Search

Laser powder bed fusion (L-PBF) was utilized to produce specimens in Ti-6Al-4V, which were subjected to a bi-lamellar heat treatment, which produces microstructures consisting of primary α-lamellae and a fine secondary α-phase inside the inter-lamellar β-regions. The bi-lamellar microstructure was obtained as (i) a direct bi-lamellar heat treatment from the asbuilt condition or (ii) a bi-lamellar heat treatment preceded by a β-homogenization. For the bi-lamellar treatment with β-homogenization, cooling rates in the range 1−500 K/min were applied after homogenization in β-region followed by inter-critical annealing in the α + β region at various temperatures in the range 850-950 °C. The microstructures were characterized using various microscopical techniques. Mechanical testing with Vickers hardness indentation and tensile testing was performed. The bi-lamellar microstructure was harder when compared to a soft fully lamellar microstructure, because of the presence of fine α-platelets inside the β-lamellae. Final low temperature ageing provided an additional hardness increase by precipitation hardening of the primary α-regions. The age hardened bi-lamellar microstructure shows a similar hardness as the very fine, as-built martensitic microstructure. The bi-lamellar microstructure has more favorable mechanical properties than the as-built condition, which has high strength, but poor ductility. After the bi-lamellar heat treatment, the elongation was improved by more than 250 %. Due to the very high strength of the as-built condition, loss of tensile strength is unavoidable, resulting in a reduction of tensile strength of ∼18 %.

This contribution addresses the effect of sub-zero Celsius treatments at cryogenic temperatures on the steel microstructure. It is shown that the formation of martensite, including the socalled isothermal, i. e. time dependent or thermally activated, martensitic product, can provide an explanation to the observations in the literature that both the temperature of cryogenic treatment and the holding time at cryogenic temperatures may have an influence on the performance of steel products in service. This review traces the most important stages in the development of cryogenic treatments along the last 95 years and describes it in parallel with the description of the current state of understanding of the kinetics of martensitic transformation in steel. In the last part of the contribution, the new insight is put into context with a practical example where various types of subzero Celsius treatments are applied.

Low temperature nitrided austenitic stainless steel exhibits strong orientation dependent case growth kinetics. Anisotropy effects during low temperature gaseous nitriding of single crystals of austenitic stainless steel with surface-normal orientations 〈100〉, 〈111〉 and 〈10 7 2〉 were investigated with thermogravimetry, X-ray diffractometry (XRD), glow discharge optical emission spectroscopy (GD-OES), nano-indentation and electron back-scatter diffraction (EBSD). Case growth kinetics were fastest for 〈100〉 and slowest for 〈111〉. Depth-profiling over the case depth was accomplished by successive layer removal combined with X-ray diffraction and nano-indentation. The results indicate that anisotropic case-growth kinetics is related to early nucleation of long-range ordered (LRO) over-stoichiometric γ’N−Me4N1+ x (Me=metal atoms) in 〈100〉, which is inseparable from short range ordered (SRO) expanded austenite, γN. Depth ranges of elasto-plastic and purely plastic accommodation of the nitrogen induced lattice expansion were distinguished. In the elasto-plastic region stress-gradient enhanced diffusion of N is observed.

The nitriding response of additively manufactured M300 maraging tool steel was investigated through a series of nitriding treatments at different temperatures in the range 360–480 °C for a duration of 40 h. The interaction between nitrogen and alloying element nitrides (Ti and Mo) and intermetallics (Ni3Ti and Ni3Mo) was studied by comparing nitriding responses of as-solution treated and subsequently aged specimens. The nitriding behavior was found to depend strongly on the initial condition of the material and the nitriding temperature applied. Ultra-low temperature nitriding at 360 °C was found to be feasible as it resulted in a nitrided case, albeit with incomplete precipitation hardening of the bulk structure, thus resulting in a softer core. The application of a post-ageing treatment resulted in full bulk hardening and distinctly changed the appearance of the hardness profile.

The present work explores for the first time additive manufacturing of powder mixtures consisting of Chromium Nitride (Cr2N) and AISI 316L with laser powder bed fusion (L-PBF). The addition of 2.5 wt% Cr2N to an AISI 316L powder resulted in the successful dissolution of both chromium and nitrogen into a fully austenitic stainless steel microstructure. The nitrogen content was augmented from 0.09 wt% in the as-delivered AISI 316L powder to 0.31 wt% in the L-PBF built part, causing a slight expansion of the austenite lattice. Elongated austenite grains with an internal cellular substructure were obtained in both the Cr2N modified 316L and the 316L specimens manufactured by L-PBF. The addition of nitrogen (and chromium) from Cr2N resulted in a Vickers hardness increase of about 40 HV0.1, mainly by interstitial solid solution strengthening. The modification of 316L by the addition of Cr2N significantly improved the corrosion resistance. The improved hardness and corrosion resistance while retaining the manufacturability and cellular microstructure illustrate the potential for modifying the composition and properties of L-PBF 316L with targeted dosing with Cr2N powders.

