Surface Self-nanocrystallization Research Articles

Gradient-structured 1060 aluminum by air pressure-driven surface mechanical rolling treatment and its effects on the friction performance

Surface mechanical rolling treatment (SMRT) is well documented as a novel and promising surface self-nanocrystallization technology. An experiment platform of air pressure-driven surface mechanical rolling treatment was established innovatively, and the gradient-structured 1060 aluminum rod samples were accordingly fabricated by air pressure-driven SMRT. The air pressure, rolling pass and rotation speed of rod sample are the three parameters used for controlling the SMRT process. A three-factor and five-level orthogonal experiment table was designed to study the effects of the process parameters on the results of air pressure-driven SMRT of 1060 aluminum rod samples in terms of the surface roughness and in-depth microhardness. The experimental results show that the different combination of the three process parameters could obtain almost the same surface roughness or in-depth microhardness. Both the surface roughness and in-depth microhardness induced by air pressure-driven SMRT of 1060 aluminum rod samples are relatively more sensitive to the air pressure. Based on the microstructure observations, the grain refinement mechanism of 1060 aluminum induced by air pressure-driven SMRT was discussed in detail. The friction and wear experiments were further conducted to investigate the influences of the surface roughness and in-depth microhardness on the dry sliding wear behavior of the air pressure-driven SMRTed 1060 aluminum rod samples. In comparison to the reduction of surface roughness, the increase of the in-depth mircohardness has the more significant impact on the dry sliding wear behavior of the SMRTed 1060 aluminum rod samples.

Surface self-nanocrystallization and -lubricating dependent wear resistance of austenitic steels at same initial hardness level

In the present paper, two austenitic wear-resistant steels, namely 110Mn13 and 0Cr17Ni10Mn5Mo2 steels, were cold-rolled to a same hardness level of ∼270 HV. Mechanical properties and wear resistance under various applied loads were tested. The deformed microstructure, worn surface morphologies and wear debris were discussed to analyze the wear resistance. Results showed that a higher density of deformation twins and dislocations were generated in the 0Cr17Ni10Mn5Mo2 steel to reach a same hardness level with the 110Mn13 steel. Consequently, the 0Cr17Ni10Mn5Mo2 steel obtained higher yield strength (736 MPa) but lower ultimate strength (896 MPa), elongation (30.5%) and strain hardening rate than the 110Mn13 steel when the impact absorption energies were at a similar level of ∼170 J. The weight loss of the 0Cr17Ni10Mn5Mo2 steel reduced by over 40% compared to that of the 110Mn13 steel under the selected wear conditions. With the cold-rolled microstructure, the worn surface of both the test steels produced nanocrystalline layers. The nanocrystalline layer thickness of the 0Cr17Ni10Mn5Mo2 steel was over 5 μm, far larger than that of the 110Mn13 steel. Meanwhile, with the wear debris containing Cr-oxide adhering to the worn surface, which could facilitate the self-lubricating effect, the 0Cr17Ni10Mn5Mo2 steel presented better wear resistance than the 110Mn13 steel.

Vacuum diffusion bonding between Ni3Al-based superalloy and S31042 steel by surface self-nanocrystallization treatment

Vacuum diffusion bonding at a relatively low temperature is a recognized method for restraining intermetallics and optimizing joint microstructure. The surface self-nanocrystallization (SSN) treatment by shot peening technology is applied on the surface before bonding a couple of Ni3Al-based superalloy and S31042 steel, which lowers the bonding temperature for achieving a sound joint by 100 °C. Moreover, the joint strength of samples bonding at 1000 °C after SSN treatment is 178 MPa higher than that of the bonded samples at 1100 °C without SSN treatment. SEM, XRD and TEM analyses indicate SSN treatment improves bonding efficiency by increasing surface micro-deformation and introducing nanocrystalline and deformed layer. The formation of disordered Ni-solution on the surface makes γ’ strengthening phase re-precipitates in the joint and hard-brittle phase of AlN is restrained by diffusion bonding at 1000 °C. The good metallurgical bonding, re-precipitated γ’ phase and the formation of refining grains zone near the bonding interface contribute to the good performance of 1000 °C-bonded samples after SSN treatment.

