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 degrees 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.

Atom location by channeling enhanced microanalysis (ALCHEMI) has been used to characterize the site distributions of Nb and Cr alloying additions in the Ll0-ordered γ phase of ternary titanium aluminides. Two alloys, Ti50Al48Cr2 and Ti50Al48Nb2 were processed by furnace cooling from 1300°C (within the α-γ two phase field) as well as by rapid solidification using twin-anvil splat quenching of electromagnetically levitated and undercooled samples. ALCHEMI studies of furnace cooled samples yield results generally consistent with those in the published literature. Nb alloying additions are found to partition exclusively to the ‘Ti’ sublattice, while Cr alloying additions exhibit an ‘Al’ sublattice preference. However, a higher degree of disorder can be achieved with rapid solidification and high solid state cooling rates (105-106 K/s). Significant distribution of the ternary elements between the ‘Ti’ and ‘Al’ sublattices has been measured in the splat quenched samples, with up to 12% of the Nb atoms occupying the ‘Al’ sublattice and the fraction of Cr atoms on the ‘Ti’ sublattice doubling to ~30%. Rapid solidification of TiAl produces an equiaxed hexagonal α phase solidification structure that transforms in a massive fashion to the tetragonal γ phase. Although the amount of massively transformed γ is dependent upon the solid state cooling rate, ternary alloying additions can more strongly influence the transformation kinetics. The Nb-modified alloy exhibits significant amounts of the massively tranformed γ, similar to the Ti50Al48 binary alloy, whereas little massively transformed γ is observed in the Cr-modified alloy. These results can be correlated with the relative atomic size, lattice distortion, and sublattice site occupancy of Nb and Cr in the Ll0 unit cell.

This study investigates the high strain-rate tensile properties of a cold-rolled medium-Mn steel (Fe-12Mn-3Al-0.05C % in mass fraction) designed to have a multi-phase microstructure and positive strain-rate sensitivity. At the intercritical annealing temperature of 585 °C, increasing the annealing time from 0.5 h to 8 h increased the phase volume fraction of ultrafine-grained (UFG) austenite from 2% to 35% by reversion. The remainder of the microstructure was composed of UFG ferrite and recovered α'-martensite (the latter resembles the cold-rolled state). Servo hydraulic tension testing and Kolsky-bar tension testing were used to measure the tensile properties from quasi-static strain rates to dynamic strain rates ( ε ˙ = 10 - 4 s - 1 to ε ˙ = 10 3 s - 1 ). The strain-rate sensitivities of the yield strength (YS) and ultimate tensile strength (UTS) were positive for both annealing times. Tensile properties and all non-contact imaging modalities (infrared imaging and digital image correlation) indicated an advantageous suppression of Lüders bands and Portevin Le Chatelier (PLC) bands (a critical challenge in multi-phase medium-Mn steel design) due to the unique combination of microstructural constituents and overall composition. Fracture surfaces of specimens annealed for 0.5 h showed some instances of localized cleavage fracture (approximately 30 μm wide areas and lath-like ridges). Specimens annealed for 8 h maintained a greater product of strength and elongation by at least 2.5 GPa % (on average for each strain rate). The relevant processing-structure-property relationships are discussed in the context of recommendations for design strategies concerning multi-phase steels such that homogeneous deformation behavior and positive strain-rate sensitivities can be achieved.

