Laser powder bed fusion of 17–4 PH stainless steel:A comparative study on the effect of heat treatment on the microstructure evolution and mechanical properties

17–4 PH (precipitation hardening) stainless steel is commonly used for the fabrication of complicated molds with conformal cooling channels using laser powder bed fusion process (L-PBF). However, their microstructure in the as-printed condition varies notably with the chemical composition of the feedstock powder, resulting in different age-hardening behavior. In the present investigation, 17–4 PH stainless steel components were fabricated by L-PBF from two different feedstock powders, and subsequently subjected to different combinations of post-process heat treatments. It was observed that the microstructure in as-printed conditions could be almost fully martensitic or ferritic, depending on the ratio of Creq/Nieq of the feedstock powder. Aging treatment at 480 °C improved the yield and ultimate tensile strengths of the as-printed components. However, specimens with martensitic structures exhibited accelerated age-hardening response compared with the ferritic specimens due to the higher lattice distortion and dislocation accumulation, resulting in the “dislocation pipe diffusion mechanism”. It was also found that the martensitic structures were highly susceptible to the formation of reverted austenite during direct aging treatment, where 19.5% of austenite phase appeared in the microstructure after 15 h of direct aging. Higher fractions of reverted austenite activates the transformation induced plasticity and improves the ductility of heat treated specimens. The results of the present study can be used to tailor the microstructure of the L-PBF printed 17–4 PH stainless steel by post-process heat treatments to achieve a good combination of mechanical properties

Charge transport and trapping in Cs-doped poly(dialkoxy-p-phenylene vinylene) light-emitting diodes

Al/Cs/MDMO-PPV/ITO (where MDMO-PPV stands for poly[2-methoxy-5-(3'-7'-dimethyloctyloxy)-1,4phenylene vinylene] and ITO is indium tin oxide) light-emitting diode (LED) structures, made by physical vapor deposition of Cs on the emissive polymer layer, have been characterized by electroluminescence, current-voltage, and admittance spectroscopy. Deposition of Cs is found to improve the balance between electron and hole currents, enhancing the external electroluminescence efficiency from 0.01 cd A-1 for the bare Al cathode to a maximum of 1.3 cd A-1 for a Cs coverage of only 1.5×1014 atoms/cm2. By combining I-V and admittance spectra with model calculations, in which Cs diffusion profiles are explicitly taken into account, this effect could be attributed to a potential drop at the cathode interface due to a Cs-induced electron donor level 0.61 eV below the lowest unoccupied molecular orbital. In addition, the admittance spectra in the hole-dominated regime are shown to result from space-charge-limited conduction combined with charge relaxation in trap levels. This description allows us to directly determine the carrier mobility, even in the presence of traps. In contrast to recent literature, we demonstrate that there is no need to include dispersive transport in the description of the carrier mobility to explain the excess capacitance that is typically observed in admittance spectra of p-conjugated materials

Setting benchmarks for modelling gas–surface interactions using coherent control of rotational orientation states

A fundamental and predictive understanding of molecule-surface interactions is challenging to obtain. Here the authors report an experimental technique allowing direct measurement of the scattering matrix, which reports on the coherent evolution of quantum states of a molecule scattering from a surface

Sub-nanometre resolution imaging of polymer-fullerene photovoltaic blends using energy-filtered scanning electron microscopy

The resolution capability of the scanning electron microscope has increased immensely in recent years, and is now within the sub-nanometre range, at least for inorganic materials. An equivalent advance has not yet been achieved for imaging the morphologies of nanostructured organic materials, such as organic photovoltaic blends. Here we show that energy-selective secondary electron detection can be used to obtain high-contrast, material-specific images of an organic photovoltaic blend. We also find that we can differentiate mixed phases from pure material phases in our data. The lateral resolution demonstrated is twice that previously reported from secondary electron imaging. Our results suggest that our energy-filtered scanning electron microscopy approach will be able to make major inroads into the understanding of complex, nano-structured organic materials

A general method for controlling and resolving rotational orientation of molecules in molecule-surface collisions

Theoretical Chemistr