Yielding behavior and its effect on uniform elongation in IF steel with various grain sizes

In the present study, IF steel specimens with different grain sizes ranging from 12 to 0.45 μm were fabricated by accumulative roll-bonding process and subsequent annealing. Tensile tests revealed that by decreasing the mean grain size down to an ultrafine range smaller than approximately 1.5 μm, yielding behavior of the IF steel gradually changed from continuous yielding to discontinuous yielding. An abrupt loss in the uniform elongation occurred, when the average grain size was smaller than about 1 μm. Hall–Petch analysis on the yield stress and uniform elongation implied that the abrupt loss in the uniform elongation in the UFG grain size range corresponded to the appearance of the discontinuous yielding behavior. As it has been found in many UFG materials, discontinuous yielding is believed to be a unique mechanical behavior of UFG materials, and it has significant importance on the uniform elongation of UFG materials.     

Ultrafine-grained Ti–Nb–Ta–Zr alloy produced by ECAP at room temperature

To ascertain the influence of severe plastic deformation (SPD) on a Ti–Nb–Ta–Zr (TNTZ) alloy, we studied the room temperature mechanical behavior and microstructural evolution of an ultrafine-grained (UFG) Ti–36Nb–2Ta–3Zr (wt%) alloy prepared via equal-channel angular pressing (ECAP) of the as-hot-extruded alloy. The tensile behavior, phase composition, grain size, preferred orientation, and dislocation density of the UFG alloy, processed under different conditions, were analyzed and discussed. Compared to the as-hot-extruded alloy, the ECAP-processed TNTZ alloy (3 passes) exhibited approximately 40 and 88 % increase in average ultimate strength and yield strength, respectively. Moreover, as the number of ECAP passes increased from 3 to 6, the TNTZ alloy exhibited not only the expected increase in ultimate and yield strength values, but also a slight increase in elongation. Our results suggest that the deformation mechanisms that govern the behavior of the as-hot-extruded coarse grained (CG) TNTZ alloy during ECAP involve a combination of stress-induced martensitic transformation and dislocation activity. In the case of the ECAP-processed UFG TNTZ alloy, the deformation mechanism is proposed to involve two components: first, dislocation activity induced by the strain field imposed during ECAP; and second, the formation of α″ martensite phase during the early stages of ECAP which eventually transforms into β phase during continued deformation. We propose that the deformation mechanism governing the room temperature behavior of the TNTZ alloy strongly depends on the grain size of the β phase.     

The effect of deformation texture on the thermal stability of UFG HSLA steel

The microstructures of ultrafine grained (UFG) metals processed by severe plastic deformation are far from the thermodynamic equilibrium thus being prone to undergo coarsening processes. Theoretical and experimental investigations revealed that the stability against discontinuous grain growth in UFG metals with high stacking fault energy strongly depends on the fraction of high angle grain boundaries (HAGBs). This means that discontinuous grain growth does not occur if the fraction of HAGBs exceeds a certain level. The present work focuses on the impact of strong deformation textures on the thermal stability of UFG microstructures in a ferritic steel processed by linear flow splitting. It shows that the expected correlation between thermal stability and fraction of HAGBs is valid up to moderate texture intensities, whereas a strong deformation texture promotes discontinuous grain growth in spite of a high fraction of HAGBs. EBSD measurements reveal that this behavior is attributed to a strain-induced grain boundary migration causing a progressive orientation pinning effect with ongoing grain growth. Thereby, a large fraction of HAGBs is transformed into low angle grain boundaries (LAGBs) with low mobility. Consequently, a microstructure with a majority of LAGBs evolves being unstable against discontinuous grain growth.     