International audience; Foils of ticanium grade 2 were carbo-oxidized in CO ac cemperarures ranging from 600 co 11 0D°C in sceps of SD°C. carbo-oxidation was performed for up co 130 hours in a thermogravin1etric serup, enabling the investigation of the kinetics of carbon and oxygen upcake from the gas mixrure. The carbo-oxidized specimens were characterized with transmission X-ray diffraction, lighc optical microscopy, scanning elecrron microscopy, elecrron probe microanalysis and nanoindentation co invescigace the microsrruccural evolmion along with the upcake of oxygen and carbon. At the surfaces of the foils TiCx developed, while the core remained h.c.p. titanium. The interstitial content in h.c.p. ticanium increased with cemperacure and firsc reached ics maxinmm solubilicy ac l00D°C and above. TiO developed within the core for high interstitial contents. The TiCxOy phase developed in-becween TiC. and the oxygen containing h.c.p. ticanium core for cen1perarures above soo•c. For cemperarures up co 8500C, the uptake of C and O in the cicanium foils (initially) obeys a parabolic time dependence. Arrhenius analysis of the thermogravimerry results indicates chat volume diffusion of oxygen in h.c.p. titanium is (initially) the race decermining scep. For cemperacures above 850°C, the activation energy is reduced by a factor 3, suggesting chat shore-circuit diffusion of species in TiCx conrrols mass increase. The hardness of h.c.p. titanium determined with nanoindentation scales with the incersticial content and the associaced c/a ratio of the h.c.p. unie cell The evolution of the microstructure with tin1e and cen1perature are discussed in relation co the observed kinetics of mass uptake.

Long-term oxidation behavior of Zr48Cu36Al8Ag8 BMG under atmospheric conditions at 600 K was investigated using in-situ X-ray diffraction analysis. Two oxide zones develop: an outer oxide zone (OOZ), consisting of a stratified distribution of CuOx, Ag, and ZrO2, and an inner oxide zone (IOZ) consisting of tetragonal ZrO2 showing Cu-enriched lines inclined to the surface. The stratified microstructure in the OOZ and the Cu enrichments in the IOZ are ascribed to micro-cracks resulting from the compressive stresses induced by ZrO2 formation. Segregation of noble elements towards these oxidation-generated free surfaces “repairs” the micro-cracks and can consistently explain the microstructural features.

Expanded austenite obtained by gaseous carburizing of stainless steel was investigated with X-ray diffraction to determine composition-depth and residual stress-depth distributions. Avoiding ghost stress effects in the analysis of X-ray diffraction data, the obtained composition- and stress-depth profiles are in excellent quantitative agreement with those obtained with other techniques. The residual stress-depth profile was attempted calculated from the composition-depth profile assuming elastic-plastic accommodation of the lattice expansion. In the model, composition-dependence of Young's modulus, yield stress and work hardening exponent were considered. Excellent quantitative agreement was achieved between the experimental and numerical residual stress-depth profiles.

A 3D elastoplastic phase-field model is used to study the effect of isochronal heating on martensitic transformation in stainless steel quenched to cryogenic temperature. The results show that the slower the heating, the larger is the martensite volume fraction developed on heating. The simulated microstructures show that the transformation is driven by autocatalysis during the early stages and by coarsening of existing martensite units during later stages of the transformation. The internal stresses are mainly relaxed by autocatalysis during the initial stages of the transformation, whereas they are relaxed by plastic deformation during the later stages of the transformation. The temperature for attaining a certain martensite fraction increases with increasing heating rate, which is consistent with a thermally activated transformation. Kissinger-like analysis of the simulated transformation curves provides an activation energy of 11.9 kJ/mol.

The present work addresses the surface hardening and heat treatment response of Ti-6Al-4V grades G5 and G23 on thermochemical surface treatment in different temperature regimes as well as in different gaseous and plasma-based media. Grades G5 and G23 were subjected to gaseous surface hardening using various gas compositions and temperature regimes. Two different series of gaseous surface treatments were carried out:1) carbo-oxidizing of G23 in mill-annealed condition was performed in CO gas at (high) temperatures ranging from 777 to 1027 °C. Treatment at 1027 °C resulted in the deepest case with the formation of a thin surface layer of titanium sub-oxides supported by a thick TiCxO1-x layer as the dominant phase. Post-nitriding of the carbo-oxidized specimen raised the hardness of the TiCxO1-x phase by the incorporation of nitrogen, yielding values up to ~2,900 HV and multi-layered structures.2) Carbo-oxidizing of G5 with a bi-modal microstructure in CO/CO2 gas mixture at (intermediate) temperatures in the range 677 - 777 °C. Treatment at 777 °C resulted in a surface hardness of ~1,900 HV and a ~60 μm-thick diffusion zone.In addition to gaseous surface hardening, also plasma-assisted methods were applied and compared to their gaseous counterparts. Intermediate- and low-temperature plasma treatments were carried out on G23:3) plasma (carbo-nitro)oxidizing was performed in a CO2 and N2 gas mixture at 750 °C and (carbo-)oxidizing in CO2 gas at 650 °C.4) Plasma nitriding was conducted in a N2/H2 gas mixture at 750 °C and 850 °C. The widely different types of treatments applied in this work represent the diversity of the gaseous and plasma-based thermochemical methods that can be applied for surface engineering of titanium alloys.

