L. Hollang

Influence of Additives on Texture Development of Nanocrystalline Nickel

Organic additives such as saccharin have been frequently used in electroplating operations to moderate deposit growth rates and to control film quality. In this study, texture development upon annealing of pulse-electroplated Nickel produced without additives is analyzed by use of electron backscatter diffraction technique. Plating without additives results in a microstructure with slightly elongated grains and a fibre texture in growth direction and this texture is conserved upon annealing up to 600°C. Structural units in form of groups of elongated grains possessing a common zone axis in growth direction and twin relationships between themselves are found in the microstructure. For revealing the influence of additives, the observations are compared with results obtained for Ni and Ni-Fe plated in the presence of additives where during abnormal grain growth the initial fibre texture changes to an energetically more favourable texture by twinning. The lack of additives is assumed to be responsible for the observed differences in texture and microstructure development.

Deformation Mechanisms in Nanocrystalline Nickel at low Temperatures

It is the aim of the present paper to quantify and visualise the grain size induced transition of the deformation mechanism in metal polycrystals from the conventional dislocation–dislocation interaction at large grain sizes to (probably) dislocation–grain boundary interaction in the “nano” region. Since both types of interaction are thermally activated, thermal activation analysis can be used to discriminate between them. For this purpose dynamic tensile tests with stress relaxation tests were performed on pure pulsed electrodeposited nickel with 140 nm grain size at temperatures between 4 and 320 K. The results clearly indicate the transition temperature to be around 77 K. A rather unexpected result is the existence of a second transition of the deformation mechanism, which is only observable at very low temperatures namely from the homogeneous deformation mode governed by conventional dislocation–dislocation interaction towards localized deformation by “catastrophic shear”.

Effect of Annealing on Microstructural Development and Grain Orientation in Electrodeposited Ni

Thick (up to 5 mm) Ni electrodeposits were produced by the pulsed electrodeposition (PED) technique. The PED-Ni was investigated in planar and cross-sections using high resolution scanning electron microscopy. Grain size and local texture were studied by electron backscatter diffraction. Thermal stability and grain growth behaviour were investigated using in-situ annealing in the transmission electron microscope. It is observed that columnar grains are present in the material and that the orientation of grains is not uniform. Textures and in-situ annealing behaviour are compared to previous data on nanocrystalline PED-Ni and Ni-Fe, where a subgrain coalescence model adopted from recrystallization is used to describe the occurrence of abnormal grain growth upon annealing and where twinning was found to be responsible for the texture development.

Deformation mechanisms in ARB processed aluminium alloy AA6016 at low temperatures

The deformation behaviour of the aluminium alloy AA6016 at low temperatures was investigated in the initial state and after 4 and 8 cycles of accumulative roll bonding (ARB). Tensile tests at 25 K, 77 K, 180 K and 296 K were performed at constant strain rate 10−4 s−1. Stress relaxation experiments performed during the tensile tests were used to determine the experimental rate sensitivity λ as a function of the flow stress σ. In all cases λ (σ) is found to be linear revealing that the Cottrell-Stokes law holds. The effect of grain size on σ can be adequately described through an additive athermal stress contribution σd, which is the higher the higher the degree of pre-deformation is. Moreover, the temperature dependence of the strain rate sensitivity m(T) indicates that the rate controlling mechanism in the initial state is local single slip. The ARB states deviate from the single slip behaviour already at 25 K. The reason probably is the occurrence of additional thermally activated slip processes in the ARB states.

Crystallographic characterization of catastrophic shear in submicron nickel at low temperatures

Nickel produced by additive-free pulsed electro-deposition with grain size of about 150 nm was deformed in tension at 320 K and 4 K. At both temperatures the specimens exhibited strong parabolic hardening and good ductility. However, at 4 K a sudden transition from homogeneous deformation to repeated catastrophic shear took place. Microstructural characterization of the sheared region showed deformed grains elongated in between tensile axis and growth direction. The texture information from the sheared region was obtained using conical dark field measurements in the transmission electron microscope. The pole figures of the deformed 4 K specimen show a typical shear texture which clearly differs from the starting texture.

Tensile properties of UFG aluminium alloy AA6016 at low temperatures

The ultrafine grained aluminium alloy AA6016 produced by accumulative roll bonding (ARB) up to eight cycles was deformed in tension at 25K, 77K, 180K and 296K at a constant strain rate. The stress-strain curves were analyzed with regard to the evolution of yield stress σp, ultimate stress σm and fracture strain ef as a function of temperature for zero, four and eight ARB cycles processed aluminium plates. For all materials σp, σm, σm−σp and f decrease with increasing temperature. With increasing number of ARB cycles, σp and σm increase, while σm-σp and f were found to decrease. Based on the experimental results the influence of temperature and number of ARB cycles is discussed.