Corrosion is normally an undesirable phenomenon in engineering applications. In the field of biomedical applications, however, implants that 'biocorrode' are of considerable interest. Deploying them ...not only abrogates the need for implant-removal surgery, but also circumvents the long-term negative effects of permanent implants. In this context magnesium is an attractive biodegradable material, but its corrosion is accompanied by hydrogen evolution, which is problematic in many biomedical applications. Whereas the degradation and thus the hydrogen evolution of crystalline Mg alloys can be altered only within a very limited range, Mg-based glasses offer extended solubility for alloying elements plus a homogeneous single-phase structure, both of which may alter corrosion behaviour significantly. Here we report on a distinct reduction in hydrogen evolution in Zn-rich MgZnCa glasses. Above a particular Zn-alloying threshold ( 28 at.%), a Zn- and oxygen-rich passivating layer forms on the alloy surface, which we explain by a model based on the calculated Pourbaix diagram of Zn in simulated body fluid. We document animal studies that confirm the great reduction in hydrogen evolution and reveal the same good tissue compatibility as seen for crystalline Mg implants. Thus, the glassy Mg60+xZn35−xCa5 (0≤x≤7) alloys show great potential for deployment in a new generation of biodegradable implants.
This paper contributes to a better understanding of processing the nickel-based superalloy IN738LC using selective laser melting (SLM). Initially, the basic workability of IN738LC using SLM is ...demonstrated. Subsequently, a comparison between a Gaussian and a doughnut profile is carried out, and clear correlations between the choice of process parameters and the resulting imperfections depending on the chosen laser beam profile are shown. Electron backscatter diffraction (EBSD) measurements show a significant influence of scan strategy and build direction on the texture of the sample, independent of the used laser beam profile. Regardless of the laser beam profile, two contradicting trends complicate the defect-free processing of IN738LC, i.e. a reduction in the crack density can only be realized with increasing porosity. Through microstructural investigations, observed hot cracks are identified to be solidification cracks. Based on a broad experimental study in combination with a numerical solidification study, a theory about the crack initiation mechanism is presented. Finally, by using atom probe tomography (APT), the element zirconium is confirmed as a possible reason for the occurrence of solidification cracks.
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•Porosity and crack density exhibit an inverse relationship.•An analysis about the chemical composition of a grain boundary is shown.•The metallurgical analysis identifies zirconium as a possible reason for hot cracking.•Cross-hatch scan strategy and directional solidification cause a cubic texture.
On the potential of aluminum crossover alloys Stemper, Lukas; Tunes, Matheus A.; Tosone, Ramona ...
Progress in Materials Science/Progress in materials science,
February 2022, 2022-02-00, 20220201, 2022-02-01, Letnik:
124, Številka:
C
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For almost a century commercial aluminum alloys were developed and optimized for high performance in a specific and narrow range of application, which commonly coincides with their industrial ...classification. Overcoming the limitations associated with the modern lightweighting concept requires new alloy design strategies that offer an expanded property portfolio with a better trade-off between formability and achievable strength. The associated materials would be key to circumventing the need for a multi material mix that diminishes the recyclability of the final product. This review summarizes current knowledge about a new class of materials, “crossover alloys”, that combine advantageous properties normally limited to certain classes of commercial aluminum alloys. It focuses on the crossover alloys AlMg/AlCuMg (5xxx/2xxx) and AlMg/AlZnMg(Cu) (5xxx/7xxx). Recently available research data provides indications for superior formability with simultaneously high age-hardening potential, which may pave the way for broader industrial application in the foreseeable future. Because these new alloys exhibit Mg as their major constituent but are – in contrast to commercial AlMg alloys – age hardenable, they do not fit into the current alloy classification scheme. This review formalizes crossover alloys as a potential new aluminum alloy class which features an innovative alloy design methodology.
► The Y-intermetallic can accelerate corrosion and Y can increase the protectiveness of the surface layer. ► In 0.1 M NaCl, the corrosion rate of Mg-Y alloys increased with increasing Y due to the Y ...intermetallic. ► In 0.1 M NaCl, there was filiform corrosion. ► In 0.1 M Na
2SO
4, the corrosion rate of Mg-Y alloys decreased with increasing Y in the range 3–7%Y. ► Hydrogen evolution was observed from particular parts of the alloy surface.
Corrosion of Mg–Y alloys was studied using electrochemical evaluations, immersion tests and direct observations. There were two important effects. In 0.1
M NaCl, the corrosion rate increased with increasing Y content due to increasing amounts of the Y-containing intermetallic. In 0.1
M Na
2SO
4, the corrosion rate decreased with increasing Y content above 3%, attributed to a more protective surface film, despite the intermetallic. The corrosion rate evaluated by electrochemical impedance spectroscopy was somewhat smaller than that evaluated from H evolution as expected from the Mg corrosion mechanism. A mechanism is proposed for filiform corrosion. Direct in situ corrosion observations revealed that a predominant feature was hydrogen evolution from particular parts of the alloy surface.
This study investigates selective laser melting (SLM) of the nickel based superalloy IN738LC and the cobalt based alloy Mar-M509, and identifies the influence of process and material parameters on ...the resulting microstructure. Comprehensive microstructural characterization was performed using electron backscattered diffraction analysis. Significant differences between IN738LC and Mar-M509 were observed with respect to grain size, grain shape and texture sharpness. Alloy IN738LC exhibits coarse and elongated grains with a sharp texture and thus a pronounced mechanical anisotropy. Alloy Mar-M509 shows smaller grains with only moderate structural and mechanical anisotropy. The different microstructural and mechanical characteristics are attributed to the different recovery and recrystallization behavior of IN738LC and Mar-M509. The high stacking fault energy (SFE) of IN738LC results in pronounced recovery of lattice defects without affecting the basic grain structure, whereas the low SFE in Mar-M509 favors recrystallization with the effect of significant grain refinement and weakening of the solidification texture. The effect of microstructure and the structural anisotropy on the orientation-dependent values of the Young’s modulus and the mechanical properties are further discussed.
