Kurdjumov-Sachs Research Articles

Faceted Kurdjumov-Sachs interface-induced slip continuity in the eutectic high-entropy alloy, AlCoCrFeNi2.1

Recently, the eutectic high-entropy alloy (EHEA), AlCoCrFeNi2.1, can reach a good balance of strength and ductility. The dual-phase alloy exhibits a eutectic lamellar microstructure with large numbers of interfaces. However, the role of the interfaces in plastic deformation have not been revealed deeply. In the present work, the orientation relationship (OR) of the interfaces has been clarified as the Kurdjumov-Sachs (KS) interfaces presenting 111B2||110FCC and 110B2||111FCC independent of their morphologies. There exist three kinds of interfaces in the EHEA, namely, (321)B2||(112)FCC, (011¯)B2||(332¯)FCC, and (231¯)B2||(552)FCC. The dominating (321)B2||(112)FCC interface and the secondary (011¯)B2||(332¯)FCC interface are both non-slip planes and atomistic-scale faceted, facilitating the nucleation and slip transmission of the dislocations. The formation mechanism of the preferred interfaces is revealed using the atomistic geometrical analysis according to the criteria of the low interfacial energy based on the coincidence-site lattice (CSL) theory. In particular, the ductility of the dual-phase alloy originates from the KS interface-induced slip continuity across interfaces, which provides a high slip-transfer geometric factor. Moreover, the strengthening effect can be attributed to the interface resistance for the dislocation transmission due to the mismatches of the moduli and lattice parameters at the interfaces.

Microstructure refinement of Ti-40Al-8Nb alloys via the decomposition of the metastable B2 phase at 1000 °C

Abstract In this study, a metastable microstructure composed of the single B2 phase was produced in Ti-40Al-8Nb (at. %) alloys by water quenching from 1400 °C. After annealing the fully-B2 microstructure at 1000 °C for 600 min, a refined microstructure composed of α2, γ and B2 phases was obtained. Detailed analysis demonstrated that both the lath-liked coherent α2 phase and equiaxed incoherent α2 phase formed in the B2 matrix within a short time. During the subsequent tempering process, nanoscale γ lamellar precipitated from the coherent α2 phase. Due to the large stored energy in the ultra-thin lamellar structures, the recrystallization of the γ phase happened, resulting in the globalization of the lamellar structures. Besides, γ twins also formed in the B2 matrix by following the Kurdjumov-Sachs (K-S) orientation relationships. With the increase of isothermal holding time, the coarsening of the α2 and γ phases occurred, leading to a refined microstructure with the average grain size of 2 μm.

<i>In Situ</i> EBSD Observation of Pearlite-Austenite Reverse Transformation in Eutectoid Steel

The reverse transformation of austenite from pearlite was observed in-situ by using a sophisticated EBSD system. Quantitative information on the nucleation sites and orientation selection of the austenite was obtained. Initially, the nucleation sites were restricted to high-angle grain boundary (HAB) and all the austenite orientations were selected by Kurdjumov-Sachs (K-S) orientation relationship. Latterly, the constraints were relaxed as new nucleation sites were involved. The γ growth was preferentially into the pearlite grains without K-S relation. Nucleation and growth at HAB edge may contribute to γ coarse grains, whereas nucleation at pre-existing γ boundary is important to grain refinement of γ structure.

Abnormal growth of columnar grains and formation of Σ3 grain boundaries in non-oriented electrical steels

The remarkably abnormal growth behavior of columnar grains and the formation of Σ3 grain boundaries (GBs) were studied via plastic deformation and annealing based on an electron backscattering diffraction (EBSD) investigation. Driven by the stored energy and GB mobility, coarse austenite columnar grains were formed by the abnormal growth of fine equiaxed grains with higher grain orientation spread (GOS) values in the reducing atmosphere of hydrogen. Moreover, extraordinarily coarse columnar grains with high GOS values and non-{1 1 1} texture were formed by microstructure inheritance during γ → α transformation. In addition, slender ferritic columnar or island grains with Σ3 GBs and low GOS values were generally obtained, and the formation mechanism was strictly associated with the Kurdjumov-Sachs (K-S) relationship, which was induced during variant selection driven by the GB mobility and surface effect in the reducing atmosphere of hydrogen.

