Substructure Evolution Research Articles

Unusual substructure evolution and post-dynamic recrystallization effects on flow softening mechanism in a [formula omitted]-free Co-base superalloy

Motivation for this study was the absence of any report of stacking fault energy (SFE) estimate and post-dynamic recrystallization (PDRX) effects at temperatures >1000 °C in γ′-free Co-base superalloys. Therefore, the substructural evolution during hot compression of Co-22Cr-22Ni-14W-2Fe-0.1C superalloy (H188) at temperatures (1050–1150 °C) and at 10−1 s−1 was studied. Unusual substructural features viz. extrinsic stacking faults, dense dislocation walls, microbands, and subgrains were observed with an increase in strain (0.01–0.70), even at 1100 °C. Denoised electron backscattered diffraction data revealed an enormous increase in overall recrystallized (PDRX+DRX) fraction from 30 to 75% with increase in temperature from 1050 to 1100 °C at ε=0.7. Comprehensive microstructural analyses suggest that continuous dynamic recrystallization is the dominant flow softening mechanism in low SFE (estimated to be 14 ± 2.0 mJ/m2) alloy H188. However, the PDRX effects triggered by the presence of higher stored energy and recrystallized nuclei contributing to a massive increase in recrystallized fraction at 1100 °C.

Cavitation erosion behaviors and damage mechanism of Ti-Ni alloy impacted by water jet with different standoff distances

The cavitation erosion of Ti-Ni alloy is explored on a cavitating liquid jet with different standoff distances. The cavitation erosion characteristics of Ti-Ni alloy specimens are analyzed, and the influences that different standoff distances of the water jet have on cavitation erosion are explored. The results of cavitation erosion outcomes under different standoff distances of 6 mm, 8 mm, 10 mm, 12 mm, 14 mm and 16 mm show slight deformation, grain boundary cracking and formation of initial micro-pits, pits of maximum depth around 50 μm, more pits of maximum depth around 80 μm with micro-cracks, obvious plastic deformation traces with some pits, bulges and slip bands, respectively. When the Ti-Ni alloy is impacted by water jet with a standoff distance of 12 mm for 4 hrs, the surface microstructure undergoes substructural evolution and new fine grains form, with obvious stress concentration and dislocation accumulation appearing on the subsurface.

Lath martensite substructure evolution in low-carbon microalloyed steels

Lath martensite substructures in as-quenched plain carbon steels exhibit dislocation-like contrast in the transmission electron microscope. More recent observations reported internal twins and nanoscale auto-tempered intra-lath carbides as additional lath substructures in ultra-low-C binary Fe–C steels. Modern microalloyed steels often have similar ultra-low C contents besides microalloying elements like Ti, Nb or V and, more recently, Mo, to achieve high strength, toughness and weldability. Nonetheless, little is known about the lath substructure evolution in the as-quenched state of microalloyed steels. This study investigates the hierarchical martensite substructure evolution post-quenching of microalloyed Nb and NbMo steels with 0.1 wt% C. Hierarchical microstructure characterization was done using scanning and transmission electron microscopy, and electron backscatter diffraction methods including parent grain reconstructions with MTEX. Thermokinetic simulations using MatCalc to determine the carbide evolution during auto-tempering were corroborated with site-specific transmission electron microscopy. Mo addition led to lowering of the martensite start temperature, yet the Nb steel showed a finer hierarchical microstructure. Finer laths with in-lath dislocations, short and long twins, and lath boundary decoration of carbides were found in the Nb steel. Conversely, laths in the NbMo were wider, with frequent intra-lath auto-tempered precipitates in the vicinity of dislocations, without twins.