On deformation twinning in a 17.5% Mn-TWIP steel: A physically based phenomenological model

TWinning Induced Plasticity (TWIP) steel is a typical representative of the 2nd generation advanced high strength steels (AHSS) which exhibits a combination of high strength and excellent ductility due to the deformation twinning mechanisms. This paper discusses the principal features of deformation twinning in faced-centered cubic austenitic steels and shows how a physically based macroscopic model can be derived from microscopic-level considerations. In fact, a dislocation-based phenomenological model, with internal state variables including dislocation density and micro-twins volume fraction describing the microstructure evolution during deformation process, is proposed to model the deformation behavior of TWIP steels. The originality of this work lies in the incorporation of a physically based model on twin nucleation and volume fraction evolution in a conventional dislocation-based approach. Microstructural level experimental observations with scanning electron microscope (SEM) and transmission electron microscope (TEM) techniques together with the macroscopic quasi-static tensile test, for the TWIP steel Fe-17.5 wt.% Mn-1.4 wt.% Al-0.56 wt.% C, are used to validate and verify the modeling assumptions. The model could be regarded as a semi-phenomenological approach with sufficient links between microstructure and the overall mechanical properties, and therefore offers good predictive capabilities. Its simplicity also allows a modular implementation in finite element-based metal forming simulations.     

Microstructure and Mechanical Properties of Austenitic Stainless Steels after Dynamic and Post‐Dynamic Recrystallization Treatment

The effects of dynamic and post‐dynamic recrystallization (DRX and post‐DRX) on the microstructure and mechanical properties of austenitic stainless steels are critically reviewed. Particularly, the paper is focused on the grain refinement and strengthening by large strain deformation including severe plastic deformation conditions. The DRX and post‐DRX microstructures are considered with close relation to the operative recrystallization mechanisms. Specific emphasis is placed upon two recrystallization mechanisms, that is, discontinuous and continuous, and their dependence on the deformation/annealing conditions. The relationships between DRX microstructures and processing conditions are summarized and their effect on post‐DRX behavior is clarified. The structural strengthening mechanisms including the grain size and the dislocation density are elaborated.

     

Nanoscale deformation of multiaxially forged ultrafine-grained Mg-2Zn-2Gd alloy with high strength-high ductility combination and comparison with the coarse-grained counterpart

Cold processing of magnesium(Mg) alloys is a challenge because Mg has a hexagonal close-packed(HCP)lattice with limited slip systems, which makes it difficult to plastically deform at low temperature. To address this challenge, a combination of annealing of as-cast alloy and multi-axial forging was adopted to obtain isotropic ultrafine-grained(UFG) structure in a lean Mg-2Zn-2Gd alloy with high strength(yield strength: ~227 MPa)-high ductility(% elongation: ~30%) combination. This combination of strength and ductility is excellent for the lean alloy, enabling an understanding of deformation processes in a formable high strength Mg-rare earth alloy. The nanoscale deformation behavior was studied via nanoindentation and electron microscopy, and the behavior was compared with its low strength(yield strength: ~46 MPa)-low ductility(% elongation: ~7%) coarse-grained(CG) counterpart. In the UFG alloy, extensive dislocation slip was an active deformation mechanism, while in the CG alloy, mechanical twinning occurred.The differences in the deformation mechanisms of UFG and CG alloys were reflected in the discrete burst in the load-displacement plots. The deformation of Mg-2Zn-2Gd alloys was significantly influenced by the grain structure, such that there was change in the deformation mechanism from dislocation slip(non-basal slip) to nanoscale twins in the CG structure. The high plasticity of UFG Mg alloy involved high dislocation activity and change in activation volume.     

Strength and strain-hardening enhancement by generating hard delta-ferrite in twinning-induced plasticity steel

ABSTRACT

A dual-phase (12?vol.% delta-ferrite?+?78?vol.% austenite) high manganese twinning-induced plasticity (TWIP) steel was produced by hot rolled and annealing treatment. In comparison with the fully austenitic TWIP steel, both the yield and ultimate tensile strength of the dual-phase TWIP steel reinforced by hard delta-ferrite are significantly increased. It was found that the delta-ferrite in dual-phase steel exhibits a high hardness owing to the formed DO₃ structured intermetallic phase within ferrite. The presence of delta-ferrite dramatically improves the strain-hardening ability of TWIP steel. This is principally attributed to the effects of strain partitioning between hard delta-ferrite and softer austenite on the kinetics of deformation twinning and/or additional geometrical necessary dislocation (GND) during the deformation process.     