Quantitative Analysis of Microstructures and Strength of Nb-Ti Microalloyed Steel with Different Ti Additions

A quantitative analysis was carried out in the present study to determine the effects of titanium (Ti) addition on microstructures and strength of Nb-Ti microalloyed steel. The obtained results revealed that strength was significantly improved with an increase in Ti content from 0.041 to 0.079 wt pct. The difference in the yield strength between the two steel samples occurred due to the different strengthening effects of grain refinement, precipitation, and dislocation strengthening, among which the grain refinement and precipitation strengthening contributions were dominating. With a further increase in the Ti content, ferrite grains became refined. Consequently, a homogeneous ferrite microstructure was attained for high Ti contents. Moreover, large-sized (Ti, Nb)C particles manifested the Kurdjumov–Sachs (KS) relationship, whereas fine (Ti, Nb)C particles held the Baker–Nutting (BN) relationship; thus, abundant fine nanoscale (Ti, Nb)C particles formed after coiling. Furthermore, the high dislocation density facilitated the precipitation of (Ti, Nb)C particles along dislocation lines.

Investigation of the relation between bainitic-ferritic microstructures and coincidence-site lattice boundaries by electron backscatter diffraction

Both bainitic-ferritic (BF) microstructures and coincidence-site lattice (CSL) boundaries are believed to be crucial for mechanical strength and corrosion resistance of steels, but little knowledge has been obtained about the relation between them. The present work systematically studied bainitic-ferritic microstructures and boundaries, with the emphasis on the relation between them by the five-parameter characterization method using the electron backscatter diffraction (EBSD). The results show that transformation from austenite to bainite leads to a hierarchical microstructure, where a preferable combination of bainitic variants form on the same habit plane and directions in the Kurdjumov-Sachs (K-S) relation. Misorientations of the hierarchical BF microstructure exibit a featured distribution with an obvious aggregation around 60°. Further investigations prove that most of them are ∑3 (60°/〈1 1 1〉) and quasi-∑33c (60°/〈1 1 0〉) boundaries with a novel preference of {1 1 0} boundary planes.

Twinned substructure in lath martensite of water quenched Fe-0.2 %C and Fe-0.8 %C steels

In the present investigation, twinned substructures within lath martensite of two water quenched steels (0.2 wt. %C and 0.8 wt. %C) were studied. The lath martensite has typical hierarchical packet-block-lath with dislocation substructure. Besides, laths that are misoriented by <011>/70.5° or <111>/60° and bordered by {011} plane, namely twinned laths, are observed, of which the density increases and the scale decreases as more carbons were presented. Such twinned laths have body centered cubic (bcc) crystal structure, belonging to twinned variants following the classical Kurdjumov-Sachs (K-S) orientation relationship with respect to the parent austenite. Unlike bcc {112}<111> twins, twinned variants produce strong double diffraction and in turn the extra diffraction spots that are commonly observed in the martensite in steels with wide range of carbon contents.

Interface dominated deformation mechanisms in two-phase fcc/B2 nanostructures: Nishiyama-Wasserman vs. Kurdjumov-Sachs interfaces

Crystallographic orientation relationship (OR) in two-phase alloy microstructures determines their heterophase interfacial structures, and may influence their corresponding deformation mechanisms. Here, using prototypical fcc-Ni and bcc-ordered B2-NiAl phases, we have compared and contrasted the effect of fcc/B2 Nishiyama-Wasserman (NW) and Kurdjumov-Sachs (KS) heterophase interfaces on the deformation mechanisms prevalent in fcc-B2 nanostructures. For this purpose, we have uniaxially deformed fcc-B2 bicrystals, and multicrystals containing B2 lamellae via molecular dynamics (MD) simulations. The NW and KS interfaces acted as source and sink for fcc and B2 dislocations, and were susceptible to shearing via fcc twins. However, both interfaces differed in their ability to influence deformation mechanisms, and maintain interfacial structure under the influence of shear stresses. Inside the fcc phase, NW interfaces preferentially promoted stacking fault tetrahedra formation during the early stages of plastic deformation, while KS interfaces facilitated {111}〈112〉fcc twining. B2 dislocation plasticity also appears to have been affected. We also learn that the {111}fcc NW interface is highly sensitive to shear stresses, and after prolonged deformation it’s interfacial structure closely resembled deformed {111}fcc KS interface. Mechanistic insights gained from MD is expected to inform alloy design approaches that aims to control the relative fractions of Nishiyama-Wasserman and Kurdjumov-Sachs interfaces.

