Combination Of High Ductility Research Articles

Change in deformation mechanism to change TRIP to TWIP strengthening in the class of 304 austenitic stainless steel sheets by trapezoidal wavy-rolling process

ABSTRACT Strengthening of steels through twinning-induced plasticity (TWIP) has several advantages like high-strength and high-ductility combination over the transformation-induced plasticity (TRIP) mechanism which gives high strength, low ductility and product phase of martensite in the single phase solid solution of austenitic matrix. But, development of TWIP effect will be more difficult unless the steel contains high stacking fault energy (SFE) (>20mJ/M2), fine grain structure and high manganese alloying (15–30 wt.%). These are very expensive processes and susceptible to metallurgical complexity in the austenitic stainless steels. Therefore, new processing technology was developed for strengthening of the austenitic stainless steel sheets by trapezoidal wavy rolling (TWR). Type 304 austenitic stainless steel was selected for this study which is known as martensite transformation, a dominant hardening mechanism due to low SFE and coarse grained structure. Wavy deformation-induced results have been studied through strain hardening behaviour by flow stress-strain curves and microstructural changes by atomic force microscope (AFM), and phase transformations by using X-ray diffractometer (XRD) analysis. It is found that this process introduced changes from TRIP to TWIP mechanism by forming plenty of mechanical twins and shear bands type structure at initial stage of deformations and partial martensite is observed in advanced stage.

Understanding the weakening of texture in multiaxial forged ultrafine-grained strong and ductile Mg-2Zn-2Gd alloy

ABSTRACT Texture is an important aspect that impacts the formability of magnesium (Mg) and its alloys. Recently, the unique characteristics of multiaxial forging of Mg-2Zn-2Gd alloy were discussed, where it was demonstrated that multiaxial forged processed alloy is characterized by high strength and high ductility combination and the texture is significantly weakened or almost texture-free. The objective of the study described here is to obtain some fundamental insights that explain the underlying reasons for the weakening of texture in the multiaxial forged Mg-2Zn-2Gd alloy.

Strong and ductile texture-free ultrafine-grained magnesium alloy via three-axial forging

We describe here the ingenious concept of combining annealing of as-cast alloy with multi-axial forging (MAF) (forging along x, y, and z-directions) that was applied to thermo-mechanically process a lean magnesium-rare earth alloy and obtain high strength and high ductility combination. This approach interestingly led to texture-free ultrafine-grained (UFG) structure with high yield strength of ∼227 MPa and elongation of ∼30 %. The objective of the study is to describe the preliminary findings and elucidate the evolution of texture in the MAF alloy in conjunction with the microstructure and mechanical properties. The discovery of the contribution of MAF in influencing texture, microstructure and mechanical properties is important from the perspective of current challenge in the processing of Mg alloys that has a hexagonal close-packed (HCP) lattice with limited slip systems, which makes it difficult to thermo-mechanically process at low temperatures.

Miscibility, crystallization and mechanical properties of poly[(3‐hydroxybutyrate)‐co‐(4‐hydroxyvalerate)]/poly(propylene carbonate)/poly(vinyl acetate) ternary blends

AbstractA ternary blend of poly[(3‐hydroxybutyrate)‐co‐(4‐hydroxyvalerate)] (P34HB), poly(propylene carbonate) (PPC) and poly(vinyl acetate) (PVAc) was initially melt‐blended in an effort to obtain an excellent balance of mechanical performance. The effect of PVAc content on the miscibility, phase morphology, thermal behavior, isothermal melt crystallization and mechanical properties of P34HB/PPC/PVAc ternary blends was systematically investigated. Dynamic mechanical analysis indicated that P34HB and PPC were immiscible, and PVAc exhibited immiscibility with PPC. PVAc was selectively dispersed in the P34HB matrix and phase morphology was refined by reducing the interfacial tension. Moreover, the PVAc component suppressed the cold crystallization and reduced the isothermal melt crystallization rate of P34HB in the ternary blends. Unexpectedly, marked increases of 32.7% and 3025% were achieved in the tensile modulus and elongation of the ternary blend containing 20 wt% PVAc compared to neat P34HB. Meanwhile, the strength did not deteriorate. The unusual combination of high ductility, strength and modulus is of great potential for expanding the practical application of sustainable and biological polymers. © 2021 Society of Industrial Chemistry.

