Characterization and Properties of Nanostructured Surface Layer in a Low Carbon Steel Subjected to Surface Mechnaical Attrition

A nanostructured surface layer was synthesized on a low carbon steel by using surface mechanical attrition (SMA) technique,The refined microstructure of the surface layer was characterized by means of different techniques, and the hardness variation along the depth was examined,Experimental results show that the microstructure is inhomogeneous along the depth ,In the region from top surface to about 40 μm depth,the grain size increases from about 100 nm to 1000 nm ,The grain refinement can be associated with the activity of dislocations After the SMA treatment, the hardness of the surface layer is enhanced significantly compared with that of the original sample ,which cam primarily be attributed to the grain refinement.     

超声冲击诱发表面纳米化及其对表面完整性的影响

采用不同超声冲击参数处理SMA490BW钢,研究了冲击后试样在低、高倍下的微观组织特征、残余应力及硬度分布等表面完整性能的变化。实验结果表明,经过超声冲击表面处理后,样品表面层晶粒细化为纳米晶,平均晶粒尺寸约为30nm;并在试样表层引入残余压应力,数值最大约为255.5MPa;超声冲击对SMA490BW钢表面能够起到明显的强化作用,与未经处理的试样相比,处理后试样表面硬度最大提高了约66.7%。超声冲击强化处理改善SMA490BW钢的表面完整性的效果与冲击电流、冲击时间之间的关系不遵循单调变化规律,超声冲击参数为20min/1.5A时,试样具有较好的表面完整性,冲击影响层深度约为320μm。     

超声冲击处理2A12铝合金焊缝表层微观组织结构变化

采用超声冲击工艺对母材为2A12铝合金焊接接头进行了焊后处理.利用扫描电镜、X射线衍射分析了超声冲击处理前后焊缝表面层的微观组织结构特征.结果表明,超声冲击处理可以在焊缝表面形成约为300μm的致密塑性变形层,晶粒明显细化.对X射线衍射峰的观察分析表明,处理后焊缝表层材料衍射峰明显宽化,这是超声冲击处理使焊缝表层材料粒子细化和晶格结构的微观应变引起的.利用谢乐方程和威尔逊公式对超声冲击处理后焊缝表层的晶粒度和微观应变进行了定量分析,结果表明,处理层的晶粒平均尺度为123nm,平均微观应变为0.134%.最后,对晶粒细化的机理进行了分析.     

X80管线钢焊接接头表面自身纳米化

采用表面机械研磨(SMAT)技术对X80管线钢的焊接接头进行了表面自身纳米化处理,利用金相显微镜(OM)、透射电子显微镜(TEM)和X射线衍射技术(XRD)研究了表面自身纳米化处理后试样表面微观结构的变化。结果表明:经SMAT处理后,可以在X80管线钢的焊接接头表面形成一定厚度的等轴、取向随机的纳米晶粒;随距处理表面深度的增加,晶粒尺寸逐渐增大;SMAT处理时间的进一步延长对表层晶粒尺寸影响不大;SMAT处理可以实现X80管线钢的焊接接头组织的连续化和均匀化。     

表面机械研磨对H13钢表面性能的影响

使用表面机械研磨（SMA）方法处理H13钢表面使其表面获得一层变形层。采用SEM,TEM及硬度试验等检测方法对变形层的厚度、晶粒大小、截面硬度梯度和热稳定性等进行了分析;同时对H13钢在SMA处理前后的渗氮行为进行了比较。结果表明,经SMA处理后,H13钢表面形成了约10μm厚的变形层,变形层内的晶粒明显细化,并且这些晶粒具有较好的热稳定性;此外,可以大大提高H13钢渗氮的效率。     

Gradient Microstructure and Properties of Surface Mechanical Attrition–Treated AZ91D Alloy: An Effect of Colliding Balls Velocity

