Showing papers on "Strain hardening exponent published in 2016"

Metastable high-entropy dual-phase alloys overcome the strength–ductility trade-off

Abstract: Metals have been mankind's most essential materials for thousands of years; however, their use is affected by ecological and economical concerns Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency However, most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength-ductility trade-off Here we present a metastability-engineering strategy in which we design nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection, the concept is overturned We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase) This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels and massive solid-solution strengthening of high-entropy alloys In our transformation-induced plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength Moreover, the increased strain hardening capacity that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials This metastability-engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys

Exceptional damage-tolerance of a medium-entropy alloy CrCoNi at cryogenic temperatures

Abstract: High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Here we examine an equiatomic medium-entropy alloy containing only three elements, CrCoNi, as a single-phase face-centred cubic solid solution, which displays strength-toughness properties that exceed those of all high-entropy alloys and most multi-phase alloys. At room temperature, the alloy shows tensile strengths of almost 1 GPa, failure strains of ∼70% and KJIc fracture-toughness values above 200 MPa m(1/2); at cryogenic temperatures strength, ductility and toughness of the CrCoNi alloy improve to strength levels above 1.3 GPa, failure strains up to 90% and KJIc values of 275 MPa m(1/2). Such properties appear to result from continuous steady strain hardening, which acts to suppress plastic instability, resulting from pronounced dislocation activity and deformation-induced nano-twinning.

Back stress strengthening and strain hardening in gradient structure

Abstract: We report significant back stress strengthening and strain hardening in gradient structured (GS) interstitial-free (IF) steel. Back stress is long-range stress caused by the pileup of geometrically necessary dislocations (GNDs). A simple equation and a procedure are developed to calculate back stress basing on its formation physics from the tensile unloading–reloading hysteresis loop. The gradient structure has mechanical incompatibility due to its grain size gradient. This induces strain gradient, which needs to be accommodated by GNDs. Back stress not only raises the yield strength but also significantly enhances strain hardening to increase the ductility.Impact Statement: Gradient structure leads to high back stress hardening to increase strength and ductility. A physically sound equation is derived to calculate the back stress from an unloading/reloading hysteresis loop.

Mechanical properties of Austenitic Stainless Steel 304L and 316L at elevated temperatures

Abstract: Austenitic Stainless Steel grade 304L and 316L are very important alloys used in various high temperature applications, which make it important to study their mechanical properties at elevated temperatures. In this work, the mechanical properties such as ultimate tensile strength ( UTS ), yield strength ( Y S ), % elongation, strain hardening exponent ( n ) and strength coefficient ( K ) are evaluated based on the experimental data obtained from the uniaxial isothermal tensile tests performed at an interval of 50 °C from 50 °C to 650 °C and at three different strain rates (0.0001, 0.001 and 0.01 s −1 ). Artificial Neural Networks (ANN) are trained to predict these mechanical properties. The trained ANN model gives an excellent correlation coefficient and the error values are also significantly low, which represents a good accuracy of the model. The accuracy of the developed ANN model also conforms to the results of mean paired t -test, F -test and Levene's test.

Strain hardening ultra-high performance fiber reinforced cementitious composites: Effect of fiber type and concentration

Abstract: An experimental work was carried out to investigate the effects of micro-steel, hooked-steel, and micro-glass fibers on the mechanical properties and ductility of Ultra-high Performance Cementitious Composites (UHPC). The aspect ratios of the of micro-steel, hooked-steel, and micro-glass fibers were 7.17, 55 and 722, respectively. At a water-binder ratio of 0.195, three groups of UHPCs containing 0.25, 0.5, 0.75, 1, 1.5, and 2% fiber volume fractions were produced and tested for compressive strength, splitting tensile strength, modulus of elasticity, flexural strength, load-displacement behavior, fracture energy, and characteristic length. Test results revealed that the mixes with 2% of micro steel fiber exhibited the best compressive strength of 180 MPa as well as the highest splitting tensile strength and modulus of elasticity. However, the mixes with 2% of hooked steel fiber displayed a strain hardening load-displacement behavior with a substantially enhanced ductility. The results also showed that the beneficial influence of using micro glass fiber began to decrease after 1.5% of the fiber volume.

