Surface Mechanical Grinding Treatment Research Articles

Enhanced precipitation hardening in nanograined CuCrZr alloy

Nanograins were induced in solid solution CuCrZr alloy by surface mechanical grinding treatment, followed by aging treatment to investigate the precipitation behavior and hardening effect of the nanograins. Experimental results showed that body-centered cubic Cr nanoparticles precipitated at nanograin boundaries accompanied with slight nanograin coarsening, yielding a considerable precipitation hardening, even twice that of coarse grains. The hardening mechanism of grain boundary nanoprecipitates in nanograins was discussed.

Achieving 2.9 GPa yield strength in Inconel 718 alloy with Cr-segregated nanograins

A gradient nanostructure was prepared in aged Inconel 718 by means of surface mechanical grinding treatment at room temperature. At the top surface layer with a thickness of ∼50 μm, the grain was refined down to extremely fine size (∼10 nm) and an abnormal Cr enrichment was detected on the grain boundaries. This Cr-segregated extremely-fine nanograin exhibits an unprecedented yield strength of ∼2935 MPa and superior GB stability against coarsening upon 800 °C annealing, providing a new sight on the preparation of superalloy with high performance.

Surface nano-crystallisation and mechanical properties of Ti[sbnd]Ta composite materials after surface mechanical grinding treatment

Surface mechanical milling generates gradient nano-structure surfacial layers that enhance the mechanical properties of titanium (Ti)–tantalum(Ta) composites for medical implants. Herein, the effect of surface mechanical grinding treatment (SMGT) on the surface nano-crystallisation of TiTa laminated composites was investigated in terms of micro-structure and Coherent Scattering Regions (CSRs) size using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. A strengthening effect was verified using a micro-hardness tester and tensile testing machine, and the fracture mechanism was analysed through fracture observation. The experimental results showed that gradient nano-structures were formed on the surface of the TiTa laminated composites after SMGT, with the smallest CSRs size of the outer surface layer of approximately 13.3 ± 0.4 nm. The plastic deformation mechanism of the TiTa laminated composites during SMGT was a dislocation cutting mechanism. The micro-hardness and tensile strength of the TiTa laminated composites increased after SMGT up to 440 HV and 1097.528 MPa, respectively. The fracture mechanism of the TiTa laminated composites after SMGT was a combination of ductile fracture and quasi-cleavage fracture mechanisms. After SMGT, the TiTa laminated composite performed well and exhibited excellent potential for use in biomaterials.

Surface grain refinement of casting A380 aluminum alloy by ultrasonic-assisted surface mechanical grinding treatment

A novel surface processing technique, ultrasonic-assisted surface mechnical grinding treatment (U-SMGT), was proposed and applied to modify the surface micriostructure of the casting aluminum alloy (A380). Comprehensive post-process chracterization techniques including 3D profilometry, optical microscope, scanning electric microscopy (SEM), and electron backscatter diffraction (EBSD) were carried out to investigate the surface morphology and microstructure of the processed region (PR). It was found that a surface layer of grain refinement with a thickness of about 50 µm was formed. In the as-received base material (BM), large clusters of dendritic eutectic phase with a size of approximately 20 µm were isolated by α-Al matrix. Nano-indentation tests showed that hardness of the eutectic and α-Al in the as-received BM was 2.15 GPa and 1.2 GPa respectively. After U-SMGT, the large dendritic eutectic phase was broken down into much fine particles with an average size of 0.57 µm. These fine particles were uniformly distributed into the α-Al matrix. Nano-indentation measurement exhibited a hardness of 2.27 GPa within the processed region. The overall high hardness in the processed region is a result of the pinning effect of uniformly distributed fine dendrites and grain refinement of Al-matrix.

Stress corrosion cracking behavior of AISI 316L stainless steel treated by surface mechanical grinding

The effects of surface mechanical grinding treatment (SMGT) on stress corrosion cracking (SCC) behavior of machined 316L stainless steel surface were investigated by comparing to those of turned and milled surfaces. The surface stress state, roughness, microstructure, and microhardness induced by the different surface machining processes were tested and analyzed. The SCC tests on the turned, milled and SMGT samples were conducted in boiling MgCl2 solution at 155 ℃, and the crack morphologies were analyzed in detail to establish the relationship between the surface integrities and the resulting SCC behavior. The findings indicated that the SMGT altered the SCC initiation behavior and improved SCC resistance of the turned and milled surfaces of 316L stainless steel, and the surface stress state, microstructure refinement and surface strengthening induced by SMGT contributed to the improvement of the SCC resistance.

