Uniform elongation and the stress-strain flow curve of steels calculated from hardness using empirical correlations

An empirical relationship between the hardness and uniform elongation of non-Austenitic hypoeutectoid steels has been developed. This new hardness-elongation relationship was combined with previously developed correlations of hardness and strength (yield and ultimate tensile strength) to predict the stressstrain flow curve from a single hardness test. The current study considers both power law hardening behavior and exponential hardening behavior. Reasonable agreement was observed between the experimental and predicted flow curves of a high strength, low alloy steel. Additionally, an empirical correlation of the flow strength at instability with hardness is provided. © ASM International.

Effect of alloying and extrusion temperature on the microstructure and mechanical properties of Mg-Zn and Mg-Zn-RE alloys

Extruded Mg-Zn-RE alloys have been shown to exhibit excellent combinations of yield strength and ductility, but it is not completely clear how adding rare earth metals to Mg-Zn alters the microstructure and affects the mechanical properties. Microstructural changes and the resulting mechanical properties from changes in composition and extrusion temperature have been investigated for Mg-. x Zn-. y RE (. x=2.5 and 5. wt.%, y=0 and 1. wt. %, and RE=Gd and Y) alloys. Adding RE to Mg-Zn increased the strength and reduced the ductility, while increasing the zinc concentration in the Mg-Zn-RE alloys had the reverse effect.

Grain growth and β-Mg17Al12 intermetallic phase dissolution during heat treatment and its impact on deformation behavior of AZ80 Mg-alloy

Microstructure evolution after solutionizing and ageing treatment of cast AZ80 Mg alloy were investigated using optical and scanning electron microscopy. Effect of these treatments on grain size, β-Mg17Al12 intermetallic phase, mechanical behavior, and flow asymmetry were investigated. The initial continuous network of β-phase found to be reduced after solutionizing. The dissolution of β-phase and simultaneous grain growth are found to be interrelated. Mechanical properties including yield strength, maximum strength (ultimate compressive strength), and maximum strain attainable in compressive found almost twice than the corresponding values obtained in tension. The asymmetry in compressive and tensile properties is found to decrease with grain size at certain solutionizing duration. Particular heat treatment found to offer best combination of tensile compressive flow properties in AZ80 Mg alloy. Aging under certain conditions found to minimize the strength asymmetry. © ASM International.

Kinetics of γ-Mg17Al12 phase dissolution and its effect on room temperature tensile properties in as-cast AZ80 magnesium alloy

As-cast AZ80 magnesium alloy consists of α-Mg, eutectic product of α-Mg and γ-Mg17Al12, with the latter present in the form of partially and fully divorce eutectic. There occurs dissolution of harder γ-Mg17Al12 phase during homogenization treatment at 400 ° and 439 °C. The proportion of the α-Mg and γ-Mg17Al12 phase was varied by solutionizing the alloy for various lengths of time at these temperatures, in order to investigate the kinetics of phase transformation and to evaluate the effect of phase proportion, size and morphology on room temperature tensile properties. It was found that the yield strength decreases with the increase in solutionizing temperature from 400° to 439 °C and at the same time, ductility in general increases with the increasing solutionizing temperature. The variation in tensile properties and the nature of fractographs were analyzed in terms of the effects of microstructure. © (2014) Trans Tech Publications, Switzerland.

Bio-inspired hydrogen-bond cross-link strategy toward strong and tough polymeric materials

It remains a huge challenge to create advanced polymeric materials combining high strength, great toughness, and biodegradability so far. Despite enhanced strength and stiffness, biomimetic materials and polymer nanocomposites suffer notably reduced extensibility and toughness when compared to polymer bulk. Silk displays superior strength and toughness via hydrogen bonds (H-bonds) assembly, while cuticles of mussels gain high hardness and toughness via metal complexation cross-linking. Here, we propose a H-bonds cross-linking strategy that can simultaneously strikingly enhance strength, modulus, toughness, and hardness relative to polymer bulk. The H-bond cross-linked poly(vinyl alcohol) exhibits high yield strength (140 MPa), reduced modulus (22.5 GPa) in nanoindention tests, hardness (0.5 GPa), and great extensibility (40%). More importantly, there exist semiquantitive linear relationships between the number of effective H-bond and macroscale properties. This work suggests a promising methodology of designing advanced materials with exceptional mechanical by adding low amounts (1.0 wt %) of small molecules multiamines serving as H-bond cross-linkers.