Titanium and its alloys could be called the “streetwalker” among commercially applied metals, because its poor nobility allows it to “pick-up” anything. Titanium is one of the metals that forms very stable borides, carbides, nitrides, oxides, and even hydrides. The solubility of nitrogen and oxygen in the hexagonal crystal structure of (metallic) titanium is very high; on the other end limited quantities of boron, carbon, and hydrogen can be dissolved. In the present contribution, we report on a significant increase in the solid solution limit of hydrogen in hexagonal close-packed (h.c.p.) titanium, where the h.c.p. crystal lattice is stabilized by deliberate interstitial alloying with high quantities of nitrogen or oxygen atoms. The presence of nitrogen/oxygen prevents the hydrogen-induced transformation of the h.c.p. titanium lattice to a face-centered cubic (f.c.c.) titanium lattice that occurs if no oxygen or nitrogen is present. The hydrogen content that can be accommodated at room temperature in h.c.p. titanium is as high as an unprecedented 50 at. pct from about 0 at. pct and causes an anisotropic expansion of the hexagonal lattice. This finding showcases that there could be an unexploited potential for combining large quantities of interstitials in titanium-based lattices.

Homogenous thin foils of titanium with nitrogen and oxygen in interstitial solid solution were characterized with X-ray diffraction, light optical microscopy and electron backscatter diffraction. Samples covering the broad solubility ranges of O and N were synthesized. The mechanical properties were assessed using nanoindentation and related to the interstitial content, h.c.p. lattice expansion and grain orientation. Effects of surface pile-up and sink-in during nanoindentation were adequately accounted for, yielding hardness and indentation modulus values for nearly the full range of concentrations, finding consistency with theoretical expressions and very limited literature values. The hardening/strengthening achieved by colossal interstitial alloying of O and N correlates with the change in c/a ratio of the h.c.p. Ti lattice as caused by interstitial occupancy.

An extra low carbon martensitic stainless steel with 16% ultrafine grained metastable reverted austenite was subjected to uniaxial tensile testing and investigated with in-situ energy-dispersive synchrotron X-ray diffraction (XRD) and in-situ electron backscatter diffraction (EBSD) to reveal the complex interplay between stress, strain and martensitic transformation. In-situ XRD demonstrated that, upon surpassing the yield strength, the fraction of reverted austenite declined linearly with increasing true stress, which was associated with transformation-induced plasticity (TRIP). EBSD and XRD consistently showed that the texture of martensite evolved from an initially weak texture towards a strong 110 α ′ fiber parallel to the tensile axis. For the first time, stress partitioning between (remaining) reverted austenite and the martensite matrix was determined quantitatively during in-situ XRD by averaging over the stress values obtained from lattice strains for multiple reflections. Martensite accommodates the majority of the applied load while reverted austenite is severely plastically deformed. XRD shows strong plastic anisotropy in austenite. In-situ forward-scatter electron imaging and advanced variant analysis of the EBSD data indicate that plastic deformation and strain-induced austenite-to-martensite transformation is concentrated along boundaries between martensite blocks and packets which are inclined up to ∼ 55° with respect to the tensile direction. These regions were preferred sites for strain-induced martensite formation.

Ti-6Al-4V was (intercritically) annealed at various temperatures in the range 1000–1250 K with intervals of 50 K, followed by cooling to room temperature at an average rate of approx. 1 K·s−1. The heat treatment procedure was intended to systematically vary the microstructure and alter the thermal stability of the β phase through the partitioning of the alloying elements between α and β phases. The annealing treatment was followed by cryogenic treatment, CT, which consisted of immersion of the samples in boiling nitrogen for durations ranging from 5 min to 24 h, followed by re-heating in air. The heat-treated material was characterized ex-situ applying light optical microscopy (LOM), synchrotron X-ray diffraction (S-XRD), and hardness Vickers indentation. A set of samples not subjected to cryogenic treatment was taken as reference. LOM revealed that the material’s microstructure after heat treatment consisted of a fraction of primary α grains and regions of lamellar α/β structure. S-XRD showed that the fraction of retained β was largest, approx. 7%, for the material treated at the highest applied annealing temperature, i.e. 1250 K, and decreased to 2% with a reduction of the annealing temperature. Hardness values varied in the range 300–330 HV and did not show a measurable effect of the annealing temperature. The applied techniques did not reveal any measurable effect of cryogenic treatment, neither on the microstructure, nor on the hardness. Additionally, CT had no measurable effect neither on the lattice parameters of the phases, nor on the density of crystallographic defects in the material. These observations are inconsistent with literature data, which report various effects of CT on the microstructure and on the mechanical properties of Ti-6Al-4V, and are attributed to a pronounced stability of the retained β phase against its conversion into α′ martensite during CT in the present heat treatment conditions.

The as-built microstructure of edge printing parameters in the laser powder-bed fusion process is investigated as well as the effect of edge parameters on the response to high temperature solution nitriding. The results show that using edge parameters leads to a zone of elongated austenite grains with shallow melt pools. Subjecting parts printed with edge parameters to high temperature solution nitriding (and high temperatures in general) results in the formation of equi-axed grains with a high density of annealing twins. Conversely, bulk parameters create a pinning effect and preserve the fine misorientation structure inside the elongated structure from the as-built condition. The results show that a high temperature solution nitriding treatment is apt to post-processing of additively manufactured 316L.

