Gradient Nanostructure Research Articles

Bending fatigue life enhancement of NiTi alloy by pre-strain warm surface mechanical attrition treatment

Many structures like medical stents made of superelastic NiTi shape memory alloy (SMA) are subjected to cyclic bending loads and show a limited fatigue life due to crack nucleation and growth in the surface layers under local tensile stress. Here, we enhance the bending fatigue life of NiTi plates by pre-strain warm surface mechanical attrition treatment (pw-SMAT) where the austenite phase is directly subjected to severe plastic deformation and grain refinement without inducing phase transformation. Amorphous and grain size gradient (5–100 nm) microstructures, as well as a maximum compressive residual stress of 1093 MPa are produced in the surface layer of the NiTi plates via the pw-SMAT. The compressive residual stress notably reduces the surface tensile stress from bending. The grain size gradient layers with improved hardness and reduced hysteresis have high fatigue crack nucleation resistance, while the middle large-grained layers of the plates have high fatigue crack growth resistance. The combined effects of the gradient nanostructure and the compressive residual stress substantially increase the bending fatigue life of the NiTi plates from an original 103 cycles to over 1.3 × 104 cycles. The results open up a new route to improve the bending fatigue life of NiTi plates by heterogenous nanostructures.

Effect of microstructure evolution on the mechanical properties of a Mg–Y–Nd–Zr alloy with a gradient nanostructure produced via ultrasonic surface rolling processing

Ultrasonic surface rolling processing (USRP) was performed on a Mg–Y–Nd–Zr alloy after equal-channel angular pressing. The result shown that the surface roughness was significantly improved, residual stress was introduced to the topmost surface, and ~450 µm gradient nanostructure (GNS) of Mg alloys was obtained after USRP treatment. The mechanism of microstructure evolution and its effect on the mechanical properties of the Mg alloy were studied in detail. There are multiple deformation mechanisms in grain refinement. In the initial stage, the coarse-grain (CG) matrix was subdivided by {101̅2}< 101̅1 > extension twins. Subsequently, the deformed grains were refined into substructures using interaction between twins, interaction between twins and dislocations, and dislocation entanglement. Moreover, the substructure contains a high density of multiple stacking faults and dislocation arrays, which can effectively block dislocation movement and promote dislocation packing. Finally, equiaxed nanograins (NGs) with high-angle grain boundaries were formed via subcrystal rotation recrystallization. With the formation of a GNS, the strength of the Mg–Y–Nd–Zr alloy considerably increased at the expense of a small amount of plasticity. The tensile test results demonstrated that the yield strength (YS) and ultimate tensile strength (UTS) of the Mg–Y–Nd–Zr alloy increased from 171.96 and 262.47 MPa to 216.06 and 330.61 MPa, respectively, and the elongation rate decreased from 20.52% to 14.65% after the USRP treatment. The YS improvement of the Mg–Y–Nd–Zr alloy after the USRP treatment was attributed to grain boundary strengthening, synergistic strengthening, dislocation strengthening, and twin strengthening. Furthermore, the microhardness of Mg–Y–Nd–Zr considerably improved after USRP treatment, and grain refinement was the primary surface strengthening mechanism of the Mg–Y – Nd – Zr alloy after the USRP treatment. The work-hardening capacity of the Mg alloy improved after the USRP treatment, and the synergistic deformation between GNS and CG matrix was the primary reason for the excellent work-hardening capacity of the Mg–Y–Nd–Zr alloy.

Revealing the effects of concomitant grain coarsening and refinement on the internal variable evolution and mechanical properties of gradient nanostructured nickel

