Exploring the transfer of plasticity across Laves phase interfaces in a dual phase magnesium alloy

The mechanical behaviour of Mg-Al alloys can be largely improved by the formation of an intermetallic Laves phase skeleton, in particular the creep strength. Recent nanomechanical studies revealed plasticity by dislocation glide in the (Mg,Al)$_2$Ca Laves phase, even at room temperature. As strengthening skeleton, this phase remains, however, brittle at low temperature. In this work, we present experimental evidence of slip transfer from the Mg matrix to the (Mg,Al)$_2$Ca skeleton at room temperature and explore associated mechanisms by means of atomistic simulations. We identify two possible mechanisms for transferring Mg basal slip into Laves phases depending on the crystallographic orientation: a direct and an indirect slip transfer triggered by full and partial dislocations, respectively. Our experimental and numerical observations also highlight the importance of interfacial sliding that can prevent the transfer of the plasticity from one phase to the other.Comment: 23 pages, 8 figures, 1 tabl

Combined κ-carbide precipitation and recovery enables ultra-high strength and ductility in light-weight steels

Light-weight high-manganese steels with high aluminum and carbon contents are known to possess high age-hardening potential due to the precipitation of nano-sized κ-carbides. Although in recrystallized material the precipitation of the κ-phase has been widely researched, the influence of pre-deformation/deformation microstructure on the formation of κ-carbides is still unknown. The present study aims at gaining an increased understanding of the underlying mechanisms (and their interplay) active during annealing after pre-deformation, i.e. concurrent strengthening by precipitation of κ-carbides and softening by recovery/recrystallization. Therefore, a Fe-29.8Mn-7.65Al-1.11C steel was age-hardened in deformed/cold-rolled as well as in recrystallized condition. The microstructure and tensile behavior of the material in both states after annealing treatments at 600–800 °C were investigated by means of light optical microscopy, (high-resolution) scanning transmission electron microscopy, x-ray diffraction and hardness measurements as well as tensile tests. The focus was put on understanding the interplay between microstructural defects (e.g. dislocations), precipitation of κ-carbides, and recovery/recrystallization. The fundamental dependence of strengthening and softening mechanisms on the precondition, annealing parameters and correlation with the mechanical properties are discussed

Mechanisms of austenite growth during intercritical annealing in medium manganese steels

The third-generation advanced high strength medium manganese (3–12 wt.) steels typically consist of ultrafine-grained dual-phase (austenite-ferrite) microstructure, obtained through the intercritical annealing of martensite at temperatures typically ≤ 0.5Tmelt, where the bulk diffusion of Mn is extremely slow. Yet, the manganese partitioning plays a prominent role in the austenite growth from the martensitic matrix during this annealing step. Therefore, the ‘short circuit’ diffusion paths provided by grain boundaries (GBs) and dislocations must be crucial to the austenite growth. However, this influence is not well understood across the literature. In the present work, we study the mechanisms of austenite growth in a cold-rolled intercritically annealed medium manganese steel of composition Fe-10Mn-0.05C–1.5Al (wt.). We provide evidence of manganese transport to austenite through GB diffusion, GB migration and dislocation pipe diffusion. Furthermore, the influence of GB misorientation on austenite growth is also reported. © 202

Atomic layer deposition and high-resolution electron microscopy characterization of nickel nanoparticles for catalyst applications

Ni nanoparticles (diameter < 10 nm) are deposited on Si and ceramic substrates of porous lanthanum-substituted strontium titanate/yttrium-stabilized zirconia (LST/YSZ) composites by a two-step process. First, NiO films are produced by atomic layer deposition at 200 °C using bis(methylcyclopentadienyl)nickel(II) (Ni(MeCp)2) and H2O as precursors. In the second step, the NiO films are reduced in H2 atmosphere at 400–800 °C. The size of the resulting Ni nanoparticles is controlled by the temperature. The largest particles with a diameter of about 7 nm are obtained at 800 °C. NiO film and Ni nanoparticles deposited on Si substrates are characterized by high-resolution electron microscopy. It was found that the Ni(MeCp)2 precursor reacts with the substrate, leading to the formation of NiSi2 precipitates beneath the surface of the Si wafer and amorphization of the surrounding area, resulting in a 10 nm thick top layer of the Si wafer. After reductive annealing, NiSi2 precipitates are preserved but Si recrystallizes and the amorphous NiO film transforms into crystalline Ni nanoparticles well distributed on the wafer surface. Process parameters were optimized for Si substrates and transfer of the process to ceramic LST/YSZ substrates is possible in principle. However, a much higher number of ALD cycles (1200 compared to 100 for Si) are necessary to obtain Ni nanoparticles of similar size and the number density of particles is lower than observed for Si substrates

Development of a Cr-Ni-V-N Medium Manganese Steel with Balanced Mechanical and Corrosion Properties

