Bain Orientation Relationship Research Articles

Microstructure evolution and phase transformation mechanism of Ti–46Al–8Nb–2.5V alloy based on phase transformation geometric models

The microstructure of conventionally solidified and rapidly solidified Ti–46Al–8Nb-2.5V alloys was analyzed with phase transformation geometric models, revealing the phase transformation mechanisms. The β→α→γ phase transformation processes follow Burgers and Blackburn orientation relationships (ORs), respectively, from which the calculated transformation between the β/β0 and γ phases follow K–S OR. Whlie, the direct β/β0→γ phase transformation can be directly observed in the microstructure, which follows a variety of ORs, mainly following the K–S OR but also following the N–W or Bain ORs, most of which deviate from the strict ORs. In cases where N–W and Bain ORs are followed, there is a large dislocation spacing between the habit planes of the β/β0 and γ phases, resulting in low static energy at the interface and narrow elongated morphology of precipitated phases. Conversely, when following K–S OR for β/β0 → γ phase transformation, interfacial migration energy is relatively lower and grain precipitated phases is larger. The rapidly solidified microstructure of Ti–46Al–8Nb-2.5V alloy consists of the massive α2 phase, massive γ phase, and basket-weave β0 phase closely resembling those found in β-type titanium alloys. After annealing at service temperature, the rapidly solidified alloy forms a microstructure highly similar to that of conventionally solidified TiAl alloy. Various metastable phases, such as the Ti2Al, L12 and Ti1.4Al phases, precipitate in the blocky γ phase, which provides a theoretical basis for studying the phase transformations of TiAl alloys in service.

Effects of Normalizing Temperature on the Precipitation of Fine Particles and Austenite Grain Growth during Carburization of Al- and Nb-Microalloyed Case-Hardening Steel

This paper deals with the precipitation behaviors of AlN and Nb(C, N) particles during normalizing of hot-forged Al- and Nb-microalloyed case-hardening steel, and investigates the effects of the number density and volume fraction of these particles on the grain growth behavior of austenite (γ) during the subsequent high-temperature carburization. When the sample was cooled from the hot forging temperature at 16°C/min, AlN did not precipitate at all and was completely supersaturated in the matrix, although fine Nb(C, N) particles precipitated during cooling. When reheated to the normalizing temperature, AlN particles started precipitation at approximately 620–640°C on the grain boundaries of the bainitic ferrite matrix and the interfaces between the cementite particles and the matrix, and the AlN particles contained Cr and Mn, and had an NaCl structure and Bain orientation relationship with the matrix. The AlN structure changed from NaCl type into wurtzite type at approximately 800°C. AlN particles grew during reheating to the normalizing temperature and holding at the temperature. Both the number density and volume fraction of the AlN particles were dramatically increased by reducing the normalizing temperature from 1070 to 900°C. The distribution of these particles strongly affected the γ grain structure formed during carburization. As the normalizing temperature was reduced from 1070 to 950 and 900°C, the γ grain structure became finer. Some abnormally large grains were formed near the carburized surface, when normalized at 1070°C. However, no abnormal grain was found when normalized below 950°C.

Dissimilar laser beam welding of AISI 2507 super duplex stainless to AISI 317L austenitic stainless steel

In this research, AISI 2507 super duplex/AISI 317L austenitic stainless steels were joined by laser beam welding technique. The effects of heat input and post weld heat treatment (PWHT) on surface, texture, microstructure and mechanical properties were investigated. Based on the experimental results, it has been determined that the weld metal (WM) exhibits a duplex microstructure. Ferrite grains, grain boundary austenite, intragranular austenite and widmanstatten austenite and dislocation formations were detected in WM. In addition, ferrite, austenite grains and stacking faults were detected in the base metal (BM) of 2507 super duplex and 317L austenitic stainless steels. While dislocation and stacking fault formations disappeared after PWHT, Cr23C6 precipitates were formed. While a K-S and N-W orientation relationship (OR) was generally seen in the samples with and without PWHT, no OR was seen in the BM of the 317L side. In addition, the PWHT result of the Bain OR seen in the WM of the welded sample was not seen. The findings showed that while the Oxygen core on the WM surface of the sample without PWHT and PWHT was at the highest level at first, it then became zero as it descended deeper. Moreover, it was determined that the levels of Fe, Cr, Ni and Mo nuclei, which were at low levels at first, increased as one went deeper. It has been determined that as the heat input increases, the tensile strength, hardness and bending force decrement, while the toughness values increase in the welded samples. Based on PWHT, there is a reduction in tensile strength, hardness and bending force; [111] orientation to the grains [101] and [001] grains, a rise in the proportion of grains with a Schmid Factor (SF) value greater than 0.4, a decrement in kernel average misorientation (KAM) values and coincidence site lattice (CSL) boundary ratios, (001)<100> and (112)<111> this component has been caused by changes such as the emergence of recrystallized γ grains causing a rise in austenite content, increase in grain size, disappearance of stacking fault occurrences and dislocations. It was determined that the rise in toughness was caused by the emergence of (113)<332> component and the increase in low angle grain boundary ratios.

