Materials Science Forum Vol. 1016

Abstract: The influence of surface topography on the corrosion behaviors of DP590 steel was studied by damp heat tests and electrochemical measurements in this paper. Results show that the corrosion behaviors of DP590 steel cannot be evaluated by using average surface roughness (Sa) due to the little difference of surface roughness. Reduced valley depth (Svk) showed a good correlation with corrosion resistance in the potentialdynamic polarization test, and the greater Svk, the better the corrosion resistance. Furthermore, DP590 sample with higher proportion of peak on the surface tended to have more corrosion points during the damp heat test since peaks were electrochemically active.

Abstract: Hot stamping, also known as press hardening in the context of sheet steel, has steadily gained relevance in the automotive industry, starting off as a specialist application and turning into a staple technique in the production of safety cage products in little more than a decade. However, despite the weight reduction offered by martensitic steels, further improvement could be obtained by substituting these components by high-performance aluminium. In this regard, the very same process of hot stamping could be employed to attain the required combination of shape complexity and mechanical properties at a reasonable cost for mass-market application, if the limitations imposed by cycle time and process window could be overcome. In this work, the feasibility of hot stamping of 6000-series aluminium alloy sheet is studied, first in dilatometry experiments and later in semi-industrial conditions in a pilot facility. A cycle time shortening strategy is employed, and compared to the conventional thermal cycle in terms of implementation and obtained results. In addition to basic characterization, aluminium thus processed is studied in terms of fracture toughness, in order to obtain data relevant to crashworthiness that can be readily compared with alternative materials.

Abstract: The performance of a vibration power generator using a single crystal core of Fe–Ga alloy was compared with that of a generator using a Fe–Ga alloy polycrystal core with a similar Ga concentration. When the generator using the polycrystal core was forcibly vibrated by 1-G acceleration, the vibration frequency dependence of the open-circuit voltage showed a peak with a maximum value of about 0.14 V at the first resonance frequency due to the inverse magnetostrictive effect. On the other hand, the generator using a single crystal core with a <100> direction parallel to the external stress direction exhibited a maximum value of about 0.26 V, about two-times larger than that of the device using the polycrystal core. Consequently, a vibration energy generator using a single crystal core of Fe–Ga alloy has advantages in performance over a generator using a polycrystal core.

Abstract: Self-organized TiO₂ nanotubes were generated on the surface of the designed alloy Ti-4.7Mo-4.5Fe (TMF55) by electrochemical anodization process to investigate the effect of nanostructured on the biocompatibility. The biocompatibility of the designed alloys showed very promising results compared to those of Ti-6Al-4V ELI alloy, especially for the untreated and nanostructured surfaces of the specimens with diameter size less than 35 nm. By increasing the diameter of nanotube, the biocompatibility is decreased. The most convenient compatible alloy was in favor of TMF8 alloy, making this V-free low-cost alloy is a promising candidate for replacing the commercial Ti-6Al-4V ELI alloy in biomedical applications. Keywords: Self-organized TiO₂ nanotubes, biocompatibility, Titanium alloys, Cell Counting Kit-8, WST-8 assay.

Abstract: The development of new low-cost alloys composed of common elements that show high biocompatibility and mechanical properties matching with human bone is the target of many researches recently. Design and controlling the mechanical properties of newly developed set of Ti-xFe-3Zr-yNb (x=3-8 & y=2-3, at.%) low-cost alloys through applying different thermomechanical treatments is the aim of this work. Fe-content in the present designed alloys is changing in the range 3 to 8 at.%. The hardness and Young's modulus of the alloys were measured for the alloys in the solution treated, hot rolled and subsequent ageing at 400 °C and 550 °C. The phases separation and hence hardness of the aged alloys at 400 °C and 550 °C are highly dependent on the Fe-content in the alloy. The Young's modulus of the alloys is also changing with the Fe-content and heat treatment, where lowest modulus (~80GPa) is shown in the Ti-5Fe-3Zr-3Nb alloy in the solution treated condition.

Abstract: The application of “masteralloys” as alloying carriers in Powder Metallurgy (PM) steels enables the introduction of highly oxygen‑sensitive alloying elements for advanced PM‑steels, by using a tailored liquid phase. The characteristics of the liquid and its interaction with the iron base powder are determining factors for final microstructure and dimensional behaviour. In this study, theoretical calculations and experimental findings are presented for the masteralloy systems Fe_Mn_Si_C, Fe_Cr_Si_C and Fe_Mn_Cr_Si_C. Lowered melting temperatures and narrow melting temperature intervals could be achieved. The interaction between Fe base material and the masteralloys was studied by infiltration and DTA experiments. It was found that by adjusting the C and Si content in the masteralloy, liquids with widely varying properties could be obtained. This might be a key for tailoring microstructures, properties and dimensional stability of advanced PM‑steels.

