Kurdjumov-Sachs Relationship Research Articles

Crystal plasticity modeling of prior austenite orientation effects on deformation behaviors of martensitic steels under different strain paths

The influence of prior austenite grain (PAG) orientation on the deformation behavior of a low-carbon martensitic steel is investigated using crystal plasticity (CP) modeling with hierarchical representative volume elements (RVEs). The Kurdjumov-Sachs (K-S) relationship is refined for accurate PAG reconstruction in martensitic steel. A robust calibration strategy for CP parameters based on nanoindentation tests is developed by integrating analytical calculations with inverse methods. Virtual polycrystalline aggregates are subsequently generated by manipulating initial PAG orientations. The simulations reveal that assigning cube texture to PAGs enhances strain hardening of martensite under plane strain tension. Moreover, the RVEs with brass and copper orientated PAGs exhibit similar deformation behavior, and a more homogeneous strain distribution is realized in the RVE with PAG cube orientation. The influence of PAG orientation on stress and strain fields is correlated with lattice rotation behavior during deformation. Notably, different strain paths elicit distinct lattice rotation trajectories, wherein uniaxial tension and plane strain tension favor grains reorientation toward hard γ-fiber, whilst equi-biaxial tension drives the crystal matrix to concentrate on 〈001〉 and 〈011〉 directions. These findings provide insights into the intricate relationship between microstructural orientation and deformation behavior in martensitic steels, which is crucial for performance optimization.

Observation of inclusions and intra-granular ferrite evolutions at the HAZ of low-carbon steel with different thermal histories

This study investigated the formation and growth of intra-granular ferrite with inclusions (TiN, MnS) in the heat-affected zone (HAZ) of a welding joint in low-carbon steels. The relationship between the cooling rate and the density and size of inclusions was analyzed. An orientation relationship akin to the Baker–Nutting relationship was identified between TiN and ferrite, while the orientation relationship between MnS and ferrite approximated the Kurdjumov–Sachs relationship. At low cooling rates, in the absence of nucleation support from TiN and MnS inclusions, the microstructure in the HAZ comprised Widmanstätten ferrite. Additionally, the shape, size, and density of primary ferrites and Widmanstätten ferrite in steels were influenced by sulfur content.

Influence of strain rate on the work hardening, strain induced martensite formation, strain partitioning, and variant selection in a medium-Mn steel

In this study, we investigated the effect of strain rate on the work hardening, strain-induced martensite transformation, strain partitioning, and crystallographic variant selection in a medium-Mn steel. Uniaxial compression tests were performed under quasi-static and dynamic loading conditions at the strain rates of 10−3, 1 and 103s−1. Besides, to determine the contribution of temperature rise due to adiabatic heating at a higher strain rate, compression tests were carried out at 409 K (estimated temperature rise) under quasi-static conditions. The work hardening rate was higher at high strain rates and it rapidly decreased at all strain rates up to a true plastic strain of ∼0.05. Beyond the true plastic strain of ∼0.05, the work hardening rate increased at a low strain rate up to a true plastic strain of ∼0.15 while at a high strain rate, it remained relatively constant and gradually decreased beyond a true plastic strain of ∼0.15. From the analysis of microstructures obtained from electron backscatter diffraction, it was observed that the strain-induced transformation of retained austenite to α′-martensite is significantly impeded at higher strain rates and in the sample tested at 409 K at low strain rates. The higher stability of retained austenite is attributed to the adiabatic heating which decreases the chemical driving force for retained austenite to martensite phase transformation and increases the stacking fault energy. Analysis of the misorientation axis and angle evolution revealed that while the Kurdjumov–Sachs relationship was followed under quasi-static conditions, it deviated at higher strain rates. Analysis of crystallographic variants related to strain-induced martensitic transformation at a true plastic strain of ∼0.15 revealed that the samples deformed at a lower strain rate did not show any noticeable variant selection. However, in samples deformed at a higher strain rate, a significant variant selection was observed. It is reasoned that the martensitic transformation nucleates initially on favorably oriented habit planes of retained austenite grains at all strain rates and other variants get activated with an increase in strain. As the transformation is impeded beyond a critical strain at a higher strain rate, the variants activated at lower strains remains predominant.

