Showing papers on "Strain hardening exponent published in 2018"

Dynamically reinforced heterogeneous grain structure prolongs ductility in a medium-entropy alloy with gigapascal yield strength

Abstract: Ductility, i.e., uniform strain achievable in uniaxial tension, diminishes for materials with very high yield strength. Even for the CrCoNi medium-entropy alloy (MEA), which has a simple face-centered cubic (FCC) structure that would bode well for high ductility, the fine grains processed to achieve gigapascal strength exhaust the strain hardening ability such that, after yielding, the uniform tensile strain is as low as ∼2%. Here we purposely deploy, in this MEA, a three-level heterogeneous grain structure (HGS) with grain sizes spanning the nanometer to micrometer range, imparting a high yield strength well in excess of 1 GPa. This heterogeneity results from this alloy's low stacking fault energy, which facilitates corner twins in recrystallization and stores deformation twins and stacking faults during tensile straining. After yielding, the elastoplastic transition through load transfer and strain partitioning among grains of different sizes leads to an upturn of the strain hardening rate, and, upon further tensile straining at room temperature, corner twins evolve into nanograins. This dynamically reinforced HGS leads to a sustainable strain hardening rate, a record-wide hysteresis loop in load-unload-reload stress-strain curve and hence high back stresses, and, consequently, a uniform tensile strain of 22%. As such, this HGS achieves, in a single-phase FCC alloy, a strength-ductility combination that would normally require heterogeneous microstructures such as in dual-phase steels.

High Strength and Ductility of Additively Manufactured 316L Stainless Steel Explained

Abstract: Structure–property relationships of an additively manufactured 316L stainless steel were explored. A scanning electron microscope and electron backscattered diffraction (EBSD) analysis revealed a fine cellular-dendritic (0.5 to 2 μm) substructure inside large irregularly shaped grains (~ 100 μm). The cellular structure grows along the 〈100〉 crystallographic directions. However, texture analysis revealed that the main 〈100〉 texture component is inclined by ~15 deg from the building direction. X-ray diffraction line profile analysis indicated a high dislocation density of ~1 × 1015 m−2 in the as-built material, which correlates well with the observed EBSD microstructure and high-yield strength, via the traditional Taylor hardening equation. Significant variations in strain hardening behavior and ductility were observed for the horizontal (HB) and vertical (VB) built samples. Ductility of HB and VB samples measured 49 and 77 pct, respectively. The initial growth texture and subsequent texture evolution during tensile deformation are held responsible for the observed anisotropy. Notably, EBSD analysis of deformed samples showed deformation twins, which predominately form in the grains with 〈111〉 aligned parallel to the loading direction. The VB samples showed higher twinning activity, higher strain hardening rates at high strain, and therefore, higher ductility. Analysis of annealed samples revealed that the observed microstructures and properties are thermally stable, with only a moderate decrease in strength and very similar levels of ductility and anisotropy, compared with the as-built condition.

Improved back stress and synergetic strain hardening in coarse-grain/nanostructure laminates

Abstract: The effect of back stress on the mechanical behaviors of heterogeneous material is studied in two modeled heterogeneous laminates, i.e. laminated structure with a nanostructured (NS) Cu-Zn alloy layer sandwiched between two coarse-grained (CG) pure Cu layers. The improved tensile ductility of NS layer is revealed and attributed to the constraint from the stable CG layers. It is found that the elastic/plastic interaction between NS and CG layers is capable of significantly improving the back stress, which makes a significant contribution to the synergetic strain hardening in low strain stage. Furthermore, a higher mechanical incompatibility permits stronger and longer mutual interaction between layers, i.e. coupling effect, which contributes to a higher back stress. These results improve our understanding about the role of back stress on mechanical behaviors of heterogeneous laminate materials.

