Currently, metallic materials such as titanium, stainless steel, magnesium, etc. are widely used in the fields of orthopedics and dentistry because of the superior mechanical properties (e.g., high tensile strength, toughness, and fracture resistance) compared with the ceramics and bio-glass [1,2].

However, for materials implanted into the body, it is important to ensure that they do not have negative effects; Willams defined this idea of biocompatibility as “the ability of a biomaterial to perform its desired function with respect to a medical therapy, without eliciting any undesirable local or systemic effects in the recipient or beneficiary of that therapy, but generating the most appropriate beneficial cellular or tissue response to that specific situation, and optimizing the clinically relevant performance of that therapy” [3]. In order to improve biocompatibility, predecessors have used micro-arc oxidation, hydrothermal synthesis, thermal spraying, electroless plating, and other methods to form a biologically active surface layer (usually a ceramic surface layer) on the metal surface [4,5,6]. Nevertheless, there is a problem: the interface between these heterogeneous coatings and the substrate can easily peel off. In this regard, severe plastic deformation (SPD)—a technique which can fabricate metals with ultrafine-grained (UFG) or nano-grained (NG) structure—shows great potential in the field of biomedicine [7,8,9,10]. It has been reported that SPD-derived ultra-/nano-structures possess a large fraction of grain boundaries and more lattice imperfections such as dislocations, vacancies, stacking faults, and twins, which has the positive effect of improving the mechanical properties (particularly the strength, according to the Hall-Petch theory) and increasing the surface energy of the metals to mediate protein adsorption, thus benefiting the subsequent cell response and tissue growth [11,12].

In general, the SPD method can refine the coarse-grained materials into UFG or NG materials through two modes: one is applying high strain on the material’s surface to generate surface grain refinement, and the other is using extremely high plastic deformation to compel the whole grain size of the bulk materials to reduce into the UFG or NG regime. In recent years, these two modes—including surface mechanical attrition treatment (SMAT), ultrasonic shot peening (USSP), friction stir processing (FSP), and equal channel angular pressing (ECAP)—have been used extensively to surface/bulk modify the metals in order to avoid the interface bonding issue and improve biocompatibility.

Surface mechanical attrition treatment (SMAT) is a physical modification method for obtaining new gradient nanometal materials [17]. It is an effective method for achieving the surface nanocrystallization of many metals, such as titanium, aluminum, nickel, steel, magnesium, and copper. The basic process is summarized as follows: Smooth spheres made of steel or ceramic, with a diameter of 3–10 mm, are placed in a chamber which is connected to a vibration generator. The vibration frequency of the chamber is 50 Hz and 20,000 Hz for I-type and II-type SMAT techniques, respectively [18]. The sample surface to be treated is impacted by a large number of balls in a short period of time, with a velocity of 1–20 m/s depending on the vibration frequency and the distance between the sample surface and the balls [13]. The impact of each ball causes severe plastic deformation (SPD) on the surface layer and facilitates the process of nanocrystallization. The thickness of the nanostructured surface layer depends very much on the processing parameters (such as ball size, vibration frequency, temperature, etc.) [13]. With flexibility and low cost, SMAT can obtain localized nanostructured surface layers in bulk materials and is capable of treating the surfaces of parts with complex shapes. In addition, this technique can produce surfaces with low roughness because of the better quality of shots and a lower impact speed [11].

The ultrasonic shot peening (USSP) process is a kind of shot peening. It can optimize the characteristics of the material surface, modify the material properties, and improve the wear and fatigue resistance. A power ultrasonic horn causes impacts with high frequency (generally higher than 20 kHz) and low amplitude. The whole process is quiet because it operates more frequently than the human ear can perceive. The high-frequency and multi-directional shock acts on the surface randomly, creating a kind of microdimple shape and forming a nanocrystalline surface layer. USSP treatment is an effective approach for forming a nanostructure layer on the surface of metallic materials [20]. However, the relatively high strain might result in residual porosities, impurities, and dimensional issues [21].

