On tension–compression asymmetry in ultrafine-grained and nanocrystalline metals

We present a physically motivated computational study explaining the tension/compression (T/C) asymmetry phenomenon in nanocrystalline (nc) and ultrafine-grained (ufg) face centered cubic (fcc) metals utilizing a variational constitutive model where the nc-metal is modeled as a two-phase material consisting of a grain interior phase and a grain boundary affected zone (GBAZ). We show that the existence of voids and their growth in GBAZ renders the material pressure sensitivity due to porous plasticity and that the utilized model provides a physically sound mechanism to capture the experimentally observed T/C asymmetry in nc- and ufg-metals.     

Cyclic creep behaviour under tension–tension loading cycles with hold time of modified 9Cr–1Mo steel

Abstract

Cyclic creep behaviour of modified 9Cr–1Mo steel was investigated by a series of cyclic creep (CC) tests at 600°C, which were performed under controlled tension–tension loading cycles with the magnitude of stress ranges in a constant stress ratio (R?=?0·1). Hold time was applied for a 10 min hold at the maximum stress (σ_max) and minimum stress (σ_min). The CC properties were compared with the static creep (SC) using Norton’s power law, Larson–Miller plot, and Monkman–Grant relation, and the microstructure was examined. For the test conditions employed in the present investigation, retardation in the CC behaviour in terms of a lower creep rate and longer rupture time compared to those in the SC was obtained. The retardation was ascribed to the effects associated with anelastic recovery during the 10 min hold time at the minimum load of the cyclic loading. The creep rupture ductility decreased with a general decrease in stress, and there was no difference in the creep ductility between the CC and SC. The steel displayed a transgranular fracture characterised by the presence of dimples resulting from micro-void coalescence. Carbide precipitation was more coarsened with increasing in exposure time in the CC tests.     

Peculiarities of high-temperature superelasticity in Ni–Fe–Ga single crystals in compression

Technical Physics Letters - The high-temperature superelasticity and temperature dependence of the yield stress of 14M and L10 martensite in [001]-oriented single crystals of Ni54Fe19Ga27 (at %) in...     

Characterising the shear–tension coupling and wrinkling behaviour of woven engineering fabrics

Modelling the forming process for engineering fabrics and textile composites using a mechanical approach, such as the finite element method, requires characterisation of the material’s behaviour under large shear deformation. For woven engineering fabrics, a coupling between in-plane tension and both shear compliance and the onset of wrinkling is to be expected. This paper focuses on a novel testing technique, the biaxial bias extension test, as a means to investigate this shear–tension coupling and fabric wrinkling. Novel methods of determining the wrinkling behaviour are demonstrated. The main difficulty with the technique lies in extracting the material contribution to the recorded signal. To do this, an experimental method is proposed and demonstrated using a plain weave glass fabric. Biaxial bias extension test results are compared against picture frame and uniaxial bias extension results.     

The effect of Ni additions on the oxidation behaviour of Co–Re–Cr high-temperature alloys

Abstract

The present study focused on the influence of Ni on the microstructure and oxidation behaviour of Co–Re–Cr-based alloys. Alloys with three different Ni contents were tested in laboratory air at 800–1100 °C. A refinement and a reduction of the σ phase volume fraction as well as a change in the matrix microstructure were observed. Thermogravimetric measurements showed that the alloys with higher Ni contents possess a better oxidation resistance when exposed to higher temperatures. All alloys suffered from continuous mass loss during oxidation at 800 °C due to the formation of porous oxides scales, consisting of Co₃O₄, Co(Ni)O and Ni-doped CoCr₂O₄, which allow the evaporation of Re-oxides. At 900–1100 °C, only the alloy with 25 at. % Ni showed parabolic oxidation kinetics after a short period of transient oxidation. This is a result of the fast formation of a protective Cr₂O₃ layer. It was also found that exposure to air at 1000 °C leads to a phase transformation of the bulk material; an oxidation-induced formation of fine hexagonal close-packed (hcp) grains was observed near the oxide scales. It is supposed that the improved oxidation resistance of Ni-containing Co–Re–Cr alloys is a result of enhanced Cr diffusion caused by the Ni addition. The extensive formation of the fcc phase in the alloy matrix had a detrimental effect on the oxidation behaviour of the Ni-containing Co–Re–Cr-based alloys.     