Insights into micro-mechanical response and texture of the additively manufactured eutectic high entropy alloy AlCoCrFeNi2.1

In the present study, the evolution of microstructure, texture and high-temperature mechanical behavior of the additively manufactured (AM) eutectic high entropy alloys (EHEA) AlCoCrFeNi2.1 have been investigated. The material was manufactured through laser engineered net shaping (LENS) process. The microstructural investigation revealed constituent phases consisting of dendritic and eutectic features, with the phases having ordered FCC (L12) and BCC crystal structures. The phase fraction of L12 was more across the build (X) face and that of the BCC was more along build plane (Z face). In both the phases, nickel constituted the base element with L12 phase being deficient in Al and the BCC deficient in Cr and rich in Al. The stability of L12 phase was attributed to Co, Cr, and Fe with near equiatomic distribution. Kurdjumov-Sachs (KS)orientation relationship was followed between the ordered L12 and BCC phases along and across the build-up direction. Strain partitioning was observed more in the BCC phase than in the L12 phase during uniaxial compression at all temperatures. Difference in hardness was observed along Z and X directions which further resulted in yield anisotropy during compression test at room temperature. High-temperature compression tests at temperatures 400 °C, 600 °C, 700 °C, and 800 °C revealed that the yield strength increased from room temperature till 400 °C and then started decreasing till 800 °C.

Phase and Microstructure Evolution of a Low-Alloyed Steel During Intercritical Annealing and Quenching

Strong, yet ductile steels can be manufactured by applying optimized thermal treatments during production. Such treatments can be performed on low-alloyed steels and their mechanical properties can be tuned within relatively wide limits. Although dual-phase steels only contain low concentrations of alloying elements due to optimized thermal processing they can reach ultimate strengths of 1000 MPa or elongations to fracture of up to 30 pct. During their production ferrite (α) partially transforms into austenite (γ) and then during quenching further into martensite (α′). So far, many assumptions had to be made about these transformations since they were difficult to observe at elevated temperatures and during quenching. In this study, we combine heating and quenching inside an SEM with EBSD measurements to track the evolution of the microstructure at the surface of a small steel sample throughout a simulated production path. The orientation relationships of austenite and martensite formation are investigated by EBSD mapping of the parent and product phases. The microstructure is analyzed before and after quenching and the orientation relationships associated with the γ–α transformation as well as the γ–α′ transformation are compared. For the first time, the orientation relationships (OR) between ferrite, austenite, and martensite are directly compared within the same location of a sample. The results show that the Kurdjumov–Sachs (KS) and Nishiyama–Wassermann(NW) ORs only approximately describe the γ–α and γ–α′ transitions. The experiment reveals the role of KS/NW boundaries in the intercritical regime and microscopically observes the consequences of the γ–α′ transition. It was found that the formation of martensite causes highly deformed ferrite in its vicinity which most likely affects the mechanical properties of the dual-phase steel. It is expected that this type of experiment will help in better understanding microstructural mechanisms during heat treatments and eventually will contribute in the development of steels with tailored microstructures.

Microstructure transformation and twinning mechanism of 304 stainless steel tube during hydraulic bulging

At room temperature, the hollow shaft of AISI 304 stainless steel tubes was produced by a hydraulic bulging process. The behavior of strain-induced austenite to martensite transformation and the twin crystallographic nature of AISI 304 stainless steel tubes at different positions after hydraulic bulging were discussed. The results have demonstrated that strain-induced austenite to martensite transformation occurred in AISI 304 stainless steel tubes during hydraulic bulging, resulting in the formation of the α′-martensite phase, and the volume fraction of martensite gradually increased with an increase in strain. The austenite and α′-martensite phases maintained lattice coherency throughout and followed the Kurdjumov–Sachs (K-S) relationship in terms of lattice coherency. During the deformation process, de-twinning occurred in the austenite and the deformation twins were formed in α′-martensite. With the increase in strain, the volume fraction of the annealing twins gradually reduced until complete disappearance in the austenite. The volume fraction of the deformation twins increased in the martensite with an increase in strain, and finally reached saturation.

Temperature Dependent Phase Transformation Kinetics of Reverted Austenite during Tempering in 13Cr Supermartensitic Stainless Steel

The formation and growth kinetics of the reverted austenite during tempering in 13Cr supermartensitic stainless steel were investigated by a combination X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM). The reverted austenite precipitated at the martensite blocks, sub-blocks, laths and grain boundaries. The growth kinetics was established by Johnson-Mehl-Avrami (JAM) kinetics equation according to the volume fraction of the equilibrium reverted austenite at room temperature. The Avrami exponent value is 0.5, and the activation energy was estimated to be 369 kJ/mol, the kinetic model indicates that the mechanism of reverted austenite is diffusion-controlled and the growth of reverted austenite closely relies on the diffusion of the nickel (Ni) element. The experimental measured orientations of the reverted austenite are in good agreement with the theoretical ones, implying that the reverted austenite has the same orientation with the surrounding martensite, which meets the Kurdjumov–Sachs (K-S) orientation relationship. The orientation relationships minimize the strain energy of the phase transformation by reducing the crystallographic mismatch between phases.