Experimental study of damage evolution in cuboid stirrup-confined concrete

This paper presents an experimental study on cuboid stirrup-confined concrete specimens under uniaxial monotonic loading and cyclic loading. The effects of stirrup volume ratio, stirrup yield strength, and concrete strength on damage evolution of the stirrup-confined concrete were investigated. The experimental results showed that the strength and ductility of concrete are improved by appropriate arrangement of the stirrup confinement. Firstly, with the increase of the stirrup volume ratio, the damage evolution of concrete can be relatively restrained. Secondly, higher stirrup yield strength usually leads to larger confining pressures and slower damage evolution. In contrast, higher concrete strength leads to higher brittleness, which accelerates damage evolution. Based on the experimental data, a plastic strain expression is obtained through curve fitting, and a damage evolution equation for stirrup-confined concrete is proposed by introducing a confinement factor (C). The comparisons results demonstrated that the proposed damage evolution curve can accurately describe the experimental results.

Microstructure and mechanical properties of wrought and additive manufactured Ti-6Al-4V cylindrical bars

Titanium alloys are widely used in various engineering design application due to its superior material properties. The traditional manufacturing of titanium products is always difficult, time consuming, high material wastage and manufacturing costs. Selective laser melting (SLM), an additive manufacturing technology has widely gained attention due to its capability to produce near net shape components with less production time. In this technical paper,microstructure,chemical composition,tensile properties and hardness are studied for the wrought and additive manufactured SLM cylindrical bar. Microstructure,mechanical properties and hardness were studied in both the longitudinal and transverse directions of the bar to study the effect of orientation. It was found that additive manufactured bar have higher yield strength, ultimate tensile strength and hardness than the wrought bar. For both conventional and SLM test samples, the yield strength, ultimate tensile strength and hardness was found to be high in the transverse direction. The difference in the properties can be attributed to the difference in microstructure as a result of processing conditions. The tensile fracture area was quantified by careful examination of the fracture surfaces in the scanning electron microscope.

Experimental and stochastic approaches to assessing the strain demand of pipelines and flexibility requirements for coatings

A criterion for selecting a coating for an energy pipeline is that the coating should have a suitable flexibility to meet the high strain demand during hydrostatic testing and during field bending. This requires knowledge of the level of strain demand for the pipeline, and also the maximum strain that could be
tolerated by the coating system. Whereas average strains imposed during manufacturing and construction are reasonably well predicted, there is insufficient understanding on the factors leading to localised deformation of the pipe. Significant work has been carried out in the past to develop tests for assessing
the coatings’ ability to handle a certain amount of strain based on bend testing, tensile testing and burst testing. However, there is a concern as to whether these tests properly represent localised micro-strains associated with construction activities including field bending and pressure testing, particularly pressure testing of pipelines designed for operation at 80% of specified minimum yield strength (SMYS). Consequently coatings considered "suitable" for modern pipelines may fail. The first issue discussed in this paper is main factors affecting strain localisation. The non-deterministic distributions of heterogeneities over the pipe provide a ground to consider the mechanisms of localisation as a stochastic process. An approach is proposed to quantify the maximum localised strain demand through cold field bending and hydrostatic experiments. Another issue discussed in this paper is the experimental assessment of coating flexibility under the effects of localised strains. Preliminary mandrel tests have been carried out to assess the uniformity of the imposed strain. Although mandrel testing has been shown to be a useful method for relative comparison of coating flexibility, it has several weaknesses that could significantly affect the reliability and reproducibility of the results.

Enhanced ductility and reduced asymmetry of Mg-2Al-1Zn alloy plate processed by torsion and annealing

The ductility and plastic asymmetry of an as-annealed magnesium alloy plate were studied in compression through combined process of torsion and subsequent annealing by optical microscope and EBSD. The yield strength (YS) and ultimate compression strength (UCS) as well as the compression ductility (CD) were simultaneously raised by prior torsion at room temperature. The CD was further enhanced by subsequent annealing. Also, the torqued sample followed by annealing experienced a rising CD with the increase in prior strain, leading to the maximum true strain of 0.279, which is twice that of the as-annealed original one. The sample showed a largely reduced tension-compression yield asymmetry by subjecting to pre-torsion alone or combined with a subsequent annealing. The enhanced ductility and reduced asymmetry are attributed to the development of a gradient microstructure with refined grains, and also randomization of the weakened texture due to torsion and subsequent annealing.

Investigation of nano-scale precipitation phenomena in Al-Fe and Al-Cu-(Fe) alloys processed via direct strip casting

The use of rapid solidification processes such as direct strip casting (DSC) is a good way to refine the Fe-intermetallics and decrease their detrimental effects. DSC creates out-ofequilibrium supersaturated microstructures. In this work, we explore the precipitation phenomena in direct strip cast Al-Fe and Al-Cu-Fe alloys and related corrosion and mechanical properties. The precipitates are characterised with differential scanning calorimetry and transmission electron microscopy. The corrosion performances are evaluated with immersion tests and weight loss measurements and the yield strength and ductility are estimated with tensile tests. A strong correlation between the microstructure and the bulk properties is revealed with a significant improvement of properties of DSC alloys.

