Surface Severe Plastic Deformation Research Articles

Surface Modifications of Biomedical Gum-Metal-Type Alloy by Nano Surface—Severe Plastic Deformation

For developing performant metallic materials for medical implants, biomechanical adaptation is important for both the bulk material and the material’s surface. Therefore, a nano surface-severe plastic deformation (NS-SPD) treatment was applied on the surface of a gum-type alloy (Ti-Nb-Zr-Fe-O) with enhanced bulk mechanical biocompatibility. Six variants with different processing parameters were tested using small impact balls accelerated by compressed air. Before the NS-SPD treatment, the alloy was first subjected to thermomechanical processing to achieve a high compositional and structural homogeneity and proper mechanical properties of the core structure. A detailed microstructural analysis was performed on the thermomechanical-processed alloy and on the surface areas. The microstructural analysis (based on scanning electron microscopy electron backscatter diffraction and x-ray diffraction) reveals the influence of the NS-SPD treatment by formation of two different surface layers: an outer, very thin layer, approximately 10 μm in width, formed of titanium oxides as a consequence of surface oxidation during ball impacts in an air atmosphere, and one approximately 100 μm to 250 μm in width, representing a zone of high-density microstructural defects.

Determination of mechanical properties of pure zirconium processed by surface severe plastic deformation through nanoindentation

Commercially pure zirconium was processed by the surface mechanical attrition treatment (SMAT), and the microstructure observation showed that a gradient structure was induced. Nanoindentation measurements were taken to obtain the load–displacement curves at different depths below the treated surface. Using dimensional analysis, the local yield stress, hardness, strain hardening exponent, and elastic modulus at the corresponding depths were derived. The results showed that the yield stress and hardness varied with depth, while the strain hardening exponent and elastic modulus were approximately invariable. The finite element method was used to simulate nanoindentation at different depths below the treated surface to verify the derivation of the local elastic–plastic constitutive relationship. Stress–strain curves were computed for the treated samples through the rule of mixtures, and they agreed well with the experimental results. The analysis showed that the surface and subsurface hardening layers as well as the transition layer shared a high load applied to the samples, even though their volume fraction was small.

Strength-ductility modulation via surface severe plastic deformation and annealing

Surface severe plastic deformation (S2PD) techniques such as surface mechanical attrition treatment (SMAT) allow processing of metals to form a gradient of stored deformation energy and microstructure. Though strengthening of S2PD metals has been investigated in the literature, little information is available on the influence of subsequent annealing on microstructure and mechanical property evolution of the S2PD metals. Here, the SMAT process of Cu followed by heating at temperatures of 175 °C, 225 °C and 275 °C for 1 h is performed. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) are utilized to investigate microstructural evolution at different depths from the treated surface. To evaluate the spatial distribution of hardness, cross-section microhardness measurements are incorporated. The stored gradient of deformation has caused a reduction in the recrystallization temperature of the SMAT treated Cu, and following the annealing, microstructural evaluation reveals a recrystallized layer near the treated surface, the depth of which increased with a higher annealing temperature. Uniaxial tension testing is utilized to analyze the mechanical properties of the SMAT processed and annealed specimen, and a substantial increase in uniform elongation is observed with the 225 °C and 275 °C heat-treated specimens when compared to the as-SMAT processed state. This increase in uniform elongation is attributed to extra work-hardening rate due to the higher dislocation storage capacity of the recrystallized layer. This study shows that with annealing after SMAT, modulated grain structure gradients can be obtained. The strength-ductility combinations of the material can be flexibly altered in order to adapt to different applications with different mechanical property requirements by using different annealing parameters.

On the comparison of graded microstructures developed through High Reduction (per pass) Cold Rolling (HRCR) and Ultrasonic Nanocrystal Surface Modification (UNSM) in nickel-base Alloy 602CA

The development of graded microstructure in Ni-base Alloy 602CA, processed through the new technique of high reduction (per pass) cold rolling (HRCR) and the severe surface plastic deformation technique of ultrasonic nanocrystal surface modification (UNSM) was investigated. Clear through-thickness gradients in residual stress, dislocation structure and boundary density with misorientation >2° were observed after the HRCR and UNSM. However, the gradient in HRCR did depend on the roll-pass schedule: relative reduction per pass, total reduction and direction of the roll pass. Applying maximum reduction per pass and minimum number of passes exhibited an improved graded microstructure for 40% total reduction in both unidirectional (UR) and cross rolling (CR) cases. The UR40 and CR40 in HRCR process led to graded microstructures that compared well with those produced by UNSM.