Prediction of Surface Microstructure, Grain Size, Martensite Content, and Microhardness of 316L Austenitic Stainless Steel in Surface Mechanical Grinding Treatment Process

Prediction of Surface Microstructure, Grain Size, Martensite Content, and Microhardness of 316L Austenitic Stainless Steel in Surface Mechanical Grinding Treatment Process

Stabilizing nanograined austenitic stainless steel with grain boundary relaxation

Grain boundary (GB) relaxation can be triggered by interaction of GB with partial dislocations during plastic deformation, which is effective to stabilize nanograined structures in a number of face-centered cubic (FCC) metals and alloys. But this mechanism is interfered by deformation-induced martensitic transformation in 304 stainless steels with a metastable FCC structure. In this work, gradient FCC nanograined structures were prepared in a 304 stainless steel by using warm surface mechanical grinding treatment. GB relaxation was triggered in the samples with grains below 60 nm in size, in which partial dislocation dominates the plastic deformation. With the GB relaxation, thermal stability increases with decreasing grain sizes, contradictory to the conventional “smaller less stable” trend. The onset temperature for coarsening of nanograins of ∼30 nm in size is as high as ∼896 °C (0.7 Tm), about 250 °C above the reported value of the steel. The GB relaxation was not detected in martensitic nanograins of the same composition, of which the instability temperatures is around 650 °C (0.54 Tm), independent of grain size from 26 nm to the submicron scale, as it is controlled by the reverse martensitic transformation.

Effects of gradient nanostructures on the tribological properties and projectile abrasion during high-speed penetration in AerMet100 steel

To reduce the mass abrasion of AerMet100 ultra-high strength steel projectiles at increasing velocities, surface mechanical grinding treatment (SMGT) was employed to prepare a gradient nanostructure (GNS) on the AerMet100 surface. The microstructure of the GNS were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and nanoindentation. It was found that the gradient nano-crystal structure was successfully prepared, and the martensite plate width was reduced to about 30 nm. As the distance from the surface depth increased, the plate size gradually increased, with the depth of the grain refinement layer measuring approximately 300 μm. The microhardness of the GNS AerMet100 was 870 Hv0.025, which was approximately 40% higher than that of the AerMet100 substrate. Additionally, the hardness was observed to decrease with increasing GNS depth. The friction and wear behavior of coarse grained (CG) AerMet100 and GNS AerMet100 were investigated. The results showed that their friction coefficients were similar, but wear volumes of GNS AerMet100 were smaller than those of CG AerMet100, the wear mechanisms of two surfaces were predominantly abrasive and adhesive wear. Furthermore, the projectiles penetration test showed that the mass loss of GNS AerMet100 was significantly lower than that of CG AerMet100 under similar velocity condition. Within the velocity range of 1200–1750 m/s, the degree of erosion and wear loss of the CG AerMet100 and GNS AerMet100 projectile noses increased with increasing penetration velocity, and the wear mechanism was mainly abrasive wear.

Integrated modeling of the gradient structure evolution during surface mechanical grinding treatment: Formation mechanism and mechanical properties

Gradient nanostructured materials are regarded as a promising class of architectures with tunable mechanical properties, primarily dependent on the optimization of well-controlled fabrication parameters. In this paper, a microvariable-based constitutive model is incorporated into an integrated finite analysis technique. This approach correlates the fabrication parameters of surface mechanical grinding treatment (SMGT) with the corresponding measured mechanical properties of gradient structured (GS) materials, quantifying the relationship between process parameters, microstructure, and mechanical properties. Through theoretical prediction and experimental verification, it is observed that grain refinement, mechanical strength, and surface hardness are enhanced by increasing processing times and reducing path spacing. The yield stress of the fabricated GS material ranges from 126.8 MPa to 162.2 MPa, an increase of above 2.5 times compared to the original material, with a slight decrease in uniform elongation by a factor of 25.8 %, indicative of an excellent strength-ductility trade-off. The underlying mechanism for improved strength-ductility synergy is discussed, focusing on the importance of the tunable spatial distribution of grain size. This work sheds light on the potential application of gradient nanograined structures at an industrial scale and advances the fundamental understanding of strengthening mechanisms in gradient nanostructures.

Surface nanocrystallization and biomedical performance of Ti-Ta laminated composite processed by surface mechanical grinding treatment

Ti-Ta laminated composites suffer from the inadequate wear resistance and deficient antibacterial properties. To promote their application, surface mechanical grinding treatment (SMGT) was employed to modify their surface physicochemical properties. This work systematically investigated the microstructure, phase composition, roughness, biological activity, friction and wear behaviors as well as anti-bacterial adhesion effect of Ti-Ta SMGT samples. The results showed that the nanocrystallization of the Ti-Ta laminated composites after SMGT was achieved, and the strength and elastic modulus met the requirements of the third-generation medical titanium alloys. The modification of the surface properties such as roughness, hardness, chemical state and surface energy improved their tribological properties in simulated body fluids and prevented the anti-bacterial adhesion for staphylococcus aureus, indicating the promising prospect of Ti-Ta SMGT samples for biomedical applications.