Achieving high damping and excellent ductility of Al[sbnd]Mg alloy sheet by the coupling effect of Mg content and fine grain structure

Fine-grained Al-xMg (x = 4.5, 6.5 and 9.2, in wt%) alloy sheets with high damping capacity and excellent ductility are prepared by high strain rate rolling (HSRR). Both the damping capacity and the mechanical properties of the alloys are improved compared with the as-homogenized states and increase with the higher Mg content. The three AlMg alloys exhibit similar strain-amplitude-independent damping capacities and the Al-9.2 Mg alloy is a little bit higher. While the strain-amplitude-dependent damping capacity increases with increasing the Mg content in a large strain amplitude range. The as-HSRRed Al-4.5 Mg, Al-6.5 Mg and Al-9.2 Mg alloys exhibit the damping values (Q−1) of 0.0140 ± 0.0005, 0.0167 ± 0.0006 and 0.0178 ± 0.0008 at the strain amplitude of 1 × 10−3. Furthermore, a combination of high ductility (42.8 ± 1.5%), moderate strength (435 ± 2 MPa) are also achieved in the Al-9.2 Mg alloy. The improved damping capacity of the as-HSRRed Al-9.2 Mg alloy are mainly attribute to the low stacking fault energy and the movement of the extended dislocation, the latter of which interaction with Suzuki atmosphere and thus increases energy consumption which plays a dominant role in the damping mechanism of the as-HSRRed alloys. The results of the present study provide new ideas for the development of high damping alloys with high mechanical property.

Tests on stainless steel frames. Part II: Results and analysis

Austenitic stainless steels offer a suitable combination of high ductility, adequate toughness, considerable strain hardening and good fire resistance, making them excellent construction materials, especially for structures subjected to accidental loading such as seismic and/or fire events. With most of the research over the recent years devoted to the behaviour of single isolated stainless steel members, advances on structural systems are scarce, yet necessary, for the further development of design codes. Hence, an extensive experimental programme on EN 1.4301 austenitic stainless steel frames including tests at different structural levels –material, cross-section, member and frame levels– has been carried out at the Laboratory of Technology of Structures and Materials at Universitat Politècnica de Catalunya. The companion paper presents the tests carried out at cross-section and member level, and the knowledge gained from the planning of the experimental set-up for frame tests, including the adopted loading schemes, auxiliary elements and instrumentation. This paper presents the results of the four tests conducted on austenitic stainless steel frames under static vertical and horizontal loading. The analysis of the test results allowed to investigate the interaction of material and geometrical nonlinearities experimentally, as well as the assessment of the EN 1993-1-4 and EN 1993-1-1 provisions for the global analysis of stainless steel frames, where the influence of material nonlinearity on second order effects is evaluated, and which is currently limited to numerical studies only.

Influence of Al Additions on the Microstructure and Mechanical Properties of a C and Si-Free High-Mn Steel

The lightweight Fe–Mn–Al–C steels have drawn considerable attention from the literature due to their outstanding combination of high ductility and specific strength. Although the mechanical behavior of such steels has been extensively studied, the effect of Al when no C and Si are added has not been investigated in detail. For this reason, the main objective of this work was to study the microstructural evolution and mechanical behavior of carbon and silicon-free high-Mn steels with different aluminum contents. Alloys with 0, 2.5, and 5 wt. % Al were processed by spray forming to ensure high homogeneity and a fully austenitic microstructure. Cold rolling and annealing were performed to obtain a fine grain-sized material. The mechanical properties were similar regardless of the Al content, especially the work hardening rate. Deformation twinning and strain-induced phase transformation were not observed for any of the compositions. Additionally, a dislocation cell-like structure was observed for all of the alloys indicating that the Al additions did not change considerably the dislocation behavior, even though it considerably changed the estimated Stacking Fault Energy (SFE) value for all the alloys studied in this work.

Simultaneous enhancement of strength and plasticity by nano B2 clusters and nano-γ phase in a low carbon low alloy steel

