Nishiyama-Wassermann Relationship Research Articles

Effects of Heat Treatment on Microstructure and Mechanical Properties of a Transformation-Induced Plasticity-Aided Economical Duplex Stainless Steel

In order to obtain steel with high tensile strength and elongation and good pitting resistance, the economical duplex stainless steel with and without the addition of tungsten (W) was prepared in this paper. The comparison and distribution of the two phases were controlled by optimizing the heat treatment process. The highest product of tensile strength and elongation of Cr19 series duplex stainless steel (DSS) DSS were obtained after solution treatment at 1050 °C for 5 min, up to 58692 Mpa%(876 Mpa * 67%), when the area f.raction of α was about 50.7%. The addition of tungsten reduces tensile strength due to the increased area fraction of α at 1050–1150 °C. Finer grain size leads to higher tensile strength after solution treatment at 1050 °C. The microstructure is characterized by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The steels exhibit an excellent transformation-induced plasticity (TRIP) effect after the solution treated is at 1050 °C, which may primarily be due to the existence of a more unstable austenite phase. The TRIP effect acts through the phase transformation of γ to α′-M during high deformation, and the orientation relationship of {111}γ || {110}α′-M & <110>γ || <001>α′-M belongs to the standard variants of Nishiyama–Wassermann relationship.

Orientation relationships between precipitates and matrix and their crystallographic transformation in a Cu–Cr–Zr alloy

The orientation relationships between Cr precipitates and matrix in a Cu–Cr–Zr alloy with various aged states were investigated by transmission electron microscopy and High-resolution transmission electron microscopy, and the crystallographic transformations of orientation relationships were also analyzed by stereographic projection and discussed. Four kinds of orientation relationships were detected in the precipitation process of Cu–Cr–Zr alloy, and coexistence of Cr precipitates with Nishiyama-Wassermann relationship and Pitsch relationship was found in the peak-aged Cu–Cr–Zr alloy. The occurrence for cube-on-cube and Pitsch relationship in Cu–Cr–Zr alloy was thought to be caused by the retardation of additions and heterogeneous nucleation of Cr precipitates on dislocations respectively. Transformations of orientation relationships were observed during precipitation and the crystallographic rotation pathways of the transformations were represented by stereographic projection. The interrelationships of various orientation relationships were established by Bain strain and crystal rotation in the Cu–Cr–Zr alloy of this study.

Orientation relationship between ferrite and austenite and its influence on ultrasonic attenuation in cast austenitic stainless steel

Cast austenitic stainless steel (CASS) is widely used in important engineering components, which has a two-phase microstructure, i.e.austenite and ferrite. With slow cooling rate during solidification procedure, the austenite grain is coarse and the morphology of ferrite is complex. Due to the remarkable elasticity anisotropy of austenite, the resulting structural noise makes the recognition of macroscopic defects quite difficult in ultrasonic testing. To improve the signal-to-noise ratio, the ultrasonic testing frequency is generally small, about 0.5-2.0 MHz, and the ultrasonic scattering effect of ferrite is ignored. However, for submillimeter or even smaller defect and damage near the surface, the ultrasonic testing frequency should be increased to achieve a higher resolution. In these cases, how the ferrite influences the ultrasonic wave propagation behavior and the testing result is still not conclusive. Therefore, CASS Z3CN20-09M is studied as an example in this paper. Based on ultrasonic propagation modeling and “in situ” experimental design, the crystal orientation relationship between ferrite and austenite in CASS is studied and the factors influencing the ultrasonic scattering attenuation are clarified. The results would be helpful for clarifying the ultrasonic response mechanism of CASS and critical for the quantitative evaluation of small defects and early-stage damage.<br/>The orientation relationship between ferrite and austenite and its influence on ultrasonic scattering attenuation in CASS are studied. The crystal orientations and their relationships between two phases are characterized by the EBSD technique. A two-dimension anisotropic model is built based on the morphology of ferrite, and the ultrasonic propagation is calculated by the time domain finite difference method. The influences of orientation relationship and morphology on the longitudinal wave attenuation are analyzed and verified by “in-situ” experiments. Results show that ferrite grains with bar or island shape are distributed on the austenite grains. The orientation relationship between ferrite and austenite is mainly Kurdjumov-Sachs relationship, and only a minority of ferrite and austenite satisfy the Nishiyama-Wassermann relationship. Numerical simulation of the ultrasonic propagation under a testing frequency of 15 MHz indicates that the orientation relationships between two phases and ferrite morphologies present collaborative effects on the ultrasonic scattering attenuation, which could not be ignored. The factors influencing the ultrasonic attenuation in <101> austenite grain are quantitatively analyzed. It is found that in single austenite grains of CASS, the inhomogeneity of crystal orientation, the orientation relationship between austenite and ferrite and the ferrite morphology play an important role in determining the total ultrasonic attenuation.<br/>The results would provide supports for clarifying the ultrasonic response mechanism of CASS and developing the quantitative evaluation methods.

