ABSTRACT
Desirable properties including strength, ductility and extrudability of 6060 Al-alloys are highly dependent on processing to control the development of microstructural features. In this study, the process chain of an extrudable 6060 Al-alloy was modeled in an Integrated Computational Materials Engineering framework and validated experimentally via quantitative SEM-EDX and TEM. All critical processing stages were considered including casting, homogenization heating and holding, extrusion cooling and two-stage aging. Segregation and intermetallics formation were accurately predicted and experimentally verified in the as-cast condition. Diffusion simulations predicted the dissolution of intermetallics and completion of ß-AlFeSi to α-AlFeSi transformation during homogenization, in excellent agreement with quantitative SEM-EDX characterization. Precipitation simulations predicted the development of a ßâ³ strengthening dispersion during extrusion cooling and aging. Needle-shaped ßâ³ precipitates were observed and analyzed with quantitative high-resolution TEM, validating predictions. Ensuing precipitation strengthening was modeled in terms of aging time, presenting good agreement with yield strength measurements. Precipitate-Free Zones and coarse, metastable ß-type particles on dispersoids and grain boundaries were investigated. The proposed integrated modeling and characterization approach considers all critical processing stages and could be used to optimize processing of extrudable 6xxx Al-alloys, providing insight to mechanisms controlling microstructural evolution and resulting properties.
ABSTRACT
Wrought magnesium alloys have received attention due to their potential application as lightweight materials. However, their use is limited by their poor corrosion resistance. Rare earth additions have the potential to enhance corrosion resistance. The present work included a microstructural investigation and corrosion testing of the alloy WE-43, containing Nd and Y, which was compared against the more conventional compositions of AZ31 and AZ61 alloys. All three alloys exhibited a recrystallized equiaxed structure after hot rolling with the presence of second phases-precipitates. The WE-43 alloy exhibited a better corrosion resistance than AZ31 and AZ61 under salt fog testing, indicated by the lower depth of attack and lower weight loss. The second phases in the microstructure of AZ31 and AZ61 alloys determined their corrosion resistance. The second phases in the AZ31 and AZ61 alloys (based on Al-Mg and Al-Mn phases) were nobler than the Mg matrix and catholically acted, thus sacrificing the Mg matrix. The superior corrosion resistance of WE43 was due to the incorporation of Y in the oxide/hydroxide film. In addition, the second phases in the WE43 consisted of Nd and Y and were less noble than the Mg-matrix. Thus, they acted as anodic sites protecting the Mg-matrix. The above results show the beneficial effect of rare earth additions to wrought Mg alloys towards increased corrosion resistance.
ABSTRACT
Cold roll forming used in the manufacturing of lightweight steel profiles for racking storage systems is associated with localized, non-uniform plastic deformations in the corner sections of the profiles, which act as fatigue damage initiation sites. In order to obtain a clearer insight on the role of existing plastic deformation on material fatigue performance, the effect of plastic pre-straining on the low cycle fatigue behavior of S355MC and S460MC steels was investigated. The steels were plastically deformed at different pre-strain levels under tension, and subsequently subjected to cyclic strain-controlled testing. Plastic pre-straining was found to increase cyclic yield strength, decrease ductility, and induce cyclic softening, which, in S460MC, degrades fatigue resistance compared to the unstrained material. In unstrained conditions, the materials present a cyclic softening to hardening transition with increasing plastic strain amplitude, which in S355MC occurs at lower strain amplitudes and degrades its fatigue resistance with regard to the pre-strained material. Pre-straining also leads to a reduction in transition life from low to high cycle fatigue. SEM fractography, performed following the onset of crack initiation, revealed that plastic pre-straining reduces the fatigue fracture section as well as striation spacing, predominantly in the S355MC steel.