Effect of recovery treatment on the wear resistance of surface hammered AZ31 Mg alloy

Surface severe plastic deformation (SPD) is an approach used to nanocrystallize metallic surfaces for enhanced resistance to wear. However, SPD alone does not generate well-defined nanocrystalline structure but one consisting of nano-sized dislocation cells with diffuse boundaries. For materials with limited slip systems such as Mg, SPD may harden the materials but deteriorate their fracture toughness. As a result, the benefit of SPD to the wear resistance of materials may not be appreciable. In this study, we nanocrystallized the surface layer of AZ31 Mg alloy by repeated hammering, followed by recovery treatment (HR). The microstructure, wear behavior, grain structure and wear debris of differently treated samples, including hammering (HM), hammering plus recovery treatment (HM-R), and as-received, were investigated and compared. Results of the study show that the SPD-HR sample had a superior nanocrystalline surface exhibiting the highest wear resistance. The hammered sample (HM) was harder than the HM-HR one but its wear resistance is lower than that of the latter, though it still performed better than the as-received one. The wear debris analysis showed that the surface of the HM-R sample was tougher than that of the HM one. The observed phenomena indicate that the wear resistance of AZ31 Mg alloys depends on both hardness and toughness to which the ductility is of importance. The recovery treatment after HM certainly helped achieving well-defined nanocrystallization for improved wear resistance of the Mg alloy.

Effects of severe plastic deformation on pre-osteoblast cell behavior and proliferation on AISI 304 and Ti-6Al-4V metallic substrates

In this study, titanium alloy (Ti-6Al-4V) and austenitic stainless steel (AISI 304) biomedical alloys were subjected to surface severe plastic deformation (SSPD) via severe shot peening (SSP) with the conditions of A28-30 Almen intensity. SSP is widely accepted as much more effective than the conventional surface modification techniques since it forms a nano-grain layer with large number of dislocations and grain boundaries. The SSP treatment in this study was led to a very thin rough layer in Ti-6Al-4V titanium alloy compared to that of AISI 304. The thicker layer of AISI 304 was created by twin-twin intersections and a martensite structure transformations. SSP treatment was resulted in a severe plastically deformed material surface and led to an increase in full width half maximum (FWHM) that expresses grain size reduction below nanometer regime. Also, SSP treatment increased the surface roughness of materials by forming pits and bumps and led to deterioration on surface topography. Besides physical and microstructural innovations, SSP treatment was improved the surface mechanical properties of the metals. On the other hand, it was noted that the effects of the alteration of surface topography and structural innovations (nano-grain crystal-deformation induced layer) were led to improved cellular behavior and increased cell proliferation regardless of material type.

Synergetic deformation-induced extraordinary softening and hardening in gradient copper

A gradient-structured Cu sample composed of grain-size gradient surface layer (GSL) and homogeneous coarse-grained (CG) core was fabricated by surface severe plastic deformation. Microhardness measurements revealed that both tension-induced softening in the top nanograined surface layer and hardening in the subsurface layer of integrated gradient sample were far more pronounced than that of a freestanding GSL, i.e. extraordinary softening and hardening occurred in gradient sample. The synergetic deformation between GSL and CG core produced accumulation of geometrically necessary dislocations and thereby back stress strengthening, resulting in extra flow stress and hardening. The extraordinary softening was attributed to the extra flow stress-assisted grain coarsening in nanograined layer. Furthermore, strain measurements proved that the CG matrix plays critical roles in stabilizing the grain coarsening and homogenizing the strain distribution in nanograined layer, which were favorable for the uniform ductility.

Improving plasticity of metallic glass by electropulsing-assisted surface severe plastic deformation

Using either electropulsing (EP) or surface severe plastic deformation (SSPD) to process metallic glasses can improve their plasticity, however, the moderate improvement in plasticity does not warrant a commercial usage. This work, for the first time, demonstrates the integration of electropulsing and surface severe plastic deformation is much more effective in improving the plasticity of metallic glasses than EP or SSPD treatment alone, opening a new avenue towards an unprecedented combination of strength and plasticity in metallic glasses. It is found that SSPD can generate microstructure heterogeneity featured by a mixture of matrix and plastically displaced regions with increased atomic volume. When applying EP and SSPD simultaneously, a synergistic effect occurs to produce a hybrid network with excess free volume and nanocrystals uniformly embedded in amorphous matrix. Molecular dynamics simulation and fracture surface analysis further reveal that the hybrid network is able to effectively reduce shear band localization, and therefore delay the fracture of metallic glasses.

