Plastic flow properties and microstructural evolution in an ultrafine-grained Al-Mg-Si alloy at elevated temperatures

An AA6082 alloy was subjected to eight passes of equal channel angular pressing at 100 °C, resulting in an ultrafine grain size of 0.2 to 0.4 µm. The tensile deformation behavior of the material was studied over the temperature range of 100 °C to 350 °C and strain rate range of 10¯⁴ to 10¯¹ s¯¹. The evolution of microstructure under tensile deformation was investigated by analyzing both the deformation relief on the specimen surface and the dislocation structure. While extensive microshear banding was found at the lower temperatures of 100 °C to 150 °C, deformation at higher temperatures was characterized by cooperative grain boundary sliding and the development of a bimodal microstructure. Dislocation glide was identified as the main deformation mechanism within coarse grains, whereas no dislocation activity was apparent in the ultrafine grains.

Structure and transport properties in an N,N-substituted pyrrolidinium tetrafluoroborate plastic crystal system

The structure and transport of N-propyl-N-methylpyrrolidinium tetrafluoroborate (P₁₃BF₄) has been investigated over a wide temperature range in consequence to exhibiting properties suitable for potential solid-state superionic electrolyte applications. Prior to melting, the organic salt, P₁₃BF₄, transforms into a plastic crystal phase. Intrinsic conductivity in this solid, phase I (45–65 °C), is comparable to that in the melt (_~10⁻³ S cm⁻¹). Ionic motion and transport properties were investigated by ¹H and ¹¹B nuclear magnetic resonance (NMR) spectroscopy. Pressure-induced plastic flow in this system may accommodate volume changes in device application and to this extent, X-ray diffraction (XRD) has been used. Scanning electron microscopy (SEM) revealed complex surface morphology and lattice imperfections associated with the strong orientational disorder of the plastic state.

Microstructural and molecular level characterisation of plastic crystal phases of pyrrolidinium trifluoromethanesulfonyl salts

Ambient temperature conductive plastic crystal phases of alkylmethylpyrrolidinium trifluoromethanesulfonyl amide (TFSA) salts are studied using positron annihilation lifetime spectroscopy (PALS) to examine the role of vacancy size and concentration in conductivity. The ethyl methylpyrrolidinium TFSA salt (P₁₂ TFSA) has larger vacancies and a greater concentration of vacancies than the dimethylpyrrolidinium TFSA salt (P₁₁ TFSA) over the temperature range investigated. The relative vacancy size and concentration vary with temperature and reflect the solid–solid transitions as measured by differential scanning calorimetry (DSC). P₁₂ TFSA has greater conductivity than P₁₁ TFSA and has furthermore been observed to exhibit slip planes at room temperature. P₁₂ TFSA has greater entropy changes associated with solid–solid phase transitions below the melting point than P₁₁ TFSA possibly indicating greater rotational freedom in P₁₂ TFSA. These results support the notion that the diffusion, conduction, and plastic flow properties of the pyrrolidinium TFSA salts are derived from the lattice vacancies.

A surface characterisation and microstructural study by scanning electron microscopy of the N-methyl-n-alkylpyrrolidinium tetrafluoroborate organic salts

A microstructural characterisation of the family of N-methyl-N-alkylpyrrolidinium tetrafluoroborate organic salts was carried out by observation of powder surface morphologies with the aim of extending the microstructure-property correlation. Inherent difficulties limiting extensive studies of organic solids by SEM, including volatility under vacuum, charging due to electron beam irradiation, and air-sensitivity were overcome with the use of a Field Emission SEM and cryostage attachment. This technique, providing considerable improvements in image quality at low accelerating voltages, enabled direct observation of complex microstructural features in samples exhibiting high temperature plastic crystalline phases (N,N-dimethylpyrrolidinium tetrafluoroborate [P11BF4]; N-methyl-N-ethylpyrrolidinium tetrafluoroborate [P12BF4]; N-methyl-N-propylpyrrolidinium tetrafluoroborate [P13BF4]). Extensive lattice imperfections including grain boundaries, slip planes and dislocation pits were observed within particles of approximately 200 mgrm diameter. The N-methyl-N-butylpyrrolidinium tetrafluoroborate (P14BF4) sample in this series revealed columnar single crystals with high aspect ratios. The origin of plastic flow properties is discussed using single crystal and polycrystalline slip observations and a relationship proposed between defect characteristics and transport properties.