Effect of Sn content on strain hardening behavior of as-extruded Mg-Sn alloys

Abstract: The effects of Sn content on strain hardening behavior of as-extruded Mg-xSn (x = 1.3, 2.4, 3.6 and 4.7 wt%) binary alloys were investigated by uniaxial tensile tests at room temperature. Strain hardening rate, strain hardening exponent and hardening capacity were obtained from the true plastic stress-strain curves. After hot extrusion, the as-extruded Mg-Sn alloys are mainly composed of α-Mg matrix and second phase Mg2Sn, which only exists in Mg-3Sn and Mg-4Sn. Average grain size decreases from 15.6 μm to 3.6 µm with Sn content increases from 1.3 to 4.7 wt%. The experimental results show that Sn content decreases strain hardening ability of as-extruded Mg-Sn alloys, but gives rise to an obvious elevation in tensile strength, yield strength and elongation of them. With increasing Sn content, strain hardening rate decreases from 3527 MPa to 1211 MPa at (σ-σ0.2) = 50 MPa, strain hardening exponent decreases from 0.21 to 0.13 and hardening capacity decreases from 1.66 to 0.63. The variation in strain hardening behavior of Mg-Sn alloys with Sn content is discussed in terms of the influences of grain size and distribution of grain orientation.

Mechanical properties and deformation twinning behavior of as-cast CoCrFeMnNi high-entropy alloy at low and high temperatures

Abstract: Tensile properties of an as-cast CoCrFeMnNi high-entropy alloy were investigated at various temperatures ranging from −160 to 1000 °C. The tensile strength and ductility did not vary significantly with loading direction, despite the alloy’s strongly preferred crystallographic orientation. The impact toughness values of the as-cast high-entropy alloy were much higher than those of many traditional alloys, particularly at low temperatures. The mechanical properties of the as-cast high-entropy alloy were compared with those of wrought high-entropy alloy and noticeable differences between the two alloys were found. The maximum tensile ductility and three different strain hardening stages were observed at 500 °C in the as-cast structure. Transmission electron microscopy observations demonstrated that the initiation of deformation twinning was very active even at 500 °C. A simple calculation suggests that very large grains of the as-cast structure induced a reduction in twinning stress, retarding the onset of strain localization.

Dislocation Networks and the Microstructural Origin of Strain Hardening

Abstract: When metals plastically deform, the density of line defects called dislocations increases and the microstructure is continuously refined, leading to the strain hardening behavior. Using discrete dislocation dynamics simulations, we demonstrate the fundamental role of junction formation in connecting dislocation microstructure evolution and strain hardening in face-centered cubic (fcc) Cu. The dislocation network formed consists of line segments whose lengths closely follow an exponential distribution. This exponential distribution is a consequence of junction formation, which can be modeled as a one-dimensional Poisson process. According to the exponential distribution, two non-dimensional parameters control microstructure evolution, with the hardening rate dictated by the rate of stable junction formation. Among the types of junctions in fcc crystals, we find that glissile junctions make the dominant contribution to strain hardening.

Microstructural characteristics and tensile behavior of medium manganese steels with different manganese additions

Abstract: Manganese is normally considered as the most important alloying element in medium Mn steels. However, the influence of Mn additions on the microstructural characteristics and the ensuring mechanical properties has not been much studied. This was addressed in the present study by investigating two variants of medium Mn steels with compositions of 0.2C-7/10Mn-3Al (in wt%), subjected to intercritical annealing (IA) and tensile testing. Results showed that a higher Mn addition can effectively increase the fraction of retained austenite (RA) at ambient temperature. However, the mechanical stability of RA was reduced, due to the lower C concentration partitioned into austenite during IA treatments. The higher fraction and lower mechanical stability of RA in the 10Mn steel gave rise to an enhanced transformation-induced plasticity (TRIP) effect, which was the main contributor to its higher strain hardening rate and improved mechanical properties, confirmed by a dislocation density-based strain hardening model. The yielding behavior and the Portevin-Le Chatelier (PLC) effect of the investigated steels were also found to be altered with different austenite mechanical stability as a consequence of different Mn additions and IA temperatures. Higher deformation-induced martensite transformation (DIMT) kinetics, i.e. lower mechanical stability of austenite, resulted in a lower yield point elongation, and eventually changed the yielding to a continuous manner due to the formation of stress-induced martensite. The PLC effect only occurred in the intermediate austenite stability range, and the critical strain for the onset of jerky flow showed a first decrease and then an increasing trend with higher DIMT kinetics. The observed beneficial effect of Mn additions on mechanical property improvement in medium Mn steels offers a significant aspect for further alloy design of such steels.

