Showing papers by "R.D.K. Misra published in 2017"

Microstructure-mechanical property relationship and austenite stability in medium-Mn TRIP steels: The effect of austenite-reverted transformation and quenching-tempering treatments

Abstract: In the present study, we fundamentally explore the reasons underlying differences in mechanical properties in hot-rolled 0.2C-1.6Al-6.1Mn-Fe TRIP steels subjected to different heat treatments. Comparing with austenite reverted transformation annealing (ART) process, quenching and tempering (QT [ART (UTS: 885–945MPa, TEL: 13–28%)]. In the ART process, long time annealing led to excessive C and Mn enrichment in austenite, which rendered austenite too stable and deteriorated TRIP effect. Furthermore, long time annealing reduced dislocation density and led to low work-hardening rate. The Q&T process enabled appropriate enrichment of elements and hence desired stability for significant TRIP effect to be observed. Thus, the steel quenched from 625 °C exhibited best combination of mechanical properties (UTS: 1038 MPa, TEL: 42%, UTS×TEL: 43.6 GPa%) because of significant contribution of TRIP effect and high dislocation density in austenite.

Deformation behavior in cold-rolled medium-manganese TRIP steel and effect of pre-strain on the Lüders bands

Abstract: Deformation behavior was studied in cold-rolled 0.2C-1.6Al-6.1Mn-Fe transformation-induced plasticity (TRIP) steel subjected to intercritical annealing. The steel intercritically hardened at 650 ℃ exhibited excellent mechanical properties, and the excellent ductility was primarily associated with the discontinuous TRIP effect. Moreover at 650 ℃, the formation of Luders bands was associated with TRIP effect and cooperative dislocation glide. The length of Luders strain was gradually reduced with increasing pre-strain, and was eventually eliminated when the pre-strain was increased to 10%. The increased average stability of retained austenite and increased dislocation density in ferrite induced by pre-strain was responsible for decrease and ultimate elimination of Luders bands. While in steel intercritically annealed at 600 ℃, ferrite and austenite was predominantly deformed, which was responsible for poor work hardening rate and inferior tensile properties.

Effect of two-step intercritical annealing on microstructure and mechanical properties of hot-rolled medium manganese TRIP steel containing δ-ferrite

Abstract: The microstructure-properties relationship, work-hardening behavior and retained austenite stability have been systematically investigated in a hot-rolled medium manganese transformation-induced-plasticity (TRIP) steel containing δ-ferrite subjected to one-step and two-step intercritical annealing. The steel exhibited tensile strength of 752 MPa and total elongation of 52.7% for one-step intercritical annealing at 740 °C, tensile strength of 954 MPa and total elongation of 39.2% in the case of intercritical quenching at 800 °C and annealing at 740 °C. The austenite obtained by two-step annealing mostly consists of refined lath structures and increased fraction of block-type particles existing at various kinds of sites, which is highly distinguished from those characterized by long lath morphology and small amounts of granular shape in one-step annealed samples. In spite of a higher C and Mn content in austenite and finer austenite laths, two-step annealing can lead to an active and continuous TRIP effect provided by a mixed blocky and lath-type austenitic structure with lower stability, contributing to a higher UTS. In contrast, one-step annealing gave rise to a less active but sustained TRIP effect given by the dominant lath-like austenite having higher stability, leading to a very high elongation. The further precipitation of vanadium carbides and the presence of both dislocation substructure and fine equiaxed grain in ferrite matrix facilitate the increase of yield strength after double annealing.

Cellular response of Staphylococcus aureus to nanostructured metallic biomedical devices: surface binding and mechanism of disruption of colonization

Abstract: Metallic materials with nanometre-sized grains provide surfaces that are different from conventional polycrystalline materials because of large proportion of grain boundaries. We describe here, the combination of materials science and engineering involving nanocrystallization and alloying with copper and cellular biology to develop our understanding of inhibition of bacterial colonization and biofilm formation on nanostructured surfaces that are significantly different from conventional coarse-grained (CG) structures. A novel process referred as phase reversion was used to induce nanostructuring in the conventional CG metallic material. Quantitative and qualitative studies of bacterial colonization indicated that the combined effect of nanostructuring and alloying with copper effectively influenced the degree of bacterial colonization, a behaviour related to passive film on the surface. The high density of grain boundaries in nanocrystalline materials provided increased diffusion path for Cr and Cu atoms ...

