Nishiyama-Wassermann Orientation Relationship Research Articles

New insights into selective leaching and ferritization in 15-15Ti austenitic steel in lead-bismuth eutectic through parent phase reconstruction

Selective leaching accompanied by ferritization is widely observed in the dissolution corrosion of austenitic steels exposed to lead-bismuth eutectic. This study proposes a refined mechanism for selective leaching involving solid-state diffusion followed by dissolution-reprecipitation processes. Pole figure analysis reveals that ferritization follows the Nishiyama-Wassermann orientation relationship, independent of corrosion environment or corrosion behavior. Moreover, austenite reconstruction analysis shows that the growth of ferrite is hindered by the misfits at random high-angle grain boundaries. Hence the above results suggest that ferritization is initially driven by solid-state diffusion. As LBE channels open within the grains, further growth of ferrite is assisted by the dissolution-reprecipitation mechanism. Additionally, the formation of dual-oriented ferrite grains at Σ3 twin boundary is observed, and the selectivity of this formation with respect to grain boundary and ferrite orientation is discussed.

Crystal-Structure Matches in Solid-Solid Phase Transitions.

The exploration of solid-solid phase transition suffers from the uncertainty of how atoms in two crystal structures match. We devised a theoretical framework to describe and classify crystal-structure matches (CSM). Such description fully exploits the translational and rotational symmetries and is independent of the choice of supercells. This is enabled by the use of the Hermite normal form, an analog of reduced echelon form for integer matrices. With its help, exhausting all CSMs is made possible, which goes beyond the conventional optimization schemes. In an example study of the martensitic transformation of steel, our enumeration algorithm finds many candidate CSMs with lower strains than known mechanisms. Two long-sought CSMs accounting for the most commonly observed Kurdjumov-Sachs orientation relationship and the Nishiyama-Wassermann orientation relationship are unveiled. Given the comprehensiveness and efficiency, our enumeration scheme provide a promising strategy for solid-solid phase transition mechanism research.

Deformation microstructures and martensitic transformation pathways in cryogenically deformed 316L stainless steel

Liquefied gas storage and transportation, as well as space propulsion, are driving increasing interest in the cryogenic temperature deformation behaviour of 316L stainless steels. This was investigated here during tensile deformation at 15, 50 and 173 K. Complex transformation pathways, including one-step γ-austenite → α′-martensite, two steps γ → ε-martensite → α′ transformation as well as twinning and stacking fault-assisted γ → α′ transformation, are observed. γ grains with a (111) plane normal direction aligned 50–65°from the loading direction appear more likely to form the ε phase. Further, high-resolution transmission Kikuchi diffraction mapping revealed that the nucleation process of α′ can be assisted by ε and stacking faults at all cryogenic temperatures, whereas twins can also serve as sites for α′ nucleation when deformed at 173 K. For two-step transformation, separate lenticular α′ nucleate following Kurdjumov–Sachs orientation relationship (OR) within the shear band, once grown out of the shear band, Pitsch OR is preferable. As for one-step transformation, irregular oval α′ nucleates directly at γ grain boundaries with Nishiyama–Wassermann OR. These findings provide new insights into the correlation between the various transformation pathways and deformation mechanisms, as well as their improved performance at low temperatures.

A strategy for simultaneously enhancing mechanical strength and hydrogen embrittlement resistance: Exceptional performance of laminated metal composite in hydrogen environments

High-strength metallic materials with low hydrogen embrittlement (HE) susceptibility have always been regarded as ideal commercial materials in the hydrogen energy field. This work proposed a low-cost laminated metal composite (LMC-CS) consisting of austenitic stainless steel (ASS) layer and martensitic steel (MART) layer, which simultaneously achieved high mechanical strength with excellent HE resistance. The ASS layers provide additional strain-hardening capability and the MART layer contributes high strength to LMC-CS after H-charging. Additionally, the composite interface displays semi-coherent characteristics, and the atomic arrangement on both sides follows the Kurdjumov-Sachs orientation relationship with (11¯0)α-Fe // (1¯11¯)γ-Fe and [111]α-Fe // [110]γ-Fe and the Nishiyama–Wassermann orientation relationship with (1¯1¯0)α-Fe // (1¯11)γ-Fe and [001]α-Fe // [011¯]γ-Fe., which coordinates deformation between layers and hinders hydrogen-induced cracks initiation at the interface. The as-designed LMC-CS achieves an ultimate tensile strength of 1102 MPa and an elongation loss of 14.8% after long-time hydrogen charging with sides sealed, outperforming a wide range of metallic materials. This work provides a fresh perspective on developing commercial materials with high strength and superior HE resistance for application in severe hydrogen environments.

