Electron backscattered diffraction to estimate residual stress levels of a superalloy produced by laser powder bed fusion and subsequent heat treatments

Metal Additive Manufacturing and Laser Powder Bed Fusion (LPBF), in particular, have come forth in recent years as an outstanding innovative manufacturing approach. The LPBF process is notably characterized by very high solidification and cooling rates, as well as repeated abrupt heating and cooling cycles, which generate the build-up of anisotropic microstructure and residual stresses. Post-processing stress-relieving heat treatments at elevated temperatures are often required in order to release some of these stresses. The effects of 1 h–hold heat treatments at different specific temperatures (solutionizing, annealing, stress-relieve and low-temperature stress-relieve) on residual stress levels together with microstructure characterization were therefore investigated for the popular Alloy 625 produced by LPBF. The build-up of residual stress is accommodated by the formation of dislocations that produce local crystallographic misorientation within grains. Electron backscattered diffraction (EBSD) was used to investigate local misorientation by means of orientation imaging, thereby assessing misorientation or strain levels, in turn representing residual stress levels within the material. The heavily constrained as-built material was found to experience full recrystallization of equiaxed grains after solutionizing at 1150◦ C, accompanied by significant drop of residual stress levels due to this grains reconfiguration. Heat treatments at lower temperatures however, even as high as the annealing temperature of 980◦ C, were found to be insufficient to promote recrystallization though effective to some extent to release residual stress through apparently dislocations recovery. Average misorientation data obtained by EBSD were found valuable to evaluate qualitatively residual stress levels. The effects of the different heat treatments are discussed and suggest that the peculiar microstructure of alloys produced by LPBF can possibly be transformed to suit specific applications

Resonance Raman kinetic spectroscopy of bacteriorhodopsin on the microsecond time scale.

Using a rotating disk with a slit of variable width, a continuous wave argon ion laser, and an Optical Multichannel Analyzer for detection, a new technique is reported which should, in principle, be capable of recording resonance Raman spectra with time resolution of 100 ns. The resonance Raman spectra of the intermediates of the photosynthetic cycle of bacteriorhodopsin are recorded on the microsecond time scale. Both the kinetic results and the resonance enhancement profile suggest that deprotonation results in an intermediate preceding bM412 that has an optical absorption maximum at a wavelength longer than that of bM412

Moessbauer and EPR study of reaction intermediates of cytochrome P450

We present a complementary Mossbauer and EPR study on reaction intermediates of substrate-free and substrate-bound cytochrome P450(cam) from Pseudomonas putida prepared by the freeze-quench method from Fe-57-labeled P450(cam) using peroxy acetic acid as oxidizing agent. When reacting the substrate-free P450(cam) for 8 ms reaction time the reaction mixture consists of similar to85% of ferric low-spin iron (Fe(III)) with g-factors and hyperfine parameters of the starting material; the remaining similar to15% are identified as ferryl iron (Fe(IV); S-Fe = 1) by its Mossbauer signature. Parallel to the ferryl iron a tyrosine radical (S-rad = 1 2) is formed. The two paramagnetic species are not exchange-coupled; however, they are close enough to significantly influence the (EPR) relaxation behavior of the radical spin. In the case of substrate-bound P450(cam) only trace amounts of the tyrosine radical are formed within 8 ms (<3%); within the accuracy of Mossbauer spectroscopy (5%) iron(IV) can not be detected. The results point to Tyr-96, which is hydrogen-bonded to the substrate camphor, as the candidate for the observed tyrosine radical

Crystal Structures of Chloroperoxidase with Its Bound Substrates and Complexed with Formate, Acetate, and Nitrate

Chloroperoxidase (CPO) is a heme−thiolate enzyme catalyzing hydrogen peroxide dependent halogenation reactions. Structural data on substrate binding have not been available so far. CPO was therefore crystallized in the presence of iodide or bromide. One halide binding site was identified at the surface near a narrow channel that connects the surface with the heme. Two other halide binding sites were identified within and at the other end of this channel. Together, these sites suggest a pathway for access of halide anions to the active site. The structure of CPO complexed with its natural substrate cyclopentanedione was determined at a resolution of 1.8