The outstanding mechanical properties of gradient nanostructured (GNS) metals and alloys such as a superior combination of strength and ductility can be attributed to two important deformation mechanisms: hetero-deformation induced (HDI) strengthening and strain hardening, as well as grain growth. Recent experimental evidence indicates that GNS Ni specimens undergo concomitant grain coarsening and refinement during tensile deformation. Nevertheless, how such grain growth behaviors affect the evolution and distribution of their internal variables such as dislocation density and HDI hardening, and how such effects are manifested in their mechanical properties, are still unknown. Therefore, this work strives to answer these questions by incorporating a modified strain gradient plasticity formulation taking into account the saturation of geometrically necessary dislocations (GND) accumulation due to the dynamic equilibrium between dislocation source formation and deactivation into a crystal plasticity finite element framework, in conjunction with two other models accounting for the evolution of statistically stored dislocations (SSD) and HDI stress, respectively, to explore the effects of grain coarsening and refinement on the internal variable evolution and mechanical properties of GNS Ni specimens with different grain size gradients under deformation. It is revealed that SSD density increases throughout the samples as a result of reduced dynamic recovery in the fine grains induced by grain coarsening as well as enhanced dislocation storage in the coarse grains due to their refinement. In contrast, grain coarsening and refinement leads to a decrease in GND density and HDI stress in the fine grains and an increase in the coarse grains as a result of the corresponding changes in strain gradient intensity. Last but not least, the overall effect of grain growth on the mechanical behaviors of GNS Ni is the outcome of the synergy and competition between its individual effects on the Hall-Petch strengthening, forest dislocation hardening and HDI hardening, respectively, and the different grain size gradients of the GNS Ni specimens give rise to the opposite variations in their mechanical behaviors with respect to grain growth.

Microstructural origin of high scratch resistance in a gradient nanograined 316L stainless steel

Surface gradient nanostructures can be implemented in metallic materials to offer outstanding resistance to sliding friction and wear, but their scratch resistance and underlying anti-scratch mechanisms remain unknown. In this study, the scratch behavior and linked structure modification of a gradient nanograined 316L stainless steel were investigated and compared to the coarse-grained and the nanograined counterparts. After a single high-load scratch, inhomogeneous nanostructures are formed ahead of the indenter in the coarse-grained and nanograined samples, predominantly leading to mechanical incompatibility, crack nucleation and subsurface peeling-off. By contrast, the gradient nanograined sample exhibits superior scratch resistance without surface and subsurface cracking, thanks to strain delocalization associated with stabilized gradient nanostructure during scratching. This work forges the links between scratch-induced microstructural origin and high scratch resistance for gradient nanograined materials under extremely high tribological stress.

Enhanced bonding property of ion-plated TiN coating on stainless steel by mechanically pre-forming a gradient nanostructure

An enhanced coating/substrate bonding property is crucial for the application of hard coatings (e.g., TiN) on soft substrates (e.g., stainless steel). In the present work, a gradient nanostructure, in which the grain size is ~50 nm in the top surface layer and increases gradually with depth, was pre-formed in 304 stainless steel by means of plate surface mechanical rolling treatment (PSMRT). Subsequently, a TiN coating was deposited on it by using a commercial arc ion plating approach. Scratch tests demonstrated that the critical load of coating detachment was significantly enhanced in the coated PSMRT sample, ~47 % higher than that in the coated coarse-grained counterpart sample. Analyses of the underlying bonding mechanisms revealed that a higher interfacial strength was achieved in the coated PSMRT sample, mostly due to the depressed formation of compounds and the accelerated diffusion of Ti in the interfacial region. In addition, such factors as a higher substrate hardness, a lower friction coefficient between the scratch tip and the coating, and a lower internal stress in the coating might also contribute to the enhanced bonding property in the coated PSMRT sample. This work provided insights on applying a simple mechanical approach to enhance the bonding properties of hard coatings on soft substrates.

Robust, Healable, Self-Locomotive Integrated Robots Enabled by Noncovalent Assembled Gradient Nanostructure.

Integration, being lightweight, and intelligence are important orientations for the future advancement of soft robots. However, existing soft robots are generally hydrogels or silicone rubber, which are inherently mechanically inferior and easily damaged and difficult to integrate functions. Here, inspired by nacre, an elastomer actuator with sulfonated graphene-based gradient nanostructures is constructed via supramolecular multiscale assembly. The resulting nanocomposite possesses an ultrahigh toughness of 141.19 MJ/m3 and high room-temperature self-healing efficiency (89%). The proof-of-concept robot is demonstrated to emphasize its maximum swimming speed of 2.67 body length per second, whose speed is comparable to that of plankton, representing the outperformance of most artificial soft robots. Furthermore, the robot can stably absorb pollutants and recover its robustness and functionality even when damaged. This study breaks the mutual exclusivity of functional execution and fast locomotions, and we anticipate that our nanostructural design will offer an effective extended path to other integrated robots that required multifunction integration.