Deformation behavior of high yield strength – High ductility ultrafine-grained 316LN austenitic stainless steel

Abstract: The concept of phase reversion annealing involving extensive cold deformation of metastable austenite to strain-induced martensite, followed by annealing at slightly elevated temperature, developed by Misra’s group in recent years [2] , [3] , [4] , [5] was used to obtain ultrafine-grained structure in a 316LN austenitic stainless steel. The primary objective of this study is to elucidate the deformation mechanisms. The study suggested that an average austenite grain size in the ultrafine regime of ~2.0 μm can be obtained using the experimental conditions described in the study, which is ~6 times finer than the grain size of commercial 316LN steel. The grain refinement led to high yield strength in ultrafine-grained 316LN steel without any significant compromise in ductility. The high plasticity of ultrafine-grained 316LN steel is attributed to the presence of mechanical twins.

High strength-toughness combination of a low-carbon medium-manganese steel plate with laminated microstructure and retained austenite

Abstract: Three different grain structures of low-carbon medium-manganese steel were prepared through appropriate controlled rolling process. The laminated microstructure with a strong //rolling direction (RD) fiber texture was characterized by ultra-fine elongated ferrite, retained austenite and martensite phase arranged alternately along the RD. The steel with equiaxed grain structure exhibited a relatively low tensile strength of 960 MPa and an extremely poor low-temperature toughness of ~ 8 J at −196 °C. An enhanced upper shelf energy (> 450 J) and low-temperature toughness (~ 105 J at −196 °C), as well as an improved tensile strength (1145 MPa) was obtained in the steel with laminated microstructure. The laminated microstructure enabled the steel to be significantly stronger and tougher along the RD, which contributed to the high tensile strength to some extent. It is concluded that the combined effect of the ultra-fine elongated laminated microstructure, the possible interface decohesion and the existence of numerous {001} cleavage planes resulted in the occurrence of delamination. The delamination fracture enhanced the upper shelf energy mainly by promoting crack branching along the RD and thus suppressing crack propagation along the v-notch direction, which finally resulted in greater plastic deformation and significant increase in absorbed energy. Besides delamination toughening, transformation-induced plasticity (TRIP) effect of metastable retained austenite is believed to be responsible for the high cryogenic toughness, which can release stress concentration of crack tips and thus blunting cracks propagation.

Cellular response of osteoblasts to low modulus Ti‐24Nb‐4Zr‐8Sn alloy mesh structure

Abstract: Titanium alloys (Ti-6Al-4V and Ti-6Al-7Nb) are widely used for implants, which are characterized by high elastic modulus (∼110 GPa) with (α + β) structure and that may induce undesirable stress shielding effect and immune responses associated with the presence of toxic elements. In this regard, we have combined the attributes of a new alloy design and the concept of additive manufacturing to fabricate 3D scaffolds with an interconnected porous structure. The new alloy is a β-type Ti-24Nb-4Zr-8Sn (Ti2448) alloy with significantly reduced modulus. In the present study, we explore the biological response of electron beam melted low modulus Ti2448 alloy porous mesh structure through the elucidation of bioactivity and osteoblast functions. The cellular activity was explored in terms of cell-to-cell communication involving proliferation, spreading, synthesis of extracellular and intracellular proteins, differentiation, and mineralization. The formation of fine apatite-like crystals on the surface during immersion test in simulated body fluid confirmed the bioactivity of the scaffold surface, which provided the favorable osteogenic microenvironment for cell-material interaction. The combination of unique surface chemistry and interconnected porous architecture provided the desired pathway for supply of nutrients and oxygen to cells and a favorable osteogenic micro-environment for incorporation (on-growth and in-growth) of osteoblasts. The proliferation and differentiation of pre-osteoblasts and their ability to form a well mineralized bone-like extracellular matrix (ECM) by secreting bone markers (ALP, calcium, etc.) over the struts of the scaffold point toward the determining role of unique surface chemistry and 3D architecture of the Ti2448 alloy mesh structure in modulating osteoblasts functions. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 859-870, 2017.

