Low-Cycle Fatigue of Ultra-Fine-Grained Cryomilled 5083 Aluminum Alloy

The cyclic deformation behavior of cryomilled (CM) AA5083 alloys was compared to that of conventional AA5083-H131. The materials studied were a 100 pct CM alloy with a Gaussian grain size average of 315 nm and an alloy created by mixing 85 pct CM powder with 15 pct unmilled powder before consolidation to fabricate a plate with a bimodal grain size distribution with peak averages at 240 nm and 1.8 μm. Although the ultra-fine-grain (UFG) alloys exhibited considerably higher tensile strengths than those of the conventional material, the results from plastic-strain-controlled low-cycle fatigue tests demonstrate that all three materials exhibit identical fatigue lives across a range of plastic strain amplitudes. The CM materials exhibited softening during the first cycle, similar to other alloys produced by conventional powder metallurgy, followed by continual hardening to saturation before failure. The results reported in this study show that fatigue deformation in the CM material is accompanied by slight grain growth, pinning of dislocations at the grain boundaries, and grain rotation to produce macroscopic slip bands that localize strain, creating a single dominant fatigue crack. In contrast, the conventional alloy exhibits a cell structure and more diffuse fatigue damage accumulation

Degassing Behavior of Nanostructured Al and Its Composites

The synthesis of bulk ultrafine-grained (UFG) and nanostructured Al via cryomilling can frequently require a degassing step prior to consolidation, partly due to the large surface area of the as-milled powders. The objective of this study is to investigate the effects associated with cryomilling with stearic acid additions (as a process-control agent) on the degassing behavior of Al powders. This objective was accomplished by completing select experiments with Al-7.5Mg, Al-6.4 wt pct Al85Ni10La5, and Al-14.3 wt pct B4C. The interaction between Al and stearic acid was determined using thermal analysis combined with Fourier transform infrared spectroscopy (FTIR). The degassing experiments were carried out under high vacuum (10−4 to ~10−6 torr) in a range from room temperature to 400 °C, with the pressure of the released gases monitored using a digital vacuum gage. The results showed that the liberation of chemisorbed water was suppressed in cryomilled Al powders and both the chemisorbed water and stearic acid were primarily released in the form of hydrogen. It was also demonstrated that under certain conditions, a nanostructure (grain size ~100 nm) can be retained following the hot vacuum degassing of cryomilled Al

Bimodal microstructure and mechanical properties of cryomilled nanocrystalline Al-7.5Mg

The microstructure and mechanical properties consisting of tensile behavior and hardness of bulk nanocrystalline Al-7.5Mg alloy were investigated. Grain refinement was achieved by cryomilling of atomized Al-7.5Mg powders, and then nanocrystalline powders blended with 15% and 30% coarse-grained Al-7.5Mg powders were consolidated by hot isostatic pressing (HIP) followed by extrusion to produce bulk nanocrystalline Al-7.5Mg alloys. Bimodal structures, which enhance ductility and toughness of nanocrystalline metals, were produced that consisted of nanocrystalline grains and elongated coarse-grain bands. Examination of indentation revealed unusual deformation mechanisms and interactions between the coarse-grain bands and nanocrystalline regions. The ductile coarse-grain bands underwent extensive plastic deformation near indentation, while nanocrystalline regions exhibited limited deformation

Cognitive Learning Styles and Digital Equity: Searching for the Middle Way

This research is driven by a desire to understand the lifelong learner in the context of styles of learning and the emerging implications of technology enhanced learning for digital equity. Recognizing cognitive learning styles is the first step educators need to take in order to be most effective in working with students of diversity and bridging across formal and informal settings. Learning environments as a characterising feature of learning styles have undergone unprecedented change over the past decade with learning environments now blending physical and virtual space. To support the increasing diversity of learners pedagogy has to be fair, culturally responsive, equitable and relevant to the ‘virtual generation’. This in turn will inform our understanding of the ‘middle way’ in recognising cognitive learning styles , associated cultural context, and the implications to digital pedagogy equity