Effect of applied stress on bainite transformation, microstructure, and properties of 15CrMo steel

The effect of external stress on the isothermal bainite transformation kinetics and microstructure of 15CrMo steel was studied using dilatometer, optical microscopes (OM), electron back-scatter diffraction (EBSD), scanning electron microscopes (SEM) and X-ray diffraction (XRD). The results show that the additional mechanical driving force provided by the external stress promotes the transformation of bainite, resulting an increase volume fraction of bainite-ferrite (BF) and causing the BF laths to become slender. The application of external stress can also induce selective orientation during the bainite transformation process, and the orientation relationship between the prior austenite and the BF under stress is closer to the Kurdjumov-Sachs (K-S) relationship. There is also an incomplete transformation of bainite transformation under stress. At the same time, the Vickers hardness of 15CrMo steel would increase after applying external stress. The mechanical driving forces calculations demonstrate that when tensile stress and compressive stress are of similar magnitudes, they provide comparable mechanical driving forces. As a result, they exert similar effects on the kinetics of bainite transformation, leading to no discernible differences in the resulting bainite microstructure.

New insights into lamellar and twinning transformations in TiAl/Nb composites with core-shell structure during high temperature annealing

Lamellar and twinning transformations play a crucial role in stabilizing the ordered phases in TiAl based composites for property improvement. In this work, we have systematically investigated the lamellar and twinning transformations in TiAl/Nb composites with core-shell structure and proposed novel insights on γ → α2 and γ → γT processes from an atomistic perspective. The results show that high temperature annealing facilitates the formation of lamellar microstructure including γ + α2 and γT twin lamellae in the vicinity of core/shell interface, due to rapid diffusion of principal atoms followed by atomic shuffling. Specifically, plenty of lenticular γT can nucleate in B2 regime by accidental misalignment of atoms on a 110B2 plane adhering to Kurdjumov-Sachs relationship, and then grow even into α2 lath driven by the release of internal stress via Shockley partial slips on the 111γ plane. The formation of γT mediated by 9R involves γ → 9R and γ ← 9R → γT transformations. Among them, a full period of atomic stacking …ABC|BCA|CAB… (LPSO) can be achieved through three Shockley partials being sequentially activated on the C, A and B atomic layers in the stacking of …ABC|ABC|ABC… (L10) with the total shift of 3δA. Then 9R will change synchronously to γ and γT once the migrating Shockley partials react with Frank partials to yield a full dislocation, i.e., δA + Dδ→DA and δA + δC + δD→BD, which can also be considered as detwinning and twinning behaviors, respectively. Additionally, the transition of bulk γ to α2 or γT lamellae is closely associated with the stacking faults, i.e. whether the activated atomic layer is located at the boundary or periphery of the updated stacking fault region.

Discontinuous yielding behavior, strengthening effect and deformation mechanism of double wall copper-brazed steel tube under various continuous sectional cooling

Unlike traditional cooling, continuous sectional cooling (CSC) consists of various cooling modes, which is of decisive significance to the microstructure and performance of double wall copper-brazed steel tube (DWBT). In this work, three CSC (CSC-A/B/C) processes are chosen to reveal strengthening and deformation mechanisms of DWBT. The results show that there are significant differences in discontinuous yield behavior, natural aging phenomenon, and tensile properties of the DWBT among three CSC processes due to diverse grain size and precipitates, but a similar texture is obtained due to Kurdjumov-Sachs relationship. For discontinuous yielding, the propagation velocity of Lüders band is a key factor to determine the yield point elongation (YPE), which reduces with decreasing grain sizes due to the interaction between dislocation and grain boundary. Thus, the CSC-C (1120 °C→4s940 °C…360 °C→16s110 °C) with small grains has a longer YPE. Meanwhile, the precipitation strengthening dominates the highest strength and lowest elongation of the CSC-A (1120 °C…100 °C) due to abundant MC carbides. Compared with CSC-B (1120 °C…170 °C), the yield strength of CSC-C is higher while ultimate tensile strength is lower, which is attributed to different contributions of precipitation and grain boundary strengthening to both. The MC carbide causes the natural aging of DWBT. Moreover, the geometrically necessary dislocations (GNDs) and low angle grain boundaries (LAGBs) raise linearly with strain. The GNDs prefer to be distributed in small grains, and uniform grain can weaken the difference of GNDs distribution in different grain sizes. The LAGBs including dense dislocation walls or microbands are formed by the grain subdivision mechanism.

Microstructure and phase composition of transition zone between low alloyed steel boiler tube and an austenitic stainless steel weld overlay produced by cold metal transfer method

The microstructural characterization, as well as phase and chemical composition of the transition zone between the 16Mo3 steel boiler tube and 310 austenitic stainless steel weld overlay produced by the Cold Metal Transfer method, were studied.It was showed that the transition zone has a complex structure related to the changing composition. Two separate areas are noticeable in the transition zone. The area near the fusion boundry is two phased (martensite and austenite). Detailed TEM investigation of this area disclosed martensitic–austenitic structure with the Kurdjumov-Sachs relationship between martensite and austenite. The microstructure of the part of the transition zone with Ms temperature lower than room temperature is austenite.The investigations showed that by using low energy welding method we can significantly reduce the width of the martensite area in the transition zone of weld overaly.