The deterministic effect of laser energy density on the microstructures of novel duplex stainless steel fabricated via in-situ alloying in laser powder bed fusion

Distinctive microstructures and distributions of novel duplex (ferrite δ + austenite γ) stainless steel can be fabricated using in-situ alloying of Fe–Cr–Mn alloy and pure Ni powders by tuning laser energy density in powder bed fusion. At low-level energy density (∼60 J/mm3), ferrite solidification dominates and results in bimodal ferrite of columnar and equiaxed grains. At medium energy density (∼100 J/mm3), fine acicular Widmanstätten austenite satisfying the Nishiyama-Wassermann orientation relationship was produced within the coarse grains of δ, which is the predominant phase although ferrite and austenite solidification occur. At high-level energy density (∼194 J/mm3), ferrite and austenite solidification occur and the resultant microstructure consists of nearly equivalent fine equiaxed-grained δ and γ. The key difference in the microstructure was attributed to the different melt pool overlapping and types, which can be characterized by the size and local ratio of Cr equivalent (Creq) and Ni equivalent (Nieq) values (Creq/Nieq). The melt pool characteristics fundamentally determine the local solidification and transformation mode at different energy densities. The testing results have shown that the novel duplex stainless steel possesses excellent mechanical properties compared to conventionally-manufactured counterparts.

Anisotropic response in mechanical and corrosion performances of UNS S31803 duplex stainless steel fabricated by laser powder bed fusion

Mechanical and corrosion behaviors perpendicular to build direction (XOY) and along build direction (XOZ) of UNS S31803 duplex stainless steel (DSS) fabricated by laser powder bed fusion (LPBF) were investigated. With the post-annealing, the steel exhibited a ferrite-austenite microstructure. For XOY plane, austenite displayed a primarily <101> texture while ferrite kept <001> preferred orientation, and mainly held a Nishiyama-Wassermann orientation relationship. However, for XOZ plane, austenite showed a mixed texture. The average size of ferrite grains was larger for XOZ plane than XOY. The austenite in XOY plane was mostly island morphology grains, while in XOZ plane, there were a large number of coarse columnar grains. A higher geometrically necessary dislocation density and proportion of low-angle grain boundaries on XOY plane were observed. The yield strength and ultimate tensile strength values perpendicular to build direction were higher than those along build direction. While the total elongation perpendicular to build direction was lower. Moreover, the XOY plane shows slightly better corrosion resistance than the XOZ.

Effect of cold rolling reduction ratio on microstructure and mechanical properties of Fe–10Mn–4Al-0.4C steel containing δ ferrite

A new low-cost low-density high-strength steel was produced by a combination of hot rolling, cold rolling and a short-time annealing process. The effect of cold rolling reduction on the microstructure and mechanical properties of Fe–10Mn–4Al-0.4C steel after annealing was studied. The microstructure of short-time annealing (750 °C/30 min) after cold rolling with different reduction consists of equiaxed α-ferrite, austenite and banded δ-ferrite. As the reduction increased, the grain size gradually decreased. After annealing, the distribution of austenite in the sample is uniform. When the reduction is 60%, the content of austenite reaches the maximum of 61%. The annealed sample with 60% reduction exhibited the highest tensile strength and elongation with a product of strength and elongation (PSE) of 40.23 GPa·%. The ferrite and martensite were distinguished in the electron backscatter diffraction (EBSD) map by the Nishiyama-Wassermann orientation relationship. After the tensile test, part of the austenite was transformed into martensite, which is banded. The strengthening mechanism was calculated according to the rule of mixtures (ROM), and the contributions of grain boundaries strengthening, dislocation strengthening, transformation induced plasticity (TRIP) effect strengthening and additional strengthening were investigated. The annealed sample with 60% reduction has favorable TRIP effect with a strengthening value of 211 MPa. EBSD analysis of the tensile sample showed that the banded δ-ferrite can accommodate a large number of dislocations.

