Gradient Structure Layer Research Articles

Microstructure and Properties of Gradient Ti(C,N)-Based Cermets by Powder Extrusion Additive Manufacturing

Ti(C,N)-based cermets are crucial for high-speed cutting tools and other high-temperature applications, yet there remains a considerable gap in their preparation controllability, fracture strength, and toughness compared to cemented carbide. Despite numerous studies having focused on modifying the hardness and toughness of Ti(C,N)-based cermets by varying process parameters and chemical compositions, this research has used gradient Ti(C,N)-based cermets produced by powder extrusion additive manufacturing (PEM) technology, which is rare. This study developed the gradient structure layer by layer using PEM. The microstructure of the printed and sintered parts was studied, and the hardness, fracture toughness, and bending strength of the gradient material were analyzed. The gradient material demonstrates superior mechanical properties compared to traditional Ti(C,N)-based cermets, with a hardness of 1760−23+39 HV20, a fracture toughness of 8.5−0.4+0.3 MPa·m1/2, and a bending strength of 2047−43+22 MPa. The research will assist researchers in assessing the potential application of PEM and broaden the application fields of gradient Ti(C,N)-based cermets.

Effects of ultrasonic shot peening followed by surface mechanical rolling on mechanical properties and fatigue performance of 2024 aluminum alloy

Aiming to improve the mechanical properties and fatigue performance of 2024 aluminum alloy, the experimental investigations on composite treatment consisting of ultrasonic shot peening (USP) followed by surface mechanical rolling (SMR) were carried out. The experimental results were compared with the specimens only treated by USP or SMR in terms of surface roughness, microhardness gradient and microstructure observation. The uniaxial tension test and low cycle fatigue test were conducted on the as-received specimen and the ones treated by USP, SMR and USP/SMR composite treatment, respectively. The tensile mechanical properties were effectively improved by USP. The introduction of SMR following USP can significantly increase the number of cycles to failure. Combining fracture morphology analysis and DEM-FEM coupling simulation of USP/SMR composite treatment, it was concluded that the improvement of tensile mechanical properties is mainly attributed to the synergistic effect of the compressive residual stresses and gradient-structured layer produced by USP, and the decrease in surface roughness resulting from SMR is the main reason for the significant improvement of fatigue performance. This work could provide an insight into the surface strengthening mechanism for USP/SMR composite treatment of materials with respect to the improvement of mechanical properties and fatigue performance.

Study on the mechanical properties and deformation mechanism of copper with different volume fractions of gradient structure

In this study, ultrasonic rolling process (USRP) were performed on the rolled copper samples (with an initial grain size of 11.4 μm) to obtain the gradient structure (GS), which comprises nanocrystalline layer (NL), deformed layer (DL), and center layer (CL). The different GS layer (NL and DL) volume fractions of samples were obtained by performing different passes of USRP after rolling, and the effect of the GS layer volume fractions on the mechanical properties was further studied. The morphology and structure of (unrolled, rolled and rolled + USRP) samples were characterized by electron backscattering diffraction (EBSD) and transmission electron microscope (TEM). In-situ tensile testing was used to investigate the tensile deformation behavior of the GS sample. The results show that the rolled + USRP (12 passes) sample with the thickest GS layer (volume fraction of 42%) has the greatest strength (yield strength (YS) 262 MPa and ultimate tensile strength (UTS) 325 MPa). The fine grains, dislocations and textures jointly strengthen the GS sample by increasing the force required for dislocation slip, and their contribution to YS is 51.3%, 44.4% and 4.3%, respectively. In addition, the nanocrystalline grains of GS layer make microcracks evenly distributed and hard to gather, to efficiently hinder cracks propagation and result in good ductility of GS samples. This study provides an effective method to obtain GS materials with good comprehensive properties and a deeper understanding of their deformation behavior and strengthening mechanism.