The influence of parent austenite characteristics on the intervariant boundary network in a lath martensitic steel

The influence of the parent austenite deformation state on the intervariant boundary network (i.e., population, plane orientation, and connectivity) of a lath martensitic microstructure was investigated using conventional EBSD mapping and five-parameter boundary analysis approach along with quantification of boundary connectivity using homology metrics. The lath martensite largely revealed a bimodal misorientation angle distribution, closely matched with the Kurdjumov–Sachs (K–S) orientation relationship. The application of deformation significantly changed the distribution, gradually reducing the intensity of the 60° misorientation angle peak. This was largely ascribed to substructure development within the parent austenite upon deformation, which stimulates particular variant/s having a habit plane (011)α′ closely parallel to the primary (111)γ and/or secondary (1overline{1 }1)γ slip plane. The interaction of these variants eventually promoted specific intervariant boundaries (e.g., 60°/[111], 10.5°/[011], and 49.5°/[110]) at the expense of 60°/[011]. The application of deformation in the parent austenite did not change the intervariant boundary plane character distribution, which mostly exhibited an anisotropic character terminated on {110} planes because of the displacive nature of the martensitic transformation. However, the extent of anisotropy progressively decreased with increasing strain in the austenite prior to transformation. The grain boundary network connectivity was markedly altered due to the local variant selection induced by the deformation. Deformation in the austenite regime generally decreased the connectivity of boundaries having a {110} plane orientation. The intervariant boundaries with the {110} twist character also displayed a similar trend, though the connectivity of {110} tilt boundaries progressively enhanced with increasing strain. The former was closely matched with a decrease in the population of 60°/[110] intervariant boundaries with the strain. The current findings suggest that the intervariant boundary network of lath martensite can be manipulated through changes in the parent austenite deformation state which, in turn, can be used to enhance key final product properties such as toughness.Graphical abstract

Influence of the Temperature-Strain Parameters on the Structure Evolution and Carbide Transformations of Cr-Ni-Ti Stainless Steel

Influence of strain accumulation schedule during hot rolling, under the thermomechanical control process (TMCP) on the structure evolution and carbide transformations of Cr-Ni-Ti austenitic stainless steel, is studied. The cellular, fragmented dislocation substructure and dynamically recrystallized (DRX) structure are observed in the steel with different strain accumulation schedules. It was found that the strain accumulation schedule, especially fractionality, affects the work-hardening and softening behavior quite significantly. The role of the strain accumulation schedule on the fragmented substructure and DRX structure evolution as well as carbide transformations and the relationship between the microstructure changes due to TMCP and the mechanical properties of studied steel, involving the recent ideas of the physics of large plastic strains, are considered.

Tailoring the tempered microstructure of a novel martensitic heat resistant steel G115 through prior cold deformation and its effect on mechanical properties

In this paper, the influences of prior cold deformation and tempering time on the precipitation behavior, substructure evolution, and mechanical properties of a novel G115 martensitic heat resistant steel were systematically investigated. It was found that the deformation reduction and tempering time had significant influences on the precipitation behavior of secondary strengthening particles, the recovery of dislocations, and the formation of subgrains, which played a crucial role in determining the tensile properties and micro-hardness. When the deformation reduction was lower than 20%, the increase of reduction could induce more dislocations or defects, which provided additional nucleation sites for M23C6 particles and obstructed the motion of dislocations and sub-boundaries, thus enhancing the tensile strength. However, when the deformation reduction was higher than 20%, the excessive reduction could accelerate the coarsening of M23C6 particles by providing more diffusion channels, thus weakening the pinning ability for boundaries and leading to substructure instability. For the G115 steel with a deformation reduction of 20%, an optimal combination of mechanical properties with an average hardness of 283.4 ± 8 HV, a tensile strength of 1102 ± 6 MPa, a yield strength of 783.6 ± 31 MPa, and a considerable ductility of 19 ± 2% could be achieved after tempering for 60 min.

The effect of microstructure evolution on the ratchetting-fatigue interaction of carbide-free bainite rail steels under different heat-treatment conditions

Low-cycle fatigue experiments emphasizing the ratchetting are performed on the carbide-free bainitic (CFB) rail steel under different heat-treatment conditions. The effects of microstructure and retained austenite (RA) on the ratchetting-fatigue interaction are investigated through macroscopic and microscopic characterizations. The results indicate that the CFB rail steel exhibits a non-saturated and strain amplitude-dependent cyclic softening feature and shows different ratchetting behaviors and failure modes at various stress levels. Ratchetting promotes the development of stable dislocation substructure and the transformation of blocky RA. The fatigue resistance of 1380 grade CFB steel under the online controlled cooling conditions is better than that of 1280 grade CFB steel under the air-cooling condition.