Heat treatment and heat input effects on the dissimilar laser beam welded AISI 904L super austenitic stainless steel to AISI 317L austenitic stainless steel: Surface, texture, microstructure and mechanical properties

In this study, AISI 904L super austenitic - AISI 317L austenitic stainless steel sheets were successfully welded by laser welding method using six different heat inputs. Welded samples went through 1 h of post-weld heat treatment (PWHT) at 1100 °C and then left to cool at room temperature. The effect of PWHT and welding heat input on surface, texture, microstructure and mechanical properties has been examined. According to the results obtained in the study, it was observed that the weld metal microstructure of the samples welded at the lowest and highest heat input values consisted entirely of austenite (dendritic + interdendritic) grains. With PWHT, Cr23C6 precipitation was observed in both grain boundaries and austenite grains. In addition, austenite as well as dislocation and stacking fault formations were observed in the microstructure. While the highest austenite content was observed in the microstructure of AISI 317L, the content of austenite increased in all samples with PWHT. It was observed that most of the grains that were orientated in the [111] direction were orientated in the direction of [101] and [001] after PWHT. However, the KAM values of the dislocation in grain boundaries decreased with the effect of PWHT. In addition, with the reduction of the ratio of dark grains and the effect of re-crystallisation, (112) <111> components representing austenite grains appeared in FCC/BCC texture. It was identified that due to these textural changes, the tensile strength and resistance to bending of the samples were negatively affected but their toughness was positively affected. AB3 sample with the highest tensile strength, hardness, bending strength and lowest toughness had values of 615 MPa, 214 HV, 1100 N and 57 J, respectively, while these values changed to 550 MPa, 181 HV, 720 N and 60 J with the effect of PWHT. Due to the presence of the chromium-oxide layer, it was identified that the density of O− ion and H− ion was at high levels on the surface of the weld metal. It was identified that the density of these ions decreases with increasing depth, while Fe−, Cr− and Ni− ions increase. Excellent K–S and N–W orientation relationship (OR) was observed between FCC/BCC-structured grains present in the microstructure of without PWHT base metals and weld metal, while Bain OR was also observed in weld metal with or without PWHT. In the microstructure of the base metals with PWHT, there was no OR relationship between grains with FCC/BCC structures.

Texture, microstructure and mechanical properties of laser beam welded AISI 2507 super duplex stainless steel

This study investigates weldability of AISI 2507 super duplex stainless steel (SDSS) sheets with laser welding. The impact of heat inputs and post-weld heat treatment (PWHT) on the mechanical properties, texture, microstructure and the surface of joints were analyzed. The microhardness and tensile strength values of the samples welded with the lowest weld heat input was detected to be greater than samples welded with the high heat input. The highest values for tensile strength, bending force and impact toughness in welded samples have been identified as 891 MPa, 1092 N and 58.4 J respectively. The effect of PWHT caused a decrease in the hardness and tensile strength of all samples, and improved the toughness and ductility properties. While dislocations and stacking faults were observed in the base material microstructure, twinning formation was also detected in the weld metal. Secondary austenite (γ2) and Cr23C6 carbides were also found after heat treatment. Variations in the coincidence site lattice (CSL) boundary proportions and the Schmid factors (SFs) directly affected the mechanical properties. In the 2D- and 3D-orientation distribution function (ODF) base material and weld metal maps, the δ-ferrite and γ-austenite textures performed high-density γ-fiber and α-fiber orientations and a low-density (110) <110> rotated cube component, and showed high-density γ-fiber and low-density α-fiber RD//<110> orientations after heat treatment. When the orientation relationships of the heat-treated sample were examined, Kurdjumov–Sachs (K–S) and Nishiyama–Wassermann (N–W) relationships were determined between the γ/α phases in the base material and weld metal, in addition to these orientation relationships, a occasionally seen (100)γ//(100)α Bain orientation relationship was also found in the non-heat-treated sample. The atomic concentrations of the non-heat-treated and heat-treated weld metals exhibited an increase in the O1s spectrum and decreases in the Ni 2p, Fe 2p, Mo 3d and Cr 2p spectra with increasing sputtering time. Diffusion of H− ion with low density to ferrite and austenite grains on weld metal structure was observed.

