•Ferritic-martensitic structure was created by controlled rolling.•Grain size was strongly dependant on the vicinity of sheet surface due to cooling by contact with rolls.•Cu interphase precipitation ...occurred in the ferrite grains close to the sheet surface.•Cu precipitation caused rise of the yield stress.•Cu precipitation occurred only if fine ferrite grains grew from probably undercooled austenite.
Two experimental 0.2C steels with 1 wt% and without Cu were studied. A dual ferritic-martensitic microstructure was prepared by controlled rolling and subsequent quenching. The heterogenous microstructure was obtained due to the severe cooling of the surface layer during the rolling process. The surface layer of the sheet had a significantly finer ferritic grain than the sheet’s core. Mechanical properties of the core were not correlated to the Cu content, whereas the surface layer revealed Cu-related strengthening due to Cu precipitation. Rows of particles typical for interphase precipitation were observed in some of the ferrite grains of the Cu-containing sample. Mode of the ferrite growth from austenite has apparently a crucial role in Cu ability to rapidly precipitate in ferrite grain during or shortly after its formation.
The key feature of Fe-Mn alloys is gradual degradability and non-magneticity, with laser power bed fusion (LPBF) parameters influencing the microstructure and chemical composition. Our study focuses ...on biodegradable Fe-Mn alloys produced by mechanically mixing pure metal feedstock powders as part of the LPBF process. The Mn content and, consequently, the γ-ε phase formation in LPBF samples are directly correlated with an adapted energy-density (E) equation by combining the five primary LPBF parameters. We varied laser power (P) in a range of 200-350 W and scanning speed at 400 and 800 mm/s, and a comprehensive study was performed on samples with similar E. The study also showed an almost linear correlation between the LPBF's laser power and the material's hardness and porosity. The corrosion resistance was significantly reduced (from 13 to 400 μm/year) for the LPBF samples compared to a conventionally produced sample due to the dual-phase microstructure, increased porosity and other defects. The static immersion test showed that the process parameters greatly influence the quantity of oxides and the distribution of their diameters in the LPBF samples and, therefore, their corrosion stability. The most challenging part of the study was reducing the amount of ε phase relative to γ phase to increase the non-magnetic properties of the LPBF samples.
The passivity of AISI 304L and AISI 316L stainless steels in a borate buffer solution, with and without the addition of chloride ions, was studied using cyclic voltammetry and potentiodynamic ...measurements. The passive layers formed by electrochemical oxidation at different passivation potentials on both the stainless steels were studied by X-ray photoelectron spectroscopy, their compositions were analysed as a function of depth, and the cationic fraction of the passive film was determined. The passive films established on the two stainless steels in the borate buffer solution at pH
=
9.3 contained the oxides of two main elements, i.e., Fe and Cr. The oxides of the alloying elements Ni and, optionally, Mo, also contribute to the passive layer. In the presence of chloride ions a strong chromium enrichment was observed in the passive layers.
•Comparison of thermal and creep exposure – influence on Laves phase (LP) growth.•Different kinetics of LP precipitation – slower rate of LP growth during early stages of creep.•Much less elongated ...LP particles formed during creep than during thermal exposure.•The state of matrix recrystallization might be the reason for the differences in the LP content and morphology.
The formation of Laves phase was studied in experimental 10Cr-3 W-3Co creep-resistant steel. The steel was exposed to the temperature of 650 °C, and just thermal and creep exposure was compared. Laves phase content, particle size, and morphology were quantitatively analysed by image analysis of micrographs. Significant differences were observed between the creep and thermal exposure. Thermal exposure resulted in fine Laves phase particle nucleation and their gradual coarsening. On the other hand, creep exposure induced the formation of coarse particles from the early stages of exposure. The differences seem to originate in a different state of the matrix, where martensite crystal boundaries facilitate the precipitation of fine Laves phase particles.
