Kurdjumov-Sachs Orientation Relationship Research Articles

Fcc → Bcc Phase Transition Kinetics in an Immiscible Binary System: Atomistic Evidence of the Twinning Mechanism of Transformation

Extensive atomistic simulations based on the quasiparticle (QA) approach are performed to determine the momentous aspects of the displacive fcc/bcc phase transformation in a binary system. We demonstrate that the QA is able to predict the major structural characteristics of fcc/bcc phase transformations, including the growth of a bcc nuclei in a fcc matrix, and eventually the formation of an internally twinned structure consisting in two variants with Kurdjumov-Sachs orientation relationship. At atomic level, we determine the defect structure of twinning boundaries and fcc/bcc interfaces, and identify the main mechanism for their propagation. In details, it is shown that twin boundaries are propagated by the propagation of screw dislocations in fcc along the α direction, while the propagation of fcc screw dislocations along coherent terrace edges is the pivotal vector of the fcc/bcc transformation. The simulation results are compared with our TEM and HRTEM observations of Fe-rich bcc twinned particle embedded in the fcc Cu-rich matrix in the Cu-Fe-Co system.

Effect of nitrogen on precipitation behavior of hyper duplex stainless steel S32707

Thermodynamic calculations, time–temperature–transformation diagram construction, microstructural characterization and precipitation kinetics analyses were conducted to investigate the effect of N content on the precipitation behavior of hyper duplex stainless steel S32707. The results demonstrate that the nose time of 3% σ phase precipitation gradually increases with increasing N content. Increasing the N content substantially reduces the quantity of the σ phase at δ/γ interfaces. Within the first 5 min of aging, the growth rate of the σ phase decreased in the order N0.35, N0.45 and N0.56. Between 5 min and 48 h, the growth rates were essentially the same. The mechanism by which N inhibits the σ phase is attributed to three aspects: (i) N reduces the quantity of favorable nucleation sites by increasing the fraction of δ/γ interfaces with low Kurdjumov–Sachs orientation relationship deviation angles; (ii) N diminishes the tendency of Cr and Mo to diffuse from the δ to γ phase by decreasing the natural partitioning coefficients; and (iii) N decreases the σ phase nucleation driving force and Cr activity. Raising the N content promotes Cr2N precipitation only after long-term aging. The role of N in promoting Cr2N is generalized in two aspects: first, N increases the diffusion rate of Cr and N in austenite based on its increased diffusion coefficient with increasing N content; second, N increases the Cr2N nucleation driving force and N activity.

Microstructure, crystallographic texture and strain hardening behavior in hot tensile tests of UNS S32304 Lean Duplex stainless steel

Duplex stainless steels can be used for applications in the nuclear industry endorsing its choice as an external shield for Type B packages. The microstructure, crystallographic texture, and the strain hardening behavior of UNS S32304 duplex stainless steel was investigated in hot tensile tests by Electron Backscattering Diffraction technique. Interrupted hot tensile tests with strain rates ε˙ = 10−2 s−1 and ε˙ = 10−1 s−1 were carried out at 700 °C using a Thermomechanical Physical simulator. The α parameter of gamma distribution was associated for the first time with strain in ferrite and austenite phases through the Grain Orientation Spread parameter (GOS). The ferrite phase did not show the dynamic equilibrium between the stored energy and recovery rates. The average GOS in the ferrite phase was larger for ε˙ = 10−1 s−1 due to a smaller time available for dislocation annihilation and rearrangement in dynamic recovery. The grain rotation sequence <100> → <100> - <101> → <101> was found in ferrite phase. In the austenite phase, the dynamic recrystallization was not observed and the <111> and <100> fiber textures parallel to the tensile direction were strengthened. The austenite → ferrite phase transformation occurred during the hot tensile tests and showed the Kurdjumov-Sachs orientation relationship. The modified Crussard-Jaoul analysis showed six-strain hardening stages which were associated with the average GOS and austenite → ferrite phase transformation.

Formation of abnormal nodular ferrite with interphase precipitation in a vanadium microalloyed low carbon steel

Abnormal nodular ferrite (NF) with vanadium carbide (VC) interphase precipitation was observed in a vanadium-microalloyed low carbon steel. The NF is characterized as a structure with a high density of low-angle boundaries, having an irrational ferrite/austenite orientation relationship (OR) with angular deviation of 15-30 ° from the exact Kurdjumov-Sachs OR with its neighboring austenite grain into which it grows. The growth of NF is substantially faster than that of normal grain boundary ferrite in the same alloy. Even in one single NF, the formation of multiple variants of VC interphase precipitation was confirmed, each of which was bordered by low-angle boundaries.

