. Compared with low strain rate, high strain rate was easier to induce deformation twins in austenitic SSs. 16 A depth-dependent gradient microstructure and elongated nano/ultrafine lamellar structures which are perpendicular to the collision direction can be observed in Figure 3A-D, and a high IQ value inside the grains indicates sufficient recovery and recrystallization in Figure 3B. Sufficient intragranular recrystallization occurred due to shock induced storage energy. In Figure 3C, the grain refinement and high ratio of the HAGBs (73%) were formed due to the SPD in high strain rate. A very small amount of deformation twins appeared in Figure 3D. Although ultra-high strain rates could induce deformation twins in pure Ni with medium stacking fault energy, the formation of deformation twins was inhibited by the temperature increasing in the Ni ball because of the high impact energy. A wide distribution of the misorientation angles is found in Figure 3D. Sufficient intragranular recrystallization with many HAGBs and significant grain refinement could both decreased dislocation activation. Especially, low dislocation activities near triple junctions could inhibit the formation of the low angle grain boundaries.

RESULTS
In contrast to near central interface region in Figure 3A-D, the microstructures near edge interface region change significantly, and many refined equiaxed grains were observed for pure Ni in Figure 4A-D. Romankov et al. 17 found that the laminated pure Ni could be transformed into an ultrafine grain during ball collisions. The larger strain and higher strain rate both should facilitate the transition of laminated structures in pure Ni to equiaxed grains. The very high intragranular IQ value also indicates that the process of dynamic recrystallization of pure Ni near edge interface region is very obvious in high-speed collisions in Figure 4B. Although the grain size was not very uniform, a large number of fine grains with HAGBs were formed in Ni ball near edge interface region after high-speed collision in Figure 4C. The large shear strain and high shear strain rate could facilitate to induce more refined grains and more homogeneous grain distribution in pure Ni in Figure 4C. Gurao et al. 18 also found that the average misorientation and the fraction of the HAGBs in compressed Ni increased with increasing strain rates. The high shear strain also cannot induce deformation twins due to the increased temperature in Ni ball in Figure 4D. Luo et al. 19 also suggested that the higher strain rate at a constant strain level increased the dislocation density and reduced the distance between deformation-induced dislocation boundaries and HAGBs, which undoubtedly facilitated the grain refinement of pure Ni in the high-speed collision process. In addition, the large shear strain and high shear strain rate could facilitate elongated ultrafine lamellar grains to transform into finer equiaxed grains. The HAGBs account for 71% and the distribution of the misorientation angles was also in a wide range in Figure 4D. When the misorientation distribution is more random, the recrystallization process could be fast enough to instantly reduce the effect of accumulated strain and thus dynamical recrystallization is easily activated. It can be seen that the main chemical constituent element distribution along the collision interface displays a large gradient, as shown in Figure 5A. This indicated that atomic diffusion was not evident between 316 L SS and Ni ball during high-speed collision. Based on the above microstructure observations and analyses, the deformation mechanism model for high-speed collision between 316 L SS and a Ni ball was proposed in Figure 5B. The 316 L SS was collided with Ni balls at a high speed, causing SPD near central interface region. On the one hand, a large number of fine grains and HAGBs were generated in the 316 L SS matrix in this area. These HAGBs can hinder dislocation movement, leading to dislocation blocking and entanglement and allowing more dislocations to accumulate in this region. On the other hand, the high-speed collision not only induced a large number of HAGBs in 316 L SS matrix, but also promoted the formation of deformation twins and α'-martensite due to high strain rates. This greatly promoted the dislocation accumulation in this area. Unlike 316 L SS, the strong shear stress and friction resulted in serious deformation and rapid temperature rise of Ni balls near edge interface region under high-speed collision. This could provide the kinetic condition for recovery through combined dislocation climbing, which decreased the dislocation density in this region and promoted to form recrystallized equiaxed grains with HAGBs. In addition, the violently normal stress in the Ni ball near central interface region during the high-speed collision resulted in SPD and the formation of elongated nano/ultrafine lamellar structures. These structures might hinder dislocation movement, resulting in a higher density of dislocations in the region. Figure 6A shows the typical nanoindentation curves of undeformed Ni ball and deformed Ni ball. Evidently, it can be seen that central interface region shows higher hardness value than edge interface region, as shown in Figure 6B. Yang et al. 20 found that the nano-lamellar microstructure had ultrahigh strengths and moderate ductile due to its lamellar boundary strengthening and a progressive work-hardening mechanism. Moreover, Zou et al. 21 reported that the heterogeneous lamellar microstructure with high-density dislocations synergized with nano/ultrafine grains, resulting in its ultrahigh yield strength. Therefore, the hardness value of the Ni ball near central interface region was higher than that near edge interface region. The higher hardness value near central interface region could be attributed to grain refinement and nano/ultrafine lamellar structures with HAGBs. On the one hand, smaller grains improved strength based on the Hall-Petch relationship. On the other hand, nano/ultrafine lamellar structures with sufficient internal recrystallization facilitated the full activation of more dislocations when a load was applied. 22 Refined equiaxed grain near edge interface region shows a lower hardness value than nano/ultrafine lamellar structures near central interface region. This is mainly because refined equiaxed grains with sufficient dynamic recrystallization cannot effectively hinder dislocation slip.