The evolution of the substructure and the distribution of interstitial elements in lath martensite during tempering in soft martensitic stainless steel X4CrNiMo16-5-1 was studied with line profile analysis of diffractograms from energy dispersive synchrotron X-ray diffraction, local chemical analysis with atom probe tomography and orientation mapping with electron backscatter and transmission Kikuchi diffraction. Martensite formation occurred below 135 °C without auto-tempering and led to a dislocation density in martensite of 3.8 ∙ 1015 m−2, as determined from X-ray line profile analysis. On tempering, carbon and nitrogen segregated to low-angle and high-angle grain boundaries. Recovery commenced above 550 °C and led to a reduction in dislocation density to a steady value of 4 ∙ 1014 m−2 from 600 to 750 °C. Further tempering led to a second increase in dislocation density at room temperature, owing to the transformation of reverted austenite, formed above 650 °C, into martensite on cooling. It was observed that the recovery of martensite competes with the formation of reverted austenite. The interpretation of the coherently diffracting domain size obtained from X-ray line profile analysis was critically discussed in the context of the internal structure in martensite.

Surface engineering of Zr 51.3 Al 8.5 Cu 31.3 Ni 4 Ti 4.9 bulk metallic glass (BMG) by gaseous oxidizing below the glass-transition temperature is investigated as a means to introduce compressive residual stress in the surface region. The ZrCuAl-based BMG was exposed to an extremely low oxygen partial pressure of 10−41 bar at 600 K for 60 h. The oxidizing treatment led to the formation of an internal oxidation zone, consisting of finely dispersed nano-crystalline cubic ZrO 2 (c-ZrO2 ), metallic regions inclined with the surface and Cu-hillocks at the surface. The stresses introduced by the volume expansion associated with oxidation were evaluated from i) the lattice strains within c-ZrO2 , as determined with an X-ray diffraction (XRD) based method, and ii) strain-relaxation as a response to annular focused ion beam (FIB) milling, as monitored with digital image correlation (DIC). XRD analysis yielded -1.5 GPa (compressive stress) in the nano-crystalline c-ZrO2 , while the strain relaxation monitored with FIB-DIC analysis indicated compressive residual stresses of −1.4 GPa in the internal oxidation zone. The strains and stresses determined with the independent measurement methods are discussed. The quantitative macro-strains are discussed in relation to the microstructural features and stress relaxation mechanisms during evolution of the internal oxidation zone.

AISI 304L stainless steel in austenitized and in solution nitrided condition was severely mechanically deformed by surface roller burnishing. High-temperature solution nitriding was applied to achieve a nitrogen-concentration depth profile, leading to a depth-gradient in the austenite stability. X-ray diffraction, electron microscopy and hardness indentation were applied for characterization of the graded microstructures obtained by combining a composition profile and a deformation profile. While severe plastic surface straining of an austenitized specimen leads to a deformation-induced transformation of austenite into martensite, the solution nitrided specimen remains austenitic upon deformation, even in the region where nanocrystallization occurs. The deformation mechanisms operable in the nitrogen-stabilized austenitic stainless steel, i.e. twinning or dislocation glide, depend on the combination of applied plastic strain/strain rate, and the nitrogen-concentration dependent stacking fault energy.

Titanium grade 2 thin foils were carbo-oxidized at 800 °C in a thermogravimetric analyzer in CO gas for 20 - 100 hours. The synthesized thin foils were characterized by infrared carbon determination for composition analysis, transmission X-ray diffraction (XRD), light optical microscopy (LOM) and nanoindentation. Thermodynamic equilibrium predictions were performed using ThermoCalc. Oxygen and carbon expanded h.c.p. α and NaCl type δ carbo-oxide were identified from the XRD patterns. Rietveld refinements of the XRD patterns revealed an increase in expansion of the h.c.p. α lattice from interstitially dissolved C and O: the c/a ratio increases from 1.588 for interstitially free h.c.p. α to 1.607 after 100 hours of exposure. The overall carbon and oxygen content after 100 hours was 4.2 wt% and 5.5 wt%, respectively; these overall compositions correspond to the two-phase α + δ region in the calculated ternary phase diagram. The microstructure from LOM revealed large grains of expanded α-phase and a surface zone consisting of porous δ carbo-oxide. The hardness, as determined by indentation, of the expanded α increased linearly with the c/a ratio, resulting in an increase to 11748 MPA after 100 hours exposure from 2193 MPa for the untreated Ti grade 2.

Three nickel electrodeposits with comparable grain size were synthesized by tailoring the electrodeposition conditions. Thorough microstructural characterizations including electron backscatter diffraction, ion channeling contrast imaging, electron channeling contrast imaging, transmission Kikuchi diffraction, transmission electron and high annular dark-field imaging were applied. The deposits contain a high density of twin boundaries with similar microstructures in terms of grain boundary character. These materials were annealed at various temperatures to study the microstructural evolution, and hence, their thermal stability. The differences in the character of twin boundaries and morphology of the grain boundaries in as-deposited state and their influence on the microstructural evolution at elevated temperatures are analyzed. The importance of incoherent twin boundaries, and the interaction of mobile general high angle boundaries with stationary boundaries are discussed. Finally, an optimal design for high thermal stability is proposed, based on the mechanisms that were inferred from the results. Keywords: Thermal stability, Twin boundaries, Annealing, Grain boundary engineering, Nickel, Electrodeposition

Magnetometry was applied to investigate the formation of α/α´martensite in 13ferrous alloys during immersion in boiling nitrogen and during re-heating to room temperature at controlled heating rates in the range 0.0083-0.83 K s-1. Data showsthat in 3 of the alloys, those that form {5 5 7}γ martensite, no martensite developsduring cooling. For all investigated alloys, irrespective of the type of martensiteforming, thermally activated martensite develops during heating. The activationenergy for thermally activated martensite formation is in the range 8‒27 kJ mol-1and increases with the fraction of interstitial solutes in the alloy

Magnetometry was applied to quantify the fraction of austenite retained in Fe-C alloys subjected to various treatments. These treatments consisted of: (i) water quenching; (ii) water quenching followed by immersion in boiling nitrogen and again in water; (iii) as for (ii) but re-heating from 77 K at a rate of 0.0083 K s− 1; (iv) as for (iii) but (re-)heating at 0.167 K s− 1 interrupted by an isothermal step. Data was coupled with hardness measurements and demonstrates that the re-heating conditions from 77 K significantly influence the fraction of austenite retained at the end of the thermal cycle.