Formation mechanism and wear behavior of gradient nanostructured Inconel 625 alloy

The formation mechanism and wear behavior of a gradient nanostructured (GNS) Inconel 625 alloy were investigated using SEM, TEM and ball-on-disc sliding wear tester. The results show that surface mechanical grinding treatment (SMGT) induced an approximately 800 μm-deep gradient microstructure, consisting of surface nano-grained, nano-laminated, nano-twined, and severely deformed layers, which resulted in a reduced gradient in micro-hardness from 6.95 GPa (topmost surface) to 2.77 GPa (coarse-grained matrix). The nano-grained layer resulted from the formation of high-density nano-twins and subsequent interaction between nano-twins and dislocations. The width and depth of the wear scar, wear loss volume, and wear rate of the SMGT-treated sample were smaller than those of untreated coarse-grained sample. Moreover, the wear mechanisms for both samples were mainly abrasive wear and adhesive wear, accompanied with mild oxidation wear. The notable wear resistance enhancement of the GNS Inconel 625 alloy was attributed to the high micro-hardness, high residual compressive stress, and high strain capacity of the GNS surface layer.

High-temperature fatigue behavior and cyclic deformation of a gradient nanostructured RAFM steel

The gradient nanostructured (GNS) surface layer was prepared to enhance the high-temperature fatigue properties of reduced activation ferritic/martensitic (RAFM) steel. The GNS-RAFM steel showed high stability of microstructures at a high temperature of 550 ℃/60 min due to the pinning effect of M23C6 and MX precipitates, which contributed to an improvement of fatigue life at 550 ℃/260 MPa by 5.5 times compared to the original one. Moreover, the high-temperature fatigue resistance enhancement in the SMRT samples was supposed to be the synergistic contribution of compressive residual stress arising, grain nanocrystallization/refinement, and the strain delocalization introduced in the gradient nanostructure.

Superior mechanical properties and deformation mechanisms of a 304 stainless steel plate with gradient nanostructure

Spatially gradient microstructures have shown a promising application in enhancing strength-ductility synergy of engineering metals such as austenitic stainless steels. However, existing approaches are limiting in producing a thick gradient nanostructured (GNS) layer with a high strengthening capability, and the underlying deformation mechanisms are still not clear in GNS austenitic stainless steels. In this work, we developed a new approach, i.e., plate surface mechanical rolling treatment, to produce a bulk gradient nanostructure in a 304 stainless steel plate of ∼1.90 mm in thickness. Uniaxial tensile tests revealed that an ultra-high yield strength of ∼1073 MPa with a considerable uniform elongation of ∼21% was achieved in the GNS sample. Subsequently, the evolutions of microstructure, phase, microhardness, and local strain distribution were systematically studied in the GNS plate during tensile tests. The results demonstrated that the mechanical incompatibilities, relating with the gradient microstructure and martensite-enclosing-austenite domains, contribute to an extra strain-hardening capability, leading to the outstanding strength-ductility synergy in the GNS 304 stainless steel. Furthermore, analyses based on experimental observations and theoretical calculations revealed that dislocation activities, instead of deformation-induced martensite transformation, microstructure refinement, and twinning, play a dominant role in the strain-hardening mechanisms of the GNS plate during tension.

Laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy with significantly enhanced wear resistance