Gradient Ultra-fine Grained Surface Layer in 6063 Aluminum Alloy Obtained by Means of Rotational Accelerated Shot Peening

Gradient ultra-fine grained surface layer in 6063 aluminum alloy was obtained by means of a novel surface self-nanocrystallization technique, namely rotational accelerated shot peening (RASP) treatment. The average grain sizes along the vertical section vary from hundreds of nanometers in the top surface to micrometers in the matrix. By using orthogonal experimental design to compare roughness values and hardness values, we synthesized the processing parameters to obtain sample of smaller roughness values and higher hardness.

Effect of surface self-nanocrystallization on friction and wear behavior of Al7075-T6511 alloy

Shot peening (SP) is an effective surface reinforced treatment, which can improve the surface properties of materials. Because of the poor wear resistance of aluminum alloys, the improvement of wear resistance has received more attention. In this research, Al7075-T6511 alloy was treated by the SP with different shot peening intensities to achieve the surface self-nanocrystallization and improve the dry friction and wear property. The surface microstructure of specimen before and after the SP was observed by using the optical microscope (OM), transmission electron microscopy (TEM) and x-ray diffraction (XRD). The surface roughness was examined by using laser confocal scanning microscope (CLSM). It has been found that the surface microhardness of the specimen was improved greatly after the SP. A nanocrystalline surface was generated, and the thickness of the severe plastic deformation layer increased with increase of the SP intensity. The results also show that the friction and wear mechanism of the SP-free specimen was severe adhesive wear and oxidation wear. While, the friction and wear mechanism of the SP-treated specimen was adhesive wear, accompanied with abrasive wear and oxidation wear.

Recent Studies of Surface Self-Nanocrystallization (SSNC) of Metallic Materials

Surface self-nanocrystallization (SSNC) is a new surface modification technology to develop new materials, which can obtain nanostructured layers with nanograins on the metals surface without changing the chemical composition of the metals. In this study, SSNC was introduced from the aspects of the preparation methods, microstructural mechanism, mechanical properties, surface roughness, corrosion resistance and applications. This paper will provide experience and reference for further comprehensive researches and industrial applications of SSNC.

The Effect of Surface Self-Nanocrystallization on Low-Temperature Gas Carburization for AISI 316L Steel

In this work, the effect of surface self-nanocrystallization on low-temperature gas carburizing for AISI316L austenitic stainless steel has been studied. The surface ultrasonic rolling processing (SURP) was used to prepare nanostructured surface layers, and then the un-SURP and SURP samples were treated by LTGC at 470 °C for 10 h, 20 h and 30 h. In order to analyze the effect of surface self-nanocrystallization on low-temperature gas carburizing, optical microscopy (OM), atomic force microscope (AFM), scanning electron probe micro-analyzer (EPMA) and nano-indentation analyzer were used. The results show depth of SURP-induced plastic deformation layer was about 330 μm. Meanwhile, the surface hardness and elastic modulus were increased but the surface roughness decreased obviously after SURP. After low-temperature gas carburizing, according to the results of the thickness, carbon concentration, nano-hardness and elastic modulus of the carburized layer, the conclusion is that surface self-nanocrystallization carried by SURP has a negative effect on the low-temperature gas carburizing for AISI316L austenitic stainless steel and with the increase of carburizing time, the greater the adverse effect on carburizing.