A single component can often benefit from being built using more than a single processing technique. Here, we investigated the hybrid additive manufacturing (HAM) of Ti6Al4V using a combination of ...powder-bed fusion (PBF) and direct-energy deposition (DED). The aim was to identify critical areas and assess the performance of the hybrid process relative to the individual processes. The PBF sub-parts were built first, and then completed by DED. The builds were in the horizontal and vertical directions, so we could observe the mechanical anisotropy relative to the build direction. X-ray computed tomography, microstructural examinations, and tensile testing coupled with digital image correlation were employed to assess the parts. The as-built PBF surface can be used to build HAM Ti6Al4V samples with DED, thus eliminating steps like machining. The HAM samples built in horizontally had intermediate tensile strengths of about 1050 MPa, and in the vertical direction, about 860 MPa, i.e., lower than the DED samples. Strength-wise, horizontally built parts exceeded the requirements. However, a reduction in deposition size (especially <102 mm2) promoted a different temperature evolution and, in the worst-case scenario, heat accumulation, which led to the formation of an undesirable microstructure and local plastic deformation in the DED part.
We have investigated the impact of the process parameters for the selective laser melting (SLM) of the stainless steel AISI 316L on its microstructure and mechanical properties. Properly selected SLM ...process parameters produce tailored material properties, by varying the laser’s power, scanning speed and beam diameter. We produced and systematically studied a matrix of samples with different porosities, microstructures, textures and mechanical properties. We identified a combination of process parameters that resulted in materials with tensile strengths up to 711 MPa, yield strengths up to 604 MPa and an elongation up to 31%, while the highest achieved hardness was 227 HV10. The correlation between the average single-cell diameter in the hierarchical structure and the laser’s input energy is systematically studied, discussed and explained. The same energy density with different SLM process parameters result in different material properties. The higher energy density of the SLM produces larger cellular structures and crystal grains. A different energy density produces different textures with only one predominant texture component, which was revealed by electron-backscatter diffraction. Furthermore, three possible explanations for the origin of the dislocations are proposed.
As a surface-hardening technique, plasma nitriding is a common procedure for improving the properties of conventional Ni-based alloys. The diffusion of nitrogen hardens a layer on the surface of the ...alloy, leading to better wear resistance and a higher coefficient of friction, as well as a higher surface hardness. This study reports the effect of plasma nitriding on additive-manufactured (AM) Inconel 625 (IN625) compared to its conventional manufactured and nitrided counterparts. The samples produced with the laser powder-bed fusion (LPBF) process were subsequently plasma nitrided in the as-built condition, stress-relief annealed at 870 °C and solution treated at 1050 °C. The plasma nitridings were carried out at 430 °C and 500 °C for 15 h. The growth kinetics of the nitride layer of the AM samples depends on the prior heat treatments and is faster in the as-built state due to the specific cellular structure. The lower nitriding temperature leads to the formation of expanded austenite in the nitride layer, while at the higher nitriding temperature, the expanded austenite decomposes and CrN precipitation occurs. The XRD and SEM analyses confirmed the presence of two layers: the surface layer and the diffusion layer beneath. The lower nitriding temperature caused the formation of expanded austenite or a combination of expanded austenite and CrN. The higher nitriding temperature led to the decomposition of the expanded austenite and to the formation/precipitation of CrN. The higher nitriding temperature also decreased the corrosion resistance slightly due to the increased number of precipitated Cr-nitrides. On the other hand, the wear resistance was significantly improved after plasma nitriding and was much less influenced by the nitriding temperature.
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•Nitriding of IN625 alloy causes combination of austenite/expanded austenite and diffusion layer.•Growth kinetics depends on production route, heat treatment, nitriding temperature.•At lower nitriding temperature expanded austenite forms in the nitride layer.•Expanded austenite decomposes at higher nitriding temperature and CrN forms.•Better corrosion performance is obtained at lower nitriding temperature.
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•Laser ablation enables controlled biodegradability of Fe–Mn alloy.•Increased corrosion appears due to laser-induced high-temperature oxides.•XPS reveals mechanism for ...biodegradability via shift in surface Fe/Mn ratio to Mn.•Corrosion rate for laser-textured Fe–Mn is 10-times higher as for Fe.•Laser-triggered corrosion is self-driven leading to biodegradability of bulk.
