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Understanding experimental design (e.g. control of variable strategy or CVS) is foundational for scientific reasoning. Previous research has demonstrated that demonstrations with cognitive conflict (e.g. asking students to evaluate and explain different experimental designs) are effective in promoting children’s scientific reasoning, however, the implementation of this approach often requires significant instructional time and resources. This study reports the impact of a brief, scalable intervention on one component of scientific reasoning, understanding experimental design, by providing brief instruction on the control-of-variable strategy (CVS), embedded in a food science activity (popping popcorn). Threehundred and seven (307) 3rd-5th graders in the midwestern US participated in either a CVS intervention or a demonstration on the science of popcorn without a CVS intervention. Performance on a pre-activity test (involving identification of good and bad experiments) did not differ between conditions. By contrast, postactivity performance was significantly greater for classes who received the CVS intervention. Thus, a brief discussion of the CVS embedded within a food-science demonstration can have a meaningful impact on children’s understanding of conducting a quality experiment. Our results demonstrate the efficacy of a simple, low-cost intervention for CVS that is potentially scalable. 

Twinning is an essential mode of plastic deformation for achieving superior strength and ductility in metallic nanostructures. It has been generally believed that twinning-induced plasticity in body-centered cubic (BCC) metals is controlled by twin nucleation, but facilitated by rapid twin growth once the nucleation energy barrier is overcome. By performing in situ atomic-scale transmission electron microscopy straining experiments and atomistic simulations, we find that deformation twinning in BCC Ta nanocrystals larger than 15 nm in diameter proceeds by reluctant twin growth, resulting from slow advancement of twinning partials along the boundaries of finite-sized twin structures. In contrast, reluctant twin growth can be obviated by reducing the nanocrystal diameter to below 15 nm. As a result, the nucleated twin structure penetrates quickly through the cross section of nanocrystals, enabling fast twin growth via facile migration of twin boundaries leading to large uniform plastic deformation. The present work reveals a size-dependent transition in the nucleation- and growth-controlled twinning mechanism in BCC metals, and provides insights for exploiting twinning-induced plasticity and breaking strength-ductility limits in nanostructured BCC metals.

 

The exchange bias effect is the physical cornerstone of applications, such as spin valves, ultra-high-density data storage, and magnetic tunnel junctions. This work studied the room temperature exchange bias effect by constructing a Ni50Mn38Sb12−xGax alloy system with coexisting martensitic phase structures. The study found that the exchange bias effect shows a non-monotonic change with the variation of Ga composition at 300 K, and an obvious room temperature exchange bias effect appears in the alloys with coexisting phase structures of 4O and L10, which is due to the strong exchange coupling between ferromagnetic and antiferromagnetic. Further research on the exchange bias effect and temperature shows that the blocking temperature is 420 K, and the exchange bias can stably exist in a temperature range of ∼200 K around room temperature. This work provides a method to engineer exchange bias effects at room temperature.

 

Everyday activities such as cooking a meal are natural opportunities for “challenging” family talk, which promotes cognitive development by prompting explanations and elaborations. Our study investigates a light intervention to increase the frequency of challenging family STEM talk during an everyday activity. Sixty-two families with children (mean age = 9.49) recorded their conversations while popping popcorn using either a standard recipe or a recipe with embedded wh-question prompts (e.g., Why did some kernels not pop?). Conversations were transcribed and coded to measure four qualities of challenging STEM talk: STEM words, STEM explanations, spontaneous questions, and elaborations (or interactive turn-taking). The results demonstrate that families who received wh-question prompts embedded into the recipe produced 3–5 times more instances of challenging STEM talk than families who received no prompts. These results provide evidence for a light intervention that increases family STEM talk through a familiar, everyday activity. 

Abstract Additively manufactured (AM) metallic materials often comprise as-printed dislocation cells inside grains. These dislocation cells can give rise to substantial microscale internal stresses in both initial undeformed and plastically deformed samples, thereby affecting the mechanical properties of AM metallic materials. Here we develop models of microscale internal stresses in AM stainless steel by focusing on their back stress components. Three sources of microscale back stresses are considered, including the printing and deformation-induced back stresses associated with as-printed dislocation cells as well as the deformation-induced back stresses associated with grain boundaries. We use a three-dimensional discrete dislocation dynamics model to demonstrate the manifestation of printing-induced back stresses. We adopt a dislocation pile-up model to evaluate the deformation-induced back stresses associated with as-printed dislocation cells. The extracted back stress relation from the pile-up model is incorporated into a crystal plasticity model that accounts for the other two sources of back stresses as well. The crystal plasticity finite element simulation results agree with the experimentally measured tension-compression asymmetry and macroscopic back stress, the latter of which represents the effective resultant of microscale back stresses of different origins. Our results provide an in-depth understanding of the origins and evolution of microscale internal stresses in AM metallic materials.