Interactive transformation mechanisms of multiple metastable precipitates in a Si-rich Al-Mg-Si alloy

ABSTRACT The precipitate evolution mechanism of the Al-Mg-Si alloys with a Si-rich composition have been a long-term challenge due to the complex transformation paths among multiple precipitates. In the present work, the interactive transformation process of multiple metastable precipitates in a Si-rich Al-Mg-Si alloy were thoroughly studied by atomic resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), in-situ heating TEM and first-principles calculation. It was revealed that the β″ phase transforms simultaneously to various types of precipitates in the over-aged stage. Two different evolution paths are proposed. Firstly, the β″ phase transforms directly to the U1, U2 and β′ phases. Secondly, the U2 phase, as an intermediate phase, transforms to the U1, β′ and B′ phases during the prolonging aging. The formation and evolution of substructures, including the QP lattice and Al–Si columns, are the key for structural transformation of precipitates. During the severe over-aging, the evolution of precipitates is governed by the dissolution of small precipitates (β′ and B′ phases) and the growth of the large ones (U1 phase). These results provide scientific basis for both property improvement of the commercial Al-Mg-Si alloys and development of new 6xxx aluminum alloys.

The correlation of c-to-a axial ratio and slip activity of martensite including microstructures during thermomechanical processing of Ti–6Al–4V alloy

This study deals with the correlation of the “c-to-a” ratio of ά-martensite and the activity of basal/secondary slip systems during thermomechanical processing of Ti–6Al–4V alloy. Two martensite including microstructures (dual phase α+ά and full ά martensite) were prepared and deformed in tensile mode at 300 °C and 600 °C. The presence of ά-martensite caused the reduction of the c-to-a ratios compared to that of the as-received α+β starting microstructure. Owing to the elemental partitioning, the amount of axial ratio reduction was considerable in the case of dual phase α+ά microstructure and caused activation of pyramidal slip during low temperature tensile deformation. This promoted the wavy configuration of dislocation and led to intensified substructure development. At higher deformation temperature, where the martensite phase was almost completely converted into the fine α+β phase through reverse transformation, the c-to-a ratio reached ∼1.599 increasing the critical resolved shear stress of the secondary slip systems.

Characterization of deformation substructure evolution in metastable Fe49Mn30Co10Cr10B1 interstitial high entropy alloy

Interstitial alloying has been utilized to overcome the strength-ductility trade-off in high entropy alloys (HEAs). In this work, boron-doping in a typical metastable FeMnCoCr HEA is chosen to investigate deformation substructure evolution by using electron backscatter diffraction and high-angle annular dark field scanning transmission electron microscopy, to understand how interstitial atoms tune the deformation modes. Boron-doping facilitates the formation of Cr2B particles, which can effectively decrease the grain size and retard the thermal-induced ε phase. Under the initial deformation (10% rolling reduction), the coexistence of dislocations slip in γ phase and of {10 1‾ 2} twins in ε phase is the dominant deformation modes. With the increasing strain (20% rolling reduction), the interaction of ε lamellas is significantly strengthened, which is favor of the formation of intersecting ε products. The occurring of intrinsic stacking faults in γ phase act as the nucleation sites for ε phase. Moreover, two types of dislocations (1/2<a+c> and <a>) are revealed in ε phase. At the ultimate deformation stage (40% rolling reduction), due to the saturation of γ → ε transformation, the coexistence of dislocations slip and twins ({10 1‾ 1} and {101‾ 3}) in ε phase can further accommodate strain, and the saturated ε phase is dominated by one persisting ε variant. Accordingly, the differences in deformation substructures of metastable FeMnCoCr HEA with or without B-doping are intrinsically arising from boron interstitial alloying effect.

