Decrease In Uniform Elongation Research Articles

Calculating the grain size effect during strain hardening through a probabilistic analysis of the mean slip distance in polycrystals

Grain refinement is an important mechanism to produce stronger alloys. Strain hardening is an essential phenomenon in metal forming processes. The interaction between grain size and strain hardening is evident: a decrease in grain size (dg)causes an increase in ultimate tensile strength but a decrease in uniform elongation. The Kocks-Mecking (KM) model for strain hardening is based on the relationship between shear strain and the path length for dislocation slip. It provides good general estimates for stress-strain curves, and empirical modifications have been made to include dg. Here, the empirical approach is substituted by theoretical probability calculations, accounting for the fact that the grain size imposes a bound on the mean slip distance, while strain compatibility defines a relationship between grain boundary-dislocation interaction and bulk storage and annihilation. The resulting differential only uses the two parameters inherent to KM. Fitting to published tensile curves for Al, Cu, and Ni produces excellent results. The fitting parameters allow to predict the tensile strength as a function of dgto good approximation, for dg>1μm. Below this limit, fundamental changes in dislocation statistics impose the activation of grain boundary dislocation sources and may induce dislocation density gradients, which seem to determine the flow stress in the sub-μm range.

Dynamic tensile behavior and constitutive model of a novel high-strength and high-toughness plate steel

The dynamic tensile behavior of a novel-developed plate steel with high strength and toughness is investigated. Uniaxial tensile tests are conducted at various strain rates varying from 0.1 s−1 to 1000 s−1, and then scanning electron microscope and electron backscattered diffraction analyses are performed on tensile specimens at failure. A large and homogeneous deformation is demonstrated during the tension and small necking is observed at failure; all test results reveal the strain rate dependency of this novel plate steel: with an increase in strain rate, the yield strength, ultimate tensile strength, and total elongation increase, whereas the uniform elongation decreases. Microstructure analyses find dimple fracture surface and considerable deformation twins and dislocations, confirming the high strength and high ductility of the novel plate steel. The density of twins and dislocations demonstrates a positive correlation with the increasing strain rate, aligning with the variations observed in the mechanical properties. Engineering stress–strain curves exhibit a necking lag effect at high strain rates, and this effect is considered when solving the true stress–strain relationships by a developed hybrid experimental–numerical method. A modified Johnson-Cook model considering the power law strain rate effect and thermal softening effect caused by adiabatic temperature rise is finally proposed and verified. This work explores the dynamic mechanical behavior of the novel plate steel at different strain rates, promoting its application in aseismic design and impact resistance engineering.

Good strength-ductility synergy achieved in a Fe62Ni16Co9Mn9Ti3Si1 Fe-high-entropy alloy with heterogeneous structures

Developing metal structural materials with a good combination of strength-ductility is of great significance for the rapid development of the aerospace industry and the energy industry. High-entropy alloys (HEAs) have attracted widespread attention due to their excellent mechanical properties, especially the excellent ductility and toughness of face-centered cubic (FCC) high-entropy alloys. However, similar to traditional alloys, there is a trade-off between strength and ductility in HEAs. In this paper, we report the successful preparation of Fe-HEA (Fe62Ni16Co9Mn9Ti3Si1 (atomic percentage, %)) with high strength-ductility synergy at room temperature by cryo-rolling and annealing at 600 °C for 10 min (CR600), which increases the yield strength by 59% compared to room temperature rolling (RR600), while the uniform elongation decreases by only 20%. The ultra-high yield strength and the not significant reduction in uniform elongation are mainly attributed to the formation of high-density dislocations, dual-phase bimodal ultra-fine grain heterostructures, (NiMn)3-xTix and (Fe, Ni)2SiTi composite nanoprecipitates, precipitation-induced chemical composition heterogeneity, and BCC→HCP phase transformation during tensile deformation during CR and subsequent annealing at a reduced temperature (600 °C). In addition, by increasing the annealing temperature of the alloy after CR to 800 °C, its uniform elongation can be increased to 53%, which is mainly due to the excellent strain hardening ability induced by changes in heterogeneous structures, nanoprecipitates, and martensitic phase transformation. Compared with other similar alloys, our CR alloy exhibits higher strength-ductility synergy. The results of this study provide a new perspective for developing HEAs with higher strength-ductility synergy.

