In-situ SEM observation of grain growth in the austenitic region of carbon steel using thermal etching.

A novel heat stage, recently developed for use within the Scanning Electron Microscope, has facilitated Secondary Electron imaging at temperatures up to 850°C. This paper demonstrates one of the applications of in-situ elevated temperature Scanning Electron Microscope imaging: observation and quantification of grain growth within the austenitic region of carbon steels. The resulting Secondary Electron data have used the technique of thermal etching to capture possible 'abnormal grain growth' in the austenitic region. Previous ex-situ and post-heating results from carbon steels indicate normal, non-linear grain growth. Therefore, this new dataset provides greater insight into the heat treatment of steels. From comparison of the in-situ data with the overall grain growth, measured ex-situ, it is further concluded that abnormal grain growth is representative of the growth at temperature. Thus, the heating and cooling parts of the heat treatment are likely to account for the non-linearity previously documented in ex-situ results and, hence, the range of powers recorded when fitting power law models for steel grain growth. The ability of data derived from in-situ thermal etching to represent the microstructure of the entire surface and the bulk material is also considered. LAY DESCRIPTION: A novel heating stage has recently been developed for use within the Scanning Electron Microscope (SEM); an instrument that uses electrons to image specimen surfaces at very high magnifications. The development of the heating stage has facilitated imaging at temperatures up to 850°C of the structure and topographic features of metals using two different detectors. This study focusses on observation and quantification of grain growth in steels at temperatures of 800  C. In Materials Science, grains refer to crystals of varying, randomly distributed, small sizes that together make up a solid metal. The temperature of 800  C is used as it is the desired temperature to heat treat steels in order to produce more favourable physical properties. It is also the temperature above which the material undergoes a phase change; phase change is a transition where the atoms rearrange from one order within a grain to another. In the case of steel, at room temperature atoms will be in what is called a ferrite phase (one order) but at 800  C, they will be in a different order within the grains, known as the austenite phase. Hence, the uniqueness of this dataset as the grain growth captured is in the high temperature steel phase of austenite. The steel samples used are made up of 0.4% Carbon, 99% iron and some manganese and other trace elements. The resulting data have, for the first time, shown so called 'abnormal grain growth' which is represented by a linear relationship between grain size and time. Abnormal grain growth is also observed in the images where it can be seen how larger grains grow at a high rate at the expense of smaller ones. Previous data taken after cooling of steels indicate normal non-linear grain growth. Therefore, it is reasonable to suggest, this new dataset provides greater insight into the heat treatment processing of steels, demonstrating that they are potentially more complex than previously thought.

Advanced microscopy analysis of the micro-nanoscale architecture of human menisci.

The complex inhomogeneous architecture of the human meniscal tissue at the micro and nano scale in the absence of artefacts introduced by sample treatments has not yet been fully revealed. The knowledge of the internal structure organization is essential to understand the mechanical functionality of the meniscus and its relationship with the tissue's complex structure. In this work, we investigated human meniscal tissue structure using up-to-date non-invasive imaging techniques, based on multiphoton fluorescence and quantitative second harmonic generation microscopy complemented with Environmental Scanning Electron Microscopy measurements. Observations on 50 meniscal samples extracted from 6 human menisci (3 lateral and 3 medial) revealed fundamental features of structural morphology and allowed us to quantitatively describe the 3D organisation of elastin and collagen fibres bundles. 3D regular waves of collagen bundles are arranged in "honeycomb-like" cells that are comprised of pores surrounded by the collagen and elastin network at the micro-scale. This type of arrangement propagates from macro to the nanoscale.

Strain softening of nano-scale fuzzy interfaces causes Mullins effect in thermoplastic polyurethane.

The strain-induced softening of thermoplastic polyurethane elastomers (TPUs), known as the Mullins effect, arises from their multi-phase structure. We used the combination of small- and wide- angle X-ray scattering (SAXS/WAXS) during in situ repeated tensile loading to elucidate the relationship between molecular architecture, nano-strain, and macro-scale mechanical properties. Insights obtained from our analysis highlight the importance of the 'fuzzy interface' between the hard and soft regions that governs the structure evolution at nanometre length scales and leads to macroscopic stiffness reduction. We propose a hierarchical Eshelby inclusion model of phase interaction mediated by the 'fuzzy interface' that accommodates the nano-strain gradient between hard and soft regions and undergoes tension-induced softening, causing the Mullins effect that becomes apparent in TPUs even at moderate tensile strains.

Experimental evidence for dendrite tip splitting in deeply undercooled, ultrahigh purity Cu.

In a recent paper we used a phase-field model of solidification in deeply undercooled pure melts to show that a kinetic instability could result in dendrite tip splitting, and we speculated that such tip splitting could give rise to the phenomenon of spontaneous grain refinement. Here we present evidence, from the as-solidified microstructure of deeply undercooled ultrahigh purity Cu, of what appears to be dendrite tip splitting during recalescence. The significance of this finding in a nongrain refined sample is discussed in terms of the current theories for spontaneous grain refinement.