Showing papers on "Strain hardening exponent published in 2014"

A fracture-resistant high-entropy alloy for cryogenic applications

Abstract: High-entropy alloys are equiatomic, multi-element systems that can crystallize as a single phase, despite containing multiple elements with different crystal structures. A rationale for this is that the configurational entropy contribution to the total free energy in alloys with five or more major elements may stabilize the solid-solution state relative to multiphase microstructures. We examined a five-element high-entropy alloy, CrMnFeCoNi, which forms a single-phase face-centered cubic solid solution, and found it to have exceptional damage tolerance with tensile strengths above 1 GPa and fracture toughness values exceeding 200 MPa·m(1/2). Furthermore, its mechanical properties actually improve at cryogenic temperatures; we attribute this to a transition from planar-slip dislocation activity at room temperature to deformation by mechanical nanotwinning with decreasing temperature, which results in continuous steady strain hardening.

Extraordinary strain hardening by gradient structure.

Abstract: Gradient structures have evolved over millions of years through natural selection and optimization in many biological systems such as bones and plant stems, where the structures change gradually from the surface to interior. The advantage of gradient structures is their maximization of physical and mechanical performance while minimizing material cost. Here we report that the gradient structure in engineering materials such as metals renders a unique extra strain hardening, which leads to high ductility. The grain-size gradient under uniaxial tension induces a macroscopic strain gradient and converts the applied uniaxial stress to multiaxial stresses due to the evolution of incompatible deformation along the gradient depth. Thereby the accumulation and interaction of dislocations are promoted, resulting in an extra strain hardening and an obvious strain hardening rate up-turn. Such extraordinary strain hardening, which is inherent to gradient structures and does not exist in homogeneous materials, provides a hitherto unknown strategy to develop strong and ductile materials by architecting heterogeneous nanostructures.

Properties of strain hardening ultra high performance fiber reinforced concrete (UHP-FRC) under direct tensile loading

Abstract: Enhanced matrix packing density and tailored fiber-to-matrix interface bond properties have led to the recent development of ultra-high performance fiber reinforced concrete (UHP-FRC) with improved material tensile performance in terms of strength, ductility and energy absorption capacity. The objective of this research is to experimentally investigate and analyze the uniaxial tensile behavior of the new material. The paper reviews and categorizes a variety of tensile test setups used by other researchers and presents a revised tensile set up tailored to obtain reliable results with minimal preparation effort. The experimental investigation considers three types of steel fibers, each in three different volume fractions. Elastic, strain hardening and softening tensile parameters, such as first cracking stress and strain, elastic and strain hardening modulus, composite strength and energy dissipation capacity, of the UHP-FRCs are characterized, analyzed and linked to the crack pattern observed by microscopic analysis. Models are proposed for representing the tensile stress–strain response of the material.

Effect of the martensite distribution on the strain hardening and ductile fracture behaviors in dual-phase steel

Abstract: In order to clarify the effects of the martensite distribution on the mechanical properties of low-carbon dual-phase steel, four types of dual-phase steel with different ferrite grain sizes and martensite distributions were prepared using a thermomechanical treatment. The tensile properties of these steels were investigated; in particular, the strain hardening and the ductile fracture behaviors were discussed in terms of the strain partitioning between the ferrite and martensite and the formation and growth of micro-voids, respectively. When the martensite grains surround the ferrite grains and form a chain-like networked structure, the strain hardenability is greatly improved without a significant loss of elongation, while the necking deformability is considerably reduced. A digital-image correlation analysis revealed that the tensile strain in the martensite region in the chain-like networked dual-phase structure is markedly increased during tensile deformation, which leads to an improvement in the strain hardenability. On the other hand, the joint part of the martensite grains in the structure acts as a preferential formation site for micro-voids. The number density of the micro-voids rapidly increases with increasing tensile strain, which would cause the lower necking deformability.

Effect of fresh martensite on the stability of retained austenite in quenching and partitioning steel

Abstract: Restrictions on fuel consumption and safety in the automotive industry have stimulated the development of quenching and partitioning (Q&P) steel. This steel is expected to have very high strength in combination with acceptable ductility owing to its microstructure consisting of martensite with a considerable amount of retained austenite. The effect of retained austenite on the mechanical properties and its transformation stability were determined by stepwise uniaxial micro-tensile testing and subsequent electron backscatter diffraction (EBSD) study of a pre-selected region. The austenite fraction evolution with increasing plastic deformation and the influence of fresh martensite on the local strain distribution were quantified based on the orientation data. The decrease of the retained austenite as a function of the applied strain was described by an exponential function with the pre-exponential and exponential factors related to the starting austenite fraction and its transformation stability respectively. It was proven that the presence of fresh martensite has a negative influence on this austenite transformation stability due to its constraining effect on the strain distribution. This effects the mechanical properties manifested by changes in the strain hardening behavior and total elongation. The results suggest that the ductility of the Q&P steels can be improved by an appropriate design of the heat treatment schedule in order to ensure high retained austenite fractions without the presence of fresh martensite in the final microstructure.

Strengthening mechanisms of T1 precipitates and their influence on the plasticity of an Al-Cu-Li alloy

Abstract: This paper presents a systematic study of the influence of the state of precipitation on the plasticity of an Al–Cu–Li alloy. By varying the heat treatment, a variety of T1 precipitate morphologies are obtained. Atomic resolution electron microscopy observations show that T1 precipitates remain shearable even during early stages of over-ageing when their thickness is of several nanometres. They appear to be sheared only by single dislocations at a given location, which prevents catastrophic strain localisation at a microscopic scale. In later stages of over-ageing, the study of macroscopic strain hardening rate and of slip line localisation strongly suggests a transition to precipitate by-passing. The influence of the strengthening mechanism on strain reversal experiments (Bauschinger effect) is discussed.