In the current work we investigate the room temperature tensile properties of a medium-Mn twinning- and transformation-induced plasticity (TWIP-TRIP) steel from quasi-static to low-dynamic strain rates ( ε ˙ = 10 - 4 s - 1 to ε ˙ = 10 2 s - 1 ). The multi-phase microstructure consists of coarse-grained recovered α' -martensite (inherited from the cold-rolled microstructure), multiple morphologies of ultrafine-grained (UFG) austenite (equiaxed, rod-like and plate-like), and equiaxed UFG ferrite. The multi-phase material exhibits a positive strain-rate sensitivity for yield and ultimate tensile strengths. Thermal imaging and digital image correlation allow for in situ measurements of temperature and local strain in the gauge length during tensile testing, but Lüders bands and Portevin Le Chatelier bands are not observed. A finite-element model uses empirical evidence from electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), plus constitutive equations to dissect the microstructural influences of grain size, dislocation density and TWIP-TRIP driving forces on tensile properties. Calibration of tensile properties not only captures the strain rate sensitivity of the multi-phase TWIP-TRIP steel, but also provides opportunity for a complete parametric analysis by changing one variable at a time (phase fraction, grain size, strain-induced twin fraction and strain-induced ε-martensite fraction). An equivalent set of high-rate mechanical properties can be matched by changing either the austenite phase fraction or the ratio of twinning vs. transformation to ε-martensite. This experimental-computational framework enables the prediction of mechanical properties in multi-phase steels beyond the experimental regime by tuning variables that are relevant to the alloy design process.

A medium-Mn steel (Fe-12Mn-3Al-0.05C wt%) was designed using Thermo-Calc (R) simulations to balance the fraction and stacking fault energy of reverted austenite. lntercritical annealing for 0.5, 8 and 48 h was carried out at 585 degrees C to investigate the microstructural evolution. X-ray diffraction (XRD), electron backscatter diffraction (EBSD), 3-dimensional EBSD, energy-dispersive spectroscopy via scanning transmission electron microscopy (STEM-EDS) and atom probe tomography (APT) enable characterization of phase fraction, grain area, grain morphology and alloy partitioning. An increase in annealing time from 0.5 h to 48 h increases the amount of ultrafine-grained (UFG) reverted austenite from 3 to 40 vol %. EBSD and TEM reveal multiple morphologies of UFG austenite (equiaxed, rod-like and plate-like). In addition, most of the remaining microstructure consists of recovered alpha'-martensite that resembles the cold-rolled state, as well as a relatively small fraction of UFG ferrite (i.e., only a small amount of martensite recrystallization occurs). Multi-scale characterization results show that the location within the cold-rolled microstructure has a strong influence on boundary mobility and grain morphology during austenite reversion. Results from APT reveal Mn-decoration of dislocation networks and low-angle lath boundaries in the recovered alpha'-martensite, but an absence of Mn-decoration of defects in the vicinity of austenite grains, thereby promoting recovery. STEM-EDS and APT reveal Mn depletion zones in the ferrite/recovered alpha'-martensite near austenite boundaries, whereas gradients of C and Mn co-partitioning are visible within some of the austenite grains after annealing for 0.5 h. Relatively flat C enriched austenite boundaries are present even after 8 h of annealing and indicate certain boundaries possess low mobility. At later stages the growth of austenite followed the local equilibrium (LE) model such that the driving force between two equilibrium phases moves the mobile interface, as confirmed by DICTRA simulations (a Thermo-Calc (R) diffusion module). The sequence of austenite reversion is: (i) formation of Mn- and C-enriched face-centered-cubic nuclei from decorated dislocations and/or particles; (ii) co-partitioning of Mn and C and (iii) growth of austenite controlled by the LE mode. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The effects of quasi-static and low-dynamic strain rate (ε̇ = 10−4 /s to ε̇ = 102 /s) on tensile properties and deformation mechanisms were studied in a Fe-25Mn-3Al-3Si (wt%) twinning and transformation-induced plasticity [TWIP-TRIP] steel. The fully austenitic microstructure deforms primarily by dislocation glide but due to the room temperature stacking fault energy [SFE] of 21 ± 3 mJ/m2 for this alloy, secondary deformation mechanisms such as mechanical twinning (TWIP) and epsilon martensite formation (TRIP) also play an important role in the deformation behavior. The mechanical twins and epsilon-martensite platelets act as planar obstacles to subsequent dislocation motion on non-coplanar glide planes and reduce the dislocation mean free path. A high-speed thermal camera was used to measure the increase in specimen temperature as a function of strain, which enabled the use of a thermodynamic model to predict the increase in SFE. The influence of strain rate and strain on microstructural parameters such as the thickness and spacing of mechanical twins and epsilon-martensite laths was quantified using dark field transmission electron microscopy, electron channeling contrast imaging, and electron backscattered diffraction. The effect of sheet thickness on mechanical properties was also investigated. Increasing the tensile specimen thickness increased the product of ultimate tensile strength and total elongation, but had no significant effect on uniform elongation or yield strength. The yield strength exhibited a significant increase with increasing strain rate, indicating that dislocation glide becomes more difficult with increasing strain rate due to thermally-activated short-range barriers. A modest increase in ultimate tensile strength and minimal decrease in uniform elongation were noted at higher strain rates, suggesting adiabatic heating, slight changes in strain-hardening rate and observed strain localizations as root causes, rather than a significant change in the underlying TWIP-TRIP mechanisms at low values of strain.