• Gradient nanostructured layer on TiZrHfTaNb refractory high-entropy alloy was fabricated by laser surface remelting technique. • The average grain size in gradient nanostructured layer is refined from ∼ 200 µm in matrix to only ∼ 8 nm in the top surface. • The microhardness is increased from ∼ 240 HV in matrix to ∼ 650 HV in the top surface layer. • The decomposition-mediated gradient refinement mechanisms were revealed mainly by HRTEM. • The gradient nanostructure shows significantly enhanced wear resistance, reducing the wear rate with an order of magnitude. Heterogeneous gradient nanostructured metals have been shown to achieve the strength-ductility synergy, thus potentially possessing the enhanced tribological performance in comparison with their homogeneous nanograined counterparts. In this work, a facile laser surface remelting-based surface treatment technique is developed to fabricate a gradient nanostructured layer on a TiZrHfTaNb refractory high-entropy alloy. The characterization of the microstructural evolution along the depth direction from the matrix to the topmost surface layer shows that the average grain size in the ∼100 µm-thick gradient nanostructured layer is dramatically refined from the original ∼200 µm to only ∼8 nm in the top surface layer. The microhardness is therefore gradually increased from ∼240 HV in matrix to ∼650 HV in the topmost surface layer, approximately 2.7 times. Noticeably, the original coarse-grained single-phase body-centered-cubic TiZrHfTaNb refractory high-entropy alloy is gradually decomposed into TiNb-rich body-centered-cubic phase, TaNb-rich body-centered-cubic phase, ZrHf-rich hexagonal-close-packed phase and TiZrHf-rich face-centered-cubic phase with gradient distribution in grain size along the depth direction during the gradient refinement process. As a result, the novel laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy demonstrates the significantly improved wear resistance, with the wear rate reducing markedly by an order of magnitude, as compared with the as-cast one. The decomposed multi-phases and gradient nanostructures should account for the enhanced wear resistance. Our findings provide new insights into the refinement mechanisms of the laser-treated refractory high-entropy alloys and broaden their potential applications via heterogeneous gradient nanostructure engineering.

Enhanced fatigue property by fabricating a gradient nanostructured surface layer in a reduced-activation steel

Reduced activation ferritic/martensitic (RAFM) steels have been selected as candidate structural materials for future advanced nuclear power systems. In the present work, the influence of a gradient nanograined surface layer on the fatigue properties of RAFM steels was studied. A gradient nanostructured (GNS) surface layer with a thickness of ∼85 ​μm was prepared on RAFM steel utilizing surface mechanical rolling treatment (SMRT). The mean grain size was approximate 43 ​nm at the topmost surface and increased gradually with depth. The results of the stress-controlled tension-compression fatigue experiments showed that the fatigue life enhanced approximately 6 times in the SMRT samples compared to the corresponding base metal counterparts. The relationship between the applied stress amplitude and the fatigue lifetime, and the fracture morphology showed that the surface strengthening and strain delocalization were caused by GNS, which suppressed surface crack initiation process, and hence the fatigue properties of RAFM steels improved. In addition, the deformation compatibility in GNS and coarse-grained boundaries leading to more dislocation interactions and accumulation during the cyclic process, also plays a crucial role in enhancing the fatigue properties of RAFM steel.

Combining synergetic effects of gradient nanotwins and nanoprecipitates in heterogeneous bronze alloy

Both strengthening effects of gradient nanostructure (especially the gradient nanotwins) and nanoprecipitates have been well-recognized to simultaneously improve the mechanical properties of metallic materials, e.g., the high strength–ductility synergy. In this study, a surface mechanical attrition treatment technique has been applied to fabricate a gradient nanostructured bronze alloy (Cu-12Sn-1.5Ni-0.6Fe) with pre-nanoprecipitates. Atomic-scale investigations were performed to capture the microstructural evolution governing the gradient refinement process along the depth direction of the treated alloy. On one hand, the body-centered cubic (bcc) Fe-rich pre-nanoprecipitates are found to be well-retained in the fabricated gradient nanostructured specimen. On the other hand, it is interesting to reveal that the semi-coherent interfacial zones between the bcc Fe-based nanoprecipitates and face-centered cubic (fcc) Cu matrix with the specific distribution of misfit dislocations could facilitate the nucleation and growth of gradient nanotwins along the depth direction. Specifically, the twinning partials could be associated with the dissociations of the misfit dislocations in the semi-coherent interfacial region, which is evidenced by the concomitant fcc-Cu/bcc-Fe/nanotwin triplex. As a result, the synergetic strengthening effects of gradient nanotwins and nanoprecipitates are combined to enhance the strength–ductility synergy and work hardening capacity. Moreover, in situ transmission electron microscope observations were conducted to understand the synergetic strengthening mechanisms of the gradient nanotwins and nanoprecipitates during the dynamic cracking process in the fabricated gradient nanostructured bronze alloy. The strategy of combining the nanoprecipitates and gradient nanostructure synergetic effects in our work might offer a novel pathway for designing high-performance metallic materials.