Effect of strain rate on tensile behavior of tailor welded blanks

The demand for weight reduction, cost savings, more fuel efficiency, and reduced greenhouse gas has led to the increasing application of tailor welded blanks (TWBs) in the automotive industry. The purpose of this study is to investigate the strain rate effect on the tensile behavior of TWB consisting of dissimilar thickness sheets. Hence, a fiber laser was used for welding St14 steel sheets. The weld zone was examined using metallographic and microhardness tests. For investigating the tensile properties, uniaxial tensile tests were performed at strain rates ranging from 0.001 to 10 s−1. It has been found that the presence of Bainitic microstructure in the weld zone increases its hardness. Tensile test results indicate that yield strength and ultimate tensile strength of base metals and TWB enhanced with increasing strain rate. The yield and ultimate tensile strength of TWB are higher than base metals at different strain rates. As the strain rate increases, the total elongation of the TWB decreases, but in base metals, it decreases first and then increases at higher strain rates. In TWB and base metals, by increasing strain rate, uniform elongation decreases, but post-uniform elongation increases. For analysis of plastic deformation of thick and thin parts of TWB, indexes of limiting thickness ratio and difference of ratios (DR) were used. DR decreases with increasing strain rate, which indicates a decrease in the plastic deformation of the thick part of TWB and is consistent with the experimental findings.

Integrated modeling of the gradient structure evolution during surface mechanical grinding treatment: Formation mechanism and mechanical properties

Gradient nanostructured materials are regarded as a promising class of architectures with tunable mechanical properties, primarily dependent on the optimization of well-controlled fabrication parameters. In this paper, a microvariable-based constitutive model is incorporated into an integrated finite analysis technique. This approach correlates the fabrication parameters of surface mechanical grinding treatment (SMGT) with the corresponding measured mechanical properties of gradient structured (GS) materials, quantifying the relationship between process parameters, microstructure, and mechanical properties. Through theoretical prediction and experimental verification, it is observed that grain refinement, mechanical strength, and surface hardness are enhanced by increasing processing times and reducing path spacing. The yield stress of the fabricated GS material ranges from 126.8 MPa to 162.2 MPa, an increase of above 2.5 times compared to the original material, with a slight decrease in uniform elongation by a factor of 25.8 %, indicative of an excellent strength-ductility trade-off. The underlying mechanism for improved strength-ductility synergy is discussed, focusing on the importance of the tunable spatial distribution of grain size. This work sheds light on the potential application of gradient nanograined structures at an industrial scale and advances the fundamental understanding of strengthening mechanisms in gradient nanostructures.

Enhanced strength-ductility synergy via high dislocation density-induced strain hardening in nitrogen interstitial CrMnFeCoNi high-entropy alloy

• Nitrogen addition induced a remarkable increase in strength but only a negligible decrease in uniform elongation of CrMnFeCoNi HEA. • First-principles calculations were carried out to analyze the effect of nitrogen on generalized stacking fault energy ( γ usf , γ isf , and γ utf ) and twinning. • The pinning effect of nitrogen on dislocations contributed to the increased dislocation density and thus the higher strain-hardening rate. The present work demonstrates that nitrogen doping inhibits the formation of deformation twins in a CrMnFeCoNi high entropy alloy, while significantly increasing the strength without sacrificing much ductility at 77 K. Microstructural characterization and first-principles calculations were employed to unveil the role of interstitial nitrogen atoms in obtaining such an excellent combination of strength and ductility at 77 K. It is found that nitrogen addition increases generalized stacking fault energy (GSFE) and reduces twinning. However, the pinning of dislocations by nitrogen atoms effectively suppresses dislocation cross-slip and dynamic recovery and in turn, promotes the accumulation of dislocations. The high dislocation density induces a high strain hardening capacity and improves uniform elongation, which compensates for the ductility loss accompanied by solid solution strengthening. The effect of nitrogen doping enriches the design concept of high- and medium-entropy alloys, providing an economical and effective strategy to develop ultra-high-performance alloys that are suitable for cryogenic applications.