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Robert (Bob) Sinclair and Nestor Zaluzec have been working for many years at the leading edge of developments in electron microscopy techniques and applications. Their distinguished careers and some of their notable scientific achievements are briefly highlighted.

In situ high-resolution electron microscopy was used to reveal information at the atomic level for the disordered-to-ordered phase transformation of equiatomic FePt nanoparticles that can exhibit outstanding magnetic properties after transforming from disordered face-centered-cubic into the tetragonal L10 ordered structure. High-angle annular dark-field imaging in the scanning transmission electron microscope provided sufficient contrast between the Fe and Pt atoms to readily monitor the ordering of the atoms during in situ heating experiments. However, during continuous high-magnification imaging the electron beam influenced the kinetics of the transformation so annealing had to be performed with the electron beam blanked. At 500°C where the reaction rate was relatively slow, observation of the transformation mechanisms using this sequential imaging protocol revealed that ordering proceeded from (002) surface facets but was incomplete and multiple-domain particles were formed that contained anti-phase domain boundaries and anti-site defects. At 600 and 700°C, the limitations of sequential imaging were revealed as a consequence of increased transformation kinetics. Annealing for only 5min at 700°C produced complete single-domain L10 order; such single-domain particles were more spherical in shape with (002) facets. The in situ experiments also provided information concerning nanoparticle sintering, coalescence, and consolidation. Although there was resistance to complete sintering due to the crystallography of L10 order, the driving force from the large surface-area-to-volume ratio resulted in considerable nanoparticle coalescence, which would render such FePt nanoparticles unsuitable for use as magnetic recording media. Comparison of the in situ data acquired using the protocol described above with parallel ex situ annealing experiments showed that identical behavior resulted in all cases.

The twinning-induced plasticity effect enables designing austenitic Fe-Mn-C-based steels with >70% elongation with an ultimate tensile strength >1 GPa. These steels are characterized by high strain hardening due to the formation of twins and complex dislocation substructures that dynamically reduce the dislocation mean free path. Both mechanisms are governed by the stacking-fault energy (SFE) that depends on composition. This connection between composition and substructure renders these steels ideal model materials for theory-based alloy design: Ab initio-guided composition adjustment is used to tune the SFE, and thus, the strain-hardening behavior for promoting the onset of twinning at intermediate deformation levels where the strain-hardening capacity provided by the dislocation substructure is exhausted. We present thermodynamic simulations and their use in constitutive models, as well as electron microscopy and combinatorial methods that enable validation of the strain-hardening mechanisms.