The coordination behaviors of alpha and beta phases with different grain sizes in Ti-6Al-4V subjected to surface severe plastic deformation

The structure characteristics of alpha and beta phases on both sides of phase boundary in Ti-6Al-4V subjected to high energy shot peening were investigated via high-resolution transmission electron microscope. The results indicated that the incoherent phase boundary was the considerably active dislocation resource and stress concentration in phase boundary owing to plastic mismatch of two phases offered larger driving force for dislocation nucleation and motion. Correspondingly, twins were completely suppressed nearby phase boundary. Because of the diversity of grain refinement levels of alpha and beta phases, the coordination deformation behaviors were analyzed based on the ratio dα/dβ, in which dα and dβ were average grain sizes of alpha and beta phases respectively. As dα/dβ was above 10 and below 30, beta phase with extremely tiny grain sizes promoted dislocation nucleation and pileup of alpha phase with larger sizes. In contrast, as dα/dβ was above 3 and below 10, alpha phase which was severely work-hardening promoted dislocation multiplication of beta phase. As dα/dβ was below 3, the phase with higher orientation factor primarily plastically deformed and promoted dislocation motion and pileup of the other phase, further resulting in crystal rotation of equiaxed ultrafine grains and grain refinement of nanograins.

Long distance chemical gradient induced by surface nanocrystallization

Natural structures are wonderful examples of what can be done with a fairly limited selection of resources. Natural structures have this feature due to their complex architecture, gradient chemical compositions and microstructure. Inspired by nature, many attempts have been made to manufacture man-made materials with similar gradients (e.g. surface nanocrystallization by severe surface plastic deformation (S2PD)). Gradient microstructure of alloys manufactured by the S2PD techniques has been very well studied. However, the distribution of chemical compositions within the alloy after the S2PD treatment has rarely been explored. The current work found that by using S2PD combined with one-year long natural aging treatments, the microstructure and chemical composition of the Al-Zn-Mg-Cu alloy (7034 Al alloy) exhibited a new gradient distribution measured from the treated surface to the pristine interior. Zn (Cu) content on the surface layer increased from 10.2 (1.3) to 57.7 (5.9) wt.%, along with an Mg surface layer depletion after treatments. The thickness of the observed chemical gradient layer was surprisingly as thick as 45μm. Also observed were the similar phenomena in other alloys after the same treatments. The findings provide potential improvements in bioinspired materials manufacturing, rare resource saving and surface functionalization.

Structure response characteristics and surface nanocrystallization mechanism of alpha phase in Ti-6Al-4V subjected to high energy shot peening

Structure features and grain refinement of alpha phase in coarse-grained Ti-6Al-4V subjected to surface severe plastic deformation by using high energy shot peening (HESP) were investigated via high-resolution transmission electron microscope. The results illustrated the novel deformation-induced structure response characteristics, including markedly reduced contribution of twinning to deformation of coarse-grained alpha phase with hexagonal close-packed (hcp) structure, dramatical rise of pyramidal <c+a>-type dislocation density accompanying an increase in grain refinement level of alpha phase, and unusual phase transformation of alpha ultrafine grains (UFGs) from hcp to face-centered cubic (fcc) structure. Based on the above-mentioned results, nanocrystallization mechanism of alpha phase was obtained as follows. The massive dislocation tangles, dislocation bands and a handful of single-system tension twins were firstly generated and subdivided the original grains into elongated UFGs, and then transversely disintegrate elongated UFGs into equiaxed UFGs, which twinning was gradually replaced by pyramidal <c+a>-type dislocation slip during the above-mentioned process. Most of equiaxed UFGs with hcp structure subsequently underwent crystal rotation to transform into randomly-oriented equiaxed nanograins. Phase transformation from hcp to fcc structure occurred in a small amount of equiaxed UFGs, in which twin-twin and dislocation-twin interactions ultimately resulted in the formation of nanograins with fcc structure.

Microstructure evolution in Ti64 subjected to laser-assisted ultrasonic nanocrystal surface modification

Abstract Surface severe plastic deformation (SSPD) can significantly improve the mechanical properties of metallic components by inducing surface nanocrystallization and beneficial compressive residual stresses. The effectiveness of the SSPD processes is significantly dependent on the plasticity of the target metals. Here, we report an innovative surface thermomechanical process called laser-assisted ultrasonic nanocrystal surface modification (LA-UNSM) that integrates localized laser heating with high strain rate plastic deformation. The laser beam locally heats the target metal and increases the local plasticity, making the SSPD treatment more effective. After LA-UNSM, a microstructure featuring a nanocrystalline layer embedded with nanoscale precipitates was achieved in Ti64, resulting in an unprecedented 75.2% increase in hardness. After LA-UNSM processing, a 25-μm severe plastic deformation layer was produced that was 2.5 times thicker than that of the room-temperature UNSM-processed material. The grains at the top surface were refined down to 37 nm, indicating a similar degree of nanocrystallization to that produced by UNSM at room temperature. Nanoscale precipitate particles with diameters in the range of 5–21 nm were non-uniformly distributed in the nanocrystalline surface layer. These precipitates were produced through laser-assisted dynamic precipitation. The extremely high surface strength obtained for the Ti64 was attributed to the composite microstructure featured by nanoscale grains embedded with nanoscale precipitates and the work-hardening.