The application of equal channel angular pressing to join dissimilar metals,aluminium alloy and steel,using an Ag–Cu–Sn interlayer

Joining cylindrical and bar-shaped components manufactured from dissimilar materials is frequently required in various industrial applications. The current study focuses on developing equal channel angular pressing (ECAP) as a severe plastic deformation process for solid state joining of tubular aluminium alloy 6061 components and SAE 1018 steel rods. The influence of using a 0.1 mm thick 60Ag–30Cu–10Sn interlayer in addition to annealing at 220, 320, 420 and 520 °C for 60 min is investigated as well. Finite element analysis (FEA) is performed in order to evaluate the deformation behaviour of the workpieces during the ECAP joining process. XRD and EDX analyses as well as nanoindentation and shear tests are carried out to evaluate the joints' characteristics. The FEA outcomes show remarkable accumulation of equivalent plastic strain with relatively low strain inhomogeneity. Moreover, the experimental results indicate that with increasing annealing temperature, joint strength exhibits improvement as well. It is also revealed that the application of an interlayer at any specific annealing temperature leads to achieving higher shear strength values. According to the results, shear strength of up to 32 MPa is feasible by having an interlayer and with subsequent annealing at 520 °C.     

Influence of Cr content on high-temperature oxidation behaviour of arc sprayed Ni–Cr coatings

In this study, the high-temperature oxidation behaviour of arc-sprayed Ni–Cr coatings with high Cr contents of 30, 45 and 50 at.% was investigated in comparison with reference AISI 1020 steel. X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy were utilised to characterise the oxide scales. The oxidation resistance of the steel substrates was found to be enhanced after the application of the Ni–Cr coatings since the oxidation kinetics followed the parabolic law. In addition, the oxidation rate of Ni–50Cr coating was 56.5% lower than that of Ni–30Cr coating, indicating that the oxidation performance of coatings was improved with increasing Cr content. The oxide layers of Ni–Cr coating were found to be a double layer structure protecting the substrate from severely oxidation, which composed of a top layer of NiO and a basal layer of Cr₂O₃ and NiCr₂O₄. The surface of Ni–30Cr coating contained lots of multi-angle NiO crystals, while the surface of Ni–50Cr coating contained a dense Cr₂O₃ structure, suggesting that the growth of NiO crystals was limited due to the large amount of Cr-rich oxides.     

Microstructural evolution and deformation behaviour of wrought Mg–Al–Zn alloys under dynamic compression

We perform dynamic compression to extruded AZ31 Mg alloys both along and perpendicular to the extrusion direction (ED) at a high strain rate of 1400?s^?1 using split Hopkison pressure bar, and study the microstructure, texture evolution and mechanical anisotropy. We find that the temperature and loading direction play an important role in affecting microstructure evolution and flow stress. The mechanical anisotropy, yield strength and hardening ability are found to decrease with the rise of temperature. Moreover, the grains are also found to be refined remarkably, and the strong basal texture is weakened substantially owing to the dynamic recrystallisation especially in the case where dynamic compression is carried out perpendicular to the ED at 473?K.     

Fracture behaviour of C–Mn steel multipass MMA weld metals at −60°C in Charpy V testing

Abstract

By observation of the fracture surfaces and of appropriate metallographic sections of C–Mn steel multipass MMA (manual metal arc) weld metals and simulated weld specimens which were fractured at ?60°C in Charpy V tests, it was found that the impact toughness of the specimen could be correlated with the length of the fibrous crack which was limited by unstable propagation of the cleavage crack; the latter could be initiated at a type of second phase particle transformed from carbon rich regions or non-metallic inclusions. The weakest zone in which the cleavage crack initiated was characterised by coarse grains of ferrite and the critical event which gave rise to unstable propagation of a cleavage crack was a crack in the ferrite grain larger than 30 μm cutting through the boundary and extending over the specimen. On the basis of these results, a model of the fracture mechanism is proposed and the effect of Mn content on increasing toughness is explained using the model.

MST/647     

Effect of strain rate on the tension–compression asymmetric responses of Ti–6.6Al–3.3Mo–1.8Zr–0.29Si

An experimental investigation is performed to explore the tension–compression asymmetry of Ti–6.6Al–3.3Mo–1.8Zr–0.29Si alloy over a wide range of strain rates. A split Hopkinson bar technique is used to obtain the dynamic stress–strain responses under uniaxial tension and compression loading conditions. Experimental results indicate that the alloy is a rate sensitive material. Both tension yield strength and compression yield strength increase with increasing strain rate. The mechanical responses of the alloy have the tension–compression asymmetry. The values of yield strength and subsequent flow stress in compression are much higher than that in tension. The yield strength is more sensitive to change with strain rate in tension than compression. The difference of the yield strength between tension and compression increases with the increase of strain rate. The tensile specimen is broken in a manner of ductile fracture presenting characteristic dimples, while the compressive specimen fails in a manner of localized shearing failure.     