Abstract: Selective Laser Melting (SLM) builds a metallic part in a layer-by-layer mode with growth occurring along the vertical axis. Metallic powder layers are melted by a laser beam by programmed scan sequences inducing specific mechanical properties in the as-built samples according to process parameters. Post heat treatments are usually performed to optimise the mechanical behaviour. In this work, the effects induced by heat treatments at 175°, 200° and 225°C on SLMed bars of Al10SiMg were investigated as function of distance from the substrate plate. The bars were 300 mm height and in the as-built condition, Vickers microhardness and tensile strength decreased along the built direction, while the elongation increased from the bottom to the top of the billet. After heat treatments, Vickers microhardness resulted lower of 10HV at the top of the bar compared to its bottom in contact with the hot substrate; microhardness decreased with time at constant temperature compared to the as-built. Tensile properties showed variations of 50 MPa and 1% elongation between the top and the bottom of the billet when aging was performed at 175°C for 4h; the strength and ductility gradients were reduced to 20 MPa and 0,5% respectively by increasing the aging time to 6h. Microstructure investigations performed by scanning electron microscopy confirmed the different evolution of Silicon particles and precipitated particles at different height of the bars.

Abstract: The aim of the current study is to design multiaxial forging (MAF) schedules in order to achieve submicron-grained (<1μm) structure in a microalloyed (MA) steel as well as an interstitial-free (IF) steel, which could impart a good combination of yield strength and tensile ductility. At the same time, an effort has been made to evaluate the fracture toughness characteristics by conducting 3-point bend tests and computing the K_Q, K_ee and J-integral values of ultrafine grained (UFG) samples and correlating them with the microstructure, besides evaluating the other mechanical properties. Fatigue strength in the high cycle fatigue (HCF) regime were also investigated and fracture mechanisms analyzed and comparison established between differently processed samples. The microstructural analysis was performed using transmission electron microscopy (TEM) and Electron backscatter diffraction (EBSD) and results corroborated with the mechanical properties. Superior combinations of yield strength (YS), ductility (% El.), fracture toughness (K_ee) and high cycle fatigue strength (σ_f) were obtained under certain conditions, i.e., i) MA steel: intercritical (α+γ) phase regime (~A_r1) controlled and 15-cycle multiaxially forged (MAFed) (YS=1027MPa, %El.=8.3%, σ_f=355MPa and K_ee=90MPa√m), and ii) IF steel: ferritic region (<A_r1) controlled 18-cycle MAFed (YS=881MPa, %El.=11.2%, σ_f=255MPa and K_ee=97MPa√m). In the case of MA steel, an enhancement of the fatigue and fracture toughness properties can be ascertained following the formation of uniformly distributed nanosized fragmented cementite (Fe₃C) particles (~35nm size) present in the submicron sized (average ~280nm size) ferritic microstructure. In contrast, in the case of IF steel, this is ascribed to the development of submicron sized ferrite grains (average ~320nm) along with a high density of dislocation substructures. These fine dislocation cells/substructures along with the nanosized Fe₃C particles could effectively block the initiation and propagation of cracks and thereby enhance the fatigue endurance and fracture toughness of the steel. Superior fracture toughness along with high mechanical properties in submicron-grained condition render the two steels highly useful for high-strength structural applications.

Abstract: The molybdenum is one of the most important refractory metals used in aerospace industry. The main disadvantage of Mo is low oxidation resistance at elevated temperature and the using of protective coatings is necessary. In present article the new types of protective coatings produced by slurry method were developed. The slurries contained Al nanopowder and Si powder as well as non-organic binder (H₂CrO₄ and water). After immersion and drying the samples with slurries were heat treated at 1000°C in Ar atmosphere. The thickness of obtained coatings was in range 10-20 μm. The presences of phases form Mo-Al as well Mo-Si systems was analyzed using scanning electron microscopy. The developed coatings were used as a bond coat for ceramic layer produced by plasma spray physical vapour deposition method (PS-PVD). In this process the columnar ceramic layer contains yttria stabilized zirconia (YSZ) was obtained wit thickness above 100 μm. The obtained results showed that it is possible to obtain TBC coating on molybdenum contained Al-Si bond coat and outer YSZ ceramic layer. The proposed coating can be used in aerospace applications.

Abstract: This study investigates the influence of the substitution of chromium (-0.8 wt.−%) by manganese (+1.3 wt.−%) in a standard quench and tempering steel and the predictability of property changes through simple simulations, only dependent on the chemical composition. The substitution of chromium by manganese leads to an increased hardness (+50 HV10) and a reduction of the critical cooling speed from 19 K s−1 for the reference alloys to 9 K s−1 for the new alloy and a nearly constant hardness of (600 HV10) after Jominy-test. The commercial software JMATPRO is used to simulate and predict key properties for the industrial production. It is shown that a successful simulation of phase transformation temperatures and the general directions of change can be predicted, but more complex properties like critical cooling rates or hardenability need more sophisticated methods.