Biodegradable hydrogel-based biomaterials with high absorbent properties for non-adherent wound dressing.

Abstract: Dressing materials involve conventional gauzes and modern materials such as hydrogels and foam-based biomaterials. Although the choice of dressing material depends on the type of wound, a dressing material is expected to be non-cytotoxic. Additionally, moist dressing is considered appropriate to accelerate epithelialisation, while dry dressing may cause tissue damage during removal. An ideal dressing material is expected to provide a moist environment and degrade and release the drug for faster wound healing. Thus, we have designed a hydrogel-based biodegradable dressing material to provide the moist environment with no cytotoxic effect in vitro. The design of the hydrogel involved alginate-collagen reinforced with whisker cellulose derived from cotton. The hydrogel was prepared via amide linkage in the presence of 1-ethyl-(dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysulfosuccinimide (NHS), followed by divalent cationic cross-linking of alginate and hydrogen bonding with cellulose. The high water retention capability of the hydrogel enables a moist environment to be maintained in the wounded area. The constituents of the hydrogel provided a microenvironment that was suitable for cell proliferation in the vicinity of the hydrogel but inhibited cell attachment on it. The MTT assay results indicated a higher fibroblast proliferation and viability in the presence of the hydrogel.

Correlation between deformation behavior and austenite characteristics in a Mn-Al type TRIP steel

Abstract: We investigate here the correlation between deformation behavior and retained austenite characteristics in a medium-Mn transformation-induced plasticity (TRIP) steel. The sample was characterized by a dual-phase microstructure consisting of ultra-fine grained ferrite and retained austenite with relatively high mechanical stability after annealing at 700 °C for 5 h. Both lath-like and blocky (granular) retained austenite with volume fraction of 38.7% and relatively inhomogeneous grain size was obtained. The tensile specimen exhibited outstanding mechanical properties with yield strength of 745 MPa, tensile strength of 1005 MPa and total elongation of 46%, as well as a distinctive work hardening behavior. The in-depth investigation on deformation behavior demonstrated that the transformation mechanism of retained austenite during deformation was strain-induced and the yielding behavior was controlled mainly by the deformation of soft ferrite phase. As to the multi-peak work hardening behavior, it is believed to be attributed to the inhomogeneous and discontinuous occurrence of TRIP effect, which resulted from the inhomogeneous stability of retained austenite. Moreover, the orientation of retained austenite (Schmid factors) was proved an important factor in determining the mechanical stability of retained austenite upon deformation, in addition to the heterogeneity of grain size. These two factors together resulted in the inhomogeneous stability of retained austenite.

Evolution of microstructure and crystallographic texture of microalloyed steel during warm rolling in dual phase region and their influence on mechanical properties

Abstract: High strength and high toughness steels can be developed by warm caliber rolling in ferrite region. However, high deformation resistance limits its application. In the present study, warm rolling was applied to plate rolling which is more suitable for industrial production to develop high strength and high toughness steels. To reduce deformation resistance, warm rolling was carried out in dual phase region. We elucidate here the evolution of microstructure and crystallographic texture and their influence on mechanical properties of microalloyed steel subjected to warm rolling. The study suggests that high strength and high toughness can also be obtained by warm rolling in the dual phase region. Elongated ultrafine microstructure and intense α-fiber texture component and γ-fiber texture component can be obtained through warm rolling. The main mechanism of microstructure evolution during warm rolling was dynamic recovery. Reducing warm rolling temperature can refine grain size, enhance α-fiber texture component and weaken γ-fiber texture component. Warm rolling can greatly enhance strength by ~64–158 MPa compared to the conventional controlled rolling (CR) process, and the warm-rolled plates had high elongation in spite of high strength. The toughness was improved because of grain refinement and delamination. Delamination can induce ductile fracture at low temperature, and delay the occurrence of brittle fracture such that high toughness is obtained in steel plates. The effect of warm rolling temperature and impact test temperature on delamination and impact property was elucidated.