Plastic deformation behavior of ultrafine-grained Al–Mg–Sc alloy

Ultrafine-grained (UFG) Al–Mg–Sc alloy was obtained by friction stir processing. The UFG alloy was subjected to uniaxial tensile testing to study the tensile deformation behavior of the alloy. An inhomogeneous yielding (Lüdering phenomenon) was observed in the stress–strain curves of UFG alloy. This deformation behavior was absent in the coarse-grained alloy. The Lüdering phenomenon in UFG alloy was attributed to the lack of dislocations in UFG microstructure. A strong dependence of uniform ductility on the average grain size was exhibited by the UFG alloy. Below a critical grain size (0.5 μm), ductility was very limited. Also, with the decrease in grain size, most of the plastic deformation was observed to be localized in necked region of the tensile samples. The negative strain rate sensitivity (SRS) observed for the UFG alloy was opposite of the SRS values reported for UFG alloys in the literature. Based on activation volume measurement, grain boundary mediated dislocation-based plasticity was concluded to be the micro-mechanism operative during plastic deformation of UFG Al–Mg–Sc alloy.     

Effect of grain refinement on strain hardening and fracture in austenitic stainless steel

The present study aims to investigate the effect of grain refinement on strain hardening behaviour and fracture surface characteristics in 316LN austenitic stainless steel (ASS). The ASSs with varying grain sizes were obtained through 90% cold rolled reduction and subsequently phase reversion annealing treatment. The results showed that the grain refinement from coarse-grained (CG) structure to ultrafine-grained (UFG) structure increased the yield strength whilst maintaining a reasonable ductility. The strain hardening curves in all the samples were divided into three stages. The fractures in all the samples were ductile fracture with dimples. The subtle differences in the strain hardening behaviour and fracture surface characteristics among the samples with various grain sizes from CG structure to UFG structure were influenced by the deformation mechanisms of austenite.     

Strain rate effects in the ultrafine grain and nanocrystalline regimes—influence on some constitutive responses

Mounting evidence is pointing to some emerging novel behaviors of metals with ultrafine-grain (UFG) and/or nanocrystalline (NC) microstructures. One such novel behavior is related to the thermodynamic and kinetic aspects of plastic response in the UFG/NC regime. Two inter-related parameters, viz., the strain rate sensitivity (SRS) and the activation volumes of plastic deformation, are used as fingerprints for the thermodynamics and kinetics of plastic deformation. Changes of these parameters with grain size may indicate transition of plastic deformation mechanisms. Therefore, investigations of these phenomena may bring out new strategies for ingenious design and synthesis of UFG/NC materials with desirable properties. In this article, we present a critical review on the experimental results and theories associated with the SRS of UFG/NC metals with different lattice structures, and the influences on some constitutive responses.     

Enhancing tensile properties of ultrafine-grained medium-carbon steel utilizing fine carbides

The aim of the present study is to evaluate the influence of nano-sized carbides upon tensile behavior in UFG medium-carbon steels and to develop a material with improved tensile properties. UFG medium-carbon steels with fine carbides were successfully fabricated by multi-pass caliber rolling at 773 K. Alloying chromium and molybdenum resulted in thinner pearlitic lamellae, which were transformed into finer particles after severe plastic deformation. The UFG steel containing the alloying elements exhibited superior tensile properties, which was attributed to the enhanced strain hardening rate by the imbedded finer particles. Subsequent annealing induced growth of grains and particles, which also recovered elongation at the expense of strength. All UFG steels investigated here showed a yield-point phenomenon due to the decreased hardening rate and lack of mobile dislocations and their sources. The deteriorating effect of particle growth overwhelmed the improving effect of grain growth after annealing of the UFG medium-carbon steel, leading to a reduced strain hardening rate. This resulted in a positive correlation between a grain size and Lüders elongation in the investigated UFG steels.     