A new orientation relationship observed between MC and ferrite in Ti Mo microalloyed steel produced by ultra-fast cooling

A new crystallographic orientation relationship (OR) between MC (M, Ti, and Mo) and ferrite has been observed in microalloyed steel produced by new-generation thermo-mechanical processing (traditional TMCP involving ultra-fast cooling). The selected area electron diffraction (SAED) patterns present strong evidence for the new OR, that is (111)MC//(110)ferrite and 1¯10MC//1¯10ferrite, which hasn't been reported previously in MC/ferrite system. While the precipitates formed in the specimens with traditional TMCP obey the well-known Baker-Nutting (BN) and Nishiyama-Wassermann (NW) ORs. The coordinate transformation matrices for BN, NW and the new OR have also been calculated, which further confirms the uniqueness of the new OR. The interfacial energy for the carbides and ferrite that obey the new OR is higher compared with that of BN and NW ORs, which can be attributed to the energy accumulated and maintained in austenite deformation and ultra-fast cooling stage, and then leads to un-equilibrium precipitates

Effects of pulsed laser surface remelting on microstructure, hardness and lead-bismuth corrosion behavior of a ferrite/martensitic steel

A typical ferritic/martensitic (F/M) steel sheet was subjected to pulsed laser surface remelting (LSR) and corrosion test in lead-bismuth eutectic (LBE) at 550 °C. There present two modification zones with distinct microstructures in the LSRed specimen: (1) remelted zone (RZ) consisting of both bulk δ-ferrite grains and martensitic plates and (2) heat-affected zone (HAZ) below the RZ, mainly composed of martensitic plates and high-density precipitates. Martensitic transformation occurs in both the RZ and the HAZ with the Kurdjumov-Sachs and Nishiyama-Wassermann orientation relationships followed concurrently, resulting in scattered orientations and specific misorientation characteristics. Hardnesses of the RZ and the HAZ are 364 ± 7 HV and 451 ± 15 HV, respectively, considerably higher than that of the matrix (267 ± 3 HV). In oxygen-saturated and oxygen-depleted LBE, thicknesses of oxide layers developed on both the as-received and the LSRed specimens increase with prolonging corrosion time (oxide layers always thinner under the oxygen-depleted condition). The corrosion resistance of the LSRed F/M steel in oxygen-saturated LBE is improved, which can be attributed to the grain-refinement accelerated formation of dense Fe–Cr spinel. In oxygen-depleted LBE, the growth of oxide layers is very low with both types of specimens showing similar corrosion resistance.

Deformation-induced phase transformation and twinning in Fe and Fe–C alloys

The phase transformations and twinning in a nano-sized face-centred cubic (FCC) crystal of iron with different carbon contents are studied by molecular dynamic (MD) simulations. The simulation cells are stretched uniaxially along the direction. It is found that the FCC matrix transforms into the body-centred cubic (BCC) phase following the Nishiyama–Wassermann orientation relationship (OR). Then, the ‘BCChexagonal close-packed (HCP)’ and ‘BCCFCC’ transformations occur following different variants of the Burgers OR and the Pitsch OR, respectively. The twins form between two neighbouring BCC grains. Additionally, the carbon atoms can destabilise the BCC phase, inhibit the phase transformation and pin phase interfaces, leading to tiny BCC twins.

The effect of Ni depletion on athermal martensitic transformation in 304 austenitic stainless steel

Chemical and microstructural analyses were performed on Ni depletion zones in 304 austenitic stainless steel sheets after annealing and cooling with three different rates (0.2, 18.6, 160.0 °C/s). The analysis was focused on the characteristics and the origin of the retained δ-ferrite and α’-martensite phases in the austenite matrix. Simultaneous Ni depletion and Cr enrichment belong to the retained δ-ferrite due to incomplete decomposition after the solidification process. Athermal α’-martensite was observed to grow within Ni-depleted zones with no Cr enrichment. The fraction of α’-martensite increases as the cooling rate increases. EPMA analysis reveals that the 1 wt% reduction of Ni contents to the bulk chemical composition promotes the thermally-activated martensitic transformation. Such athermal martensite tended to grow within twinning boundaries holding the Nishiyama-Wassermann orientation relationship with the austenite matrix confirmed with the electron diffraction patterns in TEM and the misorientation analysis in EBSD.

Effect of Processing Techniques on Microstructure and Mechanical Properties of Carbide-free Bainitic Rail Steels

Rail manufacture processing techniques, such as roller straightening, tempering and air-forced quenching, have shown to make a great influence on the microstructure and mechanical properties of rail steels. In this study, five carbide-free bainitic rail steels with same chemical composition were developed from the above rail processing routes in an industrial production line. In general, the carbide-free bainitic steels consisted of bainitic ferrite (BF) plates with distinct morphologies of retained austenite (RA) and minor occurrence of martensite in some conditions. Electron microscopy confirmed the orientation relationship between BF and RA was in line with the Nishiyama-Wassermann (N-W) orientation relationship, i.e., {111}γ//{110}α,<112>γ//<110>α. The roller straightening and tempering processes could facilitate the transformation of unstable RA. The small volume fraction of stable RA contributed to an obvious improvement of yield strength, hardness and impact toughness, but at the cost of ductility and strain hardening ability. In addition, the in-line heat treatment resulted in the refinement of bainitic ferrite plates, leading to an increase in the tensile strength of carbide-free bainitic steels. In summary, there was a clear positive relationship between the rail processing techniques and the mechanical properties of the carbide-free bainitic steels. This study can benefit the development of new bainitic steels with a combination of wear and rolling contact fatigue resistance by appropriate selection of processing techniques.

