Nanograin Layer Research Articles

Understanding the grain refinement and residual stress formation mechanisms of a Ni-based alloy during machining processes

Ni-based alloys are important materials for many industries, microstructure of the sub-surface regions of these alloys can be refined during machining processes, affecting both their machinability and service performance. Several mechanisms have been proposed to explain the machining-induced microstructure refinement of these alloys, however, sound evidences to support them are still lacking. In this research, microstructure characterization with an emphasis on the micro-texture and defects was conducted on milled surfaces of a Ni-based alloy, to promote in-depth understanding of the grain-refinement process. A nano-crystalline layer consisted a topmost region with randomly-orientated grains and annealing nano-twins and a textured region beneath it with deformed microstructure was observed in the cross-section of the alloy with the help of high resolution orientation determination techniques. These observations provide additional evidences to support the recrystallization induced grain-refinement mechanism of the nano-grains. It is also suggested that recrystallization was a by-product of the shear deformation occurred during the milling process, supported by the observation of the clear boundary between the nano-grain layer and its beneath deformed region. The deformed region was refined as a result of mechanical twining and twin-intersections, leading to significant fluctuations of the residuals stresses.

Research on influence factors of small punch test to evaluate the mechanical properties of gradient nanostructured material

Abstract The influence factors of small punch test (SPT) were investigated to evaluate the mechanical properties of gradient nanostructured (GNS) materials. The gradient nanostructure was prepared on the top layer of S30408 austenitic stainless steel by ultrasonic impact treatment (UIT). The mechanical properties of the GNS material were obtained using SPT and correlated with those obtained by standard tensile tests. The results indicate that, when the specimen thickness is 0.5 mm, the sphere diameter is 2.4 mm, the punch velocity is 0.5 mm min−1, and the gradient nano-grained layer is placed face-on in the mold, the GNS material exhibits better plastic deformability. The SPT specimen achieves better bearing capacity, and the mechanical properties of the GNS material obtained by SPT are more accurate. The yield strength and tensile strength of the GNS material were also evaluated by analytical and empirical methods in SPT. The error is approximately 10% compared with the standard tensile test results, which is within the allowable range.

Wear behavior and microstructure evolution of lamellar γ-TiAl alloy at room and elevated temperatures

The wear behavior of lamellar Ti-46Al-2Nb-2Cr was investigated by reciprocating sliding at room and service temperatures. The wear scar and subsurface microstructure were studied via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The coefficient of friction (COF) increased with the temperature. However, the wear volume loss first decreased and then increased, due to the formation of a relatively complete tribo-layer at 750 ℃. Abrasive wear, oxidative wear and adhesive wear were activated at elevated temperatures. Reciprocating sliding introduced gradient nano-grained structures covered with tribo-layers. Compared to room temperature, the oxidative tribo-layer and nano-grained layer formed at 750 ℃ were thicker. And the nano-grains were finer. Nano-grains at room temperature and 750 ℃ were related to defects and enhanced dynamic recrystallization, respectively.

Subsurface deformation mechanism and the interplay relationship between strength-ductility and fretting wear resistance during fretting of a high-strength titanium alloy

Fretting wear damage of high-strength titanium fasteners has caused a large number of disastrous accidents. Traditionally, it is believed that both high strength and excellent ductility can reduce fretting wear damage. However, whether strength and ductility are contradictory or not and their appropriate matching strategy under the external applied normal stress (Fw) are still confusing problems. Here, by analyzing the subsurface-microstructure deformation mechanism of several samples containing various α precipitate features, for the first time, we design strategies to improve fretting damage resistance under different matching relation between Fw and the tensile strength of materials (Rm). It is found that when Fw is greater than Rm or Fw is nearly equivalent to Rm, the deformation mechanism mainly manifests as serious grain fragmentation of β and αGB constituents. Homogeneous deformation in large areas only reduces damage to a limited extent. It is crucial to improve the strength to resist cracking and wear, but it is of little significance to improve the ductility. However, when Fw is far less than Rm, coordinated deformation ability reflected by ductility plays a more important role. The deformation mechanism mainly manifests as localized deformation of β and αGB constituents (kinking induced by twinning and spheroidizing). A unique composite structure of nano-grained/lamellar layer and localized deformation transition layer reduces fretting damage by five times compared with a single nano-grained layer. Only when the strength is great enough, improving the plasticity can reduce wear. This study can provide a principle for designing fretting damage resistant alloys.

