Innovation in light metals proc. and manufacture involving the use of SPD for nanostructuring, mechanical alloying and interfacial bonding

The SUP project IMPROVEMENT is related to process and alloy development of lightweight materials based on large plastic deformations (Severe Plastic Deformation (SPD)), including Equal Channel Angular Pressing (ECAP), Accumulated Roll Bonding (ARB) and High Pressure Torsion (HPT). In these processes ultra-fine-grained (UFG) materials are obtained, and new materials with unprecedented mechanical properties may be produced. With ARB also composite materials may by produced, by ?joining? materials with different properties making hybrid materials with new structural and (multi-)functional properties. With ARB a high degree of deformation are obtained by rolling stacks of sheets into one sheet during rolling through cold welding. A new test to assess the cold-weld quality has been developed. Furthermore, ARB has been used to investigate an interesting phenomenon occurring in AlMn alloys, where low-temperature annealing unexpectedly increases the strength of the rolled sheet. Finally, it has been documented that cold welding can be achieved between different metals, e.g. Al and Cu. New mechanisms on how the layers are mixed by flow instability, as the number of thin layers increase, have been found. Development and improvement of instruments and methodologies to characterize materials at the sub-micro scale, including in-situ experiments in SEM, during various thermo-mechanical treatments have been performed, - the aim being to optimize SEM/EBSD parameters at high acquisition speeds in view of image quality and spatial resolution. In particular, by optimising the operation parameters of the state-of-the-art high resolution FESEMs, an optimum EBSD resolution of 20nm have been obtained. A new compact, lightweight in-situ heating stage, optimised for high resolution EBSD analysis has been developed for studying dynamic processes (e.g. recovery, recrystallization). The methodological studies have been performed along with investigations of microstructure and mechanical properties of Al-Mg alloys after optimized ECAP technologies and in-situ heat treatments of Ti- and Al-Mg alloys. Ti is of great interest due its bio-compatibility, while Al-Mg alloys have a potential for high-strength applications. The possibility of achieving a simultaneous increase in strength and ductility in Al-Mg alloys with high Mg content (beyond conventional high-strength AlMg-alloys) subjected to room temperature ECAP combined with intermediate annealing between passes has been documented. The prominent combination of mechanical properties is attributed to the high work hardening rate and dynamic strain aging (DSA) effect occurring during deformation. The deformed microstructure has been characterized by TEM and SEM/EBSD, showing a bimodal structure with UFG grains (~100-200 nm) distributed around micrometer-sized coarse ones, which probably explains the high work hardening ability. The thermal stability and annealing response of the alloy has also been documented. The new developed Al-Mg alloys are particularly attractive in view of their high strength and good ductility and in a recycling perspective. The process and alloy development related to ARB, ECAP and HPT of Al-Mg alloys and Ti have been accompanied by advanced nano-scale characterization. This work has included establishing newly acquired hardware, ASTAR, for automated crystal orientation mapping (ACOM) in TEM (analogous to EBSD in SEM) in the NORTEM national TEM infrastructure. This technique has a lateral resolution down to the nm-scale, which makes it highly complementary with the EBSD developments described above. It is demonstrated that ASTAR can be used to extract structural information of nano-structured metals processed by SPD, not available by EBSD. Furthermore high resolution TEM (HRTEM) and crystallographic studies of precipitation of the TiH phase in Ti have been performed, as well as TEM-investigations of Ti deformed by HPT. TEM and EBSD investigations of the microstructure of commercially pure Ti processed by ECAP at room temperature have been performed and new twinning mechanisms operating under these conditions have been identified. DFT (density functional theory) calculations of interfaces in Ti processed by ECAP have been performed, for correlation with existing HRTEM observations, and for composition analysis of grain boundaries. The DFT calculations have been supplemented by first-principles molecular dynamics simulations to investigate the stability and appropriateness of the atomistic twin boundary models. These calculations have been applied to simulate accelerated heat treatment of the twin boundaries, and the initial movement of such boundaries is shown by these calculations. Impurity atoms were included in the atomistic models and their segregation tendency towards or away from the twin boundary was assessed. The results are consistent with the experiments, and draw a detailed and comprehensive picture of the twin boundaries present in these materials.

In this project, dealing with innovative light metals processing, the potentials of using severe plastic deformation (SPD) for nano-structuring, mechanical alloying, possibly also including new materials for hydrogen storage, and interfacial bonding will be fully explored and documented. The main focus is on advanced electron microscopy for nano- and microstructure characterisation, new equipment design for in situ thermo-mechanical processing in the SEM in combination with EBSD mapping, and nano-indentat ion testing for studies of processing-structure-property relationships. As a starting point, well-established SPD techniques such as accumulated roll bonding, high pressure torsion and equal channel angular pressing are used to manipulate the nano- and mi crostructure of specially designed aluminium, magnesium and titanium alloys. High-resolution field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) are then employed to study the internal grain- and dislocation stru ctures, the crystallography and connectivity of the grain/subgrain boundary networks, the length scale (layer thickness) of the laminate materials as well as the break-up of the natural oxide films at internal interfaces. The thermal and mechanical stabil ity of these nano-scale structures are, in turn, evaluated in-situ in the SEM by employing the combined hot-stage and tensile test unit being developed and tailor-made for the purpose. This structural characterisation, along with the subsequent mechanical and nano-indentation testing of the materials, provide a systematic basis for modelling the evolution of the internal grain- and dislocation structures and the interfacial bonding process during deformation. The results of this fundamental research will form the basis for coming collaborative projects with potential industry users, which aims at breaking new ground for process innovation and product developments within the Norwegian light metals industry.