Laser shock peening (LSP) is a surface modification process that uses laser beams to generate shock waves on the surface of a material. A high-energy laser is emitted from the light source, reflected by the mirror, and irradiated on the surface of the sample after being focused. The continued delivery of laser pulses rapidly heats and ionizes the vaporized material, converting it into rapidly expanding plasma [22] while a shock wave is created. If the amplitude of the shock wave is above the Hugoniot elastic limit (HEL) of the target, the material deforms plastically during the passage of shock waves, resulting in compressive residual stresses below the target surface [22]. However, due to the thermal effect, LSP is less efficient in refining the surface grains compared to the mechanical treatments. The LSP process is expensive and takes a long time to scan a large area compared with other processes [23].

FSP provides great flexibility in terms of processing conditions to tailor the localized microstructure in a material [24]. Its principle is similar to friction stir welding, which uses frictional heat to treat materials. The tool shoulder is fastened to the sample, and the tool pin is in close contact with the surface of material. In the process of FSP, the tool pin rotates at high speed and moves to the specified direction; then, severe plastic deformation occurs on the surface of sample, which is called the stir zone. Due to high frictional heat and dynamic recrystallization [25], fine grains will be generated in the processing area. The main parameters influencing FSP are rotating speed, advancing speed, plunging force, etc. In addition, microstructure and mechanical properties will also be altered with respect to the geometry of tool and the materials it uses [26]. The strict requirements of the processing conditions are a major disadvantage of FSP, hindering extensive use of this technique. In addition, the relatively high temperature during the process makes it difficult to adapt it to the production of grain structures in the nanoscale regime [27].

The UNSM technique was developed and commercialized by Design Mecha Co., Ltd. (Seoul, South Korea). It is a mechanical impact-based surface treatment applied by means of high ultrasonic vibration frequency [28]. The process includes the settings of parameters such as frequency, input amplitude, interval, horn speed, and tip (ball) diameter. The spherical tip—made of hard Al₂O₃, WC, or Si₃N₄—impacts the surface at a specific frequency. The superposition of ultrasonic low-frequency vibration on static load causes severe plastic deformation on the surface of the material, inducing grain size decreases to the nanoscale. During this process, the sample is clamped on the gripper, which reciprocates up and down (Y direction) with a constant velocity. After each Z direction scan, the silicon nitride or tungsten carbide prompts it to move a constant distance in the Z direction [29]. After treatment, the coarse-grained structure in the surface layer of the sample is refined to the nanometer level, which corresponds to the characteristics of finer grains and more uniform dislocation distribution, forming a gradient nanostructure layer with a depth of several hundreds of microns [30,31]. Moon et al. [32] found that specimens treated with the UNSM process exhibited much higher tensile strength compared to untreated specimens.

High-pressure torsion (HPT) refers to the processing of a metal which bears compressive force and simultaneous torsion force. The sample in this process is laid in the shape of a disc between two anvils. The sample is subjected to a squeezing pressure P of several GPa at room temperature; at the same time, it bears a torsion force generated by the rotation of the lower anvil [33,34]. Sometimes, in order to improve the HPT processing efficiency, the process needs to be carried out at a higher temperature; under this condition, the shearing and pressing speed should be controlled [35]. During HPT, the strain is allowed to continuously change, and it is easy to achieve high shear strains. Moreover, even materials that are relatively hard and brittle can undergo the severe deformation in this method [36].

The disadvantage of HPT is that the process usually requires high pressure and torque; otherwise, it will cause a significantly inhomogeneous microstructure in the treated sample. Therefore, HPT cannot be utilized to produce large bulk materials.

Equal channel angular pressing (ECAP) has been one of the most important machining procedures in the past few decades. It was proposed by Segal in the 1970s at an institute in Minsk in the former Soviet Union [37].Before the process, the sample needs to be lubricated; it is then squeezed through two intersecting channels (equal section) and subjected to a shearing force at the intersection of the channels [38],. The cross section of two equal-length channels is related to the angle φ (internal model/channel angle φ), whilst the angle ψ is the curvature arc at the intersection (outer curvature arc/external rotation angle ψ). The same sample is subjected to multiple extrusions, that is, repeated shear deformation; this causes a large amount of cumulative plastic strain in the material, resulting in obvious grain refinement [39]. Although the sample undergoes very strong strain when it passes through the shear plane, the cross-sectional dimension of the sample does not change, even when it finally comes out of the mold.