Textures and compressive properties of ferromagnetic shape-memory alloy Ni48Mn25Ga22Co5 prepared by isothermal forging process

A ferromagnetic shape-memory alloy Ni₄₈Mn₂₅Ga₂₂Co₅ was prepared by the induction melting and isothermal forging process. Dynamic recrystallization occurs during the isothermal forging. The deformation texture was studied by the neutron diffraction technique. The main texture components consist of (110)[112] and (001)[100], which suggested that in-plane plastic flow anisotropy should be expected in the as-forged condition. The uniaxial compression fracture strain in the forged alloy reaches over 9.5%. The final room-temperature fracture of the polycrystalline Ni₄₈Mn₂₅Ga₂₂Co₅ is controlled mainly by intergranular mode.

Crystal structures and textures in the hot-forged Ni-Mn-Ga shape memory alloys

Three ferromagnetic shape-memory alloys with the chemical compositions of Ni₅₃Mn₂₅Ga₂₂, Ni₄₈Mn₃₀Ga₂₂, and Ni₄₈Mn₂₅Ga₂₂Co₅ were prepared by the induction-melting and hot-forging process. The crystal structures were investigated by the neutron powder diffraction technique, showing that Ni₅₃Mn₂₅Ga₂₂ and Ni₄₈Mn₂₅Ga₂₂Co₅ have a tetragonal, 14/mmm martensitic structure at room temperature, while Ni₄₈Mn₃₀Ga₂₂ has a cubic, L2₁ austenitic structure at room temperature. The development of textures in the hot-forged samples shows the in-plane plastic flow anisotropy from the measured pole figures by means of the neutron diffraction technique. Significant texture changes were observed for the Ni₄₈Mn₂₅Ga₂₂Co₅ alloy after room temperature deformation, which is due to the deformation-induced rearrangements of martensitic variants. An excellent shape-memory effect (SME) with a recovery ratio of 74 pct was reported in this Ni₄₈Mn₂₅Ga₂₂Co₅ polycrystalline alloy after annealing above the martensitic transformation temperature, and the “shape-memory” influence also occurs in the distributions of grain orientations.

Crystal structures and textures of hot forged Ni48Mn30Ga22 alloy investigated by neutron diffraction technique

A ferromagnetic shape memory alloy of Ni48Mn30Ga22 prepared by induction melting was successfully hot forged. Strong textures and a large anisotropy of in plane plastic flow were developed during the hot forging process. The crystal structures, both in austenitic and martensitic states, were investigated by means of neutron powder diffraction technique. The result suggests that Ni48Mn30Ga22 has a cubic L21 Heusler structure at room temperature, the same as that in the stoichiometric Ni2MnGa. When cooled to 243 K, the Ni48Mn30Ga22 alloy changes into a seven layered orthorhombic martensitic structure. No substantial change of the neutron diffraction pattern was observed upon further cooling to 19 K, indicating that there is no intermartensitic transformation in the investigated alloy, which is different from the transformation processes in the Ni–Mn–Ga alloys with higher martensitic transformation temperatures.

Wrinkling during cup drawing with NUMISHEET2014 benchmark test

Wrinkling occurs when a blank is subjected to compressive stresses during the forming process as in the flange of a cup during drawing. Although the failure limit due to plastic flow localization can be simply defined by the FLC at each point of a continuum, the wrinkling limit cannot be defined with simple variables such as strain, stress, and thickness. Wrinkling is strongly affected by the mechanical properties of the sheet material, the geometry of the tools and blank, and contact conditions. The analysis of wrinkling initiation and growth is, therefore, difficult to perform due to the complex synergistic effects of the controlling parameters. Because of these difficulties, the study of wrinkling has generally been conducted case by case. A unique wrinkling criterion, which could be used effectively for various sheet forming processes, has not yet been proposed. There were many investigations on the effect of process parameters (BHF and friction) and geometry on wrinkling. However, there are few reported results on the influence of the material model on wrinkling. This paper shows how strain hardening and r-values affect wrinkle formation in its magnitude, initiation, and direction through the NUMISHEET2014 benchmark test for wrinkling during cup drawing.