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°.

Deformation of lamellar FCC-B2 nanostructures containing Kurdjumov-Sachs interfaces: Relation between interfacial structure and plasticity

Dual-phase lamellar microstructures containing alternating regions of plastically soft and hard phases are known to produce alloys with exceptional combination of strength and ductility. Here, by coupling high-resolution transmission electron microscopy and molecular dynamics (MD) simulations, we have investigated the deformation mechanisms prevalent in lamellar microstructures with soft fcc and harder bcc-ordered intermetallic B2 whose interfaces follow the Kurdjumov-Sachs (KS) orientation relationship. We have identified two key structural features at such an fcc/B2 KS interface. The mating KS-fcc (111) interfacial plane contains periodically arranged 1/6<112>fcc predominantly screw partial dislocations that are separated by extended dislocation “core-overlap” regions. The KS interface also contained steps and ledges with several steps exhibiting fcc-B2 lattice continuity between the {111}fcc and {011}B2. The effects of such interfaces on the uniaxial deformation of fcc-B2 multicrystal nanostructures, as a function of lamellae thickness, were studied using MD simulations. We observed that the screw-like interfacial partials facilitated the KS interfacial sliding and strain accumulation at the interphase interfaces, and reduced the yield strength of the composite material compared to a pure-fcc reference material. Deformation character depends on lamellae thickness. Thin B2 lamellae (~4 Å) sheared via twinning to drastically lowered flow stress such that the flow-strength was comparable to the pure fcc constituent phase. In contrast, thicker B2 lamellae (~12 Å) sheared via a slip-transfer mechanism, which allowed the fcc-B2 composite to maintain its flow-strength. Thus, the atomic structure of fcc/B2 KS interfaces was directly linked to dominant operative plastic deformation mechanisms.

Crystallography of Martensite in Friction-Stir-Welded 12Cr Heat-Resistant Steel

In this work, electron backscatter diffraction (EBSD) was applied for examination of the crystallography of the martensitic structure produced during friction-stir welding (FSW) of 12Cr heat-resistant steel. The martensitic transformation was found to be governed by the Kurdjumov–Sachs (K-S) relation, viz. {111}γ || {110}α′ and 〈110〉γ || 〈111〉α′, was characterized by substantial variant selection and led to considerable orientation spread within the martensite variants. Due to the latter effect, the K-S model was found to be ineffective for the reconstruction of prior-austenite grain structure. Instead, the mixed orientation relation, viz. {111}γ || {110}α′, 〈110〉γ || 〈111〉α′, and 〈112〉γ || 〈110〉α′, was shown to be more suitable for this purpose.

Effect of fusion and friction stir welding techniques on the microstructure, crystallographic texture and mechanical properties of mild steel

The present investigation aims at studying the effect of different welding techniques on the microstructure and mechanical properties of the welds. Weld zone (WZ) in the metal inert gas (MIG) welding, contains higher martensite fraction compared to arc and tungsten inert gas (TIG) welding. The micro-hardness of the WZ was found to be maximum after MIG welding (220 ± 14 HV) in comparison to arc welding (190 ± 12 HV), TIG welding (142 ± 10 HV) and friction stir welding (FSW) (202 ± 10 HV) as well as the base metal (BM) (108 ± 14 HV). The joints fabricated via MIG welding exhibited higher tensile strength (371 ± 10 MPa) as compared to other welding techniques (arc and FSW). The ductility of the parent metal was 42 ± 2% whereas the ductility of the MIG and arc joints were lower (13 ± 2% and 17 ± 3% respectively) due to the presence of martensite during fusion welding. The FSW joint shows higher yield and tensile strengths (356 ± 8 MPa) along with higher elongation (22 ± 4%) compared to fusion-welded joints due to the presence of fine, equiaxed recrystallized grains in the WZ. Fracture occurs at the HAZ/base metal interface, where the hardness is lower compared to the WZ. Compressive and tensile residual stress is obtained in the WZ upon fusion and FSW, respectively. In this paper, analytical tools are used to calculate the orientation of prior austenite (γ) in the MIG weld joint. The texture of high temperature γ phase is simulated for the different welding techniques. The occurrence of the Kurdjumov–Sachs (KS) orientation relationship (OR) is determined from the martensite grains in MIG weld. The texture investigation showed the occurrence of texture memory effect of ferrite in the HAZ after MIG welding.

