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We have examined spheroidization and coarsening of cementite in an initially pearlitic 2C-4Cr ultrahigh carbon steel containing a cementite network. Coarsening kinetics of spheroidized cementite and growth of denuded zones adjacent to the cementite network were investigated by analyzing particle sizes from digital micrographs of water-quenched steel etched with Nital. Denuded zones grew at a rate proportional to t 1/4 – t 1/5 . Spheroidization of pearlite was completed within 90 minutes at 1073 K and 1173 K (800 °C and 900 °C), and within 5 minutes at 1243 K (970 °C). Bimodal particle size distributions were identified in most of the samples and were more pronounced at higher temperatures and hold times. Peaks in the distributions were attributed to the coarsening of intragranular and grain boundary particles at different rates. A third, non-coarsening peak of particles was present at 1073 K (800 °C) only and was attributed to particles existing prior to the heat treatment. Particle sizes were plotted vs time to investigate possible coarsening mechanisms. The coarsening exponent for the growth of grain boundary carbides was closest to 4, indicating grain boundary diffusion control. The coarsening exponent was closest to 5 for intragranular carbides, indicating suppression of volumetric diffusion (possibly due to reduced effective diffusivity because of Cr alloying) and control by dislocation diffusion.

Ultrahigh carbon steels (UHCS) containing 1 wt pct Cr (1Cr UHCS) were heat treated at 1073 K, 1173 K, or 1243 K (800 °C, 900 °C, or 970 °C) for durations of 5 minutes up to 24 hours to study cementite coarsening. Results were compared to a previous study on coarsening of a UHCS with 4 wt pct Cr (4Cr UHCS) and significantly different behavior was observed. In the heat-treated 1Cr UHCS, particles clustered in zones that were tens of microns near branches of the cementite network. The opposite trend was observed in the 4Cr UHCS, in which the regions within a few microns near the cementite network were entirely denuded of particles. Causes for differences in coarsening behavior in 1Cr and 4Cr UHCS are discussed.

In this study, two different melting methods were used to investigate effects of Nb modification on microstructure in ultrahigh carbon steel (UHCS). Nb-free and Nb-modified UHCS samples were produced by melting and resolidifying an industrially produced base UHCS with and without addition of Nb powder. Microstructure was characterized using scanning electron microscopy, X-ray diffraction, and electron dispersive spectroscopy. Equilibrium computations of phase fractions and compositions were utilized to help describe microstructural changes caused by the Nb additions. Nb combined with C to form NbC structures before and during austenite solidification, reducing the effective amount of carbon available for the other phases. Cementite network spacing in the Nb-free samples was controlled by the cooling rate during solidification (faster cooling led to a more refined network). Network spacing in the Nb-modified UHCS could be enlarged by NbC structures that formed cooperatively with austenite.

We present a new microstructure dataset consisting of ultrahigh carbon steel (UHCS) micrographs taken over a range of length scales under systematically varied heat treatments. Using the UHCS dataset as a case study, we develop a set of visualization tools for interacting with and exploring large microstructure and metadata datasets. Based on generic microstructure representations adapted from the field of computer vision, these tools enable image-based microstructure retrieval, as well as spatial maps of both microstructure and related metadata, such as processing conditions or properties measurements. We provide the microstructure image data, processing metadata, and source code for these microstructure exploration tools. The UHCS dataset is intended as a community resource for development and evaluation of microstructure data science techniques and for creation of microstructure data science teaching modules.

The developing human cortex contains diverse populations of neural progenitor cells, including a large proportion of outer radial glia (ORG), a progenitor type that is rare in the mouse. The authors identify a transcriptional signature of ORG characterized by markers of neuronal lineage fate and use single-cell analyses to contrast the heterogeneity of cortical progenitors across human, mouse and ferret. The human cerebral cortex depends for its normal development and size on a precisely controlled balance between self-renewal and differentiation of diverse neural progenitor cells. Specialized progenitors that are common in humans but virtually absent in rodents, called outer radial glia (ORG), have been suggested to be crucial to the evolutionary expansion of the human cortex. We combined progenitor subtype–specific sorting with transcriptome-wide RNA sequencing to identify genes enriched in human ORG, which included targets of the transcription factor neurogenin and previously uncharacterized, evolutionarily dynamic long noncoding RNAs. Activating the neurogenin pathway in ferret progenitors promoted delamination and outward migration. Finally, single-cell transcriptional profiling in human, ferret and mouse revealed more cells coexpressing proneural neurogenin targets in human than in other species, suggesting greater neuronal lineage commitment and differentiation of self-renewing progenitors. Thus, we find that the abundance of human ORG is paralleled by increased transcriptional heterogeneity of cortical progenitors.