Matrix design of strain hardening fiber reinforced engineered geopolymer composite

Abstract: The feasibility of developing a fiber reinforced engineered geopolymer composite (EGC) exhibiting strain hardening behavior under uni-axial tension has been recently demonstrated. The effect of different alkaline activators on the matrix and composite behavior of such EGC has also been evaluated to enhance its compressive and tensile strengths with relatively low concentration activator combinations. The focus of this study, as a follow up investigation, is to evaluate the quantitative influence of geopolymer matrix properties on the strain hardening behavior of the recently developed fly ash-based EGC with the aim of selecting the appropriate type of geopolymer matrix to manufacture the strain hardening EGC with enhanced elastic modulus while maintaining the tensile ductility behavior of the composite. The effects of water to geopolymer solids ratio, sand size and sand content, as the most significant matrix-related parameters, on the matrix properties including workability, compressive strength, elastic modulus, fracture toughness and crack tip toughness, and the uni-axial tensile performance of the composite were evaluated. Experimental results revealed that lowering the water to geopolymer solids ratio and the addition of sand enhanced the elastic modulus of the geopolymer matrix and composite in all cases. However, the excessive use of fine sand and the use of coarse sand adversely affected the strain hardening behavior of the developed EGC due to the increase of the matrix fracture toughness and the first-crack strength of the composite. Only geopolymer matrices with suitable fracture toughness, as defined by the micromechanics design model, maintained the desirable tensile ductility of the developed fly ash-based EGC.

The effects of grain size on yielding, strain hardening, and mechanical twinning in Fe–18Mn–0.6C–1.5Al twinning-induced plasticity steel

Abstract: The objective of the present study was to investigate the influences of grain refinement on yielding, strain hardening, and mechanical twinning during tensile deformation in Fe-high Mn twinning-induced plasticity (TWIP) steel. For this purpose, Fe–18Mn–0.6C–1.5Al TWIP steels with average grain sizes of 2, 10, and 50 μm were tensile tested at room temperature, and their stress–strain and strain hardening rate curves, dislocation densities, and microstructures were measured and analyzed by means of transmission electron microscopy and neutron diffractometry. The stress–strain curves showed a transition from continuous to discontinuous yielding with grain refinement, which was due to a lack of mobile dislocations, not due to mechanical twinning or martensitic transformation. The grain refinement increased the dislocation density, caused the planar to non-planar slip, and retarded primary and secondary mechanical twinning. The strain hardening rate–strain curves of TWIP steels used were able to be divided into five stages by the slope change. Until the stage III, dislocation hardening was predominant; at the stages IV and V mechanical twinning became more contributive to strain hardening. The suppression of both planar dislocation slip and mechanical twinning by grain refinement is most likely due to the increase in the back stress of dislocations on a slip plane, which was caused by the rapid accumulation of dislocations by plastic deformation in the fine-grained TWIP steel. A high level of back stress narrows the width of stacking faults, facilitates the cross slip of dislocations, and reduces the interactions between partial dislocations required for mechanical twinning.

Microstructure and mechanical properties of the new Nb25Sc25Ti25Zr25 eutectic high entropy alloy

Abstract: The novel quaternary Nb25Sc25Ti25Zr25 (at.%) high entropy alloy with dual-phase structure was developed and cast by arc melting of elemental precursors. The as-cast state comprised of lamellar eutectics with alternating α(Sc, Zr) with HCP lattice plates having thickness of 30–50 nm, length up to 600 nm and inter-lamellar spacing of 5–40 nm, embedded in the β-NbZrTi matrix with BCC structure. While the α(Sc, Zr) plates showed reduced content of Nb, the β-NbZrTi matrix was particularly depleted in Sc. The unique mixture of fine hexagonal α(Sc, Zr) plates within the β-NbTiZr matrix in as-cast state led to relatively high combination of strength and ductility: the alloy with initial hardness of 418 HV reached compressive strength of 1250 MPa, yield strength of 1020 MPa and compressibility of 8.2%. The phase composition exhibited high stability with no significant changes taking place after annealing at 1000 ˚C for 24 h. The high-temperature exposure caused only coarsening of α(Sc, Zr) hexagonal plates, which reached 3 μm in width and 5-10 μm in length. The plastic strain of the annealed alloy with hardness of 202 HV increased to 20% while compression strength decreased to 670 MPa. As opposed to the as-cast state, the annealed alloy exhibited limited strain hardening during room-temperature compression.