Accelerating ferroic ageing dynamics upon cooling

Once a structural glass is formed, its relaxation time will increase exponentially with decreasing temperature. Thus, the glass has little chance of transforming into a crystal upon further cooling to zero Kelvin. However, a spontaneous transition upon cooling from amorphous to long-range ordered ferroic states has been observed experimentally in ferroelastic, ferroelectric and ferromagnetic materials. The origin for this obvious discrepancy is discussed here conceptually. We present a combined theoretical and numerical study of this phenomenon and show that the diffusive and displacive atomic processes that take place in structural glass and amorphous ferroics, respectively, lead to markedly different temperature-dependent relaxation behaviors, one being ‘colder is slower’ and the other being ‘colder is faster’.National Basic Research Program of China (2012CB619402)National Basic Research Program of China (2014CB644003)National Key Basic Research Program of China (51671156)National Basic Research Program of China 111 Project (B06025)National Science Foundation (U.S.). Division of Materials Research (DMR-1410322)National Science Foundation (U.S.). Division of Materials Research (DMR-1410636

Stress-driven crystallization via shear-diffusion transformations in a metallic glass at very low temperatures

At elevated temperatures, glasses crystallize via thermally activated diffusion. However, metallic glasses can also undergo deformation-induced crystallization at very low temperatures. Here we demonstrate the crystallization of Al[subscript 50]Fe[subscript 50] metallic glasses under cyclic deformation at 50 K using molecular dynamics simulations and reveal the underlying atomic-scale processes. We demonstrate that stress-driven nonaffine atomic rearrangements, or shear diffusion transformation (SDT) events, lead to successive metabasin-to-metabasin transitions and long-range ordering. We also illustrate that the nucleation and growth of the crystal proceed via collective attachment of ordered clusters, advancing the amorphous/crystal interface in an intermittent manner. The cooperative nature of the steplike crystallization is attributed to the large activation volume of Eshelby transformations which generate as a by-product nonaffine diffusive atomic displacements that accumulate over loading cycles. The dual nature of shear (affine) and diffusion (nonaffine) in low-temperature stress-driven SDT events thus unifies inelasticity with crystallization.National Basic Research Program of China (973 Program) (Grant 2012CB619402)National Basic Research Program of China (111 Program) (Grant B06025)National Science Foundation (U.S.) (Grant DMR-1120901)National Science Foundation (U.S.) (Grant DMR-1410636

Pressure-stabilized divalent ozonide CaO3 and its impact on Earth's oxygen cycles.

High pressure can drastically alter chemical bonding and produce exotic compounds that defy conventional wisdom. Especially significant are compounds pertaining to oxygen cycles inside Earth, which hold key to understanding major geological events that impact the environment essential to life on Earth. Here we report the discovery of pressure-stabilized divalent ozonide CaO3 crystal that exhibits intriguing bonding and oxidation states with profound geological implications. Our computational study identifies a crystalline phase of CaO3 by reaction of CaO and O2 at high pressure and high temperature conditions; ensuing experiments synthesize this rare compound under compression in a diamond anvil cell with laser heating. High-pressure x-ray diffraction data show that CaO3 crystal forms at 35 GPa and persists down to 20 GPa on decompression. Analysis of charge states reveals a formal oxidation state of -2 for ozone anions in CaO3. These findings unravel the ozonide chemistry at high pressure and offer insights for elucidating prominent seismic anomalies and oxygen cycles in Earth's interior. We further predict multiple reactions producing CaO3 by geologically abundant mineral precursors at various depths in Earth's mantle

In-Hospital Formula Feeding Hindered Exclusive Breastfeeding: Breastfeeding Self-Efficacy as a Mediating Factor