Strain localisation patterns under equal-channel angular pressing

Instabilities of plastic flow in the form of localised shear bands were experimentally observed to result from equal-channel angular pressing (ECAP) of magnesium alloy AZ31. The appearance of shear bands and their spacing were dependent on velocity of the pressing and applied back-pressure. A generic gradient plasticity theory involving second-order strain gradient terms in a constitutive model was applied to the case of AZ31 deformed by ECAP. Linear stability analysis was applied to the set of equations describing the deformation behaviour in the process zone idealised as a planar shear zone. A full analytical solution providing a dispersion relation between the rate of growth of a perturbation and the wave number was obtained. It was shown that the pattern of incipient localised shear bands exhibits a spectrum of characteristic lengths corresponding to admissible wave numbers. The interval of the spectrum of wave numbers of viable, i.e. growing, perturbations predicted by linear stability analysis was shown to be in good agreement with the experimentally observed spectrum. The effect of back-pressure applied during ECAP was also considered. The predicted displacement of the shear band spectrum towards lower wave numbers, shown to be a result of the decreased shear strain rate in the shear zone, was consistent with the experimentally observed increase of the band spacing with increased back-pressure. A good predictive capability of the general modelling frame used in conjunction with linear stability analysis was thus demonstrated in the instance of the particular alloy system and the specific processing conditions considered.

Study on yield function and plastic potential under non-associated flow for accurate earing prediction in cup drawing

Prediction of more than four ears in a cup drawing process can be successfully achieved by considering r-value and stress directionalities. Yld2004-18p based on associated flow rule and Yld2000-2D based on non-associated flow rule are the examples. The former, however, is more costly in terms of computational efficiency than the latter. In this work, an anisotropic constitutive model based on non-associated flow rule which combines two different functions, Hill (1948) and Yld2000-2d, is implemented to a user defined material model. The accuracy of the anisotropic directionalities (yield stresses and plastic strain ratios) is evaluated. Simulation of a mini-die cup drawing with a body stock alloy predicted eight ears, in good agreement with the experimental results. The use of Hill (1948) model for the yield function and Yld2000-2d for plastic potential under the framework of non-associated flow rule led to accurate prediction of up to eight ears at the lower computational cost.

Plastic deformation in the annealed Zr41Ti14Cu12.5Ni10Be22.5 bulk metal glass under indenter

Vickers and nanoindentationswere carried out on an annealed Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 bulk metallic glass (BMG), and the evolution of the shear bands in the annealed BMG was investigated and compared to that in the as-cast alloy. Results indicate that the plastic deformation in the BMG with the structure relaxation is accommodated by the semicircular (primary) and radial (secondary) as well as tertiary shear bands. Quantitatively, the shear band density in the annealed alloywas much lower than that in the as-cast alloy. The load-displacement curve of nanoindentation test for the annealed alloy exhibited a more flat serrated flow. The annihilation of free volume caused by the annealing was responsible for the embrittlement of the annealed sample.

A heuristic model selection scheme for representing hot flow data using the hot torsion test results

The problem of "model selection" for expressing a wide range of constitutive behaviour adequately using hot torsion test data was considered here using a heuristic approach. A model library including several nested parametric linear and non-linear models was considered and applied to a set of hot torsion test data for API-X 70 micro-alloyed steel with a range of strain rates and temperatures. A cost function comprising the modelled hot strength data and that of the measured data were utilized in a heuristic model selection scheme to identify the optimum models. It was shown that a non-linear rational model including ten parameters is an optimum model that can accurately express the multiple regimes of hardening and softening for the entire range of the experiment. The parameters for the optimum model were estimated and used for determining variations of hot strength of the samples with deformation.

Li-Metal symmetrical cell studies using ionic organic plastic crystal electrolyte

A low current density preconditioning process, which produces an improved lithium transport mechanism is created by the action of charge flow through a plastic crystal electrolyte (figure). A reduction in cell polarisation at high applied current density is demonstrated which approaches the rates required for these electrolytes to be used in practical devices.