Investigation of the temperature-relatedwear performance of hard nanostructured coatings deposited on a s600 high speed steel

Thin hard coatings are widely known as key elements in many industrial fields, from equipment for metal machining to dental implants and orthopedic prosthesis. When it comes to machining and cutting tools, thin hard coatings are crucial for decreasing the coefficient of friction (COF) and for protecting tools against oxidation. The aim of this work was to evaluate the tribological performance of two commercially available thin hard coatings deposited by physical vapor deposition (PVD) on a high speed tool steel (S600) under extreme working conditions. For this purpose, pin-on-disc wear tests were carried out either at room temperature (293 K) or at high temperature (873 K) against alumina (Al2O3) balls. Two thin hard nitrogen-rich coatings were considered: a multilayer AlTiCrN and a superlattice (nanolayered) CrN/NbN. The surface and microstructure characterization were performed by optical profilometry, field-emission gun scanning electron microscopy (FEGSEM), and energy dispersive spectroscopy (EDS).Funding: This research was made possible by an NPRP award NPRP 5-423-2-167 from the Qatar National Research Fund (a member of The Qatar Foundation)

Identification and Modeling of a Variable Amplitude Fatigue Experiment Apparatus with Damaged Beam Specimen

The useful remaining life of engineering structures under variable amplitude (VA) fatigue loading remains a major unresolved engineering problem. The existing proposed life prediction models are usually based on empirical approximation from experimental results (Fatemi, Yang Int J Fatigue 20(1):9–34, 1998, Santecchia et al. Adv Mater Sci Eng 2016:1–26, 2016). The variable fatigue experiment apparatus in this extended abstract was designed for simulating structural fatigue with a high testing frequency, variable R-ratio as well as modifiable experimental layout (Falco et al. J Vib Acoust 136(4):041001, 2014). In previous studies, the inherent nonlinearity of the testing rig was detected, the obtained parameters allow one to properly use this testing rig within its linear region. As damage accumulates, however, the corresponding dynamic characteristics of the specimen alter accordingly. Therefore, proper modeling considering the interaction between the inherent nonlinearity and the damage induced nonlinearity for both (1) opening crack and (2) breathing crack is necessary for future fatigue life estimation under complex fatigue loading. Here, nonlinear system identification of the lately modified variable amplitude fatigue experiment apparatus is presented based on a combination of first-principles and data-driven modeling techniques. Eventually, structure-damage interaction dynamics will be described to model the underlying fatigue evolution and structural dynamics interactions

Solid-state phase transformations in thermally treated Ti-6Al-4V alloy fabricated via laser powder bed fusion

Laser Powder Bed Fusion (LPBF) technology was used to produce samples based on the Ti-6Al-4V alloy for biomedical applications. Solid-state phase transformations induced by thermal treatments were studied by neutron diffraction (ND), X-ray diffraction (XRD), scanning transmission electron microscopy (STEM) and energy-dispersive spectroscopy (EDS). Although, ND analysis is rather uncommon in such studies, this technique allowed evidencing the presence of retained \u3b2 in \u3b1' martensite of the as-produced (#AP) sample. The retained \u3b2 was not detectable byXRDanalysis, nor by STEM observations. Martensite contains a high number of defects, mainly dislocations, that anneal during the thermal treatment. Element diffusion and partitioning are the main mechanisms in the \u3b1 \u2194 \u3b2 transformation that causes lattice expansion during heating and determines the final shape and size of phases. The retained \u3b2 phase plays a key role in the \u3b1' \u2192 \u3b2 transformation kinetics

A Review on Fatigue Life Prediction Methods for Metals

Metallic materials are extensively used in engineering structures and fatigue failure is one of the most common failure modes of metal structures. Fatigue phenomena occur when a material is subjected to fluctuating stresses and strains, which lead to failure due to damage accumulation. Different methods, including the Palmgren-Miner linear damage rule- (LDR-) based, multiaxial and variable amplitude loading, stochastic-based, energy-based, and continuum damage mechanics methods, forecast fatigue life. This paper reviews fatigue life prediction techniques for metallic materials. An ideal fatigue life prediction model should include the main features of those already established methods, and its implementation in simulation systems could help engineers and scientists in different applications. In conclusion, LDR-based, multiaxial and variable amplitude loading, stochastic-based, continuum damage mechanics, and energy-based methods are easy, realistic, microstructure dependent, well timed, and damage connected, respectively, for the ideal prediction model. 2016 E. Santecchia et al.Scopu

Microstructure and Intermetallic Strengthening in an Equal Channel Angular Pressed AA2219. Part II: Strengthening Model.

In the present work, the strengthening effect of the Fe-rich intermetallic phases in a 2219 aluminum alloy subjected to equal channel angular pressing (ECAP) has been studied. Three different deformation conditions, corresponding to the as-extruded, ECAP route A-1 pass, and ECAP route A-2 passes, were considered. Mechanical characterizations have been performed by microhardness tests and tensile tests. All the contributions to the strengthening due to solid solution affects, dislocation boundaries, fine particles, and intermetallics have been quantitatively determined using transmission electron microscopy and field-emission scanning electron microscopy. The microstructure strengthening terms were combined, and the resulting value was shown to be fully consistent with the experimental yield strength obtained either by tensile tests or by microhardness measurements. These were (i) solution hardening; (ii) dislocation hardening through grain, cell, and very-low angle (misorientation angles within 4°, and typically showing Moiré fringes on TEM) boundaries; (iii) strengthening due to the equilibrium θ (Al2Cu), either through shearable (i.e., anti-phase) or by bowing (i.e., Orowan) mechanism; (iv) GP-I, GP-II (θ″) zones, and semi-coherent θ′ pre-precipitate, generated along tangle dislocations by the combined effect of the severe plastic deformation and the adiabatic heating; and (v) coarser intermetallic particle strengthening. In particular, the effect of the Fe-rich intermetallics on the alloy has been evaluated by calculating all the characteristic terms: the so-called parameter of intermetallic appearance index, according to Shabestari, the strengthening due to the load transfer (Δσ LT), and the strengthening due to the presence of the intermetallic phases (Δσ Intermet). Very good agreement was obtained between the strengthening model developed in this study and the experimentally measured yield stress