Surface Self-nanocrystallization in Copper Electroforming

The coarse columnar grain layer generated on top of electroforming deposits leads to low surface strength and poor global performance of deposits. Here, we introduce a particle abrasive-induced surface self-nanocrystallization process, which is a part of the electroforming process rather than involving complex poststeps using special equipment. It can generate a thickness-controllable surface nanocrystalline layer on top of copper deposits for performance enhancement. In this study, surface nanocrystalline layers of 10, 20, 50 and 120 μm were fabricated on top of 500-μm-thick copper deposits. The tensile strength of copper deposits increases from 234 to 246, 330, 383 and 421 MPa, with a corresponding decrease in elongation from 26 to 25, 24, 15 and 6%, respectively. The moderately thick surface nanocrystalline layer makes the copper deposits exhibit good comprehensive mechanical properties. In addition, the surface self-nanocrystallization also makes the copper deposits show a better corrosion resistance in a neutral aqueous 0.1 M NaCl solution. This study aims to promote a convenient, controllable and reliable surface self-nanocrystallization process in electroforming for improving the global performance of deposits.

Enhanced nucleation undercooling and surface self-nanocrystallization of Al2O3-ZrO2(Y2O3) eutectic ceramics

This present work explores initially the feasibility of producing in-situ surface nanoeutectic Al2O3-ZrO2(Y2O3) (SNAZ) ceramics via the oxyacetylene flame remelting (OAFR) process. Singular cellular nanoeutectics consist of nanoscale lamellae, which prevail in the samples formed under different irradiation times. Ultrafine nanoeutectics are achieved by prolonging the irradiation time, and their formation mechanisms associated with eutectic refinement are proposed to obtain a better understanding on the melt thermal history and microstructural inheritance. The enhanced nucleation undercooling is the essential mechanism associated with the microstructural refinement. SNAZ formed by the OAFR-based rapid solidification seem to offer an inexpensive route to realize in-situ microstructure tailoring or surface shaping.

Effect of Surface Self-Nanocrystallization on Diffusion Bonding Between a Titanium Alloy and a TiAl-Based Alloy

A self-nanocrystallization layer was prepared on the surface of a Ti alloy sample via shot peening to investigate its influence on the follow-up diffusion bonding between a titanium alloy and a TiAl-based alloy. The experiment results show that surface self-nanocrystallization treatment can accelerate the interface reaction between a Ti alloy and a TiAl alloy, lowering the bonding temperature of the sound joints by more than 50 K. Moreover, the thickness of the interface reaction layer plays an important role in the interfacial bonding strength. In this work, the maximum shear strength of the joint appears to have a thickness of about 1.8 μm. As a consequence, it is found that surface self-nanocrystallization treatment is helpful for optimizing the diffusion bonding process.

Effect of surface self-nanocrystallization and Si infiltration on Si diffusion behavior, hardness and magnetic properties of pure Fe

Surface nanocrystallization of pure Fe was performed using an improved surface treatment process. The phase transformation and Si infiltration depth of the pure Fe before and after surface mechanical attrition treatment (SMAT) were compared by X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results indicated that nanocrystallization of Fe surface was achieved using SMAT, which resulted in deeper penetration of Si. Prolonging time of SMAT and Si infiltration also resulted in increasing microhardness, with the hardness first increasing with increasing distance from the surface and then decreasing. Furthermore, longer Si infiltration time, nanocrystallization of Si and longer SMAT time resulted in higher saturation magnetization (MS). The greatest Si penetration depth (150 μm), maximum hardness (280 HV), and maximum MS (1.849 × 106 A/m) were achieved after SMAT for 45 min and Si infiltration for 9 h. The interaction between adjacent grains after surface nanocrystallization leads to a region of the magnetic domain wall structure located at the grain boundary, which causes the remanence enhancement effect.

Surface self-nanocrystallization of α+β titanium alloy by surface mechanical grinding treatment

In this work, the heavy deformation was performed on Ti6Al4V alloy by using surface mechanical grinding treatment (SMGT) in order to obtain surface nanocrystalline layer. The phase structure and microstructures in the deformation zones were characterized by XRD, SEM and TEM. The nanocrystallization mechanisms of α and α phase were clarified. The results show that a gradient structure including nano grain, quasi nanograin and micro-grains was achieved within the depth of 500 μm below the surface. The depth of nanocrystallines layer was at least 20 μm. From the non deformation zone in the center to the heavy deformation zone close to treated surface, dislocation densities gradually increased. The original coarse grains gradually evolved into dislocation cell structures and subgrains through dislocation movement. In the depth of about 20 μm below the treated surface, the subgrains began to split into nano-scale grains with high angle grain boundaries by lattice rotation and tilting mechanisms. During SMGT, β phase deformed and refined prior to α phase, and inhibited the generation of twinning in α grain. The coordinated deformation between a and β phase promoted the formation of nanocrystallines.

