Showing papers by "Carl C. Koch published in 2009"

Processing and characterization of nanostructured Cu-carbon nanotube composites

Abstract: Carbon nanotube (CNT) reinforced nanostructured Cu matrix composite with a grain size less than 25 nm has been successfully fabricated via a combination of ball milling and high-pressure torsion. CNTs were found to be homogeneously dispersed into the metal matrix, leading to grain refinement with a narrow grain size distribution and significant increase in hardness.

Strong and ductile nanostructured Cu-carbon nanotube composite

Abstract: Nanocrystalline carbon nanotube (CNT)—reinforced Cu composite (grain size <25 nm) with high strength and good ductility was developed Pillar testing reveals that its strength and plastic strain could be as large as 1700 MPa and 29%, respectively Compared with its counterpart made under the same condition, an addition of 1 wt % CNTs leads to a dramatic increase in strength, stiffness and toughness without a sacrifice in ductility Microstructural analysis discloses that in the Cu matrix, CNTs could be distributed either at grain boundaries or inside grains and could inhibit dislocation nucleation and motion, resulting in an increase in the strength

The role of new particle surfaces in synthesizing bulk nanostructured metallic materials by powder metallurgy

Abstract: The role of new particle surfaces in synthesizing bulk nanostructured metallic materials by consolidation of nanostructured powders and nanopowders is analysed by developing three simple mathematical equations for calculating the α factor for different thermomechanical powder consolidation processes such as hot pressing, high pressure torsion and extrusion. The α factor is the fraction of the area of the powder particle surfaces newly formed during consolidation over the total particle surface area which includes both pre-existing surface area and the newly formed surface area. It is demonstrated that the values of the α factor calculated using these equations can be reasonably used to predict the level of inter-particle atomic bonding that is likely to be achieved through cold-welding by the above mentioned typical thermomechanical powder consolidation processes which also include high energy mechanical milling. Based on this analysis, it is clear that uniaxial hot pressing of a powder compact in a rigid die at low homologous temperatures ( T m ) is unlikely to be capable of achieving a sufficiently high level of inter-particle atomic bonding for producing a high quality consolidated material, while processes involving a large amount of plastic deformation have such capabilities.

Mechanical behavior of bulk ultra-fine-grained Zn–Al die-casting alloys

Abstract: The mechanical properties of Zn–4 wt% Al casting alloys are compared after various processing methods including sand casting, die-casting, and high energy cryogenic ball milling. For the cast structures there is an increase in strength when transitioning from a coarse sand casting microstructure to a finer grained thin-section die-casting. This is in contrast to a decrease in strength and increase in ductility seen when the cast structure is broken up by high energy cryogenic ball milling to a uniform ultra-fine grain scale. The ultra-fine grained structures produced by cryogenic ball milling subjected to a range of isothermal heat treatments follow the Hall–Petch behavior over the range of grain sizes studied.

Mechanical Properties of Nanocrystalline Cu and Cu-Zn Using Tensile and Shear Punch Tests

Abstract: Shear punch test (SPT) has been used to study the mechanical properties of Cu, Cu–10 wt.% Zn, Cu–20 wt.% Zn and Cu–30 wt.% Zn after ball milling with an average grain size in the range of 33-12nm. The strain rate sensitivity (SRS) and physical activation volume have been determined. The magnitude observed for these characteristic deformation parameters is very different from their course-grained (cg) counterpart. This suggests that the thermally activated process in nanocrystalline (nc) metal/alloys is different from the conventional forest dislocation cutting mechanism. The stacking fault energy (SFE) of Cu-Zn alloys decreased with the adding of Zn, and deformation twins are anticipated to introduce into the nc Cu-Zn alloys during process of ball milling. Dislocations could accumulate along the TBs and carry the plastic strain, so the ductility of nc Cu-Zn alloys could be improved.