A face-centered cubic (FCC) phase formed during the carbo-oxidation of hexagonal close-packed (HCP) α-Titanium at 600 to 700 °C. The FCC crystal structure was confirmed using transmission electron diffraction, X-ray diffraction and high angle annular dark field scanning transmission electron microscopy. The a lattice parameter for the FCC resulting from the three methods was consistent with FCC sublattice of titanium atoms in δ-TiH2-x. The FCC phase is hypothesized to form partly due to hydrogen pick-up, the stress state in the foil caused by the mismatch between body centered cubic iron-stabilized β-Titanium and HCP α-Titanium and the tensile stress present in front of the oxygen concentration profile. A transformation of α-Titanium into δ-TiH2-x is accomplished by introducing Shockley partial dislocations on every 2nd closest paced plane, resulting in an orientation relation that has previously been observed for thermally induced FCC γ-Ti. The TiH2-x phase was shown to decompose upon vacuum annealing at 500 °C.

Energy dispersive synchrotron X-ray diffraction was applied to investigate in situ the evolution of lattice strains and stresses in austenite and martensite during quenching and tempering of a soft martensitic stainless steel. In one experiment, lattice strains in austenite and martensite were measured in situ in the direction perpendicular to the sample surface during an austenitization, quenching, and tempering cycle. In a second experiment, the sin2 ψ method was applied in situ during the austenite-to-martensite transformation to distinguish between macro- and phase-specific micro-stresses and to follow the evolution of these stresses during transformation. Martensite formation evokes compressive stress in austenite that is balanced by tensile stress in martensite. Tempering to 748 K (475 °C) leads to partial relaxation of these stresses. Additionally, data reveal that (elastic) lattice strain in austenite is not hydrostatic but hkl dependent, which is ascribed to plastic deformation of this phase during martensite formation and is considered responsible for anomalous behavior of the 200 γ reflection.

Chromium-rich nitride precipitates in production of nickel-free austenitic stainless steel plates via pressurised solution nitriding of Fe–22.7Cr–2.4Mo ferritic stainless steel at 1473 K (1200 °C) under a nitrogen gas atmosphere was investigated. The microstructure, chemical and phase composition, morphology and crystallographic orientation between the resulted austenite and precipitates were investigated using optical microscopy, X-ray Diffraction (XRD), Scanning and Transmission Electron Microscopy (TEM) and Electron Back Scatter Diffraction (EBSD). On prolonged nitriding, Chromium-rich nitride precipitates were formed firstly close to the surface and later throughout the sample with austenitic structure. Chromium-rich nitride precipitates with a rod or strip-like morphology was developed by a discontinuous cellular precipitation mechanism. STEM-EDS analysis demonstrated partitioning of metallic elements between austenite and nitrides, with chromium contents of about 80 wt.% in the precipitates. XRD analysis indicated that the Chromium-rich nitride precipitates are hexagonal (Cr, Mo)2N. Based on the TEM studies, (Cr, Mo)2N precipitates presented a (1 1 1)γ//(0 0 2)(Cr, Mo)2N, (Formula presented.)γ//(Formula presented.)(Cr, Mo)2N orientation relationship with respect to the austenite matrix. EBSD studies revealed that the austenite in the regions that have transformed into austenite and (Cr, Mo)2N have no orientation relation to the untransformed austenite.

Expanded austenite on stainless steel with a high interstitial nitrogen content is characterized by elasto-plastic accommodation of the large composition-induced lattice expansion leading to huge compressive residual stress. The elasto-plastic accommodation as well as the (steep) concentration profile has implications for the measurement strategy to determine lattice strains and associated residual stresses with X-ray diffraction. Lattice strain measurements were performed on nitrided as well as subsequently de-nitrided expanded austenite on AISI 316L stainless steel, for various grazing incidence angles. It is demonstrated that keeping the information depth constant by choosing appropriate combinations of grazing incidence and tilt angle leads to reliable results for the 111 reflection, while the 200 reflection should be avoided. Further, it is shown for the first time that the residual stresses in expanded austenite can be tailored by de-nitriding after nitriding, such that a condition of virtually zero stress at the surface is obtained.

A 3D elastoplastic phase-field model is used to study the effect of thermal cycling on martensitic transformation as well as on mechanical strengthening of both austenite and martensite in stainless steel. The results show that with an increasing number of thermal cycles, martensite becomes more stable. Increase in strain, plastic strain and strain hardening lead to strengthening of austenite.