Isothermal Precipitation Behavior of Vanadium Carbonitride in V‐, N‐Added Low‐Carbon Steel

Herein, the isothermal precipitation behaviors of V(C,N) in ferrite are studied. At 700 °C, the dominant precipitation behavior of V(C,N) is interphase precipitation (IP), whereas at 660 and 620 °C, random precipitation (RP) is dominant. Both IP and RP show the Nishiyama–Wassermann orientation relationship (OR) with ferrite, unlike the common OR between precipitation and ferrite. The size of most precipitated particles varies in the ranges of 5–8 nm at 660 °C and 3–5 nm at 620 °C. The average values of hardness of ferrite grain formed at 700, 660, and 620 °C are 195.1, 237.6, and 278.6 HV, respectively.

New insights into the interface characteristics of a duplex stainless steel subjected to accelerated ferrite-to-austenite transformation

The ferrite-to-austenite phase transformation during water quenching of a duplex stainless steel was studied, where a duplex stainless steel was heated to 1370 °C (delta ferrite region) and quenched to room temperature. The microstructure consisted of coarse ferrite grains and fine needle-like austenite particles. The phase transformation mechanism appeared to be “diffusion-limited” displacive where shear was dominant, but also accompanied by prior or simultaneous diffusional elemental redistribution. A small fraction of the interfaces followed Kurdjumov–Sachs and Nishiyama–Wassermann orientation relationships (ORs) where austenite/ferrite interfaces terminated on {111}A//{110}F planes. The high undercooling associated with the fast cooling rates resulted in a considerable deviation from rational ORs. This was mostly due to the formation of intragranular austenite on Cr2N particles, which most likely caused a random OR with respect to the ferrite matrix. A detailed transmission electron microscopy (TEM) analysis revealed that the planar interphase boundaries characterised by the rational ORs typically contained one dominant set of parallel, regularly spaced dislocations. TEM analysis also showed the occurrence of small protrusions appearing on the edge/face of some austenite particles. Some of these did not leave a sub-boundary behind and formed a finger-like austenite morphology resulting from the instability mechanism. In some other cases, the protruded austenite possessed a low-angle grain boundary with the substrate austenite grain, which was the result of sympathetic nucleation of austenite on a pre-existing austenite particle.

From coherent to semicoherent—Evolution of precipitation crystallography in an fcc/bcc system

How are crystallographic features developed in phase transformation systems with anisotropic misfits (e.g., bcc/fcc systems)? Comprehensive knowledge about this question is still very limited, which impedes the thorough understanding and the quantitative modeling of microstructure evolution. In this work, we simulated the early stage growth of a Cr precipitate (bcc) in a Cu matrix (fcc) by combining Monte Carlo and molecular dynamics. The simulation results reveal fine details about the evolution of crystallographic features, including the orientation relationship (OR) between the two phases, the precipitate morphology, and the interfacial dislocation structures. The governing event during the evolution process is the generation of a dominant set of dislocations. The selection of the dominant dislocations is rationalized based on minimization of the interfacial energy in the major facet, which contains a single set of dislocations. The initial OR between the coherent precipitate and the matrix is close to the Nishiyama–Wassermann OR. In response to the generation of the dominant dislocations, the OR jumps towards the ideal OR corresponding to the O-line condition, which is close to the Kurdjumov–Sachs OR. This tendency reflects the experimental observations in a Cu-Cr system and provides helpful insight into the actual evolution of crystallographic features.

Texture Inheritance in the Ferrito-Martensite Structure of Low-Alloy Steel after Thermomechanical Controlled Processing

The electron back-scatter diffraction (EBSD) method was used to study the structural and textural states of low-carbon low-alloy steel after thermomechanical controlled processing (TMCP) and subsequent heat treatment (HT) including austenitization, holding at the temperature of the decomposition of the γ phase by the diffusion mechanism for different times, and quenching. As a result of the HT, structures were obtained consisting of polygonal ferrite and martensite in various ratios. The crystallographic textures of both martensite and ferrite after HT reproduced the texture of bainite formed as a result of the TMCP. The orientations that form this texture are related to the main orientations of deformed austenite grains upon the TMCP via the orientation relationships (ORs) intermediate between the Kurdjumov–Sachs and Nishiyama–Wassermann ORs. In the case of both ferrite and martensite, the orientational connection between the components of the texture components is explained by the onset of the realization of the phase transformation on crystallographically stipulated (including special) boundaries.