An atomistic simulation study of the crystallographic orientation relationships during the austenite to ferrite transformation in pure Fe

Molecular dynamics (MD) simulations on a model of pure Fe have been used in the investigation of solid-state nucleation of a body-centered-cubic (BCC) phase from a polycrystalline face-centered-cubic (FCC) matrix. A neighbor vector analysis (NVA) method has been introduced and it is shown how the NVA can be used to determine the misorientation of grain or interphase boundaries. In particular, the NVA was utilized to identify the orientation relationships (ORs) of several BCC nuclei and three special ORs were tested, namely the Kurdjumov-Sachs (KS), Nishiyama–Wassermann (NW) and Pitsch (P). From several quasi-2D simulations, it was found that all stable nuclei at grain boundaries formed at least one orientation relationship with the parent grains that was consistent with either the KS or NW relationship. Several initial MD simulation cells, which prohibited the formation of special ORs, were also examined and in these simulations no nucleation was observed after long run times. In addition, the orientation was detected in all mobile phase boundaries. Consistent with experimental findings, these observations demonstrate the importance of this high coherency atomic plane during both the nucleation and growth process. The nucleation and phase boundary characteristics identified here may provide important insights into the nucleation rate and grain orientation of more general solid state nucleation processes.

Crystallography and morphology of a lathy ferrite in Fe–Cr–Ni alloys during directional solidification

Abstract

Features of the crystal structure of disperse carbides in alpha titanium

Investigations of disperse nonmetallic inclusions in unalloyed alpha titanium VT1-0 have been performed by using transmission electron (including scanning and high-resolution) microscopy. Characteristic electron energy losses spectroscopy has shown that these inclusions are titanium carbide particles. It has been revealed that the disperse carbides are formed in the titanium hcp matrix as a phase based on the fcc sublattice of titanium atoms. The inclusion–matrix orientation relationship corresponds to the well-known Kurdyumov–Sachs and Nishiyama–Wassermann relationships $ {\left[ {2\overline {11} 0} \right]_{{\upalpha }}}\parallel {\left[ {011} \right]_{{\updelta }}}{\text{ and }}{\left( {000\overline 1 } \right)_{{\upalpha }}}\parallel {\left( {1\overline 1 1} \right)_{{\updelta }}} $ .

Design of a novel Mn-based 1 GPa duplex stainless TRIP steel with 60% ductility by a reduction of austenite stability

We report on the microstructure, texture and deformation mechanisms of a novel ductile lean duplex stainless steel (Fe–19.9Cr–0.42Ni–0.16N–4.79Mn–0.11C–0.46Cu–0.35Si, wt.%). The austenite is stabilized by Mn, C, and N (instead of Ni). The microstructure is characterized by electron channeling contrast imaging (ECCI) for dislocation mapping and electron backscattering diffraction (EBSD) for texture and phase mapping. The material has 1 GPa ultimate tensile strength and an elongation to fracture of above 60%. The mechanical behavior is interpreted in terms of the strength of both the starting phases, austenite and ferrite, and the amount, dispersion, and transformation kinetics of the mechanically induced martensite (TRIP effect). Transformation proceeds from austenite to hexagonal martensite to near cubic martensite (γ → ε → α′). The ε-martensite forms in the austenite with an orientation relationship close to Shoji–Nishiyama. The α′-martensite nucleates at the intersections of deformation bands, especially ε-bands, with Kurdjumov–Sachs and Nishiyama–Wassermann relationships. The ferrite deforms by dislocation slip and contains cell substructures.

Variant Selection during the Transformation of Deformed Austenite in a Niobium Bearing TRIP Steel

The transformation behavior of a 0.22wt%C-1.50wt%Mn-1.56wt%Si-0.045wt%Nb steel was studied after controlled rolling. Deformation below the no-recrystallisation temperature was employed to deform the prior austenite. During bainitic holding, some of the austenite was retained due to the enrichment of carbon in this phase. Using automated EBSD techniques, both the retained austenite and bainite orientations were measured and the orientation relationships between these phases are represented in Rodrigues-Frank space. The specific orientations predicted by the Bain, Kurdjumov-Sachs and Nishiyama-Wassermann relationships are compared with the measurements and the occurrence of variant selection is discussed.