A hybrid precast concrete stiffened wall substructure for residential construction on expansive soils

The use of precast systems reduces material wastage, decreases effect of weather impacts, cuts down unexpected costs, lessens skilled labour dependence and minimises construction hazards. However, the full potential of precast construction is yet to be realised in part due to most advancements being focused on its superstructure. Development of precast substructures or foundation systems for lightweight buildings, such as residential structures, needs to consider functionality and susceptibility to damage induced by the swelling movement of expansive soils causing significant structural damage and financial loss. Due to this, the main objective of this paper is to assess the ultimate strength and serviceability of a developed precast concrete wall substructure using full-scale laboratory testing and coupled finite element modelling, respectively. This study evaluates a developed hybrid precast concrete stiffened wall substructure using laboratory experiments and numerical simulations to determine its ultimate strength and serviceability, respectively. The innovative design of the developed precast substructure satisfied functional requirements for residential construction application. The slab and rib portions of the developed substructure had an acceptable ultimate strength with failure observed at 4.5 times the factored load equivalent and 2.6 times the factored load equivalent, respectively. Moreover, the developed precast system due to its configuration had reduced the active depth of expansive soils prone to moisture changes, the vertical soil movement around the prefabricated footing, and the deformation experienced by the system.

Contribution of dislocation substructures developed during cold rolling to the formation of rolling textures in Al–Mg alloys

Abstract In order to investigate the mechanism of the development of rolling textures in Al –Mg alloys, hot bands of high-purity Al, Al-3 wt. %Mg and Al-5 wt. %Mg alloys were solution-treated at 450 °C and cold-rolled by varying rolling reductions up to 97%. Rolling textures of these specimens were investigated with the orientation distribution function analysis. Comparisons of the results of the texture analysis with those of the hardness measurement and transmission electron microscopy observations revealed that, in all specimens, the texture development occurred mainly at the later half of work hardening, i. e., at rolling reductions above 80 %, where dislocation substructures were well developed. In pure Al, in which well defined, elongated cell structures were developed, {112}&lt;111&gt; and {123}&lt;634&gt; components developed remarkably, whereas in the Al-5% Mg alloys, in which only dense dislocation tangles and ill defined small cells were developed, the development of {112}&lt;111&gt; and {123}&lt;634&gt; components was strongly suppressed. All these observations thus suggested that the development of rolling textures was strongly affected by the development of dislocation substructures, which occurred concurrently during rolling, and that the formation of elongated cell structures was more favorable for the development of rolling textures. In cold rolling, elongated cell structures may be the final form of dislocation substructures, providing both mechanical stability and orientation stability.

Experimental and simulated investigation of the deformation behavior and microstructural evolution of Ti6554 titanium alloy during an electropulsing-assisted microtension process

Titanium alloy formability can be noticeably improved by electrically assisted processing. In this study, uniaxial electropulsing-assisted microtensile (EAMT) tests were performed on a new high strength-ductility near β titanium alloy Ti–6Cr–5Mo-5 V–4Al (Ti6554) at electric current densities (ECDs) from 10 to 70 A/mm2. To investigate the Joule heating, mechanical response and microstructural evolution of the Ti6554 alloy during the EAMT process, the true stress strain curves, temperature, and microstructural evolution were analyzed by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). A fully coupled electrothermomechanical finite element model (ETM-FEM) was developed to predict the stress and temperature and to decouple the thermal and athermal effects. The experimental results showed that the dynamic recrystallization (DRX), substructure and texture evolution can be strengthened when the ECD is less than 50 A/mm2, but that the dislocation density decreases with increasing ECD; the α precipitation and propagation of microcracks at higher ECDs can result in premature failure. Without considering the athermal effect, the predicted stress deviates from the tested data, but the predicted temperature agrees better. To elucidate the electroplasticity mechanism, the thermal expansion effect was qualified, and other athermal effects, including the skin effect, pinch effect, interaction between electrons and dislocations and phonon frequency, on the flow stress were discussed.

Microstructure evolution, mechanical properties and high temperature deformation of (TiB + TiC)/Ti–3.5Al–5Mo–6V–3Cr–2Sn–0.5Fe titanium alloy

