Duration-dependent effects of the BDNF Val66Met polymorphism on anodal tDCS induced motor cortex plasticity in older adults: a group and individual perspective

The brain derived neurotrophic factor (BDNF) Val66Met polymorphism and stimulation duration are thought to play an important role in modulating motor cortex plasticity induced by non-invasive brain stimulation (NBS). In the present study we sought to determine whether these factors interact or exert independent effects in older adults. Fifty-four healthy older adults (mean age = 66.85 years) underwent two counterbalanced sessions of 1.5 mA anodal transcranial direct current stimulation (atDCS), applied over left M1 for either 10 or 20 min. Single pulse transcranial magnetic stimulation (TMS) was used to assess corticospinal excitability (CSE) before and every 5 min for 30 min following atDCS. On a group level, there was an interaction between stimulation duration and BDNF genotype, with Met carriers (n = 13) showing greater post-intervention potentiation of CSE compared to Val66Val homozygotes homozygotes (n = 37) following 20 min (p = 0.002) but not 10 min (p = 0.219) of stimulation. Moreover, Met carriers, but not Val/Val homozygotes, exhibited larger responses to TMS (p = 0.046) after 20 min atDCS, than following 10 min atDCS. On an individual level, two-step cluster analysis revealed a considerable degree of inter-individual variability, with under half of the total sample (42%) showing the expected potentiation of CSE in response to atDCS across both sessions. Intra-individual variability in response to different durations of atDCS was also apparent, with one-third of the total sample (34%) exhibiting LTP-like effects in one session but LTD-like effects in the other session. Both the inter-individual (p = 0.027) and intra-individual (p = 0.04) variability was associated with BDNF genotype. In older adults, the BDNF Val66Met polymorphism along with stimulation duration appears to play a role in modulating tDCS-induced motor cortex plasticity. The results may have implications for the design of NBS protocols for healthy and diseased aged populations.

Solvent-induced dynamic single-crystal-to-single-crystal transformation of a synthetic peptide-based cyclic compound

Solvent induced single-crystal-to-single-crystal transformation has been demonstrated indicating the dynamic behavior of one dimensional arrays obtained from a self-assembled new synthetic cyclic peptide.

The evolution of ultrafine ferrite formation through dynamic strain-induced transformation

Hot torsion testing of a C–Mn–V steel was used to study the evolution of  ultrafine ferrite (UFF) formation by dynamic strain-induced transformation (DSIT) in conjunction with air-cooling for two prior austenite grain sizes. This study evaluated not only the evolution of DSIT ferrite during straining, but also the grain growth behaviour of DSIT ferrite grains during post-deformation cooling. For both austenite grain sizes, the DSIT ferrite initially nucleated on/or near prior austenite grain boundaries at an early stage of transformation followed by the grain interiors. The prior austenite grain size affected the distribution of DSIT ferrite nucleation sites at an early stage of transformation and the subsequent coarsening behaviour of the grain boundary (GB) and the intragranular ferrite (IG) grains during post-deformation cooling. For the fine prior austenite grain size, the distribution of DSIT ferrite grains was more homogenous compared with the coarse austenite and the coarsening occurred not only in the GB ferrite grains but also in the IG ferrite grains. However, the ferrite coarsening mostly occurred for the IG ferrite rather than the GB ferrite grains in the coarse austenite. The result suggests that normal grain growth occurred during the overall transformation in the GB ferrite grains for the coarse initial austenite grain size.

Formation of ultrafine grained microstructures in steel through strain induced transformation during single pass hot rolling

In the present study, wedge-shape samples were used to study the effect of strain induced transformation on the formation of ultrafine grained structures in steel by single pass rolling. The results showed two different transition strains for bainite formation and ultrafine ferrite (UFF) formation in the surface layer of strip at reductions of 40% and 70%, respectively, in a plain carbon steel. The bainitic microstructure formed by strain induced bainitic transformation during single pass rolling was also very fine. The evolution of UFF formation in the surface layer showed that ferrite coarsening is significantly reduced through strain induced transformation combined with rapid cooling in comparison with the centre of the strip. In the surface, the ferrite coarsening mostly occurred for intragranular nucleated grains (IG) rather than grain boundary (GB) ferrite grains. The results suggest that normal grain growth occurred during overall transformation in the GB ferrite grains. In the centre of the strip, there was significantly more coarsening of ferrite grains nucleated on the prior austenite grain boundaries.

The production of ultrafine ferrite in low-carbon steel by strain-Induced transformation

An investigation into the production of ultrafine (1 µm) equiaxed ferrite (UFF) grains in low-carbon steel was made using laboratory rolling, compression dilatometry, and hot torsion techniques. It was found that the hot rolling of thin strip, with a combination of high shear strain and high undercooling, provided the conditions most suitable for the formation of this type of microstructure. Although high strains could be applied in compression and torsion experiments, large volume fractions of UFF were not observed in those samples, possibly due to the lower level of undercooling achieved. It is thought that ferrite refinement was due to a strain-induced transformation process, and that ferrite grains nucleated on parallel and linear deformation bands that traversed austenite grains. These bands formed during the deformation process, and the undercooling provided by the contact between the strip and the work rolls was sufficient to drive the transformation to homogeneous UFF grains.