Properties of Metallic Materials after Surface Self Nano-Crystallization

Surface Self-nanocrystallization (SSNC) can produce nanometer grains (10~50μm depth) in the surface layer of metallic materials. And high strength, residual compressive stress as well as a mass of defects attributed to grain refinement and severe plastic deformation, greatly improve their surface properties, such as strength, wear resistance, diffusion property, fatigue performance and corrosion resistance. Now some methods have been confirmed which could realize surface nanocrystallization. This paper reviews the study of surface nanocrystallization and simply summarizes changes in their performance based on surface layer microstructure of metallic materials.

Investigation of Surface Self-Nanocrystallization in 0Cr18Ni9Ti Induced by Surface Mechanical Attrition Treatment

By means of shot peening (a form of surface mechanical attrition treatment (SMAT)), a nanostructured surface layer was formed on the cross-sectional surface of a 0Cr18Ni9Ti bar. Several characterization methods in common use, such as OM, SEM, TEM, and XRD, were adopted to systematically characterize microstructure features of the nanostructured layer in the top surface of the sample. Microstructure features of the surface layer, which appeared mainly as severely deformed and contained grains with size in nanomagnitude, could be intuitionisticly presented through OM, SEM, and TEM. XRD was a useful method and average grain size and mean microstrain could be quantitatively calculated from the XRD patterns. In addition, analyses of XRD patterns showed that there was a martensite transformation in the top surface layer. Microhardness distribution along the depth of the deformation layer from the top surface showed that the surface self-nanocrystallization (SSNC) layer is a gradient structure and that the hardness of the top surface is greatly enhanced due to grain refinement and work hardening.

Low-Temperature Pack Aluminizing of X80 Pipeline Steel through Addition of Zinc and Surface Self-Nanocrystallization

Rapid low-temperature pack aluminizing was achieved on pipeline steel X80 through combined effect of surface refinement treatment and modification of pack powder. Self-nanocrystallization surface of pipeline steel X80 was obtained by surface mechanical attrition treatment (SMAT). In addition, zinc (Zn), which has lower melting point than that of Al, was added in the pack powder, to enhance the concentration of active aluminum (Al) atom. The mechanism of the low- temperature pack aluminizing was analyzed by examining the distribution of atoms and coating thickness using EDS and SEM. The study shows that aluminizing rate of as-SMATed pipeline steel with self-nanocrystallization surface is higher than that of non-SMATed pipeline steel under the same pack condition. The addition of Zn can increase the activity of diffusion atoms and accelerate the aluminizing by changing the state of pack powder from a single solid phase to a mixture of solid phase and liquid phase. The diffusion of atoms in this low-temperature pack aluminizing is considered as bulk diffusion which is driven by the activity of diffusion atoms in an unstable state following Fick law.

Pulse Pressuring Diffusion Bonding of Ti Alloy/Austenite Stainless Steel Processed by Surface Self-nanocrystallization

Nanostructured surface layers were synthesized on the end face of Ti–4Al–2V titanium alloy and 0Cr18Ni9Ti austenite stainless steel rods by means of high energy shot peening (HESP). Making treated end surfaces as bonding interfaces, Ti–4Al–2V and 0Cr18Ni9Ti rods were bonded by pulse pressuring diffusion bonding (PPDB) on Gleeble-1500D tester at 650–750°C. Joints were tested on tensile testing machine, the fractures and microstructures of joints were researched. Results showed that the maximum tension strength of 262.0 MPa was achieved, cleavage fracture took place while tension test of joints, and the grains in the vicinity of the diffusion layer are fined. But, brittle intermetallic compounds were absence on the bonding interface.