Microstructure and work hardening behavior of Al–Cu–Li alloy (AA2198) in different temper conditions

In this paper, Al–Cu–Li alloy (AA2198) sheets have been imparted under different temper conditions to obtain different precipitate morphologies. The tensile testing of the samples in the solution treated, peak-aged, and over-aged conditions at ambient and cryogenic temperatures has been carried out to compare the hardening behavior. The strain hardening in the alloy is a strong function of the nature of obstacles present in individual temper conditions and extrinsic factors such as temperature. The decreased strain hardening capacity of peak-aged and over-aged samples is manifested in terms of a considerable decrease in uniform elongation. The effect of temperature is mainly manifested in terms of a decrease in dynamic recovery parameter and increase in dislocation storage capacity, irrespective of temper condition.

Effect of strain rate-induced microstructure on mechanical behavior of dual-phase steel

In this work, the microstructure evolution characteristics after 3% pre-strained at different strain rates are elaborated to reveal the effect on the mechanical behavior of dual-phase steel. The results showed the strain field simulated by the representative volume element model shows obvious strain heterogeneity, and the degree of strain localization intensifies with the increase of the strain rate. There is sufficient time to complete the dynamic transition among geometrically necessary dislocations (GNDs)-low-angle grain boundaries (LAGBs)-middle angle grain boundaries (MAGBs)-high angle grain boundaries (HAGBs) at low strain rates (10−4 s−1∼10−3 s−1), thereby releasing strain distortion energy. As the strain rate increases, the transformation process from LAGBs to HAGBs is hindered due to the shortened strain response time. It can promote the accumulation of high-density GNDs and LAGBs inside the grains, which intensifies strain localization. The incomplete evolution of the microstructure caused by the pre-strain history becomes a resistance to dislocation motion during re-deformation, and more importantly, the higher the strain rate, the more severe the hardening rate loss. This leads to an increase in yield strength and a decrease in uniform elongation, which drastically deteriorates the mechanical stability of the steel.

Microstructure and Mechanical Properties of Medium-Carbon Si-Rich Steel Processed by Austempering after Intercritical Annealing

In the present paper, the medium-C Si-rich steel with a quenched martensite microstructure was heated to intercritical annealing temperatures at 750 °C, 760 °C and 770 °C after warm rolling deformation to obtain ferrite with varying volume fractions. Subsequently, bainite/ferrite multiphase microstructures were attained via austempering near Ms temperature. The microstructures of the test steel after different heat treatments were characterized by scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction, and corresponding tensile and impact properties were tested. The results showed that, with the increase of intercritical annealing temperature, the austenite content increased, which limited the growth of ferrite grains, and the grain size decreased from ~1.6 μm to ~1.4 μm. In addition, the degree of ferrite recrystallization was almost complete. At the same intercritical annealing temperature, compared with austempering above Ms, prior athermal martensite (PAM) was obtained after austempering below Ms, which effectively refined the size of bainite ferrite lath. Moreover, with the increase of intercritical annealing temperature, the bainite content of the test steel increased after austempering, resulting in the increase of yield strength, tensile strength and impact energy. In contrast, while the decrease in ferrite content led to a significant decrease in uniform elongation. At constant intercritical annealing temperature, the tensile strength decreased slightly, and the impact property improved after austempering above Ms.

Effects of Yttrium on the Microstructure and Properties of 20MnSi Steel

20MnSi steel samples with different rare‐earth yttrium (Y) contents are prepared in the laboratory. Microstructure analysis is performed using the methods of microscopy and electron probe microanalysis, and the mechanical and physical properties are assessed using the tensile tester and thermal dilatometer. The results show that with increasing Y content, the thickness of the martensite layer on the surface of 20MnSi steel decreases gradually until it almost disappears, which is attributed to Y reducing martensitic transformation temperature. Y refines the size of the pearlitic lamellae, which improves the yield strength. Y‐containing composite inclusions (Y2O2S) of microsized spheres are easily formed in the steel, which assists the nucleation of ferrite, thereby resulting in an increase in the yield point elongation. In addition, the content of inclusions increases, thereby resulting in a decrease in uniform elongation.