Hexagonal e-iron nitride and e-iron carbonitride phases are formed on nitriding and nitrocarburizing of iron and steel surfaces and can exist in broad compositional ranges. Long-range nitrogen ordering and magnetic properties for e-iron nitrides and their dependence on composition have been the focus of several studies. So far, limited attention has been paid to the carbonitrides. In the current work, the effects of substitution of nitrogen by carbon on the interstitial ordering and magnetic properties in Fe2(C,N)1-z are explored using neutron diffraction, Mossbauer spectroscopy and vibrating sample magnetometry. Neutron diffraction patterns showed 001 and 301 superstructure reflections, confirming a previously proposed structural model in space group P 3 ¯ 1 m (compared to P6322 for the pure nitrides). On partial substitution of nitrogen by carbon in e-iron nitride the Curie temperature, the saturation magnetization and the hyperfine fields of the iron atoms are increased, while isomer shifts are decreased. The effects on the a and c lattice parameters indicate a change in interstitial ordering, which is related to more favorable interactions between a nitrogen and carbon atom than among nitrogen atoms. This interaction leads to additional interstitial (short-range) ordering and a decrease in the c lattice parameter, while the a lattice parameter is largely unaffected.

The martensite-to-austenite transformation in X4CrNiMo16-5-1 supermartensitic stainless steel was followed in-situ during isochronal heating at 2, 6 and 18 K min−1 applying energy-dispersive synchrotron X-ray diffraction at the BESSY II facility. Austenitization occurred in two stages, separated by a temperature region in which the transformation was strongly decelerated. The region of limited transformation was more concise and occurred at higher austenite phase fractions and temperatures for higher heating rates. The two-step kinetics was reproduced by kinetics modeling in DICTRA. The model indicates that the austenitization kinetics is governed by Ni-diffusion and that slow transformation kinetics separating the two stages is caused by soft impingement in the martensite phase. Increasing the lath width in the kinetics model had a similar effect on the austenitization kinetics as increasing the heating-rate. Keywords: Supermartensitic stainless steels, Phase transformation kinetics, Interface diffusion, Synchrotron radiation, Kinetics modeling, Reversed austenite

The effect of the scanning strategy during selective laser melting (SLM) of Ti-6Al-4V was investigated. An optimized cellular scan strategy (island scan modeled) was compared to a simple cellular scan strategy (island scan stripes) and a simple antiparallel line scanning strategy (line scan). Surface texture was investigated by optical three-dimensional (3D) surface measurements, which when combined with light optical microscopy (LOM), revealed deflections caused by the thermal stresses during the build process. Elevated edges caused by the edge-effect dominate the surface texture of all investigated specimens. The scanning strategy determines the surface texture, and the lowest surface roughness was obtained by the line scan strategy. Porosity was investigated with X-ray computed tomography-imaging. Mainly spherical porosity was observed for the line scan and island scan modeled specimens, while the island scan stripes strategy showed more lack-of-fusion defects and a higher total porosity amount. Microstructure was investigated with LOM and scanning electron microscopy (SEM). The microstructure in Ti-6Al-4V was largely martensitic &alpha, &rsquo, and prior &beta, grains. The morphology is different for the various scan strategies, and decomposition of &alpha, into lamellar &alpha, /&beta, was observed in the bottom part of the island scan specimen. Accordingly, the hardness decreased in the decomposed part of the specimen.

Additive Manufacturing (AM) enables the design of complex part geometries for high-temperature applications. Laser Powder Bed Fusion or Direct Energy Deposition of the nickel-based superalloy MAR-M247 poses a challenge in AM due to its poor weldability. Binder Jetting does not utilize a heat source and interaction with a liquid metal during build-up and thus has the potential to overcome this limitation. In this study, MAR-M247 was manufactured by Binder Jetting and subsequently characterized regarding density and microstructure. Combinations of two different Hot-Isostatic-Pressing (HIP) treatments (T = 1120–1180 °C, p = 1000–1500 bar, t = 4 h) and four different heat-treatments involving solution treatment (T = 1250 °C, t = 4 h) and different aging steps (T = 700–1000 °C, t = 12–24 h) were applied to study the densification and microstructural evolution of binder-jetted MAR-M247. The influence of the build direction in combination with HIP is studied concerning the resulting density and mechanical properties at room temperature. The results show that close-to-full densification can be achieved after HIP. Subsequent solution treatment and double-aging after HIP lead to a favorable bimodal microstructure. A process chain for binder-jetted MAR-M247 is presented, which yields tensile properties comparable to those of analogously post-processed cast material. Further possibilities of microstructural optimization and the design philosophy are discussed in the light of the Binder Jetting process-chain.

Low-temperature (470 °C) gaseous carburization is successfully employed in surface hardening of a CoCrFeNi high entropy alloy. After low-temperature gaseous carburization, a carburized case of expanded face-centered cubic (FCC) structure, i.e., a carbide-free supersaturated interstitial solid solution of carbon (maximum of ~3.0 wt%) in FCC, forms in the surface region of the high entropy alloy. Due to solid solution strengthening by interstitially dissolved carbon atoms and strain hardening caused by plastic accommodation of the lattice expansion, the surface hardness of high entropy alloy is significantly enhanced. The surface hardness reaches 1221 HV0.1N, which is about 5 times the hardness of the substrate.

The austenitization and inter-critical annealing of X4CrNiMo16-5-1 (1.4418) supermartensitic stainless steel were investigated in-situ with synchrotron X-ray diffraction (XRD), dilatometry and differential scanning calorimetry (DSC) under isochronal heating conditions. Austenitization occurred in two stages: the austenitization started at approx. 600 °C, decelerated at approx. 700 °C at 60 to 75 v.% of transformed austenite, and first resumed after heating for approx. 100 °C. This plateau in the transformation curve was more dominant for faster heating rates. Inter-critical annealing at 675 and 700 °C revealed, that austenite can to a certain extent be stabilized to room-temperature. There was good agreement for the transformation curves yielded by dilatometry and XRD. Some deviation occurred due to the different applied heating principles, different temperature monitoring and the impact of surface martensite formation on the XRD measurement. The applicable temperature range for DSC as well as the close proximity of the Ac1- and the Curie-temperature limited the usage of the technique in the present case.