A novel medium manganese (MMn) steel with additions of Cr (18%), Ni (5%), V (1%), and N (0.3%) was developed in order to provide an enhanced corrosion resistance along with a superior strength–ductility balance. The laboratory melted ingots were hot rolled, cold rolled, and finally annealed at 1000 °C for 3 min. The recrystallized single-phase austenitic microstructure consisted of ultrafine grains (~1.3 µm) with a substantial amount of Cr- and V-based precipitates in a bimodal particle size distribution (100–400 nm and <20 nm). The properties of the newly developed austenitic MMn steel X20CrNiMnVN18-5-10 were compared with the standard austenitic stainless steel X5CrNi18-8 and with the austenitic twinning-induced plasticity (TWIP) steel X60MnAl17-1. With a total elongation of 45%, the MMn steel showed an increase in yield strength by 300 MPa and in tensile strength by 150 MPa in comparison to both benchmark steels. No deformation twins were observed even after fracture for the MMn steel, which emphasizes the role of the grain size and precipitation-induced change in the austenite stability in controlling the deformation mechanism. The potentio-dynamic polarization measurements in 5% NaCl revealed a very low current density value of 7.2 × 10−4 mA/cm2 compared to that of TWIP steel X60MnAl17-1 of 8.2 × 10−3 mA/cm2, but it was relatively higher than that of stainless steel X5CrNi18-8 of 2.0 × 10−4 mA/cm2. This work demonstrates that the enhanced mechanical properties of the developed MMn steel are tailored by maintaining an ultrafine grain microstructure with a significant amount of nanoprecipitates, while the high corrosion resistance in 5% NaCl solution is attributed to the high Cr and N contents as well as to the ultrafine grain siz

Elucidating the Influence of the d-Band Center on the Synthesis of Isobutanol

As the search for carbon-efficient synthesis pathways for green alternatives to fossil fuels continues, an expanding class of catalysts have been developed for the upgrading of lower alcohols. Understanding of the acid base functionalities has greatly influenced the search for new materials, but the influence of the metal used in catalysts cannot be explained in a broader sense. We address this herein and correlate our findings with the most fundamental understanding of chemistry to date by applying it to d-band theory as part of an experimental investigation. The commercial catalysts of Pt, Rh, Ru, Cu, Pd, and Ir on carbon as a support have been characterized by means of SEM, EDX-mapping, STEM, XRD, N2-physisorption, and H2-chemisorption. Their catalytic activity has been established by means of c-methylation of ethanol with methanol. For all catalysts, the TOF with respect to i-butanol was examined. The Pt/C reached the highest TOF with a selectivity towards i-butanol of 89%. The trend for the TOFs could be well correlated with the d-band centers of the metal, which formed a volcano curve. Therefore, this study is another step towards the rationalization of catalyst design for the upgrading of alcohols into carbon-neutral fuels or chemical feedstock

Impact of precipitates on the hydrogen embrittlement behavior of a V-alloyed medium-manganese austenitic stainless steel

Abstract This paper discusses the avoidance of hydrogen embrittlement (HE) in a medium manganese stainless steel X20CrNiMnVN18–5–10. We adopted a HE-mitigation strategy that relies on improving its intrinsic resistance to hydrogen by adjusting an ultrafine microstructure (∼1.3 µm) containing a significant amount of nano-sized V- and Cr-based precipitates in the size range of 20 – ≥200 nm. The precipitation state was characterized using a high-resolution scanning transmission electron microscope. Slow strain rate tests at a strain rate of 10⁻⁶ s⁻¹ were conducted on specimens with/without hydrogen pre-charging to evaluate the HE susceptibility. Thermal desorption analysis was applied to explore the hydrogen trapping behavior in cold-rolled, annealed and hydrogen pre-charged states. Hydrogen uptake and hydrogen desorption behaviors show a dependence on the size of precipitates. It is remarked that the large precipitates trap a larger amount of hydrogen and show a higher temperature desorption peak than the small precipitates do. The high-temperature hydrogen desorption peaks (&gt;400 °C) indicate that the observed nano-sized precipitates provide irreversible trapping sites, where hydrogen uptake occurs. The investigated steel X20CrNiMnVN18–5–10 demonstrates an enhanced intrinsic resistance to HE in comparison to medium and high manganese as well as stainless steels. The findings suggest that microstructure engineering with sufficient number of hydrogen traps in an ultrafine-grained microstructure is an appropriate HE mitigation strategy that allows designing hydrogen-resistant advanced high strength steels

Co-deformation between the metallic matrix and intermetallic phases in a creep-resistant Mg-3.68Al-3.8Ca alloy

The microstructure of Mg-Al-Ca alloys consists of a hard intra- and intergranular eutectic Laves phase network embedded in a soft α-Mg matrix. For such heterogeneous microstructures, the mechanical response and co-deformation of both phases under external load are not yet fully understood. We therefore used nano- and microindentation in combination with electron microscopy to study the deformation behaviour of an Mg-3.68Al-3.8Ca alloy.We found that the hardness of the Mg2Ca phase was significantly larger than the α-Mg phase and stays constant within the measured temperature range. The strain rate sensitivity of the softer α-Mg phase and of the interfaces increased while activation volume decreased with temperature. The creep deformation of the Mg2Ca Laves phase was significantly lower than the α-Mg phase at 170 °C. Moreover, the deformation zone around and below microindents was dependant on the matrix orientation and was influenced by the presence of Laves phases. Most importantly, slip transfer from the α-Mg phase to the (Mg,Al)2Ca Laves phase occurred, carried by the basal planes. Based on the observed orientation relationship and active slip systems, a slip transfer mechanism from the soft α-Mg phase to the hard Laves phase is proposed. Further, we present implications for future alloy design strategies