In this study we report the influence of laser ablation on the controlled biodegradability of a Fe–Mn alloy developed for medical implants. After texturing by a nanosecond Nd:YAG laser, the surface expressed extreme super-hydrophilic wetting properties, since laser ablation led to micro-channels and chemical modification resulting in nanostructured metal oxides. The influence of functionalized surface properties on corrosion behavior was examined on molecular level by using X-ray photoelectron spectroscopy. Results reveal that the oxide layer after the laser texturing of Fe–Mn alloy consists mainly of Fe2O3 and FeO, with the content of Mn in the oxide layer being significantly higher than in the bulk. The results of the electrochemical measurements clearly demonstrate the superior biodegradability of the Fe–Mn alloy samples functionalized by laser ablation. Here, the laser-triggered corrosion is self-driven by further production of corrosion products that leads to biodegradability of the whole sample.
Maraging steel grade18Ni300 produced by powder bed fusion (PBF) in its as built condition was plasma nitrided at three different temperatures. The aim of the work was to investigate the impact of the ...nitriding temperature on the microstructural changes as well as on the surface properties such as hardness, wear and corrosion resistance. The microstructural features in the bulk as well as in the nitride layer were investigated using electron-backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-Ray diffraction (XRD) analysis. The bulk microstructure consists of martensite with a small amount of retained austenite, the amount of which increases with a higher nitriding temperature. The nitriding process also causes the formation of precipitates and can therefore also act as an aging treatment. A specific lamellar structure occurs on the surface during the nitriding process, which in the majority of cases consists of the Fe4N phase. The retained austenite also transforms during nitriding to the nitride phase Fe4N. It was found that nitriding at higher temperatures leads to the formation of cracks in the nitride layer. The crack formation is related to nano and micro segregation during the LPBF. These segregations lead to austenite formation, which also takes place along the grain boundaries and transforms during nitriding to Fe4N. Higher nitriding temperatures lead to a thicker nitride compound layer and to better wear resistance. The impact of the cracks on the static mechanical properties is negligible. However, the corrosion resistance is governed by the formation of cracks at higher nitriding temperatures.
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•A higher nitriding temperature increases the amount of austenite and causes cracks.•During nitriding, the austenite on the surface transforms to Fe4N and TiN.•Cracks are related to segregations and transformations of the austenite to nitrides.•A higher wear resistance is obtained at higher nitriding temperatures.•A corrosion enhancement is seen at lower nitriding temperatures.
•Surface chemistry and microstructure of titanium and CoCrMo alloys, for retrieved and new implant materials, were investigated.•Microstructure is a neglected factor in implant design, although it is ...very important in the role of prematurely failed implants.•SEM- EDS and EBSD results show two types of carbides present in the CoCrMo alloys: M23C6 and M7C3.•Thin oxide films protect the Ti alloys from pitting, intergranular and crevice corrosion and are responsible for the excellent biocompatibility.•The thin oxide film protects properly heat-treated CoCrMo alloys from intergranular and crevice corrosion, and improves the biocompatibility.
The surface chemistry and microstructures of titanium alloys (both new and used) and CoCrMo alloys used for hip and knee endoprostheses were determined using SEM (morphology), EBSD (phase analysis), AES and XPS (surface chemistry). Two new and two used endoprostheses were studied. The SEM SE and BE images showed their microstructures, while the EBSD provided the phases of the materials. During the production of the hip and knee endoprostheses, these materials are subject to severe thermomechanical treatments and physicochemical processes that are decisive for CoCrMo alloys.
The AES and XPS results showed that thin oxide films on (a) Ti6Al4V are primarily a mixture of TiO2 with a small amount of Al2O3, while the V is depleted, (b) Ti6Al7Nb is primarily a mixture of TiO2 with a small amount of Al2O3 and Nb2O5, and (c) the CoCrMo alloy is primarily a mixture of Cr2O3 with small amounts of Co and Mo oxides.
The thin oxide film on the CoCrMo alloy should prevent intergranular corrosion and improve the biocompatibility. The thin oxide films on the Ti alloys prevent further corrosion, improve the biocompatibility, and affect the osseointegration.
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