The addition of trace (TiB + TiC) reinforcements could minimize the deformation steps and decreases its costs according to recent study. In present work, the hot deformation behavior and microstructure evolution were studied via the isothermal hot compression at different temperature and strain rate. Based on the DMM model, the activation energy Q and processing map were obtained. Subsequently, two billets were hot-rolled and heat-treated, to study the microstructure and tensile properties of the alloy. Results revealed that the flow behavior of the (TiB + TiC)/Ti–3.5Al–5Mo–6V–3Cr–2Sn–0.5Fe alloy was similar with other near β titanium alloy. When deformation in β region, the deformation mechanism was associated with cDRX by lattice rotation and dynamic recovery; when deformation in (α + β) region, the deformation mechanism was associated with substructure evolution, such as DRX and dynamic recovery. The lower strain rate guarantees abundant time for microstructure transformation, while the higher strain rate leads local flow instability and cracks. The activation energies Q is 264.1 kJ/mol in (α + β) region and 181.8 kJ/mol in the β region. According to the processing map, the optimum deformation windows are 770–800°C and 850–880°C, 0.001 s −1 . In addition, the microstructure of 770R + SAT is characteristic of duplex microstructure with a certain amount of α p , considerable precipitation of fine acicular α s and continuous α GB , which is achieved a UTS of 1414 MPa with a poor ductility. Therefore, the (TiB + TiC)/Ti–3.5Al–5Mo–6V–3Cr–2Sn–0.5Fe alloy shows excellent deformability and higher strength, which is possible to achieve both high economy (less deformation step) and advancement (high mechanical properties due (TiB + TiC) reinforcements). • The deformation behavior of was associated with grain boundary sliding, dynamic recovery and dynamic recrystallization. • The UTS of 770R + SAT achieved 1414 MPa, which was slight higher than other Ti55531 or Ti1023 titanium alloy. • The fine acicular αs achieved high strength while the continuous αGB decrease the ductility. • The (TiB + TiC)/Ti–3.5Al–5Mo–6V–3Cr–2Sn–0.5Fe alloy was possible to achieve both high economy and advancement.

Development of a 3-legged jacket substructure for installation in the southwest offshore wind farm in South Korea

A suitable substructure for Offshore Wind Turbines plays an important role since it can efficiently reduce the financial costs of wind power projects. According to the track record of the Korean offshore wind farms, the 4-Legged Jacket (4LJ) substructure is the most utilized system; however, this design is still limited, specifically in the economic aspects. Thus, this study aims to design a more cost-effective substructure to enable large-scale deployment for Korean offshore projects. To achieve this objective, the new 3-Legged Jacket (3LJ) substructures together with various bracing topological forms (i.e., Pratt, Warren, and X-bracing) are developed. Results show that under the environment loads, dynamic responses obtained from the developed 3LJs are almost independent to the loading directionality; while there is a strong polarization in the case of 4LJ. Among the three cases of the 3LJs, the X-type topological form has the highest flexural stiffness together with the largest manufacturing and fabrication costs, while the Warren bracing system reaches ultimate strength earlier than the Pratt system. Therefore, the 3LJ substructure with a Pratt bracing system is suggested as a good alternative to the existing 4LJ system for Korean offshore wind farms, with reductions of up to 21% and 25% in total weight and number of weld joints, respectively.

Influence of crystallographic orientation on the recrystallization of pure tantalum through microstructure-based estimation of the stored energy

Plastic deformation and static recrystallization of pure tantalum are studied using Electron BackScatter Diffraction (EBSD) technique. Recrystallization kinetics are discussed in light of stored energy estimation from EBSD data acquired on deformed microstructures. Characterization of deformed state reveals the influence of crystallographic orientation on the plastic deformation. Dislocation substructures are formed in the γ-fiber grains and almost no substructures are observed in the θ-fiber grains. Subsequent recrystallization directly inherits this orientation dependence of deformed state. Nucleation is promoted in the γ-fiber grains because of substructure development whereas nucleation is more sluggish to occur in the θ-fiber grains. Thus, a microstructure composed mostly of γ-fiber grains recrystallizes faster than a microstructure with a less strong texture, despite a strain nearly three times lower. At the polycrystalline scale (step size of 1.20 μm in the present case), recrystallization kinetics are better described with stored energy estimated through substructures than through dislocation density. At the substructure scale (step size of 90 nm), Geometrically Necessary Dislocation (GND) density seems to be correctly estimated; still recrystallization kinetics cannot be accounted for by this density because it does not account for the substructure formation.

Influence of (Sub) Structure Development within Rotary Swaged Al-Cu Clad Conductors on Skin Effect during Transfer of Alternating Current.