The surface properties of the AZ91D alloy are altered using surface mechanical attrition treatment (SMAT), a promising method of severe surface deformation, where the role of process parameters is crucial. In this study, specimens are SMATed using ≈3 and ≈10 m s⁻¹ ball velocities (maintaining a constant percentage coverage). The SMATed specimens show higher twin density near the surface, which is reduced gradually, and twin thickness is increased with increasing depth. Further, high-velocity balls cause more twin density and better grain refinement (≈32 nm grain size at the surface). The higher ball velocity helps form a considerably thicker gradient layer (≈3500 μm) with higher hardness (≈1.98 GPa) and compressive residual stress (≈281 MPa) within a shorter SMAT duration (≈10 min). Ball velocity also influences nanomechanical properties such as nanohardness, creep resistance, strain rate sensitivity (SRS), etc. The non-SMATed alloy's SRS is about 0.037–0.040. The gradient microstructure affects SRS. The SRS value near the SMATed surface (where the reduced grain size plays a dominating role) is about 0.018–0.027; however, it drops suddenly to ≈0.01 (with a slight increase in depth), and subsequently, it rises with an increased distance in the SMATed layer (where twins play a dominating role).     

Microstructural evolution and mechanical properties of oil jet peened aluminium alloy,AA6063-T6

Grain size refinement by severe surface plastic deformation is one way of improving the surface properties. This paper describes the microstructural evolution due to severe surface plastic deformation by oil jet peening in aluminium alloy, AA6063-T6. Detail characterization of the treated surfaces using X-ray diffraction analysis and transmission electron microscopy revealed the formation of submicron size grains at and near the surface. The nozzle-traveling velocity decides the peening intensity and coverage and affects the surface properties. The specimen peened at low nozzle-traveling velocity exhibited an ultrafine grain size (∼210 nm) with high surface hardness (∼0.88 GPa), compressive residual stress (−102 ± 7 MPa) and dislocation density. The hardness is high at the surface and the depth of hardened layer is ∼400 μm. Formation of high-density dislocations and associated grain refinement resulted in increased surface hardness. Presence of surface modified layer will be beneficial in improving the fatigue and tribo behavior.     

Annealing-induced Grain Refinement in a Nanostructured Ferritic Steel

A nanostructured surface layer with a mean ferrite grain size of ～8 nm was produced on a Fe-9Cr steel by means of surface mechanical attrition treatment. Upon annealing, ferrite grains coarsen with increasing temperature and their sizes increase to ～40 nm at 973 K. Further increasing annealing temperature leads to an obvious reduction of ferrite grain sizes, to ～14 nm at 1173 K. The annealing-induced grain refinement is analyzed in terms of phase transformations in the nanostructured steel.     

Mechanical and Wear Properties of Nanostructured Surface Layer in Iron Induced by Surface Mechanical Attrition Treatment

A porosity-free and contamination-free surface layer with grain sizes ranging from nanometer to micrometer in Fe samples was obtained by surface mechanical attrition treatment (SMAT) technique. Mechanical and wear properties of the surface layer in the SMATed and annealed Fe samples were measured by means of nanoindentation and nanoscratch tests, respectively. Experimental results showed that the hardness of the surface layer in the SMATed Fe sample increased evidently due to the grain refinement. The elastic moduli of the surface layers in the SMATed and annealed Fe samples were unchanged, independent of grain size in the present grain size regime. Compared with the original Fe sample, the wear resistance enhanced and the coefficient of friction decreased in the surface layer of the SMATed Fe sample.     

Strain-induced formation of a gradient nanostructured surface layer on an ultrahigh strength bearing steel

In the present work, an ultrahigh strength bearing steel(AISI 52100) was subjected to surface mechanical rolling treatment(SMRT) at room temperature. Microstructural observations showed that martensitic laths, twins and cementite particles in the initial microstructure underwent distinct plastic strains and were gradually refined into nanostructures. Consequently, a gradient nanostructured(GNS) surface layer with a mean grain size of ~24 nm at the top surface was obtained on the bearing steel, resulting in an increment of ~20% in the surface hardness. Analyses based on microstructural evolution, phase constitution and in-depth hardness distribution revealed a mechanically induced formation mechanism of the GNS surface layer. The multiple surface severe plastic deformation under fine lubrication and cooling during SMRT contributed to the formation of a thick hardened surface layer on the bearing steel.     

Enhancing torsion fatigue behaviour of a martensitic stainless steel by generating gradient nanograined layer via surface mechanical grinding treatment

Abstract

A gradient nanograined (GNG) surface layer was formed on a martensitic stainless steel bar sample by means of the surface mechanical grinding treatment (SMGT). The average grain size is ～25 nm on the topmost surface layer and increases gradually with increasing depth. The torsion fatigue strength is elevated by 38% with the GNG surface layer compared with the original material. An additional 8% increment in fatigue strength is achieved after a post-annealing treatment of the SMGT sample. By analysing the microstructure, hardness, surface roughness and residual stress distribution in the SMGT samples, we believe that the enhanced fatigue resistances originate from the GNG structure with a hard surface layer and a high structural homogeneity.     

