Nanomaterials by Severe Plastic Deformation

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)

SPD techniques available at the institute.

 

 

 

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.

Illustration of experimental setups for HPT experiments. (a) Schematic of the “small” HPT tool with a load capacity of 400 kN. Measuring the torque permits the estimation of flow stress.  Heating with induction heating or cooling with liquid nitrogen enables processing between -196°C and 700°C. (b) Image of the small tool during an experiment. (c) “Large” HPT tool with a load capacity of 4 MN. The largest samples have a diameter of ~50mm and a maximum thickness of 10mm.

 

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.

Orientation micrographs of Ni taken at different HPT strains to illustrate the grain fragmentation. Note the different scales in the micrographs. The images represent the grain fragmentation and the formation of a steady state ultrafine-grained microstructure (saturation microstructure).  The measured torque curve during the HPT deformation (bottom right) demonstrates that the occurrence of the saturation microstructure is also visible in the saturation of deformation torque (i.e. the flow stress). The micrographs correspond to certain strains that are marked in the torque curve with colored circles.

 

Mechanical properties

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.

Examples of tensile tests performed with Hpt-deformed materials. (left) Comparison of coarse-grained copper with SPD-processed one. The benefit of marked strength increase is clearly shown as well as the deteroioration of ductility in terms of the strain at fracture. (right) An unusal phenomenon of increasing ductility (strain at fracture and reduction of area) by decreasing the testing temperature from room temperature (RT) to liquid nitrogen found in a SPD-processed refractory metal.

 

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.

Illustration of the potential of HPT consolidation and HPT transformation to a nanocomposite. (a) Illustration of the initial Fe-Cu powder mixture used for consolidation. (b) Micrograph of the Cu-Fe nanocomposite with a filament structure after the first deformation step. (c) Image of the generated nanocomposite by additional HPT after a 90° rotation of the torsion axis - second step. (d) The 2q - XRD scan illustrates the disappearance of the Cu peak proving the formation of a complete super-saturated solid solution (SSSS) of Cu-Fe. An additional schematic shows the extraction of the sample for the second deformation step from the larger HPT sample used in the first step allowing a change in the deformation path.