On coherent transformations in steel

The effective grain size of a martensitic steel is the coherence length of structure domains that share the relevant crystallographic features. These, in turn, depend on the property of interest. The crystallographic feature that governs transgranular fracture is the {1 0 0} cleavage plane while that governing dislocation plasticity is normally the {1 1 0} slip plane. However, two adjacent sub-volumes that have different orientation relationships do not necessarily mean two effective grains. In this paper, effective grain size is discussed based on 24 Kurdjumov–Sachs and 12 Nishiyama–Wassermann relationships which can be divided into three groups according to their Bain strains. Stereographic projection showed that only those from different Bain groups will have large-angle misorientations between {1 0 0} cleavage planes while dislocation glide planes {1 1 0} usually do not exhibit large-angle misorientation. Therefore, effective grain sizes for transgranular fracture and plastic deformation are not the same, and do not necessarily respond in the same way to certain grain refinement measures.

Crystallography and morphology of nanosized Cr particles in a Cu–0.2% Cr alloy

The crystallography and morphology of nanosized Cr precipitate particles in a Cu matrix have been examined using an aged Cu–0.20 mass% Cr alloy at 773 K. By HRTEM observations, the Cr particles were found to have the b.c.c. structure even for sizes smaller than 10 nm. The orientation relationships between the fine Cr particles and the Cu matrix were analyzed by observing Moiré fringes. Two distinct orientation relationships were found: one close to the Kurdjumov–Sachs relationship and the other close to the Nishiyama–Wassermann relationship. The particles exhibited major facet planes: (533) f/(134) b for the Kurdjumov–Sachs particles and (533) f/(035) b for the Nishiyama–Wassermann particles. The relative stability of the two types of Cr particles is discussed by adopting the invariant-line concept and the eigenvectors of the transformation.

Structure and intermixing in Fe/Fe(110) and Fe/Ag/Fe(110) multilayers

We have previously shown, in UHV–SEM studies of Ag/Fe(110), that Ag(111) islands grow on top of two intermediate monolayers of Ag in the Stranski–Krastanov (SK) mode. In the present work, Fe was deposited on samples consisting of (1–5 ML) annealed Ag layers on Fe(110). Growth, structure and intermixing were monitored by biased secondary electron imaging (b-SEI), reflection electron diffraction (RHEED) and Auger electron spectroscopy (AES). At room temperature (RT), Fe islands grow in a (110) orientation on (1–2 ML) Ag(111)/Fe(110). Due to kinetic limitations, these islands form facets similar to those obtained in the Fe/Fe(110) system. These limitations are overcome in the Fe/Fe(110) system by depositing at 250°C. All three orientations in the Nishiyama–Wassermann relationship are observed for Fe islands grown on thicker, but not on thinner, Ag layers. Quantitative AES indicates that there is intermixing at the monolayer level between the deposited Fe atoms and underlying Ag layers even at RT. During annealing, or for deposition at 250°C, Ag atoms undergo long range diffusion towards the top of the deposit, but kinetic limitations are not completely overcome for 5 ML Ag layers; for thinner layers, the Fe islands flatten forming additional substrate layers at the Ag/Fe(110) interface.

Growth of Ce thin film on a Mo(110) surface

The growth mode and surface structure of Ce thin films deposited on a Mo(110) surface were investigated by means of reflection high energy electron diffraction and scanning electron microscope. High temperature condensation above 500°C takes place according to the Stranski-Krastanov growth mode in which double layers grow initially, followed by the growth of Ce crystallites. The Ce two-dimensional double layers are composed of distorted fcc γ-Ce(111) layer and normal fcc γ-Ce(111) layer for the 1st and the 2nd layer, respectively. The Ce crystallites with the shape of flat islands are observed at high coverage. Epitaxial orientation relationship of 2nd monolayer and the crystallites is the Nishiyama-Wassermann relationship. However, low temperature condensation below 500°C takes place according to the Frank-van der Merwe growth mode.

Variant selection in the martensitic transformation of an Fe-30% Ni alloy with cube texture

Martensitic transformation of Fe-30% Ni with a strong cube texture was studied. According to the Nishiyama-Wassermann relationship, the corresponding relation between the 12 orientation variants and the 24 habit-plane variants was determined. Plastic deformation in different directions prior to transformation leads to selection rules for the orientation as well as for the habit-plane variants. The total variant selection is a combination of both selection rules.