In the work conducted by R.Z. Valiev [40], the following routes of billets were considered: orientation of a billet is not changed at each pass (route A); after each pass, a billet is rotated around its longitudinal axis through the angle 90° in the same direction (route B_c) or inverse direction (route B_a); after each pass, a billet is rotated around its longitudinal axis through the angle 180° (route C). Some researchers [41,42] have even pointed out that, compared with other pressing routes, the route B_c is more advantageous for obtaining ultra-fine grain (UFG) materials with a more uniform microstructure. ECAP is used to manufacture UFG structures among various SPD technologies and can produce large bulk materials for various applications, such as rod-shaped production for dental implants [43]. Additionally, ECAP can significantly economize raw materials [44]. The main disadvantage of ECAP is that the process usually happens discontinuously, with limitations in scale-up potential [45].

Based on ECAP, several variants have been developed to enhance its effect. A novel method proposed the application of vibrations with ultrasonic frequencies on the plunger to reduce the friction and forming load during ECAP; longitudinal ultrasonic vibrations were imposed onto the billet [46].

It was found that, when the plunger had ultrasound with a vibration amplitude of 2.5 μm at 20 kHz, much finer grains were achieved in the pure aluminum rods, and the calculated grain refinement efficiency of this improved the ECAP process increased by almost 25.8% [47].

Accumulative roll bonding (ARB) was first proposed in 1999 by Y. Sato et al. [48]. Compared with other bulk grain refinement methods, such as ECAP and HPT, which require large load capacities and many dies, its unique advantage is that it can be mass-produced at a lower operating cost, which makes it applicable to industrial manufacture. in this process, the stacking of materials and traditional seam welding are repeated. Firstly, place one strip aligned on top of the other, clean the mating surfaces to remove layers of sediment, and then apply a wire brush to enhance the bonding strength between the materials [49]. The two layers are joined together by rolling, just as in a traditional rolling bonding process; then, the rolled material is cut in half. The cut strip is once again surface treated, stacked, and seam welded, and the whole process is repeated. The whole process should be conducted at an elevated temperature, but below recrystallization temperature because recrystallization will cancel out the accumulated strain. Low temperature would result in insufficient ductility and bond strength [48].

Ye et al. [50] used Ni, Ti, Al, and Cu as experimental objects and studied their microstructure evolution and mechanical properties during the ARB process. With the increasing number of ARB passes, the thickness of the layers decreased gradually. It was reported that grain sizes decreased significantly with rolling reduction, and the grains in the layers near the surface possessed smaller grain sizes [51]. Furthermore, the corrosion resistance of the treated sample increased even when processed in the lower cycles [52]. Multi-layer composite metal materials can also be prepared using this method [53]. The shortcoming of ARB is that the process requires high precision and a long time; in addition, the quality of the final product is usually affected by the adhesion of the interlayers.

Multi-directional forging is a plastic deformation process that was proposed in the mid-1990s for the preparation of bulk ultrafine-grained metal materials. Its working principle is that the direction of the applied force is continuously changing during operation, and the multiple forgings in different directions of the billet are equivalent. Since its operating temperature is usually in the recrystallization temperature range of the alloy and the load is relatively low, the MDF process can be used to prepare nanostructured/ultrafine-grained materials. During the MDF process, the change of the loading direction has a great influence on the rheological behavior and microstructure evolution of the material. With the applied pressure on the material in three vertical directions changing continuously, the relationship between flow stress and strain as well as the evolution process of the microstructure of the materials can be systematically studied [57]. The advantage of this method is that nanostructures can be obtained in rather brittle materials, since the process starts at high temperatures and the specific loads applied to the product are relatively low.