It is well known that grain size has a significant effect on mechanical properties of metals and alloys. There has been a concern about the efficiency of the processing techniques for grain refinement and production of bulk artifact-free samples. Besides that, addition of solutes can dramatically change the stacking fault energy which in turn can have major effects on mechanical behavior. This research describes processing and deformation behavior of ultrafine grained and nanocrystalline copper base alloys.
Stacking fault energy was systematically changed in Cu-Zn and Cu-Zn-Al systems to study its effect on microstructure and deformation behavior. Considering the effect of deformation twins and stacking faults on deformation mechanisms and mechanical properties, different processing methods and conditions were shown to produce different results.
It was found that high energy ball milling at liquid nitrogen temperature can reduce the grain size of copper and copper base alloys down to 5nm. The grain size and hardness of the milling product saturates after 6–8h milling at liquid nitrogen temperature. A combination of cryomilling and room temperature milling resulted in production of sound artifact-free spheres with a homogeneous microstructure that were used for tensile tests. Furthermore, the product of the ball milling was used as a precursor for consolidation by HPT. It was found that ultrahigh strain HPT deformation can successfully produce bulk nanocrystaline samples. Rolling and wire drawing at liquid nitrogen temperature were also utilized to obtain microstructures in ultrafine grain size regime to be compared with their coarse grained and nanocrystalline counterparts.
It was found that lowered stacking fault energy in Cu-Zn and Cu-Zn-Al systems facilitates twinning in favor of dislocation activity. When processing temperature is concerned, deformation at liquid nitrogen temperature increases the propensity of deformation via twinning in copper and copper alloys. Therefore, the utilized processing method, depending on the degree of deformation, could produce microstructures decorated with deformation twins and stacking faults.
In situ consolidated nanocrystalline Cu-12.1at.%Al-4.1at.%Zn with a low stacking fault energy of 7 mJ/m2 was shown to have a high yield strength, 1067 MPa, compared with nanocrystalline copper with the same grain size, resulted from high density of deformation twins and stacking faults. The same composition, processed via rolling at liquid nitrogen, exhibited abundant deformation twins that did not significantly contribute to the mechanical response considering their size and distribution in the microstructure.
Nanocrystalline Cu-Zn alloys were processed with high energy ball milling followed by a consolidation step by high pressure torsion. As shown by tensile test results and electron microscopy studies the breakdown of Hall-Petch relation, for the first time, was found in Cu-30wt.%Zn. A high density of finely spaced deformation twins were observed in the microstructure that arise from high frequency multi directional forces during the ball milling process. The softening behavior and breakdown of the Hall-Petch relation was attributed to detwinning.
|Advisor:||Koch, Carl C., Scattergood, Ronald O.|
|School:||North Carolina State University|
|School Location:||United States -- North Carolina|
|Source:||DAI-B 73/12(E), Dissertation Abstracts International|
|Keywords:||Ball milling, Copper alloys, Nanocrystals, Stacking fault energy|
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