Zinc- and calcium-containing magnesium alloys, denominated ZX alloys, excel as temporary implant materials because of their composition made of physiologically essential minerals and lack of commonly ...used rare-earth alloying elements. This study documents the specific role of nanometric intermetallic particles (IMPs) on the in vitro and in vivo biocorrosion behavior of two ZX-lean alloys, Mg‒Zn1.0‒Ca0.3 (ZX10) and Mg‒Zn1.5‒Ca0.25 (ZX20) (in wt.%). These alloys were designed according to thermodynamic considerations by finely adjusting the nominal Zn content towards microstructures that differ solely in the type of phase composing the IMPs: ZX10, with 1.0 wt.% Zn, hosts binary Mg2Ca-phase IMPs, while ZX20, with 1.5 wt.% Zn, hosts ternary IM1-phase IMPs. Electrochemical methods and the hydrogen-gas evolution method were deployed and complemented by transmission electron microscopy analyses. These techniques used in concert reveal that the Mg2Ca-type IMPs anodically dissolve preferentially and completely, while the IM1-type IMPs act as nano-cathodes, facilitating a faster dissolution of ZX20 compared to ZX10. Additionally, a dynamically increasing cathodic reactivity with progressing dissolution was observed for both alloys. This effect is explained by redeposits of Zn on the corroding surface, which act as additional nano-cathodes and facilitate enhanced cathodic reaction kinetics. The higher degradation rate of ZX20 was verified in vivo via micro-computed tomography upon implantation of both materials into femurs of Sprague DawleyⓇ rats. Both alloys were well integrated with direct bone‒implant contact observable 4 weeks post operationem, and an appropriately slow and homogeneous degradation could be observed with no adverse effects on the surrounding tissue. The results suggest that both alloys qualify as new temporary implant materials, and that a minor adjustment of the Zn content may function as a lever for tuning the degradation rate towards desired applications.
In Mg‒Zn‒Ca (ZX)-lean alloys Zn is the most electropositive element, and thus requires special attention in the investigation of biocorrosion mechanisms acting on these alloys. Even a small increase of only 0.5 wt.% Zn is shown to accelerate the biodegradation rate in both simulated body conditions and in vivo. This is possible due to Zn's role in influencing the type of intermetallic particles (IMPs) in these alloys. These IMPs in turn, even though minute in size, are shown to govern the biocorrosion behavior on the macroscopic scale. The deep understanding gained in this study on the role of Zn and of the IMP type it governs is crucial to ensuring a safe and controllable implant degradation.
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Selective Laser Melting (SLM) is an additive manufacturing technology that offers significant potential for lightweight applications in space, aerospace, and automotive industries as well as in ...mechanical engineering. Structural aluminium alloys are therefore of special interest. Scalmalloy® is a scandium-modified Al-Mg alloy which displays exceptional properties when processed by SLM. These properties are predominately related to a generally very fine grained microstructure. However, the fine grained microstructure interspersed with coarser grained regions. Microstructural analyses indicate that the temperature regime and the particle precipitation behaviour are responsible for the duplex grain structures. In melt pool areas close to the pool base, numerous Al3(Sc,Zr), Al-Mg-oxides and mixed particles act as nuclei for Al matrix solidification, leading to the formation of a very fine grained microstructure. In hot melt pool areas with T>800°C the majority of particles dissolve and growth of coarse columnar grains takes place. Better understanding of the formation mechanisms for these two distinct different structures will help pave the way towards newly designed alloy compositions for the SLM process.
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•Comprehensive description of the microstructure.•Additive manufactured Scandium and Zirconium modified AlMg alloys exhibit a bi-modal grain size distribution.•The formation fine-grained microstructure is closely related to the presence of intermetallic particles in the melt pool.•Precipitate survival is affected by local temperature distribution in the melt-pool.
