V. V. Stolyarov, E. A. Prokof'ev, R. Z. Valiev
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Severe plastic deformation (SPD) has been successfully used for fabricating of nanostructures in various metals, alloys, composites and intermetallics. The possibility of this technique to attain nanostructured states in alloys with shape memory effect (SME), in particular Ti-Ni alloy, attracts special interest. Such alloys find very wide application in different scientific investigation fields and industry, especially in medicine. Grain refinement results in essential enhancement of SME properties in these materials (force developed upon SME, decrease in temperature hysteresis, and so on). In this connection the task of the given work was to investigate structure and mechanical properties of Ti49,38Ni50,62 alloy subjected to high pressure torsion (HPT) using very high imposed pressure.
The initial material for investigations was hot-rolled rod, 25 mm in diameter. Samples were cut from the transverse section of the rod and solid solution treated at 800oC prior to HPT. Grain size in this state was about 40 (m. HPT was carried out at room temperature using the rate of 1 revolution per minute. Effect of processing regimes (pressure, strain rate) and geometry of the initial sample on structure and properties of the alloy has been studied. Structure characterisation was performed using X-ray analysis and TEM. Mechanical properties were investigated by tensile tests and microhardness measurements.
Investigations of structure showed that SPD leads to formation of mixed-up structure, where proportion of amorphous and nanocrystalline (d = 10 - 20 nm) phases is determined by processing conditions. SPD results in increase of microhardness up to 6 GPa being more than 2 times higher as compared with the initial alloy. Low temperature annealing of the SPD-processed samples was accompanied by nanocrystallization of amorphous samples areas and grain growth in their crystalline regions. The present work considers the relationship: processing - nanostructures - new mechanical properties in the SPD-fabricated material.
This work was supported by INTAS Grant # 99-01741.

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