Interest in magnesium alloys and their applications has risen in recent years. This trend is mainly evident in casting applications, but wrought alloys are also increasingly coming into focus. Among ...the most common forming processes, forging is a promising candidate for the industrial production of magnesium wrought products. This review is intended to give a general introduction into the forging of magnesium alloys and to help in the practical realization of forged products. The basics of magnesium forging practice are described and possible problems as well as material properties are discussed. Several alloy systems containing aluminum, zinc or rare earth elements as well as biodegradable alloys are evaluated. Overall, the focus of the review is on the process control and processing parameters, from stock material to finished parts. A discussion of the mechanical properties is included. These data have been comprehensively reviewed and are listed for a variety of magnesium forging alloys.
A key question in materials science is how fast properties evolve, which relates to the kinetics of phase transformations. In metals, kinetics is primarily connected to diffusion, which for ...substitutional elements is enabled via mobile atomic-lattice vacancies. In fact, non-equilibrium vacancies are often required for structural changes. Rapid quenching of various important alloys, such as Al- or Mg-alloys, results for example in natural aging, i.e. slight movements of solute atoms in the material, which significantly alter the material properties. In this study we demonstrate a size effect of natural aging in an AlMgSi alloy via atom probe tomography with near-atomic image resolution. We show that non-equilibrium vacancy diffusional processes are generally stopped when the sample size reaches the nanometer scale. This precludes clustering and natural aging in samples below a certain size and has implications towards the study of non-equilibrium diffusion and microstructural changes via microscopy techniques.
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Mg-based biodegradable materials are considered promising candidates in the paediatric field due to their favourable mechanical and biological properties and their biodegrading ...potential that makes a second surgery for implant removal unnecessary. In many cases the surgical fixation technique requires a crossing of the growth plate by the implant in order to achieve an adequate fragment replacement or fracture stabilisation. This study investigates the kinetics of slowly and rapidly degrading Mg alloys in a transphyseal rat model, and also reports on their dynamics in the context of the physis and consecutive bone growth. Twenty-six male Sprague–Dawley rats received either a rapidly degrading (ZX50; n = 13) or a slowly degrading (WZ21; n = 13) Mg alloy, implanted transphyseal into the distal femur. The contralateral leg was drilled in the same manner and served as a direct sham specimen. Degradation behaviour, gas formation, and leg length were measured by continuous in vivo micro CT for up to 52 weeks, and additional high-resolution µCT (HRS) scans and histomorphological analyses of the growth plate were performed. The growth plate was locally destroyed and bone growth was significantly diminished by the fast degradation of ZX50 implants and the accompanying release of large amounts of hydrogen gas. In contrast, WZ21 implants showed homogenous and moderate degradation performance, and the effect on bone growth did not differ significantly from a single drill-hole defect.
This study is the first that reports on the effects of degrading magnesium implants on the growth plate in a living animal model. The results show that high evolution of hydrogen gas due to rapid Mg degradation can damage the growth plate substantially. Slow degradation, however, such as seen for WZ21 alloys, does not affect the growth plate more than drilling alone, thus meeting one important prerequisite for deployment in paediatric osteosynthesis.
Additive manufacturing (AM) offers significant benefits towards optimized lightweight structures for aircraft and space applications. High-strength aluminium alloys are of special interest to reach a ...maximum mass reduction. The paper presents the development of appropriate selective laser melting (SLM) processing windows for a scandium modified aluminium alloy (Scalmalloy®), reaching densities >99%. The mechanical properties of as-processed material are analyzed, pointing out a comparably low anisotropy with regard to the build orientation. A fine-grained microstructure is observed next to regions of coarser, elongated grains. The paper discusses the observed microstructure, and concludes with suggestions for innovative material design for AM.
Mechanism of low temperature deformation in aluminium alloys Gruber, Belinda; Weißensteiner, Irmgard; Kremmer, Thomas ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
09/2020, Letnik:
795
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This study investigates differences in the deformation mechanisms between room temperature (296 K) and cryogenic temperatures (77 K) and their advantages for low temperature formability in alloys EN ...AW 1085, EN AW 5182 and EN AW 6016. Compared to room temperature behaviour, tensile tests showed an overall increase in yield strength, ultimate tensile strength and uniform elongation with differences among the principal alloy types. In general, the improved mechanical properties result from higher strain hardening rates at lower temperatures. The application of an extended Kocks-Mecking approach showed a significant reduction of the dynamic recovery and suggested higher dislocation densities upon cryogenic deformation. This was confirmed via in-situ synchrotron experiments, which also reveal a higher proportion of screw dislocations. Moreover, kernel average misorientation maps from electron backscattered diffraction and in-situ cryogenic deformation in a transmission electron microscope displayed a more uniform dislocation arrangement with a reduction of slip lines and less highly misaligned areas after deformation at lower temperatures. Supported by a detailed characterization of the microstructure and its dislocation structure, the associated fundamental mechanisms we reveal, which are at the origin of the exceptional improvement in mechanical properties, are extensively discussed.
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•Cryogenic compared to RT deformation rises strength and deformability.•The reduced dynamic recovery leads to an increased dislocation density.•At cryogenic temperatures a higher fraction of screw dislocations was found.•The critical dislocation localisation is reached at higher deformations.
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