Surface severe plastic deformation induced solute and precipitate redistribution in an Al-Cu-Mg alloy

The precipitate and solute distribution in a surface severely plastically deformed Al-Cu-Mg alloy were revealed by means of transmission electron microscopy and atom probe tomography. Accompanying gradient grain structure, there is a gradient precipitate architecture which evolves from undeformed needle-like S (Al2CuMg) precipitates to shear-deformed and fragmented S precipitates along low angle dislocation boundaries within coarse grains, to reprecipitated Gunier-Preston-Bagaryatsky (GPB) zones and S precipitates in lamellar nanograins, and to completely dissolved and segregated solutes to nanograin boundaries. Dislocation activities induced precipitate dissolution and promoted solute diffusion are two concomitant processes during precipitate and solute redistribution. The variation of precipitate crystallography, including orientation relationship deviation and coherency loss between S precipitates and Al matrix, was attributed to local changes in strain fields around undissolved S precipitates and in nucleation environment for reprecipitated S phase.

Surface Severe Plastic Deformation of an Orthopedic Ti-Nb-Sn Alloy Induces Unusual Precipitate Remodeling and Supports Stem Cell Osteogenesis through Akt Signaling.

This work presents a strategy to augment the bioactivity of a new-generation metastable β-Ti-Nb-Sn alloy through surface severe plastic deformation. Foremost, the alloy was strengthened by precipitation of α phase using a well-designed thermo-mechanical processing route. Subsequently, the surface of the aged alloy was subjected to severe plastic deformation via surface mechanical attrition treatment (SMAT). Upon SMAT, a unique phenomenon of strain-induced precipitate coarsening was observed. A possible mechanism is proposed wherein the precipitates first dissolve due to significant slip transfer across the α/β-interface followed by reprecipitation along the other precipitates thereby leading to coarsening. Coarsening of the precipitates abrogated the strengthening caused by plastic deformation as a result of which the hardness did not increase significantly after SMAT in sharp contrast to other alloys. SMAT led to a decrease in the attachment of human mesenchymal stem cells because of an increase in the roughness-mediated surface hydrophobicity. On the other hand, an increase in the roughness led to the formation of more number of focal adhesions. This in turn enhanced the proliferation rate and more importantly, osteogenic differentiation of stem cells. Detailed investigation into the underlying mechanism revealed that an increase in focal adhesions activated the Akt-mediated mechano-transduction signaling pathway that enhanced the osteogenic differentiation. In summary, the potential of surface severe plastic deformation to impart bioactivity to the next-generation of orthopedic β-Ti alloys is underscored in this work.

Strain hardening and nanocrystallization behaviors in Hadfield steel subjected to surface severe plastic deformation

A gradient nanocrystalline layer with a thickness in a range of millimeter magnitude was successfully produced on the surface of Hadfield steel by a novel severe plastic deformation technology, high speed pounding. The surface hardness was measured, and the microstructure evolution during nanocrystallization process was characterized by X-ray diffraction and transmission electron microscopy. Results showed that the hardness increment and nanocrystallization in Hadfield steel were obtained at different stages under high speed pounding. The first stage was strain hardening, where surface hardness of Hadfield steel increased gradually during high speed pounding until a steady-state value was obtained. The hardening degree and rate of Hadfield steel were determined by deformation stress and strain rate, respectively. The second stage was microstructure nanocrystallization, at which twin boundaries interacted with dislocations to form general high angle grain boundaries. In this stage, the surface hardness of Hadfield steel remained basically the same. Moreover, a physical model was established to explain the strain hardening and surface nanocrystallization behaviors in accordance with the microstructure evolution at different stages in Hadfield steel.