Nano B2 FeCu ordered clusters and multiphase microstructure consisting of intercritical ferrite, tempered martensite and nano-γ phase (reverted austenite) were obtained by two-step heat treatment involving intercritical annealing and intercritical tempering. The experimental steel with nano Cu precipitates and nano-γ phase exhibited high strength and high ductility combination. The yield strength and total elongation of the experimental steel increased from 758 MPa and 16.8% to 984 MPa and 29.5% after the second step intercritical tempering for 5 min. High resolution transmission electron microscopy (HRTEM) and three-dimensional atom probe (3DAP) studies provided evidence to support that high density of nano B2 FeCu ordered clusters contributed to high strength. First principle calculations suggested that the feasibility of B2 FeCu nano-ordered clusters, and the stability of B2 structure is related to the coherent stress field at the interface between clusters and the BCC-Fe matrix. The average size of B2 FeCu nano-ordered clusters was 4 nm with a lattice constant of 0.2893 nm and an orientation relationship of (1 1 0)B2//(1 1 0)α and [0 0 1]B2//[0 0 1]α. 9R Cu without twinned structure was discovered at different tempering times. The proportion of Cu in Cu precipitates varied from 24.4 at% to 61.2 at% with change in crystal structure and increase in the size of precipitates. The significant increase in yield strength is discussed in terms of ordered domain strengthening, modulus strengthening and lattice misfit strengthening mechanisms. Bright field TEM image and selected area electron diffraction (SAED) pattern discovered and proved that spindle nano-γ phase was present at phase boundary between tempered martensite and intercritical ferrite. The size of them were among 242–375 nm in length and 52–80 nm in width. The two needle tip ends of nano-γ phase were just at phase boundaries, indicating the preferred growth direction. (1 −1 1)γ plane of nano-γ phase was parallel to (1 −1 0)α plane of the adjacent tempered martensite. The effect of nano-γ phase on enhancing the plasticity was revealed by instantaneous work hardening index curve.

Innovative processing of obtaining nanostructured bainite with high strength - high ductility combination in low-carbon-medium-Mn steel: Process-structure-property relationship

The study focused on the processing how to obtain nanoscale bainite for improving mechanical properties of a low-carbon-medium-manganese steel with nominal chemical composition of Fe–0.07C–7.9Mn–0.14Si–0.05Al–0.002S–0.003P (wt%), using warm rolling, intercritical annealing and bainite transformation. The results indicated that, after bainitic hold at 330 °C for 2 h, the steel had a yield stress (σy) of 770 MPa and tensile stress (σTS) of 1130 MPa, with a high elongation-to-failure (ɛf) of 0.62 at strain rate of 1 × 10−3 s−1. The product of σTS and ɛf was high at 70 GPa %, which is twice that the value (30 GPa %) required for third generation advanced high-strength steels. Furthermore, it is remarkably higher than the reported value of 46 GPa% for steel with identical chemical composition. Microstructural observations indicated that the steel consisted of nanoscale lamellae with characteristics of radial arrangement of bainitic ferrite and retained austenite (RA) of average thickness of ~120 nm. The volume fraction of RA was as high as 32% with carbon concentration of 1.75 wt%. The bainitic isothermal temperature and time not only affected the bainite morphology but also the volume fraction, distribution, microstructure and carbon concentration of RA, which are responsible for superior combination of high strength and good ductility of the steel.

Structure and functional properties of metastable beta Ti-18Zr-14Nb (at.%) alloy for biomedical applications subjected to radial shear rolling and thermomechanical treatment

In this preliminary study, a novel combination of radial shear rolling with thermomechanical treatment including cold rolling and post-deformation annealing at different temperatures was applied to Ti-18Zr-14Nb shape memory alloy with the objective of forming various structures in long-length bars dedicated for bone implants fabrication. The static mechanical properties and the functional fatigue behavior of Ti-18Zr-14Nb alloy in different structural states were studied. The dynamically polygonized substructure of β-phase formed as a result of radial shear rolling led to an outstanding combination of high ductility (δ = 35%) and superior functional fatigue resistance. After additional TMT (CR e = 0.3 + PDA at 500 °C), the alloy exhibited a highly-dislocated nanosubgrained structure of β-phase with subgrain size below 100 nm and a small quantity of α-phase. PDA at 550 °C led to the formation of the polygonized structure in β-phase with nano-to submicrometer-sized subgrains without α-phase present. The latter TMT improved further the functional properties of the alloy, by increasing its strength-to-modulus ratio and ensuring stable functional fatigue behavior with low level of accumulated strain.

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

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 (0.5h) → de-twinned 9R-Cu (1h) → ε-Cu (greater than 3h), which was accompanied by increase in the size of precipitates from ~ 11nm to ~ 30nm. Considering the cutting mechanism of precipitation strengthening, ε-Cu precipitation contributed to ~ 248MPa and ~ 207MPa toward yield strength for 3h and 5h tempering, respectively. The average size of niobium-containing carbides varied marginally from ~ 11–16nm 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 > 700MPa, tensile strength > 800MPa, the uniform elongation > 16% and the total elongation > 30%).