Effect of ultrasonic rolled material layer on the corrosion fatigue resistance of railway axle EA4T alloy steel

Surface treatment has been validated to improve the fatigue crack resistance of engineering components, while the coupling effect of corrosive atmosphere and external loading is relatively less reported with detailed experimental studies. In this study, gradient structure (GS) axle EA4T alloy steel specimens were prefabricated by using the ultrasonic surface rolling process (USRP), and their corrosion fatigue resistances were investigated. First, the optimal USRP parameters were determined via orthogonal tests. After the USRP treatment, the microstructure, surface topography, surface roughness, residual stress, and electrochemical corrosion behavior were measured to characterize the surface under various conditions. The electrochemical tests results indicated that the compressive residual stress (CRS) is an important factor in improving the corrosion resistance of axle steel. Rotating bending fatigue tests in both air atmosphere and corrosion liquid showed that the fatigue and corrosion fatigue performances could be significantly improved with the strengthened layer obtained via the USRP. Factor separation analyses substantiated that the remarkable enhancement of fatigue and corrosion fatigue strengths can be mainly attributed to the CRS introduced by the surface strengthening treatment. Simultaneously, the GS layer and hardened layer exhibited a synergistic effect.

Elucidating the mechanical response and microstructure evolution of the constituent layers in gradient-structured Cu alloys

The design of gradient structure (GS) in metallic materials has been reported to obtain superior mechanical properties due to the interaction between the GS layer and the coarse-grained (CG) matrix. In this study, the mechanical response and microstructural evolution of the corresponding constituent layers of GS Cu-4.5Al alloy at different pre-strains were systematically investigated. The tensile results showed that the strain hardening capability of the GS layer was improved due to the constraint of the CG matrix. Meanwhile, an extra strengthening was observed at the interface between the GS layer and the CG matrix, resulting in a higher yield strength of the CG matrix constrained by the GS layer than that of the CG matrix without the GS layer constraints. Moreover, the extra strengthening at the interface of the GS and CG matrix was gradually reduced with increasing pre-strain due to the weakening of the interaction at the interface. Microstructural observations at the interface between the GS layer and the CG matrix revealed that more dislocations and twins were activated during pre-strain in the CG matrix having GS constraints than that of the CG matrix without GS constraints. The present work provides important insights on the interaction between constituent layers, in terms of the mechanical response and microstructure evolution.

The role of microstructure modifications on electrochemical and plasma-nitriding behaviour of 316L steel produced by laser powder bed fusion

ABSTRACT The study investigates the impact of microstructure modifications on the corrosion, passivation, and plasma-nitriding behaviour of 316L steel. Microstructure modifications are achieved through the laser powder bed fusion (L-PBF) process and surface mechanical attrition treatment (SMAT). Three scanning strategies (concerning the orientation of the sample surface with scanning directions) are used in the L-PBF process, and the corresponding samples are labelled as HNS (0°), INS (45°), and VNS (90°). The scanning strategies have altered the average grain size (maximum for HNS and minimum for VNS) and porosity (HNS has the highest). Porosity disappears after SMAT. The surface of the SMATed specimen (VS) has equiaxed austenite nanograins (∼32 nm) with a fine distribution of α’-martensite, nanotwins, and high dislocation density. Microstructure affects the passivation, corrosion, and nitriding behaviour of the steel. The VNS has the lowest corrosion rate, decreasing further after SMAT. The SMATed sample exhibits the lowest nitrided layer thickness (∼65 μm). SMAT followed by nitriding causes a gradient-structured layer (with improved hardness) consisting of a nitrided layer, SMATed layer, and core. The nitrided HNS sample (∼78 μm thick nitrided layer) is dominated by γ’-Fe4N, while the nitrided VNS and VS samples have a relatively higher proportion of expanded austenite.

The effect of initial grain size on the strength property of copper with gradient microstructure

Gradient structure (GS) is beneficial for metal materials to obtain excellent comprehensive properties of strength and toughness, since the geometrically necessary dislocations induced by GS constraint can inhibit the strain localization during the deform process. Although the investigation on the GS of metal materials has gained more and more attention, there are still problems that need to be solved concerning the controllable preparation of GS and optimization of its mechanical properties, and the systematically and quantitatively research on the strengthening mechanism of GS. Herein, ultrasonic rolling process (USRP) combined with rolling was used to prepare GS of pure copper with different initial grain sizes. The GS samples (morphology, grain size and volume fraction, etc.) were characterized by scanning electron microscope (SEM) with electron backscattering diffraction (EBSD) and transmission electron microscope (TEM), and the strength and toughness were tested by universal testing machine. The results indicate that GS layer of pure copper consists of nanocrystalline layer (NL), deformed layer (DL), and center layer (CL). The thickness of GS layer first increases to the maximum and then decreases with the decrease of the initial grain size (21.2 μm–9.6 μm). The sample with the thickest GS layer (initial grain size of 11.4 μm) can obtain the highest yield strength (YS) (239 MPa) and ultimate tensile strength (UTS) (293 MPa). Compared with the raw sample, YS is improved significantly and good plasticity is simultaneously retained. In addition, the greatest contribution of strengthening effect comes from the fine grain strengthening.