A new class of austenitic steels stabilized with high Mn contents (instead of Ni) exhibits exceptional mechanical properties, such as large energy absorption and high work‐hardening rate, owing to secondary deformation mechanisms such as mechanical twinning‐induced plasticity (TWIP) and martensitic transformation‐induced plasticity (TRIP) favored for low stacking‐fault energy (SFE) [1]. The interaction of dislocations with twin boundaries and martensite interfaces during mechanical deformation enhances the work hardening, i.e., a dynamic Hall‐Petch effect, with total elongations exceeding 70% and ultimate tensile strengths in the GPa regime. The influence of the strain rate, temperature, and changes in SFE on the deformation mechanisms in high‐Mn austenitic steels has been investigated using electron backscattered diffraction (EBSD), electron‐channeling contrast imaging (ECCI), conventional bright‐field/dark‐field imaging (BF/DF), and aberration‐corrected high‐resolution scanning/transmission electron microscopy (HRTEM/HRSTEM). The TWIP/TRIP secondary deformation mechanisms are related to the low SFE exhibited in these materials. Experimentally measured SFE from weak‐beam‐dark‐field (WBDF) imaging provides the basis to understand how changes in SFE influence mechanical twinning versus transformation induced martensite [2‐3]. However, adiabatic heating during deformation at high strain rates (20 ‐10,000 s ‐1 ) increases the SFE. Quantifying the twin or martensite density by EBSD/ECCI and BF‐DF images allows for comparison of the secondary deformation at different SFE, strain rates, and total elongation, but to study the details of the deformation mechanisms requires imaging at atomic resolution using aberration‐corrected electron microscopy. Figure 1(a) shows the EBSD/ECCI experimental procedure. EBSD identifies grains with a [110] orientation that are subsequently imaged using backscattered electrons where the incident beam strongly channels except for areas with twinning and martensite plates. Although the EBSD/ECCI method provides for a statistical number of measurements, the DF imaging method shown in figure 1 (b) has the advantages of improved resolution and the ability to differentiate between the hexagonal ε‐martensite and twins. The histograms in figures 1 (c‐d) summarize the spacing between the planar defects and their thickness from a Fe‐25Mn3Al3Si alloy deformed at a strain rate of 20 s ‐1 to a total strain of 20% using these two experimental methods. Figure 3 is a HRTEM image produced with an image corrected FEI‐Titan from a Fe‐25Mn3Al3Si alloy deformed at a strain rate of 2x10 3 s ‐1 to a total strain of 18%. The image shows an example of an individual dislocation trapped at the planar defect interface as well as a local region of martenite (bottom left) together with the mechanical twins. The high‐angle dark field (HAADF) HRSTEM image in figure 3 from a Fe‐16Mn14Cr0.3N0.3C alloy deformed to a total strain of 21% at a strain rate of 10 ‐4 s ‐1 was acquired using the ER‐C PICO operating at 300 kV. Similar to the high strain rate material, the quasi‐static deformed microstructure exhibits multiple mechanical twins with evidence of local hexagonal stacking (upper right). Advantages of the HRSTEM method compared to HRTEM images are more straight forward image interpretation from the HAADF amplitude contrast, the ability to image thicker samples, and the reduced sensitivity to local variation in crystallographic orientation.

Journal Article Microstructural Characterization of a Fe-25Mn-3Al-3Si TWIP–TRIP Steel Get access J T Benzing, J T Benzing Interdisciplinary Materials Science, Vanderbilt University, Nashville TN, USA Search for other works by this author on: Oxford Academic Google Scholar J Bentley, J Bentley Microscopy and Microanalytical Sciences, PO Box 7103, Oak Ridge, TN, USA Search for other works by this author on: Oxford Academic Google Scholar W Poling, W Poling Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO, USA Search for other works by this author on: Oxford Academic Google Scholar K Findley, K Findley Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO, USA Search for other works by this author on: Oxford Academic Google Scholar D T Pierce, D T Pierce Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA Search for other works by this author on: Oxford Academic Google Scholar J M Sosa, J M Sosa Materials Science and Engineering, The Ohio State University, Columbus, OH, USA Search for other works by this author on: Oxford Academic Google Scholar H L Fraser, H L Fraser Materials Science and Engineering, The Ohio State University, Columbus, OH, USA Search for other works by this author on: Oxford Academic Google Scholar D Raabe, D Raabe Max-Planck-Institut für Eisenforschung, Max-Planck-Straβe 1, Düsseldorf, Germany Search for other works by this author on: Oxford Academic Google Scholar J E Wittig J E Wittig Interdisciplinary Materials Science, Vanderbilt University, Nashville TN, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 22, Issue S3, 1 July 2016, Pages 1962–1963, https://doi.org/10.1017/S1431927616010655 Published: 25 July 2016