Surface nanocrystallization mechanism of additive manufactured 316L stainless steel via high-energy shot peening

High-energy shot peening (HESP) is an effective way to achieve surface nanocrystallization modification of additive manufactured metal parts, significantly improving their mechanical properties. Yet, there is still a lack of deep understanding of the surface nanocrystallization evolution mechanism of additive manufactured metals. For that, this study focuses on the surface nanocrystallization mechanism of selective laser melted 316L stainless steel treated by HESP treatment. The detailed microstructural characteristics are investigated via optical microscope (OM) and transmission electron microscopy (TEM). The experimental results show that the matrix layer, the low-strain matrix layer, the elongated ultrafine grain layer, the short rod-like ultrafine grain layer, and the equiaxed nanograin layer occurred in sequence from the matrix to the topmost surface. HESP treatment enables the gradient grain refinement from the matrix with a micrometer structure of about 120 mm to the topmost surface with an average grain size of about 25 nm. The evolution of surface nanocrystallization has resulted from the interaction of high-density dislocation, dislocation tangles (DTS), and twins. This study could provide a deep understanding of the surface nanocrystallization evolution mechanism of additive manufactured metal parts treated by HESP.

Numerical investigation of internal damage in heterogeneous-structured laminates using a 3D multiple physical mechanisms based constitutive model

Heterogeneous-structured laminates (HSLs), which consist of multiple layers of metallic materials arranged in successive pairs of coarse-grained (CG) and nano-grained (NG) layers, have been recently reported to display excellent balance of strength and ductility. However, it is argued that accompanying state-of-art numerical models developed to simulate the deformation of this specific class of composite materials have limitations for investigating their underlying damage evolution. Addressing this issue is essential to support the rational design, optimisation and application of HSLs, especially when subjected to contact and dynamic processes. For this reason, a novel 3D numerical framework for HSLs is proposed and tested in this research considering published experimental findings and dislocation theories. This framework comprehensively considers the evolution of various types of dislocations and back stress while being coupled with the Johnson Cook damage criterion. The HSL specimens simulated here were made of alternating layers of CG and NG copper separated by interface affected zones. Following initial microhardness and uniaxial tensile simulations on homogenous copper with different grain sizes, simulations of HSLs were conducted to study the effect of different layer thickness and the volume fraction of the NG layer. Overall, a good correlation between numerical and experimental results was achieved. An important and distinguishing characteristic of this research is that the proposed model enables the evolution of internal damage and the synergetic effect between the CG and NG layers to be investigated. Through the evaluation of the damage accumulation factor in the NG layer, the simulations results yielded quantitative information which aligned with the following known experimental observations: 1) the smaller the layer thickness, then the smaller the internal damage and 2) the internal damage increases with the increase in volume content of the NG layer. In addition, for a set simulated strain of 10%, the developed model could be used to show that the damage accumulation factor in the NG layer was 10 times lower than that in its counterpart, i.e., a stand-alone NG layer not sandwiched between two CG layers.

New insights into microstructure refinement in crack initiation region of very-high-cycle fatigue for SLM Ti-6Al-4V via precession electron diffraction

The formation mechanism of crack initiation region of selective laser melted (SLM) Ti-6Al-4V in very-high-cycle fatigue (VHCF) was investigated by using the novel approach of transmission electron microscopy (TEM) with precession electron diffraction (PED). It is the first time to reveal the microstructure details of nanograin layer in the crack banks of both main crack and sub-crack in SLM Ti-6Al-4V via TEM with PED, which is the result of the crack closure and contacting between fracture surfaces. This revelation provides new insights into the microstructural evolution in the crack initiation region for SLM titanium alloys and other metallic materials, which is helpful for further understanding the crack initiation mechanism in VHCF.

