Kurdjumov-Sachs Orientation Relationship Research Articles

Microstructure and Crystallography of β Phase Formed through Electric Current Pulse (ECP) Treatment in Cold-Rolled Cu-40%Zn Alloy

Most of the studies on phase transformation in metallic materials have focused on transformations during cooling processes due to the easiness of the conservation of the product phase. However, for phase transformation happening during heating processes, the experimental investigations have been indirect if the product high temperature phase could not be preserved to the convenient observation temperature, for example the room temperature. The high density Electric Current Pulse (ECP) treatment allows the phase transformation during heating process and the preservation of the high temperature phase to the room temperature, offering possibilities for direct experimental examinations. Thus, in the present work, a cold-rolled Cu–40%Zn alloy was ECP treated and the microstructure of the product phase and the transformation orientation relationship were investigated. Results show that during the ECP treatment, the high temperature beta phase with BCC structure formed in the parent alpha phase with FCC structure. Especially, two kinds of orientation relationships could be detected between the parent alpha phase and the product beta precipitates. The one is the Kurdjumov-Sachs orientation relationship (K-S OR), and the other is the Nishiyama-Wasserman (N-W). In addition, the amount of beta precipitates obeying the K-S OR is more than that of precipitates obeying the N-W OR. The results of this work provide new fundamental information on phase transformation of metallic materials.

Texture, orientation, and mechanical properties of Ti-stabilized Fe-17Cr ferritic stainless steel

Microstructure and mechanical properties of the as-annealed Ti-stabilized Fe-17Cr ferritic stainless steel were investigated by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and tensile tests. Experimental results show that the size of the ferrite grain in the Fe-17Cr stainless steel is about 40 µm after rolling and annealing. Orientation distribution function (ODF) by EBSD indicates that nearly homogeneous {111} annealing texture is produced in the annealed sheets. Pole figures (PF) by EBSD show that Kurdjumov–Sachs orientation relationship between the TiN particles and ferrite grains is established in the as-annealed microstructure. Tensile tests demonstrate that the yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) of the as-annealed Ti-stabilized Fe-17Cr ferritic stainless steel are 319 MPa, 626 MPa, and 36.5%, respectively. In the SEM micrograph of the fracture, deep dimples can be observed and TiN particles can be found in the core of the dimples, which indicates that the samples undergo large deformation before fracture.

Microstructure of reverted austenite in Fe-0.3N martensite

Reverse transformation behavior during intercritical annealing was studied in Fe-0.3 mass%N binary alloy. Two morphologies of acicular and globular austenite are formed during reversion. The globular austenite has almost same orientation with one side of adjacent prior austenite grain and grows into the martensite with larger deviation angle from Kurdjumov-Sachs orientation relationship (K-S OR). On the other hand, the acicular austenite holds a near K-S OR with its surrounding martensite matrix. A large amount of retained austenite was obtained by a simple intercritical annealing due to enrichment of nitrogen into reverted austenite.

On variant selection at the prior austenite grain boundaries in lath martensite and relevant micro-mechanical implications

Prior austenite grain boundaries (PAGBs) are one of the essential microstructural constituents of lath martensite (LM). According to former studies, deformation induced micro-cracks in LM are predominantly induced along the PAGBs; a phenomenon that could be correlated either to (i) solute element segregation, or to (ii) specific crystallographic features in the vicinity of the PAGBs. In this study, we investigate the second aspect. First, a thorough crystallographic analysis is carried out on the variants adhering to the PAGBs in LM. The fracture strength of individual PAGBs is then evaluated using micro-cantilever bending experiments. The results are analyzed with respect to various variant selection rules and it is found that martensite laths at PAGBs most frequently obey a Kurdjumov-Sachs orientation relationship (K-S OR) with both their neighboring prior austenite grains (i.e. they are keeping a double K-S OR). Many of the PAGB-adhering variants also have their maximum transformation-induced strains aligned parallel to the PAGB plane for better strain accommodation. The local micro-mechanical experimental results indicate that the PAGB segments that are bound by coarser variants, and, particularly, obey the double K-S OR variant selection rule, are more resistant to crack nucleation than others. It is proposed that the double K-S OR helps to minimize the interface energy of the PAGB, and thus enhances its crack resistance.