Stability of trapped lattice dislocations on interphase boundaries in duplex stainless steel

Differences in the thermal stability of trapped lattice dislocations in interphase boundaries and grain boundaries in austenitic–ferritic steel have been studied. It was found that the distribution of diffusional properties of α/γ interphase boundaries depends on the mutual orientation of phases and differs significantly from the distribution of diffusional properties of α/α and γ/γ grain boundaries, which stems from the fact that interphase boundaries include a substantial fraction of boundaries of particularly low diffusivity. These are interfaces of austenite grains which nucleated inside ferritic grains and demonstrate Kurdjumov–Sachs or Nishiyama–Wassermann relationship with respect to the ferritic matrix.MST/1005

Stability of trapped lattice dislocations on interphase boundaries in duplex stainless steel

Differences in the thermal stability of trapped lattice dislocations in interphase boundaries and grain boundaries in austenitic–ferritic steel have been studied. It was found that the distribution of diffusional properties of α/γ interphase boundaries depends on the mutual orientation of phases and differs significantly from the distribution of diffusional properties of α/α and γ/γ grain boundaries, which stems from the fact that interphase boundaries include a substantial fraction of boundaries of particularly low diffusivity. These are interfaces of austenite grains which nucleated inside ferritic grains and demonstrate Kurdjumov–Sachs or Nishiyama–Wassermann relationship with respect to the ferritic matrix.MST/1005

Neutron diffraction pole figure measurements on iron meteorites

Neutron diffraction pole figures were measured on three iron-nickel meteorite samples, the hexahedrites Coahuila and Walker County, both belonging to group IIA, and the octahedrite Gibeon, group IVA. As expected, pole figure peaks of the hexahedrites can be assigned to reflections from a single crystal or to reflections due to twinning on {221}. Macroscopically visible fractures in Walker County sample can be recognized in the pole figures by peak splitting and broadening. Schreibersite pole figures of the Coahuila meteorite corroborate the orientation relationship between prismatic rhabdites and kamacite {110} Rh ' {210} α, <001> Rh ' <100> α). For the octahedritic specimen, taenite pole figures reveal that taenite rims have the orientation of the parent γ-phase crystal. By the kamacite pole figures we deduce the orientation between kamacite lamellae and taenite rims to be close to Nishiyama-Wassermann relationship ({110} α ' {111} γ, <001> α ' <1̄10> γ). Variant selection occurs with the kamacite crystals, and suggests the presence of strain during kamacite growth.

A microstructural characterization of solution-treated titanium alloy Ti6Al4V

A transmission electron microscopic study has been carried out to characterize the microstructural features of a titanium-6 wt% aluminum-4 wt% vanadium alloy after treatments at three different solutionizing temperatures of 900°C, 955°C, and 1065°C, followed by subsequent cooling at various rates in air, furnace, or water. Solutionization at 900°C and 955°C produced duplex microstructures consisting of equiaxed α and partially transformed β phases, while solutionization at 1065°C resulted in coarse Widmanstätten structures on furnace or air cooling and a fully martensitic structure on water quenching. The present studies revealed the morphology, size and distribution of the α, β, and martensite phases and also the presence of an interface phase with an fcc structure at almost all α-β interfaces. This interface phase has a width of about 0.15 μm and occurred in all slowly cooled specimens. Selected area diffraction patterns revealed the Burgers relationship between α and β and Nishiyama-Wassermann relationship between β and γ. The martensite is typically of lath structure with alternate laths in different orientations when quenched from 1065°C but observed as a fine platelike structure when quenched from 900° and 955°C.

Crystallography of M 23C 6 and M 6C precipitated in a low alloy steel

The orientation relationship of M 23C 6 and M 6C precipitated in a CrMoV rotor steel has been studied by means of transmission electron microscopy. A new orientation relationship, similar to the Nishiyama-Wassermann relationship for M 23C 6 α , has been found for γ α . M 6C can either form directly in ferrite or grow on the existing M 23C 6, both with a parallel orientation relationship. These are discussed in the light of matching of individual atoms on the interface between two phases.

Factors Determining the Orientation Relationships between Iron Deposits and (111) &lt;I&gt;FCC&lt;/I&gt; Substrates

Iron was vacuum deposited onto (111)fcc substrates of nickel, copper and silver. The orientation relationships between the deposit and the substrates have been examined by transmission electron microscopy and selected area diffraction. The iron/substrate interfaces take the orientation relationships of {110}Fe//{111}fcc. In the iron-silver system, the orientation relationship is [001]Fe//[110]Ag, that is the Nishiyama-Wassermann relationship. In the case of iron-copper system, the iron deposits rotate about an axis normal to the substrate and lead to the orientation relationship in the range from Nishiyama-Wassermann’s to Kurjumov-Sachs’s. For the iron on nickel, the K-S relationship is more pronounced compared to the case of iron on copper system. These results can be understood by the concept of good atomic fitting area at the interface.