Significant reduction in friction and wear of an ultrafine-grained single-phase FeCoNi alloy through the formation of nanolaminated structure

Reducing friction and wear has long posed a challenge for metallic components under dry sliding conditions. This study unveils a significant reduction in friction and wear of a single-phase ultrafine-grained FeCoNi multi-principal element alloy (MPEA). Through a comparative analysis of the sliding wear behaviors of single-phase ultrafine-grained CoCrFeMnNi and FeCoNi alloys with different stacking fault energies and detailed microstructure characterization, we found that despite having a similar initial microstructure and even a 20% lower hardness, the FeCoNi alloy exhibited a remarkable 52% reduction in coefficient of friction, and more notably, a wear rate three orders of magnitude lower than that of CoCrFeMnNi. The significantly reduced friction and wear can be attributed to the remarkable disparities in wear-induced surface microstructures between the two alloys during repetitive sliding. The sliding wear of FeCoNi alloy triggers the formation of a nanolaminated structure with excellent strain-hardening capacity and substantial deformation plasticity, as evidenced by micropillar compression tests. Conversely, the twinning-active CoCrFeMnNi alloy induces the formation of an equiaxed nanograin layer which exhibits a high compressive strength but catastrophic failure behavior after the onset of yielding, and thus deteriorates the wear resistance. The findings provide significant insights into fundamental understanding of the plastic deformation of single-phase MPEAs during wear and guide the design of wear-resistant alloys.

Microstructure features induced by fatigue crack initiation up to very-high-cycle regime for an additively manufactured aluminium alloy

Fatigue failure can still occur beyond 107 cycles, i.e. very-high-cycle fatigue (VHCF), in many metallic materials, such as aluminium alloys and high-strength steels. For VHCF of high-strength steels, a fine granular area (FGA) surrounding an inclusion is commonly identified as the characteristic region of crack initiation on the fracture surface. However, no such FGA feature and related crack initiation behaviour were observed in VHCF of conventionally cast or wrought aluminium alloys. Here, we first reported the distinct mechanisms of crack initiation and early growth, namely the microstructure feature and the role of FGA in VHCF performance for an additively manufactured (AM) AlSi10Mg alloy. The AM pores play a key role in fatigue crack initiation similar to that of the inclusions in high-strength steels, resulting in almost identical FGA behaviour for different materials under a range of mean stress with a stress ratio at R < 0 or R > 0. The profile microstructure of FGA is identified as a nanograin layer with Si rearrangement and grain boundary transition. This process consumes a large amount of cyclic plastic energy making FGA undertake a vast majority of VHCF life. These results will deepen the understanding of VHCF nature and shed light on crack initiation mechanism of other aluminium and AM alloys.

Understanding the microstructure refinement and mechanical strengthening of dual-phase high entropy alloy during ultrasonic shot peening

The ultrastrong gradient nanostructured dual-phase high entropy alloy (DP-HEA) Fe50Mn30Co10Cr10 has been fabricated by an efficient surface nanocrystallization method, e.g. ultrasonic shot peeing (USP) at room temperature. We investigated the effect of the USP duration on the microstructure of the DP-HEA Fe50Mn30Co10Cr10 by the advanced materials characterization methods, e.g. Electron Backscatter Diffraction (EBSD), X-ray Diffraction (XRD), and Transmission Electron Microscopy (TEM). We found that the microstructure of the USPed DP-HEA Fe50Mn30Co10Cr10 can be refined into several nanometers. Additionally, the FCC γ phase is transferred into the HCP ε phase during the USP process of the DP-HEA Fe50Mn30Co10Cr10 and the portion of the HCP ε phase increases gradually with the increase of the USP duration. Nanoindentation tests indicate that the nanohardness of the nanostructured DP-HEA Fe50Mn30Co10Cr10 can reach as high as 7.49 GPa, suggesting a yield strength of 2.49 GPa, which is 10 times higher than that of the coarse-grained materials. Additionally, nanocrystallization of the DP-HEA Fe50Mn30Co10Cr10 can be triggered with a short peening duration and the transformation-induced plasticity (TRIP) effect can benefit the formation of a thick gradient nano-grained layer during USP. Micro-tensile tests revealed that the gradient nano-grained layer can significantly improve the yield strength of the DP-HEA Fe50Mn30Co10Cr10 with sacrificing ductility.

Dispersed strain bands promote the ductility of gradient nano-grained material: A strain gradient constitutive modeling considering damage effect

Gradient nano-grained (GNG) metals have achieved superior strength-ductility synergy than their homogeneous counterparts. The high strength is usually attributed to the grain size effect and hetero-deformation-induced strengthening. However, incommensurate work on ductility leads to an incomplete understanding of the strength-ductility combination. In this work, a dislocation density-based strain gradient plasticity model coupling with a damage model is developed to describe the strain hardening and softening behavior of GNG material. A grain size-dependent back stress model derived from the generation of dislocation pileups is invoked to describe the widely-concerned back stress hardening. Finite element implementation of the model quantitatively predicts the tensile response of GNG nickel with various degrees of grain size gradient. The results reveal that dispersed strain bands propagate stably in the nano-grained surface layer of GNG material, which is totally different from those occurring in a freestanding nano-grained material. The stabilization of dispersed strain bands enables the nano-grained layer to deform uniformly, thus premature failure of the whole GNG material is suppressed and improved ductility is achieved. Furthermore, increasing the grain size gradient renders the strain bands more stable, leading to enhanced ductility. The method developed in this work is helpful for understanding the strength-ductility synergy of GNG materials and for optimizing the microstructure gradient in GNG materials.