Effect of isochronal annealing on the microstructure, texture and mechanical properties of a cold-rolled high manganese steel

A Fe-17Mn-3Al-2Si-1Ni-0.06C wt.% steel was subjected to cold-rolling to 42% thickness reduction and isochronally annealed for 300 s between 500 and 850 °C. The high Mn steel was characterised via electron back-scattering diffraction, transmission electron microscopy and uniaxial tensile testing. The reversion of deformation-induced ε and α′-martensite to austenite (γ) was witnessed with the formation of fine twins in reverted/recovered γ grains after annealing to 650 °C. The nucleation of new γ grains at the boundary of reverted/recovered γ grains was also noted. Upon reversion, the γ orientations originated from the ε and α′-martensite orientations by phase transformation via the Shoji-Nishiyama and Kurdjumov-Sachs orientation relationships, respectively. Upon segmenting the γ grains, the recrystallised γ grains were observed to nucleate with orientations similar to those of the reverted/recovered γ grains. Uniaxial tension on a fully recrystallised γ microstructure showed that the initial γ grain size has a more significant effect on the yield strength than the ε and α′-martensite fractions. On the other hand, the initial ε and α′-martensite fractions affect the total elongation.

Crystallographic Features of Microstructure in Maraging Steel Fabricated by Selective Laser Melting

This study characterizes the microstructure and its associated crystallographic features of bulk maraging steels fabricated by selective laser melting (SLM) combined with a powder bed technique. The fabricated sample exhibited characteristic melt pools in which the regions had locally melted and rapidly solidified. A major part of these melt pools corresponded with the ferrite (α) matrix, which exhibited a lath martensite structure with a high density of dislocations. A number of fine retained austenite (γ) with a <001> orientation along the build direction was often localized around the melt pool boundaries. The orientation relationship of these fine γ grains with respect to the adjacent α grains in the martensite structure was (111)γ//(011)α and [-101]γ//[-1-11]α (Kurdjumov–Sachs orientation relationship). Using the obtained results, we inferred the microstructure development of maraging steels during the SLM process. The results depict that new and diverse high-strength materials can be used to develop industrial molds and dies.

Comparison of rolling texture of austenite and ferrite phases of duplex steel with single-phase austenitic and ferritic steel

In this contribution, the behavior of three types of steel after rolling was investigated. Ferritic and austenitic steels or phases have different mechanical and thermal properties. Due to their mutual influence during plastic deformation, it is possible to suppose the differences between preferred orientations of austenite and ferrite phases in duplex steel and single-phase austenitic and ferritic steels. For this reason, the preferred orientations of austenite and ferrite phases of rolled duplex steel were compared with rolled single-phase austenitic and ferritic steel. The 0–50% reductions of rolled steel plates were selected. Mainly, the strength and type of preferred orientation were compared in the relation to reduction and material. For this purpose, the rolled surfaces were measured by X-ray diffraction. The significant differences between intensities and types of texture components were investigated. For austenite-ferrite interaction during plastic deformation and phase transformation, samples were analysed by electron back-scattered diffraction. The Kurdjumov-Sachs orientation relationship was investigated.

Evolution of Grain Interfaces in Annealed Duplex Stainless Steel after Parallel Cross Rolling and Direct Rolling.

Changes in various grain interfaces, including the grain boundary and phase boundary, are a strong indication of microstructural changes, particularly ultra-fined grains achieved by large strain deformation and subsequent annealing. After direct rolling and cross rolling with the same strain of ε = 2, the distributions of the interfaces in annealed UNS S32304 duplex stainless steel were investigated using electron backscatter diffraction (EBSD) in this study. The ferrite experienced continued recovery, and a high density of low-angle grain boundaries (LAGBs) was produced. The percentage and number of twin boundaries (TBs) and LAGBs varied within the austenite. TBs were frequently found within austenite, showing a deviation from the Kurdjumov-Sachs (K-S) orientation relationship (OR) with ferrite matrix. However, LAGBs usually occur in austenite, with the K-S OR in the ferrite matrix. LAGBs were prevalent in the precipitated austenite grains, and therefore a strong texture was introduced in the cross-rolled and annealed samples, in which the precipitated austenite readily maintained the K-S OR in the ferrite matrix. By contrast, more TBs and a less robust texture were found in the precipitated austenite in direct-rolled and annealed samples, deviating from the K-S OR.