The potential for grain refinement of Wire-Arc Additive Manufactured (WAAM) Ti-6Al-4V by ZrN and TiN inoculation

Wire-Arc Additive Manufacturing (WAAM) of large near-net-shape titanium components has the potential to reduce costs and lead-time in many industrial sectors including aerospace. However, with titanium alloys, such as Ti-6Al-4V, standard WAAM processing conditions result in solidification microstructures comprising large cm-scale, <001> fibre textured, columnar β grains, which are detrimental to mechanical performance. In order to reduce the size of the solidified β-grains, as well as refine their columnar morphology and randomise their texture, two cubic nitride phases, TiN and ZrN were investigated as potential grain refining inoculants. To avoid the cost of manufacturing new wire, experimental trials were performed using powder adhered to the surface of the deposited tracks. With TiN particle additions, the β grain size was successfully reduced and modified from columnar to equiaxed grains, with an average size of 300 µm, while ZrN powder was shown to be ineffective at low addition levels studied. Clusters of TiN particles were found to be responsible for nucleating multiple β Ti grains. By utilizing the Burgers orientation relationship, EBSD investigation showed that a Kurdjumov-Sachs orientation relationship could be demonstrated between the refined primary β grains and TiN particles.

Impact of crystallography at Ni/NiAl interfaces on the nucleation of Ni3Al

Early stages of phase formation at (Ni)/NiAl interfaces with different crystallographic configurations were studied. As opposed to earlier work, macroscopic diffusion couples and a low annealing temperature of 650°C were applied. The morphology of the newly forming Ni3Al phase, its crystallographic relationships to the different (Ni)/NiAl interfaces and the growth kinetics of Ni3Al were evaluated with appropriate statistics combining EBSD with STEM EDX. The Ni3Al phase forms in two layers and mainly grows into the NiAl phase, establishing a specific crystallographic relationship with respect to the adjoining phases. The majority of Ni3Al grains growing into the NiAl phase maintain Kurdjumov-Sachs orientation relationships. The grains in the second Ni3Al layer mostly grow in a near cube-on-cube orientation relationship (misorientation < 15°) towards the (Ni) phase. The formation of large Al enriched (Ni) grains at the (Ni)/Ni3Al interface is observed and interpreted as diffusion induced recrystallization.

Effect of cold rolling reduction on texture, recrystallization and mechanical properties of UNS S32304 Lean Duplex stainless steel

The effect of cold rolling reduction on the texture, extended recovery, recrystallization and mechanical properties of UNS S32304 Lean Duplex stainless steel was investigated. EBSD analysis showed that α′-martensite transformation in austenite phase caused a shear band formation in ferrite phase. An austenite grain with {110}<112> Brass component transformed to α′-martensite with {001}<110> rotated cubic component was found according to the Kurdjumov-Sachs (K-S) orientation relationship after 63% cold rolling reduction. Evidence that extended recovery and discontinuous recrystallization occurred in the ferrite phase was observed. The increase in cold rolling reduction caused an increase in the softened fraction and the fraction of grains with GOS<1° mainly due to the extended recovery in the center. The increase in cold rolling reduction causes higher stored energy and affects the discontinuous recrystallization in the austenite phase. On the other hand, the austenite texture was not influenced and a decrease in its intensity was only observed during annealing. The uniform elongation and total elongation remained approximately constant after the softened fraction reached 0.92 and the complete recrystallization of austenite phase.

Stir zone anisotropic work hardening behavior in friction stir processed EN8 medium carbon steel