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.

Comparison of SP, SMAT, SMRT, LSP, and UNSM Based on Treatment Effects on the Fatigue Properties of Metals in the HCF and VHCF Regimes

This paper aims to provide a better understanding regarding the effects of shot peening (SP), surface mechanical attrition treatment (SMAT), laser shock peening (LSP), surface mechanical rolling treatment (SMRT), and ultrasonic nanocrystal surface modification (UNSM) on the fatigue properties of metals in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes. The work in this paper finds that SMRT and UNSM generally improve the high-cycle and very-high-cycle fatigue properties of metals, while SP, SMAT, and LSP can have mixed effects. The differences are discussed and analyzed with respect to the aspects of surface finish, microstructure and microhardness, and residual stress. SMRT and UNSM generally produce a smooth surface finish, while SP and SMAT tend to worsen the surface finish on metals, which is harmful to their fatigue properties. In addition to inducing a plastic deformation zone and increasing microhardness, surface treatments can also generate a nanograin layer and gradient microstructure to enhance the fatigue properties of metals. The distribution of treatment-induced residual stress and residual stress relaxation can cause mixed effects on the fatigue properties of metals. Furthermore, increasing residual stress through SP and SMAT can cause further deterioration of the surface finish, which is detrimental to the fatigue properties of metals.

Effect of ultrasonic peening treatment on the fatigue behaviors of a magnesium alloy up to very high cycle regime

• Complete change of crack initiation site. • Quantitative evaluation of grain-boundary retarding effect. • Crack type affects the formation of FGA. Ultrasonic fatigue tests are performed on a magnesium alloy with and without ultrasonic peening treatment (UPT). Surface enhancement layer leads to the complete change of crack initiation sites. However, crack initiation mechanism keeps the same and results in a single-faceted morphology at crack initiation site. Microcracks initiate as Mode II crack within the original grain, but deflect to Mode I crack outside of the original cracked grain. A threshold SIF value is proposed to evaluate the retarding effect of grain boundary on microcrack propagation. Outside of the original cracked grain, Mode I crack propagation below the threshold ΔK σ-th is responsible for the formation of fine granular area (FGA, a nano-grain layer). Based on the Numerous Cyclic Pressing (NCP) model, it is proposed that crack type should be another necessary condition for the formation of FGA.

Ultraflexible and High-Thermoelectric-Performance Sulfur-Doped Ag2Se Film on Nylon for Power Generators.

In this work, we developed a facile method to fabricate low-cost, flexible, and high-thermoelectric-performance n-type Ag2Se1-xSx@(Ag2S1-ySey/S) composite film on a nylon membrane. The composite film was prepared by first performing wet-chemical synthesis of the S-doped Ag2Se powder, then vacuum-assisted filtration of the powder on a nylon membrane, and finally hot-pressing. Transmission electron microscopy (TEM) observation and energy-dispersive system (EDS) analysis of the film revealed that the film had a porous network-like microstructure, in which Ag2Se1-xSx sub-micron grains formed the skeleton and are coated by a ∼15 nm thick layer of S-rich Ag2S1-ySey nanograins mixed with an S amorphous phase. The film showed a power factor of ∼954.7 μW·m-1·K-2 at 300 K and superior flexibility (94.4% of the original electrical conductivity was preserved after bending 2000 times around a rod with a radius of 4 mm). Moreover, a six-leg flexible thermoelectric generator was assembled with the film and produced a maximum power of 6.67 μW (corresponding power density ∼14.8 W/m2) at a temperature difference of 38.7 K. This work reveals a novel approach to explore high-performance and low-cost flexible thermoelectric devices suitable for room-temperature applications.

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.