Effect of heat input and post-weld heat treatment on surface, texture, microstructure, and mechanical properties of dissimilar laser beam welded AISI 2507 super duplex to AISI 904L super austenitic stainless steels

In this study, AISI 2507 super duplex and AISI 904L super austenitic stainless steel sheets were successfully joined using a laser welding method with two different laser powers and laser speeds. The effects of different welding heat inputs and post-weld heat treatment on the surface, texture, microstructure and mechanical properties of laser welded joints were investigated. The results obtained in this study showed that the weld metal microstructure predominantly composed of austenite phase. In addition, the occurrence of γ-austenite (dendritic), interdendritic, ferrite, and planar, coplanar, and stacking fault was observed and also Cr23C6 precipitate was formed in the microstructure with the effect of heat treatment. In the sample joined with low heat input, an increase was observed in tensile strength, yield strength and hardness compared to the samples joined using high heat input, and a decrease in impact toughness occurred. With the effect of heat treatment, a decrease was observed in tensile, yield strength and hardness and an increase was observed in ductility and toughness. In addition, there was no significant change in the bending forces of all laser-welded samples, and the bending occurred on the super austenitic stainless steel side. While the austenite grains exhibited random orientation in the non-heat-treated and heat-treated AISI 2507 base metal and weld metal, the non-heat-treated and heat-treated AISI 904L base metal showed an orientation in the [101] and [111] directions in the transverse direction, respectively. The change in the ratio of the Schmid factor and coincidence site lattice (CSL) boundaries has a significant effect on the mechanical properties. In addition, since the kernel average misorientation (KAM) values in the interdendritic, ferrite grains and grain boundaries were higher than the austenite grains, the residual stresses were found to be higher. When the γ-austenite (dendritic), interdendritic and δ-ferrite textures were examined in the 3D-orientation distribution function maps of the non-heat-treated and heat-treated base metal and weld metal, γ-fibre and α-fibre orientations were observed at high density, and recrystallisation component (112) 〈111〉 at high density and component (113)〈332〉 with high toughness at high density appeared. The γ-austenite grains in the base metal and weld metal of the non-heat-treated and heat-treated samples showed a closer Kurdjumov–Sachs (KS) orientation relationship (OR) compared to the interdendritic and δ-ferrite grains. In the γ/δ and γ/interdendritic ORs, KS OR was generally observed, while Bain OR was observed in the heat-treated super duplex base metal. ORs known in the literature were not observed in the non-heat-treated weld metal. Densities observed in the non-heat-treated and heat-treated weld metal showed that as the sputtering time increased, the O1s spectrum decreased, and the Fe 2p, Cr 2p, Ni 2p and Mo 3d spectra increased.

Metastable orientation relationships in thin film Cu-Cr bilayers

Metastable orientation relationships (ORs) between Cu and Cr are stabilized via epitaxial thin film deposition, with the initial layer of Cu(001) or Cr(001) grown epitaxially on MgO(001). The Bain OR is observed by x-ray diffraction and scanning/transmission electron microscopy for Cu(001) / Cr(001). In contrast, three Cr/Cu ORs are found for Cr deposition on Cu(001): the Pitsch OR, and two previously unreported ORs related to the Bain and Pitsch ORs, respectively. Ab initio calculations predict the energetics of these metastable ORs, and reveal that the deformation resistance of Cr makes the Bain OR energetically unfavorable when Cr films are deposited on Cu(001). These results show that kinetic constraints imposed by controlling the substrate surface and deposition conditions can lead to metastable interfacial structures.

