How Austenitic Is a Martensitic Steel Produced by Laser Powder Bed Fusion? A Cautionary Tale

Accurate phase fraction analysis is an essential element of the microstructural characterization of alloys and often serves as a basis to quantify effects such as heat treatment or mechanical deformation. Additive manufacturing (AM) of metals, due to the intrinsic nonequilibrium solidification and spatial variability, creates additional challenges for the proper quantification of phase fraction. Such challenges are exacerbated when the alloy itself is prone to deformation-induced phase transformation. Using commonly available in-house X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) and less commonly used synchrotron-based high-energy X-ray diffraction, we characterized nitrogen-atomized 17-4 precipitation-hardening martensitic stainless steel, a class of AM alloy that has received broad attention within the AM research community. On the same build, our measurements recovered the entire range of reported values on the austenite phase fractions of as-built AM 17-4 in literature, from ≈100% martensite to ≈100% austenite. Aided by Calphad simulation, our experimental findings established that our as-built AM 17-4 is almost fully austenitic and that in-house XRD and EBSD measurements are subject to significant uncertainties created by the specimen’s surface finish. Hence, measurements made using these techniques must be understood in their correct context. Our results carry significant implications, not only to AM 17-4 but also to AM alloys that are susceptible to deformation-induced structure transformation and suggest that characterizations with less accessible but bulk sensitive techniques such as synchrotron-based high energy X-ray diffraction or neutron diffraction may be required for proper understanding of these materials.

Influence of Quenching and Partitioning Parameters on Phase Transformations and Mechanical Properties of Medium Manganese Steel for Press-Hardening Application

It has been proven that through targeted quenching and partitioning (Q & P), medium manganese steels can exhibit excellent mechanical properties combining very high strength and ductility. In order to apply the potential of these steels in industrial press hardening and to avoid high scrap rates, it is of utmost importance to determine a robust process window for Q & P. Hence, an intensive study of dilatometry experiments was carried out to identify the influence of quenching temperature (TQ) and partitioning time (tp) on phase transformations, phase stabilities, and the mechanical properties of a lean medium manganese steel. For this purpose, additional scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and energy dispersive X-ray spectroscopy (EDX) examinations as well as tensile testing were performed. Based on the dilatometry data, an adjustment of the Koistinen–Marburger (K-M) equation for medium manganese steel was developed. The results show that a retained austenite content of 12–21% in combination with a low-phase fraction of untempered secondary martensite (max. 20%) leads to excellent mechanical properties with a tensile strength higher than 1500 MPa and a total elongation of 18%, whereas an exceeding secondary martensite phase fraction results in brittle failure. The optimum retained austenite content was adjusted for TQ between 130 °C and 150 °C by means of an adapted partitioning.

Investigation of phosphoric acid and water transport in the high temperature proton exchange membrane fuel cells

A three-dimensional, non-isothermal and multiphase model of high temperature proton exchange membrane fuel cells is built to investigate water and phosphoric acid transportation, in which a spherical agglomerate model considering catalyst layer structure and liquid phase fraction is applied to determine the electrochemical kinetics in the cathode catalyst layer. Experimental polarization curve, water proportion in the anode outlet gas and phosphoric acid distribution are selected for validation. It is found that the simulated results can represent the experimental data with reasonable accuracy. Based on the model, the effects of current density and stoichiometry on the variable distributions are analyzed. The results show that water in anode is mainly from cathode by concentration diffusion of liquid water. A higher current density leads to a greater electro-migration of phosphoric acid from cathode to anode and a higher liquid phase fraction in anode,while a lower phosphoric acid concentration in the fuel cells.

Effect of electrical poling on the structural, dielectric and photoluminescence properties of small concentration of Ho+3 substituted NBT