High strength and ductile low density austenitic FeMnAlC steels: Simplex and alloys strengthened by nanoscale ordered carbides

Abstract: We introduce the alloy design concepts of high performance austenitic FeMnAlC steels, namely, Simplex and alloys strengthened by nanoscale ordered κ-carbides. Simplex steels are characterised by an outstanding strain hardening capacity at room temperature. This is attributed to the multiple stage strain hardening behaviour associated to dislocation substructure refinement and subsequent activation of deformation twinning, which leads to a steadily increase of the strain hardening. Al additions higher that 5 wt-% promote the precipitation of nanoscale L′12 ordered precipitates (so called κ-carbides) resulting in high strength (yield stress ∼1·0 GPa) and ductile (elongation to fracture ∼30%) steels. Novel insights into dislocation–particle interactions in a Fe–30·5Mn–8·0Al–1·2C (wt-%) steel strengthened by nanoscale κ-carbides are discussed.

Identification of Post-Necking Hardening Phenomena in Ductile Sheet Metal

Abstract: A combined theoretical/experimental approach accurately quantifying post-necking hardening phenomena in ductile sheet materials that initially exhibit diffuse necking in tension is presented. The method is based on the minimization of the discrepancy between the internal and the external work in the necking zone during a quasi-static tensile test. The main focus of this paper is on the experimental validation of the method using an independent material test. For this purpose, the uniaxial tube expansion test is used to obtain uniaxial strain hardening behavior beyond the point of maximum uniform strain in a tensile test. The proposed method is used to identify the post-necking hardening behavior of a cold rolled interstitial-free steel sheet. It is demonstrated that commonly adopted phenomenological hardening laws cannot accurately describe all hardening stages. An alternative phenomenological hardening model is presented which enables to disentangle pre- and post-necking hardening behavior. Additionally, the influence of the yield surface on the identified post-necking hardening behavior is scrutinized. The results of the proposed method are compared with the hydraulic bulge test. Unlike the hydraulic bulge test, the proposed method predicts a decreased hardening rate in the post-necking regime which might be associated with probing stage IV hardening. While inconclusive, the discrepancy with the hydraulic bulge test suggests differential work hardening at large plastic strains.

Strain hardening in nanolayered thin films

Abstract: Experimental results indicate that metal–ceramic multilayered thin films have unusual properties such as high strength, measurable plasticity and high strain hardening rate when both layers are nanoscale. Furthermore, the strength and strain hardening rate show a pronounced size effect, depending not only on the layer thickness but also on the layer thickness ratio. We analyze the strain hardening behavior of nanoscale multilayers using a three-dimensional crystal elastic–plastic model (3DCEPM) that describes plastic deformation based on the evolution of dislocation density in metal and ceramic layers according to confined layer slip mechanism. These glide dislocations nucleate at interfaces, glide inside layers and are deposited at interfaces that impede slip transmission. The high strain hardening rate is ascribed to the closely spaced dislocation arrays deposited at interfaces and the load transfer that is related to the layer thickness ratio of metal and ceramic layers. The measurable plasticity implies the plastically deformable ceramic layer in which the dislocation activity is facilitated by the interaction force among the deposited dislocations within interface and in turn is strongly related to the ceramic layer thickness.

Determination of the plastic properties of materials treated by ultrasonic surface rolling process through instrumented indentation

Abstract: Ultrasonic surface rolling process (USRP) is a novel surface nanocrystallization technique based on severe plastic deformation (SPD). The combination of static extrusion and dynamic impact generates intensive plastic deformation, which leads to the strengthening of the material surface. The present paper aims to investigate the mechanical properties of material surface after USRP. For this purpose, nano-indentation tests were adopted to obtain the load P and penetration depth h. Dimensional analysis of test results (P–h curves) was then performed for determining the microplasticity of the treated material locally. The values of yield strength and strain hardening exponent of the surface layer were calculated and verified by finite-element simulation. Good agreement was obtained between experimental curves and simulated ones. The microstructural evolution of the surface treated by USRP was discussed to interpret the mechanism of surface strengthening. Finally, effects of USRP parameters including static force, vibration amplitude and repeated processing numbers on the variation of plastic parameters in the surface layer were discussed.

Rheological and foaming behavior of linear and branched polylactides

Abstract: In this work, a chain extender (CE) was added to polylactide (PLA) to improve its foamability. The steady and transient rheological properties of neat PLA and CE-treated PLA revealed that the introduction of the CE profoundly affected the melt viscosity and elasticity. The linear viscoelastic properties of CE-enriched PLA suggested that a long-chain branching (LCB) structure was formed from the reaction with the CE. LCB-PLA exhibited an increased viscosity, more shear sensitivity, and longer relaxation time in comparison with the linear PLA. The LCB structure was also found to affect the transient shear stress growth and elongational flow behavior. LCB-PLA exhibited a pronounced strain hardening, whereas no strain hardening was observed for the linear PLA. Batch foaming of the linear and LCB-PLAs was also examined at foaming temperatures of 130, 140, and 155 °C. The LCB structure significantly increased the integrity of the cells, cell density, and void fraction.