ALBERT

Description: A detailed transmission electron microscopy study coupled with electron energy loss spectroscopy was conducted on AlN formed by the direct nitridation of Al particles under nitrogen atmosphere. The nitridation mechanism comprised two steps: the formation of AlN shell on Al particles and the growth of AlN with a lath type in Al droplets. Here, we found that the surface oxide layer of the Al particles acted as a channel layer, which supplied nitrogen in the atomic state to liquid Al, after being transformed into a thin AlON layer during the initial nitridation. In the Al particles, the inward growth of AlN with a shell structure occurred at the sub layer of the AlON layer. On the other hand, the extracted liquid Al droplets formed after the cracking of the AlN shell rested on the Al particles surrounded by the AlON layer. The nitridation of the droplets began at the interface between the Al particle and droplet and not at the free surface and grew outward from the droplet. Herein, based on the observation of the AlON layer formation, we propose a new mechanism for the nitridation of Al particles.

Description: In order to achieve a fine uniform grain-size distribution using the process of thin slab casting and directing rolling (TSCDR), it is necessary to control the grain-size prior to the onset of thermomechanical processing. In the companion paper, Model Fe - Al Steel with Exceptional Resistance to High Temperature Coarsening. Part I: Coarsening Mechanism and Particle Pinning Effects , a new steel composition which uses a small volume fraction of austenite particles to pin the growth of delta-ferrite grains at high temperature was proposed and grain growth was studied in reheated samples. This paper will focus on the development of a simple laboratory-scale setup to simulate thin-slab casting of the newly developed steel and demonstrate the potential for grain size control under industrial conditions. Steel bars with different diameters are briefly dipped into the molten steel to create a shell of solidified material. These are then cooled down to room temperature at different cooling rates. During cooling, the austenite particles nucleate along the delta-ferrite grain boundaries and greatly retard grain growth. With decreasing temperature, more austenite particles precipitate, and grain growth can be completely arrested in the holding furnace. Additional applications of the model alloy are discussed including grain-size control in the heat affected zone in welds and grain-growth resistance at high temperature.

Description: A 3D phase-field (PhF) model has been developed to study the formation of micropores constrained to grow within a well-developed solid network. The model accounts for the equilibrium condition at triple (solid–liquid–pore) lines and the partitioning of dissolved gases. Growth of a micropore in a representative volume element taken from X-ray tomography was simulated, which offered the possibility to study the pore evolution in temporal and spatial resolutions that are not possible in experiments.

Description: The grain coarsening temperature ( T c ) is quite low in a ferrite/pearlite (F/P) banded steel as compared with a non-banded steel. Here it is shown that the low T c in the F/P banded steel can be increased significantly by applying cold deformation prior to austenitizing. The kinetics of abnormal grain coarsening above T c is also largely retarded by cold deformation. These observations should be attributed to the uniform distribution of fine AlN precipitates caused by cold deformation.

Description: The thin slab casting and direct rolling process is a hot-rolled strip production method which has maintained commercial quality steel grades as a major material in many industrial applications due to its low processing cost. Few innovative products have however been developed specifically for production by thin slab direct rolling. Press hardening or hot press forming steel grades which are now widely used to produce structural automotive steel parts requiring ultra-high strength and formability may however offer an opportunity for thin slab direct rolling-specific ultra-high strength products. In this work, a newly designed press hardening steel grade developed specifically for thin slab direct rolling processing is presented. The press hardening steel has a high nitrogen content compared with press hardening steel grades produced by conventional steelmaking routes. Boron and titanium which are key alloying additions in conventional press hardening steel such as the 22MnB5 press hardening steel grade are not utilized. Cr is added in the press hardening steel to obtain the required hardenability. The properties of the new thin slab direct rolling-specific 22MnCrN5 press hardening steel grade are reviewed. The evolution of the microstructure and mechanical properties with increasing amounts of Cr additions from 0.6 to 1.4 wt pct and the effect of the cooling rate during die-quenching were studied by means of laboratory simulations. The selection of the optimum chemical composition range for the thin slab direct rolling-specific 22MnCrN5 steel in press hardening heat treatment conditions is discussed.

Description: Structural transitions of nanometer-sized NiMn preprecipitates during isothermal aging of Fe-Ni-Mn alloys have been faintly indicated by the measurement of hardness and electrical resistivity changes. It is shown that a precise measurement of X-ray diffraction (XRD) line breadths in association with Mössbauer spectroscopy aims a better understanding of the structural transition of the precipitation nanostructures. Changes in the XRD line breadths clearly identified three stages corresponding to the stimulated clustering of solute elements, the formation of Guinier–Preston (GP) zones and subsequent transformation to the second-phase precipitates. Effective hardening takes place when preliminary solute clusters evolve to paramagnetic GP zones.

Description: The present work analyzes the application of quenching and partitioning processing to medium Mn steel to obtain a new type of ultra-high-strength multiphase medium Mn steel. The selection of the quench temperature makes it possible to vary the ultimate tensile strength within a range of 500 MPa. The processing leads to low-carbon lath martensite matrix with a controlled volume fraction of retained austenite.

Description: U-Zr metallic fuels cladded in Fe-alloys are being considered for application in an advanced sodium-cooled fast reactor that can recycle the U-Zr fuels and minimize the long-lived actinide waste. To understand the complex fuel-cladding chemical interaction between the U-Zr metallic fuels with Fe-alloys, a systematic multicomponent diffusion study was carried out using solid-to-solid diffusion couples. The U-10 wt pct Zr vs pure Fe diffusion couples were assembled and annealed at temperatures, 903 K, 923 K, and 953 K (630 °C, 650 °C, and 680 °C) for 96 hours. Development of microstructure, phase constituents, and compositions developed during the thermal anneals were examined by scanning electron microscopy, transmission electron microscopy, and X-ray energy dispersive spectroscopy. Complex microstructure consisting of several layers that include phases such as U 6 Fe, UFe 2 , ZrFe 2 , α -U, β -U, Zr-precipitates, χ , ε , and λ were observed. Multi-phase layers were grouped based on phase constituents and microstructure, and the layer thicknesses were measured to calculate the growth constant and activation energy. The local average compositions through the interaction layer were systematically determined, and employed to construct semi-quantitative diffusion paths on isothermal U-Zr-Fe ternary phase diagrams at respective temperatures. The diffusion paths were examined to qualitatively estimate the diffusional behavior of individual components and their interactions. Furthermore, selected area electron diffraction analyses were carried out to determine, for the first time, the exact crystal structure and composition of χ , ε , and λ -phases. The χ , ε , and λ -phases were identified as Pnma (62) Fe(Zr,U), I 4/ mcm (140) Fe(Zr,U) 2 , and P 42/ mnm (136) U 3 (Zr,Fe), respectively.

Description: In situ heating, synchrotron radiation X-ray diffraction has been used to study the alloying and phase transformation behavior of TiH 2 -6Al-4V and Ti-6Al-4V alloys. Accelerated alloying and phase transformation were observed in the powder compact of the TiH 2 -6Al-4V alloy subjected to a high heating rate. In addition, an oxygen-stabilized Ti 4 Fe 2 O phase, which is present as sub-micron or nanoscaled particles, has been identified in the TiH 2 -6Al-4V alloy. The implications of these experimental findings have been discussed in terms of alloying, improved densification and oxygen scavenging in titanium and titanium alloys.