The effect of electrical poling on the room temperature structural, dielectric and photoluminescence properties of small concentration (i.e. 0.5 mole%) of Ho+3 substituted sodium bismuth titanate ferroelectric material (Na0.5Bi0.495Ho0.005TiO3 abbreviated as NBT-0.5Ho) has been investigated. Its crystal structure was found to be the mixture of two phases of rhombohedral (R3c) and monoclinic (Cc) in which monoclinic (Cc) coexisted as major phase. Comparative study of X-ray diffraction (XRD) patterns of electrically poled and unpoled specimens of NBT-0.5Ho revealed that electric field irreversibly transformed crystal structural of dominant Cc (≈-94.05% phase fraction) phase to R3c (≈70.6% phase fraction) as major phase. Dielectric value and its dispersion with frequency were significantly decreased in poled specimen which is ascribed to electric field driven structural change. Two photoluminescence (PL) emissions at 655nm and 756nm were obtained in NBT-0.5Ho. PL intensity was considerably tuned in effect of electrical poling in term of quenching. Obtain quenching is correlated with induced structural ordering towards higher symmetry phase (R3c) in effect of electric poling which is confirmed from XRD analysis. Obtained additional functionality of photoluminescence in the NBT-0.5Ho ferroelectric material and its tuning in effect of electric field opens the possibility in the material for optoelectronic devices applications.

Refinements in phase fraction determination of textured alloys from transmission diffraction data

The use of high-energy synchrotron X-ray diffraction sources has become increasingly common for high-quality phase fraction measurements and microstructural evolution experiments. While the high flux, large volume illuminated and large number of diffraction vectors should reduce common sources of uncertainty and bias, the distribution of the diffraction vectors may still cause bias in the phase fraction measurement. This hypothesis of bias was investigated with example experimental data and synthetic data. The authors found that there may be bias depending on the sample texture, the distribution of diffraction vectors and the hkl planes used in the phase fraction measurement, even for nearly complete coverage of a pole figure. The authors developed a series of geometry-based correction values that reduced the measurement bias due to sampling scheme and texture in the phase fraction measurement by an order of magnitude. The efficacy of these corrections was demonstrated with application to both experimental and synthetic data.

Tracking the Catalyst Layer Depth-Dependent Electrochemical Degradation of a Bimodal Pt/C Fuel Cell Catalyst: A Combined Operando Small- and Wide-Angle X-Ray Scattering Study

The combination of operando small- and wide-angle X-ray scattering (SAXS, WAXS) in grazing incidence configuration is presented as a new approach to provide depth-dependent insights into the changes in mean particle sizes and phase fractions occurring for fuel cell catalysts during accelerated stress tests (ASTs). As fuel cell catalyst, a bimodal Pt/C catalyst was chosen that consists of two distinguishable particle size populations. The presence of the two different sizes should favor and uncover electrochemical Ostwald ripening as the major degradation mechanism, i.e., it is expected that the size of the larger particles in the Pt/C catalyst grows at the expense of the smaller particles. The grazing incidence mode performed at the European Synchrotron Radiation Facility (ESRF) at the ID31 beamline revealed an intertwinement of the depth dependent degradation. While the larger particles show the same particle size changes close to the electrolyte-catalyst interface and within the catalyst layer, for the smaller Pt nanoparticles a different degradation scenario is observed. At the electrolyte-catalyst interface, the smaller particles increase in size while their phase fraction decreases during the AST. However, in the inner catalyst layer the phase fraction of smaller particles increases instead of decreases. The results of a depth-dependent degradation strongly suggest to employ a depth-dependent catalyst design for future improvement of the catalyst stability.

Effect of Phase Fraction Ratio Between Al3Mg2 and Mg2Si on the Oxidation Behavior of Al–Mg–Si Alloys

The effect of the phase fraction ratio between Al3Mg2 and Mg2Si on the oxidation resistance of Al–Mg–Si alloys at high temperatures was investigated. With addition of 1 mass%Si in Al-6 mass%Mg alloy, the as-cast microstructures showed formation of Mg2Si phase by eutectic reactions. With increasing Si content more than 3 mass%, the Mg2Si and Si are formed as eutectic phases with no β-Al3Mg2 phase. In addition, with an increase in the Si content from 3 mass%, significantly refined as-cast microstructures and distribution of extended eutectic phase areas were observed. The oxidized cross-sections of Al-6 mass%Mg and Al-6 mass%Mg-1 mass%Si alloys showed coarse and dark areas, which are considered as oxide clusters, nonuniformly grown into the matrix. However, Al-6 mass%Mg-3 mass%Si and Al-6 mass%Mg-5 mass%Si alloys had no significantly grown oxide clusters on the surfaces. Based on the results, it was concluded that the reduction of the ratio between β-Al3Mg2 and Mg2Si phases can reduce the rapid oxidation of Mg.