Effect of individual phase properties and volume fractions on the strain partitioning, deformation localization and tensile properties of DP steels

Deformation band localization modes, uniform tensile strength, and uniform elongation of Ferrite-Martensite Dual-Phase (DP) steels are analyzed by finite element (FE) study. Treating the microstructure inhomogeneity as the sole cause of imperfection, failure initiation is predicted as the natural fallout of plastic instability caused by load drop because of localized plastic strain in the Representative Volume Element (RVE) during straining. Strain partitioning between two phases (ferrite matrix and martensite island) are investigated on RVEs, and it reveals that the increase of martensite yield stress decreases the plastic deformation and increases the stress state in martensite. Whereas, a decrease in martensite island volume fraction (Vm) results in the reduction of plastic deformation and stress state in the island. Studies are then carried out to investigate the effects of the ferrite-martensite flow properties and martensite volume fraction on the macroscopic tensile deformation behavior and band localization of DP steels. Micromechanical based FE simulation results emphasize that an increase in initial yield strength and volume fraction of martensite increases the ultimate tensile stress (UTS) with the decrease in uniform elongation. Similarly, as the hardening rate of ferrite increases, it increases the ultimate tensile stress (UTS) and uniform elongation. Additionally, deformation band localization modes alter from inclined to perpendicular to the loading axis with an increase in martensite volume fraction and initial yield strength of martensite. The knowledge of this work can be used to design DP steels with desired mechanical properties.

The effect of strain rate on mechanical properties and microstructure of a metastable FeMnCoCr high entropy alloy

The relationship between mechanical properties and microstructures of a metastable dual-phase high entropy alloy Fe50Mn30Co10Cr10 under uniaxial tensile testing at different strain rates (10-3 s-1~103 s-1) has been studied systematically. As the strain rate increases, yield strength, ultimate tensile strength and uniform elongation decrease first and then increase. Namely, when the strain rate is 10-3 s-1, yield strength and ultimate tensile strength are 280 MPa and 720 MPa, respectively, with the uniform elongation of 64.2%. When the strain rate is increased to 1 s-1, yield strength and ultimate tensile strength are 300 MPa and 672 MPa, respectively, and uniform elongation decreases to 49.2%. When the strain rate reaches 103 s-1, yield strength and ultimate tensile strength increase to 380 MPa and 810 MPa, respectively, while uniform elongation is elevated to 67%. As dynamic deformation is affected by the adiabatic heating, the stacking fault energy of the alloy is increased by ~13 mJ m-2 at a strain rate of 103 s-1 compared with that in the quasi-static condition. Under quasi-static loading, martensitic transformation is the dominant deformation mechanism. Under dynamic loading, when the strain is low the deformation induced phase transformation dominates, whereas as the loading proceeds mechanical twinning becomes the dominant deformation mode. At the same time, the adiabatic temperature rise under dynamic tests also causes a reverse transformation from ε-martensite to austenite. Accordingly, the release of internal stress and the formation of soft and ductile austenite jointly contribute to the elevated uniform elongation of the material. Both mechanical twinning and martensitic reverse transformation promote the microstructure to be dynamically refined, so that the alloy shows the good plasticity while maintaining the high ultimate tensile strength at dynamic strain rates.

Plastic deformation mechanism of Ti–Nb–Ta–Zr–O alloy at cryogenic temperatures

The deformation behavior and mechanism of Ti–36Nb–2Ta–3Zr-0.35O alloy at room and cryogenic temperatures (77 and 200K) were investigated. The microstructure near the fracture was observed with electron backscattered diffraction analysis (EBSD) and transmission electron microscopy (TEM). The results show that the plastic deformation behavior of the alloy is sensitive to test temperature and cold deformation history of the alloy. For the alloy without cold deformation, both tensile strength and ductility have a significant enhancement at cryogenic temperatures, mainly due to the occurrence of mechanical twinning. Moreover, with the decrease of the temperature, the density of mechanical twinning increases. For the alloy cold swaged by 85%, an increase in strength accompanied by a decrease in uniform elongation was found at cryogenic temperatures. The unique temperature dependence of the deformation behavior was due to the low stability of β phase. At 200K, ω phase precipitated in β phase; while at 77K, an interesting transformation (β phase to β′ phase) was observed.