The SZ anisotropic work hardening behavior in duplex ferrite-martensite phase (DFM) microstructures has not been investigated yet. It is an important consideration for the selection of the stretch formability direction of the friction stir processed blank used for sheet metal forming applications. The anisotropic work hardening behavior is characterized by using miniature tensile specimens prepared along longitudinal (LD) and transverse (TD) directions. The tribological parameters such as coefficient of friction (COF) and wear rate at LE value of 6 are 0.57 and 0.5 × 10 − 10 m 3 / Nm . The effect of surface texture on the work hardening response is observed to be negligible. The plastic anisotropy in terms of three and multi-stage work hardening behaviors is investigated at a linear energy ( LE) value of 6. The global true stress-strain responses and fracture surface morphologies suggested that the deformation occurs in different stages. In the initial stage, the deformation is mainly localized in the soft ferrite phase followed by the distinct work hardening stages in the plastic flow curves. In the elastic and elastoplastic region of the flow curve, the soft ferrite phase plastically deformed, followed by stress transfer to the martensite/retained austenite phases in the fully plastic region. The (110) and (111) pole figure suggested that the active in-habit-plane slip system is (110) and [ 1 1 ‾ 1 ] . The strain localizes more in the V19 + V22 block having the critical resolved shear stress (CRSS) value of 445.5 MPa. The retained austenite (RA) is having ( − 11 − 1 ) A / / ( 0 − 11 ) F and [ 011 ] A / / [ 111 ] F type Kurdjumov-Sachs (K–S) orientation relationship with the ferrite phase. A BCC {112} <111> -type twinned laths is found to be present within the martensite laths. The recommended stretch formability direction (SFD) should coincide with the longitudinal or processing direction of the friction stir processed blank.

Improving surface mechanical properties of the selective laser melted 18Ni300 maraging steel via plasma nitriding

Enhancing the hardness and wear resistance of selective laser melted (SLM) maraging steel is challenging. In this work, plasma nitriding was developed to improve the surface mechanical properties of SLM 18Ni300 maraging steel based on the effect of simultaneous aging and nitriding. The nitriding behaviors of the as-fabricated sample and heat-treated (i.e., solution, aging and combined solution-aging treatments) samples were systematically analyzed using microstructure and property characterizations as well as first principles calculations. The results show that the hardness and wear resistance of all samples were notably enhanced by a nitrided layer consisting of an outermost N-rich bright white layer and an inner Mo-rich diffusion layer. Among all nitrided samples, the directly nitrided sample without heat treatment possesses the best mechanical properties, which are closely related to the strongest Mo segregation and a novel nitriding structure induced by additive manufacturing. The novel nitriding structure is an interlaced lamellar structure consisting of coarse γN lamellae along the prior SLM subgrain interfaces and parallel fine γN lamellae in the αN matrix, which obeys the typical Kurdjumov-Sachs orientation relationship between αN and γN. Compared to the traditional nitriding structure originated from the homogenized maraging steel, the novel nitriding structure benefits strengthening and toughening and encourages the formation of an Fe-O-Si self-lubricating film. The abovementioned results suggest that direct nitriding of the as-fabricated sample without heat treatment is the best method to obtain the best surface mechanical properties.

Effect of Heat Treatment Process on Microstructure and Crystallography of 20CrMnTiH Spur Bevel Gear

Obtaining excellent mechanical properties of spur bevel gear has been widely emphasized, owing to its indispensable effect on momentum. However, the conventional long-time and high-temperature heat treatment can cause microstructure coarsening, which dramatically decreases the mechanical properties. Since the normalizing process can significantly modulate the mechanical properties of spur bevel gears by adjusting their microstructure and crystallography behaviors, in the present study, the normalizing before or after carburizing heat treatment processes (NBCP and NACP, respectively) were proposed. The effect of heat treatment processes on the microstructure and crystallography of gear specimens was investigated. The results show that both NBAP and NACP had a strong effect on microstructure refinement and hardness improvement compared to the conventional process. NBCP raised the tendency for AlN precipitation, which could retard the microstructure coarsening during carburizing. Furthermore, NACP directly promoted the precipitation of globular Cr-rich M3C carbides which achieved the strongest pinning effect and brought about extremely fine microstructure. For crystallographic analysis, the 24 martensite variants in all gear specimens held Kurdjumov–Sachs orientation relationship to parent austenite. The orientations of martensite variants in gear specimens applying conventional process and NBCP were distributed regularly. However, the orientations of martensite variants in NACP gear specimen did not follow the strict rule for variant selection inside prior austenite grains owing to the distortive effect of diffuse M3C carbides on the matrix, and the adjacent martensite variants possessed less sharing of {110} habit plane compared to NBCP and conventional process.