Recently developed high-manganese steels exhibit an exceptional combination of strength and ductility and show great promise for structural applications. Understanding the relationships between manganese and carbon content, microstructure, temperature, defect formation and strain-hardening behavior is critical for alloying, design, and further optimization of these steels. The present study investigates the influence of alloy content, temperature and deformation behavior on the microstructural evolution of an austenitic Fe-14Cr-16Mn-0.3C-0.3N alloy showing twinning induced plasticity (TWIP) and of a twophase nanostructured Fe-30.5Mn-8Al-1.2C alloy exhibiting microband induced plasticity (MBIP).

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Recently developed high-manganese stainless Fe–Cr–Mn–CN steels exhibit an exceptional combination of strength and ductility and show great promise for structural applications. Understanding the relationships between temperature, stacking fault energy (SFE) and strain-hardening behavior is critical for alloying, design, and further optimization of these steels. The present study investigates the influence of temperature and SFE on the microstructural evolution to explain the deformation behavior and mechanical properties of an austenitic Fe–14Cr–16Mn–0.3C–0.3N alloy. The flow behavior is homogenous and no serrations in the flow stress occur during tensile deformation in the temperature range from −150 to 250°C. Mechanical twinning and the formation of (planar) dislocation substructures strongly influence the mechanical properties and work-hardening behavior in the intermediate temperature range from −40 to 45°C (SFE range from 17 to 24mJm−2). In the high temperature interval from 100 to 250°C the SFE ranges from 29 to 44mJm−2 and the initiation of mechanical twinning is delayed leading to reduced work-hardening in the intermediate and final stages of strain-hardening. In the low temperature regime from −150 to 100°C (SFE approximately 15mJm−2), εh.c.p.-martensite is the dominant secondary deformation mechanism, contributing to the enhanced work-hardening in the early and intermediate stages of deformation and slightly lower total elongations. The yield strength of the studied alloy is significantly larger and exhibits greater sensitivity to temperature within the thermal and athermal ranges for dislocation motion compared to conventional Fe–Mn–(Al)–C TWIP or austenitic stainless steels, which may be attributed to phenomena such as short range ordering.

Understanding the relationship between the stacking-fault energy (SFE), deformation mechanisms, and strain-hardening behavior is important for alloying and design of high-Mn austenitic transformation- and twinning-induced plasticity (TRIP/TWIP) steels. The present study investigates the influence of SFE on the microstructural and strain-hardening evolution of three TRIP/TWIP alloys (Fe–22/25/28Mn–3Al–3Si wt.%). The SFE is increased by systemically increasing the Mn content from 22 to 28 wt.%. The Fe–22Mn–3Al–3Si alloy, with a SFE of 15 mJ m−2, deforms by planar dislocation glide and strain-induced εhcp-/αbcc-martensite formation which occurs from the onset of plastic deformation, resulting in improved work-hardening at low strains but lower total elongation. With an increased SFE of 21 mJ m−2 in the Fe–25Mn–3Al–3Si alloy, both mechanical twinning and εhcp-martensite formation are activated during deformation, and result in the largest elongation of the three alloys. A SFE of 39 mJ m−2 enables significant dislocation cross slip and suppresses εhcp-martensite formation, causing reduced work-hardening during the early stages of deformation in the Fe–28Mn–3Al–3Si alloy while mechanical twinning begins to enhance the strain-hardening after approximately 10% strain. The increase in SFE from 15 to 39 mJ m−2 results in significant changes in the deformation mechanisms and, at low strains, decreased work-hardening, but has a relatively small influence on strength and ductility.