Characterization of nano-sized particles in 14%Cr oxide dispersion strengthened (ODS) steel using classical and frontier microscopy methods

Oxide dispersion strengthened (ODS) steels exhibit superior mechanical properties and irradiation resistance due to nano-sized oxides, highly dispersed in the metallic matrix. The mechanical properties are affected by the structure, composition, size and density of the nano-sized oxides. Despite numerous reports on the characterization of these oxides, ambiguity regarding their composition, crystallographic structure and orientation relationship with the matrix remains. In the present study, characterization of the crystallographic structure of oxide particles existing in 14%Cr ODS steel was performed using classical and novel transmission electron microscopy (TEM) methods. 3D dispersion, density and composition of these oxides were evaluated by atom probe tomography (APT). Three populations of particles were detected: highly dispersed, 3–20 nm Fe(Cr,Ti,Y)O particles with spinel structure; 50–150 nm YTiO3 and large (100–200 nm) particles identified as cubic TiC.The spinel-type particles displayed Bain and Kurdjumov-Sachs orientation relationships (OR) with the ferritic matrix. Applying electron diffraction tomography, the YTiO3 structure was attributed to the GdFeO3 (distorted perovskite)-type and its lattice parameters were refined as a = 5.46 Å, b = 7.66 Å and c = 5.28 Å. Orientation relationship of the YTiO3 particles and Fe matrix were determined as [110]Fe//[210]oxide and (110)Fe//(002)oxide.

Martensitic transformation at a crack under mode I and II loading

AbstractMetastable austenitic steels can undergo phase transformation. As an allotrope two crystal configurations are of interest: the softer austenitic parent phase and the martensitic phases. Here, the bain orientation relationship leads to distinct orientations for the martensitic variants with a different transformation strain [7]. A phase field approach is used to model the transformation, where a multi‐valued order parameter identifies the austenitic parent phase and the martensitic variants. This allows to define bulk and surface energies as regularized functions in terms of the order parameter and its gradient. The kinetics of the martensitic transformation are temperature dependent. Temperatures below an equilibrium temperature favour the growth of the martensitic phase, whereas temperatures above the equilibrium favour the austenitic phase. Approaching the equilibrium temperature slows down the transformation [5]. In this work we consider a static crack under mode I and mode II loading.

Precipitation of intersected plate-like γ′ phase in β and its effect on creep behavior of multiphase Ni3Al-based intermetallic alloy

The precipitation and evolution of intersected plate-like γ′ (γ′P) phase within interdendritic β during long-term aging and its relevant influence on the creep behavior of a multiphase Ni3Al-based intermetallic alloy are investigated. Results show that the precipitation and growth of the γ′P precipitates during 800 °C aging of original as-cast alloy are significantly rapid, which can be fully precipitated in 60 min to form the intersected morphology and continue to coarsen and grow during the extended aging and subsequent 800 °C creep processes. The γ′P precipitates have the Bain orientation relationship (OR) of (100)γ′P//(100)β and [0-11]γ′P//[001]β with the interdendritic β-matrix. The precipitation of intersected γ′P phase is sensitive to the elemental fluctuations in β-matrix, and the preceding high-temperature annealing treatment can significantly reduce the elemental inhomogenization and inhibit its generation within the interdendritic β. Precipitation of adequate intersected γ′P phase can effectively inhibit the dislocation proliferation and crack initiation in the brittle interdendritic β during creep, resulted in the improved creep properties. Slightly rafted γ′ after long-term aging at 800 °C transitioned into R-type γ′-raft structure during creep at 800 °C/220 MPa, and can induce larger creep resistance than N-type γ′-raft structure.

A proper assessment of TEM diffraction patterns originating from CrN nitrides in a ferritic matrix

Upon nitriding of a Fe-3%Cr alloy at 550 °C during 48 h at a nitriding potential KN = 5 atm−1/2, finely dispersed CrN grains precipitate. Considering that the nitrides developing within the nitrided layer adopt a cubic, rock-salt structure, Transmission Electron Microscopy (TEM) investigations revealed the Bain orientation relationships (also called Baker-Nutting) existing between the matrix and the precipitates. Given the latter and the structures of both ferrite (bcc) and nitrides (fcc), we have shown that group theory predicts the existence of three nitrides variants, orthogonal to each other, developing in ferrite along the 〈001〉α-Fe directions, which is in agreement with our experimental observations. The nitrides adopt a very thin platelet-like morphology, energetically corresponding to an absolute extremum. Diffraction patterns obtained along the [001]α-Fe result from the simultaneous diffraction of the ferritic matrix and the three nitrides variants. The platelet-like nitride precipitates give birth to diffuse intensity lines, originating from the two families developing along the (100) and (010) ferrite planes, as well as possible appearance of additional reflections, located at the forbidden positions {100}α-Fe and {110}CrN of the remaining variant, driving the interpretation of such diffraction patterns complicated. Foil thickness has proven to play a key role in the occurrence of this phenomenon. When the geometric diffraction conditions are favorable (i.e. very thin foil along the zone axis and very fine precipitates platelets), TEM analysis permitted to attribute these extra reflections to the nodes located in the first layer of the third variant's reciprocal space.

