Gradient Microstructure Induced by Surface Mechanical Attrition Treatment in Grade 2 Titanium Studied Using Positron Annihilation Spectroscopy and Complementary Methods

Incorporating the principles of shot peening for a better understanding of surface mechanical attrition treatment (SMAT) by simulations and experiments

Surface mechanical attrition treatment (SMAT) fabricates a nanostructured layer on the surface of 316L that possesses excellent mechanical properties. In this paper a duplex process combining SMAT with nitriding was performed, then the microstructure, hardness and corrosion properties of the treated materials were investigated by using X-ray diffraction (XRD), transmission electron microscopy (TEM), hardness tests, potentiodynamic polarization measurements, X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The results showed that this duplex surface treatment can effectively increase the hardness and corrosion resistance of the treated surface both at room temperature and in the simulated steam generator (SG) condition at high temperature and high pressure.

Reinforcement of laser-welded stainless steels by surface mechanical attrition treatment

The pure nanocrystalline Al layer and Fe containing nanocrystalline layer were fabricated on the surface of 2024 Al alloy by surface mechanical attrition treatment (SMAT) with two different mediums, ceramic balls and steel balls, respectively. The corrosion behaviour of 2024 Al alloy before and after SMAT was investigated by potentiodynamic polarisation curves and electrochemical impedance spectroscopy (EIS). The electrochemical tests demonstrated that the nanocrystalline layer fabricated by SMAT with ceramic balls improved the corrosion resistance of 2024 Al alloy, while the Fe containing layer induced by SMAT with steel balls led to the diminution in the corrosion resistance.

Direct Energy Deposition (DED), as one common type of additive manufacturing, is capable of fabricating metallic components close to net-shape with complex geometry. Surface mechanical attrition treatment (SMAT) is an advanced surface treatment technology which is able to yield a nanostructured surface layer characterized by compressive residual stresses and work hardening, thereby improving the fatigue performances of metallic specimens. In the present study, stainless steel 316L specimens were fabricated by DED and subsequently surface treated by SMAT. Both uniaxial tensile tests and uniaxial tension-compression low-cycle fatigue tests were conducted for as-built and SMAT processed specimens. The microstructure of both conditions was characterized by roughness and hardness measurements, scanning electron microscopy and transmission electron microscopy. After SMAT, nanocrystallites and microtwins were found in the top surface layer. These microstructural features contribute to superior properties of the treated surfaces. Finally, it can be concluded that the mechanical performance of additively manufactured steel under static and fatigue loading can be improved by the SMAT process.

In this paper the residual stress states induced by conventional shot peening (SP) and surface mechanical attrition treatment (SMAT) are compared. The treated part correspond to plates made of a titanium alloy. Different intensities of these two mechanical treatments are first considered: their influence on the surface characteristics (roughness, hardness...) is studied. These experimental data are then used to develop a model for the residual stress profiles with dimensional analysis, Experimental and analytical approaches are then discussed.

In the present work, the influence of surface mechanical attrition treatment (SMAT) parameters on the microstructural and mechanical properties of an aluminum-magnesium-silicon alloy AA 6061 was studied using design of experiment technique. Balls of three different diameters were used, and SMAT was done for three different durations. The microstructural features of the surface layer fabricated by SMAT were characterized by cross-sectional scanning electron microscopic observations, x-ray diffraction technique and transmission electron microscopy. The microindentation hardness, nanoindentation hardness and surface roughness were determined. Due to SMAT, nanocrystallites formed on the surface and near-surface regions, and hardness and surface roughness increased. The ball diameter was the most influencing SMAT parameter compared to the treatment duration. However, interaction between ball diameter and treatment duration could not be ignored. Regression equations were developed relating the process parameters to the surface properties. The ball diameter and treatment duration could thus be properly selected as per the required values of roughness and/or the hardness.

Boronizing is a surface thermochemical treatment in which boron atoms are made to diffuse into a metallic surface at high temperatures. A nano-crystalline surface with larger defect density assists in enhancing the diffusion rate even at low temperatures. In the present work Ni-Cr-Mo steel is subjected to a surface mechanical attrition treatment (SMAT) to activate the surface with nanocrystalline structures and crystal defects. Subsequently the samples were boronized at low temperature regime (400°C - 600°C) for 5 hours using a pack boronizing technique. The microstructure, chemical analysis and hardness of borided layers were investigated using optical microscope, SEM – EDX and Microvicker’s Hardness Tester. The SMAT treated samples showed severe plastic deformation of the surface, nano-structured grains (10-30 nm) and larger defect density illustrating mechanically activated surface for diffusion. The boronizing had clearly demonstrated the diffusion of boron even at 400°C. The thickness of diffused layer was found to be about 20 µm at 400°C and 50 µm at 600°C for SMAT samples while the untreated samples showed practically no diffusion at 400°C and 12 µm at 600°C. The SEM-EDX results had confirmed the presence of boron at the diffused layer; however the hardness was found to be low. A maximum of 650 HV0.3 was achieved by low temperature boronizing of SMAT treated samples.