Martensitic transformation dominated tensile plastic deformation of nanograins in a gradient nanostructured 316L stainless steel

We systematically investigated the tensile deformation mechanisms and mechanical properties of a gradient nanograined AISI 316 L stainless steel prepared by means of surface mechanical grinding treatment. The gradient nanograined samples exhibit a good synergy of high strength and remarkable tensile ductility at room temperature. Microstructural observations revealed that martensitic transformation co-operating with grain boundary migration accommodates the tensile plastic deformation of nanograins in the topmost layer of gradient nanograined 316 L SS, which is distinct from the traditional partial dislocation associated deformation twinning in homogeneous coarse-grained counterpart under tension loading. Accompanied by the decrease in grain boundary migration rate, the martensite content increases significantly with increasing tensile strain, reaching ∼50% at a true strain of 50%. The newly formed martensites as strengthening phase not only provide dynamic work hardening, but also effectively suppress the strain localization, resulting in the substantial tensile ductility of the gradient nanograins.

Stabilizing nanograined Fe-Cr alloy by Si-assisted grain boundary segregation

• Si promotes GB segregation of Cr in the NG Fe-8Cr-1Si treated by SMGT and annealing. • Cr segregation increases with the decrease of grain size, which is related to high- density GND. • NG Fe-8Cr-1Si has enhanced thermal stability and hardness due to Cr segregation. • Additions attractive to solute can stabilize NG binary alloys with weak segregation tendency. Intentional solute segregation at grain boundary (GB) can effectively stabilize the nanograined alloys by reducing the excess energy and mobility of GB, but it usually works for binary alloys with sufficient GB segregation tendency. Here, we found that the segregation of Cr can be enhanced in a nanostructured Fe-8Cr alloy with insufficient GB segregation tendency through the interaction of another solute Si (1.0 wt.%). After surface mechanical grinding treatment and subsequent annealing, the nanograined Fe-8Cr-1Si is more thermally stable than the nanograined Fe-8Cr, which is mainly attributed to the Si-enhanced Cr segregation as observed by Super-X energy-dispersive X-ray spectroscopy (EDS) mapping system. With the grain refinement to nanoscale, the thermal stability is further improved due to the increase of Cr content at GBs and the precipitates formed at appropriate high temperatures. The present finding provides guidance for the development of advanced nanostructured ternary alloys.

Microstructure Evolution and Formation of Gradient Structures in Single Crystal Nickel-Based Superalloy by Surface Mechanical Creep-Feed Grinding Treatment.

Gradient structures have been created in single crystal nickel-based superalloys (SX alloys) via surface mechanical creep-feed grinding treatment (SMCGT). It has been found that these gradient structures are mainly composed of nano-sized grains, sub-micron-sized grains, dislocation structures, and the matrix material of single crystals along the depth from the treated surface. In addition, the evolution of such structures is found to be dominated by the dislocation movements which run through both γ channels and γ' precipitates, subdividing the two types of microstructures into various dislocation structures, and eventually introducing the refined grains into the surface layer. Furthermore, the evolution process of gradient structures primarily originates from the mechanical effect between abrasive grits and workpiece material, owing to the large grinding force (up to 529 N) and low grinding temperature (less than 150 °C) during the unique creep-feed grinding treatment in the present investigation. Due to the typical grain refinement, the hardness of the nanostructures exhibits the largest value of around 10 GPa in the surface layer, approximately 26% higher than that of the matrix material. This study further enhances the understanding of the microstructure-property relationship of SX alloys subjected to creep-feed grinding treatment and contributes to achievement of high-performance components.

Precipitation Behavior during Aging Operations in an Ultrafine-Grained Al–Cu–Mg Alloy Produced by High-Strain-Rate Processing

An ultrafine-grained (UFG) Al-Cu-Mg alloy (AA2024) was produced by surface mechanical grinding treatment (SMGT) with a high strain rate, and the precipitation behavior inside the grain and at the grain boundary was investigated. During SMGT, element segregation at the boundary was rarely observed, since the solute atoms were impeded by dislocations produced during SMGT. During early aging, the atomic fraction of Cu at the grain boundary with SMGT alloys was approximately 2.4-fold larger than that without SMGT alloys, the diffusion rate of Cu atoms from the grain toward the grain boundaries was accelerated with SMGT alloys, because a higher local elastic stress and diffusion path were provided by high-density dislocations. The combined action, in terms of the composition of the alloy, the atomic radius, the diffusion path, and the diffusion driving force provided by high-density dislocations with SMGT alloys, led to a Cu/Mg atomic ratio of approximately 6.8 at the grain boundary. The average size of the precipitates inside the grain was approximately 2- and 10-fold larger than that formed after later aging with and without SMGT alloys, due to more nucleation sites at dislocation located inside the grain with SMGT alloys having attracted and captured numerous solute atoms during the aging process.