Lattice-switch Monte Carlo: the fcc—bcc problem

Lattice-switch Monte Carlo is an efficient method for calculating the free energy difference between two solid phases, or a solid and a fluid phase. Here, we provide a brief introduction to the method, and list its applications since its inception. We then describe a lattice switch for the fcc and bcc phases based on the Bain orientation relationship. Finally, we present preliminary results regarding our application of the method to the fcc and bcc phases in the Lennard-Jones system. Our initial calculations reveal that the bcc phase is unstable, quickly degenerating into some as yet undetermined metastable solid phase. This renders conventional lattice-switch Monte Carlo intractable for this phase. Possible solutions to this problem are discussed.

Cellular Precipitation at a 17-7 PH Stainless Steel Interphase Interface During Low-Temperature Nitridation

Cellular precipitation of Cr-rich nitrides was observed at an austenite–ferrite interface in 17-7 PH stainless steel after low-temperature nitridation. Fine-scale lamellar rocksalt-structured nitride (MN1−x , M: randomly distributed Fe, Cr, and Al) was identified at the interfaces between austenite and ferrite by local-electrode atom-probe tomography and transmission electron microscopy. The small size and spacing of the nitride lamellae reflect the low mobility of substitutional atoms under the conditions of low-temperature nitridation. Nitrides of the same structure were formed within the ferrite grain as extremely small particles. The face-centered cubic nitride precipitates in the Bain orientation relationship with the ferrite.

Irradiation-induced selective precipitation in Cu–Nb–W alloys: An approach towards coarsening resistance

A novel approach towards coarsening resistance in the precipitate-strengthened Cu-based alloys is proposed, taking advantage of selective precipitation during low-temperature ion irradiation. In the case of Cu–Nb–W, W precipitates during room temperature irradiation, forming highly ramified clusters. During subsequent thermal annealing of alloys with composition close to Cu90Nb9W1, the more mobile Nb atoms precipitate out on the W clusters, creating a core–shell structure and adopting the Bain orientation relationship within the Cu matrix. This structure is extremely resistant to coarsening. Annealing at 650°C for 1h results in a precipitates size <4nm in diameter, and annealing for an additional 9h causes no additional growth, even though Nb is highly mobile at this temperature and would coarsen in the absence of W. We attribute the remarkable stability of this precipitate structure to the strong immiscibility of W in Cu and to the highly ramified precipitate structure that W acquires during low-temperature irradiation.

Unusual nucleation and growth of γ′ iron nitride upon nitriding Fe–4.75 at.% Al alloy

The influence of substitutionally dissolved Al in ferritic Fe–4.75 at.% Al alloy on the nucleation and growth of γ′ iron nitride (Fe4N1− x ) was investigated upon nitriding in NH3/H2 gas mixtures. The nitrided specimens were characterised employing optical microscopy, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis and X-ray diffraction. As compared to the nitriding of pure ferrite (α-Fe), where a layer of γ′ develops at the surface, upon nitriding ferritic Fe–4.75 at.% Al an unusual morphology of γ′ plates develops at the surface, which plates deeply penetrate the substrate. In the diffusion zone, nano-sized precipitates of γ′ and of metastable, cubic (NaCl-type) AlN occur, having, with the ferrite matrix, a Nishiyama–Wassermann orientation relationship and a Bain orientation relationship, respectively. The γ′ plates contain a high density of stacking faults and fine ε iron nitride (Fe2N1− z ) precipitates, although the formation of ε iron nitride is not expected for the employed nitriding parameters. On the basis of dedicated nitriding experiments it is shown that the unusual microstructural development is a consequence of the negligible solubility of Al in γ′ and the obstructed precipitation of the thermodynamically stable, hexagonal (wurtzite-type) AlN in ferrite.

Nitriding of Ternary Iron-Based Alloys: Metastable Mixed Nitrides, Excess Nitrogen and Stress-Depth Profile

For the first time it could be shown that, upon nitriding Fe-Cr-Al alloys, the thermodynamical stable cubic CrN and hexagonal AlN nitrides do not develop. Instead, a mixed CrxAl1-xN nitride develops of rock-salt crystal-structure type exhibiting a Bain orientation relationship with the ferritic matrix: the Al atoms are “dragged” into the developing cubic CrN precipitates. Similar observations were made upon nitriding Fe-Cr-Ti alloys. The formation of these mixed CrxAl1-xN precipitates is associated with the incorporation of large amounts of (mobile and immobile) “excess nitrogen” and the development of large residual stresses. Creep, associated with residual stress relaxation, occurs during nitriding, giving rise to complex residual stress-depth profiles.