Cyclic deformation behavior and microstructure evolution of high-carbon nano-bainitic steel at different tempering temperatures

Bainitic steel is applied in many cyclic loading projects due to its excellent comprehensive mechanical properties. In this study, the cyclic hardening/softening behavior and microstructure evolution of high-carbon nano-bainitic steel were investigated to analyze the cyclic deformation mechanism. Results showed that the optimal tempering temperature was determined to be at low temperature. During cyclic deformation, the retained austenite is transformed into martensite according to the Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) orientation relationships, and deformation twins are generated in the retained austenite. The cyclic deformation of high-carbon steels at a total strain amplitude of 0.6% manifests no obvious cyclic softening behavior after low-temperature tempering, mainly due to the high density of dislocation tangle around martensite and deformation twins induced by cyclic deformation.

Microstructure Evolution and Orientation Relationship of Reverted Austenite in 13Cr Supermartensitic Stainless Steel During the Tempering Process.

The transformation mechanism of reverted austenite and the amount of reverted austenite during the tempering process in supermartensitic stainless steel have been investigated by X-ray diffraction (XRD), electron backscattered diffraction (EBSD), and a high-temperature laser scanning confocal microscope (HTLSCM). The results indicate that the microstructure mainly consists of tempered martensite and reverted austenite. The reverted austenite nucleates uniformly at the sub-block boundary and prior grain austenite boundary. The amount of reverted austenite strongly relies on the tempering time, showing a positive correlation in the supermartensitic stainless steel. The crystallographic orientation relationship between reverted austenite and martensite meets the Kurdjumov-Sachs(K-S) relationship and the deviation angle is mainly concentrated at about 2 degrees. The mechanism of reverted austenite transformed from martensite is a diffusion mechanism. The growth kinetics of the reverted austenite are dominated by diffusion of the Ni element and there is no shear deformation of the martensite matrix in the in situ observation. It can be deduced that the reverted austenite is formed by nickel diffusion during tempering at 620 °C for different tempering times.

Effect of low cold-rolling strain on microstructure, texture, phase transformation, and mechanical properties of 2304 lean duplex stainless steel

The effect of cold rolling deformation (4%–22%) on the microstructure, microtexture, and mechanical properties of a 2304 lean duplex stainless steel was investigated. The deformed microstructures were analyzed by electron backscattering diffraction and transmission electron microscopy. The results showed that after 4% thickness reduction, the strain produced phase transformation in austenite (γ), stacking faults (SFs), and mechanical twins. The orientation relationships of Kurdjumov–Sachs (KS) and Nishiyama–Wassermann (NW) were observed between the γ and α′-martensite. Moreover, Shoji–Nishiyama (SN) and Pitsch–Schrader (PS) were observed between γ and ε-martensite, and ε-martensite and α′-martensite, respectively. A further reduction in thickness (up to 22%) caused a reduction in total elongation and an increase in the yield and the ultimate tensile strength.

Effect of Grain Boundaries Type on Carbides Precipitates in Tempered Martensite

Different types of carbide phases and regions of their precipitation in tempered martensite of austenitic steel have been investigated with orientation microscopy (EBSD) and electron microprobe analysis. The steel structure consisted of large grains of high-temperature ferrite (~ 15%), without visible mesostructured, and martensite packages with a great number of low-angle boundaries. High-angle boundary spectrum with the most prominent coincidence site lattice (CSL) boundaries, Σ3, Σ11, Σ25b, Σ33с Σ41с, is typical for martensite. This spectrum, resulted from austenite transformation by shear mechanism according to orientation relationships (OR), intermediate between Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W). In the structure two types of carbide precipitates were observed: large MC [~ NbC] along the boundaries of former austenite grains, and dispersed M23C6 [~ (W,Mo)2(Cr,Fe)21C6] predominantly along the boundaries in martensite packages. It has been shown that under martensite tempering M23C6 precipitation was mainly at high-angle intergranular boundaries. Carbide almost did not precipitate at low-angle and special CSL Σ3 boundaries. A few carbides were detected at special CSL boundaries, Σ11, Σ25b, Σ33с Σ41с.