Thermal transport properties in amorphous/nanocrystalline metallic composites: A microscopic insight

In the last years new composite materials made of a metallic glass matrix with embedded nanocrystals have arisen as a promising alternative to the metallic glasses, due to their higher hardness. Although the effects of a partial nano-crystallization onto mechanical and magnetic properties have been widely investigated, nothing is known about thermal transport properties, interesting for recently proposed novel applications. Here we investigate how thermal transport is modified in presence of nanocrystalline inclusions in a Zr-based bulk metallic glass. By means of electric measurements and inelastic x ray scattering we are able to disentangle the effects of a partial nanocrystallization onto the two different contributions to heat transport, the electronic and the vibrational one. We show that no enhanced electrons or phonons scattering from the interfaces is observed, while the presence of crystalline nanoinclusions leads to an increase of both contributions, via an increased electric conductivity and transverse acoustic speed of sound. Surprisingly, while a gradual modification of the electric conductivity with the crystalline fraction is observed, even a low crystalline fraction is sufficient to modify elastic and vibrational properties. Our results indicate that low crystallinity composites are indeed promising alternative to both amorphous and crystalline alloys, thanks to their unique combination of high ductility, polycrystal-like vibrational properties and amorphous-like electric transport.

Phase‐Field Modeling of the Dendrite Growth Morphology with Influence of Solid–Liquid Interface Effects

High manganese steels are able to deform by the TRIP effect, TWIP effect and microbanding formation. These steels are quite promising materials for mechanical construction, once they show an unusual combination of high ductility and high tensile strength. The casting of these steels represents a technological challenge, because they are extremely prone to macro‐ and microsegregation. Segregation, on its turn, may locally impair the desired mechanical properties. Simulations by the phase‐field method may be utilized to investigate microstructure formation and the development of microsegregation patterns during solidification. Nevertheless, performing reliable microstructure simulations is only possible when reliable values for the solid–liquid interface energy are available. Through utilization of the sessile‐drop method, first measurements of the interface energy in the Fe–Mn–C alloy system were performed. By utilizing the obtained values for the interface energy as an input, phase‐field simulations were run aiming at investigating both the effect of the value for the interface energy and of the steel composition on the dendrite morphology.

Achieve high ductility and strength in an Al–Mg alloy by severe plastic deformation combined with inter-pass annealing

Metallic materials processed by severe plastic deformation generally possess very high strength but a rather low uniform elongation, usually <5%. In the present Al–7Mg alloy processed by room temperature equal channel angular pressing combined with inter-pass annealing, impressive combination of high ductility (elongation~14.5%) and high strength (UTS~600MPa) was achieved. The high ductility was rationalized by an enhanced work hardening originated from the pronounced dynamic strain aging effect and a bimodal grain structure. The high strength was mainly due to a prominent grain refinement and high dislocation density.

Phase reversion induced nanograined austenitic stainless steels: microstructure, reversion and deformation mechanisms

We describe here the attributes of a promising ‘phase reversion’ approach that results in nanograined/ultrafine grained (NG/UFG) structure in austenitic stainless steels with high strength–high ductility combination. The approach involves severe cold deformation (about 45–75%) of metastable austenite to martensite, which, on annealing for short durations, reverts to austenite via diffusional or shear mechanism, depending on the chemical composition of steel. There was, however, a need to optimise the severity of cold deformation and temperature–time annealing sequence to obtain NG/UFG structure. The fundamental criteria to obtain NG/UFG structure via ‘phase reversion’ approach were to obtain dislocation cell type martensite. In high strength NG/UFG steel, mechanical twinning contributed to the excellent ductility, while in low strength coarse grained (CG) steel, ductility was also good, but due to nucleation of strain induced martensite at shear bands. The difference in deformation mechanism between NG/UFG and CG steels was attributed to increase in the stability of austenite with decrease in austenite grain size.

Significance of control of austenite stability and three-stage work-hardening behavior of an ultrahigh strength–high ductility combination transformation-induced plasticity steel

In the context of obtaining high strength-high ductility combination in transformation-induced plasticity steels, we underscore the practical significance of controlling the stability of austenite during deformation such that strain hardening induced by martensite transformation occurs at a critical strain. An optimized intercritical hardening at 750 °C led to a ferrite–austenite microstructure that was characterized by ∼64% austenite and excellent combination of tensile strength of ∼1000 MPa and elongation of ∼65%, and a three-stage work hardening behavior.