Twin density gradient induces enhanced yield strength-and-ductility synergy in a S31254 super austenitic stainless steel

Gradient structure (GS), as a typical heterostructure, is arousing great interest for an improved synergy between the strength and ductility which are mutually conflicting. Recently, a novel design of GS is proposed by taking the density of twins in grains, instead of common grain size, as a gradient variable, showing the key role in strain hardening by the nano-scale twin boundaries. Following this idea, here, a deformation twin-density GS was produced by means of the technique of surface mechanical attrition treatment in a S31254 super austenitic stainless steel. To be specific, the GS consisted of a central coarse-grained (CG) core, with two sides sandwiched by the gradient-structured layer (GL), where the density of deformation twins appears gradient in grains along the depth towards the CG core. The tensile tests show that as compared to CG counterpart, yield strength in GS increases 80% to 0.5 GPa, along with comparable ductility of 36%. The interrupted tensile tests show the presence of mechanical hysteresis loops during each unload-reload cycle, indicative of the generation of hetero-deformation-induced (HDI) stress during tensile deformation. Furthermore, both the HDI stress and HDI strain hardening account for a large proportion of global flow stress and forest hardening. The deformation twins and their evolutions, with the emphasis on their interaction with the dislocations, are investigated in detail by means of EBSD and TEM observations to correlate the mechanical properties. The present results shed light on the crucial role of deformation twins in the twin-density gradient for the synergistic enhancement of both strength and ductility.

Corrosion resistance of surface-conditioned 301 and 304 stainless steels by salt spray test

The corrosion rate of surface-conditioned 301 and 304 stainless steels (SS) was determined by salt spray test in a controlled accelerated corrosive medium (9.5 L of pure distilled water + 500 g NaCl). By surface conditioning via mechanical attrition treatment, a gradient-structured layer was firstly generated on the surface of the samples before the salt spray test. The corrosion rate was determined by the weight loss before and after the salt spray test. Compared to the untreated 301 SS sample with a weight loss of 0.15 g, the surface-conditioned samples treated for 300 s and 1200 s experienced a lower weight loss of 0.04 and 0.02 g, respectively. A similar reduction in weight loss was achieved for 304 SS sample when treated for 5, 10, and 20 mins.

Effect of shot peening pretreatment on the high-temperature tribological behaviours of a TaN coating prepared via double-cathode glow plasma alloying

Gradient coatings with excellent physical and mechanical properties show great potential for use in engineering applications. In this study, a TaN gradient-modified layer was prepared on TA15 titanium alloy by double-cathode glow plasma alloying (DGPA) with a gradient structure pretreated by rotationally accelerated shot peening (RASP). The effects of the RASP pretreatment at different speeds on the mechanical and tribological properties and microstructure of the TaN coatings were investigated. The results indicated that the crystal size was refined to the nanometre scale (41.8–71.4 nm) by RASP pretreatment, and a gradient structure layer (30–55 μm) formed, which facilitated the adsorption and inward diffusion of Ta and N atoms. The pretreated TaN-modified layer contained an interdiffusion layer with a thin deposition layer consisting of Ta 2 N and TaN. Furthermore, the pretreated TaN-modified layer maintained good wear resistance in ball-on-disc testing at 700 K, and the wear rate was only 36.9% of that without pretreatment, which can be attributed to the high hardness, elastic modulus , toughness and thermal stability of the modified layer. This work provides insights relevant to the development of a mechanical-diffusion composite process to produce titanium alloy coatings with enhanced mechanical properties and wear resistance at elevated temperature. • TaN modified layer with a gradient change of composition was prepared on titanium alloy using RASP and DGPA. • The refined surface promotes the adsorption and diffusion of Ta and N atoms to obtain a thicker gradient-modified layer. • RASP pretreatment can improve the wear resistance of TaN modified layers at high temperatures.