Grain growth and fatigue behaviors of GH4169 superalloy subjected to excessive ultrasonic surface rolling process

In this study, the fatigue behaviors of GH4169 superalloy, which possessed a gradient nanostructure layer introduced by an ultrasonic surface rolling process (USRP) and characterized using scanning electron microscopy and transmission electron microscopy, were investigated. Rotary bending fatigue test results verified that the fatigue resistance of the alloy was enhanced due to the improved surface integrity, such as refined grains, improved microhardness and the great compressive residual stress field. However, excessive USRP treatment induced grain growth (from 58.23 to 280.7 nm) at the top surface of the sample accompanied by residual stress relaxation and decreased microhardness, leading to the degradation of fatigue resistance. In addition, the mechanism of the grain growth observed in the alloy subjected to excessive USRP treatment was revealed. It was found that the collective migration of neighboring grain boundaries caused by excessive cyclic plastic deformation promoted the formation of nanotwins, and such process played a key role in accelerating grain growth.

Combining gradient structure and supersaturated solid solution to achieve superior mechanical properties in WE43 magnesium alloy

• Surface mechanical attrition treatment was employed to successfuly produce a gradient nanostructured layer on WE43 magnesium alloy. • High-resolution TEM was mainly performed to successfully dissect the refinement mechanisms of the gradient nanostructure. • A supersaturated solid solution is formed by re-dissolving the Mg 3 RE second phase into nanostructured α-Mg matrix phase in the gradient layer. • Gradient structure and supersaturated solid solution cooperatively contribute to the extraordinary mechanical properties. In this study, surface mechanical attrition treatment was employed to sucessfully produce a gradient nanostructured layer on WE43 magnesium alloy. X-ray diffraction, energy dispersive X-ray spectrometer, and high-resolution transmission electron microscope observations were mainly performed to uncover the microstructure evolution responsible for the refinement mechanisms. It reveals that the grain refinement process consists of three transition stages along the depth direction from the core matrix to the topmost surface layer, i.e., dislocation cells and pile-ups, ultrafine subgrains, and randomly orientated nanograins with the grain size of ~40 nm. Noticeably, the original Mg 3 RE second phase is also experienced refinement and then re-dissolved into the α-Mg matrix phase, forming a supersaturated solid solution nanostructured α-Mg phase in the gradient refined layer. Due to the cooperative effects of grain refinement hardening, dislocation hardening, and supersaturated solid-solution hardening, the gradient nanostructured WE43 alloy contributes to the ultimate tensile strength of ~435 MPa and ductility of ~11.0%, showing an extraordinary strain hardening and mechanical properties among the reported severe plastic deformation-processed Mg alloys. This work provides a new strategy for the optimization of mechanical properties of Mg alloys via combining the gradient structure and supersaturated solid solution.

Synthesis and deformation mechanics of gradient nanostructured materials

The emerging gradient nanostructured metals and alloys containing spatially graded structural components with large variations in length scale and/or mechanical properties exhibit unprecedented mechanical performance. This perspective delineates the basic structural features of gradient nanostructures, i.e. structural components and spatial gradients, as well as related synthesis methods, excellent tensile properties and novel deformation mechanisms. The challenges and prospect for the development of gradient nanostructured materials in the future are also addressed.

Plastic deformation induced formation of supersaturated solid solution nanostructure in a dual-phase Cu-Ag alloy

A gradient nanostructured (GNS) surface layer was generated in a dual-phase Cu-Ag alloy by means of surface mechanical grinding treatment at liquid nitrogen temperature. With a decreasing depth in the surface layer, the coarse-grained microstructure of Cu matrix and Ag precipitates gradually converts into a nanolaminated structure of Ag- and Cu-rich phases with simultaneous chemical mixing, and finally forms a homogeneous single-phase supersaturated solid solution (SSS) nanostructure. Accordingly, a dislocation-mediated transportation mechanism was proposed to dominate the chemical mixing of Ag and Cu atoms, i.e., dislocations interacting with solution atoms penetrate the Cu/Ag interface and glide in the neighboring solvent-phase lamella.