Early stages of plastic deformation in low and high sfe pure metals

Severe plastic deformation (SPD) techniques are known to promote exceptional mechanical properties due to their ability to induce significant grain and cell size refinement. Cell and grain refinement are driven by continuous newly introduced dislocations and their evolution can be followed at the earliest stages of plastic deformation. Pure metals are the most appropriate to study the early deformation processes as they can only strengthen by dislocation rearrangement and cell-to-grain evolution. However, pure metals harden also depend on texture evolution and on the metal stacking fault energy (SFE). Low SFE metals (i.e., copper) strengthen by plastic deformation not only by dislocation rearrangements but also by twinning formation within the grains. While, high SFE metals, (i.e., aluminium) strengthen predominantly by dislocation accumulation and rearrangement with plastic strain. Thence, in the present study, the early stages of plastic deformation were characterized by transmission electron microscopy on pure low SFE Oxygen-Free High Conductivity (OFHC) 99.99% pure Cu and on a high SFE 6N-Al. To induce an almost continuous rise from very-low to low plastic deformation, the two pure metals were subjected to high-pressure torsion (HPT). The resulting strengthening mechanisms were modelled by microstructure quantitative analyses carried out on TEM and then validated through nanoindentation measurements

Nanoscale characterization of metal alloys produced by laser powder bed fusion (LPBF) technology

Advanced characterization at the nanoscale of Co-Cr-Mo-W, and Ti6Al4V alloys produced by Laser Powder Bed Fusion (LPBF) has been carried out in order to investigate the structural features responsible of the material performances. The alloys considered in this study are key materials in advanced field such as aerospace, automotive and biomedicine, while LPBF is becoming the reference for fabrication of metal parts by additive manufacturing (AM). Combining advanced metallic materials with innovative production technologies results in unexpected mechanical properties of final products. In this study, several characterization techniques including scanning (SEM) and transmission (TEM) electron microscopy, X-ray diffraction (XRD) and neutron-based techniques have been used to investigate the materials at the nanoscale

Modelling the creep behavior of an AlSi10Mg alloy produced by additive manufacturing

AlSiMg alloys produced by additive manufacturing possess an extremely fine and complex microstructure that in many ways defies the most widely used phenomenological models, which, in fact, have turned out to be poorly suited for predicting their mechanical properties. The underlying rationale for the peculiar properties of these alloys has been qualitatively established, however the need for a constitutive model with better predictive capability is still strong. To this aim, the ultra-fine microstructure was described by using a model-material (MM) consisting of soft and hard zones deforming under a similar strain rate. A physically-based set of constitutive equations which took into account also the coarsening/ripening phenomena of the Si-particles was used to predict the creep behavior of the MM. In parallel, the creep response of an AlSi10Mg alloy produced by additive manufaturing and tested in the as-deposited condition was investigated at temperatures ranging from 150 to 225 °C. The minimum creep rate curves obtained for the MM by the constitutive model were then compared with the experimental data obtained by testing the real alloy under constant load in different initial states. The excellent correlation between model curves and experimental results was discussed, taking into account the evolution of the microstructure during creep

Microstructure and Intermetallic Strengthening in an Equal Channel Angular Pressed AA2219. Part I: Microstructure Characterization

In the present work, the strengthening effect of the Fe-rich intermetallic phases in a 2219 aluminium alloy subjected to equal channel angular pressing (ECAP) has been studied. Three different deformation conditions, corresponding to the as-extruded, ECAP route A-1 pass and ECAP route A-2 passes were considered. Microstructural characterization has been performed by light microscopy, transmission electron microscopy and scanning electron microscopy observations. All the contributions to the alloy strengthening, including solid solution, dislocation boundary, very fine particle precipitation induced by the adiabatic heating, equilibrium θ = Al2Cu secondary phase particles and the effect of the Fe-rich intermetallics, are discussed in this work. The resulting strengthening was evaluated and modelled in Part II

Fatigue life and microstructure of additive manufactured Ti6Al4V after different finishing processes

Finishing methods of additive manufactured metal parts are becoming a key driver of industrial viability, increasingly with additive processes being challenged in demanding end-product applications. The same scenario stresses the requirements as to fatigue life of parts built by Additive Manufacturing (AsM). The paper addresses fatigue life of Ti6Al4V produced by Powder Bed Fusion in four finishing conditions: as-built, tool machined, after tumbling and after tumbling and subsequent shot-peening. Failure mechanisms at the micro-scale are observed in order to reinforce the mechanical results by identifying the role of different surface morphologies in crack initiation. X-ray diffraction (XRD), scanning electron microscopy (SEM) techniques and microanalysis (EDX) are used to investigate microstructural modifications generated by the different finishing methods. Results show that tumbling alone does not improve fatigue life against the as built condition, whereas tumbling and subsequent shot peening allow matching the fatigue endurance of tool machined specimens. The shot peening process causes surface amorphization and implantation of the peening media turning into subsurface inclusions. Based on the results, an optimized finishing process can be envisaged, consisting in prolonged tumbling up to the removal of a stock allowance at least equal to the powder size, before shot peening