Discontinuities in Oxidation Kinetics: A New Model and its Application to Cr–Si-Base Alloys

Some alloys such as many Cr-based systems show mass gain discontinuities during thermogravimetric measurements which strongly affect the oxidation kinetics. The behaviour cannot be described by the current models available in the literature. Thus, a novel $$k_\mathrm{para}$$ k para –$$k_\mathrm{lin}$$ k lin -P-model was developed to describe oxidation kinetics during the isothermal exposure of materials which show such behaviour. Beside the parabolic rate constant $$k_\mathrm{para}$$ k para and the linear mass loss constant $$k_\mathrm{lin}$$ k lin , the P-value and $$f_P$$ f P are introduced to take into account spontaneous rapid mass gains due to local oxide scale failure. The parameter P serves as a measure for the mass gain due to discontinuous events and $$f_P$$ f P is the frequency of such events. The both parameters can be related to oxide scale detachment and growth stresses. The application of the model is demonstrated for the oxidation of Cr–Si-based alloys in synthetic air at $$1200^{\circ }\hbox {C}$$ 1200 ∘ C for 100 h. For these alloys, the origin of the mass gain discontinuities is discussed and the meaning of P and $$f_P$$ f P is explained in more detail. Using this newly developed model, an insight into growth and nitridation resistance of oxide scales as well as scale adhesion is gained.

Variation in the Inter-Node Length During Plant Growth Influences the Tensile Strength of Maize Stem Fibres.

This study describes an investigation of the evolution of the mechanical and chemical properties of maize stem fibres with the growth stages of the plant, and how the tensile strength is influenced by the presence of nodes along the fibre length. Furthermore the variation of the tensile strength and chemical functional groups among four common maize varieties were determined. In this context, the fibres were characterised by performing tensile test, density & linear density tests, Fourier Transform infra-red spectroscopy (FTIR), X-ray diffraction (XRD), Thermo-gravimetric analysis (TGA) and surface morphology (SEM image analysis). The fibres were all extracted manually, and in some cases preceded by a water retting process for ten days. The thermal analysis, FTIR and x-ray results showed that in general the fibres from the different maize varieties and from the different growth stages are semi-crystalline in nature. The SEM micrographs revealed the presence of equi-spaced nodes along the fibre length, which are believed to be due to the growth stresses induced in the plant stem. The inter-node distance varied in relation to the growth stage of the plant, and yielded a good correlation (coefficient of 0.91) with the tensile strength of the fibres. Finally a better fibre yield was obtained from the stem at the senescence stage of the maize plant.

On a modeling stress-controlled surface growth of solids

Various processes are associated with the surface growth of solids, such as biological growth, formation of surfaces, processes accompanying additive technologies. Experiments show that the growth process of living and non-living matter can be controlled by external influences, including mechanical ones. This paper presents a surface growth model based on the expression for the configurational force obtained from the fundamental balances of mass, momentum and energy, and the second law of thermodynamics in the form of the Clausius-Duhem inequality. It is shown that the configurational force is the normal component of the tensor, called the surface growth tensor, which controls the processes of growth and adaptation to external mechanical loads. A kinetic equation in the form of the dependence of the growth rate on the growth tensor is formulated. A solid body is considered, in which a volumetric supply and subsequent diffusion of matter to the growth boundary occur. On the surface of the body, the transformation of one substance into another occurs, resulting in surface growth or resorption of the body. The surface growth process depends on the stress-strain state of the body and the concentration of the diffusing matter. In the process of growth, stresses and deformations change, affecting the configurational force and the rate of the matter supply, which also affects the configurational force. In addition, the model takes into account the growth strains that can occur in new layers of the material and affect the growth velocity. Thus, there is a coupled problem including the description of the supply, diffusion and growth processes and determination of the stress-strain state. The model was used for the problems of surface growth of various bodies under various loading conditions.

India

India has been among the world’s fastest-growing economies in recent years, lifting millions out of poverty. However, growth slowed to a six-year low in the first half of 2019, with both consumption and investment decelerating owing to weak, especially rural, income growth, stresses in the non-bank financial sector, and corporate and environmental regulatory uncertainty. On the external sector, following a rise in vulnerabilities in 2018, stability has returned, anchored by high foreign reserve buffers and a modest current account deficit.