Evolution of nano-size precipitation and mechanical properties in a high strength-ductility low alloy steel through intercritical treatment

Abstract: A two-step intercritical heat treatment was designed to obtain a multi-phase microstructure consisting of intercritical ferrite, tempered martensite/bainite and stable retained austenite in a low carbon and copper alloyed steel, characterized by high strength and high ductility combination. The evolution of copper precipitation during intercritical tempering was studied by transmission electron microscopy (TEM). Electron microscopy studies indicated that the precipitation of copper during tempering followed the sequence (as a function of time): twinned 9R-Cu (0.5 h) → de-twinned 9R-Cu (1 h) → e-Cu (greater than 3 h), which was accompanied by increase in the size of precipitates from ~ 11 nm to ~ 30 nm. Considering the cutting mechanism of precipitation strengthening, e-Cu precipitation contributed to ~ 248 MPa and ~ 207 MPa toward yield strength for 3 h and 5 h tempering, respectively. The average size of niobium-containing carbides varied marginally from ~ 11–16 nm and had a Baker–Nutting (B-N) orientation relationship with the ferrite matrix. The combination of transformation induced plasticity (TRIP) effect and nano-sized precipitation strengthening contributed to excellent mechanical properties (yield strength > 700 MPa, tensile strength > 800 MPa, the uniform elongation > 16% and the total elongation > 30%).

Grain structure dependent self-assembled bioactive coating on Mg-2Zn-2Gd alloy: Mechanism of degradation at biointerfaces

Abstract: Metallic biomedical devices with ultrafine-sized grains (UFGs) provide surfaces that are different from their coarse-grained (CG) (tens of micrometer) counterpart in terms of increased fraction of grain boundaries (UFG > 50%; CG ⁎ Corresponding author.

Microstructure-property relationship in a high strength-high toughness combination ultra-heavy gauge offshore plate steel: The significance of multiphase microstructure

Abstract: We elucidate here the microstructure-property relationship in high strength and high toughness 60 mm thick ultra-heavy gauge plate steels characterized by ferrite-bainite multi-phase microstructure and compare with the conventional bainitic microstructure. Pilot-scale results indicated that high yield strength of ~486–508 MPa with high ductility of ~10% in uniform elongation, low yield to tensile (Y/T) ratio of ~0.74–0.77, excellent low temperature toughness of ~186–228 J at −60 °C and good weldability are obtained in a multi-phase steel through combination of controlled rolling and accelerated cooling. The multi-phase microstructure steel consisted of quasi-polygonal ferrite and lath bainite with the corresponding phase proportion of 3:2 and 2:3 at 1/4 thickness (1/4 t) and 1/2 thickness (1/2 t) locations, respectively. The positive impact of multi-phase microstructure on ductility, Y/T ratio, toughness and weldability were attributed to the following aspects: (a) results from Crussard-Jaoul (C-J) analysis indicated that the soft ferrite phase in the multi-phase microstructure favorably modifies the work hardening behavior and results in high ductility and low Y/T ratio; (b) multi-phase microstructure is beneficial in enhancing toughness by enhancing initiation energy for nucleation of crack; (c) multi-phase microstructure improves the toughness of heat affected zone by altering the size and distribution of martensite/austenite (M/A) constituent.