Thixotropic deformation behavior, rapid spheroidization and solid-liquid homogenization of semi-solid Al0.8Co0.5Cr1.5CuFeNi HEA with multilevel microstructure

In this paper, powder metallurgy-rapid heating and isothermal-holding (PMRHI) method was proposed to realize the rapid spheroidization and suppression of solid grain coarsening in semi-solid Al0.8Co1.5Cr0.5CuFeNi HEA. The thixotropic behavior and microstructure after PMRHI were investigated. Shear thinning effect exists during thixotropic deformation. The bimodal characteristics of flow stress degenerates with the proceeding of solid grain sliding (SS) and flow of liquid incorporating solid grains (FLS), indicating solid-liquid homogeneity and stable thixotropic deformation. The solid-liquid distribution homogeneity can suppress voids and hot tearing caused by liquid film channels. In multilevel semi-solid microstructure, the orientation relationship between solidified liquid phase and solid grain is approximately Kurdjumov-Sachs relationship, fine Al-Ni and Fe-Cr rich spinodal plates and nano Cu precipitates are formed in solid grain. The rapid spheroidization is realized due to initial dispersed powder metallurgy microstructure. After the steady thixotropic deformation of 1225 °C /0.05 s−1, mechanical properties were significantly improved.

Microstructure evolution and mechanical properties of an annealed dual-phase CrFeCoNiAl0.7 high-entropy alloy

The present work investigated microstructures and mechanical properties of an annealed dual-phase high-entropy alloy (HEA) CrFeCoNiAl0.7 between 700 and 1100 °C. It revealed that with the increased annealing temperature, there were rod-like B2 phase precipitating from the FCC-solution matrix, and the orientation between the precipitated B2 phase and the FCC matrix obeyed Kurdjumov-Sachs (K–S) relationship {111}FCC//{011}B2 and ⟨011⟩FCC//⟨111⟩B2. The higher annealing temperature resulted in a drastic drop in yield strength, but a significant increase in fracture strain and work-hardening capacity. The positive value of the strain rate sensitivity (SRS) decreased with strain and the SRS of the HEA sample annealed at a lower temperature decreased greatly. Due to the formation of co-clusters, the activation volume of the annealed HEA presented an abnormal trend, that was, it increased with strain.

Temperature mitigates the hydrogen embrittlement sensitivity of martensitic steels in slow strain rates

The joint effect of temperature and strain rate on hydrogen embrittlement properties of martensitic steel was investigated. At 50 °C, the elongation loss first increases and then, decreases with decreasing the strain rate. It was firstly reported that at the low strain rates, hydrogen embrittlement susceptibility was mitigated by temperature due to an increase in the hydrogen effusion to the surface of the material and the release of a significant amount of hydrogen before the yield point by temperature effect. At 25 °C, elongation loss increase with decreasing the strain rate, since in lower strain rates, the hydrogen can interact with mobile dislocations, which finally leads to H-induced fracture. The Kurdjumov-Sachs relationship between martensite and austenite was established, and prior austenite EBSD micrographs were obtained. The intergranular crack was observed which can be related to acting prior austenite grain boundaries as an H-induced crack path.

Texture Memory in Si-Mn and ODS Steels Observed In Situ by Pulsed Neutron and Synchrotron X-Ray Diffractions and Prediction by Double Kurdjumov-Sachs Relation: A Concept for Intense Variant Selection

Texture Memory in Si-Mn and ODS Steels Observed In Situ by Pulsed Neutron and Synchrotron X-Ray Diffractions and Prediction by Double Kurdjumov-Sachs Relation: A Concept for Intense Variant Selection

Austenite formation and mechanical behavior of a novel TRIP- assisted steel with ferrite/martensite initial structure