Effects of Austenitizing Temperature and Cooling Rate on the Phase Transformation Texture in Hot Rolled Steels

ABSTRACTThe effects of austenitizing temperature and cooling rate on the microstructures and textures produced by phase transformations in high strength hot rolled Fe-C-Mn steel plates are investigated using orientation imagining microscopy. Samples machined from the plates are austenitized at temperatures between 820-950°C during 30 minutes and quenched in either iced-water, water or oil. Finally, the quenched samples are tempered at 450°C during 30 minutes. Characterization of microstructure and textures produced by these heat treatments was performed by conventional metallography using a reflected light microscope and orientation imaging microscopy using backscattered-electron diffraction patterns in a scanning electron microscope with thermo-ionic electron source.The results show that the microstructure and texture produced under a given combination of austenitizing temperature and cooling rate are strongly dependent on the mechanism involved in the phase transformation of the austenite (γ). High austenitizing temperatures and cooling rates promote martensitic transformation and development of textures containing significant volume fractions of Br, Cu, transformed-Cu and transformed-Br orientation components. In this case, the austenite and martensite phases are clearly related through the Kurdjumov-Sachs orientation relationship. In contrast, low temperatures and low cooling rates result in a complex mixture of transformation products, such as polygonal ferrite, Widmanstäten ferrite, martensite, bainite and pearlite. The textures formed under these conditions are quite different and contain significant volume fractions of cube, rotated-cube, Goss and rotated-Goss components, following the Bain orientation relationship.

Precipitation Process in a Cu-Ni-Be Alloy

The precipitation process in an aged Cu-1.9wt%Ni-0.3wt%Be alloy has been examined by high-resolution transmission electron microscopy. The precipitation sequence found is: Guinier- Preston (G.P.) zones → γ'' → γ' → stable γ. The disk-shaped G.P. zones and the disk-shaped γ'', γ' and γ precipitated phases are composed of monolayers of Be atoms on {100}α of the Cu matrix and alternative Be and Ni matrix layers parallel to {100}α. The γ'' phases consisting of two to eight Be-layers has a body-centered tetragonal (bct) lattice with a=b=0.24 nm and c=0.28 nm. The γ' or γ phase is bct with a=b=0.24 nm and c=0.26 nm or a=b=0.26 nm and c=0.27 nm. The γ'', γ' or γ phase aligns with the matrix according to the Bain orientation relationship. The growth kinetics of disk-shaped γ precipitates on aging at 500°C has been also investigated. The {001}α habit planes of the γ precipitates migrate by a ledge mechanism. The average thickness of the γ disks increases with aging time t as t1/2. An analysis of experimental data using a kinetic model yields the diffusivity of solute in the Cu matrix, which is in agreement with the reported diffusivity of Ni in Cu.

Observation of continuous and reversible bcc–fcc phase transformation in Ag/V multilayers

A continuous and reversible bcc–fcc phase transformation via a rotation of bcc{110} or fcc{111} planes is observed in the Bain orientation relationship in a sputter deposited V/Ag multilayers using high resolution transmission electron microscopy and analyzed using molecular dynamics simulations. As a result of the continuous phase transformation, an intermediate bct phase connecting the bcc and fcc phases coexists, giving rise to the Bain path. The periodic displacement of atoms occurs in every two adjacent Ag and V layers. The alternating shear stress created by misfit strain is responsible for generating such transformation.

Nitriding response of ferritic Fe-Ti-Cr alloys*

Abstract Specimens of ferritic iron-based Fe-Ti-Cr alloys, containing a total of 0.30 at.% alloying elements, Ti and Cr with varying Ti/Cr atomic ratio (0.45, 0.87 and 1.90), were nitrided in a NH3/H2 gas atmosphere at 580 °C. The microstructure of the nitrided zone was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Nitride platelets (length ≤ 30 nm and thickness ≤ 1 nm) developed in the nitrided zone. Particularly clear for the larger Ti/Cr atomic ratios, a tetragonally distorted ferrite develops next to the cubic ferrite, as evidenced both by XRD and TEM. The absence of separate TiN and CrN diffraction spots in the selected area electron diffraction patterns suggests the development of mixed TixCr1-xN nitride precipitates. The mixed TixCr1-xN nitride precipitates are of rock-salt crystal structure type and obey a Bain orientation relationship with the ferrite matrix.