Flow mechanisms in ultrafine-grained metals with an emphasis on superplasticity

An analysis was conducted to examine the flow behavior of ultrafine-grained (UFG) metals produced by severe plastic deformation (SPD) processing in equal-channel angular pressing. The results reveal two distinct types of behavior. At elevated temperatures, the analysis shows that superplastic flow is accurately described by the theoretical mechanism developed for coarse-grained metals so that flow in UFG materials may be interpreted using conventional flow mechanisms. By contrast, localized small-scale grain boundary sliding is observed during deformation at low temperatures and this is attributed to the movement of extrinsic dislocations in the non-equilibrium grain boundaries produced by SPD processing.     

Biomimetic nanostructured coatings on nano-grained/ultrafine-grained substrate: Microstructure, surface adhesion strength, and biosolubility

We describe here the structural morphology, nucleation and growth, surface adhesion response and biosolubility in simulated physiological fluid of nanohydroxyapatite (nHA)-based coatings (nHA, chitosan-nHA, collagen-nHA) on nano-grained (NG)/ultrafine-grained (UFG) 316L austenitic stainless steel bulk substrate. The NG/UFG stainless steel was processed by a novel procedure involving controlled phase reversion of strain-induced martensite. Electron microscopy studies indicated that the morphology of nHA on NG/UFG stainless steel is characterized by a vein-type interconnected structure consisting of fibrils that closely mimics the fibrous structure present as part of the hierarchical structure of the bone. Interestingly, the dimensions of the vein-type interconnected structure were similar to the grain size of NG/UFG stainless steel substrate. This structural morphology was retained in crystallized chitosan-nHA coating, but was modified when collagen is codeposited with nHA. The mechanisms of deposition are discussed based on the structural characteristics of coatings. Adhesion response of coating as determined by scratching tests suggested superior adhesion strength of nHA coating on NG/UFG substrate as compared to that on CG substrate and is attributed to grain size effect and wettability of the substrate.     

Heterogeneous plastic deformation in bimodal bulk ultrafine-grained nickel

Heterogeneous plastic deformation behavior of two bimodal ultrafine-grained nickel materials with different ultrafine-grained (UFG) and coarse-grained (CG) components fractions was investigated experimentally at the grain level. The prismatic specimens were deformed quasistatically up to 10% axial plastic strain using compression test at room temperature. The local microstructure of the initial and deformed samples was measured by electron backscattered diffraction pattern analysis in a scanning electron microscope. It was found that the plastic deformation of bimodal materials is highly heterogeneous and the degree of heterogeneity depends strongly on the grain size distribution and the volume fraction of the CG component. The large localized plastic strain within the coarse grains was observed during compression. The strain localization resulted in occurrence of debonding and cracks in the UFG region or in the interface between CG and UFG components.     

Stress-dependent deformation behaviour in bulk nanocrystalline titanium–nickel alloys

In this study, the deformation mechanisms operating with stress in bulk nanocrystalline (NC) titanium–nickel with an average grain size below a critical size of 10–20?nm have been investigated. We demonstrate a sequential variation of the deformation mechanism from grain boundary (GB) sliding and grain rotation to grain growth and dislocation activity with the increase of the deformation stress. These deformation mechanisms are different from the previous understanding that below a critical grain size of 10–20?nm, GB sliding and grain rotation govern plastic deformation of NC materials.     

Tensile deformation behavior of high manganese austenitic steel: The role of grain size

The tensile deformation behavior and microstructural evolutions of twinning induced plasticity (TWIP) steel with the chemical composition of Fe–31Mn–3Al–3Si and average grain sizes in the range of 2.1–72.6 μm have been analyzed. For each grain size, the Hollomon analysis and also the Crussard–Jaoul (C–J) analysis as an alternative method to describe the work hardening behavior were investigated. The results indicated that the optimum mechanical properties as a function of work hardening capacity can be obtained by changing the grain size. The microstructural observations showed that the pile-ups of planar dislocations are necessary for triggering the mechanical twinning and grain refinement suppresses the mechanical twinning in TWIP steel. Furthermore, the mechanical twinning increases with increasing applied strain. As a result, a high instantaneous work hardening due to the mechanical twin boundaries enhances the uniform elongation. The contribution from the strain of twinning and hardening due to an increase in the hardness of the twinned regions (i.e., the Basinski mechanism) may be also useful in achieving the high strength–ductility in TWIP steels.     