Improved Fatigue Behavior of Pipeline Steel Welded Joint by Surface Mechanical Attrition Treatment (SMAT)

A pipeline steel X80 with welded joint was subjected to surface mechanical attrition treatment(SMAT).After SMAT,a nanostructure surface layer with an average grain size of about 10 nm was formed in the treated sample,and the fatigue limit of the welded joint was elevated by about 13% relative to the untreated joints.In the low and the high amplitude stress regimes,both fatigue strength and fatigue life were enhanced.Formation of the nanostructured surface layer played more important role in the enhanced fat...     

Crack tip plasticity in plastically graded Ni–W electrodeposited nanocrystalline alloys

In the present paper, it is analyzed the problem of a crack approaching a plastically graded microstructure along its path. The problem regards a nanocrystalline electrodeposited material in which the plastic properties (yield stress, hardness, UTS) vary much more than the elastic ones (Young modulus and Poisson ratio).

In the present paper, the analysis has been conducted by employing ABAQUS simulations on nanocrystalline Ni–W alloys with 20 and 100 nm constant grain size; the results were compared with those of the plastically graded sheets with negative graded configuration in which the grain size varies between 20 and 100 nm from the surface to the bottom in a linear way and with positive graded configuration in which the grain size varies between 100 and 20 nm from the surface to the bottom in a linear way.

The finite element study has conducted to the conclusion that the grade in the microstructure and consequent plastic mechanical properties of nanocrystalline Ni–W alloys leads to several changes in the driving force for fracture demonstrated by the different behavior of the effective J-integral at the crack tip in the different studied configurations.     

Surface nanocrystallization of Ni3Al by surface mechanical attrition treatment

In this paper, the possibility of surface nanocrystallization of Ni₃Al intermetallic by surface mechanical attrition treatment was explored. The microstructure and hardness of treated sample were characterized through scanning electronic microscope, transmission electronic microscope, X-ray diffraction and nanoindentation examination. The results showed surface nanocrystallization was realized on Ni₃Al intermetallic by surface mechanical attrition method. The nanocrystalline in top surface was about 10 nm and a deformed layer about 10–30 μm formed. Nanoindentation examination showed the nanohardness increased sharply to a maximum and then decreased in a narrow depth till to a stable level with the depth from treated surface. The maximum nanohardness of surface was near 12 GPa.     

终轧温度对Ti-V-Mo复合微合金钢组织演变和硬度的影响

采用Gleeble3800热模拟试验机、OM、EBSD、TEM及Vickers硬度计等研究终轧温度对Ti-V-Mo复合微合金钢的组织转变、析出相和硬度的影响,并阐明了组织演变和硬度变化的原因。结果表明,不同终轧温度的Ti-V-Mo钢其组织均为多边形铁素体;随着终轧温度由1000℃降低到800℃,Ti-V-Mo钢的硬度由400HV提高到427HV;铁素体晶粒的平均尺寸由3.44μm减小到3.05μm;(Ti, V, Mo)C粒子的析出数量增加,其平均尺寸由8.38 nm减小到6.25 nm。随着终轧温度的降低,铁素体平均晶粒尺寸的减小和纳米级(Ti, V, Mo)C粒子的增多及细化是硬度增大的主要因素。在980℃以下,降低终轧温度(Ti, V, Mo)C在奥氏体中的形核率不断减小,使得其在铁素体中析出的10 nm以下的(Ti, V, Mo)C粒子不断增多,促进了硬度的提高。     

Surface and nanoindentation studies on nanocrystalline titanium diboride thin film deposited by magnetron sputtering

A detailed study on the mechanical, structural and surface characteristics of nanocrystalline TiB₂ films deposited on Si-100 substrates by direct current (DC) magnetron sputtering was carried out. X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), nanoindentaion and X-ray diffraction (XRD) studies on these films were performed. Magnetron sputtered titanium diboride coatings had a maximum hardness of 36 GPa and elastic modulus of 360 GPa. From the XRD analyses, the films were found to grow in (00l) direction-oriented perpendicular to the substrate. The AFM analysis of the films showed the variation of grain size between 30 and 50 nm. The high-resolution AFM images revealed arrangements of atoms resembling lattice and the interplanar spacings measured on the image also showed the orientation of grains in the (001) direction. Nanoindentation studies at very shallow depths showed a continuous increase of hardness and modulus with indentation depth up to 40 nm due to tip blunting and presence of oxides on the film surface (confirmed from the XPS study). The elastic recovery was approximately 69% for 100 nm depth whereas it was 52% for 1000 nm depth.     