Pack cementation was used to produce Fe1−xAl and Fe2Al5 diffusion coatings on ferritic-martensitic steel P91 and a Ni2Al3 diffusion coating on pure nickel. The performance of diffusion coatings against high-temperature corrosion induced by potassium chloride (KCl) was evaluated by exposing the samples at 600 °C for 168 h in static lab air under KCl deposit. In addition, a salt-free experiment was performed for comparison. Microstructure, chemical and phase composition of the samples were analyzed with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD) before and after the exposures. It was found that all the diffusion coatings formed protective oxides under salt-free exposure in air. Under the salt deposit, Fe1−xAl showed local failure while on large parts of the sample a protective layer had formed. Fe2Al5 was attacked over the entire surface and the dominant mode of attack was selective aluminum removal. Ni2Al3 showed excellent performance and no sign of attack was observed anywhere on the sample.

Nickel layers with 〈100〉, 〈210〉, 〈110〉 and 〈211〉 fiber textures were electrodeposited from additive-free Watts type electrolytes by adjusting both the pH and the applied current density. Quantitative crystallographic texture analysis by XRD was supplemented by micro-texture analysis applying EBSD. While XRD results correspond to absorption-weighted averages over the top part of the layer, EBSD on the cross section allowed studying the texture evolution as a function of distance to the substrate. Although layer growth started on amorphous substrates, implying that nucleation occurs unbiased by the substrate, often relatively strong fiber textures develop already at the early stage of growth. These fiber textures can further develop into other preferred fiber axis further away from the substrate. The experimental results demonstrate that already in an early stage of deposition there are major differences in the developing textures. The evolution of the substrate-adjacent textures into the texture of thick layers depends strongly on the deposition conditions.

The present work deals with the evaluation of the residual stress profiles in expanded austenite by applying grazing incidence X-ray diffraction (GI-XRD) combined with successive sublayer removal. Annealed and deformed (e eq=0.5) samples of stable stainless steel EN 1.4369 were nitrided or nitrocarburized. The residual stress profiles resulting from the thermochemical low-temperature surface treatment were measured. The results indicate high-residual compressive stresses of several GPa’s in the nitrided region, while lower-compressive stresses are produced in the carburized case. Plastic deformation in the steel prior to thermochemical treatment has a hardly measurable influence on the nitrogen-rich zone, while it has a measurable effect on the stresses and depth of the carbon-rich zone.

In the past decades, high nitrogen steels (HNS) have been regarded as substitutes for conventional austenitic stainless steels because of their superior mechanical and corrosion properties. However, the main limitation to their wider application is their expensive production process. As an alternative, high-temperature solution nitriding has been applied to produce HNS from three commercially available stainless steel grades (AISI 304L, AISI 316, and EN 1.4369). The nitrogen content in each steel alloy is varied and its influence on the mechanical properties and the stability of the austenite investigated. Both hardness and yield stress increase and the alloys remain ductile. In addition, strain-induced transformation of austenite to martensite is suppressed, which is beneficial for subsequent low-temperature nitriding of the surface of deformed alloys. The combination of high- and low-temperature nitriding results in improved properties of both bulk and surface.

A complete first-principles thermodynamic model was developed and applied to hexagonal close-packed structure e -Fe3N. The electronic structure was calculated using density functional theory and the quasiharmonic phonon approximation to determine macroscopic thermodynamic properties at finite temperatures was generalized in terms of the partition function for any lattice of interest. Specially, thermal expansion of the hexagonal close-packed e phase with two independent lattice parameters was studied by means of the present model and first-principles phonon calculations. The present predictions of thermal expansion of e -Fe3N are in good agreement with experimental data.

The carbon solubility in expanded austenite is investigated by controlled low temperaturegaseous through-carburizing of AISI 316 stainless steel thin foils with thermogravimetry and synchrotron powder diffraction analysis. Carburizing is carried out in C2H2–H2–N2 and CO–H2–N2 atmospheres at 380–420 °C and 465–470 °C, respectively. Hägg carbide (x-M5C2)develops when the carbon content in the expanded austenite exceeds the metastable solubility limit; the transformation of carbon expanded austenite into Hägg carbide occurs irrespective of carburizing temperature in the investigated temperature range (380–470 °C). The maximum solubility of carbon in expanded austenite (380 °C) is found to correspond to an occupancy (yC) of 0.220 of the interstitial octahedral sites of the austenite lattice (i.e., 4.74wt%C). Decomposition of Hägg carbide into M7C3 occurs upon prolonged carburizing treatment orthermal exposure in inert atmosphere (in situ synchrotron experiments).