Deformation anisotropy of extruded Mg–6% Al–1% Zn alloy has been investigated on specimens with different tilt angles relative to the extrusion direction. Calculations of the orientation factors for ...basal slip and of the strains caused by
{1
0
1
̄
2}
twinning were done for a slightly idealised texture. This quantification of the two dominating deformation modes was used to explain the marked mechanical anisotropy of the extruded magnesium alloy. Basal slip as well as
{1
0
1
̄
2}
twinning is inhibited in extrusion direction under tensile loads, which results in high yield strength. Any other testing direction and/or compressive loads are capable of activating slip and/or twinning and yield stress is significantly lower under such conditions. The lattice reorientation of 86.3° caused by twinning has a large influence on the deformation behaviour of a pre-deformed specimen, since the twinned areas are capable of untwinning during reloading in the opposite direction.
This study presents a design strategy for Al–Mg–Si alloys to control natural aging. Recently, trace addition of Sn was shown to suppress natural aging for up to two weeks, which was explained by the ...strong trapping of vacancies to Sn atoms. Here we explore the effect of solution treatment temperature, the combination of trace elements such as Sn and In, and the composition of main hardening elements Mg, Si and Cu on natural aging. The results are discussed based on the dissolvable amount of trace elements and their effect on diffusion retardation, and solute clustering mechanisms in Al–Mg–Si alloys. Thermodynamic calculations using the CALPHAD approach show that maximum retardation of natural aging is achievable at the highest trace element solubility, which exists at significantly different solution treatment temperatures for Sn or In. The effects of Mg, Si and Cu content on natural aging kinetics are interpreted via their influence on the Sn solubility and clustering mechanisms. It is proposed that Sn additions reduce the concentration of excess vacancies, which is most important for early Si clustering, and that the effect of Cu is comparable to the effect of Sn, but less pronounced. Based on the investigated parameter space, a design concept is proposed and an Al–Mg–Si alloy showing suppression of natural aging for >6 months and significant artificial aging potential is demonstrated.
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Biodegradable Mg alloys are of great interest for osteosynthetic applications because they do not require surgical removal after they have served their purpose. In this study, fast-degrading ZX50 ...Mg-based implants were surface-treated by micro-arc oxidation (MAO), to alter the initial degradation, and implanted along with untreated ZX50 controls in the femoral legs of 20 male Sprague–Dawley rats. Their degradation was monitored by microfocus computed tomography (μCT) over a total observation period of 24weeks, and histological analysis was performed after 4, 12 and 24weeks. While the MAO-treated samples showed almost no corrosion in the first week, they revealed an accelerated degradation rate after the third week, even faster than that of the untreated ZX50 implants. This increase in degradation rate can be explained by an increase in the surface-area-to-volume ratio of MAO-treated implants, which degrade inhomogeneously via localized corrosion attacks. The histological analyses show that the initially improved corrosion resistance of the MAO implants has a positive effect on bone and tissue response: The reduced hydrogen evolution (due to reduced corrosion) makes possible increased osteoblast apposition from the very beginning, thus generating a stable bone–implant interface. As such, MAO treatment appears to be very interesting for osteosynthetic implant applications, as it delays implant degradation immediately after implantation, enhances fracture stabilization, minimizes the burden on the postoperatively irritated surrounding tissue and generates good bone–implant connections, followed by accelerated degradation in the later stage of bone healing.
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Deformation dilatometry and semi-industrial extrusion were used to investigate the effect of different thermomechanical processing routes on the microstructure and mechanical ...properties of the low-alloy Mg alloys ZX10 (Mg–1Zn–0.3Ca) and ZX00 (Mg–0.5Zn–0.15Ca). It is shown that the deliberately adjusted formation of intermetallic particles beneficially influences dynamic recrystallization and grain growth, with the result of a fine-grained microstructure (grain size<2μm). The presence of unrecrystallized regions with its unfavorable influence on ductility and mechanical anisotropy can be controlled by the selection of an indirect extrusion mode. Meta-dynamic recrystallization generates almost fully recrystallized microstructures and hence the desired properties, which are characterized by high strength (yield strength≈240MPa), simultaneously high ductility (elongation to fracture≈30%), and low structural and mechanical anisotropy. These properties are of great interest for light-weight applications and for deployment as biodegradable implants in medical technology.