The nature of alternating current transfer via metallic materials is specific, since the current density tends to be inhomogeneous across the cross-section of the conductor and the skin effect tends to occur. However, the influence of this effect on the behaviour of the conductor can be optimized via the design and fabrication procedures. The study presents innovative design of an Al–Cu clad conductor, which is supposed to affect favourably the influence of the skin effect. The clad conductors of various diameters (20 mm, 15 mm, and 10 mm) were fabricated via rotary swaging at room temperature, and their electric characteristics were subsequently examined both experimentally and via numerical simulations. Structure analyses performed to document the effects of the swaging technology on the development of substructure and characteristic structural features were carried out by scanning electron microscopy (electron backscatter diffraction analyses), and transmission electron microscopy. The results showed that the design of the composite has a favourable effect on decreasing the power losses during alternating current transfer and that the substructure development affected favourably the electric resistance of the conductor. The highest electric resistance was measured for the composite conductor with the diameter of 20 mm (1.8% increase compared to electric resistance during transfer of direct current). This value then decreased to 0.6%, and 0.1% after swaging down to the diameters of 15 mm, and 10 mm; the 10 mm composite featured the finest grains, partially restored structure, and texture randomization compared to the 20 mm and 15 mm composites. Manufacturing of the clad composite via rotary swaging imparted advantageous combinations of both the electric and mechanical properties, as swaging also introduced increased microhardness.

Additively manufactured AlSi10Mg ultrathin walls: Microstructure and nano-mechanical properties under different energy densities and interlayer cooling times

For additive manufacturing (AM) applications of lightweight aluminum alloy parts such as radiators, it is necessary to understand the relationship between internal microstructure and local mechanical properties. The microstructure characteristics and its formation mechanism of AlSi10Mg ultrathin walls fabricated by selective laser melting (SLM) under different energy densities and interlayer cooling times were investigated, and its evolution regularity with energy density was also analyzed. The nano-mechanical properties of different regions on the ultrathin-walled cross-section and their responses with energy density were also explored. The main results show that non-banding regions and banding regions distributed alternately like sandwiches are formed in ultrathin walls, which have fine solidified cellular structure and columnar dendrite network, accompanied by nano-scale substructure formed in grains. Although the microstructure characteristics and nano-mechanical properties of ultrathin walls are insensitive to interlayer cooling time, the single-track energy density significantly affects the coarsening of solidification structure and its substructure evolution. With the increase of single-track energy density, the interaction between various strengthening mechanisms related to the evolution of microstructure comprehensively affects the nano-hardness of different macro-areas in ultrathin walls, which follows the trend of increasing first and then decreasing, and higher nano-hardness can be obtained when the dimensionless single-track energy density is between 10 and 20. The findings of this work are beneficial to optimize the SLM process of AlSi10Mg alloy to manufacture ultrathin-walled parts with ideal microstructure and mechanical properties for potential applications in aerospace, automotive and other fields.

Substructure Evolution in Deformed Austenite of 904L Stainless Steel and Its Effect on Macro Flow Stress

Substructure Evolution in Deformed Austenite of 904L Stainless Steel and Its Effect on Macro Flow Stress

Deformation mechanisms that explain the combination of high plasticity and strength in steels wires with extreme carbon content

The alloy design concepts of high performance steels, involve the knowledge on the stability of carbides associated with nanoscale phenomena present in the structure as result of the deformation suffered during the thermomechanical processes. The understanding of the nanoscale phenomena open new frontiers to understand mechanical behaviours in the steel, not totally clarified until now. The paper discuss the structure evolution during wire drawing of pearlitic steels and the impact of cementite stability on different nanoscale structure phenomena, which explain the specific mechanical behaviour to achieve ultra-high strength. Results on the kinetic and stability of carbides predicted on the base of thermodynamic simulation are correlated with thermal analysis tests results (dilatometry and differential scanning calorimetry) carried out by different authors in order to understand the cementite (Fe3C) dissolution during plastic deformation. In addition, information obtained by traditional and no traditional microscopy techniques and X ray diffraction complete the study and allows to understand more deeply the structure evolution, including nanoscale phenomena that justify the mechanical behaviour during wiredrawing and the final strength level. The dislocation substructure evolution together with the cementite dissolution during sever plastic deformation which leads to a steadily increase of the strain hardening is clarified. Copyright © 2018 VBRI Press.