DISCUSSION
Some metal materials were subjected to deformation under ultra-high strain rate during vehicle collisions. 23 Thus it was crucial to investigate the work hardening behavior of these metal materials at high strain rates. Gradient microstructures along depth from the treated surface can be generated due to a graded variation of the strain and strain rates. It was found that the microstructures evolved from dislocation cells to nanolaminated structures of about 20-100 nm thick in gradient nanostructures with increasing strains and strain rates for pure Ni after the SMGT processing at 77 K. 24 It was demonstrated that high strain rate shear deformation with large strain gradients facilitated the grain refinement of pure Ni. 25 In general, the development of ultrafine hierarchical 26 and heterogeneous 27 lamellar structures for alleviating the inverse strength-ductility dilemma of metallic materials is very promising. The local temperature increase might inhibit α'-martensite transformation in 316 L SS. 28 Sato et al. 29 found that a temperature higher than 100 • C would induce a reversion of the α'-martensite phase during conventional shot peening. Therefore, the increase in temperature due to high impact energy effectively inhibited α'-martensite transformation, as shown in Figure 2C. Nanocrystalline Ni with the grain size of 60 nm showed hardness of 4.4 GPa, and grain boundary mediated process played an important role based on activation volume. 30 The nanolaminated pure Ni with an average lamella thickness of 20 nm obtained by SMGT showed ultrahardness from 5.3 to 6.4 GPa and thermal stability. 25,31 The current hardness value near central interface region after high-speed collision is about 5.3 GPa. The deformation process of the nanolaminated structures in pure Ni should be governed by dislocation slip. It was suggested that smaller nanograins were more effective to generate GNDs in both nanodomained and heterogeneous lamella structures, which induced high strength of heterogeneous lamella structures. 32 Considering the excellent recrystallization in nano/ultrafine lamellar structures, high hardness near central interface region after high-speed collision should be attributed to intragranular dislocation activating and the HAGBs. The dislocation interaction and dislocation movement impeded by the HAGBs both significantly improved the strength of the Ni ball near central interface region.

CONCLUSIONS
The effects of the high-speed collision on the microstructure characteristics of the 316 L SS and Ni ball and the deformation mechanism were investigated. The strength of the Ni ball near the interfaces was also evaluated. It was found that the collision induced α'-martensite and ultrafine twins were formed in 316 L SS, while the elongated nano/ultrafine lamellar structures and equiaxed nano/ultrafine grains with HAGBs in Ni ball were formed near central and edge interface regions, respectively. The increase in temperature due to high impact energy inhibited α'-martensite transformation during the high-speed collision. The increase in temperature due to the high impact energy also inhibited the formation of deformation twins in Ni ball. Moreover, the high shear strain and shear strain rates could facilitate elongated nano/ultrafine lamellar structures in pure Ni to transform into finer equiaxed grains. The high hardness of 5.3 GPa for refined nano/ultrafine lamellar structures in Ni ball near central interface area should be attributed to intragranular dislocation activating and impeding effect of the HAGBs.