Gradient nanostructure, phase transformation, amorphization and enhanced strength-plasticity synergy of pure titanium manufactured by ultrasonic surface rolling

In this study, ultrasonic surface rolling process (USRP) was used to fabricate a gradient nanostructured commercial pure titanium. The site-specific microstructure, refining mechanisms and mechanical properties were investigated by high-resolution transmission electron microscopy, electron backscatter diffraction and tensile test. The results show that the surface layer exhibited multi-gradient features along the depth, including grain size gradient, deformation twin gradient, strain gradient, orientation gradient and hardness gradient. The gradient structure consisting of equiaxed nanograin layer, elongated lamellar layer, deformation twinning profuse layer, and coarse-grained layer along the depth direction. The formation mechanisms of gradient nanostructure are thoroughly elucidated, and the synergistic effect of twin and dislocation plays an important role in grain refinement. In addition, amorphous bands (generated by localized dislocations or phase transformation triggered crystalline to amorphous transition) and nano-thick lamellar with face-centered cubic (FCC) structure (<0001> HCP //<001> FCC , {1 1 ¯ 00} HCP //{2 2 ¯ 0} FCC and {11 2 ¯ 0} HCP //{220} FCC ) were formed in the nanograin layer, and the phase transformation mechanisms were elucidated. The HCP to FCC phase transformation and amorphization play a role in refining grain. Tensile test results suggested that the gradient nanostructure improved the strength (yield strength and ultimate tensile strength improved from 342 MPa and 526 MPa to 412 MPa and 598 MPa, respectively) while maintaining enough plasticity. The strength-plasticity synergy was attributed to the coordinated deformation of the gradient nanostructure and the coarse-grained core.

Synthesis and deformation mechanics of gradient nanostructured materials

Synthesis and deformation mechanics of gradient nanostructured materials

Gradient structure induced simultaneous enhancement of strength and ductility in AZ31 Mg alloy with twin-twin interactions

Gradient nanostructure was introduced to enhance the strength and ductility via deformation incompatibility accommodated by geometrical necessary dislocations for most metallic materials recently. However, few intensive researches were carried out to investigate the effect of gradient structure on the deformation twin evolution and resulting performance improvements. In the present paper, we produced gradient-structured AZ31 Mg alloy with fine-grain layers, parallel twin laminates and a coarse-grain core from two upmost surfaces to the center of plate. Surprisingly, this architected Mg alloy exhibited simultaneous enhancement of strength and ductility. Subsequent microstructural observations demonstrated that abundant twin-twin interactions resulting from higher strength and multi-axial stress state could make great contributions to the increase of work-hardening capability. This was further proved by the measurement of full-field strain evolution during the plastic deformation. Such a design strategy may provide a new path for producing advanced structure materials in which the deformation twinning works as one of the dominant plasticity mechanisms.

Effect of ultrasonic nanocrystalline surface modification process on fretting wear behavior of laser surface textured 20CrMoH steel

Laser surface texturing (LST) followed by ultrasonic surface rolling (USR) was applied to 20CrMoH steel to improve the friction and wear behavior. In this paper, the fretting friction and wear mechanisms of the modified (USR, LST + USR) 20CrMoH steel under different loads and lubrication conditions were systematically investigated. Results indicated that the regular coupling distribution of micro-texture and gradient nanostructure (GNS) was formed on the surface layer of the combined treated material. Simultaneously, the depth of the GNS was up to 22 μm and the surface microhardness was increased from 800 HV0.2 to 860 HV0.2. Besides, the surface roughness Ra value was decreased from 0.24 μm to 0.01 μm and the maximum residual compressive stress (RCS) value was −610 MPa. Under dry friction and high load conditions, compared with the untreated specimen, the average friction coefficient and worn losses of the combined treated specimen were reduced by about 10% and 30%, respectively. In comparison with the untreated specimen, the average friction coefficient of the combined treated specimen was decreased by 26% under the conditions of low load and oil lubrication. Especially, the fretting wear resistance was obviously improved and its worn loss was significantly reduced by 92.05% compared with the untreated material. The predominant wear mechanisms of the combined treated material under dry friction and oil lubrication were oxidation wear and abrasive wear, respectively.