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.
This study investigates the impact of deep cryogenic treatment (DCT) on microstructure and properties of a martensitic stainless steel AISI 431. The reasonably simplistic microstructure tailoring ...with DCT delivers a novel pathway towards advanced 3rd generation high-strength steels (AHSS), whilst utilizing existing alloys and chemistry. The resulting microstructural changes deliver improved strength and hardness of the material, whilst still sustaining good elongation properties. DCT reduces the material's toughness but improves its deformation resistance and wear resistance. The study reveals that DCT modifies the microstructural evolution during tempering. During the conventional treatment, AISI 431 develops austenite reversion transformation (ART), which allows the high strength and elongation capabilities formed with transformation-induced plasticity (TRIP). In contrast, DCT progresses the microstructure even further by retransforming the reverted austenite formed through ART into tertiary α-martensite, ε-martensite and carbides that increase the strength and deformation behavior of AISI 431. The novel microstructural modification that we have entitled cryogenic austenite retransformation (CAR), is both experimentally and theoretically assessed. The underlying transformation mechanisms of CAR and individual stages of DCT were evaluated with ex-situ electron backscatter diffraction and transmission electron microscopy. The impact of DCT on the modified transformation capabilities of AISI 431 is discussed in relation to possible implementation in automotive industry as a universally applicable treatment procedure.
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•Deep cryogenic treatment (DCT) modifies the microstructure of AISI 431.•Transformation of reverted austenite is progressed with tempering after DCT.•DCT modifies tensile response, wear resistance and fracture mechanism of AISI 431.•DCT allows novel combination of properties and microstructure tailoring.•Properly treated AISI 431 has potential as a new candidate of next generation AHSS.
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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.
We present the modification of Ti-6Al-4V surfaces with a diode end-pumped Nd:YVO4 laser by varying the distance between laser-produced micro(μ)-channels. We analyzed the influence of laser texturing ...on the morphology, microstructure, surface and corrosion properties of Ti-6Al-4V. SEM imaging reveals a characteristic μ-channel pattern with different scan line separations, while electron backscatter diffraction (EBSD) indicates that laser texturing with the current parameters influences the microstructure up to 2 µm deep with the most significant influence at the tips, where melting and rapid solidification occur. The Vickers hardness test indicates a surface hardening effect of the laser-textured compared to the as-received Ti-6Al-4V surfaces. The XPS analysis showed that the oxide layer on the laser-textured samples was considerably thicker compared to the as-received sample, at 20 and 7 nm, respectively. We observed that the wettability was strongly correlated with the scan line separation. The results show increased hydrophobicity with increased scan line separation. The corrosion resistance was improved for laser-textured surfaces compared to the as-received surface and increased with the scan line separation.
Stainless steels are important in various industries due to their unique properties and durable life cycle. However, with increasing demands for prolonged life cycles, better mechanical properties, ...and improved residual stresses, new treatment techniques, such as deep cryogenic treatment (DCT), are on the rise to further push the improvement in stainless steels. This study focuses on the effect of DCT on austenitic stainless steel AISI 304L, while also considering the influence of solution annealing temperature on DCT effectiveness. Both aspects are assessed through the research of microstructure, selected mechanical properties (hardness, fracture and impact toughness, compressive and tensile strength, strain-hardening exponent, and fatigue resistance), and residual stresses by comparing the DCT state with conventionally treated counterparts. The results indicate the complex interdependency of investigated microstructural characteristics and residual stress states, which is the main reason for induced changes in mechanical properties. The results show both the significant and insignificant effects of DCT on individual properties of AISI 304L. Overall, solution annealing at a higher temperature (1080 °C) showed more prominent results in combination with DCT, which can be utilized for different manufacturing procedures of austenitic stainless steels for various applications.
•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.