Back stress in strain hardening of carbon nanotube/aluminum composites

Abstract: As demonstrated by the loading–unloading tests and the modeling of the grain size effect and the composite effect, mainly owing to the back stress induced by CNTs, carbon nanotube/aluminum (CNT/Al) composites exhibit higher strain hardening capability than the unreinforced ultrafine-grained Al matrix. The back stress induced by CNTs should arise from the interfacial image force and the long-range interaction between statically stored dislocations and geometrically necessary dislocations around the CNT/Al interface. Therefore, this CNT-induced interfacial back stress strengthening mechanism is supposed to provide a novel route to enhancing the strain hardening capability and ductility in CNT/Al composites.

Dynamic deformation behavior of a face-centered cubic FeCoNiCrMn high-entropy alloy

Abstract: In this study, mechanical tests were conducted on a face-centered cubic FeCoNiCrMn high-entropy alloy, both in tension and compression, in a wide range of strain rates (10−4–104 s−1) to systematically investigate its dynamic response and underlying deformation mechanism. Materials with different grain sizes were tested to understand the effect of grain size, thus grain boundary volume, on the mechanical properties. Microstructures of various samples both before and after deformation were examined using electron backscatter diffraction and transmission electron microscopy. The dislocation structure as well as deformation-induced twins were analyzed and correlated with the measured mechanical properties. Plastic stability during tension of the current high-entropy alloy (HEA), in particular, at dynamic strain rates, was discussed in lights of strain-rate sensitivity and work hardening rate. It was found that, under dynamic conditions, the strength and uniform ductility increased simultaneously as a result of the massive formation of deformation twins. Specifically, an ultimate tensile strength of 734 MPa and uniform elongation of ∼63% are obtained at 2.3 × 103 s−1, indicating that the alloy has great potential for energy absorption upon impact loading.

On the mechanical response and microstructure evolution of NiCoCr single crystalline medium entropy alloys

Abstract: Unusual strain hardening response and ductility of NiCoCr equiatomic alloy were investigated through microstructural analysis of [111], [110] and [123] single crystals deformed under tension. Nano-twinning prevailed at, as early as, 4% strain along the [110] orientation, providing a steady work hardening, and thereby a significant ductility. While single slip dominated in the [123] orientation at the early stages of deformation, multiple slip and nanotwinning was prominent in the [111] orientation. Significant dislocation storage capability and resistance to necking due to nanotwinning provided unprecedented ductility to NiCoCr medium entropy alloys, making it superior than quinary variants, and conventional low and medium stacking fault energy steels.

Discrete-element modeling of nacre-like materials: Effects of random microstructures on strain localization and mechanical performance

Abstract: Natural materials such as nacre, collagen, and spider silk are composed of staggered stiff and strong inclusions in a softer matrix. This type of hybrid microstructure results in remarkable combinations of stiffness, strength, and toughness and it now inspires novel classes of high-performance composites. However, the analytical and numerical approaches used to predict and optimize the mechanics of staggered composites often neglect statistical variations and inhomogeneities, which may have significant impacts on modulus, strength, and toughness. Here we present an analysis of localization using small representative volume elements (RVEs) and large scale statistical volume elements (SVEs) based on the discrete element method (DEM). DEM is an efficient numerical method which enabled the evaluation of more than 10,000 microstructures in this study, each including about 5,000 inclusions. The models explore the combined effects of statistics, inclusion arrangement, and interface properties. We find that statistical variations have a negative effect on all properties, in particular on the ductility and energy absorption because randomness precipitates the localization of deformations. However, the results also show that the negative effects of random microstructures can be offset by interfaces with large strain at failure accompanied by strain hardening. More specifically, this quantitative study reveals an optimal range of interface properties where the interfaces are the most effective at delaying localization. These findings show how carefully designed interfaces in bioinspired staggered composites can offset the negative effects of microstructural randomness, which is inherent to most current fabrication methods.