Enhancing stress corrosion cracking resistance of machined surface via surface mechanical grinding treatment for AISI 316 L stainless steel

The mechanical properties, microstructures and corrosion resistance of turned, milled and surface mechanical grinding treatment (SMGT) processed surfaces were investigated through the surface stress measurement, microhardness, SEM, TEM, TKD, XRD and electrochemical corrosion analysis. The stress corrosion cracking (SCC) initiation behavior of three types of treated surfaces in boiling MgCl 2 solution were assessed and compared. The experimental results indicated that the SMGT processed surface inhibited the SCC initiation at the surface stress as high as 874 MPa. The enhancement of SCC resistance was attributed to the SMGT-induced dense ultrafine-grained (UFG) layer, gradient grain refinement and better corrosion resistance, resulting in the high yield strength and high critical stress of SCC initiation. • The surface mechanical grinding treatment generated a dense ultrafine-grained (UFG) layer and low residual stress on processed surface. • The dense UFG layer successfully suppressed the SCC initiation at the surface tensile stress as high as 874 MPa. • The mechanisms of SMGT improving the SCC resistance were attributed to the high yield strength combined with better corrosion resistance of the UFG layer, increasing the σ SCCof crack initiation. • The SMGT can serve as a promising strategy to enhance the SCC resistance for machined 316 L stainless steel surface.

Gradient Nanotwinned Structure and Its Formation Mechanism in Inconel 625 Alloy Produced by Surface Mechanical Grinding Treatment

Gradient Nanotwinned Structure and Its Formation Mechanism in Inconel 625 Alloy Produced by Surface Mechanical Grinding Treatment

Effects of stacking fault energy on deformation induced grain boundary relaxation in nanograined Cu alloys

Deformation induced grain boundary (GB) relaxation with resultant enhancements in thermal stability was observed in various metals with grain sizes below a critical size. In this study, effects of stacking fault energy (SFE) on the GB relaxation and thermal stability are investigated in several Cu-Ni and Cu-Al alloys with gradient nanograined samples prepared using the surface mechanical grinding treatment. For each alloy, thermal stability drops with a decreasing grain size from submicrometers to about 70 nm. However, two distinct grain size dependences of thermal stability were observed below 70 nm. For Cu-10Ni and Cu-5Ni alloys with higher SFEs than Cu, thermal stability increases for smaller grains, similar to that in pure Cu. For Cu-10Al and Cu-5Al with lower SFEs than Cu, as grain sizes decrease the thermal stability elevates firstly and then drops, exhibiting a stability peak at a certain size. The observed thermal stability in the Cu alloys below 70 nm can be attributed to the GB relaxation induced by plastic deformation dominated by partial dislocation activities. The different behaviors of GB relaxation in these Cu alloys demonstrated its obvious dependence on SFE, which determined the governing deformation mechanisms and hence the degree of relaxation of GBs.

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.

Deformation mechanism in nanolaminate FeCrAl alloys by in situ micromechanical strain rate jump tests at elevated temperatures

FeCrAl alloys have attracted extensive attention as one of the potential cladding materials for the commercial light water reactors. Although the creep behaviors of coarse-grain FeCrAl alloys have been investigated at elevated temperatures, little is known on the deformation mechanism of fine-grain FeCrAl alloys. Here, we performed surface mechanical grinding treatment to fabricate nanolaminated ferritic FeCrAl alloys. In situ micropillar compression tests were applied to determine the underlying deformation mechanism of nanolaminated FeCrAl alloys at elevated temperatures. The strain rate sensitivity and activation energy for the dominant deformation mechanism were estimated considering the presence of a threshold stress.

Deformation induced grain boundary segregation and thermal stability of nanolaminated Al-Zn-Mg-Cu alloy

In this work, nanolaminated (NL) structure with an average lamellae thickness of 27 nm and a high hardness of 2.86 GPa has been fabricated in a solution treated Al-Zn-Mg-Cu alloy by surface mechanical grinding treatment (SMGT) at room temperature, featuring strong Zn and Cu segregation to grain boundaries (GBs). The NL structure shows a good thermal stability caused by GB segregation and the pinning effect of the numerous nanoparticles at the GBs.