Pulse Pressuring Diffusion Bonding of Ti–4Al–2V/0Cr18Ni9Ti with Nanostructured Layer Synthesized by Surface Self-nanocrystallization

Nanostructured surface layer was synthesized on the end face of Ti–4Al–2V alloy and 0Cr18Ni9Ti austenite stainless steel rods by means of Surface self-nanocrystallization(SSNC). Making treated end surfaces as bonding interfaces, transition joint of Ti–4Al–2V alloy and 0Cr18Ni9Ti stainless steel bars was prepared by pluse pressuring diffusion bonding (PPDB) on Gleeble-1500D tester at 850°C for 80 s, the maximum and minimum pluse pressuring were 8 MPa and 50 MPa respectively, and cycle (N) and frequency (f) of pulse load were 40 times and 0.5 Hz respectively. Bonded joints were tensed on CMT5105 style instron. Microstructure of transition joint was investigated by scanning electron microscope (SEM) and X-ray energy dispersive spectroscope (EDS). The reaction products on the fracture were detected using X-ray diffraction (XRD). Research results showed that the maximum tensile strength reached 384.0 MPa, cleavage fracture took place while tension test of joints. Brittle intermetallic compounds such as Fe2Ti, FeTi and σ phase presented on the fracture, and on the titanium alloy side, α-Ti transformed into β-Ti in the vicinity of interface while diffusion bonding.

Nanostructured Surface Layer of Ti–4Al–2V by Means of High Energy Shot Peening

A nanostructured surface layer was formed on the ends of TA17 bars by means of high energy shot peening (HESP). Microstructures in the plastic deformation layer were systematically characterized and analyzed by using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), microhardness measurement and X-ray diffraction (XRD). Based on the experimental observations and analysis, nanograins with an average size of 35±5 nm were achieved on the top surface layer, the microhardness on the top surface is near twice that of the coarse-grained matrix. In terms of special metals materials, the stress and strain rate are critical for grain refinement in the process of surface self-nanocrystallization (SSNC). On the basis of discussion for microstructure in the plastic deformation layer, the grain subdivision mechanism of TA17 was drawn that twinning deformation transformed coarse grains into fine twinning lamellae at the low strain rate level, and the submicronic grains are broken down into randomly oriented nanograins by means of dislocation glide at high level of strain.

Formation of nanocrystalline surface of a Cu–Zn alloy under electropulsing surface treatment

A nanocrystalline surface layer of a Cu–Zn alloy was developed by electropulsing (ECP) surface treatment. The average grain size was about 20 nm on the top surface layer and was gradually augmented with the increase in depth from the top surface. Nanoindentation measurements showed that the hardness was significantly enhanced on the top surface layer compared with the as-annealed Cu–Zn sample. The mechanism for the evolution of this structure and property was related not only to a solid-state phase transformation, but also to the effect of the enhancement of the nucleation rate and the skin effect during the ECP treatment. Therefore, the ECP surface treatment provides a promising method for obtaining surface self-nanocrystallization materials.

Simulation for surface self-nanocrystallization under shot peening

Driven by high frequency and multi-directional shot peens, dislocations of various orientations proliferate into the metal, and accumulate in high density in the surface layer of a shallow depth. Migration, generation and annihilation of dislocations dictate the evolution of mobile dislocation density. Simulation for the experiment of pure iron under repeated shot peen flux of 800 times per square millimeter is carried out, and a dislocation density up to 2.17×1011 mm−2 is achieved. Dislocations of such density in the surface layer are shown to be capable of forming nano-grains whose size is about 10 nm. Molecular dynamics simulation verifies the formation of nano-grained metals at such dislocation density level. The dislocations are first regrouped to form subcrystallites, then combined to form stable nanocrystallized grains after sufficiently long time of relaxation.