Quasi-Static, Dynamic Compressive Properties and Deformation Mechanisms of Ti-6Al-4V Alloy with Gradient Structure

Gradient structure metals have good comprehensive properties of strength and toughness and are expected to improve the dynamic mechanical properties of materials. However, there are few studies on the dynamic mechanical properties of gradient structured materials, especially titanium alloys. Therefore, in this study, ultrasonic surface rolling is used to prepare a gradient structure layer on the surface of Ti-6Al-4V, and the quasi-static and dynamic compressive properties of coarse-grained Ti-6Al-4V (CG Ti64) and gradient-structured Ti-6Al-4V (GS Ti64) are investigated. The results show that a GS with a thickness of 293 µm is formed. The quasi-static compressive strength of GS Ti64 is higher than that of CG Ti64. Both CG Ti64 and GS Ti64 exhibit weak strain hardening effects and strain rate insensitivity during dynamic compression, and the compressive strength is not significantly improved. The lateral expansion of CG Ti64 is more obvious, while the lateral side of GS Ti64 is relatively straight, indicating that uniform deformation occurs in GS Ti64. The α phase in the GS produces dislocation cells and local deformation bands, and the lamellar structure is transformed into ultrafine crystals after dynamic compression. Both of them produce an adiabatic shear band under 2700 s−1, a large crack forms in CG Ti64, while GS Ti64 forms a small crack, indicating that GS Ti64 has better resistance to damage. The synergistic deformation of GS and CG promotes Ti-6Al-4V to obtain good dynamic mechanical properties.

Effect of global constraint on the mechanical behavior of gradient materials

The gradient structured (GS) samples studied here consists of gradient layer (s) with grain size gradient and a coarse-grained (CG) layer. Gradient materials have been found to have superior mechanical properties due to the hetero-deformation induced (HDI) strengthening and strain hardening. However, the effect of the interaction between the GS layer and the CG layer on the mechanical properties of the gradient materials remains to be studied. In this paper, single-sided GS Cu and double-sided GS Cu plates were prepared by surface mechanical attrition treatment (SMAT) on one side and both symmetrical sides respectively. It is found that GS samples have higher yield strength than CG samples, and the Cu samples with double-sided GS has an uniform elongation comparable to CG Cu. In addition, the global mutual constraint among different GS layers affect the mechanical behavior of gradient materials, which is revealed in the comparative study of double-sided and single-sided GS Cu. Specifically, the double-sided GS Cu exhibit higher yield strength and ductility than the single-sided GS Cu, because the former is better constrained. This study demonstrates that in addition to the local structural gradient, the symmetric constraint produce more effective HDI effect to enhance the mechanical properties. The global mutual constraints of GS layers played a positive role in producing superior mechanical properties.

The tension-compression behavior of gradient structured materials: A deformation-mechanism-based strain gradient plasticity model

Gradient structured (GS) metals can simultaneously achieve high strength and high ductility, which is the pursuit of structural materials. Abundant studies have pointed out that significant kinematic hardening is key to their strength-ductility combination. Unfortunately, few constitutive models were established to simulate and analyze the characteristic kinematic hardening behavior of GS metals. Thus, a systematic and deep understanding of the relationship between graded microstructure and the resulting macroscopic response needs further effort. In this work, we develop a strain gradient plasticity model that considers plasticity heterogeneities from the grain to the sample scale. A back stress model, which accounts for the dependency of dislocation pileups on grain size, is established to describe the cyclic deformation properties of GS materials. The established model unifies the concepts of geometrically necessary dislocations accommodating plasticity heterogeneities, grain size-dependent back stress, and reversible dislocation motion during reverse loading into a strain gradient plasticity framework. A finite element implementation of the model quantitatively predicts the uniaxial tensile and tensile-compressive responses of a GS copper bar as well as of homogeneously grained reference samples. The simulation indicates that the enhanced kinematic hardening of GS copper results mainly from fine grains in the GS layer and contributes to the considerable ductility of the GS material. The strain gradient cyclic plasticity model enables future investigation of the cyclic/fatigue behavior of GS materials, which is vital for the engineering application.

