Effect of antimony on primary graphite growth in cast iron - From Ab-initio calculations to experimental observations

Chunky graphite - interconnected strings of graphite - is a common degenerate form of nodular graphite associated with the use of rare-earth for melt treatment. Being rare-earth scavenger, antimony is used to eliminate chunky graphite in large castings while it leads to other degenerate forms in small castings. In the present work, both theoretical and experimental approaches of the effect of antimony on graphite growth were attempted. Ab initio calculations of graphite layers and of 11 21 twin boundary showed the graphite structure is modified by Sb absorption, with the twin angle being smoothed and the graphite curved. Experiments were performed with carbon saturated Fe-C-Sb alloys where primary graphite precipitates appeared as long thin curved lamellae contrasting with the thick and straight plates obtained in Fe-C alloys. This similarity in curving effect of Sb at both scales is quite striking as there are four orders of magnitude separating the calculations from the metallographic observations

The influence of temperature on the strain-hardening behavior of Fe-22/25/28Mn-3Al-3Si TRIP/TWIP steels

The influence of temperature and stacking fault energy (SFE) on the strain-hardening behavior and critical resolved shear stress for twinning was investigated for three Fe–22/25/28Mn–3Al–3Si wt.% transformation- and twinning-induced plasticity (TRIP/TWIP) steels. The SFEs were calculated by two different methods, density functional theory and statistical thermodynamic modeling. The dislocation structure, observed at low levels of plastic deformation, transitions from “planar” to “wavy” dislocation glide with an increase in temperature, Mn content, and/or SFE. The change in dislocation glide mechanisms from planar to wavy reduces the strain hardening rate, in part due to fewer planar obstacles and greater cross slip activity. In addition, the alloys exhibit a large decrease in strength and ductility with increasing temperature from 25 to 200 °C, attributed to a substantial reduction in the thermally activated component of the flow stress, predominate suppression of TRIP and TWIP, and a significant increase in the critical resolved shear stress for mechanical twinning. Interestingly, the increase in SFE with temperature had a rather minor influence on the critical resolved shear stress for mechanical twinning, and other temperature dependent factors which likely play a more dominant role are discussed.This work is sponsored by the National Science Foundation Division of Materials Research, USA, under grants DMR0805295 and DMR1309258, by the Ministry of Science and Innovation of Spain, under Grant MAT2012–39124, and under a Center for Nanophase Materials Science user proposal CNMS2014–291 at Oak Ridge National Laboratory. DTP gratefully acknowledges support for extended visits to CSIC, Madrid and MPI, Düsseldorf during his time as a graduate student at Vanderbilt University where most of this research was performed. The authors would also like to acknowledge Easo George and Josh Cicotte for reviewing the manuscript

The influence of temperature on the strain-hardening behavior of Fe–22/25/28Mn–3Al–3Si TRIP/TWIP steels

The influence of temperature and stacking fault energy (SFE) on the strain-hardening behavior and critical resolved shear stress for twinning was investigated for three Fe–22/25/28Mn–3Al–3Si wt. transformation- and twinning-induced plasticity (TRIP/TWIP) steels. The SFEs were calculated by two different methods, density functional theory and statistical thermodynamic modeling. The dislocation structure, observed at low levels of plastic deformation, transitions from “planar” to “wavy” dislocation glide with an increase in temperature, Mn content, and/or SFE. The change in dislocation glide mechanisms from planar to wavy reduces the strain hardening rate, in part due to fewer planar obstacles and greater cross slip activity. In addition, the alloys exhibit a large decrease in strength and ductility with increasing temperature from 25 to 200 °C, attributed to a substantial reduction in the thermally activated component of the flow stress, predominate suppression of TRIP and TWIP, and a significant increase in the critical resolved shear stress for mechanical twinning. Interestingly, the increase in SFE with temperature had a rather minor influence on the critical resolved shear stress for mechanical twinning, and other temperature dependent factors which likely play a more dominant role are discussed. © 202

Ab initio calculations of elastic properties of Ru1-xNixAl superalloys

Contains fulltext : 76051.pdf (publisher's version ) (Open Access)3 p

Impact of nanodiffusion on the stacking fault energy in high-strength steels

A key requirement of modern steels – the combination of high strength and high deformability – can best be achieved by enabling a local adaptation of the microstructure during deformation. A local hardening is most efficiently obtained by a modification of the stacking sequence of atomic layers, resulting in the formation of twins or martensite. Combining ab initio calculations with in situ transmission electron microscopy, we show that the ability of a material to incorporate such stacking faults depends on its overall chemical composition and, importantly, the local composition near the defect, which is controlled by nanodiffusion. Specifically, the role of carbon for the stacking fault energy in high-Mn steels is investigated. Consequences for the long-term mechanical properties and the characterisation of these materials are discussed

Strengthening and strain hardening mechanisms in a precipitation-hardened high-Mn lightweight steel

We report on the strengthening and strain hardening mechanisms in an aged high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C, wt.%) studied by electron channeling contrast imaging (ECCI), transmission electron microscopy (TEM), atom probe tomography (APT) and correlative TEM/APT. Upon isothermal annealing at 600 °C, nano-sized κ-carbides form, as characterized by TEM and APT. The resultant alloy exhibits high strength and excellent ductility accompanied by a high constant strain hardening rate.In comparison to the as-quenched κ-free state, the precipitation of κ-carbides leads to a significant increase in yield strength (∼480 MPa) without sacrificing much tensile elongation. To study the strengthening and strain hardening behavior of the precipitation-hardened material, deformation microstructures were analyzed at different strain levels. TEM and correlative TEM/APT results show that the κ-carbides are primarily sheared by lattice dislocations, gliding on the typical face-centered-cubic (fcc) slip system {111}, leading to particle dissolution and solute segregation. Ordering strengthening is the predominant strengthening mechanism. As the deformation substructure is characterized by planar slip bands, we quantitatively studied the evolution of the slip band spacing during straining to understand the strain hardening behavior. A good agreement between the calculated flow stresses and the experimental data suggests that dynamic slip band refinement is the main strain hardening mechanism. The influence of κ-carbides on mechanical properties is discussed by comparing the results with that of the same alloy in the as-quenched, κ-free state

Strengthening and strain hardening mechanism in a high-Mn lightweight steel

Proceedings: Wolfgang Bleck; Dierk RaabeEditing: Sonja Brühl4th HMnS / IEHK, RWTH Aachen UniversityProceedings (2019) 180 - 183RWTH-2019-0375