Origins of high strength and ductility combination in a Guinier-Preston zone containing Mg-Al-Ca-Mn alloy

The excellent strength and ductility combination of a microalloyed Mg-Al-Ca-Mn, in the peak-aged condition, has been reported previously. However, a mechanistic explanation has not been reported. The present work shows that an enhanced strain-rate sensitivity is responsible for this behavior. Increased post-uniform elongation compensates for the decrease in uniform elongation upon aging, resulting in a total elongation comparable to the solutionized material. The thermally activated nature of dislocation interactions with Guinier-Preston (GP) zones are discussed, and the potential for optimizing mechanical properties by engineering the strain-rate sensitivity through microstructural modifications is highlighted.

Deformation behavior and microstructural evolution in ultra-high-strength dual-phase (UHS-DP1000) steel with different strain rates

The dynamic tensile behavior and deformation mechanism of ultra-high-strength dual-phase (UHS-DP1000) steel were investigated over a wide range of strain rates from 10−4 to 103 s−1. As the strain rate increases, the transition strain decreases from 2.73 to 1.92, and the martensite plastic deformation starts earlier. At strain rate of 10−4–0.5 s−1, the inhomogeneous plastic deformation ability increases because the dislocation density in the ferrite matrix increases. This leads to a decrease in uniform elongation and an increase in fracture elongation. When the strain rate increases from 0.5 to 500 s−1, the amount of mobile dislocation increases, which is the main reason for the enhancing uniform elongation and fracture elongation. Meanwhile, because the dislocation motion resistance rapidly increases, the yield strength and ultimate tensile strength also increase. When the strain rate is higher than 500 s−1, the hardening behavior caused by the dislocation motion resistance has not been offset by softening due to the mobile dislocation and adiabatic heating. The voids at the early stage of deformation could not uniformly form and grow, and thus the homogeneous plastic deformation ability decreases.

Effect of Strain Aging on Tensile Behavior and Properties of API X60, X70, and X80 Pipeline Steels

The effect of strain aging on tensile behavior and properties of API X60, X70, and X80 pipeline steels was investigated in this study. The API X60, X70, and X80 pipeline steels were fabricated by varying alloying elements and thermomechanical processing conditions. Although all the steels exhibited complex microstructure consisting of polygonal ferrite (PF), acicular ferrite, granular bainite (GB), bainitic ferrite (BF), and secondary phases, they had different fractions of microstructures depending on the alloying elements and thermomechanical processing conditions. The tensile test results revealed that yielding behavior steadily changed from continuous-type to discontinuous-type as aging temperature increases after 1% pre-strain. After pre-strain and thermal aging treatment in all the steels, the yield and tensile strengths, and yield ratio were increased, while the uniform elongation and work hardening exponent were decreased. In the case of the X80 steel, particularly, the decrease in uniform elongation was relatively small due to many mobile dislocations in PF, and the increase in yield ratio was the lowest because a large amount of harder microstructures such as GB, BF, and coarse secondary phases effectively enhanced work hardening.

Effects of strain rate on mechanical properties and deformation behavior of an austenitic Fe-25Mn-3Al-3Si TWIP-TRIP steel