Gradient plasticity in gradient nanocrystalline metals: Extra toughness from dislocation migration

Gradient nanocrystalline (GNC) metals exhibit an unprecedented combination of high strength and high ductility. In this study, GNC copper was obtained using ultrasonic nanocrystalline surface modification (UNSM), and its plasticity mechanism was investigated using tensile tests. It was found that UNSM treatment can grant copper superior yield strength and ductility which is beyond the capacity of conventional cold-working. Moreover, the UNSM-treated copper has a reduced strain-hardening capacity which does not lead to early necking and deteriorated ductility. The strain energy analysis shows that the yield strength of GNC material can be estimated using the root mean square of yield strengths of the coarse-grained (CG) layer and the gradient structure (GS) layer. The strain energy analysis also predicted a strong interaction between GS layer and CG layer during tensile deformation. Furthermore, a dislocation-based constitutive model was applied to understand the interaction between CG layer and GS layer. The migration of dislocations from the CG layer to the GS layer was proposed to explain the interaction. The migration of dislocations induces strain softening and, thus, releases internal strain energy. The proposed mechanism is supported by our experimental observations: the reduction in strain-hardening capacity is positive-related to the depression in strain localization in UNSM-treated GNC copper. The improved ductility is attributed to the depressed strain localization and the refreshed capacity for dislocation accumulation, both resulted from dislocation migration from the CG layer to the GS layer.

A superior strength-ductility combination in gradient structured Cu–Al–Zn alloys with proper stacking fault energy and processing time

This article aims to investigate the mechanical properties of the gradient structured (GS) Cu–Al–Zn alloys. To prepare the GS samples, three kinds of Cu–Al–Zn alloys with different stacking fault energies (SFEs) were processed by surface mechanical attrition treatment (SMAT) at cryogenic temperature. The results show that the Cu-5.5%Al-4.5%Zn alloy having the lowest SFE processed by SMAT for 5 min exhibits an optimized combination of strength and ductility. For both the initial annealed sample and the SMAT samples processed for the same processing time, uniform elongation (UE) and ultimate tensile strength (UTS) increase with decreasing SFE, whereas the yield strength (YS) is not sensitive to SFE. In the case of the same SFE, as the SMAT processing time increases, the YS and UTS of the GS alloy increase and the UE decrease. The optimized SMAT processing time can sufficiently refine the grains in the GS layer, which can increase the strength and maintain good ductility. Also, sufficient grain refinement promotes the better accumulation of geometrically necessary dislocations (GNDs), thus forming rather effective hetero-deformation induced (HDI) stress strengthening and HDI hardening to enhance strength while maintaining good ductility. The combination of low SFE and optimized SMAT processing time is beneficial to obtain an optimized combination of strength and ductility. Additionally, since the near-surface layer of the GS material is plastically deformed earlier than the sub-surface layer, the accumulation of GNDs at the interface between the two layers also contributes to HDI strengthening and HDI hardening to improve strength and ductility.

Microstructure characterization and tensile properties of processed TC17 via high energy shot peening

The microstructure and tensile properties of the processed TC17 via high energy shot peening (HESP) were investigated, in which special attention was paid to reveal the fracture behaviour of the HESP processed surface layer. The results indicated that the severe plastic deformation (SPD) zone with the nanograins appeared in the HESP processed surface layer of TC17, in which the SPD thickness increased with the increasing of air pressure while it increased firstly and then decreased with the prolonging of peening duration, and the finer nanograins could be obtained at the higher air pressure and longer peening duration. Due to the generation of the gradient structure layer, the HESP processed TC17 had ruptured in a hybrid fracture. The fracture features corresponding to nanograined structure, ultrafine-grained structure and coarse-grained structure were the brittle fracture, the mixed fracture and the ductile fracture respectively. Comparing with the properties of the annealed TC17, the HESP processed TC17 possessed a better strength-ductility balance owing to combined effects of grain refinement, high-density dislocations, work hardening and compressive residual stress (CRS), in which the yield strength (YS) and the ultimate tensile strength (UTS) were increased by 10.1% and 13.9%, respectively.