A high-temperature Auger electron spectrometer setup and its application to reactive wetting experiments at 1700 K

A high-temperature Auger electron spectroscopy setup and its in situ application to sessile drop experiments of molten silicon on oxide substrates are presented. The experimental setup allows for measurements of previously inaccessible surface reactions at temperatures up to 1700 K. Auger electron spectra of SiO₂, MgO, and liquid Si are presented. Furthermore, the areas of the substrates that have been transiently wetted by the silicon melt are investigated. The results are discussed with respect to questions concerning reactive wetting of oxides by metal melts, which are important for the material science of joining processes.     

An improved constitutive model for high-temperature flow behaviour of the Fe–Mn–Al duplex steel

The high-temperature flow curves of the Fe–Mn–Al duplex steel showed an uncommon yield-like behaviour and an abnormal dynamic recrystallisation behaviour that occurred at low temperatures rather than high temperatures. The interaction of strain partitioning and unsynchronised softening behaviour in δ-ferrite and austenite caused this peculiar flow behaviour. By discussing the stress exponent and apparent activation energy, respectively, at low and high temperatures, a modified hyperbolic sine function was developed to predict the characteristic stresses. By simplifying the material constant θ and compensating the microstructural evolution in the exponential saturation work-hardening law, an improved constitutive model was developed to predict the transient stress. The comparison between the experimental and calculated values confirmed a high prediction accuracy of this improved model.     

Mesoscale simulation on the shock compression behaviour of Al–W-Binder granular metal mixtures

The mesoscale characteristics of granular metal mixtures have considerable influence on their shock compression properties. However, it is difficult to use traditional research methods, such as theoretical analysis and experiments, to investigate granular metal mixtures in mesoscale. In this study, a mesoscale simulation method, which follows the real morphology distribution, is introduced to investigate the shock compression behaviour of granular metal mixtures based on the mesoscale characteristics. Numerical studies on the compressive processes of typical granular metal mixtures were conducted in mesoscale. The relationship between particle velocity (U_p) and shock velocity (U_s), which is the bridge between mesoscale and macroscale, is established and validated against the experimental results. The influence of mesoscale characteristics of an Al–W-Binder granule metal mixture on the shock compression behaviour has been analyzed based on the simulation results. It has been shown that the simulation results correlate reasonably well with the corresponding experimental results. The numerical studies presented can be used to predict dynamic response of granular metal mixtures with different mesoscale characteristics over a wide range of pressure.     

Violation of Schmid's law and tension–compression asymmetry in L12 alloys: role of ordered structure

Abstract

Hypostoichiometric Ni₃Al(B) undergoes a structural transformation from its initial L1₂ structure to DO₂₂ structure during cold rolling. It has been found that this DO₂₂ structure reverts back to L1₂ structureduring annealing. The transformation to DO₂₂ structure as well as back to L1₂ at higher temperature is very much dependent on the degree of cold reduction. Based on thesefindings, the possible reason for abnormal behaviour of these L1₂ordered alloys, i.e. violation of Schmid's law and orientation dependence of flow stress is hypothesised.     

Strength evaluation of unidirectional carbon fiber-reinforced plastic laminates based on tension–compression biaxial stress tests

Biaxial stress tests of carbon fiber-reinforced plastic (CFRP) laminates were performed to investigate failure criteria under biaxial loads. Specimens of unidirectional CFRP laminates were subjected to a tensile load in the longitudinal fiber direction and a compressive load in the transverse fiber direction. An exclusive jig was used to perform biaxial stress tests with a commonly used single-axis testing machine. Measurements were obtained by controlling the displacement ratio between compressive and tensile displacements. The critical tensile and compressive stresses were then calculated using a constitutive equation. The critical longitudinal tensile stress markedly dropped with increasing the compressive load. The failure criteria of the biaxial stress tests were expressed as the ellipse, of which the major and minor axes were the longitudinal tensile/transverse compressive strengths or fracture strains, respectively. Scanning electron microscope observations suggest that fiber/matrix interfacial debonding due to the compressive load could decrease the critical longitudinal tensile stress.     

Dynamic spheroidisation behaviour of the lamellar Ti–22Al–25Nb alloy during hot compression

The dynamic spheroidisation behaviour of the lamellar Ti–22Al–25Nb alloy during hot compression at 970°C/0.01?s^?1 and strain of 30%, 50% and 70% was investigated. The results showed the lamellar-O phase that participated in the B2 matrix could be obtained by hot pressed sintering from pre-alloy powders. The corresponding scanning electron microscopy, electron backscattered diffraction and transmission electron microscopy observations indicated that the dynamic spheroidisation process can be divided into three stages, the first is the primary interphase high-energy defect stage, introduced by dislocation migration twinning and localised shearing. The second is the separation, and the last is the spheroidisation stages, which could be explained by boundary splitting and termination migration.