奥氏体不锈钢晶粒细化对形变机制和力学性能的影响

利用相逆转变原理采用冷变形使得亚稳奥氏体转变为形变马氏体,采用不同温度和时间退火分别获得纳米晶/超细晶和粗晶奥氏体不锈钢。通过拉伸实验得到不同晶粒尺寸的奥氏体不锈钢力学性能,采用透射电镜观察形变组织结构并利用扫描电镜观察断口特征。结果表明:高屈服强度纳米晶/超细晶奥氏体不锈钢通过形变孪晶获得优良塑性;而低屈服强度的粗晶奥氏体不锈钢发生形变诱导马氏体效应,得到良好的塑性;两组具有不同形变机制的奥氏体不锈钢拉伸断口均为韧性断裂。形变机制由形变孪晶转变为形变诱导马氏体归因于晶粒细化导致奥氏体稳定性大幅度提高。     

A constitutive model of nanocrystalline metals based on competing grain boundary and grain interior deformation mechanisms

In this work, a viscoplastic constitutive model for nanocrystalline metals is presented. The model is based on competing grain boundary and grain interior deformation mechanisms. In particular, inelastic deformations caused by grain boundary diffusion, grain boundary sliding and dislocation activities are considered. Effects of pressure on the grain boundary diffusion and sliding mechanisms are taken into account. Furthermore, the influence of grain size distribution on macroscopic response is studied. The model is shown to capture the fundamental mechanical characteristics of nanocrystalline metals. These include grain size dependence of the strength, i.e., both the traditional and the inverse Hall–Petch effects, the tension–compression asymmetry and the enhanced rate sensitivity.     

Mechanical properties of ultrafine grained ferritic steel sheets fabricated by rolling and annealing of duplex microstructure

A new route to fabricate ultrafine grained (UFG) ferritic steel sheets without severe plastic deformation is proposed in this article. A low-carbon steel sheet with a duplex microstructure composed of ferrite and martensite was cold-rolled to a reduction of 91% in thickness, and then annealed at 620–700 °C. The microstructure obtained through the process with annealing temperatures below 700 °C was the UFG ferrite including fine cementite particles homogenously dispersed. The grain size of ferrite matrix changed from 0.49 to 1.0 μm depending on the annealing temperature. Dynamic tensile properties of the produced UFG steels were investigated. The obtained UFG ferrite–cementite steels without martensite phase showed high strain rate sensitivity in flow stress. The UFG ferritic steels are expected to have high potential to absorb crash energy when applied to automobile body.     

Plastic accommodation at homophase interfaces between nanotwinned and recrystallized grains in an austenitic duplex-microstructured steel

The plastic co-deformation behavior at the homophase interfaces between the hard nanotwinned grain inclusions and the soft recrystallized matrix grains in a duplex-microstructured AISI 316L austenitic stainless steel is examined through the analysis of long-range orientation gradients within the matrix grains by electron backscatter diffraction and transmission electron microcopy. Our analysis reveals that the mechanical accommodation of homophase interfaces until a macroscopic strain of 22% is realized within a small area of soft grains (about four grains) adjacent to the homophase interface. The activation of deformation twinning in the first two grain layers results in the occurrence of a ‘hump’ in the orientation gradient profile. We ascribe this effect to the role of deformation twinning on the generation of geometrically necessary dislocations. The smooth profile of the orientation gradient amplitude within the first 10 grain layers indicates a gradual plastic accommodation of the homophase interfaces upon straining. As a consequence, damage nucleation at such interfaces is impeded, resulting in an enhanced ductility of the single phase duplex-microstructured steel.