Literature review on the mechanical properties of materials after surface mechanical attrition treatment (SMAT)

Most of the challenges experienced by many engineering materials originate from the surface which later leads to total failure, hence affecting the resultant mechanical properties and service life. However, these challenges have been addressed thanks to the invention of a novel surface mechanical attrition treatment (SMAT) method which protects the material surface by generating a gradient-structured layer with improved strength and hardness without jeopardizing the ductility. The present work provides a comprehensive literature review on the mechanical properties of materials after SMAT including the hardness, tensile strength and elongation, and residual stress. Firstly, a brief introduction on the different forms of surface nanocrystallization is given to get a better understanding of the SMAT process and its advantages over other forms of surface treatments, and then the grain refinement mechanisms of materials by SMAT from the matrix region (base material) to the nanocrystallized layer are explained. The effects of fatigue, fracture, and wear of materials by the enhanced mechanical properties after SMAT are also discussed in detail. In addition, the various applications of SMAT ranging from automotive, photoelectric conversion, biomedical, diffusion, and 3D-printing of materials are extensively discussed. The prospects and recent research trends in terms of mechanical properties of materials affected by SMAT are then summarized.

An investigation of fretting fatigue behavior and mechanism in 17-4PH stainless steel with gradient structure produced by an ultrasonic surface rolling process

A gradient structure layer was fabricated on 17-4PH specimens using an ultrasonic surface rolling process. The separation-factor methods confirmed that residual compressive stress played a critical role in improving fretting fatigue (FF) life, while gradient nano-crystalline structure and “fish scale-like” surface topography also had a beneficial influence. Residual compressive stress could enhance crack closures and reduce effective tensile stress. Nano-grains grew, and dislocation density reduced after FF experiment, which could accommodate an externally applied strain and neutralize any stress concentration. Additionally, the “fish scale-like” topography reduced total contact area between the specimen surface and fretting pad, which alleviated surface damage.

Effect of stacking fault energy on microstructural feature and back stress hardening in Cu-Al alloys subjected to surface mechanical attrition treatment

Three Cu-Al alloys with nominal Al contents of 2.2, 4.5 and 6.9 wt% were subjected to surface mechanical attrition treatment (SMAT) to investigate the effect of stacking fault energy (SFE) on the microstructural feature of gradient structure (GS) layers produced by SMAT and back stress hardening during Bauschinger unloading-reloading tests. In the GS layer, the alloys with different SFE exhibit various grain morphologies, grain size distributions and densities and distributions of geometrically necessary dislocations (GNDs). Because of these differences in the microstructural features of the GS layers, the alloys show different variations associated with SMAT during the Bauschinger unloading-reloading tests, i.e. the increment of true stress decreases sharply with a decrease of SFE at small strains, while it slightly decreases at large strains. It is found that SFE can significantly affect grain boundary strengthening and back stress hardening to synthetically improve the tensile strength of GS Cu-Al alloys to different levels.

Study of the austenitic stainless steel with gradient structured surface fabricated via shot peening

A gradient-structured layer with initially nanoscale grain size increased gradually to original coarse microscale (∼50 μm) was fabricated on the surface of austenitic (γ) stainless steel SS304 via ultrasonic nanocrystallization surface modification (UNSM) peening. Modified cross-sectional and depth-specific plan-view (DSPV) sample preparation methods were explored with the analysis scales from macro to atom by synchrotron radiation XRD, EBSD, and TEM to determine the depth (strain) dependent deformation microstructures and grain refinement mechanism. The depth-dependent deformation microstructures were ascribed to strain-induced martensitic-transformed (SIMT) α′- and ε-martensite, deformation nanotwins (NT), and dislocation grids. The grain nanocrystallization mechanism was suggested as the formation of α′ and ε grain boundaries that divided the original coarse grains (CG) to the nanoscale. The strains also appeared to play a key role in the crystallographic orientation relationship (OR) of the γ/α′: Kurdjumov-Sachs (K-S) and Pitsch in low strain region and Nishiyama-Wassermann (N-W) in the high strain region.