The effects of quasi-static and low-dynamic strain rate (ε̇ = 10−4 /s to ε̇ = 102 /s) on tensile properties and deformation mechanisms were studied in a Fe-25Mn-3Al-3Si (wt%) twinning and transformation-induced plasticity [TWIP-TRIP] steel. The fully austenitic microstructure deforms primarily by dislocation glide but due to the room temperature stacking fault energy [SFE] of 21 ± 3mJ/m2 for this alloy, secondary deformation mechanisms such as mechanical twinning (TWIP) and epsilon martensite formation (TRIP) also play an important role in the deformation behavior. The mechanical twins and epsilon-martensite platelets act as planar obstacles to subsequent dislocation motion on non-coplanar glide planes and reduce the dislocation mean free path. A high-speed thermal camera was used to measure the increase in specimen temperature as a function of strain, which enabled the use of a thermodynamic model to predict the increase in SFE. The influence of strain rate and strain on microstructural parameters such as the thickness and spacing of mechanical twins and epsilon-martensite laths was quantified using dark field transmission electron microscopy, electron channeling contrast imaging, and electron backscattered diffraction. The effect of sheet thickness on mechanical properties was also investigated. Increasing the tensile specimen thickness increased the product of ultimate tensile strength and total elongation, but had no significant effect on uniform elongation or yield strength. The yield strength exhibited a significant increase with increasing strain rate, indicating that dislocation glide becomes more difficult with increasing strain rate due to thermally-activated short-range barriers. A modest increase in ultimate tensile strength and minimal decrease in uniform elongation were noted at higher strain rates, suggesting adiabatic heating, slight changes in strain-hardening rate and observed strain localizations as root causes, rather than a significant change in the underlying TWIP-TRIP mechanisms at low values of strain.

Influence of martensite volume fraction and hardness on the plastic behavior of dual-phase steels: Experiments and micromechanical modeling

Model dual-phase microstructures were developed to decouple the effect of martensite volume fraction and martensite hardness on the plastic behavior of dual-phase steels. The martensite volume fraction ranges from 11% to 37%, involving two levels of martensite hardness. The yield strength and tensile strength increase with increasing martensite volume fraction, while the uniform elongation decreases. The martensite hardness has a weak impact on the initial yield strength, but it significantly affects the flow behavior for sufficiently large martensite volume fraction. Increasing the hardness of the martensite leads to higher tensile strength combined with only a limited impact on uniform elongation, resulting in an improved strength/ductility balance. The experimental results are successfully captured using finite element based micromechanical analysis. Among others, periodic cell calculations show very good predictive capabilities of the overall plastic response when the stage-IV hardening of the ferrite is taken into account. Our numerical analysis reveals that an accurate description of the elasto-plastic behavior of the martensite is a key element to rationalize the mechanical behavior of DP steels. This modeling approach provides a framework for designing dual-phase steels with optimized plastic flow properties.

Effect of austenite grain structure on the strength and toughness of direct-quenched martensite

Abstract The effect of prior austenite grain structure on the microstructure and properties of two low alloyed hot-rolled and direct-quenched martensitic steels was investigated. Strength properties were determined using uniaxial tensile testing, while toughness properties were characterized by using Charpy-V and fracture toughness tests. Microstructures were characterized using OM, SEM, EBSD and a novel EBSD-based image quality (IQ) technique. It was found that an increase in the rolling reduction in the non-recrystallization temperature regime of austenite was an effective way to improve the strength, impact and fracture toughness without a significant decrease in uniform elongation. In addition, this approach also decreased the in-plane anisotropy of the tensile and toughness properties. Refinement of the equiaxed austenite grain structure during rolling in the recrystallization temperature regime (roughing) also improved the strength and toughness properties, but the effect was found to be weaker than the effect of rolling in the non-recrystallization regime. In toughness testing, a correlation was found between the Charpy-V 28J transition temperature and the fracture toughness characteristic temperature T 0 . However, the correlation differs significantly from that reported in the literature for lower strength ferritic steels. In all cases, the steel microstructures consisted of mainly auto-tempered martensite and lower bainite. A brief discussion of the microstructural features controlling the strength and toughness properties is given.

Size effects on tensile and fatigue behaviour of polycrystalline metal foils at the micrometer scale

Tensile and fatigue properties of as-rolled and annealed polycrystalline Cu foils with different thicknesses at the micrometer scale were investigated. Uniaxial tensile testing results showed that with decreasing foil thickness the uniform elongation decreases for both as-rolled and annealed foils, whereas the yield strength and ultimate tensile strength increase for as-rolled foils, but decrease for the annealed foils. For both the as-rolled or annealed foils, bending fatigue resistance decreases with decreasing the foil thickness. Deformation and fatigue damage behaviour of the free-standing foils were characterised as a function of foil thickness. In addition, the fatigue strength of various small-scale Cu foils was compared to understand the physical mechanisms of size effects on mechanical properties of the metallic material at micrometer scales.