Activating dispersed strain bands in tensioned nanostructure layer for high ductility: The effects of microstructure inhomogeneity

How to suppress strain localization in tensioned nanograined layer and make it ductile? This remains a great challenge. Here we explore the effects of microstructure inhomogeneity on the plastic behavior of nanostructured layer. A nanostructured CrMnFeCoNi high entropy alloy layer supported by coarse-grained substrate (in gradient architecture) is taken as an example. In the tensile deformation, strain examinations find that the nanostructure layer experiences significant strain delocalization by activating dispersed stable strain bands (SBs) and promoting the deformation of non-strain banding zone. Finite element analysis reveals that it is the intra-layer microstructure inhomogeneity which enables dispersed SBs nucleation from the mechanical weak sites. The inter-layer microstructure inhomogeneity introduces extra shear constraints from the coarse-grained substrate, which stabilizes SBs and improves the stress of non-strain banding zone, thereby suppressing the early catastrophic strain localization. Importantly, the strong nanostructure layer indeed becomes ductile (enables further work hardening) due to the activation of hierarchical nanotwinning and dislocation activity.

Wetting and interfacial reaction of liquid CuAg-Ti alloy on SiO2-BN ceramic

Interfacial reactions have been recognized to play a decisive role in the wettability of liquid alloys on SiO2-BN ceramic; however, the priority between interfacial reactions and spreading of droplet during wetting is still puzzling. In this work, this confusion was experimentally clarified by the sessile drop method associated with a designed wetting structure and the instantaneous solidification for a CuAg-Ti/SiO2-BN wetting system. New phases formed at the wetting interface as a result of the reactions between active Ti and hexagonal BN component while the droplet had not yet spread, confirming that the interfacial reactions occur prior to the spreading of liquid alloys. Moreover, the wettability of the system depended on the reaction products of the TiN-TiB2 nanograin layer formed at the wetting interface, while the spreading was sensitive to the interfacial reaction process.

The oxidation behavior of uranium treated by ultrasonic surface rolling process

The oxidation behavior of uranium treated by ultrasonic surface rolling process (USRP) was investigated. Optical microscopy (OM), transmission electron microscopy (TEM), x-ray diffraction (XRD), x-ray photo-electron spectroscopy (XPS), auger electron spectroscopy (AES), scanning electron microscopy (SEM), focused ion beam (FIB) and ultraviolet-visible reflectance spectroscopy were used to characterize the microstructure and oxidation behavior of uranium. The oxidation kinetics of USRP treated uranium with pure O2 at 343–388 K was subdivided into two different regimes: a linear-like stage and a parabolic-like stage. The calculated activation energies of USRP treated uranium were approximately 52.3kJ/mol and 75.1kJ/mol for linear and parabolic stages, respectively. The oxide thickness of untreated uranium generally followed a parabolic growth model and the activation energy was 61.4kJ/mol. After the USRP treatment, the nano-grained layer and the close-packed U(002) texture were formed, which promoted the formation of a passive and stable oxidation film on the surface. Meanwhile, the improved surface integrity and the residual compressive stress were obtained, which slowed down the rupture of the oxide layer. As a result, it was concluded that the enhancement of oxidation resistance after USRP treatment was attributed to the nano-grained microstructure, the U(002) texture, the residual compressive stress and the smooth surface.

Gradient Enhanced Strain Hardening and Tensile Deformability in a Gradient-Nanostructured Ni Alloy.

Gradient-nanostructured material is an emerging category of material with spatial gradients in microstructural features. The incompatibility between gradient nanostructures (GNS) in the surface layer and coarse-grained (CG) core and their roles in extra strengthening and strain hardening have been well elucidated. Nevertheless, whether similar mechanisms exist within the GNS is not clear yet. Here, interactions between nanostructured layers constituting the GNS in a Ni alloy processed by surface mechanical rolling treatment were investigated by performing unique microtension tests on the whole GNS and three subdivided nanostructured layers at specific depths, respectively. The isolated nanograined layer at the topmost surface shows the highest strength but a brittle nature. With increasing depths, isolated layers exhibit lower strength but enhanced tensile plasticity. The GNS sample’s behavior complied more with the soft isolated layer at the inner side of GNS. Furthermore, an extra strain hardening was found in the GNS sample, leading to a greater uniform elongation (>3%) as compared to all of three constituent nanostructured layers. This extra strain hardening could be ascribed to the effects of the strain gradients arising from the incompatibility associated with the depth-dependent mechanical performance of various nanostructured layers.