Positron states for the bulk and vacancy in intermetallic compounds CoAl and CoTi crystallizing in the B2 structure have been calculated using the DV-Xα electronic structure calculation and compared with experimental results. The calculated positron lifetimes in CoAl can explain the compositional dependence of the positron lifetimes in CoAl. The calculated positron lifetime at the Ti vacancy in CoTi is longer than that at the Co vacancy. The charge transfer from Ti to Co leads to the difference of the localization of the positron wave function at the vacancy. At the Co vacancy site surrounded by the positively charged Ti atoms, the positron wave function does not localize as well as at the Ti vacancy site, which decrease the positron lifetime. The calculated results suggest that the positron lifetimes in intermetallic compounds can not be directly correlated to those of the pure component and the charge transfer between the constituent atoms affects the localization of the positron wave function. Intermetallic compounds are of considerable interest due to their physical and mechanical properties. In intermetal- lic compounds having a wide range of concentrations around the stoichiometry, structural defects are introduced in order to compensate deviations from the stoichiometry. Several prop- erties show compositional dependence originating from the structural defects. Main structural defects are vacancies on the sublattice and anti-structure atoms occupying the other sublattice. Positron annihilation spectroscopy is a sensitive tool for vacancy-type defects in solids. In a perfect crystal, the positron wave function is delocalized in the interstitial region because of the repulsion from the ion cores. If vacancy-type defects exist in solids, the positron wave function is localized at the defect where the electron density is lower than the other regions. The localization of the positron at the defect results in a longer lifetime of the positron compared to the bulk life- time because the positron lifetime is inversely proportional to the electron density where the positron annihilated. Positron annihilation spectroscopy is very useful to study the behav- ior of the structural defects in intermetallic compounds. If the vacancy is mainly responsible for the deviation from the stoichiometry, longer positron lifetimes should be measured compared to the bulk lifetime. However, interpretation of the experimental positron lifetime for intermetallic compounds is more complicated than for pure metals, because several types of vacancies may exist in intermetallic compounds. The dif- ferent types of vacancies may yield the different positron life- times. In order to characterize measured positron lifetimes in intermetallic compounds, a theoretical analysis is indispens- able. However, little theoretical work has been done for inter- metallic compounds. In this work, we have performed calculations of positron lifetimes in intermetallic compounds CoAl and CoTi with the B2 structure based on first-principles electronic structure cal- culations. The calculated results are compared with the exper- imental positron lifetimes in order to identify the defect type in intermetallic compounds and estimate the reliability of the calculated positron lifetimes. 2. Computational Method

Plasma nitriding of AISI 304 stainless steel: Role of surface mechanical attrition treatment

The Portevin-Le Châtelier effect of gradient nanostructured 5182 aluminum alloy by surface mechanical attrition treatment

The excellent corrosion properties of copper justify its use in a large number of industrial applications, in spite of its low strength in the traditional state. To overcome this problem and to enhance the mechanical behavior of this material, the Surface Mechanical Attrition Treatment (SMAT) seems to be well-adapted [1]. As shown in several previous works, SMA-Treatment induces a grain refinement up to the nanometer scale in the top surface layer of the treated sample, through severe plastic deformation performed at high strain rate. While the grain size of the untreated material is in the micrometer scale, nanograins are generated beneath the treated surface. Between this nanocrystallised layer and the bulk of the sample, a transition layer is also present which is characterised by a grain size gradient: the grain size grows from the nanometer to the micrometer scale as the depth below the treated surface increases. The treated metal can thus be seen as a multilayered material. All these microstructural changes lead to strong modifications of the plastic properties of the samples subjected to SMAT [2] [3]. In the last years, several methods have been proposed to determine the Young’s modulus, the yield stress and the strain hardening coefficient using microindentation tests. More recently, Cao and Lu [4] [5] developed a new inverse method to extract plastic properties of metallic materials based on instrumented spherical indentation.

The effect of mechanical incompatibility and strain delocalization on the mechanical properties of gradient structure Cu-4.5Al alloy

Effect of Nanocrystalline Surface and Iron-containing Layer Obtained by SMAT on Tribological Properties of 2024 Al Alloy

Strength-ductility modulation via surface severe plastic deformation and annealing