Formation of residual stresses during quenching of Ti17 and Ti 6Al 4V alloys Influence of phase transformations

The formation of internal stresses during quenching of titanium alloys from the amp; 946; phase field are investigated both experimentally and by simulation, in order to show the effects of phase transformations. Two titanium alloys are considered the amp; 946; metastable Ti17 alloy and the amp; 945; amp; 946; Ti 6Al 4V alloy. During the quench into water of laboratory scale samples 40 mm diameter cylinders , no phase transformations occurred in the Ti17 alloy because of its amp; 946; metastable character and the fast cooling. However, amp; 946; amp; 8594; amp; 945; amp; 946; and amp; 946; amp; 8594; amp; 945; amp; 8242; phase transformations occurred in the Ti 6Al 4V sample. Both alloys are compared in order to highlight the effects of the phase transformations. Residual stresses were determined by neutron diffraction, by the contour method and at the surface by the hole drilling method. A model for the coupled thermal, mechanical and metallurgical evolutions is established in order to simulate the quenching operations. The material model for the Ti17 alloy was established in a previous study. Regarding the Ti 6Al 4V alloy, modeling approaches and experimental data from literature are utilized to build the material model. From both experiment and simulation, it is found that the internal stress evolutions are governed by the thermal gradients. The phase transformations have a weak impact because of the small deformation strains induced by the phase change. Nevertheless, good prediction of the phase transformation kinetics is necessary for accurate simulations, because the amp; 945; and amp; 945; phases strengthen the alloy, thereby limiting the plastic straining at the origin of the residual stresses. As most plastic strains are cumulated at high temperature, the thermomechanical model should be established accurately over the temperature ranges in which there is a significant proportion of amp; 946; phas

Integrating the weight of history into the alignment framework: the case of distributed generation technologies

Whereas a centralized energy supply system is still dominant today, the energy sector is currently witnessing the development of small-scale and more geographically dispersed generation units, so-called distributed generation technologies. So-called "alignment framework" proposes a very useful approach to look at this evolution. Yet, we argue in this paper that this framework does not fully take into account the inertia associated with past technological and institutional choices that may hinder future changes. Relying on the concept of socio-technical lock-in, we illustrate this point with the case of the diffusion of distributed generation technologies. Based on this analysis, we propose an adaptation of the alignment framework to integrate these elements

Dual targeting is the rule for organellar aminoacyl-tRNA synthetases in Arabidopsis thaliana

In plants, protein synthesis occurs in the cytosol, mitochondria, and plastids. Each compartment requires a full set of tRNAs and aminoacyl-tRNA synthetases. We have undertaken a systematic analysis of the targeting of organellar aminoacyl-tRNA synthetases in the model plant Arabidopsis thaliana. Dual targeting appeared to be a general rule. Among the 24 identified organellar aminoacyl-tRNA synthetases (aaRSs), 15 (and probably 17) are shared between mitochondria and plastids, and 5 are shared between cytosol and mitochondria (one of these aaRSs being present also in chloroplasts). Only two were shown to be uniquely chloroplastic and none to be uniquely mitochondrial. Moreover, there are no examples where the three aaRS genes originating from the three ancestral genomes still coexist. These results indicate that extensive exchange of aaRSs has occurred during evolution and that many are now shared between two or even three compartments. The findings have important implications for studies of the translation machinery in plants and on protein targeting and gene transfer in general