Ultrafine grained structure formation in steels using dynamic strain induced transformation processing

The refinement of ferrite grain size is the most generally accepted approach to simultaneously improve the strength and toughness in steels. Historically, the level of ferrite refinement is limited to 5-10 μm using conventional industrial approaches. Nowadays, though, several thermomechanical processes have been developed to produce ferrite grain sizes of 1-3 μm or less, ranging from extreme thermal and deformation cycles to more typical thermomechanical processes. The present paper reviews the status of the production of ultrafine grained steels through relatively simple thermomechanical processing. This requires deformation within the Ae3 to Ar3 temperature range for a given alloy. Here, the formation of ultrafine ferrite (UFF) involves the dynamic transformation of a significant volume fraction of the austenite to ferrite. This dynamic strain induced transformation (DSIT) arises from the introduction of extensive intragranular nucleation sites that are not present in conventional controlled rolling. The DSIT route has the potential to be adjusted to suit current industrial infrastructure. However, there are a number of significant issues that have been raised, both as gaps in our understanding and as obstacles to industrial implementation. One of the critical issues is that it appears that very large strains are required. Combined with this concern is the issue of whether a combination of dynamic and static transformation can be used to achieve an adequate level of refinement. Another issue that has also become apparent is that grain sizes of 1 μm can lead to low levels of ductility and hence many workers are attempting to obtain 2-3 μm grains, or to introduce a second phase to provide the required ductility. There are also a number of areas of disagreement between authors including the role of dynamic recrystallisation of ferrite in the production of UFF by DSIT, the reasons for the low coarsening rate of UFF grains, the role of microalloying elements and the effects of austenite grain size and strain rate. The present review discusses these areas of controversy and highlights cases where experimental results do not agree.

Dynamic adjustment of ferrite grains during dynamic strain induced transformation

The dynamic adjustment of ferrite grains formed during 'dynamic strain induced transformation (DSIT)' is an important feature of this mechanism that has not been addressed previously. A novel experimental method was applied to follow the effect of deformation at different stages on ferrite formed initially through DSIT. It is shown that while the continuous dynamic recrystallisation (CDRX) appears to be an acceptable mechanism for re-refinement of coarser grain size (i.e. dα>2dDSIT), it cannot explain the steady state grain size for finer ferrite grains (i.e. dα<2dDSIT). Other potential mechanisms involved in this phenomenon are examined.

Formation of ultrafine ferrite by dynamic strain-induced transformation

In the current study, the role of dynamic strain induced transformation on ferrite grain refinement was investigated using different thermomechanical processing routes. A Ni-30Fe austenitic model alloy was also employed to study the evolution of the deformation structure under different deformation conditions. It was shown that the extreme refinement of ferrite is more likely due to the formation of extensive high angle intragranular defects in the austenite through deformation. Among the different thermomechanical parameters, the deformation temperature had a significant effect on the intragranular defect characteristics. There was a transition where the cell dislocation structure changed to laminar microband structures with a decrease in the deformation temperature. Moreover, the ultrafine grained structure was also successfully produced through static transformation using warm deformation process; in other words, concurrent deformation and transformation are not necessary for ultrafine ferrite formation.

Mechanisms of dynamic strain induced transformation and post-deformation evolution

In this study, a novel experimental approach was applied to study the mechanism of the equiaxed shape retention in dynamic strain induced ferrite during deformation. The post-deformation ferrite evolution in both static and dynamic transformation was studied. The refinement potential and the origin of their differences in both mechanisms were analysed.

Deformation induced phase transformation during machining of Ti-5553

Ti-5553 is a relatively new titanium alloy with applications particularly in the aerospace industry for such key structural components as landing gear. However, during machining of Ti-5553, the elevated temperature and high strain at tool-workpiece interface may alter workpiece microstructure and result in ß to a phase transformation. During phase transformation, some intermediated phase such as w phase may form which is brittle and hard to machine, and it could reduce the fatigue life of machined components. The aim of this research work is to optimize the machining condition for Ti-5553, in which its hot deformation behavior in terms of ß to a phase transformation could be fully understood. Analysis of variables such as micrographs of phase components and cutting zone temperature demonstrates that the cutting temperature governs the formation of final phase components and to some extent this variation has been quantified to allow for further and more detailed investigation.

Dynamic strain-induced ferrite transformation during hot compression of low carbon Si-Mn steels

The dynamic strain-induced transformation (DSIT) of austenite to ferrite was investigated under different undercooling conditions using three low carbon Si-Mn steels. The undercooling of austenite (ΔT) was controlled by varying the cooling rate between austenitization and deformation temperatures. Uniform DSIT ferrite grains (∼2.3 μm) were produced at a relatively high deformation temperature above 840°C using a low carbon high Si steel (0.077C-0.97Mn-1.35Si, mass%) in connection with a larger ΔT. The critical conditions for DSIT were determined based on the flow stress-strain curves measured during hot compression tests. Influence of deformation temperature on DSIT of low carbon Si-added steel was also discussed.