Low-Temperature Nitridation of 2205 Duplex Stainless Steel

Low-temperature gas-phase nitridation has been studied in $$\updelta $$-ferrite in 2205 duplex stainless steel. High-resolution spatially-resolved compositional and structural analysis revealed two competitive responses to nitridation. Some regions revealed a nitrogen atom fraction approaching 25 at. pct—greater than $$10^6$$ times the equilibrium solubility limit at room temperature. Remarkably, there is no expansion or distortion of the body-centered cubic lattice. This is similar to the response of $$\updelta $$-ferrite in this alloy to low-temperature carburization. In conventional transmission electron microscopy bright-field images, the supersaturated ferrite grains show no diffraction contrast—resembling the appearance of amorphous structures—suggesting an unusually high defect density. These grains exhibit spinodal decomposition of the ferrite to nanometer-scale Fe-rich and Cr-rich ferrite domains. High-resolution imaging reveals pristine Fe-rich nanocrystals, whereas the Cr-rich domains are apparently amorphous. Elsewhere in the nitrogen-rich case, an isothermal ferrite-to-austenite phase transformation occurred. The austenite transformation product formed martensitically with a high-aspect-ratio plate-like morphology in the Nishiyama–Wassermann orientation relationship to the ferrite matrix.

Roaldite in the iron-meteorite São Julião de Moreira

Roaldite – Fe4N – has been identified in the São Julião de Moreira iron meteorite using electron backscatter diffraction (EBSD) and simultaneously acquired energy-dispersive x-ray spectroscopy (EDS). Mean-periodic-number images derived from raw EBSD patterns confirm this phase by an even higher spatial resolution compared to EDS.Roaldite appears in the form of systematically and repetitively aligned plates. Despite the locally heavy plastic deformation, it is shown that the origin of the oriented precipitation of roaldite is linked to the orientation of the kamacite matrix. Roaldite can be considered to be precipitated from kamacite using an inverse Kurdjumov-Sachs (K-S) or Nishiyama-Wassermann (N-W) orientation relationship. A more accurate discrimination is impossible due to the accumulated shock deformation, which blurs the local reference orientation of kamacite. The habit plane of roaldite is found to be {112}R, which is most likely parallel to {120}K of kamacite. Some of the roaldite plates contain two orientation variants which repeatedly alternate. Their misorientation angle is about 12°.

Atomic Models of Mechanical Twinning and &lt;110&gt;-Reorientations in BCC-Crystals

Atomic models of twinning and formation of -reorientation bands in bcc-crystals via bcc→fcc→bcctransformations accompanied by a change in the reverse transformation system are proposed. It is shown that {112} deformation twins are formed in the course of these transformations, when the shears and directions of homogeneous deformation of the reverse transformation occur in the crystallographically equivalent directions, making 60° angles with the initial direction (during the forward transformation) and the Kurdyumov–Sachs relations are valid. A fulfillment of the Nishiyama–Wassermann orientation relationships or a change in the type such dependence in the course of the reverse transformations gives rise to reorientation of the crystal lattice of these microbands around the -type directions by the angles 60° or (60 ± 5.23)°. An important feature of these models is a considerable contribution of homogeneous transformation deformation of the martensitic type into the value of plastic deformation of the twin.

Effects of C Addition on the Microstructures of As-Cast Cu-Fe-P Alloys.

Effects of C addition on the microstructures of as-cast Cu–Fe–P (mass fraction) alloys were systematically investigated. The results show that C addition can refine the matrix microstructure and make Fe particles finer. The Fe particles observed in both the non-C-alloyed and C-alloyed specimens are α-Fe particles, which possess a body-centered cubic (bcc) structure with a Nishiyama–Wassermann orientation relationship with the matrix. C is reported to be an γ-Fe stabilizer in the literature. The reason for the difference between the phases of Fe particles observed in this study, and that reported in the literature, are finally discussed. Additionally, C addition facilitates the decomposition of the supersaturated solid solution which occurs by the simultaneous precipitation of very fine Fe particles. Such initial decomposition product has an face-centered cubic (fcc) structure with a cube-on-cube orientation relationship with the matrix.