Abstract A newly developed TRIP-assisted steel with ferrite/martensite initial structure was designed to investigate the microstructural evolution during overall annealing cycle. The results were compared with the traditional cold-rolled TRIP steel. The characteristic of austenite formation from different initial microstructure was clarified based on combining the experiments and modeling. Results indicated that the new austenite obtained during intercritical annealing was fine-grained and uniformly-distributed. It comprised of acicular structure formed in pre-existing martensite (Mpre) matrix and blocky structure occurred at phase interfaces or prior austenite grain boundaries. The kinetics of austenite formation in specimens involving Mpre are similar in the early heating stage, and then decreased with decreasing Mpre content at temperatures greater than 770 °C. Both experimental and DICTRA results indicated that the austenite fractions of specimens with 90% and 100% Mpre fraction attained almost the maximum at the end of heating, and decreased with isothermal holding time during the intercritical annealing. Moreover, the retained austenite at interfaces of ferrite and pre-existing phase followed the Kurdjumov-Sachs (K–S) relationship with adjacent tempered martensite, while only part of the retained austenite had identical relationship with neighboring ferrite. In addition, the amount of retained austenite increased with Mpre content, although their stability against martensite transformation during tensile testing was similar and greater than that of cold-rolled specimen. As a consequence, the yield strength was increased from 590 MPa to 753 MPa and the tensile strength increased slightly from 995 MPa to 1046 MPa with Mpre fraction increasing from 50% to 100%, and the total elongation was ~30%, similar to the third generation advanced high strength steels.

Homogenisation effect on mechanical and pitting behaviour of 2205 Duplex Stainless Steel

We investigated the effect of homogenisation on the grain boundary character distribution and its influence on the mechanical behaviour and pitting resistance of an ultrafine grained 2205 duplex stainless steel after cold rolling and annealing. The results revealed that the cold rolling process induced the formation of a strain-induced martensite (SIM). Additionally, the Kurdjumov–Sachs relationship between the martensite laths and the metastable austenite was established through texture analysis. Furthermore, the homogenisation process enhanced the ductility of the steel. An increase in the pitting resistance after homogenisation was attributed to the grain boundary character distribution. This improvement corresponds to the increase in the ∑3 boundaries in the austenite and increase in the ∑17b boundaries fraction in the ferrite.

Acicular ferrite nucleation and growth in API5L-X65 steel submerged arc welded joints

In the present study, acicular ferrite nucleation and growth were investigated from a crystallographic point of view in API5L-X65 steel weld metals. It was observed that acicular ferrite was nucleated on inclusions in weld metal. The titanium-enriched layer around inclusions was characterised as the TiO phase by selected area diffraction patterns. A scanning electron microscope equipped with an electron backscatter diffraction technique was applied to investigate the crystallographic orientation relationship. The results showed that acicular ferrite lathes had a Baker–Nutting orientation relationship with the TiO layer on the inclusion surface in the nucleation of acicular ferrite. However, it is confirmed that the growth of acicular ferrite lathes depends on the deviation from the Kurdjumov–Sachs relationship between lathes and primary austenite.

Hardness, microstructure and texture of friction surfaced 17-4PH precipitation hardening stainless steel coatings with and without subsequent aging

Precipitation hardening stainless steel 17-4PH coatings were successfully deposited on the AISI 304 substrate via friction surfacing (FS) for enhancing the surface hardness. Through FS, the incoherent precipitates (Cu and CrN, 25–50 nm) originally existed in the as-received (AR) 17-4PH consumable rod (345 HV1) were re-dissolved into the martensitic matrix of the FSed 17-4PH coatings (400 HV1). The strain incompatibility caused by severe plastic deformation (SPD) in FS induced the formation of Cr-rich (Ni and Cu-depleted) δ-ferrite (1.8–2.7%) along the plastic flow direction with stress concentration. The shear deformation and dynamic recrystallization (DRX), as evidenced by the−J shearing and RTc’ recrystallization components in the FSed coatings, refined the grain size of the prior austenite. Increase in hardness of the FSed coatings was mainly attributed to the high dislocation density introduced by SPD. There was 4.8% acicular reversed austenite formed displacively, which possessed Kurdjumov-Sachs (K-S) relationship with surrounding martensite in the AR 17-4PH rod after solutionized treatment (ST) and aging (H900). The hardness of the AR 17-4PH after H900 was increased (439 HV1) because of the increment of dislocation density and micro-strain in the martensitic matrix introduced by the early-stage precipitation of coherent Cu. Owing to existence of δ-ferrite in the FSed coatings, some globular reversed austenite formed diffusively besides acicular ones after H900. There was more austenite (9.2%) in the FSed coating after H900 (456 HV1). The globular reversed austenite, which nucleated on the shear deformed martensitic grains in the FSed coatings, possessed {110}fcc//{120}bcc with adjacent δ-ferrite. SPD-induced dislocations in the FSed coating were annihilated during H900 leading to decrease in hardness but compensated by the precipitation hardening of coherent Cu. After FS, the micro-scale MnS and NbP2S8 inclusions in the AR 17-4PH were broken into nano-sized fragments which were dissolved into martensitic matrix during H900.