Atomic-scale investigation of deep hydrogen trapping in NbC/α-Fe semi-coherent interfaces

The precipitation of niobium carbide (NbC) is a superior approach to mitigating hydrogen embrittlement (HE). The role of the semi-coherent interface between NbC and α-Fe on hydrogen trapping and HE resistance in high-strength tempered martensitic steel was investigated in this study. High-resolution transmission electron microscopy observations are performed to reveal the atomic-scale crystallographic orientation relationship, atomic arrangements, and associated crystalline defects in the NbC/α-Fe semi-coherent interface. We observed the Kurdjumov–Sachs orientation relationship with (11¯1¯)NbC//(101)α−Feand [01¯1]NbC//[1¯11]α−Febetween the NbC and α-Fe phases. Noticeably, two sets of misfit dislocations with Burgers vectors of b(1)=ab/2[111]on (011¯)α-Fe planes and b(2)=ab/2[11¯1]on (110) α-Fe planes (ab is the lattice constant of α-Fe), which would be the deep hydrogen trapping sites, were characterized in the NbC/α-Fe semi-coherent diffuse interface. In addition, density functional theory-based first-principles calculations revealed that the deep binding energy between the NbC/α-Fe semi-coherent interface and hydrogen is 0.80 eV, which well matches the hydrogen desorption activation energy of 81.8 kJ/mol determined via thermal desorption spectroscopy experiments. These demonstrate that the nature of the deep hydrogen trapping sites of the NbC/α-Fe semi-coherent interface is the misfit dislocation core. Distinguished HE resistance was obtained and ascribed to the deep hydrogen trapping of uniformly dispersed NbC nanoprecipitates with an average diameter of 10.0 ± 3.3 nm. The strategy of deep hydrogen trapping in the NbC/α-Fe semi-coherent interface is beneficial for designing HE-resistant steels.

Microstructure and texture of friction stir welds in low carbon steel using prior austenite reconstruction method

ABSTRACT Friction stir welding (FSW) was performed on thick plates of high-strength low-alloy (HSLA) steel with polycrystalline cubic boron nitride (PCBN) tools. To investigate the mechanism of microstructure formation in the stir zone (SZ), the prior austenite structure undergone through a thermo-mechanical history during the FSW process was investigated. The prior austenite structure was reconstructed by the local crystal orientations of prior austenite calculated from the bainite orientations by using the Kurdjumov–Sachs orientation relationship between the prior austenite and bainite. The reconstructed austenite revealed a heterogeneity in the SZ. The austenite grain size in the advancing side (AS) of SZ was larger than that of the retreating side (RS). Thus, a reheated temperature of AS was considered to be higher than that of RS. The textures of reconstructed austenite were also different in SZ. The rolling texture was observed in RS, but the shear texture was observed in the centre and AS. The heterogeneous microstructure and texture in SZ was considered to be caused by the material flow behaviour in FSW process.

Strong Interactions between Austenite and the Matrix of Medium-Mn Steel during Intercritical Annealing.

The effects of heat treatment on the microstructure evolution was studied in regards to austenite nucleation and grain growth. It was found that the austenite nucleation and matrix recrystallization kinetics of samples annealed at 675 °C for different times were revealed, implying a strong interaction between the ferrite matrix and austenite. The recrystallization of the matrix during annealing provided favorable conditions for austenite nucleation and growth, and the formation of austenite during this process reduced the matrix recrystallization kinetics, thus delaying the recrystallization process of the matrix around the austenite grains. The statistical results for the austenite grain size under different annealing temperatures indicated that the average grain size of the austenite slightly increases with increasing of the annealing temperature, but the austenite with the largest grain size grows faster at the same temperature. This difference is attributed to the strict Kurdjumov Sachs (KS) orientation relationship (OR) between the austenite grains and the matrix, because the growth of austenite with a strict KS OR with the matrix is often inhibited during annealing. In contrast, the austenite maintains a non-strict KS OR with the matrix and can grow preferentially with increasing annealing temperature and time.

Coalescence of martensite under uniaxial tension of iron crystallites by atomistic simulations

Molecular dynamics simulations are used to study the effects of tensile loading on nucleation and subsequent growth of bcc phase in pure fcc iron. The results show that orientation variant selection occurs during the stress-induced fcc-to-bcc transformation, which leads to the coalescence of neighbouring bcc platelets with identical orientation. The bcc phase nucleates mainly following Nishiyama–Wassermann and Kurdjumov–Sachs orientation relationships with the parent fcc phase. The present simulations contribute to a better understanding of mechanisms controlling mechanically induced martensitic transformation as well as coalescence of bcc platelets in steels.