Microstructure and functionality of a uniquely graded super duplex stainless steel designed by a novel arc heat treatment method

A novel arc heat treatment technique was applied to design a uniquely graded super duplex stainless steel (SDSS), by subjecting a single sample to a steady state temperature gradient for 10 h. A new experimental approach was used to map precipitation in microstructure, covering aging temperatures of up to 1430 °C. The microstructure was characterized and functionality was evaluated via hardness mapping. Nitrogen depletion adjacent to the fusion boundary depressed the upper temperature limit for austenite formation and influenced the phase balance above 980 °C. Austenite/ferrite boundaries deviating from Kurdjumov–Sachs orientation relationship (OR) were preferred locations for precipitation of σ at 630–1000 °C, χ at 560–1000 °C, Cr2N at 600–900 °C and R between 550 °C and 700 °C. Precipitate morphology changed with decreasing temperature; from blocky to coral-shaped for σ, from discrete blocky to elongated particles for χ, and from polygonal to disc-shaped for R. Thermodynamic calculations of phase equilibria largely agreed with observations above 750 °C when considering nitrogen loss. Formation of intermetallic phases and 475 °C-embrittlement resulted in increased hardness. A schematic diagram, correlating information about phase contents, morphologies and hardness, as a function of exposure temperature, is introduced for evaluation of functionality of microstructures.

First-principles simulations of binding energies of alloying elements to the ferrite-austenite interface in iron

The kinetics of the ferrite-austenite (bcc-fcc) phase transformation in steels are markedly affected by substitutional alloying elements. However, the detailed mechanisms of their interaction with the bcc-fcc interfaces are not fully understood. In this study, the effects of common alloying elements (e.g., Nb, Mo, Mn, Si, Cr, and Ni) on the structure, segregation, and magnetic properties of bcc-fcc interfaces in Fe are systematically investigated using spin-polarized Density Functional calculations within a generalized gradient approximation to the exchange correlation potential and a super cell approach with a Kurdjumov-Sachs orientation relationship between bcc and fcc. The calculation results are in semi-quantitative agreement with the experimental results, i.e., Nb has the largest binding energy to the bcc-fcc interface in Fe followed by Mo.

Orientation of austenite reverted from martensite in Fe-2Mn-1.5Si-0.3C alloy

The orientation of austenite (γ) reverted from lath martensite has been studied using electron backscatter diffraction (EBSD) in Fe-2%Mn-1.5%Si-0.3%C (mass%) alloy. Two morphologies of γ, acicular and globular, are formed during reversion. Nucleation sites for acicular γ are lath, block and sub-block boundaries, while those for globular γ are prior γ grain boundaries, as well as inside packets. Both acicular and globular γ hold a near Kurdjumov-Sachs (K-S) orientation relationship with at least one of its surrounding tempered martensite blocks. Most acicular γ have almost identical orientations with the prior γ, while only a part of acicular γ have a specific twin-related variant with the prior γ. Furthermore, grain boundary globular γ has almost identical orientations with the prior γ into which it does not grow. The formation of acicular and grain-boundary globular γ was attributed to strong variant selection during γ nucleation at lath, block and sub-block boundaries. On the other hand, orientation relationship analyses among intragranular globular γ, tempered martensite and cementite suggest that the nucleation of γ at cementite and tempered martensite interfaces causes weakening of the variant selection for the formation of intragranular globular γ.

Atomistic modeling of Mg/Nb interfaces: shear strength and interaction with lattice glide dislocations

Using a newly developed embedded-atom-method potential for Mg–Nb, the semi-coherent Mg/Nb interface with the Kurdjumov–Sachs orientation relationship is studied. Atomistic simulations have been carried out to understand the shear strength of the interface, as well as the interaction between lattice glide dislocations and the interface. The interface shear mechanisms are dependent on the shear loading directions, through either interface sliding between Mg and Nb atomic layers or nucleation and gliding of Shockley partial dislocations in between the first two atomic planes in Mg at the interface. The shear strength for the Mg/Nb interface is found to be generally high, in the range of 0.9–1.3 GPa depending on the shear direction. As a consequence, the extents of dislocation core spread into the interface are considerably small, especially when compared to the case of other “weak” interfaces such as the Cu/Nb interface.