The morphology of the eutectic chromium carbides in the microstructure of as-cast AISI D2 tool steel was modified by adding small amounts of rare-earth elements (REEs) to the melt. As a result of ...these REE additions the eutectic carbide morphological type was changed from lamellar to globular. Similar phenomena have already been reported for various tool steels, but no complete theoretical explanation has been provided. Here, we propose a new model that is derived from first-principles thermodynamic calculations based on the phase-field modeling of the eutectic reaction. Using this new approach, where the decomposition of the phase-boundary surface-energy term is divided into the isotropic and anisotropic parts, we were able to account for the transition from a lamellar to a globular eutectic morphology in REE-modified AISI D2 tool steel.
Lab-scale investigations on the processing of small powder volumes are of special importance for applications in additive manufacturing (AM) techniques. Due to the technological importance of ...high-silicon electrical steel, and the increasing need for optimal near-net-shape AM processing, the aim of this study was to investigate the thermal behavior of a high-alloy Fe-Si powder for AM. An Fe-6.5wt%Si spherical powder was characterized using chemical, metallographic, and thermal analyses. Before thermal processing, the surface oxidation of the as-received powder particles was observed by metallography and confirmed by microanalysis (FE-SEM/EDS). The melting, as well as the solidification behavior of the powder, was evaluated using differential scanning calorimetry (DSC). Due to the remelting of the powder, a significant loss of silicon occurred. The morphology and microstructure analyses of the solidified Fe-6.5wt%Si revealed the formation of needle-shaped eutectics in a ferrite matrix. The presence of a high-temperature phase of silica was confirmed by the Scheil-Gulliver solidification model for the ternary model Fe-6.5wt%Si-1.0wt%O alloy. In contrast, for the binary model Fe-6.5wt%Si alloy, thermodynamic calculations predict the solidification exclusively with the precipitation of b.c.c. ferrite. The presence of high-temperature eutectics of silica in the microstructure is a significant weakness for the efficiency of the magnetization processes of soft magnetic materials from the Fe-Si alloy system.
Glow plugs in the automotive industry are based on heating resistors made from FeCrAl alloys, often referred to as Kanthal. Metals and alloys exposed to high temperatures tend to react with the ...surrounding atmosphere, often resulting in high-temperature corrosion. Some high-temperature alloys use aluminium to form an Al2O3 protective layer in order to improve the oxidation resistance. In the temperature range above 1200 °C, SiO2 and Al2O3 are the two oxides capable of forming an efficient, protective barrier against further oxidation. The alloy nominally requires more than 4 wt % Al to form a continuous protective layer. In this paper we have investigated the Al2O3 formation and growth on the surface of iron aluminides prepared using different heat-treatment parameters to achieve a continuous protective layer on the surface of the heating resistors. SEM/EDX mapping analyses were performed to confirm the elemental distribution on the cross-section of the resistors heat treated in air or pure oxygen, or in a humid H2/Ar atmosphere, for different times and temperatures to achieve a continuous coverage of the oxide layer. The results of different heat treatments and their influences on the oxide growth are compared.
•Development of stabile surface oxide for increasing lifetime of material.•High-temperature corrosion resistance of the applied material.•Temperature as a main factor for thickness of the surface protective oxide.•Controlled atmosphere for non-permeable Al oxide surface protection.
Laser-powder bed fusion (LPBF) is one of the preferred techniques for producing Co-Cr metal structures for dental prosthodontic appliances. However, there is generally insufficient information about ...material properties related to the production process and parameters. This study was conducted on samples produced from three different commercially available Co-Cr dental alloys produced on three different LPBF machines. Identically prepared samples were used for tensile, three-point bending, and toughness tests. Light microscopy (LM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) analyses of microstructure were performed after testing. Differences were observed in microstructures, which reflected statistically significant differences in mechanical properties (one-way analysis of variance (ANOVA) and Scheffé post hoc test (α = 0.05)). The material produced on the 3D Systems DMP Dental 100 had 24 times greater elongation ε than the material produced on the Sysma MySint 100 device and the EOS M100 machine. On the other hand, the material produced on the EOS M100 had significantly higher hardness (HV0.2) than the other two produced materials. However, the microstructure of the Sysma specimens with its morphology deviates considerably from the studied group. LPBF-prepared Co-Cr dental alloys demonstrated significant differences in their microstructures and, consequently, mechanical properties.
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