Microstructure and mechanical properties of friction stir welded joint of TRIP steel

Friction stir welding (FSW), as a new type of solid-solid welding method, was utilized to join transformation-induced plasticity (TRIP) steel. This method can avoid the solidification structural defects caused by fusion welding technology and improve the welding quality. In this study, the microstructural transformation and deformation behavior of TRIP steel during FSW were evaluated by optical microscopy, electron backscatter diffraction, and room-temperature tensile test. The results show that after FSW, martensite transformation occurred in the stir zone (SZ) and thermomechanically affected zone (TMAZ), and the retained austenite content decreased. The grain refinement and ratio of low-angle boundaries (LABs) increased. In heat-affected zone (HAZ), the retained austenite was thermally spheroidized and decomposed, the grains were coarsened, and the ratio of LABs decreased. No obvious preferred grain orientation was observed in each region of joint, and the texture strength decreased by phase transformation and grain rotation. After the tensile deformation of FSW joint, the retained austenite disappeared, the grains were refined, and the proportion of LABs increased in the base metal (BM) and HAZ. The content of retained austenite in SZ remained unchanged, but decreased in TMAZ. The grains were refined, and the ratio of LABs did not change significantly in SZ and TMAZ. FSW increased the microhardness of SZ and TMAZ by 218 HV and 93 HV, respectively, while the microhardness of HAZ decreased by 4 HV. The yield strength, tensile strength, and elongation of joint were 563 MPa, 942 MPa, and 12.8 %, respectively, reaching 96%, 104%, and 59%of BM, respectively. The tensile fracture exhibited ductile fracture characteristics. In addition, the joint exhibited strain nonuniformity during the tensile deformation, and the order of deformation degree was as follows: HAZ > TMAZ > SZ. Therefore, HAZ became a weak zone of joint mechanical properties. Continuous dynamic recrystallization and discontinuous dynamic recrystallization were the main mechanisms of grain refinement, and the martensite transformation conformed to the Kurdjumov-Sachs orientation relationship.

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.

Diffusional and displacive transformations in interstitial-free steel within the scope of a critical assessment of the mechanical property

Interstitial–free steel is fully ferritic at room temperature, extremely soft and ductile. The strengthening is a key focus of the present study through heat treatment. The samples are soaked in the austenitic temperature domain followed by different rates of cooling in furnace, air, and water quenching. The cooling curves obtained from the experiments are superimposed in the computed CCT diagram to predict and discuss various microstructure formations by diffusional and displacive mechanisms of the phase transformations which is lacking in the system. Essentially, the best combination of strength-ductility is obtained in the water quenched sample, compared to the reported techniques, for a detailed analysis. The electron microscopy hints at a lath type morphology with high dislocation density. The selected area diffraction pattern has indexed the lattice of the product phase as BCC. Further, the image quality (IQ) map, grain boundary misorientation, as well as Kurdjumov-Sachs (K–S) orientation relationships with parent austenite confirm the lath as martensite through variant selections. For the mechanical property assessment with EBSD, the role of the hierarchical structures of martensite (e.g., packet, block and sub-block boundaries) to the dislocation pinning is discussed against microstructural contributions to resolve ambiguity in the strengthening process.

Accurate determination of orientation relationship between ferrite and (Fe, Mn)3AlC in an Fe-Mn-Al-C eutectoid alloy

The orientation relationship between ferrite and (Fe,Mn)3AlC has been investigated in an Fe-3.5Mn-5.8Al-1.05C based eutectoid alloy. Electron diffraction pattern analysis shows two possible orientation relationships between both phases. Accurate analysis using convergent beam electron diffraction reveals the deviation of [001]ferrite pole from [110](Fe,Mn)3AlC pole in Nishiyama-Wasserman orientation relationship and the coincidence of [111]ferrite and [110](Fe,Mn)3AlC poles in Kurdjumov-Sachs orientation relationship. Transmission Kikuchi diffraction resolves the orientation of several tens nanometer scales of ferrite+(Fe,Mn)3AlC lamellar. Kurdjumov-Sachs orientation relationship between ferrite and (Fe,Mn)3AlC is clarified by the comparison of deviation angle from both orientation relationships. Coherency at the interphase boundaries is further discussed.

&lt;i&gt;In Situ&lt;/i&gt; 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.

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.