Breastfeeding self-efficacy (BSE), defined as a mother’s confidence in her ability to breastfeed, has been confirmed to predict the uptake of exclusive breastfeeding (EBF). Early experiences during the birth hospital stay, especially in-hospital formula feeding (IHFF), can impact both EBF and maternal breastfeeding confidence. Therefore, our objective was to examine the association between IHFF and EBF outcomes and investigate whether this association is influenced by BSE. The study included 778 infants from a larger cohort study conducted in 2021, with a one-year follow-up in rural areas of Sichuan Province, China. We used a causal mediation analysis to estimate the total effect (TE), natural direct (NDE), and nature indirect effects (NIE) using the paramed command in Stata. Causal mediation analyses revealed that IHFF was negatively associated with EBF (TE odds ratio = 0.47; 95% CI, 0.29 to 0.76); 28% of this association was mediated by BSE. In the subgroup analysis, there were no significant differences in the effects between parity subgroups, as well as between infant delivery subgroups. Our study found that IHFF hindered later EBF and that BSE mediated this association. Limiting the occurrence of in-hospital formula feeding or improving maternal breastfeeding self-efficacy is likely to improve exclusive breastfeeding outcomes

Coupled deformation-diffusion-fracture theories for solids : application to polymeric gels and hydrogen embrittlement in steels

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.Cataloged from PDF version of thesis.Includes bibliographical references (pages 207-216).Solids with deformation-diffusion coupling are ubiquitous in engineering applications. Understanding and modeling the fracture of such solids is vitally important. This thesis addresses the theoretical formulation, numerical implementation, and application of fully-coupled deformation-diffusion-damage theories for two different classes of materials: (i) polymeric gels and (ii) hydrogen embrittlement in steels, as elaborated below. (i) Fracture of polymeric gels: We first introduce a field called "stretch of Kuhn segments and/or crosslinks", which is necessary for understanding and modeling of the fracture in polymeric materials. Together with this newly introduced field, we formulate a thermodynamically consistent phase-field type theory for fracture of gels. A central feature of our theory is the recognition that the free energy of polymeric materials is not entirely entropic in nature, there is also an energetic contribution from the deformation of the backbone bonds in a chain and/or the crosslinks. It is this energetic part of the free energy that drives the progressive damage and fracture of polymeric materials. We have implemented our theory in a finite element code, and used this simulation capability to study some interesting phenomena in failure of elastomers and gels. (ii) Fracture of steels due to hydrogen embrittlement: We have formulated a thermodynamically consistent theory for the diffusion of hydrogen coupled with the large elastic-plastic deformations, and a phase-field type theory to model ductile fracture of metals. The theory accounts for the macroscopic effects due to the generation and agglomeration of microscopic hydrogen-vacancy complexes. We have implemented our fully coupled theory in a finite element program, and calibrated the material parameters in the theory by using experimental data available in the literature. Finally we have utilized our simulation capability to study the process of fracture due to hydrogen embrittlement in some technically relevant notched-components made from steel.by Yunwei Mao.Ph. D

Fracture of Elastomeric Materials by Crosslink Failure

If an elastomeric material is subjected to sufficiently large deformations, it eventually fractures. There are two typical micromechanisms of failure in such materials: chain scission and crosslink failure. The chain scission failure mode is mainly observed in polymers with strong covalent crosslinks, while the crosslink failure mode is observed in polymers with weak crosslinks. In two recent papers, we have proposed a theory for progressive damage and rupture of polymers with strong covalent crosslinks. In this paper, we extend our previous framework and formulate a theory for modeling failure of elastomeric materials with weak crosslinks. We first introduce a model for the deformation of a single chain with weak crosslinks at each of its two ends using statistical mechanics arguments, and then upscale the model from a single chain to the continuum level for a polymer network. Finally, we introduce a damage variable to describe the progressive damage and failure of polymer networks. A central feature of our theory is the recognition that the free energy of elastomers is not entirely entropic in nature; there is also an energetic contribution from the deformation of the backbone bonds in a chain and/or the crosslinks. For polymers with weak crosslinks, this energetic contribution is mainly from the deformation of the crosslinks. It is this energetic part of the free energy which is the driving force for progressive damage and fracture of elastomeric materials. Moreover, we show that for elastomeric materials in which fracture occurs by crosslink stretching and scission, the classical Lake-Thomas scaling - that the toughness Gc of an elastomeric material is proportional to 1/√G₀, with G₀ = NκBγ the ground-state shear modulus of the material - does not hold. A new scaling is proposed, and some important consequences of this scaling are remarked upon