Effect of deep cryogenic treatment on structure-property relationship in an ultrahigh strength Mn-Si-Cr bainite/martensite multiphase rail steel

Abstract: The bainite/martensite (B/M) multiphase microstructure was studied in 0.22C–2.0Mn–1.0Si–0.8Cr–0.8(Mo+Ni) (wt%) bainitic steel subjected to deep cryogenic treatment (DCT) to elucidate the positive effect of DCT on structure and mechanical properties. The study indicates that DCT can improve mechanical properties and wear resistance. It reduces the content of blocky martensite/austenite (M/A) constituents by eliminating unstable retained austenite (RA). At the same time, RA is relatively more enriched in carbon after DCT, compared to the tempering process. During DCT, the brittle martensite is also avoided, since the enhanced recovery reduces the carbon concentration during tempering. Meanwhile, the contraction of unit cell at low temperature promotes the precipitation of fine dispersed carbides and contributes to wear resistance.

Effect of copper precipitates on mechanical and magnetic properties of Cu-bearing non-oriented electrical steel processed by twin-roll strip casting

Abstract: Microstructure, texture, precipitation, mechanical and magnetic properties in a twin-roll strip casting Cu-bearing non-oriented electrical steel were studied by a combination of X-ray diffraction (XRD), electron back-scattered diffraction (EBSD), optical microscopy (OM) and transmission electron microscopy (TEM). The results indicated that the average grain size and main texture component were not affected by aging time, while the size and crystal structure of copper precipitates were influenced. In the peak strength condition, copper precipitation contributed ~ 207 MPa to the yield strength of the final product without deteriorating the magnetic properties. In summary, a non-oriented electrical steel with excellent mechanical and magnetic properties was successfully processed via twin-roll strip casting.

Microstructrual evolution and mechanical properties of the stir zone during friction stir processing a lean duplex stainless steel

Abstract: A lean duplex stainless steel was friction stir processed at 300 rpm-100 mm/min using a W-Re stirring tool. The microstructural evolution on the advancing side, the center, and the retreating side of the stir zone were studied by a combination of electron microscopy and electron backscattered diffraction. The study indicated that the direction of fiber structure on the advancing side, center, retreating side were 0°, 90°, 45° relative to the processing direction because of the stirring effect of the tool. Given that the strain rate and temperature were different on the advancing side, center and retreating side, the phase fraction, misorientation, texture components varied in different parts of the stir zone. The deformation mostly occurred in the center and retreating side and the material on the advancing side almost did not deform during tensile test. As a result, strain induced martensitic transformation occurred in the center and retreating side. The voids initiated at the interface of ferrite and transformed martensite on the retreating side, and cracks also propagated along this interface during tensile test. Furthermore, because of the 45° direction of the fiber structure on the retreating side, the tensile strength of the FSP sample was slightly less than the base metal.

Microstructural evolution and mechanical properties of duplex TRIP steel produced by strip casting

Abstract: In the present study, a duplex TRIP steel (Fe-0.232C-2.0Si-2.39Mn-2.01Al wt%) was successfully produced by a novel processing route involving twin-roll strip casting (TRSC), heat treatment, cold rolling and annealing for the first time. The evolution of microstructure during the entire processing route was extensively studied that included determination of mechanical properties and fractography of tensile specimens of annealed sheets. It was observed that the as-cast strip microstructure consisted of ferrite and martensite. The morphology at the surface and the center was dendritic and equiaxed, respectively. On using an appropriate heat treatment schedule (heat treatment at 780 °C for 10 min followed by direct quenching), a duplex structure of ferrite and retained austenite (RA) in the heat-treated strip was obtained that was suitable for cold rolling. Annealing temperature was very critical for microstructure and mechanical properties of the annealed sheets. Annealing at 780 °C for 5 min followed by direct quenching yielded a duplex structure of ferrite and RA (volume fraction of ~22%), which exhibited an excellent combination of tensile strength and total elongation of 945 MPa and 28.56%, respectively.

Tunable TiO2–pepsin thin film as a low-temperature electron transport layer for photoelectrochemical cells

Abstract: Electron transport layer (ETL) in perovskite-based solar cells (PSCs) is attractive from the perspective of photon-to-electron conversion efficiency. TiO2 thin film is preferred in situatio...