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.

Microstrain and boundary misorientation evolution for recrystallized super DSS after deformation

After deformation, super duplex stainless steel (DSS) was annealed at 1100 °C for a two-phase solid–solution region to avoid σ formation. Preferred orientations in δ and γ phases changed to (111)δ, (100)δ, and (101)γ from the rough random orientation of as-received super DSS. The kernel average misorientation (KAM) angle was less than 5° implying the grain boundary misorientation strain shifted to a higher value after cold rolling, which was consistent with the X-ray diffraction (XRD) microstrain results. After annealing, the misoriented angle was restored to a low angle of approximately 0.5°, which implied elimination of grain boundary misorientation strain when the annealing time increased from 15 to 30 min. The calculated microstrain based on XRD peaks demonstrated that the γ phase deformed before the δ phase at 50% CR. The recrystallized δ grains with slow growth and high staking fault energy (SFE) mainly showed dynamic recovery and a wide misorientation distribution of high angle grain boundary (HAGB) in the range between 15° and 58°, whereas the recrystallized γ grains with low SFE exhibited rapid dynamic recrystallization; the angles of the most HAGB grains were greater than 50°, because of the dissipation of the stored strain energy. This recrystallized austenite adhered to the Kurdjumov–Sachs relationship with ferrite.

Microstructures and hardnesses of AlCoCr0.5FexNi2.5 high entropy alloys with equal valence electron concentration

A group of high entropy alloys (HEAs) in equal valence electron concentration (VEC), AlCoCr0.5FexNi2.5 (x = 0.5, 1.5, 2.5, 3.5), were melted and prepared. The microstructures and hardnesses were analyzed after homogenizing treatment. The results indicated that all of the equal-VEC HEAs consisted of FCC phase and BCC phase, their orientations obey Kurdjumov–Sachs relationship. This work also implied that the atomic size difference (Δr), like VEC, is an important parameter influencing the phase selection and worth considering in the HEA design. Fe is an FCC phase stabilizer for AlCoCr0.5FexNi2.5 HEAs and the ordered FCC phase (L12) gradually disappear with the Fe addition increased. The increasing FCC phase and the decreasing BCC phase resulted in the decreased hardness of HEAs.

Development of Fe100-x(NiCoMn)x magnetostrictive alloys with good mechanical properties

In this work, a series of Fe100-x (NiCoMn)x (13 ≤ x ≤ 45, at.%) cast alloys are developed. The mechanical properties and magnetostriction are intimately related to the phase constituent and crystal structure of these alloys. In the body-centered-cubic phase region (x < 25), the magnetostriction and tensile strength are increasing monotonically with increasing x value, both of which drop when the face-centered-cubic phase appears. The crystallography relationship between the face-centered-cubic and body-centered-cubic phase satisfies the Kurdjumov-Sachs relationship. In Fe75(NiCoMn)25 polycrystalline alloy, a magnetostriction of 50 ppm with a tensile strength more than 1000 MPa is achieved.

Hydrogen-Induced Cracking of Laser Beam and Gas Metal Arc Welds on API X65 Steel

Hydrogen-induced cracking (HIC) behavior was analyzed for gas metal arc (GMA) and laser beam (LB) welds on American Petroleum Institute (API) X65 steel. The GMA welds consisted of acicular ferrite (AF) with some widmanstatten ferrite (WF), while the LB welds had bainitic ferrite (BF) with some AF. The welds and heat affected zone (HAZ) of GMA exhibited a hardness of 220-250 HV, while those of the LB had a hardness of 230-290 HV. The LB welds and HAZ exceeded the hardness limit of 250 HV for pipeline steel, defined by the National Association of Corrosion Engineers (NACE) standard. Slow strain rate tests (SSRT) were performed in air and in-situ with hydrogen to observe HIC behavior. The ultimate tensile strength of the GMA welds decreased by 13%, while that of the LB welds decreased by 16% after hydrogen charging. Both welds showed a dimple fracture in the center and quasi-cleavage fracture along the edges. When austenite transforms to BF, it is known to grow along the directions of twins or the Kurdjumov-Sachs relation. These directions are strongly related to the coincidence site lattice, specifically Σ3 and Σ13b, and have good crack resistance. Because of this grain boundary characteristic, the LB welds with a BF microstructure showed good HIC behavior compared to the GMA welds. (Received May 7, 2019; Accepted June 12, 2019)