There remains much interest in the development of FePt for magnetic recording media.For almost a decade we have performed extensive studies of FePt multilayers and nanoparticles, especially epitactic growth on layers of MgO and the phase transformation from a disordered solid solution to the L10 ordered structure that exhibits attractive magnetic properties [1][2][3][4][5][6][7].Many studies incorporated atomic-resolution high-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) imaging which readily discriminates the Pt and Fe because of the large difference in their atomic numbers.The studies at Oak Ridge National Laboratory also included in-situ annealing with a prototype Protochips Aduro heating holder in a probe-corrected JEOL 2200 operated at 200kV.Complex annealing behavior was frequently observed including unexpected L1 2 ordering which implied a local composition nearer to Pt3Fe than FePt.It was unclear whether some of these observed effects were artifacts caused by the electron beam or were due to incorrect composition of as-synthesized nanoparticles.Some evidence of the latter was obtained but composition measurements by energy-dispersive X-ray spectrometry (EDS) in a Philips (FEI) CM200FEG with probes of ~1 nA were compromised by beam damage, frequently with a dramatic loss of Fe.This effect was somewhat unexpected because 200 kV is well below the displacement threshold for Fe in FePt.At surfaces atomic bonding may be weaker but it is unknown if that is a major contribution to the responsible mechanism.Certainly, hole drilling at beam energies well below the (bulk) displacement threshold has been a commonly observed effect since soon after the introduction of FEG-STEMs and is frequently more pronounced in the presence of oxide on the specimen surface.Conventional wisdom is that surface carbon (contamination) is actually helpful in preventing hole drilling, perhaps as a sacrificial surface species that self-heals by surface diffusion as it is sputtered away.However, in the case of FePt nanoparticles deposited on a carbon support film it was postulated that, under the action of the electron beam, the carbon might react with the Fe exacerbating, if not directly causing, loss of Fe from FePt.

The stacking fault and interfacial energies of three transformation- and twinning-induced plasticity steels (TRIP/TWIP) (Fe–22/25/28Mn–3Al–3Si wt.%) were determined by experimental and theoretical methods. Analysis of Shockley partial dislocation configurations in the three alloys using weak-beam dark-field transmission electron microscopy yielded stacking fault energy (SFE) values of 15 ± 3, 21 ± 3 and 39 ± 5 mJ m−2 for alloys with 22, 25 and 28 wt.% Mn, respectively. The experimental SFE includes a coherency strain energy of ∼1–4 mJ m−2, determined by X-ray diffraction, which arises from the contraction in volume of the stacking fault upon the face-centered cubic (fcc) to hexagonal close-packed (hcp) phase transformation. The ideal SFE, computed as the difference between the experimental SFE and the coherency strain energy, is equal to14 ± 3, 19 ± 3 and 35 ± 5 mJ m−2, respectively. These SFE values were used in conjunction with a thermodynamic model developed in the present work to calculate the free energy difference of the fcc and hcp phases and to determine a probable range for the fcc/hcp interfacial energy in the three Fe–Mn–(Al–Si) steels investigated. In addition, the interfacial energies of three Fe–18Mn–0.6C–0/1.5(Al/Si) TWIP and five Fe–16/18/20/22/25Mn binary alloys were also determined from experimental data in the literature. The interfacial energy ranged from 8 to 12 mJ m−2 in the TRIP/TWIP steels and from 15 to 33 mJ m−2 in the binary Fe–Mn alloys. The interfacial energy exhibits a strong dependence on the difference in Gibbs energy of the individual fcc and hcp phases. Accordingly, an empirical description of this parameter is proposed to improve the accuracy of thermodynamic SFE calculations.