高能喷丸处理对7A52铝合金表面微观组织结构及性能的影响

随着金属表面自纳米化技术的不断发展,金属材料的表面性能得到了明显提升。通过SEM、EBSD、TEM和HRTEM等分析测试手段,对高能喷丸处理后的7A52铝合金表面纳米化机理进行了分析。结果表明,随喷丸时间的延长铝合金表面硬度显著增大,当喷丸时间为50min时,表面硬度约为270HV,与基体相比提高了近1.5倍。此时铝合金外表面均匀分布着平均晶粒尺寸约为14.16nm的等轴晶,距表面约60μm处分布着以小角度晶界为主的微米级亚晶。7A52铝合金表面纳米级晶粒的形成机理主要是:晶粒变形使晶内逐渐形成高浓度位错,位错的缠绕、塞积对基体晶粒进行了初步分割细化,在往复载荷的作用下最终在表面形成了等轴的随机取向分布的纳米级晶粒。     

Mo离子注入对纯铜表面纳米层稳定性的影响

采用表面机械研磨处理(SMAT)对纯铜进行表面改性,通过金属蒸汽真空弧离子注入技术在纳米表层注入Mo离子。利用光学显微镜(OM)、X射线衍射分析仪(XRD)和扫描电镜(SEM)观察SMAT处理效果,表面存在纳米层和变形层,通过原子力显微镜(AFM)表征纳米层的晶粒尺寸。结果表明:晶粒尺寸得到了显著的抑制,表面纳米层的晶粒在退火后长大到163nm,而注入了Mo离子的只长大到72nm。此外,SMAT并离子注入后材料表面的硬度仅达到SMAT试样的3.5倍,是纯Cu基体硬度的7倍左右。Mo离子的分散和由SMAT及离子注入引入晶体缺陷的反应促使了这些优化现象的产生。     

Influence of Surface Nano-structured Treatment on Pack Boriding of H13 Steel

In order to lower the boriding temperature of hot work steel H13, method of surface mechanical attrition treatment （SMAT）, which can make the grain size of the surface reach nano-scale, was used before pack boriding. The growth of the boride layer was studied in a function of boriding temperature and time. By TEM （transmission electron microscopy）, SEM （scanning electron microscopy）, XRD （x-ray diffraction） and microhardness tests, the grain size, thermal stability of the nano-structured （NS） surface and the thickness,appearance, phases of the surface boride layer were studied. Kinetic of boriding was compared between untreated samples and treated samples. Results showed that after SMAT, the boride layer was thicker and the hardness gradient was smoother. Furthermore, after boriding at a low temperature of 700℃ for 8 h, a boride layer of about 5 μm formed on the NS surface. This layer was toothlike and wedged into the substrate, which made the surface layer combine well with the substrate. The phase of the boride layer was Fe2B. Research on boriding kinetics indicated that the activation energy was decreased for the treated samples.     

Laser Surface Alloying of (Pre-placed) SiC and Ni-SiC Coating on Commercially Pure Titanium

Results of laser alloying of 100% SiC and 50% Ni + 50% SiC on commercially pure titanium were presented in this investigation. The high hardness Hv800-1200 obtained at 100% SiC and 50% Ni + 50% SiC alloying conditions were due to the presence of various intermetallic phases such as TiC, TiSi, Ti₅Si₃ and NiTi₂. These intermetallic phases present in the laser alloyed surface were validated by EDXRD analysis and the diffusion of Ni, Si, C in titanium responsible for these phase formations was identified by SIMS study. The alloyed layer microstructure consists of dendrites and its density level depends on laser processing conditions. At low level power density the alloyed layer depth was about 0.5 mm with a constant hardness level, whereas at higher level powerdensity the depth of alloyed layer touched a maximum of 1.6 mm with large fluctuation in hardness.