The crystal structure and magnetic properties of austenitic stainless steel with a colossal interstitial content, so-called expanded austenite, are currently not completely understood. In the present work, the magnetic properties of homogeneous samples of expanded austenite, as prepared by lowerature nitriding of thin foils, were investigated with magnetometry and Mössbauer spectroscopy. At room temperature, expanded austenite is paramagnetic for relatively low and for relatively high nitrogen contents (yN = 0.13 and 0.55, respectively, where yN is the interstitial nitrogen occupancy), while ferromagnetism is observed for intermediate nitrogen loads. Spontaneous volume magnetostriction was observed in the ferromagnetic state and the Curie temperature was found to depend strongly on the nitrogen content. For the first time, X-ray diffraction evidence for the occurrence of long-range interstitial order of nitrogen atoms in expanded austenite was observed for high nitrogen contents.

Kurzfassung Die Austenit-zu-Martensit-Umwandlung unterhalb Raumtemperatur wurde in situ mit Magnetometrie in Fe-N-, Fe-Cr-C- und Fe-Cr-Ni-basierten Legierungen untersucht. Nach dem Abschrecken von der Austenitisiertemperatur auf Raumtemperatur wurden die Legierungen in siedendem Stickstoff abgeschreckt. Anschließend wurde die martensitische Umwandlung während der Erwärmung auf Raumtemperatur verfolgt. Verschiedene konstante Erwärmraten zwischen 0,5 K/min und 10 K/min wurden untersucht. Eine Probe wurde als Referenz schockartig in Wasser aufgeheizt, um zu überprüfen, ob die Martensitbildung bei einer hohen Erwärmrate unterdrückt werden kann. Thermisch aktivierte Bildung von Martensit während dem Erwärmen wurde für alle getesteten Legierungen mit einer von der Erwärmrate abhängigen Umwandlungskinetik überzeugend demonstriert. Außerdem konnte mittels Magnetometrie gezeigt werden, dass die Erwärmrate den Martensitanteil während der Wärmebehandlung beeinflusst. Die Aktivierungsenergie für thermisch aktivierte Martensitbildung wurde mit der Methode nach Kissinger zu 11–18 kJ/mol bestimmt und nimmt mit dem Anteil der Zwischengitterelemente zu.

Vibrating sample magnetometry, VSM, was applied to follow the evolution of the austenite-to-martensite transformation in AISI D2 tool steel at cryogenic temperatures and to determine the associated changes in magnetic hardness. Data shows that the transformation can progress during cooling from room temperature (RT) to −193 °C and, more surprising, during re-heating from −193 °C to RT, indicating the formation of, so called, isothermal or thermally activated martensite. The thermally activated conversion of austenite into martensite occurred in the temperature range −33 °C ≤ T ≤ −173 °C and was fastest at −113 °C. Additionally, VSM revealed that the magnetic hardness decreases in correspondence with the formation of martensite. Data is interpreted in terms of continuous growth of the (ferromagnetic) martensite units at cryogenic temperatures that reduces the number of (paramagnetic) austenite regions retained in the material. Eventually, the work is put in perspective to arrive at an understanding of the effect of cryogenic treatment on the microstructure and performance of martensitic steels.

Low-temperature gaseous carburization is a surface modification method for austenitic stainless steels. In order to investigate the effects of low-temperature gaseous carburization on the fatigue behavior of AISI 316L, fully reversed axial fatigue tests were performed at room temperature on specimens with various remaining case depths. The fatigue performance of AISI 316L could be significantly improved; a 15% higher endurance limit is achieved after low-temperature gaseous carburization. After removal of the outer, brittle part of carburized case by electropolishing, the improvement of the fatigue performance is reduced. Fractography showed that for untreated specimens, fatigue cracks always initiated on the surface. For the carburized specimens, however, the locations of crack initiation sites depend on the applied stress levels. Compressive residual stresses in the case move the crack initiation site to the sub-surface; the lower the applied stress, the deeper the initiation site. A quantitative analysis of the effect on fatigue behavior forms the basis for a life prediction model, which can accurately predict the fatigue life of AISI316L steel after low temperature carburization.

The present work investigates the response of different gaseous thermochemical treatments on selective laser melted (SLM) Ti-6Al-4V. The resulting microstructures after thermochemical treatment were investigated with X-ray diffraction, light optical microscopy, scanning electron microscopy and Vickers-microhardness indentation. Nitriding, performed at 1000–1050 °C resulted in a diffusion zone of nitrogen in solid solution and surface compound layers consisting of TiN (and Ti2N at 1000 °C). Below the compound layer Al-enrichment of the α-zone was observed. Carbo-oxidising in a CO atmosphere at 1000–1050 °C resulted in deep diffusion zones and thick compound layers of the ternary compound TiC1-xOx. Both surface hardness and layer depth were found to increase with temperature and treatment time. Chemically controlled carbo-oxidising, applying the gas redox system CO-CO2, was performed at temperatures in the range 850–1050 °C, resulting in carbo-oxides and formation of oxides with increasing Ti:O ratio with increasing temperatures (rutile and Magnéli phases). Nitriding followed by (carbo-)oxidising treatment resulted in higher surface hardness owing to the formation of mixed interstitial compounds TiC1-x-yNxOy in the compound layer. The compound layer grew into the Al-rich zone as elongated structures.The improvement of wear by nitriding, carbo-oxidising and duplex nitriding/(carbo-)oxidising on SLM Ti-6Al-4V was evaluated by dry sliding wear testing. Lowering of the wear volume by up to a factor of 450 compared to an annealed reference sample was realised. Carbo-oxidising in CO at 1000 °C offered the best wear resistance and resulted in a lowering of the friction coefficient, averaging μ = 0.22, compared to μ = 0.45 for an annealed reference sample.