Morphology, orientation relationships and formation mechanism of TiN in Fe-17Cr steel during solidification

Morphology, orientation relationships, and formation mechanism of TiN in Fe-17Cr ferritic stainless steel were investigated theoretically and experimentally by scanning electron microscopy, X-ray diffractometer, and electron backscatter diffraction. Two types of structures of TiN, without cores and with cores, were observed in the solidified microstructure. The dimensionless entropy of fusion (ΔSm/R) of TiN is calculated as 2.5, which suggests that TiN will exhibit regular faceted morphology. The planar disregistry between (111) plane of TiN and (110) plane of ferrite is calculated as 17.1%, which indicates that it is not sure if TiN can act as the heterogeneous site for ferrite nucleation during solidification. Pole figures of TiN and ferrite by EBSD show that Kurdjumov-Sachs orientation relationship between TiN and ferrite exists, which confirms that TiN can act as the heterogeneous site for ferrite. In addition, based the EDS mapping, TiN in the microstructure should form during solidification rather than solid-state transformation.

Analysis of crack tip transformation zone in austenitic stainless steel laser-MIG hybrid welded joint

In this study, fatigue crack growth rates in weld center and Heat Affected Zone (HAZ) of laser-MIG hybrid welded joint are compared using standard compact tension (CT) specimens. Electron backscatter diffraction (EBSD) is applied to analyze crack tip transformation zone by characterizing phases, plastic strains and crystallographic orientations. This study provides interesting insights into the growth of fatigue cracks in austenite steel welded joint at micro-scale. Analysis of the plastic strain and Schmid factor indicates that mechanical energy provided by external load transforms into internal crystal energy firstly, which is stored within grains in the form of strain energy, then partly relieves to provide mechanical driving force for martensite transformation. With plastic strain accumulating within individual grain, dislocations begin to glide on slip system resulting in formation of persistent slip bands (PSB), then metastable austenite grains transform into martensite. Thus, strain-induced martensite transformation increases resistance to crack growth by dissipating energy and crack closure effect. Different crack growth rate in weld center and HAZ is attributed to variant volume of strain-induced martensite. Shielding effect produced by neighboring harder ferrite decreases martensite in weld center, as confirmed by nanoidentation and Atomic Force Microscope (AFM). Additionally, martensite phase is found to have a Kurdjumov-Sachs (K-S) orientation relationship with austenite phase according to Inverse Pole Figure (IPF) and Pole Figures (PF) derived by EBSD.

Validation and Analysis of the Parameters for Reconstructing the Austenite Phase from Martensite Electron Backscatter Diffraction Data

This work focuses on the validation of a method for reconstructing the fcc crystallographic data from martensite orientation electron backscatter diffraction (EBSD) maps based on the “γ nuclei identification” and “γ nuclei spreading strategy.” To that end, an Fe-30Ni alloy was employed. The martensite transformation start temperature (M s ) of this material is close to or below room temperature; therefore, during hot deformation and after water quenching, it presents an fcc austenitic microstructure, while after subzero quenching, austenite-to-martensite transformation takes place. Accordingly, the reconstruction procedure can be applied to the martensitic EBSD crystallographic data, and the morphological and orientation results of the reconstruction can be validated by comparison with the original crystallographic fcc data. Torsion tests were performed to produce recrystallized and deformed austenite microstructures. Although applying the Kurdjumov–Sachs orientation relationship (OR) resulted in reconstructed area fractions larger than 75 pct, the reconstruction quality improved significantly when other ORs closer to the Greninger–Troiano OR were applied. The analysis carried out on the recrystallized microstructure shows that the method is robust against variation in the different parameters involved in the reconstruction. Good angular and morphological reconstruction results were obtained in both recrystallized and deformed microstructures, including the ability to reconstruct twins.

A molecular dynamics study of the nucleus interface structure and orientation relationships during the austenite-to-ferrite transformation in pure Fe

ABSTRACTMolecular dynamics simulations using an embedded-atom method potential for pure iron (Fe) were performed to investigate the solid-state transformation of a body-centred-cubic phase from a polycrystalline face-centred-cubic matrix. A Kurdjumov–Sachs orientation relationship accompanied with a step-ledge disconnection structure was detected in high mobility phase boundaries of a full 3D classical and barrier-free nucleation specimen. The results imply that interface coherency is an important factor for both nucleation and growth processes, which may significantly affect the transition rate of the austenite–ferrite transformation.