Gradient microstructure and texture in wedge-based severe plastic burnishing of copper

Abstract

Gradient Nanostructured Tantalum by Thermal-Mechanical Ultrasonic Impact Energy

Microstructural evolution and wear performance of Tantalum (Ta) treated by ultrasonic nanocrystalline surface modification (UNSM) at 25 and 1000 °C were reported. The UNSM treatment modified a surface along with subsurface layer with a thickness in the range of 20 to 150 µm, which depends on the UNSM treatment temperature, via the surface severe plastic deformation (S2PD) method. The cross-sectional microstructure of the specimens was observed by electron backscattered diffraction (EBSD) in order to confirm the microstructural alteration in terms of effective depth and refined grain size. The surface hardness measurement results, including depth profile, revealed that the hardness of the UNSM-treated specimens at both temperatures was increased in comparison with those of the untreated ones. The increase in UNSM treatment temperature led to a further increase in hardness. Moreover, both the UNSM-treated specimens with an increased hardness resulted in a higher resistance to wear in comparison with those of the untreated ones under dry conditions. The increase in hardness and induced compressive residual stress that depend on the formation of severe plastically deformed layer with the refined nano-grains are responsible for the enhancement in wear resistance. The findings of this study may be implemented in response to various industries that are related to strength improvement and wear enhancement issues of Ta.

Effect of ultrasonic nanocrystal surface modification on elevated temperature residual stress, microstructure, and fatigue behavior of ATI 718Plus alloy

Ultrasonic Nanocrystal Surface Modification (UNSM) is a mechanical surface treatment that induces large compressive residual stresses and near-surface microstructural changes in the material by using repetitive impacts at ultrasonic frequencies with a WC tip. In this study, we investigate the use of UNSM to improve the fatigue life of ATI 718 Plus (718Plus) at high temperature of 650 °C. UNSM induced severe surface plastic deformation which led to nano-sized crystallites, twins, and high dislocation density in the near surface regions, coupled with an increase in surface hardness and high magnitude of compressive residual stresses. This change in the near-surface microstructure remained stable at elevated temperatures. In addition, UNSM retained ∼−680 MPa near-surface residual stress (56% of its initial residual stress) even after 140 h exposure to 650 °C. The retained near-surface residual stresses and the stable microstructure from the UNSM increased the yield strength by ∼11% (∼105 MPa) and endurance limit by ∼8% (∼70 MPa) in corresponding tests at 650 °C. This improvement in fatigue life was attributed to near-surface microstructure changes, material hardening and high compressive residual stress induced by UNSM. The estimated crack growth rates were ∼60% lower for UNSM-treated 718Plus as compared with untreated material. The thermal-mechanical residual stress relaxation indicates the effectiveness of UNSM in improving the fatigue life of 718Plus at 650 °C.

Fatigue Fracture Characteristics of Ti6Al4V Subjected to Ultrasonic Nanocrystal Surface Modification

The influence of ultrasonic nanocrystal surface modification (UNSM) on the fatigue fracture characteristics of Ti6Al4V was investigated. Two groups of specimens were separated due to different heat treatment conditions. Group one was stress-relief annealed at 650 °C, and group two was then treated with solid solution-aging. UNSM with the conditions of a static load of 25 N, vibration amplitude of 30 μm, and 36,000 strikes per unit produced about 40 μm surface severe plastic deformation (SPD) layers on both groups of specimens. UNSM improved the microhardness and the compressive residual stress. UNSM also helped achieve a neat surface, almost without changing the surface roughness. The fatigue strengths of these two groups were improved by 7% and 11.7%, respectively. After UNSM, fatigue cracks mainly initiated from the surface of the specimen before the fatigue life of 106 cycles, while they appeared at the internal compress deformed α-phase at the zone between the SPD layer and the core after the fatigue life of 106 cycles. The cracks usually extended along the deformation overflow bands and the process traces on the surface. Through the change of micro-dimples in the fatigue final rupture region, nanocrystals were achieved in the SPD layer. The crystal slip and the surface remodeling together influenced the energy field of crack evolution.

Strain-induced formation of a gradient nanostructured surface layer on an ultrahigh strength bearing steel

In the present work, an ultrahigh strength bearing steel (AISI 52100) was subjected to surface mechanical rolling treatment (SMRT) at room temperature. Microstructural observations showed that martensitic laths, twins and cementite particles in the initial microstructure underwent distinct plastic strains and were gradually refined into nanostructures. Consequently, a gradient nanostructured (GNS) surface layer with a mean grain size of ∼24 nm at the top surface was obtained on the bearing steel, resulting in an increment of ∼20% in the surface hardness. Analyses based on microstructural evolution, phase constitution and in-depth hardness distribution revealed a mechanically induced formation mechanism of the GNS surface layer. The multiple surface severe plastic deformation under fine lubrication and cooling during SMRT contributed to the formation of a thick hardened surface layer on the bearing steel.