Severe plastic deformation at ESI
In the last 15 years several different SPD techniques have been installed and partly improved at the ESI:
- Equal Channel Angular Pressing (ECAP)
- Cyclic Channel Die Compression (CCDC)
- High Pressure Torsion (HPT)
- Accumulative Roll Bounding (ARB)
In principal all these techniques can be used at the institute but HPT plays currently the most important role. The developed new design of HPT by the ESI group, especially regarding the anvil design, has approached an international standard. The advantages of HPT:
- almost no restrictions in applicable strains
- applicable to materials which are difficult to deform
- simple variation of processing parameters, such as temperature, strain rate, pressure and strain path (cyclic HPT)
- simple tool for powder consolidation into bulk form
- transformation of consolidated powders into nanocomposites
- possibility to estimate the evolution of the flow stress by measuring the applied torque.
Most advantages are founded in the typically high hydrostatic stress component, which prevents crack formation. Due to advantages and the successful up-scaling of our HPT-tool this technique, namely HPT, is now the preferred SPD-process at the institute.
The process of refinement
The plastic deformation in crystals is realized by the generation and movement of defects (dislocations). They are partly stored in the crystals and form a cell like structure, where most of the dislocations are in the cell walls and only few in the cell interior. The dislocations in the cell walls cause a difference in the crystal orientation of neighboring cells. With increasing plastic deformation the size of the cells decreases and the difference in the orientation increases. Through this process submicron- or nanocrystalline micro-structures develop. The controlling physical phenomena, materials- and processing parameters are one of the research topics at the ESI. An example is presented underneath.
SPD materials have an inherent high strength compared to standard coarse grained materials. A major roadblock for structural application areas may be a loss in ductility and toughness compared to the coarse-grained counterparts and more importanatly compared to competitive established alloys. A systematic analyses of the effect of SPD on ductility and fracture toughness in different types of SPD materials has been performed at the institute to understand the underlying phenomena. The goal is to develop a microstructural design concept to improve the ductility of submicron and nanocrystalline materials. Our large HPT device is a very helpful tool to generate the necessary SPD materials with sufficient size to overcome size limitations in material testing.
Bulk mechanical alloying
It has been shown at the institute that SPD can be used to transform a coarse two phase alloy in a nanocomposite. A certain or complete solution of even immiscible phases has been found.
It has been shown successfully that such nanocomposites or alloys which can not be generated by classical metallurgical ways can be generated by powder consolidation and subsequent HPT deformation. The principal is similar to mechanical alloying however no sintering is necessary because one obtains a bulk sample. The underlying phenomena and the generation of technically interesting new materials are the topic in this area.