Orientational Equidistance Method for Solving Orientation Relationship From Product Variants: Application to Steel

A three-step procedure has been evolved for finding the orientation relationship (OR) between parent austenite and product ferrite phase, from the room temperature microtexture data of ferritic/martensitic steel. In the first step, the frequently occurring misorientations between product ferrite variants are sought from the electron backscatter diffraction (EBSD) dataset. This is carried out by calculating the ‘misorientation angle-rotation axis’ values across all neighboring pair of ferrite pixels, and plotting them in a parametric space to detect the prominent misorientation peaks. Identification of ‘concurrent relative orientation’ of product variants, i.e., the second step, is resolved by comparing the observed misorientation peak positions with the inter-variant relation set of Kurdjumov–Sachs (KS) OR. The final crucial step of solving the OR based on the ‘Orientational Equidistance’ principle of parent phase from all its product phase variants is implemented in the Rodrigues–Frank parametric space. The procedure is illustrated using an large area EBSD dataset of normalized and tempered 9Cr–1Mo–0.1C steel, and the optimum orientation relation is determined to be ~44.0°[0.151 0.137 0.979], which is 2.4° deviated from KS OR.

Microstructure and deformation behavior of metastable duplex stainless steel at high rolling reductions

The current investigation is a part of project concerning microstructure and texture in a series of ferrite-austenite stainless steels after deformation and annealing. The present paper focuses on deformation behavior of constituent phases in a metastable duplex type steel subjected to cold-rolling in the range of high reductions. Optical microscopy was used to examine changes in the morphology and the formation of ferrite-austenite band-like structure. Transmission electron microscopy was applied to analyze the microstructure and deformation mechanisms within the bands of austenite and ferrite. Additional research included the examination of texture changes in both constituent phases. Special attention was paid to deformation mechanisms operating within ferrite-austenite banded structure at high rolling reductions, including the formation of micro-shear bands, the occurrence of micro-twinning as well as deformation induced martensitic transformation. Transmission electron microscopy analysis indicates that the phase transformation occurred most likely through direct (γ→α') mechanism and obeyed predominantly Kurdjumov-Sachs orientation relationship. Within strongly fragmented two-phase microstructure at high strains the formation of α'-martensite proceeded in most cases at internal interfaces in micro-bands from the operating deformation systems.

Digital identification scheme for steel microstructures in low-carbon steel

This study proposes a scheme for digital identification of a steel microstructure based on crystallographic features. A 2D space for classifying mixed microstructures of bainite and martensite formed by seven different thermal cycles is defined using two dimensions related to variants in Kurdjumov–Sachs orientation relationships and dislocations. As a dimension relating to variants, the minor variant pair characterized by a rotation axis of 〈011〉 and a rotation angle of 52° is most useful in classifying the steel microstructure. Furthermore, the kernel average misorientation also served as a suitable dimension for the 2D space. The reasons for using the minor variant pair to classify steel microstructures are discussed from the viewpoint of phase transformation phenomena. The present study demonstrates the possibility for steel microstructures to be digitally identified.

Investigation of Parent Austenite Grains from Martensite Structure Using EBSD in a Wear Resistant Steel

Crystallographic reconstruction of parent austenite grain boundaries from the martensitic microstructure in a wear resistant steel was carried out using electron backscattered diffraction (EBSD). The present study mainly aims to investigate the parent austenite grains from the martensitic structure in an as-rolled (reference) steel sample and samples obtained by quenching at different cooling rates with corresponding dilatometry. Subsequently, this study is to correlate the nearest cooling rate by the dilatometer which yields a similar orientation relationship and substructure as the reference sample. The Kurdjumov-Sachs orientation relationship was used to reconstruct the parent austenite grain boundaries from the martensite boundaries in both reference and dilatometric samples using EBSD crystallographic data. The parent austenite grain boundaries were successfully evaluated from the EBSD data and the corresponding grain sizes were measured. The parent austenite grain boundaries of the reference sample match the sample quenched at 100 °C/s (CR100). Also the martensite substructures and crystallographic textures are similar in these two samples. The results from hardness measurements show that the reference sample exhibits higher hardness than the CR100 sample due to the presence of carbides in the reference sample.