Nanoscale deformation experiments on the strain rate sensitivity of phase reversion induced nanograined/ultrafine-grained austenitic stainless steels and comparison with the coarse-grained counterpart

We have recently adopted the concept of phase-reversion to obtain nanograined/ultrafine-grained (NG/UFG) structure with high strength–high ductility combination in some austenitic Cr–Ni stainless steels. The controlled deformation-annealing schedule involves cold deformation (∼45–77%) of metastable austenite to generate strain-induced martensite that on annealing transforms back to austenite with nano (NG) or ultrafine grain (UFG) size. In the present study, we describe the first report on systematic experiments carried out to investigate the strain rate sensitivity of deformation in an Fe–17Cr–7Ni steel characterized by NG/UFG structure and compare with the conventional coarse grained (CG) counterpart, using nanohardness measurements and extensive transmission electron microscopy of the plastically deformed nanoindented regions. The observations reveal that the NG/UFG structure exhibited greater strain rate sensitivity and smaller activation volume compared to that of the CG structure, under identical test conditions. Furthermore, there was clear distinction and fundamental transition in the deformation mechanism from twinning and stacking faults in NG/UFG structure to strain-induced martensite formation at the intersection of shear bands in the CG structure. The transition in the deformation mechanism is discussed in terms of stacking fault energy and grain size.

ODS ferritic steel engineered with bimodal grain size for high strength and ductility

An attractive way to enhance the ductility of ODS ferritic steels is to develop an alloy with a bimodal grain size distribution, in which the micron-sized coarse grains provide high ductility. The nanometer-sized fine grains enhance the tensile strength. The microstructures were obtained by blending the gas-atomized powders and mechanical alloyed powders, followed by hot forging and annealing. The homogeneously distributed nanometer-sized oxide nanoparticles can also be detected. Mechanical properties tests revealed a great improvement in ductility in comparison with other ODS ferritic steels, and high strength over the whole range of test temperatures, owing to the fine grains and oxide nanoparticles. The combination of high ductility and high strength makes this ODS ferritic steel much promising in high-temperature application.

Stimulated Cellular Response of Novel Hybrid Network Structure Elastomers with Inorganic Short Chain Cross‐Links for Soft Tissue Reconstruction

AbstractThe flexible and highly ductile silicone elastomer widely used as a soft‐tissue substitute for joint reconstruction and replacement fragments prematurely because of low tensile strength and inadequate bone build‐up around the implants. In this regard, an excellent vehicle to favorably modulate cellular response and retain intrinsic elastic recovery of soft tissue implants is to consider hybrid network structure elastomers with inorganic short chain cross‐links. The concept involves covalently linking nanocrystalline titania with a bi‐functional agent, acrylic acid, which has a carboxylic group to coordinate with titania and a vinyl group to form short chain cross‐links as an integral part of the silicone network during curing at elevated pressure. This approach provides high strength‐at‐break and undiminished intrinsic ductility of silicone elastomer and high cytocompatibility. Interestingly, cellular response is favorably modulated, as described here. The biologic response in terms of initial cell attachment, cell viability, and proliferation was consistently greater in relation to stand alone silicone elastomer such that cell spreading, morphology, and density were distinctly different. Pre‐osteoblasts grown on hybrid network structure elastomers were widely spread on the surface as a sheet after a culture time of 24 h. In contrast, these features were less pronounced on the stand alone silicone elastomer. Furthermore, immunofluorescence study illustrated distinct fibronectin expression level, stronger vinculin focal adhesion contacts associated with abundant actin stress fibers in pre‐osteoblasts grown on hybrid network structure elastomer compared to stand alone silicone elastomer, implying enhanced cell–substrate interactions. This finding was consistent with the observation of total protein content and sodium dodecyl sulfate polyacrylamide gel analysis.Based on the study described here, our hypothesis is that silicone–titania network structure elastomer with high strength–high ductility combination and favorable cellular response is a model hybrid network structure elastomer for joint reconstruction and soft tissue substitution. The integration of cellular and molecular biology with materials science and engineering aspects described here provides a new direction toward the development of new generation of hybrid network structure silicone elastomer. This subject is germane from the view point of fundamental understanding and from the view of hybrid elastomers as soft tissue reconstruction.

Probing deformation processes in near-defect free volume in high strength–high ductility nanograined/ultrafine-grained (NG/UFG) metastable austenitic stainless steels

The deformation behavior of a nanograined/ultrafine-grained (NG/UFG) austenitic stainless steel characterized by high strength–high ductility combination is investigated via nanoindentation and electron microscopy , and the behavior compared with its coarse-grained (CG) counterpart. In NG/UFG steel, mechanical twinning was an active deformation mechanism , while in CG steel, nucleation of strain-induced martensite at the shear bands occurred. The differences in deformation mechanisms of NG/UFG and CG steels are reflected in the discrete burst in the force–displacement plots and attributed to austenite stability.