This study first provides a concise review of natural aging in Al–Mg–Si alloys and its effect on artificial aging. The second part investigates prolonged natural aging at different temperatures for ...>500days of commercial and trace element added alloys. Together, the two parts improve the picture of underlying mechanisms and refine suggestions regarding the five stages of natural aging. Trace Sn- or Sn+In-added alloys show a trend towards higher activation energies of clustering, and a higher temperature dependency of natural aging than commercial alloys. This is attributed to an additional contribution of a thermally activated vacancy release from Sn- and In-vacancy pairs. Sn and In additions are suggested to decrease cluster number density while increasing cluster size. Prolonged natural aging increasingly retards artificial aging kinetics. This is interpreted according to increasingly slower cluster dissolution kinetics and slower preferential growth of β´´ needles. The reachable strength however seems to strongly depend on the ratio and size of coarse β´´ needles, preferentially grown, and the number of fine re-precipitated β´´ needles. Artificial aging after prolonged natural aging at 45°C increases the artificial aging peak hardness due to a lower density of larger clusters than at lower temperatures.
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•A refined overall mechanistic picture of natural aging of Al–Mg–Si alloys is presented.•Vacancy trapping trace elements retard natural aging (up to several years in special cases).•Trace elements increase the temperature dependency of natural aging.•Prolonged natural aging generally retards artificial aging kinetics.
In this study we describe the effect of the main alloying elements Si, Cu and Ni on the thermal properties of hypoeutectic and near-eutectic Al–Si foundry alloys. By means of systematic variations of ...the chemical composition, the influence of the amount of ‘second phases’ on the thermal conductivity, thermal expansion coefficient, and thermal shock resistance is evaluated. Thermodynamic calculations predicting the phase formation in multi-component Al–Si cast alloys were carried out and verified using SEM, EDX and XRD analysis. The experimentally obtained data are discussed on a systematic basis of thermodynamic calculations and compared to theoretical models for the thermal conductivity and thermal expansion of heterogeneous solids.
▶ We discovered that addition of small amounts of Y has a very positive effect on the hot tearing susceptibility (HTS) of Mg–Zn–Zr alloys. ▶ We attribute the reduced HTS to the effect of Y on the ...solidification path at the terminal period of solidification; Y addition generates an increase in the solidus temperature. ▶ We explain the increase in the solidus temperature by means of a simple “virtual element enrichment” approach: it is caused by the formation of the ternary phase Mg
3YZn
6.
The influence of Y additions on the hot tearing behaviour of Mg–Zn alloys was investigated in this study. In permanent mould castings and in direct chill cast ingots, alloying of a few wt.% Y results in a significant reduction of hot tearing susceptibility. The reduced susceptibility is attributed to the effect of Y on the solidification path at the terminal period of solidification: it increases the solidus temperature and thus shortens the solidification path, which in turn reduces the terminal freezing range. Via thermodynamic calculations it is shown that this is caused by the formation of the ternary phase Mg
3YZn
6. Using a simplified version of Clyne and Davies’ model, the influence of the terminal freezing range on hot tearing susceptibility is clearly illustrated.
With the aim of obtaining materials with high thermal conductivities for solid state thermal management applications, metal-matrix composites were produced by reinforcing aluminum and ...aluminum–silicon with diamond single crystals via two different liquid metal infiltration techniques – gas pressure infiltration and mechanically assisted infiltration (squeeze casting). The obtained composites exhibited thermal conductivities as high as 670
W/mK, but also as low as 130
W/mK. The large variation in the thermal conductivities can be related to the microstructural characteristics of the interface between diamond and the metal-matrix. On fracture surfaces of the composites, it was found that preferential adhesion between aluminum and diamond occurs on the {1
0
0} faces of diamond. Chemical and electrochemical etching treatments of the composites along with TEM observations of interfacial cross-sections suggest that this adhesion may be attributed to the local formation of aluminum carbide at the diamond surface. The contact time between melt and diamond during processing and also the addition of silicon to the matrix material were found to significantly affect the thermal conductivity of the composites by modification of the interface.