Influence of backpressure during ECAP on the monotonic and cyclic deformation behavior of AA5754 and Cu99.5

Ultrafine-grained (UFG) metals produced by equal channel angular pressing (ECAP) exhibit outstanding mechanical properties. They show high strength under monotonic loading as well as strongly enhanced fatigue lives in the Wöhler S-N-plot compared to their coarse grained (CG) counterparts. It could be shown that the fatigue lives can be significantly enhanced further by applying backpressure during ECAP. Besides the positive effect of backpressure on the processability of hard to deform materials via ECAP, the hydrostatic stress induced by backpressure also influences the mechanical properties under monotonic and cyclic loading. Therefore the influence of backpressure on ECAPed Cu99.5 and on the ECAPed aluminum alloy AA5754 was investigated. It is shown that backpressure has no effect on the hardness and grain size in Cu99.5 but changes the grain boundary misorientation to higher fractions of low angle grain boundaries. Also the temperature dependency of the yield strength as well as the hardening behavior under monotonic compression is affected. The cyclic deformation behavior of Cu99.5 is not strongly influenced by backpressure, but the mean stress level changes drastically. The fatigue life increases with the application of backpressure at low plastic amplitudes due to a change in the crack initiation and propagation. Aim of this work is the investigation of the influence of backpressure during equal channel angular pressing (ECAP) on the mechanical properties under monotonic and cyclic loading. Therefore we performed hardness measurements, compression, and fatigue tests on ECAPed Cu99.5 and AA5754. The results are discussed in terms of microstructure and relevant deformation and damage mechanisms.

Equal-channel angular pressing and annealing of a twinning-induced plasticity steel : microstructure, texture, and mechanical properties

In this work, a high-manganese Fe-23Mn-1.5Al-0.3C Twinning-Induced Plasticity (TWIP) steel was subjected to plastic shear deformation using Equal-Channel Angular Pressing (ECAP) at 300 °C following route BC and additional annealing. The microstructure evolution during both deformation by ECAP and subsequent annealing was investigated and correlated with the mechanical properties. The successive grain refinement during ECAP was promoted by two parallel mechanisms, namely dislocation driven grain fragmentation and twin fragmentation, and accounted for the ultra-high strength. In addition, due to the relatively low volume fraction of deformation twins after ECAP at 300 °C, further contribution of deformation twinning during room temperature deformation allowed additional work-hardening capacity and elongation. During subsequent recovery annealing the ultra-fine grains and deformation twins were thermally stable, which supported retainment of the high yield strength along with regained uniform elongation. For the first time, the texture evolution during ECAP and during the following heat treatment was analyzed. After 1, 2, and 4 ECAP passes a transition texture with the characteristic texture components of both high- and low-SFE materials developed. During the following heat treatment the texture evolution proceeded similar to that observed in the same material after cold rolling. Retaining of the ECAP texture components due to oriented nucleation at grain boundaries and triple junctions as well as annealing twinning accounted for the formation of a weak, retained ECAP texture after recrystallization.

The effect of ageing on the compressive deformation of Mg-Sn-Zn-Na alloy

The influence of ageing on deformation twinning in an extruded Mg-6Sn-3Zn-0.04Na alloy is investigated. In-situ compression tests have been carried out using high resolution synchrotron X-Ray Diffraction (XRD) to measure the influence of precipitates on twining activity. Synchrotron experiments revealed the increase in the critical resolved shear stress of twinning with ageing. The compressive yield strength (along the extrusion direction) of the aged sample increased by ∼ 150% over the non-aged specimen. To obtain statistical insight into the twinning activity, the microstructure of the non-aged and aged samples (200°C, 24 hours) deformed up to ∼1% plastic strain was studied using optical microscopy. A higher number of thinner twins were observed in the microstructure of the aged sample compared to the non-aged sample.

Effect of coiling treatment on microstructural development and precipitate strengthening of a strip cast steel

The effect of a simulated coiling treatment on a strip cast Nb-containing steel has been investigated. A lath ferritic supersaturated microstructure was observed in the as-cast condition with no coiling. The microstructure remained lath like during coiling at high temperature (850 °C) and the formation of chemically complex Nb-rich precipitates containing C, N, Si and S was observed. Coiling at an intermediate temperature (700 °C) caused the formation of polygonal ferrite with a dendritic morphology due to chemical micro-segregation. The polygonal ferrite contained Nb(C,N) precipitates. The microstructure remained lath like at the lowest coiling temperature (600 °C). In the latter case the precipitation of Nb-rich clusters was observed, and atom probe tomography revealed them to be ∼85% Fe. Small angle neutron scattering and transmission electron microscopy were used to quantify precipitation kinetics during coiling and the mechanical properties were evaluated with a shear punch apparatus. A yield strength model was developed to describe the observed mechanical behaviour, and this showed that the two largest contributors to strength were the bainitic microstructure and the Nb-rich precipitates. Strategies to further strengthen these materials are suggested.